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

   https://doi.org/10.1098/rsos.201380  

   Clark, E. G.; Hutchinson, J. R.; Briggs, D. E. (December 2020, Royal Society Open Science)

   null (Ed.)

   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. 

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2. Integration of biplanar X-ray, three-dimensional animation and particle simulation reveals details of human ‘track ontogeny’

   https://doi.org/10.1098/rsfs.2020.0075  

   Hatala, Kevin G.; Gatesy, Stephen M.; Falkingham, Peter L. (October 2021, Interface Focus)

   null (Ed.)

   The emergence of bipedalism had profound effects on human evolutionary history, but the evolution of locomotor patterns within the hominin clade remains poorly understood. Fossil tracks record in vivo behaviours of extinct hominins, and they offer great potential to reveal locomotor patterns at various times and places across the human fossil record. However, there is no consensus on how to interpret anatomical or biomechanical patterns from tracks due to limited knowledge of the complex foot–substrate interactions through which they are produced. Here, we implement engineering-based methods to understand human track formation with the ultimate goal of unlocking invaluable information on hominin locomotion from fossil tracks. We first developed biplanar X-ray and three-dimensional animation techniques that permit visualization of subsurface foot motion as tracks are produced, and that allow for direct comparisons of foot kinematics to final track morphology. We then applied the discrete element method to accurately simulate the process of human track formation, allowing for direct study of human track ontogeny. This window lets us observe how specific anatomical and/or kinematic variables shape human track morphology, and it offers a new avenue for robust hypothesis testing in order to infer patterns of foot anatomy and motion from fossil hominin tracks. 

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3. Convergence of undulatory swimming kinematics across a diversity of fishes

   https://doi.org/10.1073/pnas.2113206118  

   Di Santo, Valentina; Goerig, Elsa; Wainwright, Dylan K.; Akanyeti, Otar; Liao, James C.; Castro-Santos, Theodore; Lauder, George V. (December 2021, Proceedings of the National Academy of Sciences)

   Fishes exhibit an astounding diversity of locomotor behaviors from classic swimming with their body and fins to jumping, flying, walking, and burrowing. Fishes that use their body and caudal fin (BCF) during undulatory swimming have been traditionally divided into modes based on the length of the propulsive body wave and the ratio of head:tail oscillation amplitude: anguilliform, subcarangiform, carangiform, and thunniform. This classification was first proposed based on key morphological traits, such as body stiffness and elongation, to group fishes based on their expected swimming mechanics. Here, we present a comparative study of 44 diverse species quantifying the kinematics and morphology of BCF-swimming fishes. Our results reveal that most species we studied share similar oscillation amplitude during steady locomotion that can be modeled using a second-degree order polynomial. The length of the propulsive body wave was shorter for species classified as anguilliform and longer for those classified as thunniform, although substantial variability existed both within and among species. Moreover, there was no decrease in head:tail amplitude from the anguilliform to thunniform mode of locomotion as we expected from the traditional classification. While the expected swimming modes correlated with morphological traits, they did not accurately represent the kinematics of BCF locomotion. These results indicate that even fish species differing as substantially in morphology as tuna and eel exhibit statistically similar two-dimensional midline kinematics and point toward unifying locomotor hydrodynamic mechanisms that can serve as the basis for understanding aquatic locomotion and controlling biomimetic aquatic robots. 

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4. Effects of shell morphology on mechanics of zebra and quagga mussel locomotion

   https://doi.org/10.1242/jeb.053348  

   Peyer, Suzanne M.; Hermanson, John C.; Lee, Carol Eunmi (July 2011, Journal of Experimental Biology)

   SUMMARY Although zebra mussels (Dreissena polymorpha) initially colonized shallow habitats within the North American Great Lakes, quagga mussels (Dreissena bugensis) are becoming dominant in both shallow- and deep-water habitats. Shell morphology differs among zebra, shallow quagga and deep quagga mussels but functional consequences of such differences are unknown. We examined effects of shell morphology on locomotion for the three morphotypes on hard (typical of shallow habitats) and soft (characteristic of deep habitats) sedimentary substrates. We quantified morphology using the polar moment of inertia, a parameter used in calculating kinetic energy that describes shell area distribution and resistance to rotation. We quantified mussel locomotion by determining the ratio of rotational (Krot) to translational kinetic energy (Ktrans). On hard substrate, Krot:Ktrans of deep quagga mussels was fourfold greater than for the other morphotypes, indicating greater energy expenditure in rotation relative to translation. On soft substrate, Krot:Ktrans of deep quagga mussels was approximately one-third of that on hard substrate, indicating lower energy expenditure in rotation on soft substrate. Overall, our study demonstrates that shell morphology correlates with differences in locomotion (i.e. Krot:Ktrans) among morphotypes. Although deep quagga mussels were similar to zebra and shallow quagga mussels in terms of energy expenditure on sedimentary substrate, their morphology was energetically maladaptive for linear movement on hard substrate. As quagga mussels can possess two distinct morphotypes (i.e. shallow and deep morphs), they might more effectively utilize a broader range of substrates than zebra mussels, potentially enhancing their ability to colonize a wider range of habitats. 

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5. Limb Ecometrics Show Limited Applicability for Quantifying Ecological Novelty in the Deep Evolution of Synapsida

   Lungmus, J. K.; Angielczyk, K. D.; Luo, Z. X. (January 2020, Scoiety for Integrative aand Comparative Biology 2020 Abstract Book)

   Mammalia are the only living members of the larger clade known as Synapsida, which has a fossil record spanning from 320mya to today. Despite the fact that much of the ecological diversity of mammals has been considered in light of limb morphology, the origin of broader synapsid limb diversity and its influence on ecological diversity has received less attention. Here we present shape analyses of the forelimbs of the multiple fossil synapsid radiations in comparison to a broad sample of extant Mammalia. Previous work by the authors has shown that shape broadly is not informative of specific locomotor ecomorphologies in earliest fossil Synapsida. Considering the broader scientific use of limb morphology in testing for fossil ecomorphologies, we sought to better understand at what juncture in synapsid evolutionary history do limb metrics begin to show utility in ecomorphological analyses. Shape data on humeri and ulnae elements from an extant sample representing known ecomorphologies provided the framework for a comparative study of extinct ecomorphologies, associated specifically with locomotion. We conducted linear and geometric morphometric comparisons between the extant sample and five taxonomic subsampled radiations moving crown-ward along the synapsid lineage. Taxonomic designations were the PermoCarboniferous "pelycosaurs”, both Permian and Triassic therapsids, "Non-mammaliaforme cynodonts”, and "Mammaliaformes”. Results show that many limb ecomorphological metrics commonly used are not effective designators until close to the origin of crown Mammalia, as late as the Jurassic. This brings into question the overall utility of using extant analogues to test for ecological signal in a given tetrapod group's deepest fossil ancestors. 

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