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Structural consequences of skull flattening in crocodilians
... The functional morphology, biomechanics, and feeding ecology of the Crocodylia, Phytosauridae, Lissamphibia, and Temnospondyli have been studied extensively using qualitative and quantitative observation of extant species, plus morphometric, histological, and computational methods to assess the extinct taxa (e.g., Beecher, 1979;Jupp and Warren, 1986;Hunt, 1989;Busbey, 1995;McHenry et al., 2006;Rayfield et al., 2007;Pierce et , 2008;Kimmig, 2009;Kimmig and Arp, 2010;Fortuny et , 2011;Kimmig and Spielmann, 2011;Walmsley et , 2013;Piras et , 2014;Fortuny et , 2015;2017;Witzmann and Werneburg, 2017;Konietzko-Meier et al., 2018;Gignac et al., 2019;Lemanis et al., 2019). ...
... Our metrics are similar, but not identical, to those used by Busby (1995) in his study of skull flattening in the Crocodylia. We chose midline suture length (MSL) as opposed to Busbey's (1995) basal skull length (BSL = length of the ventral midline including the basioccipital condyle) as a measure of skull length because many fossil specimens are prepared on slabs or are otherwise visible in dorsal view only. We chose skull width at the anterior narial borders (ANW) as opposed to Busbey's (1995) premaxillary width because it is more readily identifiable in fossils. ...
... We chose midline suture length (MSL) as opposed to Busbey's (1995) basal skull length (BSL = length of the ventral midline including the basioccipital condyle) as a measure of skull length because many fossil specimens are prepared on slabs or are otherwise visible in dorsal view only. We chose skull width at the anterior narial borders (ANW) as opposed to Busbey's (1995) premaxillary width because it is more readily identifiable in fossils. ...
We have investigated the skull morphology of numerous aquatic and semiaquatic predators, paying particular attention to the rostrum. Cluster analysis, using the ratios of a few skull or mandible measurements, shows that their skull shapes do not exist on a continuum that varies from relatively long to relatively short rostra and/or relatively narrow to relatively broad rostra. Rather, there are separate, distinct morphological clusters that represent various trophic guilds—groups with similar feeding habits. Three major guilds emerge from the analysis: ambush predators, generalists, and fish specialists. Similar analysis of mandible morphology also indicates clustering, but the guilds are not as well-defined and separated as in the skull data. The study specimens are dominated by metoposaurs, which are of principal interest, and other temnospondyls, as well as numerous other aquatic predators, including members of the Crocodylia, Phytosauridae, and a few salamanders and turtles for comparison. Results show that the brevirostral metoposaur-like amphibians form a guild of probable sit-and-wait ambush predators that may have also had active hunting capability. Suction feeders form a distinct group within the ambush predators, distinguished by extreme brevirostry. The generalist feeders comprise two overlapping guilds—one more alligator-like, such as Mastodonsaurus, Sclerocephalus and Eryops, and another more crocodile-like, including the robust phytosaurs and the long-snouted temnospondyls, Eocyclotosaurus, and Wellesaurus. A third distinct guild comprises the fish-specialists, including the gracile phytosaurs and the gharial-like crocodylids that possess long, tubular rostra. We found that there is a high degree of individual variation in skull shapes of the Metoposauridae and some other taxa. These individual shape variations often outweigh specific or generic morphological differences. Although the metoposaur-like amphibians have often been judged to fill an alligator-like feeding niche or to be suction feeders, our analysis shows that their skull morphology and presumably their feeding method is quite distinct from the Alligatoridae, and they are also morphologically distant from the suction feeders and other predatory temnospondyls. We establish the boundaries of these morphological clusters so this simple morphometric method may be used as a diagnostic tool to help categorize the feeding habits of fossil species.
... Here, robustness characters specifically refer to the proportions of the mandibular symphysis (MSL/ML and MSD/ML) and maximum depth of the lower jaw, normally at the coronoid process (CPD/ML). These features describe in first approximation the cross-sectional area of the lower jaw, a key factor to determine the mechanical resistance to bending and torsion in the absence of more reliable measurements of I (second moment of area) and J (polar moment of inertia) [39,55,56]. In extant crocodylians and toothed cetaceans, gracile lower jaws with long shallow mandibular symphyses are associated with feeding on small-bodied prey items caught with agile lateral movements [52,[57][58][59]. ...
Mesozoic marine ecosystems were dominated by diverse lineages of aquatic tetrapods. For over 50 Ma in the Jurassic until the Early Cretaceous, plesiosaurians, ichthyosaurians and thalattosuchian crocodylomorphs coexisted at the top levels of trophic food webs. We created a functional dataset of continuous craniomandibular and dental characters known from neontological studies to be functionally significant in modern aquatic tetrapods. We analysed this dataset with multivariate ordination and inferential statistics to assess functional similarities and differences in the marine reptile faunas of two well-sampled Jurassic ecosystems deposited in the same seaway: the Oxford Clay Formation (OCF, Callovian–early Oxfordian, Middle–Late Jurassic) and the Kimmeridge Clay Formation (KCF, Kimmeridgian–Tithonian, Late Jurassic) of the UK. Lower jaw-based macroevolutionary trends are similar to those of tooth-based diversity studies. Closely related species cluster together, with minimal overlaps in the morphospace. Marine reptile lineages were characterized by the distinctive combinations of features, but we reveal multiple instances of morphofunctional convergence among different groups. We quantitatively corroborate previous observations that the ecosystems in the OCF and KCF were markedly distinct in faunal composition and structure. Morphofunctional differentiation may have enabled specialization and was an important factor facilitating the coexistence of diverse marine reptile assemblages in deep time.
... Mekosuchinae is an extinct clade of Australasian crocodylians that are particularly diverse and abundant in the Oligo-Miocene deposits of Queensland and the Northern Territory Willis, 1997Willis, , 2001Yates, 2017;Ristevski et al., 2023a). Mekosuchines are remarkable in that they present a number of distinct ecomorphological types that match, to greater and lesser degrees, the ecomorphological categories of crocodyliforms that have been established on the basis of rostral shape and other correlated skull characters (Busbey, 1995;Brochu, 2001;Drumheller and Wilberg, 2020). Basal genera such as Australosuchus Willis and Molnar, 1991a, Kambara Willis et al., 1993, and Kalthifrons Yates and Pledge, 2017 have moderately elongate (mesorostrine), dorsoventrally flattened (platyrostral) rostra that are broad-based and roughly triangular in dorsal view. ...
... longirostrine skull). For further information see Clark (1994) and Busbey (1995). Taxa are predominately based either on a node-or stem-based definition. ...
... In addition to bones concealed by matrix and overlying elements, CT scanning allows visualisation of sutures, the contacts between cranial and mandibular bones. Sutures exhibit numerous different morphologies, and data from both in vivo bone strain experiments and biomechanical modelling have found correlations between specific suture shapes and certain loading regimes: for example, butt joints are associated with tension and bending; interdigitated sutures are associated with compression; and scarf joints are associated with complex load regimes (torsion, shear, or tension and compression) (Herring & Mucci 1991;Busbey 1995;Rafferty & Herring 1999;Herring & Teng 2000;Markey et al. 2006). CT scanning has permitted more recent anatomical descriptions of early tetrapods to use suture shape to predict skull load regimesand infer the feeding strategies used to generate these loadsin several early tetrapods and extant analogues (Markey & Marshall 2007;Porro et al. 2015aPorro et al. , 2015bPorro et al. , 2023Gruntmejer et al. 2019;Rawson et al. 2021). ...
The early tetrapod Eoherpeton watsoni is known from the mid- to late Carboniferous (late Viséan to Namurian, approximately 346–313 Ma) of Scotland. The holotype is made up of a nearly complete but crushed skull with postcranial fragments. The skull anatomy of Eoherpeton was first described over 40 years ago; however, many details are obscured due to deformation of the specimen, including internal bone surfaces, the palatal bones and dentition, and suture morphology. Most phylogenetic analyses place Eoherpeton as an embolomere/reptilomorph on the lineage leading to amniotes, making it a key taxon for understanding anatomical changes during the fish-tetrapod transition. In this paper, we scanned the holotype using micro-computed tomography and digitally prepared the specimen. Based on these data, we present a revised description of the skull, including sutural morphology, that supplements and amends previous descriptions. New anatomical findings include the presence of a previously unknown tooth-bearing vomer, additional information on the shape of the basipterygoid processes and jaw joint, the ability to visualise the full extent of the pterygoid, and confirmation of the arrangement of the coronoid series. We also note the size of the pterygoid flange, which is larger than previously described for Eoherpeton . The pterygoid flange is widely considered to be characteristic of amniotes and serves as the origin of the medial pterygoideus muscle. The differentiation of the adductor muscles and appearance of medial pterygoideus are thought to have permitted a static pressure bite in amniotes, potentially resulting in greater bite forces and increased dietary range. Thus, the presence and extent of the pterygoid flange in Eoherpeton suggests this feature (and associated changes in feeding mechanism) may have evolved earlier than previously thought. Finally, the skull was digitally repaired and retrodeformed to create a new, hypothetical three-dimensional reconstruction of the skull of Eoherpeton .
... This may allow Alligator to employ the high bite force generated at the posteriormost teeth, which aligns with the inferred functional morphology of its dental arrangement. Modern crocodylians display a significant degree of heterodonty, with generalist species typically exhibiting a gradient of tooth types along their jaws, ranging from caniniform to molariform teeth (D'Amore et al., 2019;Sellers et al., 2019), favoring the observed generalist diet of the species (Busbey, 1989(Busbey, , 1995. Prominent molariform teeth such as those seen in Alligator are not only commonly associated with specialized feeding on hard prey or larger animals (Erickson et al., 2003a(Erickson et al., , 2003b) but also possess a higher concentration of enamel, making them ideal for processing tougher prey with crushing bites (Sellers et al., 2019). ...
Proterochampsids are a group of South American nonarchosaurian
archosauriforms whose general morphology has been historically likened to that of the extant Crocodylia, which purportedly exhibited similar habits by convergence. Taxa from the genus Proterochampsa, for example, show platyrostral skulls with dorsally faced orbits and external nares and elongated snouts that might indicate a feeding habit similar to that of crocodilians. Nonetheless, some aspects of their craniomandibular anatomy are distinct. Proterochampsa has comparatively larger skull temporal fenestrae, and a unique morphology of the mandibular adductor chamber, with a remarkably large surangular shelf and a fainter retroarticular region in the mandible. In light of this, we conducted biomechanical tests on a 3-dimensional model of Proterochampsa nodosa including the first Finite Element Analysis for proterochampsians and compared it with models of the extant crocodylians Tomistoma schlegelii and Alligator mississippiensis. Our analyses suggested that, despite the differences in adductor chamber, Proterochampsa was able to perform bite forces comparable to those modeled for Alligator and significantly higher than Tomistoma. However, the morphology of the surangular shelf and the adductor chamber of Proterochampsa renders it more prone to accumulate stresses resulting from muscle contraction, when compared with both analogs. The elongated lower jaw of Proterochampsa, like that ofTomistoma, is more susceptible to bending, when compared with Alligator. As a result, we suggest that Proterochampsa might employ anteriorly directed bites only when handling small and soft-bodied prey. In addition, Proterochampsa exemplifies the diversity of arrangements that the adductor musculature adopted in different diverging archosauromorph groups.
... An osseous secondary palate has convergently evolved in different groups of amniotes, such as mammals, turtles and crocodilians. In these groups, the evolution of this structure has been linked to feeding, since it is involved in resisting loads and reinforcing the skull 20,21 . A secondary palate has also evolved in different lineages of dinosaurs, such as spinosaurid theropods 59 and ankylosaurs 17 . ...
Ankylosaurs were important megaherbivores of Jurassic and Cretaceous ecosystems. Their distinctive craniodental anatomy and mechanics differentiated them from coexisting hadrosaurs and ceratopsians, and morphological evidence suggests dietary niche partitioning between sympatric ankylosaurids and nodosaurids. Here, we investigate the skull biomechanics of ankylosaurs relative to feeding function. First, we compare feeding functional performance between nodosaurids and ankylosaurids applying finite element analysis and lever mechanics to the skulls of Panoplosaurus mirus (Nodosauridae) and Euoplocephalus tutus (Ankylosauridae). We also compare jaw performance across a wider sample of ankylosaurs through lever mechanics and phylogenetic comparative methods. Mandibular stress levels are higher in Euoplocephalus , supporting the view that Panoplosaurus consumed tougher foodstuffs. Bite force and mechanical advantage (MA) estimates indicate that Panoplosaurus had a relatively more forceful and efficient bite than Euoplocephalus . There is little support for a role of the secondary palate in resisting feeding loads in the two ankylosaur clades. Several ankylosaurs converged on similar jaw mechanics, while some nodosaurids specialised towards high MA and some ankylosaurids evolved low MA jaws. Our study supports the hypothesis that ankylosaurs partitioned dietary niches in Late Cretaceous ecosystems and reveals that the two main ankylosaur clades evolved divergent evolutionary pathways in skull biomechanics and feeding habits.
Shape ontogenetic changes of the lower jaw in crocodylians are poorly understood. In order to answer some questions related to the inter- and intraspecific morphological variation of the mandible of two extant Caiman species, we performed a three-dimensional geometric morphometric approach. For this purpose, we used landmarks and semilandmarks on two ontogenetic mandibular series of 48 and 15 post-hatching specimens of C. yacare and C. latirostris , respectively. We have also examined the relationship between these anatomical transformations and ontogenetic shifts in diet. We performed a principal component analysis (PCA) for the two species, and regression and partial least squares (PLS) analyses for each species, separately. As a result, species were segregated along the PC1 with specimens of C. yacare showing more gracile mandibles, and specimens of C. latirostris more robust ones. The PC2 and regression analyses showed an age gradient and represented ontogenetic shape changes. Adult caiman mandibles are higher and wider than juvenile ones, and shape changes are more conspicuous in C. latirostris . The PLS analyses showed a significant relationship between shape and diet. Morphological changes of the PLS1 of block-1 match with those of the regression analysis for both species. We have detected morphological transformations in areas where the musculature in charge of mandibular movements is attached. Common morphological changes occurring during ontogeny seem to reflect the same mechanical properties required for crushing and killing in both species, driven by an ontogenetic shift in the diet from invertebrates to vertebrates. Additionally, interspecific differences were also found to be correlated to ontogenetic changes in diet and could be related to dissimilar feeding mechanical requirements ( e.g ., stiffness and toughness of the item consumed), and to different habitat preferences. Robust mandibles would be more suitable for shallow and fully vegetated environments, as it can be seen in C. latirostris , whereas slender jaws seem to be more suitable for more aquatic species such as C. yacare .
Crocodyliform palaeontology in Australasia has a productive research record that began in the late nineteenth century and continues today. In this study, we summarize the current understanding on the taxonomic diversity and phylogenetic relationships of Australasian crocodyliforms based on first-hand knowledge of relevant fossil material and a review of the published literature. The currently known fossil record of Crocodyliformes in Australasia spans more than 113 million years, from the Early Cretaceous to the Holocene, and largely consists of body fossils discovered on continental Australia. Whilst only two crocodyliform genera are recognized from Australasia’s Mesozoic, the Cenozoic is distinguished by a remarkable taxonomic diversity of crocodylian crocodyliforms. By far the most common crocodylians from Australasia are members of Mekosuchinae, whose fossils are unambiguously known from the early Eocene until the Holocene. In addition to mekosuchines, during the Cenozoic Australasia was also inhabited by gavialoids and species of Crocodylus, with four extant species of the latter being the only surviving crocodylians in Australia and New Guinea. The phylogenetic relationships of Australasia’s crocodylians, particularly mekosuchines, have been a topic of interest to palaeontologists for over two decades. We performed several phylogenetic analyses to test the relationships of Mekosuchinae and other extinct crocodylians. Most results from our analyses found Mekosuchinae as a basal crocodyloid clade within Longirostres. However, some of the results recovered an alternative position for the majority of mekosuchines outside of Longirostres and the Late Cretaceous–early Paleogene Orientalosuchina as its deeply nested subclade. These results suggest that Mekosuchinae had its origins in Asia during the Cretaceous, and that mekosuchines arrived from southeast Asia into Australia no later than the late Paleocene. If this hypothesis is correct, then Mekosuchinae would no longer be an Australasian endemic clade since mekosuchines also seem to have persisted on continental Asia until the late Eocene.
The early tetrapod Crassigyrinus scoticus was a large aquatic predator known from the lower- to mid-Carboniferous (upper Tournasian to upper Visean/lower Serpukovian, approximately 350–330 Ma) of Scotland and Canada. Crassigyrinus is enigmatic in terms of its phylogenetic position due to its unusual morphology, which features a mixture of primitive and derived characters. Previous reconstructions, based on five incomplete and deformed specimens, have suggested a dorsoventrally tall skull with a short and broad snout, large orbits and external nares, and an extended postorbital region. In this study, we scanned four specimens using computed tomography and segmented imaging data to separate bone from matrix and individual bones from each other. Based on these data, we present a revised description of the upper and lower jaws, including sutural morphology and abundant new anatomical information. Damage was repaired and the skull retrodeformed to create a hypothetical three-dimensional reconstruction of the skull of Crassigyrinus that is dorsoventrally flatter than earlier reconstructions, yet still morphologically unique amongst early tetrapods. Overall skull shape, the size and distribution of the teeth, sutural morphology, and the specialized anatomy of the jaw joint and mandibular symphysis all suggest that Crassigyrinus was a powerful aquatic predator capable of hunting and subduing large prey.
