The epipterygoid of crocodyliforms and its significance for the evolution of the orbitotemporal region of eusuchians

Abstract

ABSTRACT-A broad survey of crocodyliform archosaurs and their outgroups was conducted to explore the evolutionary and morphological patterns of the orbitotemporal region, which is a highly apomorphic but poorly understood portion of the head. Data were gathered on the topological similarity and phylogenetic congruence of the epipterygoid, laterosphenoid, and temporal region as a whole, including relevant osteological correlates and such inferred soft tissues as the trigeminal nerves and jaw musculature. Despite the complete suturing of the palatocranial junction, the epipterygoid remained a consistent cranial element throughout crocodyliform evolution, only to be replaced by the topologically analogous, but developmentally neomorphic lateral bridge of the laterosphenoid during the early evolution of eusuchians. These changes led to a unique morphology of the region surrounding the exit of the trigeminal nerve. Mesoeucrocodylian taxa exhibit a diversity of epipterygoid morphologies including waisted (e.g., Araripesuchus), overlapping (e.g., Sarcosuchus), and isolated (e.g., Goniopholis, Leidyosuchus) forms. The isolated form, in which the epipterygoid is uncoupled from the pterygoid and does not to cover the cavum epiptericum laterally, represents a key transition to the extant condition of loss of the epipterygoid. Changes in the epipterygoid coincide with the migration of M. pseudotemporalis superficialis onto the laterosphenoid outside of the dorsotemporal fossa and the topological change in the intermuscular path of the maxillary nerve, both of which are apomorphies found in extant crocodylians. These data reflect a diverse and potentially homoplastic distribution of orbitotemporal morphologies among mesoeucrocodylians and indicate that the epipterygoid was only recently eliminated in crocodyliform evolution.

Skull Anatomy of the Bizarre Crocodylian Mourasuchus nativus (Alligatoridae, Caimaninae)

“…In M. nativus, the trigeminal fossa is huge and divided by a conspicuous bony bridge from the quadrate separating V 2 and V 3 openings similar to that found in some large crocodylians (Crocodylus palustris; Holliday and Witmer, 2009, Fig. 4C) (Figs.…”

“…The lateral and caudal bridges of laterosphenoid are present. As in other caimanines (e.g., Caiman, Paleosuchus, and Melanosuchus), the caudal bridge is typically short, robust, and encloses the supraorbital branch of the trigeminal nerve (Holliday and Witmer, 2009;Bona and Desojo, 2011;Figs. 3C,D, 5C,D).…”

“…3C,D; Holliday and Witmer, 2009). Dorsally, the postorbital process of the laterosphenoid (the primary site of the attachment for the pseudotemporalis superficialis muscle in crocodiles; Holliday and Witmer, 2009: Fig.…”

“…The surface of the dorsal aspect of laterosphenoid bears a marked rostral longitudinal oblique crest for the attachment of the pseudotemporalis muscle (Iordansky, 1964;Holliday and Witmer, 2007;Bona and Desojo, 2011). In M. nativus, this crest (cotylar crest of laterosphenoid; Holliday and Witmer, 2009) is more vertically oriented than in other caimans, extending from the dorsal aspect of the postorbital process of laterosphenoid ventrally toward the cranial margin of the laterosphenoid lateral bridge (Fig. 3C,D).…”

“…3C) and forms the caudal margin of the trigeminal foramen and the medial wall of the trigeminal fossa. The prootic also forms the rostral margin of the lateral foramen for the passage of the tympanic trigeminal branch (Holliday and Witmer, 2009), which is bounded by the quadrate caudally. In M. nativus, this foramen is particularly large (it size is almost equal to that of the trigeminal foramen), completely exposed in lateral view and horizontally aligned with the trigeminal foramen (Fig.…”