Romano et al.—Early Triassic Fishes of Elko County (Nevada)


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it consists of minute teeth (odontodes; Ørvig, 1978). The internal lamina extends from the rostral end of the maxilla until the level of the posterior end of the dentigerous margin of the dentary (Fig. 3). Lower jaw.—The mandible is largely complete, with only


the anteriormost part missing (Fig. 3). Most of the lateral surface is weathered, meaning that the margins between the dentary, angular, and surangular represent the medial ones (the dentary usually covers a large area of the angular laterally; e.g., Romano and Brinkmann, 2009). The dentary is composed of an external plate as well as a


dorsal and a ventral internal lamina. The external plate is bounded by a gently convex ventral margin and a nearly straight, tooth-bearing dorsal border. The dorsal internal lamina runs close to the upper confinement of the external plate, whereas the ventral internal lamina parallels the lower margin of the bone. The angular forms the posterior and posteroventral borders of the lower jaw. This bone, too, is composed of an external plate and a large internal lamina. The plate-like part of the angular is confined by a long, convex border ventrally, and by an S-shaped margin posteriorly, which is dorsally concave and ventrally convex. The posterior and ventral margins run suborthogonally and together form a rounded posteroventral corner on the mandible. The suture between the external plate of the dentary and the surangular runs obliquely from anterodorsal to posteroventral, except in the most dorsal segment; there, the posterior margin of the dentary forms a recess, which houses the pointed anterior part of the surangular (Fig. 3). The upper portion of the boundary between the external plates of the angular and dentary is vertical, whereas the lower portion is distinctly inclined, running from posterodorsal to anteroventral. The wedge-shaped surangular exhibits a coronoid process (Fig. 3), without contribution of the dentary. The mandibular sensory canal traverses the external plate of the angular near its caudal and ventral borders and continues through the external plate of the dentary near its ventral margin, probably along the base of the ventral internal lamina. The conspicuous internal lamina of the angular projects


Figure 4. Birgeria americana n. sp. (NMMNH P-66225, holotype) from the Smithian of the Winecup Ranch, Elko County, Nevada, USA. (1) Tentative restoration of the skull of P-66225; (2) close-up view (position indicated in 4.1) of the dentition (above), and drawing thereof (below); the large lingual teeth growing on the internal lamina of the maxilla and the dorsal internal lamina of the dentary, respectively, are highlighted in dark gray; teeth of the labial row (white arrows) and intermediate row are colored white; (3) close-up view of a lingual tooth of the maxilla (position shown in 4.1) illustrating the surface ornamentation of the base and acrodin cap (photographed using a Leica MZ16F camera mounted on a stereomicroscope, contrast enhanced with ammonium chloride); the white arrow in 4.3 points to the demarcation between the tooth base and the acrodin cap; anterior is right (1–3). Scale bars = 10mm (total) (2), or 5mm (total) (3).


from the medial side of the plate-like part (Fig. 3). The base of this internal lamina follows the ventral and posterior borders of the external plate, but is offset with regard to these margins. In the posterior region of the lower jaw, the internal lamina is oriented mediocaudally, forming an obtuse angle with the external plate. In contrast, the external plate and internal lamina run suborthogonally within the ventral part of the angular. The ventral component of the medial lamina is lateromedially less broad than the posterodorsal portion of the lamina. The posterior part of the lamina becomes successively broader from the posteroventral angle of the angular to the level of the jaw joint, forming a sizeable posteromedial projection at the caudal end of the mandible (Fig. 3). The internal lamina is also curved medially. Dentition.—Macroscopic teeth are developed on the dentary


and the maxilla, whereas the surangular lacks such teeth. The dentition of the maxilla and dentary consists of conical teeth that are arranged in three longitudinal rows: a lingual, an intermediate, and a labial one. The lingual row consists of fairly large, stout teeth growing on the internal lamina of the maxilla, and the dorsal internal lamina of the dentary, respectively. In the


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