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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