Sclerocephalus (Permian temnospondyl) skeletal growth record + evolution of diagonal-couplet gait in tetrapods (free pdfs)
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Ben Creisler
May 6, 2026, 3:42:32 PM (10 days ago) May 6
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Ben Creisler
Recent papers:
Free pdf:
Nicole Klein & Dorota Konietzko-Meier(2026)
Intraskeletal variability in osteohistology and growth record in a single individual of Sclerocephalus nobilis (Temnospondyli; Early Permian)
Royal Society Open Science 13(5): 252244
doi: https://doi.org/10.1098/rsos.252244
https://royalsocietypublishing.org/rsos/article/13/5/252244/481601/Intraskeletal-variability-in-osteohistology-and
The osteohistology and growth record of eight bones from a single individual of the Permian temnospondyl Sclerocephalus nobilis were analysed. This individual grew dominantly with a parallel-fibred matrix, which can be histologically differentiated into an early ontogenetic (larval) and a later ontogenetic (juvenile) stage. The histological observation of two life phases concurs with the morphological assessment of this individual as a late juvenile. The number of growth marks interpreted as being annual ranges from five (humerus) to 10 (tibia). The bones show a variety of growth marks with diverging sequences (single, double and multiple rest lines). Thus, the number of growth marks does not necessarily reflect actual age but may nevertheless provide an indication of the time involved. A cautious interpretation suggests that the two ontogenetic phases observed in this S. nobilis individual each lasted several seasons, possibly years. More speculatively, considering the relatively simple sequence of the growth marks of both humeri and the dermal bones, the larval stage may have persisted for 4 to 5 years, followed by a juvenile stage lasting for another year before death. The overall slow growth rate observed in this specimen of S. nobilis is probably related to a strong seasonal growth in a harsh environment.
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Free pdf:
Tsutomu Miyake, Kanto Nishikawa, Masamitsu Iwata, Hiroko Kamiyama, Kohtaro Ozaki, Hiroshi Koie, Arito Yozu, Tetsuya Hirasawa & Naoto Kobayashi (2026)
From Fin to Limb: Orientational Shift and Evolution of Diagonal-Couplet Gait in Tetrapods
Integrative Organismal Biology, obag020
doi: https://doi.org/10.1093/iob/obag020
https://academic.oup.com/iob/advance-article/doi/10.1093/iob/obag020/8670345
The transition from fins to limbs represents a pivotal evolutionary shift that enabled vertebrates to engage terrestrial environments through coordinated transformations in appendicular structure and neuromuscular control. Here, we propose that this transition can be understood as a biomechanical reorganization of force transmission across multi-joint musculoskeletal linkages. Rather than a simple rotational modification transforming fins directly into forelimbs, this process involved progressive reorganization of sarcopterygian appendages, including axial rotation, directional force production, and coordinated stance-swing dynamics that likely emerged prior to fully terrestrial locomotion. This review applies the two-joint link model, originally developed for human limb biomechanics, to extant sarcopterygians such as the coelacanth (Latimeria) and to early-diverging tetrapods, including archosaurs such as the American alligator (Alligator mississippiensis). The model demonstrates how sequential activation of biarticular and monoarticular muscles generates predictable, axis-aligned endpoint force outputs, providing a mechanistic explanation for how ancestral fins and early limbs could support weight bearing, propulsion, and postural stabilization during the water-to-land transition. The framework further clarifies the evolution and persistence of diagonal-couplet lateral sequence gait, a locomotor pattern widespread among tetrapods and consistent with Paleozoic trackway evidence. Integration of electromyographic data with conserved spinal circuitry including central pattern generators, interneurons, and the topographic organization of lateral motor column motor pools reveals how intrinsic spinal architecture governs muscle activation sequences and limb-level force redirection. Although supraspinal pathways modulate locomotion, core coordination of diagonal-couplet gait emerges from spinal mechanisms. A central property of the model is musculoskeletal redundancy: multiple muscle combinations can generate equivalent endpoint forces within linked multi-joint systems, thereby preserving functional capacity across postural variation. This robustness arises from the four-bar linkage organization of biarticular muscles, a design principle conserved across vertebrate musculoskeletal systems. Together, these findings position the two-joint link model as a unifying neuromechanical framework for understanding fin-to-limb evolution, the emergence of tetrapod gait, and the conserved spinal organization underlying vertebrate locomotion.
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Tsutomu Miyake, Kanto Nishikawa, Masamitsu Iwata, Hiroko Kamiyama, Kohtaro Ozaki, Hiroshi Koie, Arito Yozu, Tetsuya Hirasawa & Naoto Kobayashi (2026)
From Fin to Limb: Orientational Shift and Evolution of Diagonal-Couplet Gait in Tetrapods
Integrative Organismal Biology, obag020
doi: https://doi.org/10.1093/iob/obag020
https://academic.oup.com/iob/advance-article/doi/10.1093/iob/obag020/8670345
The transition from fins to limbs represents a pivotal evolutionary shift that enabled vertebrates to engage terrestrial environments through coordinated transformations in appendicular structure and neuromuscular control. Here, we propose that this transition can be understood as a biomechanical reorganization of force transmission across multi-joint musculoskeletal linkages. Rather than a simple rotational modification transforming fins directly into forelimbs, this process involved progressive reorganization of sarcopterygian appendages, including axial rotation, directional force production, and coordinated stance-swing dynamics that likely emerged prior to fully terrestrial locomotion. This review applies the two-joint link model, originally developed for human limb biomechanics, to extant sarcopterygians such as the coelacanth (Latimeria) and to early-diverging tetrapods, including archosaurs such as the American alligator (Alligator mississippiensis). The model demonstrates how sequential activation of biarticular and monoarticular muscles generates predictable, axis-aligned endpoint force outputs, providing a mechanistic explanation for how ancestral fins and early limbs could support weight bearing, propulsion, and postural stabilization during the water-to-land transition. The framework further clarifies the evolution and persistence of diagonal-couplet lateral sequence gait, a locomotor pattern widespread among tetrapods and consistent with Paleozoic trackway evidence. Integration of electromyographic data with conserved spinal circuitry including central pattern generators, interneurons, and the topographic organization of lateral motor column motor pools reveals how intrinsic spinal architecture governs muscle activation sequences and limb-level force redirection. Although supraspinal pathways modulate locomotion, core coordination of diagonal-couplet gait emerges from spinal mechanisms. A central property of the model is musculoskeletal redundancy: multiple muscle combinations can generate equivalent endpoint forces within linked multi-joint systems, thereby preserving functional capacity across postural variation. This robustness arises from the four-bar linkage organization of biarticular muscles, a design principle conserved across vertebrate musculoskeletal systems. Together, these findings position the two-joint link model as a unifying neuromechanical framework for understanding fin-to-limb evolution, the emergence of tetrapod gait, and the conserved spinal organization underlying vertebrate locomotion.
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