Neuroanatomy of the mekosuchine crocodylianTrilophosuchus rackhamiWillis, 1993

Abstract

Although our knowledge on crocodylomorph palaeoneurology has experienced considerable growth in recent years, the neuroanatomy of many crocodylomorph taxa has yet to be studied. This is true for Australian taxa, where thus far only two crocodylian crocodylomorphs have had aspects of their neuroanatomy explored. Here, the neuroanatomy of the Australian mekosuchine crocodylian Trilophosuchus rackhami is described for the first time, which significantly increases our understanding on the palaeoneurology of Australian crocodylians. The palaeoneurological description is based on the taxon's holotype specimen (QMF16856), which was subjected to a μCT scan. Because of the exceptional preservation of QMF16856, most neuroanatomical elements could be digitally reconstructed and described in detail. Therefore, the palaeoneurological assessment presented here is hitherto the most in‐depth study of this kind for an extinct Australian crocodylomorph. Trilophosuchus rackhami has a brain endocast with a distinctive morphology that is characterized by an acute dural peak over the hindbrain region. While the overall morphology of the brain endocast is unique to T. rackhami, it does share certain similarities with the notosuchian crocodyliforms Araripesuchus wegeneri and Sebecus icaeorhinus. The endosseous labyrinth displays a morphology that is typical for crocodylians, although a stand‐out feature is the unusually tall common crus. Indeed, the common crus of T. rackhami has one of the greatest height ratios among crocodylomorphs with currently known endosseous labyrinths. The paratympanic pneumatic system of T. rackhami is greatly developed and most similar to those of the extant crocodylians Osteolaemus tetraspis and Paleosuchus palpebrosus. The observations on the neuroanatomy of T. rackhami are also discussed in the context of Crocodylomorpha. The comparative palaeoneurology reinforces previous evaluations that the neuroanatomy of crocodylomorphs is complex and diverse among species, and T. rackhami has a peculiar neuromorphology, particularly among eusuchian crocodyliforms.

Neuroanatomy of the crocodylomorph Portugalosuchus azenhae from the late cretaceous of Portugal

“…The degree of their sigmoidal shape varies depending on the flexion angles between the forebrain-midbrain (cephalic flexion, CF) and the midbrain-hindbrain (pontine flexion, PF) (Figure 9f). This flexure varies from straighter and more tubular brains (high CF and PF values), as in thalattosuchians such as Plagiophtalmosuchus, Macrospondylus, Cricosaurus, Pelagosaurus and 'Metriorhynchus', to more flexed sigmoidal brains (low CF and PF values), as in crocodylians such as Alligator (Erb & Turner, 2021;Pierce et al, 2017) or Trilophosuchus (Ristevski, 2022) (Erb & Turner, 2021;Pierce et al, 2017). This greater angle in pontine flexion produces that the dorsal region of the cerebellum is approximately at the same height of the dorsal surface of the cerebrum in contrast with most nongavialoid crocodylians.…”

Cranial anatomy of the mekosuchine crocodylianTrilophosuchus rackhamiWillis, 1993

“…The maximum dorsoventral thickness/depth of the bone at its anterior contact with the frontal is ~8 mm. Internally, the parietal hosts a considerable pneumatic recess (character 165, state 0; Figure 3 ; for a description, see Ristevski, 2022 ).…”

“…From an anterior view, the cavum epiptericum has an elliptical outline, having a length of ~2 mm along its major axis and ~0.9 mm along its minor axis. Internally, the body of each laterosphenoid is occupied by a substantial pneumatic recess (see Ristevski, 2022 ).…”

“…Small sections of each prootic are also visible through the external auditory meatus (Figure 2c,d ). Internally, the prootic is pneumatized by the prootic facial recess (Ristevski, 2022 ). As proposed by Kuzmin et al ( 2021 ), the crocodylian prootic may be subdivided into several portions, with the same subdivisions being applicable to the prootic of T. rackhami as well.…”

“…Therefore, the aim of this study is to provide an in‐depth description of its cranial osteology with the aid of high‐resolution μCT scan data. A detailed description of the neuroanatomy of QMF16856 is provided by Ristevski ( 2022 ). Based on the results from this study, we also reassess the phylogenetic relationships of T. rackhami and Mekosuchinae.…”