JAW MORPHOLOGY IN THE LOWER CRETACEOUS HYBODONT SHARK TRIBODUS LIMAE FROM BRAZIL

The visceral skeleton (including a complete mandibular and hyoid arch and branchial arches) and teeth of the Lower Cretaceous hybodont shark Tribodus limae is described based on well-preserved fossil material, and its jaw suspension and musculature are reconstructed (representing the first reconstruction of jaw musculature in a hybodont shark). The jaw suspension of Tribodus is similar to batoids and advanced galeomorphs in lacking direct cranio-palatine articulations and having skeletal jaw support by the hyoid arch alone (unlike most other hybodonts), but differs from batoids in that the hyoid arch is intact. The jaws are short anteriorly, as in the hybodonts Asteracanthus and Lonchidion, and were connected symphysially but not fused. CT scanning reveals the presence of supportive endochondral struts in force-bearing regions of the jaws, representing the first report of these structures in a fossil elasmobranch. Five branchial arches were present, of which pharyngobranchial, epibranchial, and ceratobranchial elements were observed (although hypobranchials and basibranchials were presumably also present). A pharyngobranchial blade is present as in some other hybodonts (e.g., Lissodus) and extant galeomorphs (e.g., Heterodontus), and the posteriormost pharyngobranchials are unfused. Tribodus is considered durophagous, based on presence of endochondral struts and a weakly heterodont monognathic pavement dentition of flattened hexagonal teeth, as in extant myliobatoid rays. SEM examination shows that teeth of Tribodus are anaulacorhize with a double layer of single crystallite enameloid (SCE), and confirms the presence of columnar osteodentine, supporting the previous placement of Tribodus as the sister group to Asteracanthus.