The case of Paraeurypterus anatoliensis is far from being unique. Paleontology historically benefited a lot from colonialism (which used it in turn for cultural soft power), a dynamic that led to underrepresented of interest & works on non-European or non-North American fossils & people. The discoveries from Africa, South Africa, Asia & Oceania, made these last decades more & more by people from these areas unraveled ground breaking advances, like, in today’s case, for eurypterids. The entire old vision of the group, now false due to bias from historical & political choices, has led to analytical errors and ignorance of potential fossil-rich localities, and it’ll take time to recover from it & rebalance north & south knowledge.

Is this paleoart accurate ? This one is a blend of the holotype & phylogenetic bracketing, I drew it with the goal to give it a basal dolichopteroid vibe. The prosoma & mesosoma are taken straight from the original description, including for the lil’ scales on the mesosoma. Since the appendages are unknown, I chose to go with a classic Dolichopterus-like look, but still made the paddles relatively small & slender since it’s a basal species. The metasoma & telson I chose to add are of the most generic type possible for a semi-derived eurypterine, with a sword-like telson and basic segments bearing epimeres on their sides.

REFERENCES • Lamsdell J.C, 2025. Codex Eurypterida: A Revised Taxonomy Based on Concordant Parsimony and Bayesian Phylogenetic Analyses. Bulletin of the American Museum of Natural History 473, 195 pp. · Lamsdell J.C., Hoşgör I., Selden P.A., 2013. A new Ordovician eurypterid (Arthropoda: Chelicerata) from southeast Turkey: Evidence for a cryptic Ordovician record of Eurypterida. Gondwana Research 23, p. 354–366. · Monarrez P.M., Zimmt J.B., Clement A.M., Gearty W., Jacisin J.J., Jenkins K.M., Kusnerik K.M., Poust A.W., Robson S.V., Sclafani J.A., Stilson K.T., Tennakoon S.D. & Thompson C.M., 2022. Our past creates our present: a brief overview of racism and colonialism in Western paleontology. Paleobiology 48, p. 173–185. · Perinçek D., Duran O., Bozdoğan N. & Çoruh T., 1992. Stratigraphy and Paleogeographical Evolution of the Autochthonous Sedimentary Rocks in Southeast Turkey. Ozan Sungurlu Symposium, Proceedings. Tectonics and Hydrocarbon Potential of Anatolia and Surrounding Regions. Turkish Petroleum Corporation - Turkish Association of Petroleum Geologists, p. 274-305. The ICS international chronostratigraphic chart 2025. Episodes 2025. Online at https://stratigraphy.org/chart Colorado Plateau Geosystems Inc., 2026. Global Series. In DeepTimeMaps. Online at https://deeptimemaps.com/map-lists-thumbnails/global-series/ Dunlop J. A., Penney D. & Jekel D. 2023. A summary list of fossil spiders and their relatives. In World Spider Catalog. Natural History Museum Bern, online at http://wsc.nmbe.ch, version 23.5.

I wanted to talk about this species a lot also because it was one of my awakenings of how occidental dominance & colonialism affected paleontology's history, and how we're still recovering from it (+drawing feedback & refs).

Thank you for reading & I’ll see you on the next #Cheliceratime!
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As its name implies, Paraeurypterus anatoliensis was considered at first as a close relative to Eurypterus. With Pentlandopterus minor (previously called “Eurypterus minor”), both species were regarded as part of a grade linking more derived groups (mostly Eurypterus and the diploperculates) with more basal families, and were seen as some sort of transitional form between dolichopteroid-like groups and more derived clades. Questions still remained on the exact placements of P. anatoliensis, which still remained outside of any large group while pushing back ghost lineages of several groups in the Ordovician. The slide also comprise a tree of the first phylogenetical placement (2013) of Paraeurypterus anatoliensis as a relative of higher derived groups like diploperculates

However, a super extensive work by James Lamsdell in 2025 changed the narrative, retrieving Paraeurypterus anatoliensis as the most basal dolichopterid, far from the original Eurypterus-like idea. This new relationship matches better with the prosoma & eyes shape, as with the few appendages’ bits known. It also provides concrete evidence of an Ordovician lineage for this family that diverged relatively early in eurypterines history, and reinforces the sampling of Ordovician eurypterids that show ancient roots to the groups that radiated during the Silurian. The slide also showcase a phylogenetic tree of eurypterids from 2025 where Paraeurypterus anatoliensis sits as the sister group to all other dolichopterids, far from diploperculates

Despite its relatively generic look, P. anatoliensis is a big deal for our understanding of eurypterids’ evolution. Eurypterids were traditionally seen as an only-North American & European group (the paleocontinents of Laurentia, Baltica & Avalonia) that originated here and later dispersed to other continents (an hypothesis that was reserved only to species thought to be able of oceanic dispersion). Back in 2013, the discovery of P. anatoliensis made it really special for an eurypterid: it was at the time only the second named species from an area that was part of the Gondwana, and it lived before the Silurian eurypterids’ golden age, contradicting a lot of what was theorized. In the decade that followed, more and more eurypterids from the Southern Hemisphere were brought to light.

These discoveries completely shattered the traditional consensus, and a new scenario of eurypterids’ evolution is slowly emerging. The probability of the group, due to several basal and ancient gondwanan members, to have originated in the shallow seas of Gondwana, is growing more and more. Some families would have secondarily ended up in Laurentia & Baltica, at first a selected few in the Ordovician like Megalograptids, and then all the order radiated during the Silurian in the Euramerican paleocontinents, giving rise to the most iconic eurypterids’ genera like Eurypterus, Pterygotus & Mixopterus. What’s interesting is how many of these families, even those thought to not be capable of oceanic dispersal, lived all around the world, like mixopterids & pterygotids, which indicate these arthropods truly were widespread predators with cryptic gondwanan origins. Lower-Mid Ordovician: First eurypterid diversification in Gondwana Upper Ordovician-Lower Silurian: Eurypterids well established in Gondwana, first Euramerican occurences by the South, oceanic dispersion Mid-Upper Silurian: Eurypterids distributed worldwide, Euramerican radiation

When this species shines is when talking about eurypterids' evolution, its discovery changed a lot by shattering previous assumptions and showing a unknown possible evolutionary path for eurypterids that could change a lot in how we study them.

#Cheliceratime

Paraeurypterus anatoliensis is an eurypterid species named in 2013, known by a single specimen found in Southeast Turkey, near the border with Iraq. The fossil was discovered in rocks from the Şort Tepe Formation, a stratigraphic unit of Upper Ordovician age.

Paraeurypterus anatoliensis lived sometime during mid-Katian stage (~450 million years ago) in waters that covered what would become the northern parts of the Arabian plate, making today some territories of southern Anatolia (the Asian part of Turkey). This makes P. anatoliensis a gondwanian eurypterid that lived in the Southern Hemisphere. The Şort Tepe Formation was at this time an outer continental shelf, at the edge of a large chunk of land covered by water in a marine transgression context.

The only known specimen is an incomplete individual made of a prosoma, 7 opisthosomal segments and some legs’ remains. Telson’s morphology is still unknown. The prosoma is in the shape of a rounded square, typical of derived non-diploperculates eurypterids. There are also diagnostic lil’ scales on the axis of opisthosomal segments. The eyes are well-developed with a strong crescentic/ semi-circular shape with enlarged palpebral lobes, and sit in the front half of the prosomal carapace.

The legs’ morphology is unknown, only small bits of the 3°, 4° and 5° legs being visible. The 5° one seems to be enlarged, which suggests the presence of larger paddle-like legs. A leg spine, coming from on the front legs, is also visible. All these features make P. anatoliensis a medium-size eurypterid looking a lot like dolichopterids, Eurypterus or Erieopterus. Due to these scattered remains, this species’ ecology is hard to assess. It was probably a semi-swimming hunter of bottom dwellers.

First, there's already a lot to say regarding when & where it lived and what it looked like, even tho only one specimen is known at this day.

#Cheliceratime
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This turkish eurypterid, despite its relatively generic look, is of great importance in the understanding of how these animals evolved before their Silurian golden age, both when looking at species relationships, paleobiogeography & science’s history. Size : a bit more than 10cm of body length Time period : Late Ordovician Paleoart speculativometer : Mostly based on another taxon The animal drawn looks a bit like a scorpion but without claws, short spiny legs at the front, a last pair of legs enlarged and used as fins, and a long sword-like telson in place on the sting

What time is it? It’s #Cheliceratime & #Fossilfriday!
Today, one of my favourite eurypterids for all it represents, Paraeurypterus anatoliensis!
All the basic infos are here but if you want to learn more, there’s more below!⬇️🧵

#eurypterid #ordovician #turkey #paleoart #sciart #bugsky #invert
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REFERENCES (part 1) · Costa S.G.S, Tolstikov A., Saboori A., Batista-Ribeiro D., Noei J., Harvey M.S., Shaw M.D., Klimov P.B., Zhang Z.-Q., Pepato A.R., 2024. One-way ticket to the blue: A large-scale, dated phylogeny revealed asymmetric land-to-water transitions in acariform mites (Acari: Acariformes). Molecular Phylogenetics and Evolution 177, 107626. · Klimov P.B., OConnor B.M., Chetverikov P.E., Bolton S.J., Pepato A.R., Mortazavi A.L., Tolstikov A.V., Bauchan G.R. & Ochoa R., 2018. Comprehensive phylogeny of acariform mites (Acariformes) provides insights on the origin of the four-legged mites (Eriophyoidea), a long branch. Molecular Phylogenetics and Evolution 119, p. 105-117. · Krantz G.W. & Walter D.E., 2009. A Manual of Acarology. 3° Edition, Texas Tech University Press, 816 pp. · Linné C. von & Salvius L., 1758. Caroli Linnaei...Systema naturae per regna tria naturae :secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Holmiae: Impensis Direct. Laurentii Salvii. · Makol J., 2000. Catalogue of the World Trombidiidae (Acari: Actinotrichida: Trombidioidea). Annales Zoologici (Warszawa) 50(4), p. 599-625.

REFERENCES (part 2) · Makol J. & Felska M., 2024. The syntopic occurrence of velvet mites Trombidium spp. (Acariformes: Trombidiidae) with the first characteristics of the larva of Trombidium heterotrichum. Scientific Reports 14, 32155, 14 pp. · Makol J. & Wohltmann A., 2000. A redescription of Trombidium holosericeum (Linnaeus, 1758) (Acari; Actinotrichida: Trombidioidea) with charasterics of all active instars and notes on taxonomy and biology. Annales Zoologici (Warszawa) 50(1), p. 67-91. · Schmidt J.O. & Schmidt L.S., 2022. Big, bad, and red: Giant velvet mite defenses and life strategies (Trombidiformes: Trombidiidae: Dinothrombium). The Journal of Arachnology 50, p. 175-180. INaturalist, 2026. Trombidium holosericeum. Online at https://www.inaturalist.org/taxa/58397-Trombidium-holosericeum GBIF - Trombidium holosericeum https://www.gbif.org/fr/species/4540860

And of course: some references, thank you for reading I’ll see you on the next #Cheliceratime!

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Trombidium holosericeum was one of the first mites ever given a scientific name, its first mention dating back to the Systema Naturae of Carl von Linné in 1758. At that time, it was housed like all mites in the genus Acarus, and its name was “Acarus holosericeus”. The second part of its name referred to its velvet look, and is derived from the Ancient Greek ὅλος, holos, "whole" and σηρικ-, serik-, "silken". The genus name Trombidium was later introduced by Fabricius in 1775, Trombidium holosericeum gaining its definitive name in the process and being chosen as the type species (species of reference) for the genus. Over time, some authors also used alternative genera, like Atomus by Latreille in 1795 or Metathrombidium by Oudemans in 1909.

But, no surprise here, due to these different names used and the quick outdated description of Linné that mushed all mites together, what is the genus Trombidium turned out to be really blurry. These problems were solved in an extensive revision in 2000, giving a fresh start to the species. The original specimen being lost or destroyed (IF there even was a cataloged specimen), this revision was a total reboot. New specimens of reference were appointed, the different stages of the T. holosericeum life cycle were extensively described and numerically measured, and the new description followed the precise observation level needed for mites, like by making setae morphology mandatory. The naming history of this species is a prime example of how important good definition of described species are important, as well as knowing taxonomic correspondences through science history & preservation of specimens are crucial in biology.

Due to how frequently humans encounter this animal & its close lookalike relatives, common names can vary across languages. This animal is among the few arachnids to have more than one common name in some languages, sometimes of diverse origins. The German “Rote Samtmilbe” is a simple association animal<->feature, while the French “araignée rouge” keeps its color while identifying it as a larger animal (in this case, a spider). Most languages highlight its size or velvet-like look. The slide also comprise a chart summarizing some of the common names of this mite: Bulgarian/български => Кадифен кърлеж English => Red velvet mite, True velvet mite, Rain bug French/Français => Araignée rouge, Acarien rouge, Trombidion soyeux German/Deutsch => Rote Samtmilbe Hungarian/Magyar => Közönséges bársonyatka, bíboratka Dutch/Nederland => Fluweelmijt Swedish/Svenska => Braae, Bråde, Bråe, Bråfrö

Is this drawing accurate ? It’s more a Trombidium sp. than a Trombidium holosericeum to be honest, but it still fits well for this species. The idiosoma’s dorsal side was a pain in the *ss to be honest due to how many refs have different looks, so I did simple bumps. I had trouble to properly draw the chelicerae so that’s why they’re hidden under the pedipalps. Could have done better on the claws.

First mentions of this species go all the way back to 1758, and how its scientific name evolved is interesting to talk about, as much as its numerous common names across languages (+the usual drawing auto-feedback)

#Cheliceratime
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Like all velvet mites, this species occupies different ecological niches depending on its growth stage. Like said before, larvae are ectoparasites acting as ticks on humans but on an arthropod scale. They can be found on virtually any type of arthropod, from harvestmen and flies to grasshopers, beetles, aphids, spiders and many others. They are well visible by looking like red bubbles on the parasitized arthropod. Full grown adults, thanks to their size, are predators capable of taking down large arthropods relative to their size, like small insects, larvae, other mites & spiders, although they can often feed on arthropod eggs. Adults are also good burrowers and can hide in soil for a long time.

Thanks to extensive studies on its biology, the life cycle of this mite is well-known, and is similar to other species of Trombidium. This species can in theory live all year round, but the mating period in northern spring often restrains the yearly periodicity of its life cycle, and low temperatures can also inhibit growth. Reproduction EGGS Hatch (after 1 month) LARVAE: At first mobile, search host to feed. Can survive more than a month without feeding, lasts from 1 to 3 months. Length : 0,2-1 mm Short active phase, then immobile state for a month DEUTONYMPH (Second nymphal stage): Mobile at first with its 8 legs, stage ending with an immobile growth period. Lasts 1 month and a half. Length : 1-2 mm Immobile growth for 1 month before final molt ADULTS: Full size, sexual maturity. Females last longer (months) than males. Length : 2,5-5 mm TOTAL LIFESPAN: +/- 1 to 2 years

Trombidium holosericeum belongs to a group called Parasitengona, a hypo-order of mites traditionally placed in the anystines (which monophyly isn’t always recovered), a large cohort of predatory trombidiformes mites. Most parasitengones are predators easily distinguishable from other mites by their classic epiparasitic larval feeding stage, even tho this feature wasn’t that developed until more derived families appeared. The slide also displays a phylogenetic tree of acariform mites showing parasitengones as part of the anystines in the trombidiformes

Parasitengones contains a pletra of lineages, today’s species being part of the Trombidioids, more precisely Trombidioids stricto sensu. Of the 5 families making up this group, as its name implies, Trombidium holosericeum is the center point of the family Trombidiidae. This family houses the largest mites in the world non-harmful to humans: the Trombidium genus itself is of average size with 2 to 5 mm in length at adulthood, but Dinothrombium can exceed 1cm in length. The slide also showcase a phylogenetic tree of parasitengones in which tombidiidae are deeply nested as some of the most derived families of the whole group

Being big and one of the most documented mites in the world, this species' biology & relationships have been extensively studied, especially on the life cycle side.

#Cheliceratime
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Trombidium holosericeum is a species of velvet mites, a family nicknamed thus in reference to high concentration of setae on their exoskeleton, giving them a velvet-like look. It’s a really large (often 2 to 5 mm long) mite with a heavy build based on the classical mite body plan: 4 pairs of 7-segmented walking legs densely covered with setae, clustered by 2 at the front & at mid-length. The femur is split into 2. A front part called the gnathosoma, bearing the mouthparts (scissor-like chelicerae & sensory pedipalps). Numerous setae on idiosoma’s dorsal side, of 2 types & arranged in parallel rows. A fused prosoma & opisthosoma, forming a large idiosoma that bears the walking legs.

The Trombidium genus houses many different species, a lot looking similar to one another. The precise species’ determination, like it’s often the case among arachnids, relies on precise criteria of genital organs, like shape’s details and setae configuration. These small details are of great importance when needed to be sure of the species and not staying with a complex of species, T. holosericeum and T. geniculatum being for example almost identical if not for reproductive morphology details. The bright red coloration results from carotenoids (red-orange pigments), and as such color can evolve among animals, it serves as a warning to deter predators by indicating significant toxicity if consumed. This kind of flashy advertisement strategy is called aposematism, and is a major driver for red or orange coloration to evolve among non-apex predator species.

Like other mites, T. holosericeum goes through several phases during its life, mainly 3 being important: Larva, Deutonymph & Adult, and molt each time it reaches a new growth stage. Fun fact, like all mites, the larval stage only displays 6 legs, a feature commonly used to recognize mites’ larvae (and that, with its long legs here, make it similar to small insects if no further attention given). The fourth pair pops out later during the nymphal stage, when they look like mini-mites, before reaching their final size as adults. The larval stage is distinctive, the young mite behaving like an ectoparasite, feeding upon other arthropods in a fashion similar to how ticks can feed on us humans. The larva can more than double its size in the process.

Trombidium holosericeum has been found across a lot of the Palearctic biogeographical realm, most occurrences focusing on Europe. Due to the fact that Trombidium’s species can look undistinguishable to the naked eye at first glance, many other species can be confused with today’s criter, so to make it simple: if you’re seeing a mite looking like T. holosericeum outside of Europe, there are good chances for it to be of another species. This tiny arthropod is an edaphic species, living almost all its life at ground level on humid soils with moderate humidity. It can be found in a large diversity of environments, from forests & forest edges to anthropic environments like parks & gardens.

First, let's talk about its morphology: from sheer anatomy, size, color, body changes through its life, there's a lot to say! And some details on its distribution.

#Cheliceratime
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Like other velvet mites, this species is among the largest mites in the world, being visible to the naked eye and capable of hunting other smaller mites. Size: 0,5-ish mm of body length Time period: Holocene (present day) Conservation status: Least concern The animal drawn is a mite with a large red main body & 8 short but thin legs

What time is it? It’s #Cheliceratime!
One of the biggest mites in the world in Cheliceratime today, the bright red Trombidium holosericeum!

All the basic infos are here but if you want to learn more, there’s waaay more below!⬇️🧵

#mite #arachnid #sciart #bugsky #invert

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Is this drawing accurate ? It’s an old one of mine from 5 years ago, it’s a bit simple but it still works well. Seems like I forgot to clearly draw the limit between the patella & tibia on each leg The eyes could be a bit bigger and with a denser outer ornamentation. I chose this position in respect to how the spider handles its web

REFERENCES • Bradley R.A., 2012. Common Spiders of North America. University of California Press, 624 pp. · Blest A. D., 1978. The rapid synthesis and destruction of photoreceptor membrane by a dinopid spider: A daily cycle. Proceedings of the Royal Society of London Series B. Biological Sciences 200, p. 463–483. · Coddington J.A., Matjaž K. & Opell B.D., 2012. Systematics of the spider family Deinopidae with a revision of the genus Menneus. Smithsonian Contributions to Zoology 636, 61 pp. · Da Ponte R.P., Stefani V. & Vasconcellos-Neto J., 2021. Natural history of the ogre-faced spider Deinopis cf. cylindracea (Araneae: Deinopidae): revealing its phenology. Studies on Neotropical Fauna and Environment 56, p. 210–219. · Goldstein L.M., Lietzenmayer L.B. & Taylor L.A., 2022. Ogre-faced Spider, Net Casting Spider, Gladiator Spider Deinopis spinosa (Marx, 1889) (Arachnida: Araneae: Deinopidae): EENY-779/IN1356, 4/2022. EDIS 2022, 4 pp. • Jocqué R. & Dippenaar-Schoeman A.S., 2007. Spider Families of the World. 2° Edition, Royal Museum of Central Africa, 336 pp. · Kulkarni S., Wood H.M., Hormiga G., 2023. Advances in the reconstruction of the spider tree of life: A roadmap for spider systematics and comparative studies. Cladistics 39, p. 479–532. · Stevenson D.J., Brown G., Chandler H., Daniel D.D., Garza C., McWhorter M., Moore M. & Thomas A., 2018. Recent Noteworthy Distribution Records for Deinopis spinosa (Marx, 1889) (Araneae: Deinopidae) in the Southeastern United States. Southeastern Naturalist 17, p. 28–33. INaturalist, 2026. Deinopis spinosa. Online at https://www.inaturalist.org/taxa/528913-Deinopis-spinosa Deinopis spinosa - World Spider Catalog https://wsc.nmbe.ch/spec-data/15290/Deinopis_spinosa

And of course, the usual drawing auto-feedback & some references, thank you for reading I’ll see you on the next #Cheliceratime!

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Deinopis spinosa is a nearctic species & the northernmost Deinopis’ species. It’s known in Texas & South East U.S.A. It lives in coastal forests & plains, with no habitat vegetation preferences: pine forests, swamps, coastal hammocks, oak forests, and so on. Understanding this species’ precise distribution is still a work in progress. Sightings in the Caribbean have also been reported. Specimens are mostly observed from spring to early winter. Like many spiders, this species has a 1 year-ish life cycle.

This spider is often tricky to spot in its environment, spending the daytime with its legs aligned in the form of a stick (even tho males tend to be less cryptic). When nighttime comes, it can be found with its thin web between plants, some individuals being almost at ground level. Deinopis spinosa feeds upon a large range of arthropods, crickets being the most seen on the Internet, but close species have been found to be good hunters of ants & beetles too for example. This diversity makes it an euryphageous species, a species with a super generalist diet. Interestingly, studies on close species showed these spiders as species are adaptable to some environmental changes but need time to process them.

Apart from their eyes, deinopids are famous for their unique hunting style. Like many spiders, they are of the sit-and-wait type of hunters, but it’s there that it becomes weird: They scrutinize their surroundings, waiting for a prey to come by and hold between their legs a small web... and when it happens, they cast on their prey the said web to grab them and bring it up to them, only then using venom & surrounding the prey with more regular silk threads. This unique hunting style is what owns them their name of “net casting spiders” or “gladiator spiders”, and is a hunting behavior of a mean 50% efficiency.

The hunting web has the particularity of being sticky... without being made of sticky silk fibers. It’s a type of silk called “cribellate silk”, made by a complex entanglement of fibers through a structure called cribelum, making that in the end, simply by the physical structure of the threads, they stay on the prey & make the web adhesive by sheer physic trick. True sticky silk, lined up with sticky fluids, is exclusive to the group of spiders called “araneoids”, which already diverged from deinopids long before the net casting behavior evolved, and contains other species like orbweavers, widows & long jawed spiders. Cribellate silk on the other hand, looking sometimes a bit like a “wooly silk”, is the default setting among spiders for developing sticky webs since it only needs morphological adaptations of their spinnerets & mastery of some spining techniques, which can be both evolved convergently.

This north american species is famous for its hunting style of casting its web on its prey, but is a rather discrete animal on the field.

#Cheliceratime
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Deinopis spinosa is a rather large species of net casting spider, and exhibits the classic body plan of this spider family. It has a long & thin body (narrow prosoma & tubular opisthosoma) with slim legs and has a brown-greyish irregular coloration that helps it to blend in its environment as a twig. Like all spiders from this family, it has extremely enlarged eyes. The “ogre spider” name came from its super large eyes, giving it a distinct “face” for a spider. These eyes are the origin story of the name Deinopis, meaning “terrible eyes”. It’s sometimes called “Dinopis” despite being an incorrect phrasing, the confusion between deino/dino being a frequent mistake for many unrelated scientific names.

Both males & females look similar and are of roughly similar size to a few millimeters, most specimens ranging between 1,5 & 2,5 cm of main body length. Sexual dimorphism is mostly visible not by size, but by adult morphology. Males are way thinner than females & have smaller eyes, and spend their time to look for a mate thanks to airborne cues from mature females. They also stop hunting, making large eyes not so needed anymore. Females, on their side, have really large eyes & enlarged femur.

This spider’s eyes deserve their own slide because there’s a lot to talk about them. First, even tho they look a bit like jumping spiders large front eyes, they aren’t the same: Net casting spiders’ large eyes are in fact median posterior eyes, meaning their big eyes are enlarged eyes normally more on the back, that migrated to a more upfront position, while the median front ones became smaller. These eyes are the most light sensitive of any spiders, 2000 times more than our own eyes or those of jumping spiders. This oddity grants them an amazing sight but comes at the cost of being an only nocturnal species, daylight being too harmful for an arthropod needing low light to see well. This is even more impressive due to the total absence of tapetum lucidum (a type of light-reflective membrane that many nocturnal-leaning animals have, like cats): instead, the light-sensitive membrane inside the eyes is destroyed each morning and then rebuilt every night, making these eyes some of the most biologically active areas of this spider’s whole body.

Net casting spiders (Deinopidae) diverged early on from other entelegyne lineages (spiders with complex reproductive organs, making up like ⅔ of current species), taking part in the grade of various enetelegyne lineages from which emerged later the RTA-spiders (like jumping spiders, ground spiders, lycosoids, etc). They are close relatives to Oecobiioids, a group encompassing families with derived features at the opisthosoma end, such as long setae or elongated spinerrets. Deinopis spinosa itself is often recovered as a basal member of this family. The slide showcase a cladogram in which deinopids are the sister group to oecobiioids, this group itself being the sister group ot more derive entelegynes like jumping spiders & wolf spiders. They are far from the araneoids (orbweavers) and synspermiates (like daddy long legs)

You know the drill, let's talk about its morphology, unusual traits & phylogeny first to meet this spider

#Cheliceratime
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This spider & other close species are famous for their outlandish sight capabilities and unusual hunting style, throwing their hunting web like a net on their prey to catch them. Size: 2-ish cm of body length Time period: Holocene (present day) Conservation status: Least concern/Near threatened The animal drawn is a spider with a slim tubular body, long thin legs and giant eyes, as big as its fangs

What time is it? It’s #Cheliceratime & #spidersaturday !
And today's time to talk about net casting spiders with Deinopis spinosa!

All the basic infos are here but if you want to learn more, there’s more in the thread below!⬇️🧵

#spider #arachnid #sciart #bugsky #invert

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Paleoarts accuracy For extinct species, since I grew “a bit” bored of how childish both paleoartists & scientists can sometimes get when thinking the other side is always wrong when reconstructing extinct organisms, I decided to be 100% honest about my approach. Each post about an extinct species features a “Paleoart speculativometer”, to indicate the amount of scientific fidelity of the drawing and at what degree my take may reflect what these animals could have looked like. I may not be a professional researcher, but I’m still a science person & a paleoartist at the same time. I have no problem with speculations, they’re an essential part of paleontology & its history, but being honest about my own work quality & relevance doesn’t make them lesser pieces. And I think it’s even more important when a certain number of the entries I publish feature the first paleoarts of some extinct species, it’s simple transparency & honesty. 5 levels of the speculativometer: Pure speculation Mostly based on another taxon Some missing parts Few missing details Almost a carbon copy

Science content I apply to the posts the same bibliographic & research methods I practiced in academia. I also consider a variety of sources (publications, books, databases, podcasts, etc) to cross sources as much as possible and publish detailed work. If I’m wrong (I’m human, we all fail sometimes, that happens): unless it’s a gigantic error that would require to delete the post, I’ll had an erratum in comments. You are welcome to report mistakes, I may be confident in my capacities but I know that I’m still a generalist compared to some super-specialized folks among you. If there are personal takes, I try to make them as explicit as possible, like in the case of the euchelicerates’ phylogenetical mess being gradually solved these last years.

AI & Chelcieratime If you were hoping to see AI slop here, you’re not on the right account. I’m strongly opposed to this sh*t that steals people’s work, drains vital resources, has no soul and harms the very environment we’re supposed to love as nature enjoyers. All the work you’ll see on Cheliceratime is AI-free, from the drawings to the texts & references used, you have my word. Art is about trying again & again, learning (here both on the scientific & artistic sides), expressing yourself, your tastes, how you perceive the world, etc. No one asks of you to be perfect (‘cause a perfect art doesn’t exist, each take is unique in its own way), and if drawing isn’t your strong suit reach out to people who love to do that. We can do a lot on our own, but, even if it’s hard sometimes, acting together by acknowledging our flaws & strengths is when we grow and accomplish wonders.

No backing down on important topics I’m gonna be 100% honest: science IS political, it’s shaped by our choices as humans, therefore it IS political. It’s not by putting important subjects under the carpet that we deal with them properly and move forward as a society towards a fairer future for us, the biosphere and the planet. So, if such topics are to be discussed when talking about chelicerates, I’ll talk about them without detours. Each extant species’ post also features a pictogram of its conservation status. The 6 levels of conservation status used in Cheliceratime: Least Concern Near Threatened Vulnerable Endangered Critically endangered Extinct in the wild (I hope I’ll never have to use this one) Some examples of cases where humans' actions have to be talked about: Poecilotheria metallica (the peacock tarantula) Soon to be extinct in the wild due to deforestation and wild specimens collects. Burmathele biseriata (cretaceous segmented spider) Discovered in Burmese amber, which exploitation is directly linked to armed conflicts and child labor. Modern horseshoe crabs May be driven to extinction by the pharmaceutical industry and pollution.

It's the serious part of the thread: some words about my views on the paleoart accuracy you'll see here, science content, AI use (spoiler: no AI slop here) and politics when it comes to scientific topics.

#Cheliceratime
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Posts’ structure Each post contains: A front cover summarizing the take home message. Slides about morphology, ecology, phylogeny & other topics, it’s the core of the post. A slide to talk about the drawing process (more on that in a few slides) 1 or 2 last slides for references of the works used to make the post Posts’ length is variable, it often gravitates around 9 to 12 slides, but depending on the subject it can reach 20 slides. The drawing process slide isn’t part of the thematic posts.

Drawings & paleoart I make a point to draw each chelicerate I talk about, one of the goals of Cheliceratime being to give these animals more representation, and to use the knowledge I have to create the most accurate paleoarts possible for extinct species. Apart from some diagrams made digitally, I have a strong preference for traditional hand drawing. All the species you’ll see in the posts will have been drawn with pencils & markers, and colored with color pencils, giving the whole drawings & posts a field notebook vibe.

The drawing auto-feedback This part is, in my eyes, one of the most important of the species’ posts. It’s where I’m fully transparent on the strengths and flaws of the drawing that served to illustrate the whole post, and where I can talk about the choices made, especially for extinct species where speculation plays a bigger part in the drawing process. Art isn’t about being perfect, but always trying!

Drawing galery & use You can find all the animals drawn for the posts on my DeviantArt page. You are free to use my drawings for non-monetized projects, I just ask of you proper credit with the image (just ©Amypteride or ©Cheliceratime will suffice). If commercial use or any profitable form of use is at play, I knidly ask you to discuss with me about it. In any case, if you have questions about the drawings, whether it’s use-related or not, feel free to ask, I’ll happily chat about it with you and/or work with you.

More details on the post structure and the drawings I make for cheliceratime, they are an essential piece to the posts so I'd like to talk a bit about them!

#Cheliceratime
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What are chelicerates Chelicerates are mobile & bilateral animals, part of the arthropods. They have an exoskeleton, grow through molting and have segmented bodies & appendages. This large group comprises various animals, most of them being carnivorous and walking on 8 or 10 legs: spiders, scorpions, mites, harvestmen, vinegaroons for example, all terrestrial and called “arachnids”, but also the aquatic horseshoe crabs, pycnogonids (=”sea spiders”), the extinct eurypterids & many others. Often called insects, they are in fact far from being ones: they all lack antennae, replaced instead by pincers or fangs called chelicerae. These chelicerae are what bear the fangs in spiders.

A diverse group today... Pycnogonids/"Sea spiders" Palpigrades Mites => Acariformes => Parasitiformes Solifugids/"Camel spiders" Harvestmen Ricinuleids Xiphosurans/"Horseshoe crabs" Pseudoscorpions/"False scorpions"/Book scorpions" Scorpions Spiders Uropygids/"vinegaroons"/"whip scorpions" Schizomids Amblypygids/"Whyp spiders"/Tailless whip scorpions"

... as well as a diverse group in the past Mollisonids & Habelids Chasmataspidids 'Synziphosurines' Eurypterids/"Sea scorpions" Phalangiotarbids Trigonotarbids Haptopods Uraraneids & other stem-spiders And other weirdos

Cheliceratime entries are also all summarized in the form of a large phylogenetic tree, allowing you to perceive easier how these animals are related to each other. All entries have their respective links to the different social media, and are updated frequently. If you’re on Instagram or Bluesky, check the linktree in bio to access it. If you’re seeing this post on TikTok, look in the comments for the link.

A lil' word about the main subjects of Cheliceratime with a bried introduction of what they are, their diversity, and you can find all the entries in the tree right here!
=> www.canva.com/design/DAG4G...

#Cheliceratime
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Who's behind the account? Hi, my name’s Amy ! (Amypteride on the Internet) I’m a French Scicom educator, I talk to others about natural sciences (mostly paleontology) on a daily basis for work. I also have a Master degree in Paleontology and I worked during my time in academia on fossil arachnids, and I’ve always loved these little eight-legged critters. I could have continued in academia, but I think that if creating knowledge is cool, spreading it is just as important and it belongs to everyone. I’m also a huge Superman nerd & cosplayer sometimes and I firmly believe in truth, love & kindness but we’re getting out of the topic xD

What is cheliceratime? Cheliceratime is a drawing & sci-com project that aims to popularize science about the animals called chelicerates, like spiders, mites, horseshoe crabs & many others. The project aims to propose homemade drawings of extant species and paleoarts of extinct ones. I both love to draw and to talk about these animals, so Cheliceratime combines both into a sci-com & art project. Each post is the opportunity to discover a new topic or a new species, what makes it unique, if it’s an extant or extinct arthropod, and for me to draw it, some of them for the first time ever, and to talk about the choices I made to draw them.

Where to find Cheliceratime? Cheliceratime is available on 3 social media platforms: Instagram, where it has its own account TikTok, where it has its own account Bluesky, on my personal profile and the dedicated feed

Types of posts There are 2 main types of posts you’ll find across the cheliceratime entries: Species posts, in which I present a species of chelicerate (extant as extinct). At least once per week. Thematic posts, in which I talk about broader topics than a single species. Rarer, once per month.

What time is it? It’s #Cheliceratime !
Today's a special post because it's time to properly introduce myself & Cheliceratime!

Who's behind the account, what this account aims to do, what you can expect to see here, where to find Cheliceratime, all your answers are in this thread! ⬇️ 🧵

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Trigonotarbida Forwarded clypeus hiding the chelicerae 2 median eyes & little pairs on the sides Complex dorsal lock between the prosoma & the narrowed 1° opisthosomal segment Last 10, 11 & 12 segments folded on the underside, the last to in a sort of “postabdomen” Ventral sac 2 pairs of book lungs 3 claws: 2 large ones & a smaller one Last segment of the chelicera turned into a fang Trilobed opisthosoma with Lateral Lobe/Plate & Median Lobe/Plate

Araneae (spiders) Group represented: Araneomorphs Last segment of the chelicera turned into a venomous fang Palpal bulbs for reproduction in males Primitively 8 eyes in 2 rows, but eye pattern highly variable Fovea Patella often shorter than Femur & Tibia No muscles to extend the legs, extension is done by self-regulation of internal hydraulic pressure 2 claws by default, 3 among web-spinning species Opisthosoma often without external segmentation 2 to 6 spinnerets from which silk is spinned Book lungs (1 pair in araneomorphs, 2 in tarantulas & mesotheles) Tracheae (absent in tarantulas & mesotheles) Epygine (female genital opening) Narrow pedicel (“wasp waist”) Sternum Labium Endite (extension of the pedipalp’ coxae to process food) Notes: Spiders are highly diverse, this is an overall generalised body plan. Uraraneids and other stem spiders lineages are based on the same general model but with sometimes a flagellum and doubts about the presence of true spinnerets

Pedipalpi (Vinegaroons, schizomids & amblypygids/”whip spiders”) 2 median eyes in & little lateral pairs for species with sight Prosoma in one block for vinegaroons & amblypygids, subdivided like solifuges & palpigrades for schizomids Last 3 segments of the opisthosoma are smaller & forming a postabdomen Telson mondified into a flagellum (lost in amblypigids & reduced in schizomids) 2 pairs of book lungs (only one in schizomids) Tarsus is always subdivided into tarsomeres First pair of legs elongated and used as antennae, loss of their walking function Large raptorial pedipalps Note: This diagram is a mix of both schizomids, vinegaroons & amblypygids to show the numerous features shared by these 3 orders despite their differences.

And for the end of the thread, the pantetrapulmonates: trigonotarbida, araneae (spiders) and pedipalpi (vinegaroons, amblypygids, schizomids)

Thank you for reading & I’ll see you on the next #Cheliceratime!

#arachnid #spider #trigonotarbid #vinegaroon #amblypygid #schizomid
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Ricinulei Group represented: Neoricinuleids Forwarded cucullus hiding the chelicerae 2-segmented small chelicerae Carapace densely granulated Males’ 2° femur enlarged Double trochanter (3° & 4° pairs of walking legs) Complex dorsal lock between the prosoma & the narrowed 1° opisthosomal segment Trilobed opisthosoma with Lateral Lobe/Plate & Median Lobe/Plate Last 3 segments of the opisthosoma form a short postabdomen Pedipalps ending with a small pincer claw Males’ 3° legs modified for reproduction Tracheal openings hidden among the legs Pedipalp folded under the body & ending with a pincer claw

Phalangiotarbida Genus represented: Mesotarbus Small leg-like pedipalps Extremely small chelicerae, exact morphology unsure 6 eyes arranged in 3 pairs on a trifoliate tubercle First 6 segments of the opisthosoma are densely packed on the dorsal side Broad 3-segment end of opisthosoma (often fused) Dorsal anal opening Ogive-shaped main body Crab-like heavy walking legs 1 pair of spiracled linked to unknown respiratory organs reproductive’s morphology unknown Note: This group is still poorly known; further research could change several details radically.

Pseudoscorpiones Group represented: Cheliferoids Pedipalps turned into pincer claws (with venomous glands in iocheirates species) Small 2-segmented chelicerae with a small spinneret called “galea” on the moveable digit linked to a silk gland 0 to 4 simple eyes on the forward rims of prosoma Patella usually is longer than the femur Exoskeleton can be heavily ornamented with tubercles Opisthosoma ending by an anal cone, no telson 2 pairs of tracheae Large genital plate First opisthosomal segment non-visible on ventral side Endite (extension of the pedipalp’ coxae to process food)

Scorpiones Group represented: Orthosternians Pedipalps turned into pincer claws 2 median eyes in & little lateral pairs Legs telescoped backwards Legs’ coxae well-developed & forming a stomotheca (a pre-oral chamber before the mouth) Sternum Genital opening & Pectines (ventral sensory appendages close to the ground) closely grouped 4 pairs of book lungs Opisthosoma subdivided into a mesosoma of 7 segments & a metasoma (=”tail”) of 5 segments Telson turned into a venomous sting Acculeus (sting) Vesicle (contains the venom) Note: Scorpions changed a lot since the Silurian, many of the paleozoic species didn’t have stomotheca for example. This diagram is mostly representative of today’s species.

Orders of unclear affinities & scorpion-like figures: ricinulei, phalangiotarbida, pseudoscorpiones and scorpiones

#Cheliceratime #arachnid #ricinulei #phalangiotarbida #pseudoscorpion #scorpion
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Opiliones (Harvestmen) Large chelicerae useful for identification Leg-like pedipalps of various morphologies, useful for identification Odoriferous gland for defense pruposes 2 median eyes sitting on an ocularium Prosoma & opisthosoma broadly joined, often merged together Dorsal side often covered by a +/- long scutum (shield) Legs often elongated, with tarsus split in numerous tarsomeres (up to 100) Anus often can be way more forward than in other arachnids Genital opening thrust forward between the legs and also bears the tracheal respiratory system Legs’ coxae well-developed & forming a stomotheca (a pre-oral chamber before the mouth) Note: Harvestmen show a wide diversity of morphologies concerning the prosoma-opisthosoma fusion and their ventral side, so the one here doesn’t exist & is a fusion of several types to reflect the broad average of what you can find among these arthropods.

Palpigradi Large projected forward chelicerae No eyes, presence of specific sensory organs at the body front (called “frontal organ” & “lateral organ”) Prosomal carapace subdivided from front to back into: propeltidium, mesopeltidium, metapeltidium Last pair of legs 8-segmented Last 3 segments of the opisthosoma smaller & forming a postabdomen Telson modified into a flagellum of 15-ish segments, can easily break Ventral sacs (if Eukoeneniidae, absent in Prokoeneniidae). No external respiratory system, the animal is small enough for gas exchanges to occur through the weakly sclerotinized exoskeleton Genital plate extending backward Long pedipalps, used for walking, retention of 3 claws at the appendage’s end The first pair of legs is 12-segmented and mainly used as sensory appendages while walking

Solifugae 2-segmented super enlarged chelicerae 2 median eyes on a high spot Prosomal carapace subdivided from front to back into: propeltidium (larger & raised), mesopeltidium, metapeltidium Patella is usually longer than the femur Tarsus divided into tarsomeres, usually 3 tarsomeres but sometimes more (like 6 among solpugids) Double trochanter on the 3° & 4° walking legs Soft 10-segmented opisthosoma 3 pairs of tracheal openings on the underside of the opisthosoma malleoli/racquet organs on the coxae & trochanter of the last pair of legs, acting as sensitive organs close to the ground 1 pair of tracheal openings between the 2° & 3° walking legs First pair of walking legs significantly shorter than pedipalps Long & strong leg-like pedipalps, covered with highly sensitive setae Adhesive organ at pedipalps’ tip

The mite body plan Although not forming a single monophyletic group and having a morphological diversity way too vast to be exhaustively synthesised in a single drawing, all chelicerates called “mites”, from flour mites to ticks, share a broad body plan easily recognizable, even tho there are many variations depending on which group you’re looking at. The prosoma’s part bearing chelicerae & pedipalps is detached from the body and is called “gnathosoma” Prosoma & opisthosoma fused, forming a new body part called “idiosoma”. Some groups have dorsal shields. The front half bearing the first 2 pairs of legs is called “propodosoma”, and the back half “hysterosoma) Small eyes can be present, most often on the idiosoma forward sides Legs’ segment count is HIGHLY variable depending on the considered group Anal plate Genito-ventral plate Breathing through tracheae if present (openings usually directed to the front half of the body) Legs’ coxae can be fused to the idiosoma in some groups Caruncle/ Pre-tarsus Sternal plate

Some apulmonate arachnids: Opiliones (Harvestmen), Palpigradi, Solifugae and the mite body plan

#Cheliceratime #arachnid #harvestmen #solifuge #palpigrade #mite #tick
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Xiphosura (Horseshoe crabs) Group represented: Limulines Cardiac lobe extending forward Ophthalmic ridge The first & second segments of the opisthosoma are functionally linked to the prosoma, making horseshoe crabs the sole chelicerates where using the word “cephalothorax” is 100% correct scientifically. The rest of the opisthosoma is called “thoracetron” More rigid thoracetron with fused segments for all post-Paleozoic species Thoracetron sides often spiny, modern species having articulated spines Sword-shaped telson with wide motion range 5 pairs of book gills. They can be flapped to aid in swimming on the back when needed First opisthosomal legs reduced to non-functional appendages stuck between prosomal walking legs & called “chilaria” Large genital operculum at the end of the cephalothorax Last prosomal legs modified & called “pusher legs”, with a small highly modified exopodite serving as a cleaning structure. 4 first walking legs ending with pincer claws Uniramous prosomal appendages covered by the vaulted prosomal shield 3-segmented chelicerae

Chasmataspidida Group represented: Diploaspidids lineage 5 pairs of uniramous only-walking prosomal legs Unknown chelicerae, supposed to be small & 3-segmented Last segment of 5° legs modified into a small claw or a leg joint Opisthosoma subdidived into typical pattern of a shorter & broader mesosoma of 4 segments, and a longer narrower metasoma (=tail) of 9 segments. The 3 largest segments of the mesosoma are grouped under the term of “buckler”. Reduced pointy/rounded telson Genital appendage tubular and stemming from the 2° operculum, similar to what’s seen in eurypterids Respiratory organs hidden under 3 opercula and of unknown morphology, probably book gills like horseshoe crabs & eurypterids. Possibly 3 supplementary operculas on the first 3 segments of the metasoma. First opisthosomal segment super reduced and squished against the prosoma, overlapping in ventral view on the prosoma in the form of a metastoma between the 5° legs.

Eurypterida ("sea scorpions") Group represented: Derived eurypterines Chelicerae small on average, rarely visible beyond the prosomal rim in most species Large lateral eyes with excellent sight Morphologies of prosoma & uniramous legs are extremely diverse, key features for eurypterid identification & classification Eurypterines specifically have an additional segment to their 5° legs, and the said legs are modified in most eurypterines into large flippers aiding in swimming Opisthosoma subdidived into typical pattern of a broader mesosoma of 6 segments, and a narrower metasoma (=tail) of 6 segments. Sometimes a 7/5 preabdomen/postabdomen denomination is used Telson of various shapes depending on the group considered Complex & segmented tubular +/- long genital appendage called “zipfel” and steming from the fused 1° & 2° operculum. 5 pairs of book gills hidden under opercula called “blattfüsse” First opisthosomal segment squished against the prosoma, only lasting as an overlapping metastoma between the 5° legs in ventral view 1° & 2° opercula fused, ensuring both reproduction & respiration

The arachnid body plan Due to multiple terrestrialization events, arachnids in their traditional sense aren’t monophyletic and therefore, didn’t originate from a single terrestrial ancestor. However, the historic grouping of these animals in a single class relied on a lot of morphological similarities that seem to be convergently shared by default by all terrestrial euchelicerates. The mouth is pushed forward to accommodate the changes in food processing between land & water The first pair of legs is modified as manipulatory & sensory appendages, now called pedipalps Shortened chelicerae, often only 2-segmented 0 to 8 eyes, pattern highly variable Prosoma-opisthosoma connexion is often less rigid and can be morphologically diverse Opisthosoma made of max 12 segments, the ancestral first one being lost Telson reduced, most frequently lost, or turned into a flagellum Spiracles (air-breathing openings) usually on the ventral first half of the opisthosoma, internal respiratory system Genital opening often in the shape of a plate Prosoma’s ventral side often shows fused plates, rigidifying the body for terrestrial walk Loss of the gnathobases & reduction of the masticatory role of the coxae, the legs mainly serve to move Uniramous appendages

Aquatic main euchclicerates groups: Xiphosura (Horseshoe crabs), Chasmataspidida, Eurypterida & the arachnid body plan

#Cheliceratime #horseshoecrab #chasmataspidid #eurypterid #seascorpion #arachnid
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Nomenclature & legends To make them easier to understand, all diagrams will have the same color coding, helping with identifying which body parts are the same across chelicerates’ diversity. MAIN BODY Euchelicerates : Prosoma-opisthosoma-telson Pycnogonids: cephalosome-thorax-abdomen (+telson for some fossils) The genital opening/external organs will be indicated in pink, and the breathing system will be in white. APPENDAGES The first arthropods had double legs called “biramous legs”, with 2 branches attached to a single coxa (trilobites are a good example of this condition). The endopodite, the more inward one, was used for walking, while the exopodite, the more outward one, served mostly for respiration. Most chelicerates conserved only one of them, more often to walk, and heavily modified the breathing ones. Chelicera, Birmous legs made of endopodite (walking) & endopodite both attached to a coxa, and other significant appendages are in purple. Leg’s segments name abreviations Cx: Coxae Tr: Trochanter Fe: Femur Pa: Patella Ti: Tibia Mt: Metatarsus Ta: Tarsus Pr: Propod

Chelicerata No one knows for sure what the ancestral chelicerate looked like, the group as a whole being really rare before many of its main lineages became well established in the fossil record (and already too different from one another). The diagram here is an attempt to summarize the common traits of all chelicerates on a theoretical profile reminiscing of a cambrian habellian-like arthropod. Front appendages are chelicerae, claws (or later fangs) made of 4 or more segments at the beginning (now reduced to 3 or 2 in all current chelicerates) Accute senses with a well- developed nervous system, eyes and numerous setae (=hairs) sensing vibrations No clear distinction at first of anterior & posterior tagma. At least the first 4 segments (acron+3 first legs-bearing segments) were already fused together, & generally the front of the body is more dedicated to walking & handling food, while the body end is often non-walking. The exact number of body segments is unknown, most chelicerates (except pycnogonids) having ancestraly around at least 15 segments post-chelicerae. The mouth opening is located between the chelicerae & the first pair of legs Gnathobase (serrated extension of the pedipalp’ coxae to process food, lost in pycnogonids & terrestrial forms) Ancestral forms have biramous legs with a walking endopodite & a breathing exopodite. Endopodites become more and more reduced past the front part of the animal, and quasi all living chelicerates have lost the biramous state for uniramous legs. Segmented body covered by an unmineralized exoskeleton made of chitine, growing through molting. Body ending with an accessory telson, located after the anal opening.

Pycnogonida (sea spiders) Group represented: Pantopods, family Nymphonidae Body in 3 tagma: cephalosome, thorax & abdomen Mouth proned forward in a proboscis 1° walking legs on the cephalosome 4 eyes on an ocular tubercle Legs host part of the digestive tract and their large surface serve for breathing Thorax bearing 2°, 3° & 4° pairs of walking legs Small abdomen, almost non-existent to absent in modern species Long, 9-segmented legs with tripled coxae, attached to the main body by a lateral process Gonopores (reproductive openings) on the 2° legs 3° appendages used only for handling eggs, called ovigers 2° appendages used for manipulation, called palps Notes: Paleozoic pycnogonids are extremely diverse morphologically, often exhibiting a more developed abdomen & more primitive appendages structure. Larvae morphology is also typical and is called “protonymphon”

Euchelicerata 3-segmented chelicerae Primitively 2 simple median eyes (ocelli) & 2 lateral compound eyes Often show a raised median part on the prosoma called cardiac lobe, corresponding to vascular & nervous system clustering Body divided into a prosoma & an opisthosoma, often called respectively “cephalothorax” & “abdomen” in popular science 13-segmented opisthosoma, mostly dedicated to hosting non-locomotory & non-sensitive systems. Most derived groups only show 12 segments or less, the first segment being often reduced or squished into the prosoma in modern groups. Pointy telson Exopodite modified into a booked structure to grant more surface for gas exchanges for respiration, similar to simpler horseshoe crab’s book gills All extant euchelicerates have their genital opening placed on their 2° opisthosomal segment, which was in third position before being reduced or squished into the prosoma in modern groups First opisthosomal segment with a pair of walking legs, reduced to non-functional appendages or just absent in most euchelicerates Prosoma bearing chelicerae & 5 pairs of walking legs. Endopodites reduced (if not absent in most cases, most euchelicerates having uniramous prosomal legs) Mouth placed more at the center of the prosoma Note: This diagram doesn’t represent a specific group but illustrates the template from which horseshoe crabs, eurypterids, chasmataspidids & arachnids evolved. The overall shape is inspired by species of synziphosurines.

Some info about drawings' legends, and here we go, we begin with chelicerata, pycnogonida & euchelicerata

#Cheliceratime #chelicerata #pycnogonida #seaspider #anatomy
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A QUICK VISUAL TOUR OF CHELICERATES’ ANATOMIES Chelicerates’ morphologies are widely variable: from spiders to horseshoe crabs or mites & scorpions, it can be hard to navigate, even more so with the numerous words used to describe these animals’ body parts. This post aims to serve as a simplified tool for you to better understand Cheliceratime’s entries by summarizing the main information about each large chelicerate groups, so don’t forget to swipe if you’re looking for a group in particular!

What time is it? It’s #Cheliceratime !
This post aims to serve as a simplified tool for you to better understand Cheliceratime’s entries by summarizing the main information about each large chelicerate groups, so unroll the thread to find the group you're looking for!⬇️ 🧵

#sciart #arachnid
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Chelicerates & humans Human’s perception of chelicerates is... complicated. Arachnids (and especially spider-looking ones) are often subjects to arachnophobia, and the disconnection between humans and chelicerates often makes it hard for many people to sympathize with them. They are also harvested economically to the point of risking extinction, like for horseshoe crabs & many spiders. However, of the more than a hundred thousand species, very few pose a threat to humans, and we still know little of their behaviors for many species. Whether we like it or not, they are a vital part of Earth’s ecosystems, spiders’ regulation of insects making them the most important predators for land ecosystems. Most of these views are tied to our link with nature and how we perceive it, arachnids being typically non-charismatic animals for numerous people. And I hope your time on these posts will help you see them from a new perspective, not as monsters, but incredibly diverse & fascinating animals :)

REFERENCES (part 1) · Aria C. & Caron J.-B., 2019. A middle Cambrian arthropod with chelicerae and proto-book gills. Nature 573, p. 586–589. · Benavides L.R., Cosgrove J.G., Harvey,M.S. & Giribet G., 2019. Phylogenomic interrogation resolves the backbone of the Pseudoscorpiones tree of life. Molecular Phylogenetics and Evolution 139, article 106509. • Bicknell R.D.C. & Pates S., 2020. Pictorial Atlas of Fossil and Extant Horseshoe Crabs, With Focus on Xiphosurida. Frontiers in Earth Science 8, Article 98, 60 pp. • Cloudsley-Thompson J.L., 1977. Adaptational biology of Solifugae (Solpugida). Bulletin of the British Arachnological Society 4, p. 61-71. · Clouse R.M et al., 2017. First global molecular phylogeny and biogeographical analysis of two arachnid orders (Schizomida and Uropygi) supports a tropical Pangean origin and mid‐Cretaceous diversification. Journal of Biogeography 44, p. 2660–2672. · De Miranda G.S., Kulkarni S.S., Tagliatela J., Baker C.M., Giupponi A.P.L., Labarque F.M., Gavish-Regev E., Rix M.G., Carvalho L.S., Fusari L.M., Harvey M.S., Wood H.M. & Sharma P.P., 2024. The Rediscovery of a Relict Unlocks the First Global Phylogeny of Whip Spiders (Amblypygi). Systematic Biology 73, p. 495–505. • Derkarabetian S., Lord A., Angier K., Frigyik E. & Giribet G., 2023. An Opiliones-specific ultraconserved element probe set with a near-complete family-level phylogeny. Molecular Phylogenetics and Evolution 187, article 107887. · Dunlop J.A. & Lamsdell J.C., 2017. Segmentation and tagmosis in Chelicerata. Arthropod Structure & Development 46, p. 395–418. • Dunlop J.A. & Penney D., 2012. Fossil Arachnids. Monograph Series Vol.2, Siri Scientific Press. 192 pp. • Garwood R.J., Dunlop J.A., 2023. Consensus and conflict in studies of chelicerate fossils and phylogeny. Arachnologische Mitteilungen: Arachnology Letters 66(1), p. 2-16.

REFERENCES (part 2) • Garwood R.J. & Dunlop J.A., 2014. Three-dimensional reconstruction and the phylogeny of extinct chelicerate orders. PeerJ 2, e641, 33 pp. • Jocqué R. & Dippenaar-Schoeman A.S., 2007. Spider Families of the World. 2° Edition, Royal Museum of Central Africa, 336 pp. · Klimov P.B., OConnor B.M., Chetverikov P.E., Bolton S.J., Pepato A.R., Mortazavi A.L., Tolstikov A.V., Bauchan G.R. & Ochoa R., 2018. Comprehensive phylogeny of acariform mites (Acariformes) provides insights on the origin of the four-legged mites (Eriophyoidea), a long branch. Molecular Phylogenetics and Evolution 119, p. 105-117, · Kulkarni S., Wood H.M., Hormiga G., 2023. Advances in the reconstruction of the spider tree of life: A roadmap for spider systematics and comparative studies. Cladistics 39, p. 479–532. • Lamsdell J.C, 2025. Codex Eurypterida: A Revised Taxonomy Based on Concordant Parsimony and Bayesian Phylogenetic Analyses. Bulletin of the American Museum of Natural History 473, 195 pp. · Penney D. & Selden P.A., 2012. Fossil Spiders The Evolutionary History of a Mega-diverse Order. Monograph Series Vol.1, Siri Scientific Press. 128 pp. • Sabroux R., 2018. Biodiversité et histoire évolutive des Pycnogonides (Arthropoda, Pycnogonida). Museum national d'histoire naturelle - MNHN PARIS, 345 pp. • Sato S., Derkarabetian S., Valdez-Mondragón A., Pérez-González A., Benavides L.R., Daniels S.R., Giribet G., 2024. Under the hood: Phylogenomics of hooded tick spiders (Arachnida, Ricinulei) uncovers discordance between morphology and molecules. Molecular Phylogenetics and Evolution 193, article 108026. • Sharma P.P. & Gavish-Regev E., 2025. The Evolutionary Biology of Chelicerata. Annual Review of Entomology 70, p. 143–163. Dunlop J. A., Penney D. & Jekel D. 2023. A summary list of fossil spiders and their relatives. In World Spider Catalog. Natural History Museum Bern, online at http://wsc.nmbe.ch, version 23.5.

To end this thread, I wanted to add a lil' note on humans' perception about chelicerates, and of course some references, I hope this thread helped when it comes to understand what is said in Cheliceratime's posts!^^

Thank you for reading and I’ll see you on the next #Cheliceratime!

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The aquatic chelicerates Chelicerates history began in the sea, and numerous extinct groups show this diversification through the Paleozoic. Most of them are euchelicerates, and this part of the tree is still shaky due to how water to land transition happened more than once. This diversity has become very scarce, only pycnogonids and horseshoe crabs remaining today. The slide showcase a large cladogram of marine chelicerates relationships (basal chelicerates, synziphosurines, chasmataspidids, xiphosurans (horseshoe crabs), eurypterids, and pycnogonids). Pycnogonids are separated from all other groups, while the relationships of euchelicerates are unclear, modern takes suggesting horseshoe crabs, eurypterids & chasmataspids being maybe closer to scorpions & spiders than other arachnids orders.

The arachnopumonates The most stable & monophyletic part of the sub-phylum, made of terrestrial chelicerates breathing ancestrally with book lungs (sometimes modified in tracheae, like with pseudoscorpions & many spiders) and they have a duplicated genome. Arachnopulmonates are mainly divided into panscorpiones (scorpions & pseudoscorpions) & pantetrapulmonates (spiders, vinegaroons, amblypygids, schizomids, haptopods & trigonotarbids)

The apulmonates arachnids Terrestrial chelicerates breathing with tracheae. This part of the chelicerate phylogenetic tree is still unclear and could change drastically in the following years (apart for cephalosomates), ricinuleids being more closely related to arachnopulmonates & fossils lacking to understand these orders’ evolution. They all share non-flagellate spermatozoids. Phalangiotarbids, parasitiform mites & harvestmen are of unknown affinities, solifuges, palpigrades & acariforme mites are grouped together in the cephalosomates, and ricinuleids are closer to horseshoe crabs & arachnopulmonates

The mites Long considered a single group forming the subclass Acari, mites are now strongly separated into 2 unrelated groups, the acariformes and parasitiformes. Their similar morphology and small size appear to be the result of convergent evolution. Arachnologists often consider these 2 groups as orders, but here in Cheliceratime we’ll adopt the ranks acarologists use to reflect how diverse these animals are: 2 super-orders, and at least 6/7 orders (the tree below is super simplified). Even tho some groups are well defined morphologically, numerous large clades are based on genetic & molecular evidence. Acariformes comprise eriophyoids (four-legged mites), various endeostigmates, sarcoptiformes (oridatids & astigmates) and trombidiformes. Parasitiformes are made of opilioacariformes, mesostigmates, holothyrides & ixodides (=ticks)

The many groups of chelicerates have ancient and sometimes still unclear relationships to one another. The 4 slides here summarize where we are on the topic, with some grey areas that could change in the coming years.

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Chelicerates' broad anatomy These animals have complex inner anatomies, with more centralized nervous systems than many arthropods and dorsal hearts. The arrangement of their mouthparts is highly variable depending on the group, from the claws of scorpions to the giant chelicerae of solifuges. Respiration is ensured thanks to gills or skin in water, and book lungs/tracheae/air sacs/skin for land species. Sight is of unequal quality across chelicerates, terrestrial groups having often simple eyes, and sound communication is poorly researched for these arthropods. All chelicerates rely much more on their sense of touch, the numerous setae (=hairs) on their bodies making them super sensitive to their surroundings. This coverage is an essential part of how they perceive the world, and is much more than just hairs like we humans have. It’s the main reason why spiders can look sometimes hairy, but since they don’t have “hair”, let’s say they are fluffy, it’s cute :3

Chelicerates' lifestyles These animals are primarily carnivorous, herbivory being a secondary condition observed mostly among mites. They comprise some of the most important predators in earth’s ecosystems, like the more than 53000 current spider species or the extinct eurypterids that were the apex predators of the Silurian period. Hunting styles are various, with a large amount being sit-and-wait type of hunters, some being scavengers sometimes. Terrestrial groups can’t swallow hard food and need to liquify or squish it before eating. Apart from pycnogonids & several mites, chelicerates larvae often look like tiny adults with softer bodies. Reproduction varies greatly depending on the group considered, even among orders. Life expectancy is widely variable, from a few days for some mites to more than 20 years for certain tarantulas. Many species in temperate regions live 1 year.

"The great arachnid issue" Long thought to be a clean natural group born from a single water to land transition, the traditional view of arachnids has been challenged in the last 15 years. Genomic data suggested new relationships among arachnids, reflecting multiple events of land colonization, with horseshoe crabs right in the middle. At first difficult to believe, this new normal has been globally accepted due to a large amount of unrelated works retrieving the same general trends. In Cheliceratime, the position chosen will be to consider the group called “Arachnida” as similar to Prosomapoda (=crow group euchelicerates) until further research clarifies this part of the tree. It absolutely doesn’t mean that horseshoe crabs evolved from a spider, but that names are just names that can be redefined with new knowledge, and that the arachnid type body plan evolved several times. “Arachnids” as a common name will refer in the posts to the traditional view to encompass easier topics like ecology, general body plan, functional morphology, etc.

An overview of chelicerates' relationships Chelicerates’ phylogeny is... a bit of a mess. Some groups & each order themselves are strongly supported, but it’s still a work in progress with numerous uncertainties concerning terrestrialization events. For the sake of comprehension, they’ll be treated in the further slides in 4 pools, symbolized here by the colors below. AQUATIC CHELICERATES ARACHNOPULMONATES APULMONATE ARACHNIDS MITES The image is an educational infographic: the main content is a stylized phylogenetic tree that runs vertically and branches toward the right. Branch lines are drawn in bright, clean colors — blue, purple, yellow and green — and some branches are dashed or have question marks, signaling uncertainty. Small colored nodes on the left of the tree label higher-level groups (a green dot and a blue dot are visible), and dotted connectors show alternative placements for some groups. Along the right edge of the tree a vertical strip of illustrations shows representative animals aligned with their respective branches: tiny sea spider–like figures near the top, a horseshoe crab & eurypterid-like forms, then scorpion, spider, whip scorpion/amblypygid & vinegaroon, harvestman/daddy-longlegs, camel spider/solifuge, and smaller icons of ticks and mites toward the bottom. The artwork is detailed enough to distinguish those general body plans (long legs, segmented tail and pincers for the scorpion, round body and many legs for mites/ticks).

The biology of chelicerates is an extensive topic covering anatomy, lifecycle, ecology and so on, and recent discoveries have reshaped how we perceived their relationships, tearing up the traditional view of what is an arachnid.

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A diverse group today... Pycnogonids/"Sea spiders" Palpigrades Mites => Acariformes => Parasitiformes Solifugids/"Camel spiders" Harvestmen Ricinuleids Xiphosurans/"Horseshoe crabs" Pseudoscorpions/"False scorpions"/Book scorpions" Scorpions Spiders Uropygids/"vinegaroons"/"whip scorpions" Schizomids Amblypygids/"Whyp spiders"/Tailless whip scorpions"

... as well as a diverse group in the past Mollisonids & Habelids Chasmataspidids 'Synziphosurines' Eurypterids/"Sea scorpions" Phalangiotarbids Trigonotarbids Haptopods Uraraneids & other stem-spiders And other weirdos

Chelicerates' evolutionary history Part 1: 538,8-251,9 million years ago Cambrian (538,8-486,8 Mya) First euchelicerates & pycnogonids appear during the Cambrian explosion as small bottom-dwellers and background species, shadowed by other more ancient groups. Ordovician (486,8-443,1 Mya) Evolutionary radiation, chelicerates become major predators & scavengers in various environments: first eurypterids, chasmataspidids & horseshoe crabs, cryptic terrestrializations, numerous bottom dwellers. Silurian (443,1-419,6 Mya) Eurypterids’ golden age, first members of known firmly terrestrial arachnid lineages (scorpions, trigonotarbids) Devonian (419,6-358,8 Mya) Arachnids radiate on land (first harvestmen, mites & tetrapulmonates) and establish themselves as vital parts of terrestrial ecosystems. Modern pycnogonids appear. Slow decrease in mean size and ecological importance of aquatic euchelicerates (horseshoe crabs, eurypterids, chasmataspidids for the main trio). Carboniferous (358,8-298,9 Mya) Freshwater horseshoe crabs & eurypterids. Confirmed ocurrences of most known arachnids orders with ground-based & arboreal members. Radiation of the tetrapulmonates: first spiders, vinegaroons & amblypygids. Pangea assemblage allows terrestrial & freshwater groups to be distributed worldwide. Permian (298,9-251,9 Mya) Cryptic diversification among mites and other arachnids orders. Permian-Triassic Mass extinction: extinction of many paleozoic groups like eurypterids, trigonotarbids & many primitive lineages, acting as a bottleneck for chelicerates’ diversity.

Chelicerates' evolutionary history Part 2: 251,9 million years ago-Today Triassic (251,9-201,4 Mya) Post extinction recovery & diversification: large cryptic radiation of spiders post Permian-Triassic extinction (potential apparition of the true spider webs), horseshoe crabs radiation in salt & freshwaters. Jurassic (201,4-143,1 Mya) Continuation of Triassic radiations: horseshoe crabs limited to saltwater, spiders already well implemented in ecosystems with various ecological types, modern pycnogonids groups present. Pangea break-up begins to isolate groups in certain continents and leave marks that will guide arachnids’ evolution up to today. Cretaceous (143,1-66 Mya) Huge ecological diversification of spiders linked to the rise of flowering plants & more modern insect fauna: sticking webs become more widespread, development of ground-based species. Intense period of evolutionary radiation on land for groups like schizomids, spiders or amblypygids, facilitated by Pangea break-up and geographic isolation, most of the modern arachnid families (or at least their ancestors) are supposed to appear during this time. Paleogene (66-23 Mya) Araneoids & RTA spiders’ radiation, including the first jumping spiders. Recovery post-extinction, transitional phase between mesozoic and modern faunas. Aquatic chelicerates now restricted to marine horseshoe crabs & pycnogonids. Neogene & Quaternary (23-0 Mya) Modern arachnofauna on land. Scorpion’s venom becomes potent to mammals. Orbweavers, jumping & wolf spiders and several other spider groups become diverse & worldwide.

Chelicerates comprise diverse groups extinct as extant from spiders to mites, eurypterids or other weirdos, and have a rich evolutionary history going back to the cambrian: they've known a time without land plants, the dinosaurs golden age & now they walk alongside us.

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A brief overview Chelicerates are mobile & bilateral animals, part of the arthropods. Like all arthropods, chelicerates have an exoskeleton, grow through molting and have segmented bodies & appendages. If we only look at today’s arthropods, they are the sister group of all the other lineages, meaning they aren’t insects, crustaceans or millipedes, but an entirely separated group. Their origins go way back to the Cambrian explosion, more than 500 million years ago. Chelicerates form a sub-phylum comprising various animals, most of them being carnivorous and walking on 8 or 10 legs: spiders, scorpions, mites, harvestmen, vinegaroons for example, all terrestrial and called “arachnids”, but also the aquatic horseshoe crabs, pycnogonids (=”sea spiders”), the extinct eurypterids & many others.

The chelicera: chelicerates’ most unique feature Chelicerates stand out among arthropods by having no antennae (again, they aren’t insects), instead replaced by their first pair of appendages, called chelicerae (singular chelicera). These are primitively small claws placed before the mouth, and are the prime mouthparts of these animals. They happened to be modified several times across chelicerates’ evolution, being turned into venomous fangs in spiders for example. Scorpions’ large claws are modified manipulatory appendages, their chelicerae are their most upfront little pincers. The slide also display the 3 morphological types of chelicerae: 2 segmented & fangs-like, like in spiders, 2 segmented & claw-like, like in amblypygids, pseudoscorpions & solifuges, and 3 segmented & claw-like, like in horseshoe crabs, pycnogonids, harvestmen or eurypterids

A group of arthropods... Chelicerates share a common ancestor with other arthropods that lived long ago (maybe 540/530 million years ago) and are kinda unique among this giant phylum. According to the most accepted hypothesis (supported for more than 20 years now), a primitive group of arthropods had spines on their first appendages’ pair. Quickly, they shortened, spines grew stronger, segments began to articulate differently and the appendages, now turned into pincer claws, lost their initial sensory function for a mouthpart role: chelicerae were born, and so was the group we call chelicerates. This feature makes chelicerates the only living arthropods without antennae. The slide also features a phylogenetic tree in which chelicerates close to the extinct megacheirians, this clade itself being the sister group to trilobites & mandibulates (millipedes, crustaceans, insects...)

...with specific features Other than chelicerae, chelicerates often exhibit the following features: A body usually subdivided into a functional walking & manipulatory front part, and a raised non-legged posterior part. Front appendages specialized for sensory and feeding purposes. An unmineralized exoskeleton, even among the marine forms. 4 to 5 pairs of walking appendages, sometimes only the last 3 pairs are used to walk. Uniramous appendages, either used to walk & manipulate objects, or for breathing/other life assists.

First, let's begin with a brief introduction and see the main morphological features useful to recognize these animals and where they sit among arthropods.

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What time is it? It’s #Cheliceratime !
No particular species today, we're doing a bit of a recap before going further: in the end, what is a chelicerate?

Answers in the thread below! ⬇️ 🧵

#arthropod #scicom #arachnid #horseshoecrab #pycnogonid #eurypterid #invert #bugsky

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REFERENCES • Dunlop J.A., Poschmann M. & Anderson L.I., 2001. On the Emsian (Early Devonian) arthropods of the Rhenish Slate Mountains: 3. The chasmataspidid Diploaspis. Paläontologische Zeitschrift 75, p. 253–269. · Dunlop J.A. & Lamsdell J.C., 2017. Segmentation and tagmosis in Chelicerata. Arthropod Structure & Development 46, p. 395–418. • Garwood R.J. & Dunlop J.A., 2023. Consensus and conflict in studies of chelicerate fossils and phylogeny. Arachnologische Mitteilungen: Arachnology Letters 66(1), p. 2-16. • Lamsdell J.C. & Briggs D.E.G., 2017. The first diploaspidid (Chelicerata: Chasmataspidida) from North America (Silurian, Bertie Group, New York State) is the oldest species of Diploaspis. Geological Magazine 154, p. 175–180. · Marshall D.J., Lamsdell J.C., Shpinev E. & Braddy S.J., 2014. A diverse chasmataspidid (Arthropoda: Chelicerata) fauna from the Early Devonian ( Lochkovian) of Siberia. Palaeontology 57, 631–655. · Selden P.A., Lamsdell J.C. & Liu Q., 2015. An unusual euchelicerate linking horseshoe crabs and eurypterids, from the Lower Devonian (Lochkovian) of Yunnan, China. Zoologica Scripta 44 p. 645–652. · Naimark E., Sizov A., 2025. Three groups of arthropods (Chasmataspidida, Offacolidae (?), and Euthycarcinoidea) cohabited a tidal zone in the late Cambrian paleobasins (495–488 Ma) of Eastern Siberia, Journal of Asian Earth Sciences 289, article 106595. Colorado Plateau Geosystems Inc., 2026. Global Series. In DeepTimeMaps. Online at https://deeptimemaps.com/map-lists-thumbnails/global-series/

And of course: some references, thank you for reading I’ll see you on the next #Cheliceratime!

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