Hillary C. Maddin

The anatomy and development of the claws of Xenopus laevis (Lissamphibia: Anura) reveal alternate pathways of structural evolution in the integument of tetrapods.

Digital end organs composed of hard, modified epidermis, generally referred to as claws, are present in mammals and reptiles as well as in several non‐amniote taxa such as clawed salamanders and frogs, including Xenopus laevis. So far, only the claws and nails of mammals have been characterized extensively and the question of whether claws were present in the common ancestor of all extant tetrapods is as yet unresolved. To provide a basis for comparisons between amniote and non‐amniote claws, we investigated the development, growth and ultrastructure of the epidermal component of the claws of X. laevis. Histological examination of developing claws of X. laevis shows that claw formation is initiated at the tip of the toe by the appearance of superficial cornified cells that are dark brown. Subsequent accumulation of new, proximally extended claw sheath corneocyte layers increases the length of the claw. Histological studies of adult claws show that proliferation of cornifying claw sheath cells occurs along the entire length of the claw‐forming epidermis. Living epidermal cells that are converting into the cornified claw sheath corneocytes undergo a form of programmed cell death that is accompanied by degradation of nuclear DNA. Subsequently, the cytoplasm and the nuclear remnants acquire a brown colour by an as‐yet unknown mechanism that is likely homologous to the colouration mechanism that occurs in other hard, cornified structures of amphibians such as nuptial pads and tadpole beaks. Transmission electron microscopy revealed that the cornified claw sheath consists of parallel layers of corneocytes with interdigitations being confined to intra‐layer contacts and a cementing substance filling the intercorneocyte spaces. Together with recent reports that showed the main molecular components of amniote claws are absent in Xenopus, our data support the hypothesis that claws of amphibians likely represent clade‐specific innovations, non‐homologous to amniote claws.

A new large caseid (Synapsida, Caseasauria) from the Permian of Rodez (France), including a reappraisal of “Casea” rutena Sigogneau-Russell & Russell, 1974

ABSTRACT The description of a new large caseid, Ruthenosaurus russelbrum n. gen., n. sp. from the Early Permian (upper Cisuralian to lower Lopingian) of the Rodez Basin, France, is based on a partial postcranial skeleton that was initially collected at the same time as the holotype of “Cásea” rutena Sigogneau-Russell & Russell, 1974. Despite its distinctly larger size than “C.” rutena, the holotype of Ruthenosaurus n. gen. clearly represents an immature individual, as shown most clearly by the lack of fusion of the neural arches to their respective vertebral centra and incomplete ossification of the ends of the limb elements, including the absence of an ossified olecranon on the ulna. Nonetheless, Ruthenosaurus n. gen. is diagnosed by several autapomorphic characters, including dorsal vertebrae with anteriorly tilting neural spines and a diamond-shaped outline in transverse section, a first sacral rib with a dorsoventrally expanded distal head, and a low iliac blade with a poorly developed anterior process. The new taxon is further distinguishable from the only other known French caseid, “Casea” rutena, by the shape of the distal part of the humerus, including an ectepicondylar notch rather than a fully enclosed foramen, the specific shape of the ulna, and the overall robustness of the specimen. The taxonomic status of “Cased” rutena is discussed and it is concluded that this species should be moved into a new genus named Euromycter n. gen. The occurrence of the large-sized Ruthenosaurus n. gen. in France increases our knowledge about the early diversity of this clade in Europe.

A redescription of Carrolla craddocki (Lepospondyli: Brachystelechidae) based on high‐resolution CT, and the impacts of miniaturization and fossoriality on morphology

Some recent morphological analyses have brought into question the monophyly of Lissamphibia (frogs, salamanders, and caecilians). In these analyses, brachystelechid “microsaurs” are found to be sister group to caecilians. To test this hypothesis, the holotype specimen of the brachystelechid Carrolla craddocki was submitted to high‐resolution X‐ray computed tomography to gain insight into the nature of the morphology supporting the potential relationship between brachystelechids and caecilians. This analysis enabled us to conduct a detailed description of the internal anatomy such as the braincase and otic capsule endocast (the first of its kind for a lepospondyl), and new information regarding the architecture of the skull. Our results suggest brachystelechid cranial morphology is strongly influenced by miniaturization (enlarged sensory organs, anterior placement of the jaw articulation, and combination of both reduced‐ and hyper‐ossifications) and burrowing habits (co‐ossified braincase with broad, sloping occipital surface, overlapping joints between skull roof bones, and well‐ossified anterior braincase). Characteristics of brachystelechids that appear unrelated to size‐reduction and burrowing are the diamond‐shaped skull and possible pedicellate dentition. We provide a revised diagnosis for Carrolla and identify possible new characters within the anatomy of the braincase and inner ear. Several characters currently uniting caecilians and “microsaurs” are among those associated with either miniaturization or burrowing, demonstrating that future efforts should continue to focus on fine details of anatomy minimally affected by these influences to contribute to the resolution of the question of the origin of caecilians. J. Morphol. 2011.

Evolution of the Amphibian Ear with Implications for Lissamphibian Phylogeny: Insight Gained from the Caecilian Inner Ear

Abstract The inner ear is a complex structure consisting of the vestibular and auditory systems. Across vertebrates, morphological variation in the inner ear provides a source of homologous features (characters) that may aid in resolving phylogenetic relationships. The morphology of the inner ear in extant frogs and salamanders is well known, and has been extensively studied from functional perspectives. However, the ability of its form and features to shed light on the broader question of lissamphibian origins and relationships has not been as thoroughly explored. Herein we review the morphology of the inner ear of the least well-known lissamphibian group, the caecilians, and present three-dimensional reconstructions of otic capsule endocasts and of soft-tissue labyrinths. We use these data to explore previous statements about the structure of the caecilian inner ear and its evolutionary significance. The postulate that the periotic canal has a posterior path is corroborated, and the periotic sacs of each ear are observed to extend into the brain cavity, where they are applied to a fluid-filled compartment that is located ventral to the brain. These features are shared with frogs and salamanders. Additionally, it is hypothesized that the regression of two endorgans in caecilians is correlated with the secondary loss of the two middle ear auditory pathways, the tympanum–stapes and opercularis hearing pathway, suggesting that the lissamphibian-type ear is present, but in a derived state in caecilians. Identification of osteological correlates of this lissamphibian-type ear permits the interpretation of the evolution of this distinct ear type in the context of the three competing hypotheses of lissamphibian phylogeny. The distribution of traits is shown to be most parsimoniously explained when optimized onto the phylogenetic pattern that incorporates a monophyletic temnospondyl-derived Lissamphibia. This interpretation is consistent with a single origin of a lissamphibian-type tympanic ear. Therefore, characters of the ear seemingly provide synapomorphies that unite lissamphibians with amphibamid temnospondyls, potentially improving the resolution of concepts about the affinities of frogs, salamanders, and caecilians and clarifying issues of tetrapod ear evolution.

Evolutionary development of the neurocranium in Dissorophoidea (Tetrapoda: Temnospondyli), an integrative approach.

SUMMARY Ontogenetic data can play a prominent role in addressing questions in tetrapod evolution, but such evidence from the fossil record is often incompletely considered because it is limited to initiation of ossification, or allometric changes with increasing size. In the present study, specimens of a new species of an archaic amphibian (280 Myr old), Acheloma n. sp., a member of the temnospondyl superfamily Dissorophoidea and the sister group to Amphibamidae, which is thought to include at least two of our modern amphibian clades, anurans and caudatans (Batrachia), provides us with new developmental data. We identify five ontogenetic events, enabling us to construct a partial ontogenetic trajectory (integration of developmental and transformation sequence data) related to the relative timing of completion of neurocranial structures. Comparison of the adult amphibamid morphology with this partial ontogeny identifies a heterochronic event that occurred within the neurocranium at some point in time between the two taxa, which is consistent with the predictions of miniaturization in amphibamids, providing the first insights into the influence of miniaturization on the neurocranium in a fossil tetrapod group. This study refines hypotheses of large‐scale evolutionary trends within Dissorophoidea that may have facilitated the radiation of amphibamids and, projected forward, the origin of the generalized batrachian skull. Most importantly, this study highlights the importance of integrating developmental and transformation sequence data, instead of onset of ossification alone, into investigations of major events in tetrapod evolution using evidence provided by the fossil record, and highlights the value of even highly incomplete growth series comprised of relatively late‐stage individuals.

Deciphering morphological variation in the braincase of caecilian amphibians (Gymnophiona).

High levels of morphological homoplasy have hindered progress in understanding morphological evolution within gymnophione lissamphibians. Stemming from the hypothesis that the braincase has the potential to yield phylogenetic information, the braincases of 27 species (23 genera) of gymnophione amphibians were examined using high‐resolution micro‐computed tomography and histologically prepared specimens. Morphology of the brain and its relationship to features of the braincase is described, and it is shown that eight different patterns exist in the distribution of foramina in the antotic region. The distribution of variants is congruent with molecule‐based phylogeny. Additionally, all variants are shown to correspond directly to stages along developmental continua, suggesting that the evolutionary truncation of development in the antotic region at various stages has driven the evolution of morphology in this region. Attempts to correlate the observed morphology with proxies of putative heterochronic events (including those attributable to burrowing, life history, and size) fail to explain the distribution of morphology if each proxy is considered separately. Thus, it is concluded that either currently unrecognized causes of heterochrony or combinations thereof have influenced morphology in different lineages independently. These data identify clades whose morphology can now be reconsidered in light of previously unrecognized heterochronic events, thereby providing a foundation for future analyses of the evolution of morphology within Gymnophiona as a whole. Most significantly, these data confirm, for the first time in a lissamphibian group, that the braincase can preserve important phylogenetic information that is otherwise obscured in regions of the skull that experience strong influences from functional constraints. J. Morphol., 2011.

Restorative Regeneration of Digital Tips in the African Clawed Frog (Xenopus laevis Daudin)

Localized, specialized structures are carried on the tips of digits of many amniote and certain non‐amniote tetrapods. The pes of some members of Pipidae represents a rare example among tetrapods of differential expression of digital tip form (clawed vs. non‐clawed digits). As a step towards understanding how such localized forms are generated and maintained, we conducted a series of amputation experiments and observed, through the process of regeneration, the potential for reconstitution, at the tissue and organ level, of the different digit tip morphologies. Results of this study reveal that immediately following metamorphosis specialized digit tip structures are regenerated with a high degree of structural and spatial accuracy by a process that essentially replicates normal development in recently metamorphosed Xenopus laevis froglets. Furthermore, this regenerative capacity is maintained in juveniles 4 months beyond metamorphosis, and also in adults of 1 year or more in age, indicating that metamorphosis‐specific conditions do not exclusively facilitate regeneration of digit tips. In addition, regenerative capacity is maintained through repeated bouts of amputation and regeneration, indicating deep‐seated digit identity and retention of the distinct digit tip developmental programs within the digits. Together, these data suggest that the developmental programing responsible for the formation of the discrete digital tip morphologies is located regionally within each digit, and that it is retained through time. Our results suggest that Xenopus can serve as a model organism for exploring the molecular underpinnings of digit tip formation because regeneration leads to morphologically identical structures to those of the original digit tips. Anat Rec, 2011.