BLACKBURN, D.C.; Harvard University: The effect of miniaturization on skeletal morphology within the African frog family Arthroleptidae

Miniaturization�the evolution of extremely small body size�has broad implications for many aspects of vertebrate evolution. Its dramatic effects include novel morphologies, increased morphological variation, and changes in proportions and anatomical organization. Within the frog family Arthroleptidae, which is endemic to sub-Saharan Africa, miniaturization is associated with a suite of skeletal morphologies. The similarities among small-sized species were proposed by Laurent (1979) to be derived characters of a single miniaturized lineage (genus Schoutedenella). Long-standing taxonomic difficulties in arthroleptid systematics suggest that miniature species do not necessarily form a monophyletic lineage. This raises the possibility that different arthroleptid lineages have converged on similar skeletal morphology via independent miniaturization, possibly through paedomorphosis. My research reveals that many, but not all, small-sized species (<25 mm snout-vent length) are characterized by carpal fusions, loss of a tarsal, novel sesamoids associated with the mesopodials, and proportions of the skull and pectoral girdle that differ from those of larger arthroleptid species. Interestingly, two miniature arthroleptid species appear to be polymorphic for fusion of carpal elements and may also exhibit left-right asymmetries in carpal fusions. This pattern is reminiscent of that found by Hanken (1985) in the miniature plethodontid salamander Thorius. Based on ontogenetic data from both large and miniature arthroleptid species, the skulls of miniature species do not appear to be strictly paedomorphic with respect to larger species. Phylogenetic hypotheses constructed using data independent of morphology (i.e., mitochondrial DNA sequence) will be necessary to evaluate the monophyly of miniature species and to determine the extent to which the skeletal morphologies associated with miniaturized species represent derived characteristics of miniature lineages. Supported by NSF EF-0334846 (AmphibiaTree) and the Museum of Comparative Zoology (Harvard University).