Intraspecific Variation in Cranial Ossification in the Tailed Frog, Ascaphus truei

We describe the sequence of cranial ossification in the Tailed Frog, Ascaphus truei, and examine intraspecific variation in the timing and sequence of cranial development. Because tadpoles of A. truei lack the subarticular tubercles that normally delineate Gosner stages 39 and 40, we provide a modified Gosner staging system more appropriate for this species. Ossification sequence was highly variable among the populations and greater than that reported for other anuran species. In general, early ossification events in A. truei are similar to that reported for other anuran species. However, compared with other taxa, both onset of initial ossification events and appearance of specific elements appear relatively later in the development of Ascaphus.