Romano et al.—Early Triassic Fishes of Elko County (Nevada)
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Figure 8. Interspecific variation in the dentition of Birgeria Stensiö, 1919. Whereas species with two rows of teeth on the maxilla and dentary occur throughout the Triassic, species with three discrete rows are restricted to the Early Triassic. See text for details. Specimens shown: Birgeria americana n. sp. from the Smithian of Elko County, Nevada, USA (NMMNH P-66225, holotype; posterior part of dentary, middle part of maxilla); B. aldingeri Schwarz, 1970 from the Smithian of Spitsbergen (PMU P 1421, holotype, cf. Stensiö, 1932; suborbital part of maxilla—the internal lamina and the lingual teeth are not preserved because this is an external mold); B. stensioei Aldinger, 1931 from the Anisian and Ladinian of Monte San Giorgio and Besano, Swiss-Italian borderland (PIMUZ T 1; posterior part of dentary); Birgeria sp. (B. acuminata?) from the middle Norian of Cene, Bergamo, Italy (MPUM 9334, cf. Lombardo and Tintori, 2005; posterior part of dentary). Anterior is right in all photographs. Specimens not to scale. Arrows point to teeth of the lingual (black arrow), the intermediate (gray arrow), and the labial row (white arrow). U/Pb ages for the Early Triassic: a after Burgess et al. (2014), b after Galfetti et al. (2007a), c/d after Ovtcharova et al. (2006, 2015). Abbreviations of intervals: Di. = Dienerian, G. = Griesbachian, I. = Induan, Ladin. = Ladinian, Olenek. = Olenekian, Sm. = Smithian.
odontodes on the external side of the jaw bones become larger and tooth-like in vicinity to the marginal teeth, exhibiting both an acrodin cap and a pulp cavity. It is conceivable that the labial teeth developed from such odontodes. The present paper provides additional evidence that Birgeria
encompasses both species with three rows of teeth and species with two rows of teeth along the oral margins of the maxilla and dentary. Based on the current state of knowledge, it appears that birgeriids with three well-developed tooth rows as well as widely spaced intermediate teeth with varying height are restricted to the Early Triassic. On the other hand, species with only two principal rows of teeth and small, close-set, equal-sized intermediate teeth
occur throughout the Triassic. Whether the reduction of the intermediate and labial tooth rows on the maxilla and dentary represents an evolutionary trend in Birgeria requires further comparative studies. Compared to the intermediate teeth, the size distribution
pattern of the lingual teeth is seemingly more conservative between species. The same size distribution pattern is found in B. americana n. sp., B. stensioei, and B. acuminata (Aldinger, 1931; Boni, 1937; Schwarz, 1970; Romano and Brinkmann, 2009; Figs. 3, 4). In both B. stensioei and B. acuminata, the lingual teeth in the caudal part of the dentary are curved towards the anterior (Boni, 1937; Bürgin and Furrer, 1992; Romano and
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