#chasmataspidid
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amypteride.bsky.social
What time is it? It’s #Cheliceratime !
For this entry we're talking about a cute little chasmataspidid, Diploaspis casteri from the Devonian

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

#arthropod #devonian #paleoart #sciart #bugsky #invert
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This medium-size boxy chasmataspidid was the second species named from this order. It’s known from several specimens, showing a wide range of shapes depending on how they fossilized. Size : 3 to 4-ish cm of body length Time period : Late Cretaceous Paleoart speculativometer : Some Missing parts The animal drawn looks a bit like a boxy shrimp without antennae with an overall brown color
March 23, 2026 at 5:02 PM
amypteride.bsky.social
What time is it? It’s #Cheliceratime !
For this entry we're talking about a russian chasmataspidid, Octoberaspis ushakovi !

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

#arthropod #devonian #paleoart #sciart #bugsky #invert
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This medium size chasmataspidid is among the most derived species of its order. It was named in reference to the October Revolution Island, a large Russian island named after the 1917 October revolution in Russia that led to the rise of the USSR. Size : 3 to 4-ish cm of body length Time period : Late Cretaceous Paleoart speculativometer : Few missing details The animal drawn looks like an eurypterid with a body looking like a mix of a horseshoe crab at the front and shrimp at the back
July 15, 2026 at 4:08 PM
fossildetective.bsky.social
For the first #FossilFriday of the year, here is an undescribed Ordovician chasmataspidid from Manitoba, Canada. Referred to as a eurypterid in publications, the species looks very similar to Hoplitaspis, which is known from contemporaneous rocks in Michigan. ⚒️🧪
A fossil of an arthropod preserved on a beige rock. The animal is seen in side view, with its body armed upwards. Large limbs, including a broad paddle, are preserved on the underside of the head. The first three segments of the body are fused together with a single large plate on the underside.
January 5, 2024 at 9:02 PM
amypteride.bsky.social
You know the drill, let's talk about its morphology, discovery & paleoenvironment first to meet this chasmataspidid, which is among the best known chasmataspidid species.

#Cheliceratime
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Diploaspis casteri was the second species of chasmataspidid to be named in 1972, alongside Heteroaspis novojilovi. Those 2 small-sized, eurypterid-like devonian chasmataspidid contrasted a lot at the time with the larger ordovician horseshoe crab-like Chasmataspis laurencii, which was the only known member of the group until then. The two species were synonymized in 2001 when H. novojilovi was reconsidered to be a junior variant of D. casteri, which is the sole name in use today. Diploaspis casteri lived during the Early Devonian in a large area that comprises now Germany & Siberia, on a period of time extending from 415 (Late Lochkovian) to 405 million years ago (Early/Mid Emsian). D. casteri lived in a wide range of environments that all seemed to be shallow waters. The Renish Massif in Germany was around the same latitude as South Africa and was covered with estuaries and river deltas throwing water into the Proto-Tethys Ocean, while the Siberian specimens may have lived in more marine settings. Diploaspis casteri was most probably wandering on the bottom to hunt small animals. Diploaspis casteri is a small/medium size chasmataspidid, reaching around 3 to 4 cm in length for the largest specimens. Despite looking a bit like a compressed chiby eurypterid, its anatomy fit perfectly the chasmataspidid standards. The prosoma looks like a rounded square and bears 2 pairs of eyes in a pattern similar to eurypterids. The legs are short and pointy like those of eurytperids, the last one being flattened and possibly helping in swimming. The opisthosoma shows the unique segmentation pattern of chasmataspidids: first a boxy mesosoma of 4 large segments (one of them being compressed under the prosoma while the remaining 3 are larger), followed by a thinner metasoma of 9 segments (here taking a triangular shape). The body ends with a small telson. This species is known by a large array of specimens, several of them having been described after the species was initially named. The degree of body shape variations between these individuals is a great case of how taphonomy & fossilization prossesses as a whole can make things harder to understand when looking at fossils, all of them being from the same species but with deformations enough for many to see different species.
March 23, 2026 at 5:02 PM
amypteride.bsky.social
You know the drill, let's talk about its morphology, discovery & paleoenvironment first to meet this chasmataspidid, which is among the best known chasmataspidid species.

#Cheliceratime
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Octoberaspis ushakovi is a species of chasmataspidid that was named in 2002, found in Devonian rocks from the Severnaya Zemlya Formation, a Russian geological Formation of the October Revolution Island. Its name is a reference to several influences. The ushakovi part is an homage to Georgi Ouchakov, an arctic explorer, while the october part of the genus’ name is derived from the October Revolution Island. This large island is one of the several Russian ones in the Arctic circle, and was named after the October’s Revolution of 1917, which overthrow the Russian ruling class and led to the birth of the USSR. Octoberaspis ushakovi lived during the Early Devonian in the shallow waters of the paleocontinent of Siberia during the Early Lockovian stage 419/415 million years ago, not so long after the Silurian/Devonian boundary. O. ushakovi lived in a rich ecosystem that comprised many aquatic vertebrates such as acanthodians and various armored jawless fishes groups (osteostracans, anaspids...), and several arthropods like ostracods and phyllocarids. The presence of plant debris suggests this environment wasn’t too far from the coast. Most of the known arthropods were discovered in oval-shaped nodules. Octoberaspis ushakovi is a medium-sized chasmataspidid, reaching 3 to 4 cm in length for the largest specimens. Unlike other devonian chasmataspidid with short & boxy bodies, this animal looked a lot like an eurypterid. The prosoma looks like half of an ovale and bears 2 pairs of eyes in a pattern similar to eurypterids: a pair of large ones on the side, and 2 little ones in the middle of the carapace. The legs are short and pointy like those of eurypterids, the last one being flattened and possibly helping in swimming. The opisthosoma shows the unique segmentation pattern of chasmataspidids: first a boxy mesosoma of 4 large segments (one of them being compressed under the prosoma while the remaining 3 are larger) sometimes called a buckler and with dense granulation on its sides. It’s followed by a thinner metasoma of 9 segments. The body ends with a small pointy telson preceded by a pretelsonic segment with spines, giving a fork-like look to the animal’s body end.
July 15, 2026 at 4:08 PM
fossildetective.bsky.social
This #FossilFriday I am looking at the Devonian arthropod Diploaspis. Diploaspis is a chasmataspidid, a strange group of arthropods related to eurypterids. The earliest forms had only walking legs, but later ones like Diploaspis convergently evolve eurypterid-like paddles. 🧪⚒️
A photograph of a small dark fossil preserved as black, very crumpled cuticle on a dark grey rock. The fossil preserves a rounded headshield, a rectangular 'buckler' just behind the head, and a series of small, narrow abdomen segments behind that.
September 22, 2023 at 2:34 PM
amypteride.bsky.social
And of course: some references, thank you for reading I’ll see you on the next #Cheliceratime!

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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/
March 23, 2026 at 5:02 PM
amypteride.bsky.social
This chasmataspidid is among the most complete of the group, and provided numerous informations regarding chasmataspidids evolution and their links to eurypterids.

#Cheliceratime
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This species, thanks to its really good conservation, is among the most eurypterid-looking among chasmataspidids, and in fact helped to bridge some gaps between the 2 groups by showing several features weren’t specific to eurypterid, either having stemmed from a common ancestry or convergent evolution: Paddle-like 5th legs (convergence) A metastome plate, possibly derived from the compressed first opisthosomal segment pointy epimeras on the last opisthosomal segment, something way more common among eurypterids Tubular reproductive appendage Eurypterid-like operculum Chasmataspidids sometimes look like a mosaic of eurypterid & horseshoe crab features, but are often retrieved closer to eurypterids: both orders share similar traits and have deep paleozoic origins, more than probably during the Late Cambrian considering how diverse & derived some eurypterids already were in the Ordovician and how chasmataspidid seemed to appear in the Late Cambrian. The links of both groups with freshwaters, as well as several possible land exaptations, make their study of certain importance in assessing chelicerate terrestrialization. The slide also showcase a putative phylogeny proposed to reconcile paleontological & molecular data, in which eurypterids & chasmataspidids may be close to arachnopulmonates (spiders, scorpions, vinegaroons...) Chasmatapidids inner phylogeny is in the process of being revised, so to be clear, the names in the cladogram below aren’t fully official yet and come from an available but unpublished master thesis that is the current most up-to-date work on the topic. The order is usually split into an older early paleozoic bunch of species looking more like horseshoe crabs, like Chasmataspis laurencii, and a younger group mainly known from more recent periods. This later lineage comprises numerous Ordovician to Devonian species, all exhibiting a more eurypterid-like body: smoother profiles (especially for the prosoma), large eyes positioned foreward and a last pair of legs modified into paddle, just like many derived genera of eurypterines developed during the Ordovician & Silurian. The slide also showcase a phylogeny of chasmataspidids showing the split between more horseshoe crab-like lineages and more eurypterid-like ones, in which the diploaspidids belong Octoberaspis ushakovi belongs to this later group, and is usually seen as a relative of the best known genus Diploaspis within the family Diploaspididae. It seems however that it was closer to other genera that could form all together a group called Octoberaspidinae, characterized by a more rigid front opisthosoma and ridges running on their prosomal carapace, some being noticeable on O. ushakovi when looking near its large lateral eyes. These latter chasmataspidids seem to have had a large distribution across North America & Europe, from brackish to fully marine setup. The slide also showcase a phylogeny of diploaspidids showing Octoberaspis belonging to the sub-family octoberaspidinae, in which can be found too Heteroaspis, Nahlyostaspis, Loganamaraspis & Dvulikiaspis
July 15, 2026 at 4:08 PM
fossildetective.bsky.social
A recent paper has described Late Cambrian arthropods from Irkutsk, including a synziphosurine and chasmataspidid. These would be the oldest known; however, I think these identifications need to be treated with caution. 🧵[1/5] 🧪⚒️

https://link.springer.com/article/10.1134/S1028334X2360127X
A figure from the scientific paper showing two fossils on a red and grey rock. The first fossils (a supposed synziphosurine) shows a series of maybe for or five broad segments and three or four smaller ones, but is poorly preserved. The second fossil (described as a euthycarcinoid) resembles a long smear on the rock. An image from the paper showing two fossils described as chasmataspidids consisting of a small 'head' region and then an area composed of maybe three to five broad segments, then a long abdomen of seven or eight narrower segments.
August 23, 2023 at 1:24 PM
amypteride.bsky.social
Chasmataspidid evolution is a fascinating toping yet still poorly understood, and many uncertainties remain on their origins and evolutionary trends (+ the usual drawing feedback)

#Cheliceratime
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Chasmataspidids share traits with both horseshoe crabs & eurypterids, making attempts to classify them sometimes difficult, even more with the few species known from this group. They are often retrieved close to eurypterids: both orders share similar traits and have deep paleozoic origins, more than probably during the Late Cambrian considering how diverse & derived some eurypterids already were in the Ordovician and how chasmataspidid seemed to appear in the Late Cambrian. The links of both groups with freshwaters, as well as several possible land exaptations, make their study of certain importance in assessing chelicerate terrestrialization. The slide also showcase a putative phylogeny proposed to reconcile paleontological & molecular data, in which eurypterids & chasmataspidids may be close to arachnopulmonates (spiders, scorpions, vinegaroons...) Chasmatapidids inner phylogeny is soon to be revised, so we won’t talk much about it, it’ll be the topic of another time. The order is usually split into an older ordovician bunch of species looking more like horseshoe crabs, like Chasmataspis laurencii, and a younger devonian group. Diploaspis casteri is the historical center of this latter group, and exhibits like all these species a more eurypterid-like body: smoother profiles (especially for the prosoma), large eyes positioned foreward and a last pair of legs modified into paddle, just like many derived genera of eurypterines developed during the Ordovician & Silurian. Like some of the most derived members of this subgroup, the Diploaspis genus exhibits a more compact & shortened body. The slide also showcase a phylogeny of chasmataspidid without the families' names due to the potential future revision of the order, Diploaspis being of the most derived genus of the group Chasmataspidids appear to be one of the most ancient groups of euchelicerates, and growing evidence suggests the group was already well implemented at the end of the Cambrian & beginning of Ordovician near the landmasses that ended forming today’s North America & Europe. Devonian forms show important morphological changes but what happened to these arthropods during the Late Ordovician & Silurian is still unknown, best guess being a ecological specialization allowing the group to last longer alongside other more successfull relatives. The group seems to disappear after the Devonian, never to be seen again. Their eurypterids relatives endured until the end of the Permian but in relictual freshwater forms. The low diversity of chasmataspidids may have been one of the reasons behind their extinction, making them too vulnerable to the ecological changes occurring during the Devonian. The slide also comprise a timescale featuring the 2 major pools of chasmataspidids being stratigraphically isolated, complicating the understanding of the group's evolution Is this paleoart accurate? My little sister told me it looked like the Pokemon Kabuto while I was drawing it, so since it wasn’t that extravagant for an aquatic arthropod I put the same colors on it^^ The overall morphology is taken straight from Dunlop et al. (2001), it’s the most detailed & up-to-date description available. The end curve here may seem a bit forced but it gives a movement to the drawing & makes it more balanced.
March 23, 2026 at 5:02 PM
amypteride.bsky.social
Aquatic main euchclicerates groups: Xiphosura (Horseshoe crabs), Chasmataspidida, Eurypterida & the arachnid body plan

#Cheliceratime #horseshoecrab #chasmataspidid #eurypterid #seascorpion #arachnid
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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
March 28, 2026 at 5:47 PM
amypteride.bsky.social
Chasmataspidid evolution is a fascinating topic yet still poorly understood, and many uncertainties remain on their origins and evolutionary trends

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

4/4
Despite looking a lot like an eurypterid, O. ushakovi differs from them on several key features. The most obvious is the body segmentation, the thick 4-segmented buckler at the front half of the opisthosoma being unique to chasmataspidids. Another interesting one is the morphology of the paddle-like 5th legs, a gold example of convergent evolution since it looks similar in both groups despite being built differently. It most probably derived from a pincer-like appendage for chasmataspidids, while eurypterids evolved theirs from pointy legs. Chasmataspidids are among the oldest groups of euchelicerates, and may have originated in shallow waters of the landmasses that ended forming today’s North America & Europe. This rather rare group has a sparse fossil record showing the replacement of the old horseshoe crab-like forms by more derived eurypterid-like forms through the Ordovician and Silurian, best guess being an ecological specialization allowing the group to last longer alongside other more successfull relatives. The group seems to disappear after the Devonian, never to be seen again. Their eurypterids relatives endured until the end of the Permian but in relictual freshwater forms. The low diversity of chasmataspidids may have been one of the reasons behind their extinction, making them too vulnerable to the ecological changes occurring during the Devonian. The slide also comprise a timescale featuring the 2 major pools of chasmataspidids being stratigraphically isolated, complicating the understanding of the group's evolution Is this paleoart accurate? Did I put yellow & reddish colors on it just because its name is a reference to a communist revolution? You bet I did xD (but it looks cool nonetheless and it’s not an aberration). The overall morphology is taken straight from Dunlop (2002), it’s the most detailed & up-to-date description available. I could have extended a bit the paddle-like 5th legs, but I think it’s better to stay safe on this one. REFERENCES · Dunlop J.A., 2002. Arthropods from the Lower Devonian Severnaya Zemlya Formation of October Revolution Island (Russia). Geodiversitas 24(2), p. 349-379. · 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., Gunderson G.O. & Meyer R.C., 2019. A common arthropod from the Late Ordovician Big Hill Lagerstätte (Michigan) reveals an unexpected ecological diversity within Chasmataspidida. BMC Evolutionary Biology 19(8), 24 pp. · 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. · 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. · Voss B.D., 2024. The phylogeny of Chasmataspidida. Graduate Theses, Dissertations, and Problem Reports 12475 44 pp. Colorado Plateau Geosystems Inc., 2026. Global Series. In DeepTimeMaps. Online at https://deeptimemaps.com/map-lists-thumbnails/global-series/
July 15, 2026 at 4:08 PM
fossildetective.bsky.social
The 'chasmataspidids' do somewhat resemble the Devonian chasmataspidid Octoberaspis, but the head is very small and it is difficult to tell how many segments are in the anterior broad region. Overall, I don't find a chasmataspidid affinity for these fossils that compelling. [3/5]
A black and white photograph of two fossils of Octoberaspis, from Dunlop 2002. The fossils preserve a large head, three broad body segments, and then a narrow abdomen of 9 segments.
August 23, 2023 at 1:25 PM