Chasmataspidida is an order of peculiar arthropods, which seem to resemble both xiphosurans and eurypterids. Their phylogenetic placement
is still very uncertain because of this. Therefore, I decided to learn more about these creatures in order to see if I would be able to come up
with my own hypothesis regarding their phylogenetic placement.
with my own hypothesis regarding their phylogenetic placement.
Chasmataspidida consists of two clades: Chasmataspididae (one genus) and Diploaspididae (10 genera). I decided to focus on Chasmataspididae
first. The only genus in Chasmataspididae is Chasmataspis. It was first described by Caster and Brooks, and later redescribed by Dunlop et al.
first. The only genus in Chasmataspididae is Chasmataspis. It was first described by Caster and Brooks, and later redescribed by Dunlop et al.
Reconstruction of Chasmataspis according to Caster and Brooks. Something that sets Chasmataspidids apart from Eurypterids is the clear separation of the carapace, abdomen and
postabdomen, making them resemble Xiphosurans a lot more.
postabdomen, making them resemble Xiphosurans a lot more.
Reconstruction of Chasmataspis by Dunlop et al. Dunlop et al. noted that Chasmataspis would have likely held its legs under its carapace in a horseshoe crab-like fashion, instead of the
sprawled posture suggested by Caster and Brooks, where the legs would extend beyond the carapace.
sprawled posture suggested by Caster and Brooks, where the legs would extend beyond the carapace.
To give Chasmataspis (and with it all of Chasmataspididae) a proper phylogenetic placement, I’ve looked at it’s anatomical features and I have concluded
that Chasmataspididae is most likely closer to Xiphosura than to Eurypterida. First of all, the legs (and their posture) of Chasmataspis resembles those
of Xiphosurans much more than those of Eurypterids.
that Chasmataspididae is most likely closer to Xiphosura than to Eurypterida. First of all, the legs (and their posture) of Chasmataspis resembles those
of Xiphosurans much more than those of Eurypterids.
The legs of Chasmataspis. Note the serrated coxa, which is characteristic to Xiphosurans.
Just like many Xiphosurans, Chasmataspis possesses chelate claws on its appendages, a feature not seen in at all in Eurypterids. Furthermore, the legs
of Chasmataspis also possess odd stalk-like structures on their coxa, called flabella. These are also exclusively seen in Xiphosura.
of Chasmataspis also possess odd stalk-like structures on their coxa, called flabella. These are also exclusively seen in Xiphosura.
The leg of Chasmataspis. The stalk extending from the coxa is the flabellum.
Secondly, the anatomy of the tergites resemble that of early Xiphosurans. Caster and Brooks, aswell as Dunlop et al. noticed that the first tergite in
Chasmataspidids is almost completely overlapped by the carapace. This feature was considered unique to Chasmataspidida. However, this is also
the case in the primitive Xiphosuran Dibasterium durgae.
Chasmataspidids is almost completely overlapped by the carapace. This feature was considered unique to Chasmataspidida. However, this is also
the case in the primitive Xiphosuran Dibasterium durgae.
The Silurian Xiphosuran Dibasterium durgae. It is clearly visible that the first tergite is covered by the carapace. Briggs et al. also noted this.
In conclusion, my research suggests that Chasmataspidida nests closer to Xiphosura than to Eurypterida. However, my hypothesis has still got several
flaws. For example, the amount of tergites in Chasmataspids (14) is much higher than those in Xiphosurans (12), and closer to those of Eurypterids (13).
Furthermore, The abdomen of Chasmataspids consists of only 4 tergites, while those of Xiphosurans consists of 8 tergites. Last but not least, the overall
anatomy of Diploaspids resembles that of Eurypterids much closer, looking nothing like Xiphosurans at all.
flaws. For example, the amount of tergites in Chasmataspids (14) is much higher than those in Xiphosurans (12), and closer to those of Eurypterids (13).
Furthermore, The abdomen of Chasmataspids consists of only 4 tergites, while those of Xiphosurans consists of 8 tergites. Last but not least, the overall
anatomy of Diploaspids resembles that of Eurypterids much closer, looking nothing like Xiphosurans at all.
Reconstruction of Forfella mitchelli, a Diploaspid.
However, I am still convinced that Chasmataspids are more closely related to Xiphosurans than to Eurypterids. The fact that Xiphosurans have less tergites
than Chasmataspids might be because these were either lost or fused. We can see in modern Xiphosurans that the abdomen and postabdomen consists
entirely out of fused tergites. And for Diploaspids, well, these might not even be Chasmataspidids after all…
than Chasmataspids might be because these were either lost or fused. We can see in modern Xiphosurans that the abdomen and postabdomen consists
entirely out of fused tergites. And for Diploaspids, well, these might not even be Chasmataspidids after all…
but more on that in part two.
References:
References:
- A redescription of Chasmataspis laurencii Caster & Brooks, 1956 (Chelicerata: Chasmataspidida) from the Middle Ordovician of Tennessee, USA, with remarks on chasmataspid phylogeny
- New fossils from the Canadian-Chazyan (Ordovician) hiatus in Tennessee
- A common arthropod from the Late Ordovician Big Hill Lagerstätte (Michigan) reveals an unexpected ecological diversity within Chasmataspidida
- A new chasmataspid (Chelicerata: Chasmataspida) from the Lower Devonian of the Midland Valley of Scotland
- Silurian horseshoe crab illuminates the evolution of arthropod limbs
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