DEVELOPMENTAL PARALLELISMS AND PAEDOMORPHOSIS DROVE CONVERGENT EVOLUTION BETWEEN CHASMATASPIDID AND EURYPTERID CHELICERATES

Chasmataspidids are rare fossil chelicerates known from only 14 species but exhibiting a broad temporal and geographic distribution, ranging from the Middle Ordovician to Middle Devonian of North America, Scotland, Germany, Norway, and Siberia. The group has a convoluted taxonomic history with their monophyly strongly questioned due to the xiphosuran-like nature of the Ordovician Chasmataspis in contrast to the more eurypterid-like morphology of the Diploaspididae that comprise the remaining chasmataspidid species. Despite these previous doubts as to the validity of Chasmataspidida as a natural group, the clade is now considered to be a valid taxonomic unit diagnosed by the possession of an ankylosed four-segmented buckler that resolves phylogenetically as sister to a clade comprising eurypterids and arachnids. Chasmataspidids therefore demonstrate remarkable convergence toward a eurypterid morphology which entailed a radical shift from that of Chasmataspis, although it has been unclear how this was accomplished.

Restudy of the original material of Chasmataspis revealed one of the paratype specimens, originally referred to as a “small postabdomen”, to be a complete juvenile individual which also preserves fragments of the prosomal appendages, including the chelicera. The juvenile is approximately 25 mm in length, while the largest adults are approximately 60 mm long, and has a much smaller prosoma and buckler in relation to the postabdomen than observed in the adults, resulting in the juvenile having proportions identical to those of the eurypterid-like Hoplitaspis, Loganamaraspis, and Dvulikiaspis. Comparison with juveniles of Hoplitaspis indicates that diploaspidids developed their convergent eurypterid-like morphology through paedomorphosis (the retention of juvenile traits into adulthood). Hoplitaspis exhibits a number of traits similar to waeringopterid and pterygotid eurypterids, both clades which are thought to have developed their distinct morphology through paedomorphic heterochronic shifts. In combination with the recent discovery of a eurypterid-like ontogenetic trajectory in the Ordovician xiphosurid Lunataspis, the convergent similarities between chasmataspidids and eurypterids may be due to developmental parallelisms.