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| Eunotosaurus africanus with in-set labyrinth in Evers, Panigot, Petermann, Rubidge, Bever et Lyson, 2026. Life reconstruction by Andrey Atuchin. |
Abstract
The Middle Permian reptile Eunotosaurus africanus is a key taxon for understanding reptile evolution because it has been proposed to be one of the earliest stem turtles, but it could alternatively represent an early sauropsid. Alternative evolutionary interpretations are based on morphological character observations, and comparative anatomy of previously undocumented body parts may yield novel character evidence. Based on a previously unreported specimen of Eunotosaurus africanus, we provide descriptions of its inner ear anatomy, which we compare to a diversity of amniotes including turtles, millerettids and novel segmentations of Youngina capensis and Champsosaurus lindoei. The inner ear of Eunotosaurus africanus has plesiomorphic neodiapsid inner ear features that contrast with turtle ear morphology, including a radius of curvature of the anterior semicircular canal (ASC) that is larger than that of the posterior semicircular canal (PSC), narrow intercanal angles, slender vertical semicircular canals, and a bend in the central section of the PSC. Eunotosaurus africanus shares with turtles a large common crus cross-section and a thickened lateral semicircular canal. We provide a landmark-based comparison of amniote labyrinth shapes, which shows principal differences in the semicircular canals of birds, mammals and turtles as end members of a gradient of semicircular canal geometries. Other extant amniote groups (crocodiles, lepidosaurs), fossil lineages (e.g. sauropterygians, captorhinids, millerettids, younginiforms, varanopids, Eunotosaurus africanus) and stem lineages of extant groups (e.g. pterosaurs, non-avian dinosaurs, phytosaurs, other extinct pseudosuchians, mosasaurs, ‘pelycosaurs’, dicynodonts) show semicircular canal geometries that are generally intermediate between extreme shapes of birds, mammals and turtles, so that the entire morphospace shows gradational morphologies. The observed variation provides evidence of convergent evolution of labyrinth features, but also suggests phylogenetic signal is widespread in labyrinth morphology and that bird, mammal, and turtle labyrinth shapes may have evolved from a plesiomorphic shape that was retained in many reptilian stem lineages.
Keywords: neuroanatomy, inner ears, emicircular canal geometry, amniote phylogeny, turtle origins
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| Photographs of Eunotosaurus africanus (BP/1/7852). A, dorsal view. B, ventral view. Arabic numerals indicate position of vertebra or rib. Note that the spacing between pre- and postzygapophyses of dorsal vertebra seven in A demonstrates the elongate vertebral centra. Abbreviations: cor, coracoid; cra, cranium; cvs, cervical vertebral series; d, dorsal vertebra; hu, humerus; icl, interclavicle; man, mandible; poz, postzygapophysis; prz, prezygapophysis; sc, scapula; tr, thoracic rib. |
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| Life reconstruction of Eunotosaurus africanus with in-set labyrinth reconstruction by Andrey Atuchin. |
Serjoscha W. Evers, Eldon Panigot, Holger Petermann, Bruce S. Rubidge, Gabriel S. Bever and Tyler R. Lyson. 2026. Labyrinth Morphology of Eunotosaurus africanus in the Context of Semicircular Canal Shape Variation across Amniotes. Journal of Systematic Palaeontology. 24(1); 2634330. DOI: doi.org/10.1080/14772019.2026.2634330 [30 Mar 2026]
