Romano et al.—Early Triassic Fishes of Elko County (Nevada)
moderately posteriorly inclined, the caudally following ones are more distinctly so. Suborbital and infraorbital series.—The bones of the sub-
orbital series are preserved in situ, albeit mostly in a poor state of preservation. Their morphology and arrangement agree with those of other species of Birgeria, meaning that they are elongate, obliquely and serially arranged bones with a broad posterodorsal part and a slender anteroventral portion. About three suborbitals are visible (Fig. 3), the posterodorsal parts of which are still intact and nested below the anterior process of the preoperculum. The anteroventral parts of the two, posteriormost suborbitals of P-66225 are fragmentarily preserved, but it is evident that they extended rostrad until the level of the third lingual tooth of the maxilla (which would mark the level of the posterior border of the orbital opening; Fig. 4.1). A subtriangular bone fragment located anterodorsal to the
suborbitals (Fig. 3) may belong to the dermosphenotic (this bone is usually subdivided into two or more elements in Birgeria, see Stensiö, 1932; Romano and Brinkmann, 2009). Other elements of the circumorbital series and the sclerotic ring are perhaps visible at the rostral end of the fossil, but they are too poorly preserved for accurate description. Splanchnocranium.—The quadratum and articular are pre-
served in situ (Fig. 3). The quadratum is partly covered by the maxilla laterally, but the articulation condyle is exposed. The articular is mostly hidden by the angular and only its dorsalmost portion can be seen. The hyomandibula is only partly visible, being exposedwhere the preoperculumis damaged (Fig. 3). Some additional bones situated just medial to the postmandibular branchiostegal rays possibly belong to the hyomandibula or the symplectic, but they are not well exposed. The dermohyal is preserved in situ between the preoperculum, the operculum, and the antoperculum (Fig. 3). This bone is elongate and wedge-like, with a pointed posterior end, and a rounded, club-shaped anterior termination. The dermohyal is marked by a longitudinal crest running along its dorsal side. A few elements of the branchial skeleton are partly exposed (Fig. 3). For instance, a portion of an epibranchial is probably visible medially to the operculum. Asegment of a possible ceratobranchial is seen at the posterior end of the concretion. Several rod-like bones preserved anteroventral to the lower jaw likely belong to the hyoid arch or the branchial arches.
Pectoral girdle.—The right cleithrum is the only bone of the
shoulder girdle that is visible (Fig. 3). Only the large lower branch of the cleithrum is preserved, the anterodorsal portion of which could not be prepared. The cleithrum fragment has a roughly triangular outline, being confined by a long, gently convex ventral border, and a long dorsal margin, being deeply concave in its caudalmost segment. The posterodorsal corner of the cleithrum fragment tapers, marking the transition to the missing upper branch of this bone. The external surface of the cleithrum is divided into a large lower portion, facing laterally, and a subhorizontally oriented upper portion.
Etymology.—The species name refers to its provenance.
Remarks.—The morphology of NMMNH P-66225 agrees well with that of the Triassic genus Birgeria Stensiö, 1919. Species of Birgeria are essentially differentiated by cranial features, but
1033
also by a few postcranial traits, such as the arrangement pattern of the dorsal fin pterygiophores (Romano and Brinkmann, 2009). Diagnostic characters justifying the erection of a new species for the Nevada material are the less reduced dermal gill cover (i.e., presence of an antoperculum), very elongate suboperculum, and relatively numerous postmandibular branchiostegal rays, including a rudimentary ray situated within the gap of the branchiostegal series (see Discussion). Based on the length of the preserved part (26 cm) of P-66225,
corresponding to ~70–75% of the snout to shoulder girdle length in birgeriids with elongate postorbital skull portions (e.g., Nielsen, 1949), a snout to shoulder girdle length of ~34.5–37cm for P-66225 is estimated. In large species of Birgeria, the skull plus shoulder girdle length usually corresponds to ~20% of the total length (see Liu et al., 2006; Romano and Brinkmann, 2009), suggesting a total length of ~172–185 cm for P-66225.
Birgeria sp. Figure 5
Occurrence.—From upper lower Smithian to lower upper Smithian strata (Thaynes Group), ~2.75km south-southeast of the Winecup Ranch, east-central Elko County, Nevada, USA (Fig. 1).
Description.—NMMNH P-77117 (Fig. 5) is a cranial fragment with a length of 17 cm. The fossil is seen in left aspect and preserved in a limestone nodule as part (P-77117 a) and counterpart (P-77117 b), with the fracture surface going through the bones of the left cheek. The specimen is associated with bivalves, one of which resembles Crittendenia (personal communication to CR, M. Hautmann, 2016). Palatoquadratum and its dermal bones.—Several upper jaw
bones are exposed: the maxilla, dermopalatine, entopterygoid, ectopterygoid, and ‘dermometapterygoid’ (Fig. 5). The quad- ratum is probably present, though not well visible. The maxilla is only fragmentarily preserved, with portions of the postorbital blade and the internal lamina discernible. The dermopalatine is an elongate, low element situated in the anteroventral region of the upper jaw. This bone exhibits its maximum depth rostrally. The height of this element tapers caudally. Only one dermopalatine is visible. Dorsal to the dermopalatine is the entopterygoid, of which only the rostral portion is preserved. The ectopterygoid lies posterior to the dermopalatine and posteroventral to the entopterygoid. The ectopterygoid is a large, rostrocaudally elongate element that extends posteriorly until the jaw joint area. The boundary between the ectopterygoid and the entopterygoid runs obliquely from anteroventral to posterodorsal. Another, albeit incompletely preserved, element situated posterior to the entopterygoid and posterodorsal to the ectopterygoid, represents the ‘dermometapterygoid’ (termi- nology of Nielsen, 1949; but see Arratia and Schultze, 1991). Lower jaw.—The mandible is incompletely preserved. Its
anterior tip is missing and the lateral surface is broken off (Fig. 5). The exposed margins between the dentary, angular, and surangular represent the medial ones, and the morphologies of these bones are similar to those ofNMMNHP-66225 (holotype of B. americana n. sp.; Fig. 3). The angular of P-77117 (Fig. 5) is equipped with a well-developed internal lamina. The dorsal and
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