Cobelodus enters the LRT, but not at the base of all sharks and ratfish

Carroll 1988 reported,
“the simple skull surrounded by enlarged gill elements apparently evolving to become jaws, hyoids and the hyomandibular arch.”

It would seem so, but toothless jaws appeared on another taxon, Chondrosteus, phylogenetically much earlier than in Cobelodus.”

Cobelodus aculeatus (Zangerl and Case 1976, originally Styptobasis aculeata Cope 1891; FMNH UF576; Pennsylvanian, Middle to Late Carboniferous; 2m) is a large, shark-like taxon originally considered a holocephalian. Here it nests with Falcatus as ‘the best known Paleozoic elasmobranch’ (ca 1976). Zangerl and Case note, “In the differentiation of the neurocranium and the visceral skeleton, Cobelodus presents a morphological condition not previously documented in any chondrichthyan, or, for that matter, any other fish.” The hyoid arch is not connected by ligaments in any with the mandibular arch. The authors considered this pirmitive, but reported, “Not everyone, nowadays, would agree that the aphetohyoidian condition in Cobelodus and its allies is primitive.” Large eyes suggest a deep sea niche. The pectoral fins have a 30cm trailing feeler supported by flexible cartilage. No anal fin is present. The pelvic fins are elongate.

References
Coates M, Gess R, Finarelli J, Criswell K and Tietjen K 2016. A symmoriiform chondrichthyan braincase and the origin of chimaeroid fishes. Nature. 541: 208–211.
Zangerl R 1981. Chondrichthyes I – Paleozoic Elasmobranchii. Handbook of Paleoichthyology. Stuttgart: Gustav Fischer Verlag. pp. i–iii, 1–115.
Zangerl R and Case GR 1976. Cobelodus aculeatus (Cope), an anacanthous shark from Pennsylvanian black shales of North America.
Zangerl R and Williams ME 1975. New evidence on the nature of the jaw suspension in Palaeozoic anacanthous sharks. Palaeontology, 18(2), 333–341.