EVOLUTIONARY MECHANISMS OF LIMB LOSS IN TETRAPODS

The Energetic Cost of Limbless Locomotion

The net energetic cost of terrestrial locomotion by the snake Coluber constrictor, moving by lateral undulation, is equivalent to the net energetic cost of running by limbed animals (arthropods, lizards, birds, and mammals) of similar size. In contrast to lateral undulation and limbed locomotion, concertina locomotion by Coluber is more energetically expensive. The findings do not support the widely held notion that the energetic cost of terrestrial locomotion by limbless animals is less than that of limbed animals.

Behavior, body types and the irreversibility of evolution

A functional approach to evolutionary morphology is emphasized in this paper. This perspective differs from the current 'structuralist' trend, which emphasizes the constraining role of developmental paths. In addition, the present approach agrees with the adaptationist paradigm. It is further argued that three types of phenomena are better understood in this light: i. The existence of evolutionary trends, ii. The maintenance of certain structural features within a given taxon, and iii. The irreversibility of evolution.

Fibular a/hypoplasia: review and documentation of the fibular developmental field

Fibular aplasia and/or hypoplasia is documented as a developmental field defect and the extent of the fibular developmental field is delineated. The term fibular a/hypoplasia denotes the clinical spectrum of fibular deficiency in different patients and also implies that aplasia can be present in one limb and hypoplasia in the other. Causal heterogeneity of fibular a/hypoplasia is demonstrated, thereby defining it as a developmental field defect. Most cases of fibular a/hypoplasia are isolated, sporadic events. An autosomal dominant form of isolated fibular a/hypoplasia with ankle joint anomaly is reviewed. Fibular a/hypoplasia may be part of more complex sporadic dysostoses; sporadic syndromes, an aneuploidy syndrome; several autosomal dominant and autosomal recessive conditions. Fibular a/hypoplasia is also postulated to occur as a result of disruption or teratogenic insult; in animals, fibular development can be disturbed by radiation, busulfan, and retinoic acid. Clinical data allow evaluation of the extent of the fibular developmental field of the lower limb. This appears to include the pubic portion of the pelvis, proximal femur (distal half being apparent tibial developmental territory), patella, anterior cruciate ligament, and lateral and/or axial foot rays (but "never" the hallux and almost never associated with polydactyly). The rare cases of fibuloulnar dimelia allow confirmation of the well known homology of mesomelic limb segments responsible for concordant ulnar and fibular (and radial and tibial) defect, if both upper and lower limbs are involved in a given condition. Because fibular a/hypoplasia is the commonest of the mesomelic paraxial hemimelias, is usually nonsyndromal, and in most cases is apparently nongenetic (ie, with negligible recurrence risk), we propose that in humans, as in several other tetrapods, the fibula is undergoing regressive evolution and hence is developmentally especially labile.

Appendicular skeletal morphology in minute salamanders, genus Thorius (amphibia: Plethodontidae): Growth regulation, adult size determination, and natural variation

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.

Skeleton of Diacodexis, Oldest Known Artiodactyl

A nearly complete skeleton of early Eocene Diacodexis, the oldest known member of the mammalian order Artiodactyla, is described. Its slender, elongate limb elements indicate that Diacodexis was highly cursorial and closer in postcranial adaptations to tragulids and other primitive ruminants than to living or extinct nonruminant artiodactyls. Its skeletal specializations call into question the widespread notion that Diacodexis was the ancestor of all later artiodactyls.

Is a new evolutionary synthesis necessary?

The current (synthetic) theory of evolution has been criticized on the grounds that it implies that macroevolutionary processes (speciation and morphological diversification) are gradual. The extent to which macroevolution is gradual or punctuational remains to be ascertained. Macroevolutionary processes are underlain by microevolutionary phenomena and are compatible with the synthetic theory of evolution. But microevolutionary principles are compatible with both gradualism and punctualism; therefore, logically they entail neither. Thus, macroevolution and microevolution are decoupled in the important sense that macroevolutionary patterns cannot be deduced from microevolutionary principles.