Diverse stem-chondrichthyan oral structures and evidence for an independently acquired acanthodid dentition

Abstract

The teeth of sharks famously form a series of transversely organized files with a conveyor-belt replacement that are borne directly on the jaw cartilages, in contrast to the dermal plate-borne dentition of bony fishes that undergoes site-specific replacement. A major obstacle in understanding how this system evolved is the poorly understood relationships of the earliest chondrichthyans and the profusion of morphologically and terminologically diverse bones, cartilages, splints and whorls that they possess. Here, we use tomographic methods to investigate mandibular structures in several early branching ‘acanthodian’-grade stem-chondrichthyans. We show that the dentigerous jaw bones of disparate genera of ischnacanthids are united by a common construction, being growing bones with non-shedding dentition. Mandibular splints, which support the ventro-lateral edge of the Meckel's cartilage in some taxa, are formed from dermal bone and may be an acanthodid synapomorphy. We demonstrate that the teeth of Acanthodopsis are borne directly on the mandibular cartilage and that this taxon is deeply nested within an edentulous radiation, representing an unexpected independent origin of teeth. Many or even all of the range of unusual oral structures may be apomorphic, but they should nonetheless be considered when building hypotheses of tooth and jaw evolution, both in chondrichthyans and more broadly.

The pharynx of the iconic stem-group chondrichthyanAcanthodes(Agassiz, 1833) revisited with micro computed tomography

“…2-4, 6C-F). A large lateral fossa provided an insertion site for the mandibular adductor while a ventrolateral groove would have carried the dermal mandibular splint, which matches the description of Dearden & Giles (2021). The mandibles are separate, but the expanded anterior tip suggests a well-developed symphysis, which may be a character uniting some stem-group chondrichthyans (Dearden & Giles, 2021).…”

“…This includes in aspects of the specialised mandibular apparatus: the expanded anterior end of the Meckel's cartilage appears to be present in Halimacanthodes (Burrow et al, 2012;Dearden & Giles, 2021), while a double-faceted palatoquadrate has been reported in Cheiracanthus (Miles, 1973a). Although Acanthodes is often characterised as the last gasp of the doomed acanthodians, it is perhaps better framed as the last member of a 100 million year-old lineage with a success based in an innovative feeding mechanism.…”

“…Despite there being numerous, well-studied fossils in museum collections preserving the pharyngeal skeleton of Acanthodes its anatomy remains uncertain. Interpretations of the pharynx of Acanthodes remain based almost entirely (but see (Dearden & Giles, 2021)) on acid-prepared moulds, studied by making casts either physically (Miles, 1973a) or more recently digitally (Brazeau & de Winter, 2015). While many detailed studies of these casts of A. confusus have been carried out, the mouldic nature of the specimens have resulted in interpretational ambiguities, further exacerbated by the segmented ossification pattern of the visceral skeleton (Reis, 1896;Dean, 1907;Watson, 1937;Miles, 1964Miles, , 1965Miles, , 1968Miles, , 1973aNelson, 1968;Jarvik, 1977)).…”

The skeletal completeness of the Palaeozoic chondrichthyan fossil record

“…Of these mineralized fragments, isolated teeth consist of layers of dentine and enameloid which are some of the hardest biological tissues, often resulting in the only remains left of a decayed and fragmented cartilaginous fish. Crownward chondrichthyans furthermore grow, shed and replace their teeth throughout their lifetime which may further complicate this bias [ 99 , 100 ]. Rates of replacement in fossil taxa are mostly unknown and suggested to have been slow [ 101 ] and there is good evidence of tooth retention throughout life in Palaeozoic cladodonts [ 99 ].…”