Detailed anatomy of the braincase ofMacelognathus vagansMarsh, 1884 (Archosauria, Crocodylomorpha) using high resolution tomography and new insights on basal crocodylomorph phylogeny

Abstract

BackgroundMacelognathus vagans Marsh, 1884 from the Late Jurassic Morrison Fm. of Wyoming was originally described as a dinosaur by Marsh and in 1971 Ostrom suggested crocodilian affinities. In 2005, Göhlich and collaborators identified new material of this species from Colorado as a basal crocodylomorph. However, a partial skull found in association with mandibular and postcranial remains was not described.MethodsDue to the small size and delicate structures within the braincase, micro CT studies were performed on this specimen. The new anatomical information was incorporated in a phylogenetic dataset, expanding both character and taxon sampling.ResultsThis new material reinforces the non-crocodyliform crocodylomorph affinities of Macelognathusas it bears a large otic aperture, unfused frontals and lacks ornamentation on the dorsal cranial bones. The internal structures also support these affinities as this specimen bears traits (i.e., heavily pneumatized and expanded basisphenoid; the presence of additional pneumatic features on the braincase; and the otoccipital-quadrate contact) not present in most basal crocodylomorphs. Furthermore, the presence of a wide supraoccipital and a cranioquadrate passage are traits shared with Almadasuchus from the early Late Jurassic of Argentina. Macelognathus was recovered as one of the closest relatives of crocodyliforms, forming a clade (Hallopodidae) with two other Late Jurassic taxa (Almadasuchus and Hallopus).DiscussionThe clade formed by Almadasuchus + Hallopus + Macelognathus, the Hallopodidae, is characterized by a higher degree of suturing of the braincase, posteriorly closed otic aperture (paralleled in mesoeucrocodylians) and cursorial adaptations. Also, the phylogenetic position of this lineage of derived crocodylomorphs as the sister group of Crocodyliformes implies a large amount of unsampled record (ghost lineage), at least 50 million years.

Peer Review #1 of "Braincase and endocranial anatomy of two thalattosuchian crocodylomorphs and their relevance in understanding their adaptations to the marine environment (v0.1)"

“…The development of pneumatizations anterior to the tympanic crest is not so well documented in non-crocodyliform crocodylomorphs, although pneumatic anterior foramina have been reported in several taxa (e.g. Terrestrisuchus, Dibothrosuchus, Junggarsuchus, Almadasuchus, Macelognathus), and in some of them the connection with the middle ear cavity was confirmed (Leardi, Pol & Clark, 2017). As it was observed with the peculiar shape of the otic aperture, the lack of a subtympanic foramen (sensu Montefeltro, Andrade & Larsson, 2016) and an associated infundibular diverticulum is common to most thalattosuchians (e.g.…”

“…Macelognathus, C. latirostris) or multiple (e.g. Junggarsuchus, P. richardsoni, Notosuchus) (Leardi, Pol & Clark, 2017). Thus, the subtympanic foramina are the external osteological correlates of these quadrate pneumatizations (Dufeau & Witmer, 2015).…”

“…The cranioquadrate canal is a structure present in hallopodid crocodylomorphs, mesoeucrocodylians and derived crocodyliforms (i.e. Fruitachampsa, Gobiosuchus, Hsiosuchus) (Clark, 1994;Leardi, Pol & Clark, 2017). The cranioquadrate canal connects the middle ear space with the posterior aspect of PeerJ reviewing PDF | (2018:05:28033:1:1:NEW 26 Jul 2018)…”

“…Most extant and extinct crocodylomorphs have, at least, a well-marked mid-hindbrain flexure, giving the encephalon a curved profile in lateral view (e.g. Pholidosaurus meyeri, MB.R.2027; Macelognathus, Leardi, Pol & Clark, 2017;Sebecus icaeorhinus, Colbert, 1946a: pl. 14A;Crocodylus johnstoni, Witmer et al, 2008: Fig.…”

“…The posterior pneumatization of the otic capsule is present and well-developed in derived non-crocodyliform crocodylomorphs and crocodyliforms (Pol et al, 2013;Leardi, Pol & Clark, 2017), where the otoccipital diverticulum is dorsally extended, exceeding the dorsal border of the Manuscript to be reviewed foramen magnum. This contrasts with the thalattosuchian condition, in which the diverticulum is mostly restricted to the ventral part of the otoccipital.…”