New austrolimulid from Russia supports role of Early Triassic horseshoe crabs as opportunistic taxa

Abstract

Horseshoe crabs are extant marine euchelicerates that have a fossil record extending well into the Palaeozoic. Extreme xiphosurid morphologies arose during this evolutionary history. These forms often reflected the occupation of freshwater or marginal conditions. This is particularly the case for Austrolimulidae—a xiphosurid family that has recently been subject to thorough taxonomic examination. Expanding the austrolimulid record, we present new material from the Olenekian-aged Petropavlovka Formation in European Russia and assign this material to Attenborolimulus superspinosus gen. et sp. nov. A geometric morphometric analysis of 23 horseshoe crab genera illustrates that the new taxon is distinct from limulid and paleolimulid morphologies, supporting the assignment within Austrolimulidae. In considering Triassic austrolimulids, we suggest that the hypertrophy or reduction in exoskeletal sections illustrate how species within the family evolved as opportunistic taxa after the end-Permian extinction.

An earliest Triassic age forTasmaniolimulusand comments on synchrotron tomography of Gondwanan horseshoe crabs

“…Extending CFD studies to fossil xiphosurids will facilitate comparative studies of the hydrodynamic properties of the carapaces of extinct members of the clade, in addition to elucidating the effects of bizarre morphologies, such as the hypertrophied genal spines, on fluid flow. Such spines have been hypothesised to represent an adaptation to movement through unidirectional fluid flow in primarily freshwater or marginal marine environments ( Lamsdell, 2016 ; Lamsdell, 2021 ; Bicknell & Pates, 2019 ; Bicknell & Shcherbakov, 2021 ; Bicknell et al, 2022a ); CFD provides the most compelling method for evaluating the likelihood of this hypothesis. Due to compression of the fossils (consider Dubbolimulus peetae ) CFD models of compressed xiphosurids would need to be retro-deformed, likely using modern forms as a proxy for inflation, to account for taphonomic alteration.…”

On the morphospace of eurypterine sea scorpions

“…The geometric morphometric analysis presented here considers the carapace morphology of 115 specimens. This approach draws on previous applications of geometric morphometrics to the closely related Xiphosurida (Bicknell 2019;Bicknell & Pates 2019;Bicknell & Shcherbakov 2021;Lustri et al 2021). The carapace was examined because it is commonly preserved and is important for eurypterid systematics (Tollerton 1989;Cuggy 1994).…”

“…This approach draws on previous applications of geometric morphometrics to the closely related Xiphosurida (Bicknell 2019; Bicknell et al . 2019; Bicknell & Pates 2019; Bicknell & Shcherbakov 2021; Lustri et al . 2021).…”

Peer Review #2 of "An earliest Triassic age for Tasmaniolimulus and comments on synchrotron tomography of Gondwanan horseshoe crabs (v0.2)"

“…While fossil arthropods have received comparatively less attention than the aforementioned taxa (e.g., Pates et al, 2021;Song et al, 2021), CFD studies have modelled lift and drag experienced by modern xiphosurids (Bicknell & Pates, 2019;Davis et al, 2019). Extending CFD studies to fossil Manuscript to be reviewed marginal marine environments (Lamsdell, 2016(Lamsdell, , 2021Bicknell & Pates, 2019;Bicknell & Shcherbakov, 2021;Bicknell et al, 2022a); CFD provides the most compelling method for evaluating the likelihood of this hypothesis. Due to compression of the fossils (consider Dubbolimulus peetae) CFD models of compressed xiphosurids would need to be retro-deformed, likely using modern forms as a proxy for inflation, to account for taphonomic alteration.…”