Debeerius ellefseni (Fam. Nov., Gen. Nov., Spec. Nov.), an autodiastylic chondrichthyan from the Mississippian bear gulch limestone of Montana (USA), the relationships of the chondrichthyes, and comments on gnathostome evolution.

Debeerius ellefseni is an autodiastylic, operculate chondrichthyan from the 320-million-year-old Bear Gulch limestone (Heath Formation, Big Snowy Group, Upper Chesterian) of Montana, USA. Cranial and postcranial morphologies show strong affinities to the holocephalan cochliodonts and Chimaeriformes. The heterodont dentition is, however, selachian in plan. Debeerius ellefseni's cranial, postcranial, and suspensorial characters identify this fish as a paraselachian, an early chondrichthyan with a morphology intermediate to the chimaeroid and selachian plans. They also support the division of Chondrichthyes into the subclasses Elasmobranchii and Euchondrocephali (Paraselachii + Holocephalimorpha). Details of the anatomy of D. ellefseni are reviewed in light of recent advances in understanding vertebrate splanchnocranial development and, thus, permit a discussion of historically problematic craniate features, including labial cartilages and the nature of the mandibular arch relative to hyoid and branchial arches. Developmental and evolutionary considerations of these characters are consistent with an embryonic body plan shared by both lampreys and gnathostomes. Debeerius ellefseni's suspensorium corresponds to the plesiomorphous gnathostome condition theorized by DeBeer and Moy-Thomas in 1935. The description of this autodiastylic condition is clarified to include observations of the hyoid arch, which is complete with a pharyngohyal and provides support for the primary opercular valve. The confirmation of an autodiastylic suspensorium requires a reexamination of the commonly accepted paradigm for jaw evolution. The selachian, chimaeroid, and actinopterygian conditions are all derivable from this plesiomorphous state; the placoderm and sarcopterygian conditions are related and probably similarly derived. The comparable osteichthyan suspensorium is best represented by the suspensorial condition of coelacanths. Copyright 2000 Wiley-Liss, Inc.