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Brigham Young University Science Bulletin, Biological Series

Abstract

The differences observed from the osteology and myology are few and subtle.

Neither species is consistent in having all of the anatomical characters equal. E. skiltonianus, however, is more variable than gilberti. This species: 1. exhibits a wider range of variation in the number of slips of the intermandibularis anterior; 2. has two distinct bundles of the depressor mandibularis; and 3. has variation in the anterior suture pattern of the frontal, nasal, prefrontal and maxilla elements. E. gilberti on the other hand has: 1. only one distinct bundle of the depressor mandibularis; 2. a frequent reduction of the relative size of the postorbitals when compared to other adjacent bones; and 3. a more limited suture pattern in the anterior portion of the skull. The major anatomical difference between these two species seems to be size. From the specimens used in this study the average snout-vent length of gilberti is approximately 30mm greater than that of skiltonianus.

We believe that skiltonianus shows more anatomical plasticity than gilberti. Plasticity in this case would indicate a more recent gene flow between peripheral and central individuals, and genetic inconstancy which would provide more variation and more flexibility for adaptation and selection. When the geographical distribution of these two species is considered it also appears that skiltonianus has more flexibility.

Taylor (1935) has suggested that these species may have arisen simultaneously from a common ancestral stock. On the basis of this anatomical study there is little reason to doubt their close relationship. However, it seems logical to propose an alternate suggestion; namely, that the anatomical variations of E. skiltonianus, when compared with similar characters in E. gilberti, indicate that E. gilberti may have arisen from E. skiltonianus.

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