Date of Award

Summer 7-12-1991

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Biological Sciences

First Advisor

Cannatella, D.C.

Second Advisor

Good, David A.

Third Advisor

Hafner, Mark S.

Abstract

The ontogenetic sequence of ossification of the cranium, including descriptions of the development of individual elements, from initial appearance of bone through adulthood, is described for two species of amphibian: the Spotted Salamander, Ambystoma maculatum, and the Tailed Frog, Ascaphus truei. This study is based on cleared-and-stained developmental series of 269 specimens of Ambystoma and 325 of Ascaphus. Nine stages of cranial development are defined for Ambystoma maculatum. By metamorphosis, all bones of the skull of Ambystoma except the articular have begun to ossify. The ossification sequence of Ambystoma shows little interindividual variation; only 6 specimens do not follow the modal sequence. Comparisons with other salamanders reveals basic similarities in the developmental pattern of the caudate skull. In Ascaphus truei, geographic variation in the sequence of cranial ossification is greater than previously reported for any species of amphibian. In addition, intraspecific variation within populations is also higher than previously reported. The number of bones present at any developmental stage shows a poor coorelation with external morphology. As many as 13 Gosner stages may pass after initial appearance of a given bone before it appears in all specimens in the sample. Relative to other anurans, ossification of many cranial elements is delayed in Ascaphus. This delay is discussed in terms of larval specialization, and as a possible consequence of heterochrony. Differences in the developmental patterns of ossification in Ambystoma maculatum and Ascaphus truei reflect basic differences in the evolution of larval life histories.

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