Aspects of gorgonopsian paleobiology and evolution: insights from the basicranium, occiput, osseous labyrinth, vasculature, and neuroanatomy

Abstract

Synapsida, the clade including therapsids and thus also mammals, is one of the two major branches of amniotes. Organismal design, with modularity as a concept, offers insights into the evolution of therapsids, a group that experienced profound anatomical transformations throughout the past 270 Ma, eventually leading to the evolution of the mammalian bauplan. However, the anatomy of some therapsid groups remains obscure. Gorgonopsian braincase anatomy is poorly known and many anatomical aspects of the brain, cranial nerves, vasculature, and osseous labyrinth, remain unclear. We analyzed two gorgonopsian specimens, GPIT/RE/7124 and GPIT/RE/7119, using propagation phase contrast synchrotron micro-computed tomography. The lack of fusion between many basicranial and occipital bones in GPIT/RE/7124, which is an immature specimen, allowed us to reconstruct its anatomy and ontogenetic sequence, in comparison with the mature GPIT/RE/7119, in great detail. We explored the braincase and rendered various skull cavities. Notably, we found that there is a separate ossification between what was previously referred to as the “parasphenoid” and the basioccipital. We reinterpreted this element as a posterior ossification of the basisphenoid: the basipostsphenoid. Moreover, we show that the previously called “parasphenoid” is in fact the co-ossification of the dermal parasphenoid and the endochondral basipresphenoid. In line with previous descriptions, the anatomy of the osseous labyrinth is rendered in detail, revealing a unique discoid morphology of the horizontal semicircular canal, rather than toroidal, probably due to architectural constraints of the ossification of the opisthotic and supraoccipital. In addition, the orientation of the horizontal semicircular canal suggests that gorgonopsians had an anteriorly tilted alert head posture. The morphology of the brain endocast is in accordance with the more reptilian endocast shape of other non-mammaliaform neotherapsids.

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.…”

Endocranial Casts of Pre-Mammalian Therapsids Reveal an Unexpected Neurological Diversity at the Deep Evolutionary Root of Mammals

“…Well before the dinosaurs, these species were among the first tetrapods to occupy most terrestrial ecosystems [Rubidge and Sidor, 2001;Kemp, 2005]. Recent enhancement of CT scanning techniques has encouraged an international effort to improve the taxonomic sampling of NMT for palaeoneurological studies [Castanhinha et al, 2013;Laass et al 2014Laass et al , 2015Benoit et al, 2015Benoit et al, , 2016aBenoit et al, , b, 2017bRodrigues et al, 2014;Araújo et al, 2017;Laass and Kaestner, 2017]. Along with studies demonstrating the variety of neurological structures, which are usually believed to be very conservative in NMT, such as the pineal tube and foramen [Roth et al, 1986;Benoit et al, 2015Benoit et al, , 2016a, the bony labyrinth [Castanhinha et al, 2013;Laass, 2015b;Benoit et al, 2017b], the volume of the olfactory bulbs , or that of the brain endocast itself [Laass, 2015a;Laass and Kaestner, 2017], a closer look at NMT here demonstrated unexpected neuroanatomical diversity.…”

“…Accordingly, previous attempts to measure brain size and determine brain shape in therapsids resulted more in approximations than true estimations as there was a reliance on a variety of indirect approaches, ranging from geometric simplifications to conjectural visual approximations of brain morphology [Cluver, 1971;Kemp, 1969Kemp, , 1979Kemp, , 2009Jerison, 1973;Hopson, 1979;Quiroga, 1980Quiroga, , 1984Rowe et al, 2011;Rodrigues et al, 2014;Laass, 2015a]. Even the most recent CT scan-based surveys have encountered difficulties in obtaining a reliable picture of the therapsid braincase [Kemp, 2009;Rowe et al, 2011;Castanhinha et al, 2013;Rodrigues et al, 2014;Laass, 2015a;Araújo et al, 2017]. Accordingly, most endocranial volumes have been calculated on indirect evidence [Jerison, 1973;Hopson, 1979;Quiroga, 1980Quiroga, , 1984Kielan-Jaworowska et al, 2004;Rowe et al, 2011;Rodrigues et al, 2014;Laass, 2015a; but see Laass and Kaestner, 2017], and resulting estimations of the encephalisation quotient (EQ, a measure of brain volume relative to body mass [Jerison, 1973]) are controversial [see Kemp, 2009].…”

A potential link between lateral semicircular canal orientation, head posture, and dietary habits in extant rhinos (Perissodactyla, Rhinocerotidae)

“…Potential correlations between the LSC orientation and head posture(s), and their usage to reconstruct behavior and locomotion of extant and extinct animals were broadly investigated and discussed for different mammals, therapsids, and dinosaurs [e.g., xenarthrans (Coutier, Hautier, Cornette, Amson, & Billet, 2017); rodents, carnivores, marsupials, etc. (Berlin, Kirk, & Rowe, 2013); dinocephalians (Benoit, Manger, Norton, Fernandez, & Rubidge, 2017); gorgonopsians (Ara ujo et al, 2017); mammals, birds, and sauropods, (Sereno et al, 2007;Taylor et al, 2009)].…”