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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. 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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4. Investigating the effect of host plant identity on instar number in fall webworm, a common generalist herbivore

   https://doi.org/10.1093/ee/nvad126  

   Tanino-Springsteen, Mykaela M; Vyas, Dhaval K; Mitchell, Audrey; Durso, Catherine; Murphy, Shannon M (January 2024, Environmental Entomology)

   Ranger, Christopher (Ed.)

   Abstract For herbivorous insects with a broad diet breadth, host plant identity can influence larval development by either accelerating or delaying growth. For some species of Lepidoptera, the number of larval instars varies depending on the host plant’s identity. Fall webworm (Hyphantria cunea, Drury) is a polyphagous herbivore that feeds on over 450 host plants worldwide. Of the 2 morphotypes (red- and black-head) of fall webworm, the number of instars for the red-head fall webworms has not been characterized. Given its broad diet breadth, fall webworm developmental stages may vary with plant identity. We investigated whether host plant identity affected the number of instars observed during red-head fall webworm development. We measured the head capsules of over 6,000 fall webworm larvae reared on 6 different plants commonly eaten by fall webworms in Colorado. We modeled head capsule widths as Gaussian mixture models, with a Gaussian distribution that corresponded to each instar. We show that our red-head fall webworms varied in number of instars depending on the identity of their host plant upon which they fed. We found that red-head fall webworm exhibited 7 instars on 5 of the host plants and 8 instars on 1 host plant that we studied. Our results for the number of instars for red-head fall webworm are consistent with reports of the number of instars for black-head fall webworm. Our research provides insight into the influence of host plant identity on fall webworm development, which can be used to advance lab and field research of this species. 

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5. Long-term clade-wide shifts in trilobite segment number and allocation during the Palaeozoic

   https://doi.org/10.1098/rspb.2022.1765  

   Hopkins, Melanie J.; To, Rebecca (December 2022, Proceedings of the Royal Society B: Biological Sciences)

   Arthropods are characterized by having an exoskeleton, paired jointed appendages and segmented body. The number and shape of those segments vary dramatically and unravelling the evolution of segmentation is fundamental to our understanding of arthropod diversification. Because trilobites added segments to the body post-hatching which were expressed and preserved in biomineralized exoskeletal sclerites, their fossil record provides an excellent system for understanding the early evolution of segmentation in arthropods. Over the last 200 years, palaeontologists have hypothesized trends in segment number and allocation in the trilobite body, but they have never been rigorously tested. We tabulated the number of segments in the post-cephalic body for over 1500 species, selected to maximize taxonomic, geographical and temporal representation. Analysis reveals long-term shifts in segment number and allocation over the 250-million-year evolutionary history of the clade. For most of the Palaeozoic, the median number of segments in the body did not change. Instead, the total range decreased over time and there was long-term increase in the proportion of segments allocated to the fused terminal sclerite relative to the articulated thoracic region. There was also increased conservation of thoracic segment number within families. Neither taxonomic turnover nor trends in functionally relevant defensive behaviour sufficiently explain these patterns. 

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