An earliest Triassic age for Tasmaniolimulus and comments on synchrotron tomography of Gondwanan horseshoe crabs

Abstract

Constraining the timing of morphological innovations within xiphosurid evolution is central for understanding when and how such a long-lived group exploited vacant ecological niches over the majority of the Phanerozoic. To expand the knowledge on the evolution of select xiphosurid forms, we reconsider the four Australian taxa: <i>Austrolimulus fletcheri</i>, <i>Dubbolimulus peetae</i>, <i>Tasmaniolimulus patersoni</i>, and <i>Victalimulus mcqueeni</i>. In revisiting these taxa, we determine that, contrary to previous suggestion, <i>T. patersoni</i> arose after the Permian and the origin of over-developed genal spine structures within Austrolimulidae is exclusive to the Triassic. To increase the availability of morphological data pertaining to these unique forms, we also examined the holotypes of the four xiphosurids using synchrotron radiation X-ray tomography (SRXT). Such non-destructive, in situ imaging of palaeontological specimens can aid in the identification of novel morphological data by obviating the need for potentially extensive preparation of fossils from the surrounding rock matrix. This is particularly important for rare and/or delicate holotypes. Here, SRXT was used to emphasize <i>A. fletcheri</i> and <i>T. patersoni</i> cardiac lobe morphologies and illustrate aspects of the <i>V. mcqueeni</i> thoracetronic doublure, appendage impressions, and moveable spine notches. Unfortunately, the strongly compacted <i>D. peetae</i> precluded the identification of any internal structures, but appendage impressions were observed. The application of computational fluid dynamics to high-resolution 3D reconstructions are proposed to understand the hydrodynamic properties of divergent genal spine morphologies of austrolimulid xiphosurids.