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1032


Journal of Paleontology 91(5):1025–1046


rostral part of these bones, the large lingual teeth are fully exposed (but partly weathered), whereas farther posteriorly only the apical parts are visible (Figs. 3, 4). The lingual teeth are aligned almost perfectly, forming a palisade (although gaps may occur where teeth fell out in vivo). The largest lingual teeth of the maxilla are located near the rostral end of the bone. From there, they slowly decrease in size in caudal direction. Conversely, the lingual teeth of the dentary slightly increase in height in posterior direction. The posteriormost lingual teeth of the dentary could not be reached with the air pen. The intermediate series of teeth line the ventral margin


of the external plate of the maxilla, and the dorsal margin of the external plate of the dentary, respectively (they are lost in the anterior parts of both of these bones due to weathering). The teeth of the intermediate row are always smaller than the neighboring teeth of the lingual row. They are irregularly distributed, but consecutive teeth are mostly separated by wide interspaces. The intermediate teeth of themaxilla decrease in size posteriorly, becoming minute at the level of the caudal end of the dentigerous margin of the dentary. Farther posteriorly, within the convex part of the maxilla that laterally covers the lower jaw, the intermediate teeth of the maxilla are larger again (Fig. 3). The intermediate teeth in the anterior segment of the dentary are about the same size as their antagonists on the maxilla, whereas those in the posterior segment of this bone are distinctly larger (Fig. 4). Close to the caudal end of the dentigerous margin of the dentary, the intermediate teeth rapidly decrease in size. The teeth of the labial row are smaller than those of the


intermediate row (Fig. 4). Like the intermediate teeth, the labial teeth are irregularly distributed along the jaw margins. They are quite frequent in the posterior part of the dentary, but a few labial teeth are also intermittently developed on the maxilla, especially in the anterior segment of this bone. The labial teeth are bordered by minute teeth laterally (odontodes; Ørvig, 1978), forming the ornamentation of the maxilla and the dentary. Whereas the apicobasal axes of the lingual teeth are


inclined medially (and gently caudally), those of the inter- mediate teeth are more or less oriented dorsoventrally (except in the caudal part of the maxilla, where they are medially tending). The apicobasal axes of the labial teeth are inclined laterally, whereas the lateral leaning varies between individual labial teeth. The apicobasal axis of some labial teeth is nearly perpendicular to that of the lingual teeth (Fig. 4.2). The teeth of the three longitudinal rows are all linguolabially compressed and curved medially. All teeth comprise an acrodin cap (Ørvig, 1978) and a high


base (Fig. 4.2, 4.3). The acrodin cap usually makes up only a small part of the total tooth height in lingual teeth (probably about one-fifth to one-sixth), whereas in teeth of the intermediate and labial rows the acrodin cap measures at least one-third of the total height. The surface of the acrodin cap ismostly smooth, except for its basal segment,which shows a fewwidely spaced,meandering, apicobasally oriented ridges (Fig. 4.3). Anterior and posterior cutting edges are present on teeth of all three rows. The surface of the tooth base, where preserved, is ornamented with very fine, subvertical striae. The striae are close-set and frequently anastomose (Fig. 4.3).


Preoperculum and operculogular series.—The incompletely


preserved preoperculum (Fig. 3) adjoins the postorbital plate of the maxilla dorsally and posteriorly. The boomerang-shaped preoperculum is composed of a long anterior shank and a short posteroventral branch, which meet in an obtuse angle. The rostral margin of the anterior shank is deeply concave and the anterodorsal part of the bone protrudes rostrally. The poster- oventral branch of the preoperculum adjoins the straight upper part of the posterior margin of the maxilla, but does not reach farther ventrally, thus exposing the quadratum laterally. The caudal margin of the preoperculum is convex. A section of the preopercular sensory canal is visible in the lower shank. The canal traverses the bone centrally (Fig. 3). The bones of the operculogular series (Fig. 3) are arranged


in the characteristic manner for Birgeria. The operculum is situated dorsally to the anterior shank of the preoperculum. It is tilted in a way that its external side now faces dorsally. The operculum is incompletely preserved, but had an oblong, ovoid outline with acute anterior and posterior ends. Rostral to the operculum is another small, plate-like bone, interpreted as the antoperculum. Caudally, the operculum reaches to the knee of the preoperculum, where it is still articulated with the suboperculum (sensu Romano and Brinkmann, 2009). The vertically arranged suboperculum is elongate and slender, being morphologically indistinguishable from the posteriorly adjoin- ing branchiostegal rays. The suboperculum has a triangular outline and is confined by very long anterior and posterior borders and a short dorsal margin contacting the operculum. The lateral side of the suboperculum is convex. The branchiostegal series is divided into a postmandibular


and a submandibular series, which are separated by a gap (Fig. 3). Five postmandibular radii branchiostegii are preserved ventrocaudal to the suboperculum. The postmandibular branchiostegals are coalesced neither with each other nor with the suboperculum. The postmandibular rays are slender, elongate elements. The first postmandibular branchiostegal ray is dorsoventrally almost as long as the suboperculum. The caudally following postmandibular rays are much shorter and their length consecutively decreases towards the last of these bones. Moreover, whereas the first and second rays are dorsoventrally oriented, like the suboperculum, the subsequent


ones are more and more caudally inclined, with the fourth and fifth ray being somewhat anteroposteriorly arranged. Each ray has an acute ventral/anterior end and a rounded dorsal/posterior termination (except for the posteriormost of these elements, which has a pointed caudal end). Their lateral surface is convex. In addition to the five postmandibular rays situated behind the suboperculum, a small, thin, sixth postmandibular radiumbranchiostegium is seen isolated about halfway between the postmandibular and the submandibular series (Fig. 3). Mediolaterally, this ray is situated at the same level as the other branchiostegals. Nine slender submandibular branchiostegal rays are preserved


medioventral to the lower jaw (Fig. 3), though their original number was higher. Each of these elements is plate-like and has a quadrangular outline, whereas their anterolateral corner protrudes rostrad. The anteroposterior width of these bones subsequently decreases from the first (anterior) to the last one. Furthermore, while the anterior submandibular branchiostegals are only


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