Ontogenetic endocranial shape change in alligators and ostriches and implications for the development of the non‐avian dinosaur endocranium

Abstract

Birds and crocodiles show radically different patterns of brain development, and it is of interest to compare these to determine the pattern of brain growth expected in dinosaurs. Here we provide atlases of 3D brain (endocast) reconstructions for Alligator mississippiensis (alligator) and Struthio camelus (ostrich) through ontogeny, prepared as digital restorations from CT scans of stained head and dry skull specimens. Our morphometric analysis confirms that ostrich brains do not change significantly in shape during postnatal growth, whereas alligator brains unfold from a cramped bird‐like shape in the hatchling to an elongate, straight structure in the adult. We confirm that birds exhibit paedomorphic dinosaur endocranial traits such as retaining an enlarged and compact brain shape in the adult, whereas crocodiles show peramorphic traits where the brain elongates with growth as the skull elongates. These atlases of ontogenetic stages of modern bird and crocodilian endocrania provide a basis for comparison of non‐avian dinosaur endocasts and consideration of the divergence of the “avian” and “crocodilian” modes of brain development and heterochronic change on phylogenies.

Ontogenetic variation in the crocodylian vestibular system

“…Crocodylian proportional brain volume also varies considerably through ontogeny, with the brain occupying a smaller proportion of the endocast in later ontogenetic stages (Watanabe et al, 2019). In contrast, birds show relatively little ontogenetic variation in their brain endocasts (Hu et al, 2020; Kawabe et al, 2015), probably due at least in part to their rapid maturation from hatchling to adult. The previously recognised changes in brain shape and size during ontogeny, and the changes in vestibular labyrinth size and shape that we recognise here, together indicate that there is a general pattern of neurosensory system transformation seen throughout crocodylian ontogeny.…”

“…Finally, our finding that crocodylians changed labyrinth size and shape during ontogeny corresponds to other observed changes in neurosensory systems as crocodylians grow. Recent work shows that Alligator brain endocasts unfold during ontogeny, with hatchlings having a bird‐like shape (an S‐shaped endocast with the forebrain region located anterodorsal to the hindbrain region) and adults a more elongated and straight endocast (Hu et al, 2020; Jirak & Janacek, 2017). Crocodylian proportional brain volume also varies considerably through ontogeny, with the brain occupying a smaller proportion of the endocast in later ontogenetic stages (Watanabe et al, 2019).…”

“…Sensory systems, however, also undergo significant changes during ontogenetic growth although these have received far less study. In crocodylians, the brain (and thus the brain endocast) becomes anteroposteriorly more elongated from juvenile to more mature individuals (Hu et al, 2020; Jirak & Janacek, 2017), the tubes for the pharyngotympanic and median pharyngeal sinus structures become vertically longer (Dufeau & Witmer, 2015), and the semicircular canals have recently been shown to undergo negative allometric growth in relation to the bony otic capsule (Kuzmin et al, 2021). Thus, it can be hypothesized that most endocranial systems are affected by ontogeny, but much work remains to document and interpret these changes.…”

Morphology and postnatal ontogeny of the cranial endocast and paranasal sinuses of capybara (Hydrochoerus hydrochaeris), the largest living rodent

“…Besides comparing semaphoronts of different taxa, the threedimensional reconstruction of the endocranial spaces of individuals of distinct ontogenetic stages can provide key information on the growth and ontogenetic variation in a non-invasive way (Dumont et al, 2020;Hoffmann et al, 2019;Hu et al, 2020;Hublin et al, 2015;Hurlburt et al, 2013;Iurino et al, 2020;Lautenschlager & Hübner, 2013;Macrini et al, 2007;Petroviĉ et al, 2018;Picasso et al, 2010). Therefore, the study of ontogenetic growth of cranial cavities is not only important for understanding species developmental patterns, but also for identifying age-related bias when applying these methodologies to fossil taxa.…”

Neuroanatomy of the mekosuchine crocodylianTrilophosuchus rackhamiWillis, 1993

“…This is because of the relatively thick dural envelope that surrounds the brain, such that the dura mater and its dural venous sinuses are significant contributors to the endocast's contours, proportions, and volume (Jirak & Janacek, 2017 ; Watanabe et al, 2019 ; Witmer et al, 2008 ). Additionally, the crocodylian brain undergoes ontogenetic changes that affect its shape (and consequently, the shape of the brain endocast; see Beyrand et al, 2019 ; Hopson, 1979 ; Hu et al, 2021 ; Jirak & Janacek, 2017 ; Lessner & Holliday, 2020 ) as well as ratio of brain volume versus endocast volume, where in juveniles the brain occupies more endocranial space than in mature individuals (Hopson, 1979 ; Hu et al, 2021 ; Hurlburt et al, 2013 ; Jirak & Janacek, 2017 ; Rogers, 1999 ; Watanabe et al, 2019 ; see Figure S2.3). Hence, the description is based on the brain endocast rather than the brain itself, which is obviously neither preserved nor accurately represented by the endocast of the fossil.…”