Rafinesque, 1832
Gopher tortoises
Recognition
The four living species of North American gopher tortoises are closely related to the extinct genus Stylemys (Eocene-Miocene) and show numerous adaptations for burrowing. They have a hingeless, oblong, dorsally flattened carapace which lacks a cervical indentation and may have flared and serrated posterior marginals. The cervical scute is about as broad as long, and there are usually 11 marginals on each side, with a single, undivided supracaudal scute. No submarginal scutes are present. The nuchal bone lacks lateral costiform processes, and there are 11 peripheral bones on each side. Anterior neurals are usually alternately tetragonal or octagonal, but sometimes hexagonal. Two suprapygal bones are present. The hingeless plastron is well-developed and has a deep posterior notch. Its gulars are paired and project anteriorly. The entoplastron is anterior to the humero-pectoral seam. Axillary and inguinal buttresses are short and barely touch the costal bones. The skull is short and broad with a nonprojecting snout and a slightly hooked upper jaw. A medial ridge lies on both the premaxillae (a feature shared only with Stylemys) and maxillae. The maxillae do not contribute to the roof of the palate, and the anterior orbito-nasal foramina are small and concealed from ventral view. The temporal arch is moderately developed. The prootic bone is well-exposed dorsally and anteriorly, and the quadrate bone usually encloses the stapes. In G. flavomarginatus and G. polyphemus, the sacculus contains a large otolithic structure and the inner ear chambers are hypertrophied, but in G. agassizii and G. berlandieri, the sacculus contains only a small otolithic mass and the inner ear chambers are only slightly inflated. In flavomarginatus and polyphemus, the cervical vertebrae are short and stout with enlarged, closely joined pre- and postzygapophyses, and a specialized, interlocking joint is present between cervical 8 and the 1st dorsal vertebra, but in agassizii and berlandieri, these vertebrae are not appreciably shortened, their pre- and postzygapophyses not enlarged, and the 8th cervical vertebra does not form a specialized, interlocking joint with the 1st dorsal vertebra (Bramble, 1982). The 1st dorsal vertebra is also attached to a distinct, bony strut on the nuchal bone in flavomarginatus and polyphemus, but not in agassizii and berlandieri. Instead, there are a small zygapophysis and a neural arch sutured to the 1st neural (Bramble, 1982). Forelimbs are flattened for digging with two to four subradial carpals and five claws; hind limbs are club shaped. No toe webbing occurs, and the tail lacks a large terminal scale.
Little karyotypical variation occurs with the genus Gopherus (Dowler and Bickham, 1982). The diploid chromosomes numbers of G. agassizii, G. berlandieri and G. polyphemus are 52 (26 macrochromosomes and 26 microchromosomes): 20 metacentric and submetacentric, 10 subtelocentric, and 22 acrocentric and telocentric (Stock, 1972; Dowler and Bickham, 1982).
Taxonomic Comments
On the basis of differences in carpus and cranial structure, as well as other anatomical features, two species-groups of Gopherus may be recognized (Auffenberg, 1966a, 1976; Crumly, 1987, 1994): a polyphemus group, comprising G. polyphemus and G. flavomarginatus, and an agassizii group, comprising G. agassizii and G. berlandieri. Further evidence of the close relationship of G. agassizii and G. berlandieri was presented by Woodbury (1952), who described hybrids produced by a mating in captivity of these two species. In the polyphemus group, the sacculus contains a large otolithic structure and the inner ear chambers are hypertrophied, but in the agassizii group the sacculus contains only a small otolithic mass and the inner ear chambers are only slightly enlarged. In flavomarginatus and polyphemus, the cervical vertebrae are short and stout with enlarged, closely joined pre-and postzygapophyses, and a specialized, interlocking joint links the eighth cervical and the first dorsal vertebra. In agassizii and berlandieri, these vertebrae are not appreciably shortened, their pre- and postzygapophyses not enlarged, and the eighth vertebra does not form a specialized, interlocking joint with the first dorsal vertebra (Bramble, 1982). The first dorsal vertebra is also attached to a distinct, bony strut on the nuchal bone in the polyphemus group, but not in the agassizii group. Instead, there are a small zygapophysis and a neural arch sutured to the first neural (Bramble, 1982). Mental glands are better developed in the agassizii group than in the polyphemus group (Winokur and Legler, 1975). Other characters differentiating the two groups are discussed in Auffenberg (1976) and Crumly (1987, 1994).
Germano (1993) examined the morphometric differences among the four living species of Gopherus and found that carapace length differs significantly among species. After adjusting for size differences, carapace shape is more similar between the largest species, flavomarginatus, and the smallest species, berlandieri, than between the two intermediate-sized tortoises, agassizii and polyphemus. His results are not congruent with other studies of morphological variation (Winokur and Legler, 1975; Auffenberg, 1976; Bramble, 1982; Crumly, 1987, 1994) and genetic variation (Lamb et al., 1989), nor are they correlated with environmental data.
Remark
A controversy has arisen as to what is the proper generic name of the North American gopher tortoises. Bramble (1982) assigned the two species, agassizii and berlandieri, from the western United States to his newly created genus, Scaptochelys, while he retained polyphemus and flavomarginatus in the genus Gopherus. Bour and Dubois (1984b) proved that the name Scaptochelys is a junior subjective synonym of Xerobates Agassiz, 1857, and thus not available. They proposed Xerobates as a valid name for the genus, and some researchers have followed this designation. Anatomical studies by Crumly (1987, 1994), however, have shown that these four tortoise species share a unique suite of characters unknown in most other turtles: a medium premaxillary ridge (shared with the extinct Stylemys), prefrontal pits, class I type mental glands (Winokur and Legler, 1975), and absence of dorsal crest on the postzygapophyses of cervical vertebrae 6-8 (dorsal crests are almost absent in the African Malacochersus). Because of this, he recommended that all living gopher tortoises be retained in the genus Gopherus. We follow his recommendation.
Species identification
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