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 Grou p, 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 ellefse ni's cranial, postcranial, and suspensorial characters identify this fish a s a paraselachian, an early chondrichthyan with a morphology intermediate t o the chimaeroid and selachian plans. They also support the division of Cho ndrichthyes into the subclasses Elasmobranchii and Euchondrocephali (Parase lachii + Holocephalimorpha). Details of the anatomy of D. ellefseni are rev iewed in light of recent advances in understanding vertebrate splanchnocran ial development and, thus, permit a discussion of historically problematic craniate features, including labial cartilages and the nature of the mandib ular arch relative to hyoid and branchial arches. Developmental and evoluti onary considerations of these characters are consistent with an embryonic b ody plan shared by both lampreys and gnathostomes. Debeerius ellefseni's su spensorium corresponds to the plesiomorphous gnathostome condition theorize d by DeBeer and Moy-Thomas in 1935. The description of this autodiastylic c ondition is clarified to include observations of the hyoid arch, which is c omplete with a pharyngohyal and provides support for the primary opercular valve. The confirmation of an autodiastylic suspensorium requires a reexami nation of the commonly accepted paradigm for jaw evolution. The selachian, chimaeroid, and actinopterygian conditions are all derivable from this ples iomorphous state; the placoderm and sarcopterygian conditions are related a nd probably similarly derived. The comparable osteichthyan suspensorium is best represented by the suspensorial condition of coelacanths. (C) 2000 Wil ey-Liss, Inc.