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Abstract Jaw muscles are key features of the vertebrate feeding apparatus. The jaw musculature is housed in the skull whose morphology reflects a compromise between multiple functions, including feeding, housing sensory structures, and defense, and the skull constrains jaw muscle geometry. Thus, jaw muscle anatomy may be suboptimally oriented for the production of bite force. Crocodylians are a group of vertebrates that generate the highest bite forces ever measured with a flat skull suited to their aquatic ambush predatory style. However, basal members of the crocodylian line (e.g.,Prestosuchus) were terrestrial predators with plesiomorphically tall skulls, and thus the origin of modern crocodylians involved a substantial reorganization of the feeding apparatus and its jaw muscles. Here, we reconstruct jaw muscles across a phylogenetic range of crocodylians and fossil suchians to investigate the impact of skull flattening on muscle anatomy. We used imaging data to create 3D models of extant and fossil suchians that demonstrate the evolution of the crocodylian skull, using osteological correlates to reconstruct muscle attachment sites. We found that jaw muscle anatomy in early fossil suchians reflected the ancestral archosaur condition but experienced progressive shifts in the lineage leading to Metasuchia. In early fossil suchians, musculus adductor mandibulae posterior and musculus pterygoideus (mPT) were of comparable size, but by Metasuchia, the jaw musculature is dominated by mPT. As predicted, we found that taxa with flatter skulls have less efficient muscle orientations for the production of high bite force. This study highlights the diversity and evolution of jaw muscles in one of the great transformations in vertebrate evolution. 

Abstract Almadasuchus figariiis a basal crocodylomorph recovered from the Upper Jurassic levels of the Cañadón Calcáreo Formation (Oxfordian–Tithonian) of Chubut, Argentina. This taxon is represented by cranial remains, which consist of partial snout and palatal remains; an excellently preserved posterior region of the skull; and isolated postcranial remains. The skull of the only specimen of the monotypicAlmadasuchuswas restudied using high‐resolution computed micro tomography.Almadasuchushas 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 carotids) 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 projected floccular recess as in other basal crocodylomorphs. Internally, the skull ofAlmadasuchusis heavily pneumatized, where different air cavities invade the bones of the suspensorium and braincase, both on its dorsal or ventral parts.Almadasuchushas a large basioccipital recess, which is formed by cavities that excavate the basioccipital and the posterior surface of the basisphenoid, and unlike other crocodylomorphs is connected with the basisphenoid pneumatizations. Ventral to the otic capsule, a pneumatic cavity surrounded by the otoccipital and basisphenoid is identified as the rhomboidal recess. The quadrate ofAlmadasuchusis 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) basisphenoid 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 hallopodids and crocodyliforms. The marine thallatosuchians exhibit a reduction of the pneumaticity of the braincase and this reduction is evaluated considering the two phylogenetic positions proposed for the clade. 

Major evolutionary transitions, in which animals develop new body plans and adapt to dramatically new habitats and lifestyles, have punctuated the history of life. The origin of cetaceans from land-living mammals is among the most famous of these events. Much earlier, during the Mesozoic Era, many reptile groups also moved from land to water, but these transitions are more poorly understood. We use computed tomography to study changes in the inner ear vestibular system, involved in sensing balance and equilibrium, as one of these groups, extinct crocodile relatives called thalattosuchians, transitioned from terrestrial ancestors into pelagic (open ocean) swimmers. We find that the morphology of the vestibular system corresponds to habitat, with pelagic thalattosuchians exhibiting a more compact labyrinth with wider semicircular canal diameters and an enlarged vestibule, reminiscent of modified and miniaturized labyrinths of other marine reptiles and cetaceans. Pelagic thalattosuchians with modified inner ears were the culmination of an evolutionary trend with a long semiaquatic phase, and their pelagic vestibular systems appeared after the first changes to the postcranial skeleton that enhanced their ability to swim. This is strikingly different from cetaceans, which miniaturized their labyrinths soon after entering the water, without a prolonged semiaquatic stage. Thus, thalattosuchians and cetaceans became secondarily aquatic in different ways and at different paces, showing that there are different routes for the same type of transition. 

Background Macelognathus 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. Methods Due 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. Results This new material reinforces the non-crocodyliform crocodylomorph affinities of Macelognathus as 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 ). Discussion The 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.