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On the phylogenetic placement of Chasmataspidida, Part 2

In part one, we discussed the phylogenetic placement of Chasmataspididae and concluded that Chasmataspididae is closer to Xiphosura than to Eurypterida. However, Chasmataspidida consists of two clades: Chasmataspididae and Diploaspididae. Surprisingly enough, the case of Diploaspididae is entirely different than that of Chasmataspididae.


Even though they are relatively unknown by the public, their fossil record is quite extensive. Unlike Chasmataspididae, which only contains one genus, Diploaspididae consists of as much as 10 genera, all well-spread across the globe (ranging from the USA to Russia). Recent discoveries have revealed that Diploaspidids filled in more ecological niches as previously thought.
A reconstruction of Hoplitaspis, a suprisingly well-developed predator. It even has forward-placed eyes, providing improved binocular vision. Figure obtained from Lamsdell et al. (2019).
When looking at Diploaspidids, the resemblance to Eurypterids quickly becomes obvious. Similarities can be seen in the often rectangular shaped carapace, spiked feeding appendages and most of all, the paddle-shaped appendage VI, also seen in Eurypterines. These features, especially the latter, caused many researchers to place Chasmataspidida closer to Eurypterida, and sometimes even within Eurypterida. However, there is something not quite right about this…
When comparing appendage VI in Eurypterids and Diploaspidids, one notices a significant difference. The way the paddle is constructed differs in Diploasidids from Eurypterids. It seems that the paddles are actually a product of convergent evolution rather than divergent evolution.
Appendage VI of Chasmataspidids and Eurypterids compared. Chasmataspidids on the left, Eurypterids on the right. Note that Lamsdell et al. (2019) believes Chasmataspidida is a monophyletic group, and thus included appendage VI of Chasmataspis. Figure obtained from Lamsdell et al. (2019).
As you can see, Diplaspidid paddles clearly differ from their Eurypterid counterparts. In Eurypterids, the paddle consists mostly of the enlarged basitarsus (green) and telotarsus (red), plus the ‘podomere’ 7a (an extra podomere supposedly coming from the basitarsus). Podomere 8 is strongly reduced in size and becomes almost entirely vestigial in later forms. In Diploaspidids, the paddle also consists mostly of enlarged basitarsa and telotarsa, however here podomere 8 gets integrated into the paddle. This makes the inclusion of Chasmataspidida in Eurypterida unlikely.
OK, so no lumping of Chasmataspidida in Eurypterida. Does this mean that Diploaspidids are not at all closely related to Eurypterids? Certainly not. Even though the paddles differ in both groups, they still have a lot in common with each other.


A reconstruction of Loganamaraspis dunlopi. It is the only known Chasmataspidid with a preserved genital appendage.
In 2003, a new Diploaspidid was described: Loganamaraspis dunlopi. Loganamaraspis is unique for having a preserved, three segmented genital appendage; a feature otherwise exclusively seen in Eurypterids. The genital appendage resembles the type A genital appendage of Eurypterids. The fact that no other Chelicerate arthropod possesses these appendages, strongly suggests a close relationship of Diploaspidids and Eurypterids. The genital appendage is likely to be a homologous structure. Other characteristics of Diplosapidids also seem to match Eurypterid morphology, such as the usually big coxae, arrangement of the operculum and the possession of a metasoma.


There is still one problem. One of the main reasons Diploaspidids were included in Chasmataspidida, is the fact that they have a strongly reduced, partially covered tergite 1. This feature is not present in Eurypterids. However, Tetlie & Braddy (2003) suggested that the small tergite 1 in Diploaspidids is probably a primitive trait, that got lost in the more advanced Eurypterids.
Because of all the evidence we have for a closer affinity of Diploaspididae to Eurypterida, and closer affinity of Chasmataspididae to Xiphosura, I am convinced that Chasmataspidida is a polyphyletic group. Instead, I think we should nest Chasmataspididae sister to (or maybe even include in) Xiphosura, and have Diploaspididae sister to Eurypterids, as a more primitive group.
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