Water vascular system architecture in an Ordovician ophiuroid

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

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.

Integrating morphology and in vivo skeletal mobility with digital models to infer function in brittle star arms

Brittle stars (Phylum Echinodermata, Class Ophiuroidea) have evolved rapid locomotion employing muscle and skeletal elements within their (usually) five arms to apply forces in a manner analogous to that of vertebrates. Inferring the inner workings of the arm has been difficult as the skeleton is internal and many of the ossicles are sub-millimeter in size. Advances in 3D visualization and technology have made the study of movement in ophiuroids possible. We developed six virtual 3D skeletal models to demonstrate the potential range of motion of the main arm ossicles, known as vertebrae, and six virtual 3D skeletal models of non-vertebral ossicles. These models revealed the joint center and relative position of the arm ossicles during near-maximal range of motion. The models also provide a platform for the comparative evaluation of functional capabilities between disparate ophiuroid arm morphologies. We made observations on specimens of Ophioderma brevispina and Ophiothrix angulata. As these two taxa exemplify two major morphological categories of ophiuroid vertebrae, they provide a basis for an initial assessment of the functional consequences of these disparate vertebral morphologies. These models suggest potential differences in the structure of the intervertebral articulations in these two species, implying disparities in arm flexion mechanics. We also evaluated the differences in the range of motion between segments in the proximal and distal halves of the arm length in a specimen of O. brevispina, and found that the morphology of vertebrae in the distal portion of the arm allows for higher mobility than in the proximal portion. Our models of non-vertebral ossicles show that they rotate further in the direction of movement than the vertebrae themselves in order to accommodate arm flexion. These findings raise doubts over previous hypotheses regarding the functional consequences of ophiuroid arm disparity. Our study demonstrates the value of integrating experimental data and visualization of articulated structures when making functional interpretations instead of relying on observations of vertebral or segmental morphology alone. This methodological framework can be applied to other ophiuroid taxa to enable comparative functional analyses. It will also facilitate biomechanical analyses of other invertebrate groups to illuminate how appendage or locomotor function evolved.

The function of the ophiuroid nerve ring: how a decentralized nervous system controls coordinated locomotion

Echinoderms lack a centralized nervous control system yet each extant echinoderm class has evolved unique and effective strategies for locomotion. Brittle stars (Ophiuroidea) stride swiftly over the seafloor by coordinating motions of their five muscular arms. Their arms consist of many repeating segments, requiring them to use a complex control system to coordinate motions among segments and between arms. We conducted in vivo experiments with brittle stars to analyze the functional role of the nerve ring, which connects the nerves in each arm. These experiments were designed to determine how the ophiuroid nervous system performs complex decision-making and locomotory actions under decentralized control. Our results show that brittle star arms must be connected by the nerve ring for coordinated locomotion, but information can travel bidirectionally around the nerve ring so that it circumvents the severance. Evidence presented indicates that ophiuroids rely on adjacent nerve ring connections for sustained periodic movements. The number of arms connected via the nerve ring is correlated positively with the likelihood that the animal will show coordinated locomotion, indicating that integrated nerve ring tissue is critical for control. The results of the experiments should provide a basis for the advancement of complex artificial decentralized systems.