Cross-species chromosome painting and repetitive DNA mapping illuminate the karyotype evolution in true crocodiles (Crocodylidae).

Crocodilians have maintained very similar karyotype structures and diploid chromosome numbers for around 100 million years, with only minor variations in collinearity. Why this karyotype structure has largely stayed unaltered for so long is unclear. In this study, we analyzed the karyotypes of six species belonging to the genera Crocodylus and Osteolaemus (Crocodylidae, true crocodiles), among which the Congolian endemic O. osborni was included and investigated. We utilized various techniques (differential staining, fluorescence in situ hybridization with repetitive DNA and rDNA probes, whole chromosome painting, and comparative genomic hybridization) to better understand how crocodile chromosomes evolved. We studied representatives of three of the four main diploid chromosome numbers found in crocodiles (2n = 30/32/38). Our data provided new information about the species studied, including the identification of four major chromosomal rearrangements that occurred during the karyotype diversification process in crocodiles. These changes led to the current diploid chromosome numbers of 2n = 30 (fusion) and 2n = 38 (fissions), derived from the ancestral state of 2n = 32. The conserved cytogenetic tendency in crocodilians, where extant species keep near-ancestral state, contrasts with the more dynamic karyotype evolution seen in other major reptile groups.

Forelimb muscle activation patterns in American alligators: Insights into the evolution of limb posture and powered flight in archosaurs.

The evolution of archosaurs provides an important context for understanding the mechanisms behind major functional transformations in vertebrates, such as shifts from sprawling to erect limb posture and the acquisition of powered flight. While comparative anatomy and ichnology of extinct archosaurs have offered insights into musculoskeletal and gait changes associated with locomotor transitions, reconstructing the evolution of motor control requires data from extant species. However, the scarcity of electromyography (EMG) data from the forelimb, especially of crocodylians, has hindered understanding of neuromuscular evolution in archosaurs. Here, we present EMG data for nine forelimb muscles from American alligators during terrestrial locomotion. Our aim was to investigate the modulation of motor control across different limb postures and examine variations in motor control across phylogeny and locomotor modes. Among the nine muscles examined, m. pectoralis, the largest forelimb muscle and primary shoulder adductor, exhibited significantly smaller mean EMG amplitudes for steps in which the shoulder was more adducted (i.e., upright). This suggests that using a more adducted limb posture helps to reduce forelimb muscle force and work during stance. As larger alligators use a more adducted shoulder and hip posture, the sprawling to erect postural transition that occurred in the Triassic could be either the cause or consequence of the evolution of larger body size in archosaurs. Comparisons of EMG burst phases among tetrapods revealed that a bird and turtle, which have experienced major musculoskeletal transformations, displayed distinctive burst phases in comparison to those from an alligator and lizard. These results support the notion that major shifts in body plan and locomotor modes among sauropsid lineages were associated with significant changes in muscle activation patterns.

Central African dwarf crocodiles found in syntopy are comparably divergent to South American dwarf caimans.

Recent molecular taxonomic advancements have expanded our understanding of crocodylian diversity, revealing the existence of previously overlooked species, including the Congo dwarf crocodile (Osteolaemus osborni) in the central Congo Basin rainforests. This study explores the genomic divergence between O. osborni and its better-known relative, the true dwarf crocodile (Osteolaemus tetraspis), shedding light on their evolutionary history. Field research conducted in the northwestern Republic of the Congo uncovered a locality where both species coexist in sympatry/syntopy. Genomic analysis of sympatric individuals reveals a level of divergence comparable to that between ecologically similar South American dwarf caimans (Paleosuchus palpebrosus and Paleosuchus trigonatus), suggesting parallel speciation in the Afrotropics and Neotropics during the Middle to Late Miocene, 10-12 Ma. Comparison of the sympatric and allopatric dwarf crocodiles indicates no gene flow between the analysed sympatric individuals of O. osborni and O. tetraspis. However, a larger sample will be required to answer the question of whether or to what extent these species hybridize. This study emphasizes the need for further research on the biology and conservation status of the Congo dwarf crocodile, highlighting its significance in the unique biodiversity of the Congolian rainforests and thus its potential as a flagship species.

Neuroanatomy of the crocodylomorph Portugalosuchus azenhae from the late cretaceous of Portugal.

We present the first detailed braincase anatomical description and neuroanatomical study of Portugalosuchus azenhae, from the Cenomanian (Late Cretaceous) of Portugal. This eusuchian crocodylomorph was originally described as a putative Crocodylia and one of the oldest representatives of this clade; however, its phylogenetic position remains controversial. Based on new data obtained from high resolution Computed Tomography images (by micro-CT scan), this study aims to improve the original description of this taxon and also update the scarce neuroanatomical knowledge of Eusuchia and Crocodylia from this time interval, a key period to understand the origin and evolution of these clades. The resulting three-dimensional models from the CT data allowed a detailed description of its well-preserved neurocranium and internal cavities. Therefore, it was possible to reconstruct the cavities of the olfactory region, nasopharyngeal ducts, brain, nerves, carotid arteries, blood vessels, paratympanic sinus system and inner ear, which allowed to estimate some neurosensorial capabilities. By comparison with other crocodylomorphs, these analyses showed that Portugalosuchus, back in the Cenomanian, already displayed an olfactive acuity, sight, hearing and cognitive skills within the range of that observed in other basal eusuchians and crocodylians, including extant species. In addition, and in order to test its disputed phylogenetic position, these new anatomical data, which helped to correct and complete some of the original observations, were included in one of the most recent morphology-based phylogenies. The position of Portugalosuchus differs slightly from the original publication since it is now located as a "thoracosaurid" within Gavialoidea, but still as a crocodylian. Despite all this, to better contrast these results, additional phylogenetic analyses including this new morphological character coding together with DNA data should be performed.

Ontogenetic variability of the intertympanic sinus distinguishes lineages within Crocodylia.

The phylogenetic relationships within crown Crocodylia remain contentious due to conflicts between molecular and morphological hypotheses. However, morphology-based datasets are mostly constructed on external characters, overlooking internal structures. Here, we use 3D geometric morphometrics to study the shape of the intertympanic sinus system in crown crocodylians during ontogeny, in order to assess its significance in a taxonomic context. Intertympanic sinus shape was found to be highly correlated with size and modulated by cranial shape during development. Still, adult sinus morphology distinguishes specimens at the family, genus and species level. We observe a clear distinction between Alligatoridae and Longirostres, a separation of different Crocodylus species and the subfossil Malagasy genus Voay, and a distinction between the Tomistoma and Gavialis lineages. Our approach is independent of molecular methods but concurs with the molecular topologies. Therefore, sinus characters could add significantly to morphological datasets, offering an alternative viewpoint to resolve problems in crocodylian relationships.

Discovery of facultative parthenogenesis in a new world crocodile.

Over the past two decades, there has been an astounding growth in the documentation of vertebrate facultative parthenogenesis (FP). This unusual reproductive mode has been documented in birds, non-avian reptiles-specifically lizards and snakes-and elasmobranch fishes. Part of this growth among vertebrate taxa is attributable to awareness of the phenomenon itself and advances in molecular genetics/genomics and bioinformatics, and as such our understanding has developed considerably. Nonetheless, questions remain as to its occurrence outside of these vertebrate lineages, most notably in Chelonia (turtles) and Crocodylia (crocodiles, alligators and gharials). The latter group is particularly interesting because unlike all previously documented cases of FP in vertebrates, crocodilians lack sex chromosomes and sex determination is controlled by temperature. Here, using whole-genome sequencing data, we provide, to our knowledge, the first evidence of FP in a crocodilian, the American crocodile, Crocodylus acutus. The data support terminal fusion automixis as the reproductive mechanism; a finding which suggests a common evolutionary origin of FP across reptiles, crocodilians and birds. With FP now documented in the two main branches of extant archosaurs, this discovery offers tantalizing insights into the possible reproductive capabilities of the extinct archosaurian relatives of crocodilians and birds, notably members of Pterosauria and Dinosauria.

An intermediate crocodylian linking two extant gharials from the Bronze Age of China and its human-induced extinction.

A solid phylogenetic framework is the basis of biological studies, yet higher level relationships are still unresolved in some major vertebrate lineages. One such group is Crocodylia, where the branching pattern of three major families (Alligatoridae, Crocodylidae and Gavialidae) has been disputed over decades due to the uncertain relationship of two slender-snouted lineages, gavialines and tomistomines. Here, we report a bizarre crocodylian from the Bronze Age of China, which shows a mosaic of gavialine and tomistomine features across the skeleton, rendering support to their sister taxon relationship as molecular works have consistently postulated. Gavialine characters of the new Chinese crocodylian include a novel configuration of the pterygoid bulla, a vocal structure known in mature male Indian gharials. Extinct gavialines have repeatedly evolved potentially male-only acoustic apparatus of various shapes, illuminating the deep history of sexual selection on acoustic signalling in a slender-snouted group of crocodylians. Lastly, a cutmark analysis combined with accelerator mass spectrometry (AMS) radiocarbon dating of bone remains demonstrated that two individuals from Shang and Zhou dynasties in Guangdong, China, suffered head injuries and decapitation. Archaeological evidence together with historical accounts suggests the human-induced extinction of this unique crocodylian only a few hundred years ago.

Neuroanatomy of the mekosuchine crocodylian Trilophosuchus rackhami Willis, 1993.

Although our knowledge on crocodylomorph palaeoneurology has experienced considerable growth in recent years, the neuroanatomy of many crocodylomorph taxa has yet to be studied. This is true for Australian taxa, where thus far only two crocodylian crocodylomorphs have had aspects of their neuroanatomy explored. Here, the neuroanatomy of the Australian mekosuchine crocodylian Trilophosuchus rackhami is described for the first time, which significantly increases our understanding on the palaeoneurology of Australian crocodylians. The palaeoneurological description is based on the taxon's holotype specimen (QMF16856), which was subjected to a µCT scan. Because of the exceptional preservation of QMF16856, most neuroanatomical elements could be digitally reconstructed and described in detail. Therefore, the palaeoneurological assessment presented here is hitherto the most in-depth study of this kind for an extinct Australian crocodylomorph. Trilophosuchus rackhami has a brain endocast with a distinctive morphology that is characterized by an acute dural peak over the hindbrain region. While the overall morphology of the brain endocast is unique to T. rackhami, it does share certain similarities with the notosuchian crocodyliforms Araripesuchus wegeneri and Sebecus icaeorhinus. The endosseous labyrinth displays a morphology that is typical for crocodylians, although a stand-out feature is the unusually tall common crus. Indeed, the common crus of T. rackhami has one of the greatest height ratios among crocodylomorphs with currently known endosseous labyrinths. The paratympanic pneumatic system of T. rackhami is greatly developed and most similar to those of the extant crocodylians Osteolaemus tetraspis and Paleosuchus palpebrosus. The observations on the neuroanatomy of T. rackhami are also discussed in the context of Crocodylomorpha. The comparative palaeoneurology reinforces previous evaluations that the neuroanatomy of crocodylomorphs is complex and diverse among species, and T. rackhami has a peculiar neuromorphology, particularly among eusuchian crocodyliforms.

Feeding behaviour and functional morphology of the neck in the long-snouted aquatic fossil reptile Champsosaurus (Reptilia: Diapsida) in comparison with the modern crocodilian Gavialis gangeticus.

The extinct freshwater choristoderan reptiles Champsosaurus and Simoedosaurus are characterised by large body size and an elongated snout. They have often been considered as eco-analogues of crocodilians based on superficial similarities. The slender-snouted Champsosaurus has been described as a 'gavial-like reptile', which implies it feeds underwater with a lateral swipe of the head and neck, as in the living slender-snouted crocodilians such as Gavialis gangeticus. In contrast, the short-snouted Simoedosaurus is often compared with short-snouted living crocodilians and is considered to take single prey items. However, the neck mobility and flexibility needed for feeding movements are poorly understood even in extant crocodilians. This study explores the relationship between cervical morphology and neck flexion, focusing particularly on lateral and dorsal movements in G. gangeticus by comparison with shorter-snouted crocodilians. The paper also describes a method to estimate the maximum angle of neck dorsiflexion in choristoderes based on the cervical morphology of extant crocodilian species. Three indices were used in this study, of which Index 3 is newly proposed, to compare cervical morphology and intervertebral joint flexibility: (1) Enclosed zygapophyseal angles (EZA) as an index of dorsoventral/ bilateral flexibility, (2) moment arm (M) of dorsiflexor muscles as an Index of resistance against ventroflexion and (3) the orientations of zygapophysial facets for a maximum angle of dorsiflexion. These Indices were validated using µCT scanning of fresh specimens of G. gangeticus and Caiman latirostris in lateral and dorsal flexion. A unique mechanism of lateral flexion was identified in G. gangeticus that uses a combination of the following features: (1) lateral flexion mainly restricted to the anterior cervical vertebrae (v2/v3: high EZA, with more horizontal zygapophyses) and (2) high degree of dorsiflexion at the v3/v4 and v4/v5 joints with potential for dorsal flexibility through the middle-posterior neck, which is used in inertial feeding. In contrast, Champsosaurus and Simoedosaurus possess relatively short cervical vertebrae, as in short-snouted crocodilians. The middle-posterior cervical vertebrae of Champsosaurus are specialised for lateral flexion (high EZA), and there is only limited capacity for dorsiflexion throughout the neck. Like G. gangeticus, therefore, Champsosaurus may have used its slender snout to grab fish from shoals using lateral sweeping motions of the head and neck, but the movement is through the neck, not the craniocervical joint. However, inertial feeding is less likely to have occurred in this genus, and the aligned palatal dentition may have aided the lingual transport of prey into the mouth. Simoedosaurus, on the other hand, appears to have been less specialised, with a neck that combined lateral and dorsolateral flexion, a move that could have been effective in catching both terrestrial and aquatic prey. Where these two choristoderan genera occurred in the same place, they may have divided their niche by prey types.

Ontogenetic variation in the crocodylian vestibular system.

Crocodylians today live in tropical to subtropical environments, occupying mostly shallow waters. Their body size changes drastically during ontogeny, as do their skull dimensions and bite forces, which are associated with changes in prey preferences. Endocranial neurosensory structures have also shown to change ontogenetically, but less is known about the vestibular system of the inner ear. Here we use 30 high-resolution computed tomography (CT) scans and three-dimensional geometric morphometrics to investigate the size and shape changes of crocodylian endosseous labyrinths throughout ontogeny, across four stages (hatchling, juvenile, subadult and adult). We find two major patterns of ontogenetic change. First, the labyrinth increases in size during ontogeny, with negative allometry in relation to skull size. Second, labyrinth shape changes significantly, with hatchlings having shorter semicircular canal radii, with thicker diameters and an overall dorsoventrally shorter labyrinth than those of more mature individuals. We argue that the modification of the labyrinth during crocodylian ontogeny is related to constraints imposed by skull growth, due to fundamental changes in the crocodylian braincase during ontogeny (e.g. verticalisation of the basicranium), rather than changes in locomotion, diet, or other biological functions or behaviours.

The impact of molecular data on the phylogenetic position of the putative oldest crown crocodilian and the age of the clade.

The use of molecular data for living groups is vital for interpreting fossils, especially when morphology-only analyses retrieve problematic phylogenies for living forms. These topological discrepancies impact on the inferred phylogenetic position of many fossil taxa. In Crocodylia, morphology-based phylogenetic inferences differ fundamentally in placing Gavialis basal to all other living forms, whereas molecular data consistently unite it with crocodylids. The Cenomanian Portugalosuchus azenhae was recently described as the oldest crown crocodilian, with affinities to Gavialis, based on morphology-only analyses, thus representing a potentially important new molecular clock calibration. Here, we performed analyses incorporating DNA data into these morphological datasets, using scaffold and supermatrix (total evidence) approaches, in order to evaluate the position of basal crocodylians, including Portugalosuchus. Our analyses incorporating DNA data robustly recovered Portugalosuchus outside Crocodylia (as well as thoracosaurs, planocraniids and Borealosuchus spp.), questioning the status of Portugalosuchus as crown crocodilian and any future use as a node calibration in molecular clock studies. Finally, we discuss the impact of ambiguous fossil calibration and how, with the increasing size of phylogenomic datasets, the molecular scaffold might be an efficient (though imperfect) approximation of more rigorous but demanding supermatrix analyses.

Braincase anatomy of extant Crocodylia, with new insights into the development and evolution of the neurocranium in crocodylomorphs.

Present-day crocodylians exhibit a remarkably akinetic skull with a highly modified braincase. We present a comprehensive description of the neurocranial osteology of extant crocodylians, with notes on the development of individual skeletal elements and a discussion of the terminology used for this project. The quadrate is rigidly fixed by multiple contacts with most braincase elements. The parabasisphenoid is sutured to the pterygoids (palate) and the quadrate (suspensorium); as a result, the basipterygoid joint is completely immobilized. The prootic is reduced and externally concealed by the quadrate. It has a verticalized buttress that participates in the canal for the temporal vasculature. The ventrolateral processes of the otoccipitals completely cover the posteroventral region of the braincase, enclose the occipital nerves and blood vessels in narrow bony canals and also provide additional sutural contacts between the braincase elements and further consolidate the posterior portion of the crocodylian skull. The otic capsule of crocodylians has a characteristic cochlear prominence that corresponds to the lateral route of the perilymphatic sac. Complex internal structures of the otoccipital (extracapsular buttress) additionally arrange the neurovascular structures of the periotic space of the cranium. Most of the braincase elements of crocodylians are excavated by the paratympanic pneumatic sinuses. The braincase in various extant crocodylians has an overall similar structure with some consistent variation between taxa. Several newly observed features of the braincase are present in Gavialis gangeticus and extant members of Crocodylidae to the exclusion of alligatorids: the reduced exposure of the prootic buttress on the floor of the temporal canal, the sagittal nuchal crest of the supraoccipital projecting posteriorly beyond the postoccipital processes and the reduced paratympanic pneumaticity. The most distinctive features of the crocodylian braincase (fixed quadrate and basipterygoid joint, consolidated occiput) evolved relatively rapidly at the base of Crocodylomorpha and accompanied the initial diversification of this clade during the Late Triassic and Early Jurassic. We hypothesize that profound rearrangements in the individual development of the braincases of basal crocodylomorphs underlie these rapid evolutionary modifications. These rearrangements are likely reflected in the embryonic development of extant crocodylians and include the involvement of neomorphic dermal anlagen in different portions of the developing chondrocranium, the extensive ossification of the palatoquadrate cartilage as a single expanded quadrate and the anteromedial inclination of the quadrate.

Anatomy, ontogeny, and evolution of the archosaurian respiratory system: A case study on Alligator mississippiensis and Struthio camelus.

The avian lung is highly specialized and is both functionally and morphologically distinct from that of their closest extant relatives, the crocodilians. It is highly partitioned, with a unidirectionally ventilated and immobilized gas-exchanging lung, and functionally decoupled, compliant, poorly vascularized ventilatory air-sacs. To understand the evolutionary history of the archosaurian respiratory system, it is essential to determine which anatomical characteristics are shared between birds and crocodilians and the role these shared traits play in their respective respiratory biology. To begin to address this larger question, we examined the anatomy of the lung and bronchial tree of 10 American alligators (Alligator mississippiensis) and 11 ostriches (Struthio camelus) across an ontogenetic series using traditional and micro-computed tomography (µCT), three-dimensional (3D) digital models, and morphometry. Intraspecific variation and left to right asymmetry were present in certain aspects of the bronchial tree of both taxa but was particularly evident in the cardiac (medial) region of the lungs of alligators and the caudal aspect of the bronchial tree in both species. The cross-sectional area of the primary bronchus at the level of the major secondary airways and cross-sectional area of ostia scaled either isometrically or negatively allometrically in alligators and isometrically or positively allometrically in ostriches with respect to body mass. Of 15 lung metrics, five were significantly different between the alligator and ostrich, suggesting that these aspects of the lung are more interspecifically plastic in archosaurs. One metric, the distances between the carina and each of the major secondary airways, had minimal intraspecific or ontogenetic variation in both alligators and ostriches, and thus may be a conserved trait in both taxa. In contrast to previous descriptions, the 3D digital models and CT scan data demonstrate that the pulmonary diverticula pneumatize the axial skeleton of the ostrich directly from the gas-exchanging pulmonary tissues instead of the air sacs. Global and specific comparisons between the bronchial topography of the alligator and ostrich reveal multiple possible homologies, suggesting that certain structural aspects of the bronchial tree are likely conserved across Archosauria, and may have been present in the ancestral archosaurian lung.

Allometric relations of respiratory variables in Amniota: Effects of phylogeny, form, and function.

Biological variables are frequently described by analyzing scaling relationships of the variable against body mass (MB). Respiratory variables are no exception and allometric relations for oxygen consumption, pulmonary ventilation, tidal volume, breathing frequency, and lung volume have been described in the literature. While the allometric relations of respiratory variables given for mammals and birds are very consistent among different studies, scaling relationships for non-avian reptiles have only been scarcely described and show considerable variation between studies. Since no comprehensive study of allometric relations of respiratory variables has been carried out comparing the different groups of non-avian reptiles, we analyzed morphological and physiological variables of the respiratory system of crocodilians, chelonians, lizards, snakes, birds, and mammals, regarding the allometric relations of each variable from a phylogenetic perspective as well as related to lung morphology. Our results indicated that few respiratory variables possess significant phylogenetic signals and that tidal volume, breathing frequency (except mammals), and air convection requirement were independent of phylogeny. Contrary to the literature, lung volume of amniotes scaled isometrically to MB, with the exception of lizards (MB0.78). Air convection requirement scaled isometrically in mammals and birds, but was more variable among non-avian reptiles, from a taxonomic perspective and in regard to different lung structures. In conclusion, respiratory variables among non-avian reptiles scaled more variably than previously expected, both according to phylogeny and to lung type, warranting future studies to explore structure-function relations of the reptilian respiratory system, especially regarding snakes and crocodilians, since these groups had very few data available for analysis.

Metallic elements in aquatic herpetofauna (Crocodylia; Testudines) from a lentic Atlantic rainforest environment in northeastern Brazil.

In the present study, Fe, Cu, Cr, Cd, Pb, Ni, and Al concentrations in Caiman latirostris and Testudines blood from the Tapacurá reservoir, Pernambuco, Brazil, were investigated. Blood was acid digested with HNO3, and metals were determined by ICP-OES and FAAS. Lead showed concentrations below the established limit of detection. Eighty animals were evaluated, forty from each group. The levels of all elements were statistically significant when compared between the two studied taxa (p < 0.05). In caimans, significant differences between young and adults were observed for chromium (p = 0.0539) and aluminum (p = 0.0515). Testudines showed no statistically significant differences for the variable age structure. Gender did not influence metal concentrations detected in the present study for either group. Differences between species of testudines were significant for Fe between Mesoclemmys tuberculata vs Phrynops geoffroanus (p = 0.0932) and Kinosternon scorpioides vs Phrynops geoffroanus (p = 0.063). The inter-elementary correlations showed statistically significant differences between the elements Cr vs Al (R2 = 0.52), Cr vs Cd (0.43), Cd vs Cu (R2 = 0.41), Ni vs Cu (R2 = 0.31), Ni vs Cr (R2 = 0.30), Al vs Cd (R2 = 0.27), and Cd vs Fe (R2 = 0.26). It is concluded that blood is an excellent predictor of metals in crocodilians and testudines in the Tapacurá reservoir, with statistically significant differences when correlated to concentrations such as size and species studied. In addition, it evidenced data that prove the exposure of these animals to metals, with strong inter-elementary correlations and opening doors for future studies that seek to understand possible biological effects caused in the studied taxa.

Pliocene crocodiles from Kanapoi, Turkana Basin, Kenya.

Three crocodylid species are known from the Pliocene Kanapoi locality in the western Turkana Basin. One of these, Crocodylus thorbjarnarsoni, includes material previously referred to Crocodylus niloticus (the modern Nile crocodile currently living in Lake Turkana) and Rimasuchus lloydi. C. thorbjarnarsoni was a gigantic horned crocodile similar in overall shape to most other generalized crocodylids, but its closest known relative is another extinct species, Crocodylus anthropophagus from the Pleistocene of Olduvai Gorge in Tanzania. It is not closely related to C. niloticus. The second is an extinct form of sharp-nosed crocodile (Mecistops), a group of slender-snouted crocodylids currently restricted to western and central Africa. The third is Euthecodon, a crocodylid with an extremely long, slender, and distinctively notched snout. Euthecodon and C. thorbjarnarsoni are known from substantial numbers of specimens, but only one Mecistops specimen has been identified from the locality. The crocodylian fauna at Kanapoi is taxonomically similar to that of most other Plio-Pleistocene fluviolacustrine deposits in the Turkana Basin. Crocodylian diversity in the Turkana region contracted from a peak of five co-existing species in the late Miocene to one today; this contraction was underway by the early Pliocene, but crocodylian diversity remained stable at three species until well into the Quaternary.

Heterochronic shifts and conserved embryonic shape underlie crocodylian craniofacial disparity and convergence.

The distinctive anatomy of the crocodylian skull is intimately linked with dietary ecology, resulting in repeated convergence on blunt- and slender-snouted ecomorphs. These evolutionary shifts depend upon modifications of the developmental processes which direct growth and morphogenesis. Here we examine the evolution of cranial ontogenetic trajectories to shed light on the mechanisms underlying convergent snout evolution. We use geometric morphometrics to quantify skeletogenesis in an evolutionary context and reconstruct ancestral patterns of ontogenetic allometry to understand the developmental drivers of craniofacial diversity within Crocodylia. Our analyses uncovered a conserved embryonic region of morphospace (CER) shared by all non-gavialid crocodylians regardless of their eventual adult ecomorph. This observation suggests the presence of conserved developmental processes during early development (before Ferguson stage 20) across most of Crocodylia. Ancestral state reconstruction of ontogenetic trajectories revealed heterochrony, developmental constraint, and developmental systems drift have all played essential roles in the evolution of ecomorphs. Based on these observations, we conclude that two separate, but interconnected, developmental programmes controlling craniofacial morphogenesis and growth enabled the evolutionary plasticity of skull shape in crocodylians.

The sex-determination pattern in crocodilians: A systematic review of three decades of research.

Sex in crocodilians is not determined by chromosomes, but by egg incubation temperature, where different temperatures produce different clutch sex ratios. Two patterns have been proposed to describe these changes in sex ratios: a 100% female proportion at low and high temperatures with male predominance at intermediate ones (FMF) or a simpler pattern with a single female-to-male transition (FM). Over the last three decades, researchers have provided empirical information to support either of these two patterns in different species; however, no consensus has been reached partly because data have not been analysed as a whole. Here, we aimed at gathering the existing data on these patterns to provide models of temperature-dependent sex determination in those crocodilians studied so far. Potentially relevant publications were searched on Web of Knowledge, Scopus, Scielo and Science Direct. Studies that reported results on the sexual identity of crocodilian hatchlings obtained from constant temperature incubation treatments were considered. Using statistical models varying in their underlying assumptions, we evaluated which sex-determination pattern was best supported for the studied crocodilians and constructed species-specific and latitude-specific models. Based on the 8,458 sexed hatchlings studied throughout 31 studies, we show that the evidence supports a shared FMF pattern in all the crocodilian species for which enough data are available. We find that such pattern changes between species and at different latitudes. These results suggest a lability of the FMF crocodilian sex-determination pattern, a key feature under the present climate change scenario.

Archelosaurian Color Vision, Parietal Eye Loss, and the Crocodylian Nocturnal Bottleneck.

Vertebrate color vision has evolved partly through the modification of five ancestral visual opsin proteins via gene duplication, loss, and shifts in spectral sensitivity. While many vertebrates, particularly mammals, birds, and fishes, have had their visual opsin repertoires studied in great detail, testudines (turtles) and crocodylians have largely been neglected. Here I examine the genomic basis for color vision in four species of turtles and four species of crocodylians, and demonstrate that while turtles appear to vary in their number of visual opsins, crocodylians experienced a reduction in their color discrimination capacity after their divergence from Aves. Based on the opsin sequences present in their genomes and previous measurements of crocodylian cones, I provide evidence that crocodylians have co-opted the rod opsin (RH1) for cone function. This suggests that some crocodylians might have reinvented trichromatic color vision in a novel way, analogous to several primate lineages. The loss of visual opsins in crocodylians paralleled the loss of various anatomical features associated with photoreception, attributed to a "nocturnal bottleneck" similar to that hypothesized for Mesozoic mammals. I further queried crocodylian genomes for nonvisual opsins and genes associated with protection from ultraviolet light, and found evidence for gene inactivation or loss for several of these genes. Two genes, encoding parietopsin and parapinopsin, were additionally inactivated in birds and turtles, likely co-occurring with the loss of the parietal eye in these lineages.

Ontogeny of bite force in a validated biomechanical model of the American alligator.

Three-dimensional computational modeling offers tools with which to investigate forces experienced by the skull during feeding and other behaviors. American alligators (Alligator mississippiensis) generate some of the highest measured bite forces among extant tetrapods. A concomitant increase in bite force accompanies ontogenetic increases in body mass, which has been linked with dietary changes as animals increase in size. Because the flattened skull of crocodylians has substantial mediolaterally oriented muscles, crocodylians are an excellent model taxon in which to explore the role of mediolateral force components experienced by the feeding apparatus. Many previous modeling studies of archosaur cranial function focused on planar analysis, ignoring the mediolateral aspects of cranial forces. Here, we used three-dimensionally accurate anatomical data to resolve 3D muscle forces. Using dissection, imaging and computational techniques, we developed lever and finite element models of an ontogenetic series of alligators to test the effects of size and shape on cranial loading and compared estimated bite forces with those previously measured in vivo in A. mississippiensis We found that modeled forces matched in vivo data well for intermediately sized individuals, and somewhat overestimated force in smaller specimens and underestimated force in larger specimens, suggesting that ontogenetically static muscular parameters and bony attachment sites alone cannot account for all the variation in bite force. Adding aponeurotic muscle attachments would likely improve force predictions, but such data are challenging to model and integrate into analyses of extant taxa and are generally unpreserved in fossils. We conclude that anatomically accurate modeling of muscles can be coupled with finite element and lever analyses to produce reliable, reasonably accurate estimate bite forces and thus both skeletal and joint loading, with known sources of error, which can be applied to extinct taxa.