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1. Absolute axial growth and trunk segmentation in the early Cambrian trilobite Oryctocarella duyunensis

   https://doi.org/10.1017/pab.2020.63  

   Dai, Tao ; Hughes, Nigel C. ; Zhang, Xingliang ; Fusco, Giuseppe ( October 2021 , Paleobiology)

   Patzkowsky, M. (Ed.)

   Abstract A short stratigraphic interval near Bulin in western Hunan (China) yields multiple specimens of the ~514-Myr-old oryctocarine trilobite Oryctocarella duyunensis . Size data obtained from these specimens indicate that, from meraspid degree 1 onward, degrees represent successive instars. Meraspid growth persisted until a terminal stage was reached, providing the first example of determinate growth in trilobites and, notably, in an early Cambrian species. The sample contains three varieties of such terminal stages, recognized as holaspids, with 9, 10, or 11 thoracic segments, respectively. During the meraspid phase, growth rates were not constant in this species. The pattern of growth seen in the Bulin assemblage differs modestly from that reported in the same species from two other localities, attesting to microevolutionary variation in developmental patterns among these collections. 

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2. Developmental and functional controls on enrolment in an ancient, extinct arthropod

   https://doi.org/10.1098/rspb.2023.0871  

   Esteve, Jorge ; Hughes, Nigel C. ( June 2023 , Proceedings of the Royal Society B: Biological Sciences)

   Three-dimensional models reveal how the mechanics of exoskeletal enrolment changed during the development of a model organism for insights into ancient arthropod development, the 429-million-year-old trilobite Aulacopleura koninckii. Changes in the number, size and allocation of segments within the trunk, coupled with the need to maintain effective exoskeletal shielding of soft tissue during enrolment, necessitated a transition in enrolment style about the onset of mature growth. During an earlier growth phase, enrolment was sphaeroidal, with the venter of the trunk fitting exactly against that of the head. In later growth, if lateral exoskeletal encapsulation was to be maintained trunk length proportions did not permit such exact fitting, requiring an alternative, non-sphaeoridal enrolment style. Our study favours the adoption of a posture in later growth in which the posterior trunk extended beyond the front of the head. This change in enrolment accommodated a pattern of notable variation in the number of mature trunk segments, well known to characterize the development of this species. It suggests how an animal whose early segmental development was remarkably precisely controlled was able to realize the marked variation in mature segment number that was related, apparently, to life in a physically challenging, reduced oxygen setting. 

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3. Scaling of axial muscle architecture in juvenile Alligator mississippiensis reveals an enhanced performance capacity of accessory breathing mechanisms

   https://doi.org/10.1111/joa.13523  

   Rose, Kayleigh A. R. ; Tickle, Peter G. ; Elsey, Ruth M. ; Sellers, William I. ; Crossley, II, Dane A. ; Codd, Jonathan R. ( July 2021 , Journal of Anatomy)

   Quantitative functional anatomy of amniote thoracic and abdominal regions is crucial to understanding constraints on and adaptations for facilitating simultaneous breathing and locomotion. Crocodilians have diverse locomotor modes and variable breathing mechanics facilitated by basal and derived (accessory) muscles. However, the inherent flexibility of these systems is not well studied, and the functional specialisation of the crocodilian trunk is yet to be investigated. Increases in body size and trunk stiffness would be expected to cause a disproportionate increase in muscle force demands and therefore constrain the basal costal aspiration mechanism, necessitating changes in respiratory mechanics. Here, we describe the anatomy of the trunk muscles, their properties that determine muscle performance (mass, length and physiological cross‐sectional area [PCSA]) and investigate their scaling in juvenileAlligator mississippiensisspanning an order of magnitude in body mass (359 g–5.5 kg). Comparatively, the expiratory muscles (transversus abdominis,rectus abdominis,iliocostalis), which compress the trunk, have greater relative PCSA being specialised for greater force‐generating capacity, while the inspiratory muscles (diaphragmaticus,truncocaudalis ischiotruncus,ischiopubis), which create negative internal pressure, have greater relative fascicle lengths, being adapted for greater working range and contraction velocity. Fascicle lengths of the accessorydiaphragmaticusscaled with positive allometry in the alligators examined, enhancing contractile capacity, in line with this muscle's ability to modulate both tidal volume and breathing frequency in response to energetic demand during terrestrial locomotion. Theiliocostalis, an accessory expiratory muscle, also demonstrated positive allometry in fascicle lengths and mass. All accessory muscles of the infrapubic abdominal wall demonstrated positive allometry in PCSA, which would enhance their force‐generating capacity. Conversely, the basal tetrapod expiratory pump (transversus abdominis) scaled isometrically, which may indicate a decreased reliance on this muscle with ontogeny. Collectively, these findings would support existing anecdotal evidence that crocodilians shift their breathing mechanics as they increase in size. Furthermore, the functional specialisation of thediaphragmaticusand compliance of the body wall in the lumbar region against which it works may contribute to low‐cost breathing in crocodilians.

    

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4. Low-dimensional organization of angular momentum during walking on a narrow beam

   https://doi.org/DOI:10.1038/s41598-017-18142-y  

   Chiovetto, E ; Huber, M ; Sternad, D ; Giese, M ( January 2018 , Scientific reports)

   Walking on a beam is a challenging motor skill that requires the regulation of upright balance and stability. The difficulty in beam walking results from the reduced base of support compared to that afforded by flat ground. One strategy to maintain stability and hence avoid falling off the beam is to rotate the limb segments to control the body’s angular momentum. The aim of this study was to examine the coordination of the angular momentum variations during beam walking. We recorded movement kinematics of participants walking on a narrow beam and computed the angular momentum contributions of the body segments with respect to three different axes. Results showed that, despite considerable variability in the movement kinematics, the angular momentum was characterized by a low-dimensional organization based on a small number of segmental coordination patterns. When the angular momentum was computed with respect to the beam axis, the largest fraction of its variation was accounted for by the trunk segment. This simple organization was robust and invariant across all participants. These findings support the hypothesis that control strategies for complex balancing tasks might be easier to understand by investigating angular momentum instead of the segmental kinematics. 

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5. Biomechanical Responses of Neonatal Brachial Plexus to Mechanical Stretch

   https://doi.org/10.1055/s-0038-1669405  

   Singh, Anita ; Shaji, Shania ; Delivoria-Papadopoulos, Maria ; Balasubramanian, Sriram ( January 2018 , Journal of Brachial Plexus and Peripheral Nerve Injury)

   Abstract This study investigated the biomechanical responses of neonatal piglet brachial plexus (BP) segments—root/trunk, chord, and nerve at two different rates, 0.01 mm/second (quasistatic) and 10 mm/second (dynamic)—and compared their response to another peripheral nerve (tibial). Comparisons of mechanical responses at two different rates reported a significantly higher maximum load, maximum stress, and Young's modulus (E) values when subjected to dynamic rate. Among various BP segments, maximum stress was significantly higher in the nerve segments, followed by chord and then the root/trunk segments except no differences between chord and root/trunk segments at quasistatic rate. E values exhibited similar behavior except no differences between the chord and root/trunk segments at both rates and no differences between chord and nerve segments at quasistatic rate. No differences were observed in the strain values. When compared with the tibial nerve, only mechanical properties of BP nerves were similar to the tibial nerve. Mechanical stresses and E values reported in BP root/trunk and chord segments were significantly lower than tibial nerve at both rates. When comparing the failure pattern, at quasistatic rate, necking was observed at maximum load, before a complete rupture occurred. At dynamic rate, partial rupture at maximum load, followed by a full rupture, was observed. Occurrence of the rate-dependent failure phenomenon was highest in the root/trunk segments followed by chord and nerve segments. Differences in the maximum stress, E values, and failure pattern of BP segments confirm variability in their anatomical structure and warrant future histological studies to better understand their stretch responses. 

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