Chasmataspidida
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, while 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 suggesting that they were also present during the late Cambrian. Chasmataspidids are most easily recognised by having an opisthosoma divided into a wide forepart and a narrow hind part 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 unambiguously comprises species from different periods. There was also a trace fossil composed of resting imprints with Chasmataspis-like outline discovered from late-Cambrian stratum in Texas, which might suggest an earlier occurrence of chasmataspidids. In addition, likely fossils from the Jiangshanian have been recorded from the Kimiltei site. However, affinities of those Cambrian records are questioned in 2023 study, identifying imprint from Texas as made by phyllocarid, and suggesting Kimiltei fossils are also phyllocarids, while original authors argues.Morphology
Most chasmataspidids are small arthropods with a body length that did not exceed 3 centimeters, with the ordovician species being exceptionally large, ranging between 10 and 29 centimeters.The streamlined body of a chasmataspidid is composed of a rigid prosoma and an externally 13-segmented opisthosoma. As in eurypterids, the 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. Chasmataspidids are readily distinguished from other chelicerates by the subdivision of the 13 opisthosomal segments into a widened, 4-segmented preabdomen and a slender, 9-segmented postabdomen. the tergite of the first opisthosomal or preabdominal segment is retained as a narrow element known as 'microtergite', which is not observed in eurypterids. The posterior three preabdominal segments are well developed, forming a rigid box-like section called a 'buckler'. The postabdominal segments are cylindrical, and the last segment terminates 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 a 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 ventral side of preabdomen, each pair originated from one preabdominal 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 operculum pair bore a medially positioned genital appendage that extend until the posterior region of second operculum pair. Some of the opercula may have book gills just like those of xiphosurans and eurypterids, but the evidence are equivocal. Previous reports of a large operculum cover the whole ventral surface of buckler are most likely a misinterpretation of the ventral buckler wall, which were originally enclosed by the opercula in life. The metastoma, opercula and genital appendage are shared characters between chasmataspidid and eurypterid, but unlike the fused first and second operculum pair of eurypterid, the two operculum pairs 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 postabdominal 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.