Chasmataspidid


Chasmataspidids, sometime referred to as chasmataspids, are a group of extinct chelicerate arthropods that form the order Chasmataspidida. Chasmataspidids are probably related to horseshoe crabs and/or sea scorpions, with more recent studies suggest that they form a clade with Eurypterida and Arachnida. Chasmataspidids are known sporadically in the fossil record through to the mid-Devonian, with possible evidence suggest that they also present during the late Cambrian. Chasmataspidids are most easily recognised by having an abdomen divided into a short forepart and a longer hindpart each comprising 4 and 9 segments respectively. There is some debate about whether they form a natural group.

Distribution

Chasmataspidids survived at least since Ordovician to mid-Devonian in age. As of 2019, most chasmataspidids are known from the Devonian strata, while the preceding Silurian and Ordovician period each have 3 and 2 species being described. Diploaspis is the only genus of chasmataspidids that unambigiously comprises species from different periods. There was also trace fossil compose of resting imprints with Chasmataspis-like outline discovered from late-Cambrian stratum, which might suggest an earlier occurrence of chasmataspidids.

Morphology

Most chasmataspidids are small arthropods with body length did not exceed 3 centimeters, with the ordovician species being exceptionally large, ranging between 10 and 29 centimeters.
The streamlined body of chasmataspidid compose of a rigid prosoma and an externally 13-segmented opisthosoma. Like eurypterid, dorsal side of the prosoma was covered by a rigid carapace that bore a pair of larger lateral eyes and a pair of tiny median ocelli. Chasmataspidid readily distinguish from other chelicerates by the subdivision of opisthosoma into a widen, 4-segmented mesosoma and a slender, 9-segmented metasoma. the first opisthosomal/mesosomal tergite retain as a narrow element known as 'microtergite', which is not observable in eurypterid. The posterior 3 mesosmal segments are well developed, forming a rigid box-like section called 'buckler'. The metasomal/postabdominal segments are cylinderal and the last segment terminated with a spine/plate-like telson, which is usually relatively short.

Appendages

Since the appendages of chasmataspidid are rarely preserved in the fossil, most species have only fragile or even no appendicular structures had been described. Based on available materials, the prosoma compose of 6 appendage pairs just like most euchelicerates, which were 1 pair of small chelicerae and 5 pairs of limb-like appendages, although the detail morphology of the former is still unclear. The coxae of appendage II-VI bore gnathobases. At least the posteriormost appendage pair of prosoma seems to be differ between families. Appendage of Chasmataspididae known only from 2 disarticulated specimens of appendages which interpreted as appendage VI of Chasmataspis. the appendage bore exopod-like structure on the base and terminated with a chelate, similar to those of a xiphosuran. On the other hand, Appendage VI modified into a paddle that strikingly resemble to those of an eurypterine was discovered in some species of Diploaspididae, but the basal diploaspidid Loganamaraspis possibly did not possess this character on Appendage VI. the limb-like appendage II-V of diploaspidids are either featureless or bore rows of spines.
Opisthosomal appendages are even rarely being observed and only known from a few diploaspidid materials. they are at least present on the mesosomal region, each pair originated from one mesosomal segment. the anteriormost appendicular structure of opisthosoma was metastoma, a plate-like structure interpreted as a fused appendage pair of first opisthosomal segment, situated between the gnathobase of prosomal appendage VI. Beyond the metastoma were 3 pairs of plate-like opercula originated from the 3 'buckler' segments, with the first opercula bore a medially positioned genital appendage that extend until the posterior region of second opercula. The metastoma, opercula and genital appendage are shared characters between chasmataspidid and eurypterid, but unlike the fused first and second opercula of eurypterid, the opercula seems to be unfused in chasmataspidid. Possible chasmataspidid trace fossil from cambrian have imprints resembling 6 pairs of opercula. If the interpretation is true, chasmataspidid may had extra 3 pairs of opercula on the first 3 metasomal segment as well.

Representative genera

''Chasmataspis''

The first chasmataspidid to be discovered was Chasmataspis laurencii, described by the American palaeontologists Kenneth E. Caster and H. K. Brooks in 1956. These Ordovician fossils come from the site of the Douglas Dam in Tennessee, USA. They are the most xiphosuran-like of the known chasmataspidid species, with a horseshoe-shaped carapace. Caster & Brooks raised a new family, Chasmataspididae, to accommodate these specimens. The species was redescribed by Jason Dunlop and colleagues in 2004.

''Diploaspis''

The next species to be discovered were Diploaspis casteri and Heteroaspis novojilovi; both described by the Norwegian palaeontologist Leif Størmer from the early Devonian of Alken an der Mosel in Germany in 1972.
A revision by Markus Poschmann and co-workers in 2005 recognised H. novojilovi as a synonym of D. casteri. The two species appear to actually be preservational variants of the same species. Poschmann et al. also described a second species as Diploaspis muelleri.
A third species, Diploaspis praecursor, was described by Lamsdell and Briggs in 2017.

''Forfarella''

Forfarella mitchelli from the early Devonian of the Forfar region in the Midland Valley of Scotland was described by Jason Dunlop and colleagues in 1999; although the fossil had actually been recognised as a chasmataspidid and provisionally labelled as such some years previously by Charles Waterston. Forfarella mitchelli is not very well preserved, but does show the characteristic chasmataspidid body plan.

''Acahanarraspis''

The stratigraphically youngest chasmataspidid is Achanarraspis reedi, described by Lyall Anderson and colleagues in 2000, from the mid-Devonian Achanarras quarry in Caithness, Scotland; a famous fossil fish locality.

''Octoberaspis''

Well preserved chasmataspidids were recovered from the early Devonian of October Revolution Island, part of the Severnaya Zemlya group in the Russian Arctic. Originally briefly described as eurypterids, they were formally described as Octoberaspis ushakovi by Jason Dunlop in 2002. Octoberaspis is one of the few chasmataspidids with well-documented opisthosomal appendages, reveal some characters previously though to be eurypterid-exclusive were also shared by chasmataspidid as well.

''Loganamaraspis''

Loganamaraspis dunlopi discovered from a famous Silurian fossil locality near Lesmahagow in Scotland. Described by Erik Tetlie and Simon Braddy in 2003, it was placed in Diploaspididae, but interpreted as being somewhat more intermediate in form between the Chasmataspis and Diploaspidid body plans.

''Dvulikiaspis''

Fossils of Dvulikiaspis menneri discovered from the Imangda River of Taymyr Peninsula were originally interpreted as a species of eurypterid genus Stylonurus, and formally described as a new genus of chasmataspidid by David J. Marshall and co-authors in 2014. Dvulikiaspis menneri is one of the few well-preserved chasmataspidid, with distal morphology of appendage II-VI had been revealed.

''Hoplitaspis''

Hoplitaspis hiawathai is the second known species of Ordovician chasmataspidid, discovered from the Big Hill Lagerstätte of Michigan in United States, described by James C. Lamsdell and co-authors in 2019. With nearly complete set of appendages being observable, Hoplitaspis hiawathai is the most complete chasmataspidid known at that time. Each of the paddle of Hoplitaspis hiawathai has a claw instead of an intersegmental element like those of other diploaspidids, providing clues on the relationship between the appendage VI of Chasmataspis and diploaspidids.

Classification

Phylogenetic position

Chasmataspidids have a controversial phylogenetic position within Chelicerata. The first species to be discovered were thought to be unusual fossil xiphosuran, while later species were often based on specimens initially misidentified as eurypterids. Chasmataspidids had been interpreted as relatives/members of either xiphosurans or eurypterids, or forming a clade with eurypterids and arachnids. Some studies even suggest that chasmataspidids may not represent a monophyletic taxon - for example as a paraphyletic grade where the eurypterids arose; or a polyphyletic group with Chasmataspis and diploaspidids more closely related to xiphosuans and eurypterids, respectively. The polyphyletic hypothesis was based on the xiphosuran-like characters of Chasmataspis and eurypterid-like characters found on diploaspidid genera. However this interpretation could be unreliable, as the characters are either partially shared by both xiphosuans and eurypterids or more likely represent a result of parallel evolution. Additionally, the monophyly of chasmataspidids could be supported by the unique component of 4-segmented mesosoma and 9-segmented metasoma as well. As of 2010s, many studies supports the monophyly of Chasmataspidida and Dekatriata.

Interrelationships

As of 2019, up to 12 genera had been associated within Chasmataspidida. With the exception of Diploaspis which compose of 3 species since 2017, all chasmataspidid genera are monotypic. The order Chasmataspidida subdivided into two families: Chasmataspididae and Diploaspididae. the former comprise of Chasmataspis while the latter include the remaining genera. Chasmataspididae is defined by a horseshoe-shaped carapace with distinct genal spines and a completely fused mesosoma; while Diploaspididae is defined by a semicircular to subquadrate carapace and a mesosoma with curved, nontrilobate segments.
†Chasmataspidida Caster & Brooks, 1956