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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