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Journal of Paleontology 91(5):1025–1046
Brinkmann, 2009; Fig. 8), which is not the case in B. americana n. sp., however. Nielsen (1949) noted that the lingual teeth of B. stensioei are more widely spaced than in B. groenlandica. Although this is true for some specimens of B. stensioei (e.g., the lectotype), the teeth are more densly distributed in other specimens (Aldinger, 1931; Schwarz, 1970; Fig. 8). Tooth loss cannot account for such regular distribution patterns, but more studies are necessary to better assess the taxonomic value of tooth spacing. Previous workers (Stensiö, 1932; Lehman, 1952; Schwarz,
1970) suggested interspecific variation in the ornamentation of the acrodin cap, ranging from completely smooth, to only basally striated, to fully striated. Nevertheless, the taxonomic value of tooth ornamentation is doubtful, as this character also shows intraspecific variability. For instance, in B. stensioei the acrodin cap is fully striated in PIMUZ T 4780 (Schwarz, 1970), but only basally striated in PIMUZ T 1 (Fig. 8). Nielsen (1949) also mentions variability concerning this character in B. groenlandica. Crown ornamentation is also subjected to wear. The dentitions of the rostropremaxilla, maxilla, and dentary
are supplemented by at least one row of macroscopic teeth on the prearticular, coronoid, ectopterygoid, and dermopalatine, respectively, and myriads of minute teeth also cover the lingual surfaces of the prearticular, ectopterygoid, entopterygoid, parasphenoid, and the bones of the branchial arches (Stensiö, 1921; Nielsen, 1949; Lehman, 1952; Bürgin and Furrer, 1992; Romano and Brinkmann, 2009; this study). All species are characterized by a strong dentition, which together with the weakly developed bones of the operculogular series, suggest that Birgeria was a ram feeder (contra Lombardo and Tintori, 2005; Tintori et al., 2014a), meaning that prey was chased and bitten rather than engulfed through current action (Schaeffer and Rosen, 1961). The coronoid process on the mandible— developed in convergence to holosteans—reduced torque on the jaw joint (Schaeffer and Rosen, 1961). Birgeria is often allied with saurichthyids and the
Acipenseriformes (sturgeons and paddlefish), even though they share only a few characters (e.g., reduced squamation, posterior elongation of the parasphenoid; Bemis et al., 1997), some of which are also present in other actinopterygians. The suggested close affiliation goes mainly back to the ‘Stockholm school’ (Schultze, 2009), whose influential works repeatedly high- lighted similarities between Birgeria and Acipenseriformes, some of which were later called into question (e.g., nerve sac groups, Ørvig, 1978). Jessen (1972) also doubted a close affinity due to differences in the pectoral girdle skeleton, and according to Coates’ (1999) cladistic analyses, Birgeria is resolved as closely related to Acipenser only if endocranial characters are omitted. A close relationship among Birgeria, Saurichthys, and Acipenseriformes was, nonetheless, recovered in the cladistic analysis of Gardiner et al. (2005). Nielsen (1949), among others, compared the peculiar ray-
like elements posteroventral to the operculum of B. groenlandica with the lobate suboperculum of Polyodon. However, as pointed out by Romano and Brinkmann (2009), only the anteriormost subopercular ray ofNielsen (1949) borders on the operculum, and the same condition is also seen in B. nielseni, B. stensioei,and
B.americana n. sp. (Beltan, 1980;Romano andBrinkmann, 2009; Fig. 3). Although Nielsen (1949) stated that the rays are
proximally fused in B. groenlandica, like in Polyodon (e.g., Bemis et al., 1997), fusion is not evident in our material. The present study supports the view that the ‘suboperculum’ of Nielsen (1949) is a composite element and that it is not homologous with the suboperculum of the American paddlefish. The homology of the slender suboperculum of Birgeria with the suboperculum of other actinopterygians requires further study; a homology with the ‘accessory operculum’ of early ray-fins (e.g., Cheirolepis) would also be possible.
Saurichthys Agassiz, 1834.—Saurichthys is known from Triassic sites around the world, both marine and freshwater, and with over forty named species (Kogan and Romano, 2016a and references therein), it is much more speciose than
Birgeria.After its first appearance in the latest Permian, Saurichthys rapidly reached global distribution and high species richness during the Early–Middle Triassic, but later became less diverse and geo- graphically more restricted (Mutter et al., 2008; Romano et al., 2012). Within the United States, Saurichthys has previously been described from the early late Smithian Anasibirites beds west of Georgetown, Bear Lake County, Idaho (Romano et al., 2012; a second skull, PIMUZ A/I 4621, was found in 2013, and a cranial fragment, NMMNH P-77359, in 2015 by JJ). Saurichthys was also described from the Middle Triassic of Pershing County, Nevada (Sander et al., 1994; Rieppel et al., 1996). Three-dimensionally preserved skulls or skull fragments of
Saurichthys (preserved as either body fossils or external molds) are frequently found in strata of Early Triassic age (Stensiö, 1925; Lehman, 1952; Schaeffer and Mangus, 1976; Beltan and Janvier, 1978; Minikh, 1981, 1982; Mutter et al., 2008; Romano et al., 2012; Kogan and Romano, 2016a; this study) or Middle Triassic age (Frech, 1903–1908; Hennig, 1909; Beltan et al., 1979; Rieppel, 1985; Wu et al., 2015), but are seemingly rare in younger deposits. Although saurichthyid crania contain some diagnostic features (e.g., Romano et al., 2012; Werneburg et al., 2014), species are predominantly differentiated by postcranial characters (e.g., squamation or fin segmentation pattern, morphology of the axial skeleton; e.g., Rieppel, 1985; Mutter et al., 2008; Tintori, 2013; Tintori et al., 2014b; Maxwell et al., 2015; Kogan and Romano, 2016a, b). Moreover, comparisons of dermal skull bone patterns between species are complicated due to the fusion of bones during ontogeny. However, PIMUZ A/I 4397 from the latest Smithian of Palomino Ridge shows some features useful for comparison, such as the elongate rostrum, the anterior extension of the lachrymal, or the distinct lateral flange of the dermopterotic. In addition to Saurichthys sp. from Palomino Ridge,
a dorsoventrally and anteroposteriorly expanded lateral flange on the dermopterotic is found in S. toxolepis Mutter, Cartanyà, and Basaraba, 2008 from the Early Triassic of Canada, S. obrutchevi Minikh, 1981 from the Early Triassic of Russia, and some specimens ascribed to S. wimani Woodward, 1912, S. ornatus Stensiö, 1925, and S. elongatus Stensiö, 1925 from the Smithian of Spitsbergen. The same condition is possibly also seen in S. cf. elongatus from the early late Smithian of Idaho (Romano et al., 2012). In contrast, the lateral flange of the dermopterotic is more reduced in other Early Triassic species, such as S. madagascariensis Piveteau, 1945, S. cf. ornatus from Greenland, and S. wimani (Stensiö, 1925;
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