Thorius

Abstract: Cranial skeletal morphology, ontogeny and variation are examined in five species of Thorius, a genus of diminutive plethodontid salamanders that are among the smallest, extant, tailed tetrapods The skull of adull Thorius is characterized by: (1) limited development or absence of several ossified elements and dentition; (2) increased inter-and intraspecific variability; (3) novel morphological configurations of the braincase and jaw suspensorium Posthatching cranial mineralization in all species of Thorius is truncated precociously with respect to that typical of larger and more generalized plethodontid genera, such as Pseudoeurycea These features implicate paedomorphosis as a predominant mechanism responsible for the evolution of decreased size in Thorius from larger plethodontid ancestors Interspecific differences in cranial morphology are evident; species may be characterized by greater or lesser degrees of truncated development However, there is no consistent relationship between degree of paedomorphosis and mean adult body size in interspecific comparisons Adult morphology of several individual elements represent potentially useful taxonomic characters for distinguishing species Reduction, increased variability, and morphological novelty are common to many lineages of dwarfed taxa They represent a null hypothesis for examination of the developmental mechanisms and morphological consequences of miniaturization in other groups

Abstract: Patterns of growth and variation of the appendicular skeleton were examined in Thorius, a speciose genus of minute terrestrial plethodontid salamanders from southern Mexico. Observations were based primarily on ontogenetic series of each of five species that collectively span the range of adult body size in the genus; samples of adults of each of seven additional species provided supplemental estimates of the full range of variation of limb skeletal morphology. Limbs are generally reduced, i.e., pedomorphic, in both overall size and development, and they are characterized by a pattern of extreme variation in the composition of the limb skeleton, especially mesopodial elements, both within and between species. Fifteen different combinations of fused carpal or tarsal elements are variably present in the genus, producing at least 18 different overall carpal or tarsal arrangements, many of which occur in no other plethodontid genus. As many as four carpal or tarsal arrangements were observed in single population samples of each of several; five tarsal arrangements were observed in one population of T. minutissimus. Left-right asymmetry of mesopodial arrangement in a given specimen is also common. In contrast, several unique, nonpedomorphic features of the limb skeleton, including ossification of the typically cartilaginous adult mesopodial elements and ontogenetic increase in the degree of ossification of long bones, are characteristic of all species and distinguish Thorius from most related genera. They form part of a mechanism of determinate skeletal growth that restricts skeletal growth after sexual maturity. Interspecific differences in the timing of the processes of appendicular skeletal maturation relative to body size are well correlated with interspecific differences in mean adult size and size at sexual maturity, suggesting that shifts in the timing of skeletal maturation provide a mechanism of achieving adult size differentiation among species. Processes of skeletal maturation that confer determinate skeletal growth in Thorius are analogous to those typical of most amniotes - both groups exhibit ontogenetic reduction and eventual disappearance of the complex of stratified layers of proliferating and maturing cartilage in long bone epiphyses - but, unlike most amniotes, Thorius lacks secondary ossification centers. Thus, the presence of secondary ossification centers cannot be used as a criterion for establishing determinate skeletal growth in all vertebrates.

Abstract: Five new species of diminutive salamanders of the endemic Mexican genus Thorius (Plethodontidae) are described from the Sierra de Juirez in northern Oaxaca. The species are diagnosed by adult body size, external proportions, dentition, osteology and coloration. The three species that have been studied using protein electrophoresis are genetically unique; all differ from T. macdougalli, the only species of the genus previously known from these mountains. Each of the six species studied has distinct geographic and elevational ranges, and there is a complex pattern of geographic overlap and replacement. As many as three species co-occur locally at elevations up to 2955 m on Cerro Pelon, and each species is sympatric with at least one other. One species descends to approximately 800 m, which is the lowest known elevational record for the genus. The new taxa include the full size range of the genus, with two large and three small species.