Evolution of skull shape in the family Salamandridae (Amphibia: Caudata)

Larval Body Shape and Eye Size Variation in Japanese Toads (Anura: Bufonidae: Bufo): An Ecological Implication for Lentic and Lotic Habitats

Variation in morphological traits of anurans has evolved due to the pressures imposed by inhabiting different environments. The Japanese stream toad, Bufo torrenticola, breeds in running water, such as mountain streams, where the larvae grow. This lotic-breeding habit of B. torrenticola was suggested to have evolved from a lentic-breeding ancestor. Evolutionary shift of breeding habit from lentic- to lotic-breeding caused larval morphological changes to adapt to the stream habitat. However, morphological adaptation associated with the larval habitat of these three types of Japanese toads has not been explored well. In this study, we applied geometric morphometrics and distance measurements to compare body and eye characters among the tadpoles of three Japanese toads. The results showed that B. torrenticola has more dorsally and posteriorly positioned eyes, and a longer and wider rostrum because of having a larger mouth and more-developed oral muscles than its close relatives. These characters might be related to lotic lifestyle. Meanwhile, tadpoles of B. torrenticola and its sister taxon B. japonicus japonicus showed similar eye growth pattern, supporting their close phylogenetic relationship. Some of the lotic-adapted characters have also been reported in other lotic tadpoles, which is indicative of convergent evolution among stream-adapted tadpoles.

Morphological variation in the vomer of aquatic and terrestrial spelerpini salamanders

The vomer is an important tooth-bearing cranial bone in the lungless salamanders (Caudata: Plethodontidae) that serves different functional roles in aquatic versus terrestrial feeding. Vomerine tooth rows that run parallel with the maxillary teeth are thought to help grasp prey while expelling water from the mouth, while posterior extensions of the tooth row may help terrestrial taxa bring prey down the throat. We hypothesize that these two general morphological types will correlate with the habitat (aquatic vs. terrestrial) of adult salamanders. Alternatively, variation in form may be due to taxonomic effects, such that closely related species will have similar vomer morphology regardless of adult habitat. To test this hypothesis, we examined vomer shape on a set of species of the morphologically diverse tribe Spelerpini, in which two of the five genera (Eurycea and Gyrinophilus) include both aquatic and terrestrial species. Data were collected using micro computed tomography (micro-CT) scans from specimens from the Field Museum of Natural History and the Illinois Natural History Survey; additional data was obtained from public online repositories including Morphosource.org. Two-dimensional geometric morphometric analyses were performed to capture shape variation of both the vomer and the vomerine tooth row. We found clear separation between aquatic and terrestrial taxa, with most of the variation due to differences in the vomerine tooth row. Differences ascribed to habitat use likely correspond to feeding behavior, and the functional role of the vomer in prey processing warrants further investigation in this species-rich salamander family.

Cranial skeletogenesis of one of the largest amphibians, Andrias japonicus, provides insight into ontogenetic adaptations for feeding in salamanders

Skeletal development in salamanders is greatly influenced by their complex life history. Although the relationship between skull development and life history has been investigated in a variety of salamander species, few studies have compared skull development among lineages. Here, we describe the cranial skeletogenesis of the Japanese giant salamander, Andrias japonicus (family Cryptobranchidae), for the first time. To conduct interspecific comparisons, we also describe the cranial skeletogenesis of Hynobius nebulosus (Hynobiidae), Pleurodeles waltl (Salamandridae) and Ambystoma mexicanum (Ambystomatidae). Our comparative analyses reveal interspecific differences in cranial skeletogenesis, particularly in the timing of the onset of ossification, the direction of ossification (e.g. from anterior to posterior) and the ossification pattern of the hyobranchial skeleton. The early onset of jaw ossification, posterior extension of the maxilla and posterolateral inclination of the squamosal in An. japonicus might be adaptations that allow An. japonicus to gulp feed, primarily using the suction force generated by opening the mouth wide, while also supplementing that force by depressing the hyobranchial skeleton. Multivariate regression of skull shape on log10-transformed centroid size revealed that the unique skull shape of adult and subadult An. japonicus forms through hypermorphosis.

Cranial shape evolution of extant and fossil crocodile newts and its relation to reproduction and ecology

The diversity of the vertebrate cranial shape of phylogenetically related taxa allows conclusions on ecology and life history. As pleurodeline newts (the genera Echinotriton, Pleurodeles and Tylototriton) have polymorphic reproductive modes, they are highly suitable for following cranial shape evolution in relation to reproduction and environment. We investigated interspecific differences externally and differences in the cranial shape of pleurodeline newts via two‐dimensional geometric morphometrics. Our analyses also included the closely related but extinct genus Chelotriton to better follow the evolutionary history of cranial shape. Pleurodeles was morphologically distinct in relation to other phylogenetically basal salamanders. The subgenera within Tylototriton (Tylototriton and Yaotriton) were well separated in morphospace, whereas Echinotriton resembled the subgenus Yaotriton more than Tylototriton. Oviposition site choice correlated with phylogeny and morphology. Only the mating mode, with a random distribution along the phylogenetic tree, separated crocodile newts into two morphologically distinct groups. Extinct Chelotriton likely represented several species and were morphologically and ecologically more similar to Echinotriton and Yaotriton than to Tylototriton subgenera. Our data also provide the first comprehensive morphological support for the molecular phylogeny of pleurodeline newts.

Postmetamorphic ontogenetic allometry and the evolution of skull shape in Nest-building frogs Leptodactylus (Anura: Leptodactylidae)

Allometry constitutes an important source of morphological variation. However, its influence in head development in anurans has been poorly explored. By using geometric morphometrics followed by statistical and comparative methods we analyzed patterns of allometric change during cranial postmetamorphic ontogeny in species of Nest-building frogs Leptodactylus (Leptodactylidae). We found that the anuran skull is not a static structure, and allometry plays an important role in defining its shape in this group. Similar to other groups with biphasic life-cycle, and following a general trend in vertebrates, ontogenetic changes mostly involve rearrangement in rostral, otoccipital, and suspensorium regions. Ontogenetic transformations are paralleled by shape changes associated with evolutionary change in size, such that the skulls of species of different intrageneric groups are scaled to each other, and small and large species show patterns of paedomorphic/peramorphic features, respectively. Allometric trajectories producing those phenotypes are highly evolvable though, with shape change direction and magnitude varying widely among clades, and irrespective of changes in absolute body size. These results reinforce the importance of large-scale comparisons of growth patterns to understand the plasticity, evolution, and polarity of morphological changes in different clades.

Morphological evolution and modularity of the caecilian skull

BACKGROUND

Caecilians (Gymnophiona) are the least speciose extant lissamphibian order, yet living forms capture approximately 250 million years of evolution since their earliest divergences. This long history is reflected in the broad range of skull morphologies exhibited by this largely fossorial, but developmentally diverse, clade. However, this diversity of form makes quantification of caecilian cranial morphology challenging, with highly variable presence or absence of many structures. Consequently, few studies have examined morphological evolution across caecilians. This extensive variation also raises the question of degree of conservation of cranial modules (semi-autonomous subsets of highly-integrated traits) within this clade, allowing us to assess the importance of modular organisation in shaping morphological evolution. We used an intensive surface geometric morphometric approach to quantify cranial morphological variation across all 32 extant caecilian genera. We defined 16 cranial regions using 53 landmarks and 687 curve and 729 surface sliding semilandmarks. With these unprecedented high-dimensional data, we analysed cranial shape and modularity across caecilians assessing phylogenetic, allometric and ecological influences on cranial evolution, as well as investigating the relationships among integration, evolutionary rate, and morphological disparity.

RESULTS

We found highest support for a ten-module model, with greater integration of the posterior skull. Phylogenetic signal was significant (K_mult = 0.87, p < 0.01), but stronger in anterior modules, while allometric influences were also significant (R² = 0.16, p < 0.01), but stronger posteriorly. Reproductive strategy and degree of fossoriality were small but significant influences on cranial morphology (R² = 0.03-0.05), after phylogenetic (p < 0.03) and multiple-test (p < 0.05) corrections. The quadrate-squamosal 'cheek' module was the fastest evolving module, perhaps due to its pivotal role in the unique dual jaw-closing mechanism of caecilians. Highly integrated modules exhibited both high and low disparities, and no relationship was evident between integration and evolutionary rate.

CONCLUSIONS

Our high-dimensional approach robustly characterises caecilian cranial evolution and demonstrates that caecilian crania are highly modular and that cranial modules are shaped by differential phylogenetic, allometric, and ecological effects. More broadly, and in contrast to recent studies, this work suggests that there is no simple relationship between integration and evolutionary rate or disparity.

Chewing or not? Intraoral food processing in a salamandrid newt

Food processing refers to any form of food mechanical breakdown prior to swallowing. Variations of this behaviour are found within all major gnathostome groups. Chewing is by far the most commonly used intraoral processing mechanism and involves rhythmic mandibular jaw and hyobranchial (tongue) movements. Chewing occurs in chondrichthyans (sharks and rays), actinopterygians (ray-finned fishes), dipnoi (lungfishes) as well as amniotes and involves similarities in the patterns of muscle activity and movement of the feeding apparatus. It has been suggested that amniote chewing, which involves the interaction of movements of the mandibular jaw and the muscular tongue, has evolved as part of the tetrapod land invasion. However, little is known about food processing mechanisms in lissamphibians, which might have retained many ancestral tetrapod features. Here, we identify a processing mechanism in the salamandrid newt, Triturus carnifex, which after prey capture displays cyclic head bobbing in concert with rhythmic jaw and tongue movements. We use high-speed fluoroscopy, anatomical reconstructions and analyses of stomach content to show that newts, although not using their mandibular jaws, deploy a derived processing mechanism where prey items are rasped rhythmically against the dentition on the mouth-roof, driven by a loop-motion of the tongue. We then compare patterns and coordination of jaw and tongue movements across gnathostomes to conclude that food processing in this newt species shares traits with processing mechanisms in fish as well as amniotes. This discovery casts salamanders as promising models for reconstructing the evolution of intraoral processing mechanisms at the fish-tetrapod split.