Virtual reconstruction of the endocranial anatomy of the early Jurassic marine crocodylomorphPelagosaurus typus(Thalattosuchia)

Abstract

Thalattosuchians were highly specialised aquatic archosaurs of the Jurassic and Early Cretaceous, and represent a peak of aquatic adaptation among crocodylomorphs. Relatively little is known of their endocranial anatomy or its relevance for the evolution of sensory systems, physiology, and other aspects of biology. Nevertheless, such data have significance for two reasons: (1) thalattosuchians represent an important data point regarding adaptation to marine life in tetrapods; and (2) as early-diverging members of the crocodylian stem-lineage, thalattosuchians provide information on the evolutionary assembly of the brain and other endocranial structures in crocodylomorphs. Here we use µCT data to virtually reconstruct the endocranial anatomy of Pelagosaurus typus, an early thalattosuchian with plesiomorphic traits of relevance to the split between the two major subgroups: Teleosauroidea and Metriorhynchoidea. Interpretation of these data in a broad comparative context indicate that several key endocranial features may be unique to thalattosuchians, including: a pyramidal morphology of the semicircular canals, the presence of an elongate endosseous cochlear duct that may indicate enhanced hearing ability, the presence of large, paired canals extending anteriorly from an enlarged pituitary fossa, a relatively straight brain (possibly due to the presence of large, laterally placed orbits), and an enlarged venous sinus projecting dorsally from the endocast that is confluent with the paratympanic sinus system. Notably, we document a large expansion of the nasal cavity anterior to the orbits in Pelagosaurus as an osteological correlate of an enlarged salt gland previously only documented in Late Jurassic metriorhynchoids. This is the first anatomical evidence of this structure in early thalattosuchians. Pelagosaurus also shares the presence of paired olfactory bulbs with metriorhynchoids, and shows an enlarged cerebrum, which may also be present in teleosauroids. Taken together, our findings indicate that physiological and sensory adaptations to marine life occurred early in thalattosuchian evolution, predating the origins of flippers, tail flukes, and hydrodynamic body forms seen later in metriorhynchoids.

Convergence and functional evolution of longirostry in crocodylomorphs

“…The lifestyle of this species is supported by its derived physiological, neuroanatomical and sensory adaptations, such as pyramidal semicircular canals, an elongated cochlear duct and possibly nasal salt glands (Pierce et al . ). Pierce et al .…”

“…The anatomy of Pelagosaurus has been thoroughly described (Pierce & Benton , Pierce et al . ), although the use of CT scanning enabled us to study bones that were previously unknown, such as the braincase and the medial elements of the mandible. The skull of Pelagosaurus was digitally reconstructed by labelling individual bones of the left side of the skull in the segmentation editor (Fig.…”

Sensory Evolution and Ecology of Early Turtles Revealed by Digital Endocranial Reconstructions

“…As in other amniotes, the portion between the fore-and midbrain is the most voluminous, but this is achieved exclusively by an increase in height, since the endocast is nearly constant in width over its entire length (Figure 1). Another striking feature is the pendant pituitary fossa, which is very common in archosaurs (Witmer et al, 2008;Lautenschlager and Butler, 2016;Araújo et al, 2017;Pierce et al, 2017), but does not occur in extant turtles, in which the dorsum sellae and the sella turcica are aligned, positioning the pituitary fossa approximately at the same level as the posterior portions of the endocast (Figures 2, 3). Although the pituitary fossa of turtles can also house other smaller structures (e.g., internal carotid and abducens nerve) the size of the pituitary gland should be at least partially responsible for the larger size of the fossa in P. quenstedti.…”

“…Outlines were smoothed ten times to eliminate pixel noise, and 23 Fourier harmonics were found to describe the outlines of all sampled taxa sufficiently (average Fourier power > 99%) (see also Supplementary Material). In addition to the reconstructed endocasts, further outlines of 52 taxa were collected from the literature (Hopson, 1979;Franzosa, 2004;Neenan and Scheyer, 2012;Bona and Paulina-Carabajal, 2013;Carabajal et al, 2013;George and Holliday, 2013;Herrera et al, 2013;Holloway et al, 2013;Lautenschlager and Butler, 2016;von Baczko and Desojo, 2016;Jirak and Janacek, 2017;Laaß et al, 2017;Paulina-Carabajal et al, 2017;Pierce et al, 2017; Digimorph 1 ) for different turtle, archosauromorph, lepidosauromorph and other amniote taxa (for list of taxa see Table S1). These outlines were redrawn in Adobe Illustrator to ensure sufficient resolution for the digitization process.…”

Endocranial anatomy and life habits of the Early Triassic archosauriform Proterosuchus fergusi

“…Compared to other archosauriforms the olfactory region is relatively short, analogous to the thalattosuchian Pelagosaurus typus (Pierce et al . ), but considerably larger than the ornithosuchid Riojasuchus tenuisceps (von Baczko & Desojo ).…”