Neurosensory and Sinus Evolution as Tyrannosauroid Dinosaurs Developed Giant Size: Insight from the Endocranial Anatomy of Bistahieversor sealeyi

Abstract

Tyrannosaurus rex and other tyrannosaurid dinosaurs were apex predators during the latest Cretaceous, which combined giant size and advanced neurosensory systems. Computed tomography (CT) data have shown that tyrannosaurids had a trademark system of a large brain, large olfactory bulbs, elongate cochlear ducts, and expansive endocranial sinuses surrounding the brain and sense organs. Older, smaller tyrannosauroid relatives of tyrannosaurids developed some, but not all, of these features, raising the hypothesis that tyrannosaurid‐style brains evolved before the enlarged tyrannosaurid‐style sinuses, which might have developed only with large body size. This has been difficult to test, however, because little is known about the brains and sinuses of the first large‐bodied tyrannosauroids, which evolved prior to Tyrannosauridae. We here present the first CT data for one of these species, Bistahieversor sealeyi from New Mexico. Bistahieversor had a nearly identical brain and sinus system as tyrannosaurids like Tyrannosaurus, including a large brain, large olfactory bulbs, reduced cerebral hemispheres, and optic lobes, a small tab‐like flocculus, long and straight cochlear ducts, and voluminous sinuses that include a supraocciptal recess, subcondyar sinus, and an anterior tympanic recess that exits the braincase via a prootic fossa. When characters are plotted onto tyrannosauroid phylogeny, there is a two‐stage sequence in which features of the tyrannosaurid‐style brain evolved first (in smaller, nontyrannosaurid species like Timurlengia), followed by features of the tyrannosaurid‐style sinuses (in the first large‐bodied nontyrannosaurid tyrannosauroids like Bistahieversor). This suggests that the signature tyrannosaurid sinus system evolved in concert with large size, whereas the brain did not. Anat Rec, 303:1043–1059, 2020. © 2020 American Association for Anatomy

Baby tyrannosaurid bones and teeth from the Late Cretaceous of western North America1

“…As a result, they are well studied and much is known of their anatomy, diversity, growth, and evolution (Brusatte et al 2010). Recent discoveries have further elucidated the origin of their distinctive body plans (Xu et al 2004, Lü et al 2014, Nesbitt et al 2019, sensory apparatus (Brusatte et al 2016a, Carr et al 2017, McKeown et al 2020, and large body sizes (Erickson et al 2004, Woodward et al 2020. Osteohistological data have enabled detailed analyses of tyrannosaurid growth rate and life history (Erickson et al 2004, Horner and Padian 2004, Woodward et al 2020, showing that tyrannosaurids grew at high but inconsistent rates in the later stages of their lives.…”

The endocranium and trophic ecology of Velociraptor mongoliensis

“…Medium‐ and large‐bodied carnivorous theropods (e.g. tyrannosaurids (Witmer and Ridgely, 2009; Bever et al ., 2011; Brusatte et al ., 2016; Kundrát et al ., 2018; McKeown et al ., 2020), abelisaurids (Carabajal and Succar, 2015), carcharodontosaurids (Franzosa and Rowe, 2005; Brusatte and Sereno, 2007; Carabajal and Canale, 2010), megaraptorans (Carabajal and Currie, 2017), and allosaurids (Rogers, 1999; Gleich et al ., 2005)) as well as small and medium‐sized maniraptorans—e.g. oviraptorosaurs (Kundrát, 2007; Balanoff et al ., 2018), therizinosaurs (Lautenschlager et al ., 2012), and others (Walsh et al ., 2009; Zelenitsky et al ., 2011)—have been the focus of neurosensory studies.…”

Two braincases of Daspletosaurus (Theropoda: Tyrannosauridae): anatomy and comparison1

“…In both CMN 8506 and TMP 2001.36.1, the olfactory tract is clearly separated from the cerebral hemispheres by a transverse constriction. In TMP 2001.36.1, the tract is markedly narrow both vertically and transversely with respect to the cerebral region and olfactory bulbs, similar to the endocasts of Alioramus, Bistahieversor, and Gorgosaurus (Witmer and Ridgely 2010;Bever et al 2013;McKeown et al 2020). However, CMN 8506 is more consistent with other large tyrannosaurines in having an olfactory tract that is not so bottlenecked as in TMP 2001.36.1 (Osborn 1912;Brochu 2000;Witmer and Ridgely 2009;Bever et al 2013).…”

“…The exception may be Allosaurus, as at least one specimen (UUVP 294) has a flocculus that almost extended through both anterior and posterior loops of the semicircular canals (Hopson 1979;Rogers 1998). The medulla oblongata is markedly convex ventrally in both CMN 8506 and TMP 2001.36.1, reflecting the greater midbrain flexure in Daspletosaurus compared with Alioramus, Tarbosaurus, and Tyrannosaurus (Saveliev and Alifanov 2007;Ridgely 2010, 2010;Bever et al 2013), but which is similar to Bistahieversor and Gorgosaurus (Witmer and Ridgely 2009;McKeown et al 2020).…”

“…The braincases of Allosaurus from a single bonebed also have high frequencies of variation (Chure and Madsen 1996). In an extreme example of standing variations in the pneumatic connections, the anterior tympanic and subcondylar recesses are broadly confluent in a mature individual of Bistahieversor (NMMNH P-27469) (McKeown et al 2020), even though the lower portion of the anterior tympanic recess is labeled as part of subcondylar recess in that reconstruction.…”