Morphospace occupation in thalattosuchian crocodylomorphs: skull shape variation, species delineation and temporal patterns

Abstract: Skull shape variation in thalattosuchians is examined using geometric morphometric techniques in order to delineate species, especially with respect to the classification of Callovian species, and to explore patterns of disparity during their evolutionary history. The pattern of morphological diversity in thalattosuchian skulls was found to be very similar to modern crocodilians: the main sources of variation are the length and the width of the snout, but these broad changes are correlated with size of supratemporal fenestra and frontal bone, length of the nasal bone, size of the orbit and premaxilla and position of the narial opening. Patterns of shape variation, in combination with discrete‐state morphology and stratigraphic and geographic range data were used to distinguish nine species of teleosaurid and 14 species of metriorhynchid, with the four currently recognized Callovian species being split into eight. Metriorhynchids were found to be more disparate from the average shape of morphospace than teleosaurids. However, short‐snouted metriorhynchids and long‐snouted teleosaurids showed the greatest amount of disparity with respect to snout morphotypes, indicating that each group tended to explore opposite areas of morphospace. Phylogeny was found to have a moderate influence on the pattern of morphospace occupation in metriorhynchids, but little effect in teleosaurids suggesting that other factors or constraints control the pattern of skull shape variation in thalattosuchians. A comparison of thalattosuchians with dyrosaur/pholidosaurids shows that thalattosuchians have a unique skull morphology, implying that there are multiple ways to construct a ‘long snout’. Moreover, the skull geometry of the problematic species Pelagosaurus typus was found to converge on the teleosaurid area of morphospace. Finally, the temporal distribution of thalattosuchian species and morphotypes demonstrate a clear and highly correlated relationship with sea level curves and mass extinction events through the Jurassic and the Early Cretaceous.     

Mitogenomic Analyses Place the Gharial (Gavialis gangeticus) on the Crocodile Tree and Provide Pre-K/T Divergence Times for Most Crocodilians

Based on morphological analyses, extant members of the order Crocodylia are divided into three families, Alligatoridae, Crocodylidae, and Gavialidae. Gavialidae includes one species, the gharial, Gavialis gangeticus. In this study we have examined crocodilian relationships in phylogenetic analyses of seven mitochondrial genomes that have been sequenced in their entirety. The analyses did not support the morphologically acknowledged separate position of the gharial in the crocodilian tree. Instead the gharial joined the false gharial (Tomistoma schlegelii) on a common branch that was shown to constitute a sister group to traditional Crocodylidae (less Tomistoma). Thus, the analyses suggest the recognition of only two Crocodylia families, Alligatoridae and Crocodylidae, with the latter encompassing traditional Crocodylidae plus Gavialis/Tomistoma. A molecular dating of the divergence between Alligatoridae and Crocodylidae suggests that this basal split among recent crocodilians took place ≈140 million years before present, at the Jurassic/Cretaceous boundary. The results suggest that at least five crocodilian lineages survived the mass extinction at the KT boundary. [Reviewing Editor: Dr. Nicolas Galtier]     

Microevolutionary patterns in the common caiman predict macroevolutionary trends across extant crocodilians

Both extinct and extant crocodilians have repeatedly diversified in skull shape along a continuum, from narrow‐snouted to broad‐snouted phenotypes. These patterns occur with striking regularity, although it is currently unknown whether these trends also apply to microevolutionary divergence during population differentiation or the early stages of speciation. Assessing patterns of intraspecific variation within a single taxon can potentially provide insight into the processes of macroevolutionary differentiation. For example, high levels of intraspecific variation along a narrow‐broad axis would be consistent with the view that cranial shapes can show predictable patterns of differentiation on relatively short timescales, and potentially scale up to explain broader macroevolutionary patterns. In the present study, we use geometric morphometric methods to characterize intraspecific cranial shape variation among groups within a single, widely distributed clade, Caiman crocodilus. We show that C. crocodilus skulls vary along a narrow/broad‐snouted continuum, with different subspecies strongly clustered at distinct ends of the continuum. We quantitatively compare these microevolutionary trends with patterns of diversity at macroevolutionary scales (among all extant crocodilians). We find that morphological differences among the subspecies of C. crocodilus parallel the patterns of morphological differentiation across extant crocodilians, with the primary axes of morphological diversity being highly correlated across the two scales. We find intraspecific cranial shape variation within C. crocodilus to span variation characterized by more than half of living species. We show the main axis of intraspecific phenotypic variation to align with the principal direction of macroevolutionary diversification in crocodilian cranial shape, suggesting that mechanisms of microevolutionary divergence within species may also explain broader patterns of diversification at higher taxonomic levels.     

Why the Long Face? The Mechanics of Mandibular Symphysis Proportions in Crocodiles

Background

Crocodilians exhibit a spectrum of rostral shape from long snouted (longirostrine), through to short snouted (brevirostrine) morphologies. The proportional length of the mandibular symphysis correlates consistently with rostral shape, forming as much as 50% of the mandible’s length in longirostrine forms, but 10% in brevirostrine crocodilians. Here we analyse the structural consequences of an elongate mandibular symphysis in relation to feeding behaviours.

Methods/Principal Findings

Simple beam and high resolution Finite Element (FE) models of seven species of crocodile were analysed under loads simulating biting, shaking and twisting. Using beam theory, we statistically compared multiple hypotheses of which morphological variables should control the biomechanical response. Brevi- and mesorostrine morphologies were found to consistently outperform longirostrine types when subject to equivalent biting, shaking and twisting loads. The best predictors of performance for biting and twisting loads in FE models were overall length and symphyseal length respectively; for shaking loads symphyseal length and a multivariate measurement of shape (PC1– which is strongly but not exclusively correlated with symphyseal length) were equally good predictors. Linear measurements were better predictors than multivariate measurements of shape in biting and twisting loads. For both biting and shaking loads but not for twisting, simple beam models agree with best performance predictors in FE models.

Conclusions/Significance

Combining beam and FE modelling allows a priori hypotheses about the importance of morphological traits on biomechanics to be statistically tested. Short mandibular symphyses perform well under loads used for feeding upon large prey, but elongate symphyses incur high strains under equivalent loads, underlining the structural constraints to prey size in the longirostrine morphotype. The biomechanics of the crocodilian mandible are largely consistent with beam theory and can be predicted from simple morphological measurements, suggesting that crocodilians are a useful model for investigating the palaeobiomechanics of other aquatic tetrapods.     

The evolution of cranial form and function in theropod dinosaurs: insights from geometric morphometrics

Theropod dinosaurs, an iconic clade of fossil species including Tyrannosaurus and Velociraptor, developed a great diversity of body size, skull form and feeding habits over their 160+ million year evolutionary history. Here, we utilize geometric morphometrics to study broad patterns in theropod skull shape variation and compare the distribution of taxa in cranial morphospace (form) to both phylogeny and quantitative metrics of biting behaviour (function). We find that theropod skulls primarily differ in relative anteroposterior length and snout depth and to a lesser extent in orbit size and depth of the cheek region, and oviraptorosaurs deviate most strongly from the "typical" and ancestral theropod morphologies. Noncarnivorous taxa generally fall out in distinct regions of morphospace and exhibit greater overall disparity than carnivorous taxa, whereas large-bodied carnivores independently converge on the same region of morphospace. The distribution of taxa in morphospace is strongly correlated with phylogeny but only weakly correlated with functional biting behaviour. These results imply that phylogeny, not biting function, was the major determinant of theropod skull shape.     

Variation in crocodilian dorsal scute organization and geometry with a discussion of possible functional implications

Dermal ossifications, including osteoderms, are present in many vertebrates and are frequently interpreted as a defense against predators. Nevertheless, osteoderms remain ubiquitous in adult crocodilians while being absent in hatchlings, even though adults rarely experience predation. In other biological systems, increased variation, particularly fluctuating asymmetry, have proven useful for identifying biological structures likely to have evolved under relaxed selection, which in turn may inform their function. Therefore, using the keratinous scutes as proxies for the underlying osteoderm morphology, I investigated the average intraspecific variability of geometry and fluctuating asymmetry in dorsal scutes in five species of crocodilians. I first tested for differences in variability of scute length and width, then for differences in bilateral fluctuating asymmetry of scute number, before finally investigating scute distribution patterns for each species compared to hypothetical rectangular and hexagonal scute arrangements. The American crocodile, Crocodylus acutus, shows significantly more asymmetry than other species, which is consistent with relaxed selection on osteoderms in this species. A suspected decrease in intraspecific aggression within Crocodylus acutus, in conjunction with the inferred relaxed selection, suggests that, in general, crocodilian osteoderms function primarily as defensive armor in aggressive encounters with conspecifics. The smooth‐fronted caiman, Paleosuchus trigonatus, exhibits increased variation in scute dimensions linked to the mediolateral offset of osteoderms in adjacent rows, possibly resulting in a more rigid carapace. Unfortunately, comparative data on crocodilian behavior, physiology, and development is extremely limited and restricts the ability to explore other potential explanations for the patterns observed, highlighting the need for more research on rare and cryptic crocodylians.     

A phylogenetic view on skull size and shape variation in the smooth newt (Lissotriton vulgaris,Caudata, Salamandridae)

In this study, we explore skull size and shape variation in the smooth newt, a taxon with substantial morphological differentiation and complex phylogeographic relations. By projecting phylogenies into the morphospace of the skull shape, we explore the variation in and differentiation of this complex morphological structure within a phylogenetic framework. For these analyses, we used a dataset that covers the most southern part of the species’ distribution range, including all conventionally recognized subspecies. The study revealed different patterns of divergence in skull shape between sexes, which is paralleled by intraspecific differentiation. The divergence in dorsal skull shape is concordant with the phylogenetic divergence, as the most diverged clades of the smooth newt (Lissotriton vulgaris kosswigi and Lissotriton vulgaris lantzi) exhibit a skull shape that significantly diverges from the smooth newt’s mean shape configuration. The results of this study also indicate that ventral skull portion, which is more directly related to feeding and foraging, shows higher variation between populations than dorsal skull portion, which appears to be less variable and phylogenetically informative.     

Historical Zoogeography of the Eusuchian Crocodilians: A Physiological Perspective

The historical zoogeography of eusuchian crocodilians has rarelybeen reviewed in any detail and yet is of increasing interestto students of crocodilian biology as large amounts of comparativeinformation on a wide range of species come to hand. Previousinterpretations of crocodilian zoogeography have been basedon one or another of two assumptions–that the major continentalland masses have remained more or less fixed in position, andthat the eusuchians have had only very limited powers of dispersalacross marine barriers. Both of these assumptions are inappropriatein light of our present knowledge of continental drift and crocodilianphysiology. In this paper we attempt a reinterpretation of eusuchian zoogeographybased on new information on their systematic relationships,physiological capacity for marine dispersal, and fossil history.We postulate that anatomical and physiological adaptations toa marine existence have played an important role in eusuchianhistory. We propose that Gavialis and Tomistoma, now restrictedto fresh waters, may have been derived secondarily from ancestorsadapted to salt water. In the case of Tomistoma, similaritiesin lingual gland and buccal cavity anatomy to the true crocodiles(Crocodylus and Osteolaemus) suggest that marine adaptationspredated the divergence of tomistomine and crocodyline stocks.The buccal morphology of Gavialis suggests it also has a marineancestry. Its systematic affinities are uncertain, lying perhapswith Tomistoma or, on other interpretations, with the Mesosuchia.In both cases, the fossil record is not inconsistent with thispossibility. Palaeontological information now available is inadequate toreconstruct the evolutionary history of the Eusuchia in detail.However, saltwater adapted eusuchians are more common in thefossil record than is widely recognized and the likelihood ofdispersal across marine "barriers" by non-alligatorid crocodilianscannot be ignored.     

Intraspecific variation in the skull morphology of the black caiman Melanosuchus niger (Alligatoridae,Caimaninae)

Melanosuchus niger is a caimanine alligatorid widely distributed in the northern region of South America. This species has been the focus of several ecological, genetic and morphological studies. However, morphological studies have generally been limited to examination of interspecific variation among extant species of South American crocodylians. Here, we present the first study of intraspecific variation in the skull of M. niger using a two‐dimensional geometric morphometric approach. The crania of 52 sexed individuals varying in size were analysed to quantify shape variation and to assign observed shape changes to different types of intraspecific variation, that is, ontogenetic variation and sexual dimorphism. Most of the variation in this species is ontogenetic variation in snout length, skull depth, orbit size and the width of the postorbital region. These changes are correlated with bite force performance and probably dietary changes. However, a comparison with previous functional studies reveals that functional adaptations during ontogeny seem to be primarily restricted to the postrostral region, whereas rostral shape changes are more related to dietary shifts. Furthermore, the skulls of M. niger exhibit a sexual dimorphism, which is primarily size‐related. The presence of non‐size‐related sexual dimorphism has to be tested in future examinations.     

Italian Cenozoic crocodilians: taxa, timing and palaeobiogeographic implications

Crocodylian remains are collected in 39 fossil-bearing localities but only in seven localities specimens with reliable taxonomic attributions, at least to genus level have been collected. Three species have been reported from the early Lutetian Purga di Bolca site: Pristichampsus cf. Pristichampsus rollinati, Asiatosuchus sp., Hassiacosuchus sp. (=Allognathosuchus sp.). The three crocodilians discovered at Purga di Bolca have been reported also from Geiseltal and Messel (Middle Eocene, Germany). Bolca at that time was part of a Tethysian archipelago and no mammals have been found there till now. Crocodilians and turtles clearly arrived from the European mainland across a marine water barrier. Among the other fossiliferous localities of Veneto, very interesting is the Monte Zuello site, of late Middle Eocene age, yielding a longirostrine crocodilian, Megadontosuchus arduini, a tomistomine species. Tomistomines are known in contemporaneous sediments of both Europe and Africa, but the European forms Dollosuchus and Kentisuchus seem the closest taxa. Remains of Oligocene age have been collected in Veneto and Liguria, but the fossils discovered in the second region are teeth or fragmented bones. The fossil crocodilians of Monteviale (Veneto), of Early Oligocene age, have been assigned to two species but they have been recently all identified as Diplocynodon ratelii, known from several European sites of Late Eocene, Oligocene and Miocene age. This species arrived in the Monteviale area from the European mainland across a narrow sea. Several crocodilian fossils of Miocene age are very fragmentary or represented by isolated teeth. In the Middle and Late Miocene of Sardinia, a well-established species, Tomistoma calaritanum is present. Remains of Tomistoma of the same age have been reported in some localities in Tuscany, Apulia, Sicily and Malta. In the Mediterranean area, the genus is known from European and African sites (of older age). The colonisation of Europe by this genus is the result of a dispersion from Africa (or less probably from Asia). During Late Miocene Sardinia and Tuscany belong to the same palaeobioprovince characterized by the Oreopithecus-Maremmia fauna. In Tuscany, a crocodilian identified as Crocodylus bambolii is present in the late Miocene site of Monte Bamboli. If the generic attribution of this form is correct, its ancestors must have arrived from Africa. Another fossil assemblage of Late Miocene age characterizes the Apulia-Abruzzi palaeobioprovince (Hoplitomeryx-Microtia fauna) and testifies complete isolation between the two palaeobioprovinces. In this last area, remains of Crocodylus sp. have been collected in coastal sandstones at Scontrone (Abruzzi) and in several fissure fillings of Gargano of slightly younger age. The ancestors of this species arrived from Africa while no African elements are present among the mammalian fauna. The dispersion of the genus Crocodylus in the Italian palaeoislands may have taken place once, with allopatric differentiation of the two populations (Tuscany-Sardinia and Apulia-Abruzzi) or twice with independent colonisation of each area.     

Niche partitioning and morphospace in early stages of two sympatric Diogenichthys species (Myctophidae)

Diet and morphospace of larval stages of two sympatric lanternfish Diogenichthys atlanticus and D. laternatus from the south-east Pacific Ocean were compared and the covariance between both variables was assessed for each species. Diogenichthys atlanticus stomach contents consisted mainly of copepod nauplii and digested remains and this species had a broader niche than D. laternatus, in which stomach contents were highly digested. No dietary overlap was found between both species. The covariance between skull shape and diet for D. atlanticus was given by a wider mouth gape related to the presence of copepod nauplii, whilst for D. laternatus, a shorter snout and posteriorly displaced eye were related to the presence of highly digested stomach contents. Interspecific differences between diets and skull shapes suggest that both species may have undergone morphological or niche divergence to avoid competition, such as feeding at different hours or depth stratification.     

Sexual size dimorphism and allometric growth of Morelet's crocodiles in captivity

Few studies have conducted morphological analyses of crocodilians, and little information exists on differences between size-classes and sexes in Neotropical crocodilians. In this study, we measured nine morphological traits in 121 captive Morelet's crocodiles Crocodylus moreletii (81 females and 40 males). Our results revealed that individuals < 2 m total length do not exhibit sexual dimorphism in morphometric characteristics. However, for crocodiles over 2 m in length, males were significantly larger than females in terms of dorsal-cranial length, cranial width, snout width and snout-ventral length. In general, morphological traits demonstrated a strongly significant relationship with total length at the smaller size class of 150-200 cm length. However, in the highest size class of 250-300 cm length (large adult males), morphological traits were no longer significantly related with total length. Male crocodiles demonstrated allometric growth of cranial morphology with significantly greater increase in cranial width, snout width, and mid-snout width relative to total length at higher size classes. Morphological dimorphism and allometric growth may be associated with adaptive strategies for reproductive success.     

Head‐shape polymorphism in Japanese eels Anguilla japonica in relation to differences of somatic growth in freshwater and brackish habitats

The age, total length (L_T), head shape and skull shape were investigated for 379 Japanese eels Anguilla japonica sampled in freshwater and brackish areas of the Kojima Bay–Asahi River system, Okayama, Japan, to learn about the differentiation process of head‐shape polymorphism. The relative mouth width (ratio of mouth width to L_T) of A. japonica > 400 mm L_T collected in fresh water was significantly greater than that of fish collected in brackish water. Growth rates of mouth width and the distance from the snout to the midpoint of the eyes (the ratio of width and distance to age, respectively) were not significantly different between freshwater and brackish‐water samples, whereas the somatic growth rate (the ratio of L_T to age) of freshwater samples was significantly lower than that of brackish‐water eel samples. These results suggest that the factors affecting head and somatic growth of A. japonica are not identical. According to these results and feeding patterns in each habitat reported by another study, it is suggested that somatic growth appears to play a significant role in the differentiation process of the head‐shape polymorphism in A. japonica, with the slow‐growing fish in fresh water becoming broad‐headed and the fast‐growing fish in brackish water becoming narrow‐headed.     

Un atoposauridé (Crocodylia,mesosuchia) du portlandien de la Meuse (est de la France)

A small crocodilian skull from the lower Portlandian (Cyrena rugosa beds) of Brauvilliers (Meuse) is referred to an atoposaurid of the genus Alligatorium, apparently closely related to the species A. paintenense from the Tithonian of Franconia. It is the first reportof one of these small continental crocodilians from north-eastern France.     

Morphological divergence in Indian oil sardine,Sardinella longiceps Valenciennes, 1847– Does it imply adaptive variation?

The Indian oil sardine, Sardinella longiceps, is an important pelagic species in Indian waters, and shows divergent morphology while in sympatry. The reasons behind this divergent morphology were investigated using morphometric, genetic and nutritional analyses. Twenty‐one morphometric characters (as percentage of standard length) and eight meristic characters were studied in the three variants to assess whether they are significantly diverged. Distinct clustering of morphotypes was evident in the principal component analysis on log‐transformed ratios of morphological characters with PC1 and PC2, explaining 50.7% and 17.6% of the total morphological variation, respectively. PC1 was highly correlated with the distance from snout to anal origin, depth at dorsal, distance from snout to pelvic and distance from snout to first dorsal. PC2 was highly correlated with head length, caudal width and anal depth. Analysis of similarities (ANOSIM) was conducted using log‐transformed morphometric ratios, with the results showing the clusters to be well differentiated (R = 0.511; P < 0.01). Similarity of percentage analysis (SIMPER) analysis showed that the differences in depth at the dorsal, anal base length, caudal width, distance from pelvic to anal origin, anal depth and eye diameter accounted for 52% of variations between variant 1 and 2. Differences in caudal width, distance from pelvic to anal origin, anal base length, depth at dorsal and anal depth accounted for 56% of the variation between variant 2 and 3. Differences in caudal width, eye diameter, anal base length, anal depth, distance from pelvic to anal origin accounted for 50% of the variation between variant 1 and 3. Genetic divergence was not significantly based on mitochondrial cytochrome c oxidase I (COI) or control region sequences. Proximate composition analyses showed significantly high fat content in variants 1&3 and significantly high protein content in variant 2, probably due to dissimilar dietary preferences. The study shows that morphotypes of the Indian oil sardine may be the result of divergent selection and adaptive variations, which need further investigation using a long‐term sampling design.     

The biomechanical consequences of longirostry in crocodilians and odontocetes

Unrelated clades of aquatic tetrapod have evolved a similar range of skull shapes, varying from longirostrine (elongate and narrow rostrum) to brevirostrine (short rostrum). However, it is unclear which aspects of organismal performance are associated with this convergence in the range of skull shapes. Furthermore, it is not known how fundamental anatomical differences between groups influence these relationships. Here we address this by examining the load bearing capabilities of the skulls of two of the most diverse groups of living aquatic tetrapod: crocodilians and odontocetes. We use finite element analysis to examine the abilities of different cranial morphologies to resist a range of biologically relevant feeding loads including biting, shaking and twisting. The results allow for form/function relationships to be compared and contrasted between the two groups. We find that cranial shape has similar influences on performance during biting, shaking or twisting load cases at the anterior tooth positions, e.g. brevirostrine species experienced less strain than longirostrine species. The pattern of this form/function relationship is similar for both crocodilians and odontocetes, despite their fundamentally different anatomies. However, when loading teeth at the posterior end or middle of the tooth row the results do not follow the same pattern. Behavioural differences in bite location plays a key role in determining functional abilities in aquatic tetrapod taxa.     

Convergence and divergence in the evolution of cat skulls: temporal and spatial patterns of morphological diversity

Background

Studies of biological shape evolution are greatly enhanced when framed in a phylogenetic perspective. Inclusion of fossils amplifies the scope of macroevolutionary research, offers a deep-time perspective on tempo and mode of radiations, and elucidates life-trait changes. We explore the evolution of skull shape in felids (cats) through morphometric analyses of linear variables, phylogenetic comparative methods, and a new cladistic study of saber-toothed cats.

Methodology/Principal Findings

A new phylogenetic analysis supports the monophyly of saber-toothed cats (Machairodontinae) exclusive of Felinae and some basal felids, but does not support the monophyly of various saber-toothed tribes and genera. We quantified skull shape variation in 34 extant and 18 extinct species using size-adjusted linear variables. These distinguish taxonomic group membership with high accuracy. Patterns of morphospace occupation are consistent with previous analyses, for example, in showing a size gradient along the primary axis of shape variation and a separation between large and small-medium cats. By combining the new phylogeny with a molecular tree of extant Felinae, we built a chronophylomorphospace (a phylogeny superimposed onto a two-dimensional morphospace through time). The evolutionary history of cats was characterized by two major episodes of morphological divergence, one marking the separation between saber-toothed and modern cats, the other marking the split between large and small-medium cats.

Conclusions/Significance

Ancestors of large cats in the ‘Panthera’ lineage tend to occupy, at a much later stage, morphospace regions previously occupied by saber-toothed cats. The latter radiated out into new morphospace regions peripheral to those of extant large cats. The separation between large and small-medium cats was marked by considerable morphologically divergent trajectories early in feline evolution. A chronophylomorphospace has wider applications in reconstructing temporal transitions across two-dimensional trait spaces, can be used in ecophenotypical and functional diversity studies, and may reveal novel patterns of morphospace occupation.     

Metamorphosis of cranial design in tiger salamanders (Ambystoma tigrinum): A morphometric analysis of ontogenetic change

While ontogenetic analyses of skull development have contributed to our understanding of phylogenetic patterns in vertebrates, there are few studies of taxa that undergo a relatively discrete and rapid change in morphology during development (metamorphosis). Morphological changes occurring in the head at metamorphosis in tiger salamanders (Ambystoma tigrinum) were quantified by a morphometric analysis of cranial osteology and myology to document patterns of change during metamorphosis. We employed a cross-sectional analysis using a sample of larvae just prior to metamorphosis and a sample of transformed individuals just after metamorphosis, as well as larvae undergoing metamorphosis. There were no differences in external size of the head among the larval and transformed samples. The hyobranchial apparatus showed many dramatic changes at metamorphosis, including shortening of ceratobranchial 1 and the basibranchial. The subarcualis rectus muscle increased greatly in length at metamorphosis, as did hypobranchial length and internasal distance. A truss analysis of dorsal skull shape showed that at metamorphosis the snout becomes wider, the maxillary and squamosal triangles rotate posteromedially, and the neurocranium shortens (while maintaining its width), resulting in an overall decrease in skull length at metamorphosis. These morphometric differences are interpreted in light of recent data on the functional morphology of feeding in salamanders. Morphological reorganization of the hyobranchial apparatus and shape changes in the skull are related to the acquisition of a novel terrestrial feeding mode (tongue projection) at metamorphosis. Metamorphic changes (both internal and external) that can be used to judge metamorphic condition are discussed.     

Regression equations between body and head measurements in the broad-snouted Caiman (Caiman latirostris)

In the present study, regression equations between body and head length measurements for the broad-snouted caiman (Caiman latirostris) are presented. Age and sex are discussed as sources of variation for allometric models. Four body-length, fourteen head-length, and ten ratio variables were taken from wild and captive animals. With the exception of body mass, log-transformation did not improve the regression equations. Besides helping to estimate body-size from head dimensions, the regression equations stressed skull shape changes during the ontogenetic process. All age-dependent variables are also size-dependent (and consequently dependent on growth rate), which is possibly related to the difficulty in predicting age of crocodilians based on single variable growth curves. Sexual dimorphism was detected in the allometric growth of cranium but not in the mandible, which may be evolutionarily related to the visual recognition of gender when individuals exhibit only the top of their heads above the surface of the water, a usual crocodilian behavior.