Three-dimensional visualization as a tool for interpreting locomotion strategies in ophiuroids from the Devonian Hunsrück Slate

Abstract

Living brittle stars (Echinodermata: Ophiuroidea) employ a very different locomotion strategy to that of any other metazoan: five or more arms coordinate powerful strides for rapid movement across the ocean floor. This mode of locomotion is reliant on the unique morphology and arrangement of multifaceted skeletal elements and associated muscles and other soft tissues. The skeleton of many Palaeozoic ophiuroids differs markedly from that in living forms, making it difficult to infer their mode of locomotion and, therefore, to resolve the evolutionary history of locomotion in the group. Here, we present three-dimensional digital renderings of specimens of six ophiuroid taxa from the Lower Devonian Hunsrück Slate: four displaying the arm structure typical of Palaeozoic taxa ( Encrinaster roemeri, Euzonosoma tischbeinianum, Loriolaster mirabilis, Cheiropteraster giganteus ) and two ( Furcaster palaeozoicus , Ophiurina lymani ) with morphologies more similar to those in living forms. The use of three-dimensional digital visualization allows the structure of the arms of specimens of these taxa to be visualized in situ in the round, to our knowledge for the first time. The lack of joint interfaces necessary for musculoskeletally-driven locomotion supports the interpretation that taxa with offset ambulacrals would not be able to conduct this form of locomotion, and probably used podial walking. This approach promises new insights into the phylogeny, functional morphology and ecological role of Palaeozoic brittle stars.

A Silurian ophiuroid with soft‐tissue preservation

“…On this basis Glass & Blake (2004) inferred that podial locomotion was likely for the Devonian protasterid Bundenbachia beneckei , and Clark et al . (2020) extended this to Palaeozoic ophiuroids without fused segments. Movement in the Herefordshire specimen may have been facilitated in the same way although the tube feet are slender, as in living ophiuroids, compared with those of B .…”

“…The interpretation of locomotion in fossil asterozoans, especially those outside crown group Ophiuroidea and Asteroidea, is not straightforward; morphology may not be a definitive guide to function (Dean Shackleton 2005, p. 59; Clark et al . 2020). Oegophiurids such as Protaster had offset ambulacra and their arms were presumably less flexible than those of living ophiuroids.…”

Gilsonicaris from the Lower Devonian Hunsrück slate is a eunicidan annelid and not the oldest crown anostracan crustacean

“…Instead, clear morphological similarities with polychaete annelids can be identified, such as many segmented body lateral outgrowths/appendages ( parapodia), a head with differentiated prostomium and peristomium formed of two rings (which houses a ventral mouth) and a multi-element jaw apparatus that is comparable with that of eunicidan polychaetes. µCT has so far been applied only on rare occasions to Hunsrück fossils, firstly to an machaeridian annelid [7] and later to two arthropods, a vertebrate, a mollusc and an echinoderm [8][9][10][11][12]. Considering the level of detail and information it revealed for such a limited but wide range of organisms, including specimens as small and flattened as Gilsonicaris, our data further highlights the potential of µCT for uncovering new insights into the systematics and palaeoecology of the Hunsrück fauna, and also by extension into mid-Palaeozoic marine ecosystems.…”

Back to life: Techniques for developing high-quality 3D reconstructions of plants and animals from digitized specimens

“…CT imaging also permits the visualization of internal and external anatomical features as they are arranged in situ, which can be useful for interpreting function and behavior [e.g., [6][7][8][9]. CT imaging can be especially useful in the visualization of fossil taxa that may be embedded within a rocky matrix [e.g., [10][11][12].…”