The visceral skeleton and jaw suspension in the durophagous hybodontid shark Tribodus limae from the Lower Cretaceous of Brazil

The visceral skeleton (including complete mandibular, hyoid, and branchial arches) and teeth of the Lower Cretaceous hybodontid shark Tribodus limae are described based on well preserved fossil material. Jaw suspension and musculature are reconstructed, representing the first reconstruction of jaw musculature in a hybodont. 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 an intact hyoid arch is present. As in Asteracanthus and Lonchidion, the jaws do not extend to the snout, and were connected symphysially but not fused. CT scanning reveals the presence of supportive 'trabecular cartilage' struts in force-bearing regions of the jaws, representing the first report of these structures in an extinct chondrichthyan. Five branchial arches are present, of which pharyngobranchial, epibranchial, and ceratobranchial elements are 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 "trabecular cartilage' struts and a weakly heterodont monognathic pavement dentition of flattened hexagonal teeth, as in extant myliobatoid rays. SEM examination shows that teeth of T. limae are anaulacorhize with a double layer of single crystallite enameloid (SCE), and confirms the presence of columnar osteodentine, as in other Acrodontidae.