Neurosensory anatomy of Varanopidae and its implications for early synapsid evolution

Abstract

Varanopids are a group of Palaeozoic terrestrial amniotes which represent one of the earliest‐diverging groups of synapsids, but their palaeoneurology has gone largely unstudied and recent analyses have challenged their traditional placement within synapsids. We utilized computed tomography (CT) to study the virtual cranial and otic endocasts of six varanopids, including representative taxa of both mycterosaurines and varanodontines. Our results show that the varanopid brain is largely plesiomorphic, being tubular in shape and showing no expansion of the cerebrum or olfactory bulbs, but is distinct in showing highly expanded floccular fossae. The housing of the varanopid bony labyrinth is also distinct, in that the labyrinth is bounded almost entirely by the supraoccipital‐opisthotic complex, with the prootic only bordering the ventral portion of the vestibule. The bony labyrinth is surprisingly well‐ossified, clearly preserving the elliptical, sub‐orthogonal canals, prominent ampullae, and the short, undifferentiated vestibule; this high degree of ossification is similar to that seen in therapsid synapsids and supports the traditional placement of varanopids within Synapsida. The enlarged anterior canal, together with the elliptical, orthogonal canals and enlarged floccular fossa, lend support for the fast head movements indicated by the inferred predatory feeding mode of varanopids. Reconstructed neurosensory anatomy indicates that varanopids may have a much lower‐frequency hearing range compared to more derived synapsids, suggesting that, despite gaining some active predatory features, varanopids retain plesiomorphic hearing capabilities. As a whole, our data reveal that the neuroanatomy of pelycosaur‐grade synapsids is far more complex than previously anticipated.

Endocasts of the basal sauropsid Captorhinus reveal unexpected neurological diversity in early reptiles

“…The pronounced flexures of the brain observed in the captorhinids described here are similar to the condition described in some later sauropsids, including placodonts, phytosaurs, turtles, and several dinosaurian clades (Evans et al, 2009; Lautenschlager et al, 2012, 2018; Lautenschlager & Butler, 2016; Neenen & Scheyer, 2012; Schade et al, 2020; Sereno et al, 2007). This contrasts with the more tubular brain shape observed in therizinosaurian dinosaurs and varanopid synapsids (Bazzana et al, 2022; Lautenschlager et al, 2012). The amorphous nature of the cranial endocast in captorhinids, wherein the boundaries between various regions of the brain (e.g., optic lobes, medulla) are largely indiscernible, is similar to that seen in other early amniotes, stem amniotes, and many non‐mammalian taxa (Bazzana et al, 2022; Hopson, 1979; Schade et al, 2020) and is consistent with the lower degree of encephalization that is likely the plesiomorphic condition for crown Amniota.…”

“…This contrasts with the more tubular brain shape observed in therizinosaurian dinosaurs and varanopid synapsids (Bazzana et al, 2022; Lautenschlager et al, 2012). The amorphous nature of the cranial endocast in captorhinids, wherein the boundaries between various regions of the brain (e.g., optic lobes, medulla) are largely indiscernible, is similar to that seen in other early amniotes, stem amniotes, and many non‐mammalian taxa (Bazzana et al, 2022; Hopson, 1979; Schade et al, 2020) and is consistent with the lower degree of encephalization that is likely the plesiomorphic condition for crown Amniota.…”

“…As in many early amniotes (Bazzana et al, 2022;, the posterior braincase is wellpreserved but the metopic fissure remains open, such that the ventral portion of the endocast is only preserved posterior to the level of the pituitary fossa. Although the ventral portion of most of the braincase is absent, the margins of the endocranial cavity can be roughly delineated from the impressions on the overlying skull roof in OMNH 73281a, revealing an approximation of the outlines of the cerebrum, olfactory tract, and olfactory bulbs (Figure 2).…”

Early Origins of Divergent Patterns of Morphological Evolution on the Mammal and Reptile Stem-Lineages

“…S2 of the Supplementary material available on Dryad). Parareptiles are found as the sister to neodiapsids and varanopids are found as reptiles, suggesting that support for this contentious phylogenetic hypothesis (e.g., Benoit et al 2021 ; Bazzana et al 2021 ) remains high even given a larger sample of early synapsids. The addition of further pelycosaurian-grade synapsids produced results broadly consistent with recent analyses of this grouping ( Brocklehurst and Fröbisch 2018 ; Maddin et al 2020 ; Berman et al 2020 ).…”

An earliest Triassic age forTasmaniolimulusand comments on synchrotron tomography of Gondwanan horseshoe crabs

“…Nonetheless, non-destructive three-dimensional imaging using SRXT will likely continue to play a role in anatomical studies of fossil xiphosurids, following the rapid adoption of this imaging modality across palaeontology. Furthermore, NCT is being used more commonly in palaeontology, owing to the ability of neutrons to penetrate through typically radiopaque minerals such as iron pyrite, a high sensitivity to hydrogenous material, and thus to residual organic remains, ( Gee et al, 2019 ; Gee, Bevitt & Reisz, 2019 ; Na et al, 2021 ; Smith et al, 2021 ; Bazanna et al, 2021 ), and to increasing availability of high-quality neutron imaging facilities at nuclear research reactors and spallation neutron sources around the world (see https://www.isnr.de/index.php/facilities/user-facilities ). Finally, techniques that can more readily distinguish areas with very small differences in radiopacity, such as phase-contrast enhanced imaging, show promise for more detailed examination of muscles and other internal structures in suitably well-preserved specimens.…”