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Journal of Paleontology 91(5):1025–1046
characterized, for instance, by the possession of an extended, cleaver-shaped maxilla (reflecting the large gape size and the far forward location of the eyes), the arrangement of jaw teeth in parallel rows, the remote position of the dorsal fin, and the heterocercal caudal fin (e.g.,Aldinger, 1931, 1937;Nielsen, 1949; Lehman, 1952). Likemany other early actinopterygians, Birgeria possesses a dermohyal (Romano and Brinkmann, 2009; this study), but the presence of this bone remained undetected in earlier descriptions. In B. americana n. sp., B. nielseni, B. aldingeri,and B. stensioei, the dorsal side of the dermohyal is marked by a longitudinal crest (Stensiö, 1932; Schwarz, 1970; Beltan, 1980; Romano and Brinkmann, 2009; this study). Apart from the aforementioned plesiomorphic traits,
Birgeria stands out by a set of derived features, such as the advanced reduction of the squamation, the presence of an unpaired rostropremaxilla, the posterior extension of the frontals medially separating the parietals, or the large internal lamina on the angular (e.g., Stensiö, 1921, 1932; Nielsen, 1949; Lehman, 1952; Schwarz, 1970; Jin, 2001; Liu et al., 2006; Romano and Brinkmann, 2009). A further, autapomorphic condition is the arrangement of the elements of the operculogular series. The gill cover of Birgeria consists of a low, anteroposteriorly elongated operculum, located dorsal to the maxillary blade, a narrow, vertically arranged suboperculum (sensu Romano and Brinkmann, 2009; i.e., first ray of the ‘suboperculum’ of Nielsen, 1949; Beltan, 1980), and a branchiostegal series that is divided into separate postmandiular and submandibular series (e.g., Nielsen, 1949; Lehman, 1952; Beltan, 1980; Romano and Brinkmann, 2009). These highly specialized modifications are attributable to the extreme obliquity of the suspensorium (Aldinger, 1937; Schwarz, 1970), related with the tremendous enlargement of the gape size (Nielsen, 1949), pushing the jaw joint close to the pectoral girdle. Interspecific variation within the operculogular series
concerns chiefly the number of postmandibular branchiostegal rays (sensu Romano and Brinkmann, 2009; i.e., second to last ray of the ‘suboperculum’ of Nielsen, 1949; Beltan, 1980). Whereas three to five postmandibular branchiostegals are
developed in B. groenlandica (cf. Nielsen, 1949) and four to five in B. nielseni (cf. Beltan, 1980), none or at most one is usually preserved in B. stensioei (Romano and Brinkmann, 2009). In these species, all postmandibular branchiostegals are vertically arranged, like the suboperculum, whereas in B. americana n. sp. the posterior ones are caudally tending (Fig. 3). The operculogular series of B. americana n. sp. also displays a rudimentary branchiostegal ray between the main postmandibular and the submandibular series, which has thus far not been observed in other species. In addition, an antoperculum is developed in the Nevada taxon, a bone that is present in several other early actinopterygians, but until now was unknown in Birgeria (e.g., Aldinger, 1937; Lehman, 1952; Gardiner and Schaeffer, 1989). The gill cover of B. americana n. sp. is less reduced compared to other birgeriids. Other interspecific differences within the skull concern,
among others, the outlines of the parasphenoid and maxilla (Boni, 1937; Schwarz, 1970). Many Early Triassic birgeriids possess a maxilla with a relatively low, elongate postorbital blade (e.g., B. groenlandica, B. aldingeri, B. cf. aldingeri, and B. americana n. sp.). Furthermore, the anterior margin of the
postorbital blade is much more oblique (e.g., B. groenlandica, B. aldingeri, B. cf. aldingeri, B. nielseni, and B. americana n. sp.) than, for instance, in the Middle Triassic B. mougeoti or B. stensioei (Agassiz, 1834–1843; Stensiö, 1919, 1932; Aldinger, 1931; Nielsen, 1949; Schwarz, 1970; Romano and Brinkmann, 2009; Fig. 3). Additionally, the dorsal and ventral margins of the postorbital blade are subparallel in Middle and Late Triassic species (B. mougeoti, B. stensioei, and B. acuminata) and some Early Triassic species (B. nielseni and Birgeria sp. from British Columbia; Lehman, 1952; Schaeffer and Mangus, 1976), whereas in most Early Triassic forms they clearly converge rostrad (e.g., B. americana n. sp., B. groenlandica, B. aldingeri, B. cf. aldingeri, Birgeria sp. from Russia, and some specimens referred to B. nielseni; Stensiö, 1919, 1932; Boni, 1937; Nielsen, 1949; Lehman, 1952; Berg et al., 1964; Schwarz, 1970; Beltan, 1980; Romano and Brinkmann, 2009; Fig. 3). In general, Griesbachian–Smithian birgeriids have low, elongate skulls, whereas Middle–Late Triassic species possess higher, shorter crania (see Schwarz, 1970). A reduction of the postorbital skull length has also been documented in Saurichthys during the Early–Middle Triassic transition (Mutter et al., 2008; Romano et al., 2012), being a possible case of parallelism between these two predatory actinopterygians. Some comments are necessary concerning the number of
tooth rows and the size distribution pattern of teeth on the maxilla and dentary. Stensiö (1921, 1932), Nielsen (1949), Lehman (1952), Savage and Large (1966), and Bürgin and Furrer (1992) described two rows of teeth on the maxilla and/or dentary of B. groenlandica, B. nielseni, B. aldingeri,
B.cf. aldingeri, and B. acuminata, whereas Schwarz (1970) observed three rows in B. stensioei. Three discrete rows of teeth are also developed in B. americana n. sp. (Fig. 4) and in the poorly known Middle?– Late Triassic B.? costata (Münster, 1839), whose generic attribution has been questioned (Boni, 1937; Bürgin and Furrer, 1993). Based on unpublished computer tomography generated images of the holotype of B. groenlandica (ZMUC VP 3176; personal communication to CR, T. Argyriou, 2016), it is evident that most of the large lingual teeth are not exposed on the surface, but two rows of teeth are confirmed here. Regarding B. aldingeri, only the lateral imprints of the intermediate and labial teeth of the maxilla are visible, with the internal lamina and lingual teeth not preserved (personal observation, C. Romano, 2016; Fig. 8; Stensiö, 1932), thus three rather than two rows are developed in this species. Notably, only some Early Triassic birgeriids (B. aldingeri
and B. americana n. sp.) possess intermediate teeth that are variable in size but predominantly high, and chiefly widely spaced, whereas in the Middle Triassic B. stensioei and the Late Triassic B. acuminata and Birgeria sp. they are close-set and of relatively small, uniform size throughout the length of the jaw (Woodward, 1889; Boni, 1937; Savage and Large, 1966; Schwarz, 1970; Bürgin and Furrer, 1992; Lombardo and Tintori, 2005; Fig. 8). The labial teeth are small but distinct in B. aldingeri and B. americana n. sp., whereas in stratigraphi- cally younger taxa they are either very small or absent (Fig. 8), with the exception of the problematic B.? costata (Bürgin and Furrer, 1993). Ørvig (1978), confirming observations by previous authors (e.g., Stensiö, 1919), showed that the
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