TEETH AND TAXONOMY; CONJUNCTION AND CHONDRENCHELYS (TRAQUAIR, 1888)

Chondrichthyans figure prominently in studies of gnathostome evolution; increasingly so in the fields of evolutionary developmental and molecular evolution. However, parallel and largely independent treatments of data drawn from the numerous records of isolated teeth, on the one hand, and from more rarely preserved skeletal remains, on the other, create problems when attempting to understand early diversity. Taking advantage of two nearly complete specimens from Mumbie Quarry, Scotland, UK and a previously misidentified specimen from the Hunterian Museum collections, we redescribe the dental anatomy of Chondrenchelys, a Mississippian (Viséan) holocephalan. A more complete understanding of the morphology of early taxa such as Chondrenchelys provides a new perspective on the evolution of the specialized dentitions of modern chimaeroids. In Chondrenchelys, the palatal dentition consists of a single pair of large, upper posterior toothplates and two pairs of smaller, anterior toothplates. The mandibular dentition consists of one pair of large posterior toothplates and a single pair of anterior toothplates. Remarkably, all three new specimens preserve a previously undescribed extra-oral arcade of teeth, in conjunction with these toothplates. Furthermore, these “accessory teeth” are similar in gross morphology and histology to those described in early chondrichthyans such as Debeerius and petalodonts. Re-examination of several classical Chondrenchelys specimens reveals further, and previously unrecognized, poorly preserved examples of this heterodont suite of teeth. It is also noteworthy that upper and lower posterior toothplates in Chondrenchelys show an ontogenetic shift in the orientation of tooth replacement, such that early replacement proceeded linguo -labially, as in Helodus and chochliodonts, but shifted to an antero-posteriorly directed sequence, as in modern chimaeroids.