Abstract
Rhyacotriton olympicus is a stream-dwelling ambystoma- tid salamander having a mosaic of primitive (mainly structural) and derived (mainly ecological, developmental and physiological) features. Aspects of the morphology and ontogeny of middle to late larvae are considered. Proportional relationships during ontogeny, and external and microscopic features of the skin of the head and body are recorded. Skulls are well developed at an early age, especially those parts con- cerned with feeding. The last bones to appear are, in order, orbito- sphenoids, prefrontals, lacrimals (from two ossification centers) and septomaxillaries, and formation of lacrimals and septomaxillaries seems to be associated with metamorphosis. Nasal and coronoid bones never form. The anterior part of the otic capsule develops from two centers of ossification. Changes in dentition are marked and involve increases in the numbers of tooth loci on all but the premaxillary bone, during the period of growth studied. The rate of increase is greatest on the dentary. Most larvae lack palatopterygoid teeth, and all lack coronoid teeth; these are derived features in the family. Mineralized gill rakers are few in number. The hyobranchial apparatus is similar to that of other amby- stomatids. The vertebral column develops early, but some caudal verte- brae are added during later stages of larval life. From 42-46 vertebrae comprise the column, usually with 16 trunk and three caudosacral verte- brae. Limbs and girdles are well developed at relatively early ages. Coloration is analyzed, and development of bright yellow ventral pigment in late larval life is documented. The characters of Rhyacotriton are discussed relative to their evolu- tionary significance. The specializations mainly relate to heterochronic modes of evolution, favored by selection pressures associated with life in torrential streams and possibly related to such differential development as the early establishment of feeding and locomotor systems and the retardation of lung development.
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