Braincase anatomy of Almadasuchus figarii (Archosauria, Crocodylomorpha) and a review of the cranial pneumaticity in the origins of Crocodylomorpha

LEARDI, JUAN MARTÍN; POL, DIEGO; CLARK, JAMES MATTHEW

JOURNAL OF ANATOMY

WILEY-BLACKWELL PUBLISHING, INC

Año: 2020

0021-8782

Almadasuchus figarii is a basal crocodylomorph recovered from the Upper Jurassic lev- els of the Cañadón Calcáreo Formation (Oxfordian?Tithonian) of Chubut, Argentina. This taxon is represented by cranial remains, which consist of partial snout and pala- tal remains; an excellently preserved posterior region of the skull; and isolated post- cranial remains. The skull of the only specimen of the monotypic Almadasuchus was restudied using high-resolution computed micro tomography. Almadasuchus has an apomorphic condition in its skull shared with the closest relatives of crocodyliforms (i.e. hallopodids) where the quadrates are sutured to the laterosphenoids and the otoccipital contacts the quadrate posterolaterally, reorganizing the exit of several cranial nerves (e.g. vagus foramen) and the entry of blood vessels (e.g. internal ca- rotids) on the occipital surface of the skull. The endocast is tubular, as previously reported in thalattosuchians, but has a marked posterior step, and a strongly pro- jected floccular recess as in other basal crocodylomorphs. Internally, the skull of Almadasuchus is heavily pneumatized, where different air cavities invade the bones of the suspensorium and braincase, both on its dorsal or ventral parts. Almadasuchus has a large basioccipital recess, which is formed by cavities that excavate the basi- occipital and the posterior surface of the basisphenoid, and unlike other crocody- lomorphs is connected with the basisphenoid pneumatizations. Ventral to the otic capsule, a pneumatic cavity surrounded by the otoccipital and basisphenoid is identi- fied as the rhomboidal recess. The quadrate of Almadasuchus is highly pneumatized, being completely hollow, and the dorsal pneumatizations of the braincase are formed by the mastoid and facial antra, and a laterosphenoid cavity (trigeminal diverticulum). To better understand the origins of pneumatic features in living crocodylomorphs we studied cranial pneumaticity in the basal members of Crocodylomorpha and found that: (a) prootic pneumaticity may be a synapomorphy for the whole clade; (b) ba- sisphenoid pneumaticity (pre-, postcarotid and rostral recesses) is a derived feature among basal crocodylomorphs; (c) quadrate pneumatization is acquired later in the history of the group; and (d) the rhomboidal sinus is a shared derived trait of hal- lopodids and crocodyliforms.