The Ossification Sequence of Aneides lugubris, with Comments on Heterochrony

Abstract

Post-hatching ontogeny is described for the skeleton of a large, direct-developing, terrestrial plethodontid salamander, Aneides Iugubris. This species appears to have the most pro- longed ontogeny of the plethodontids. Until the end of the second year of life, osteological ontog- eny is similar to that of other species, but beyond that point major changes occur. Ossification of the long bones continues throughout life and is never completed. Elements such as the pelvic plate ossify continuously, but never completely. Mesopodial elements remain cartilaginous throughout life. The most dramatic changes relate to the feeding system. The anterior part of the skull becomes grotesquely enlarged in the largest specimens, with extensive coossification. Maxillary and dentary teeth decrease in number but increase in size and complexity. Features identified as supportive adaptations appear during ontogeny, more so in the feeding than in the locomotory mechanism. Data are interpreted in the framework of recent studies of heterochrony. Salamanders of the family Plethodon- tidae present an unusually good oppor- tunity for investigating the role of devel- opmental modes of evolution in the production of phylogenetic patterns. The family includes a wide array of adaptive morphologies. Several species (in the gen- era Gyrinopkilus, Haideotriton, Eurycea, and Typklornolge) are permanently aquatic and never metamorphose from their essen- tially larval morphology (Dunn, 1926; Wake, 1966). In contrast, over two-thirds have abandoned the aquatic larval stage, have direct terrestrial development, and occupy a vast array of terrestrial habitats, from temperate deciduous forests to des- ert canyons and tropical rain forests. The terrestrial species range from surface- dwelling salamanders to fossorial and arboreal forms. Such developmental modes of evolution as heterochrony have been very important, as is evident from the neoteny of the permanently larval species. However, heterochrony is also of