A bristle-nosed Jurassic ray-finned fish (Actinopterygii) bears true dermal odontodes on its snout

Abstract

Comparison of shared morphological characteristics between living and extinct species is essential to reconstructing repeated phenotypic evolution in deep time. For example, the extinct Triassic–Jurassic redfieldiid ray-finned fishes possess tubercles on their snout which resemble the enlarged dermal odontodes that give some extant ray-finned fishes “bristly” snouts. The three-dimensional anatomy of redfieldiid snout tubercle-bearing bones is typically obscured in fossils, leaving the anatomical identity and homology of these snout tubercles uncertain. We describe new aspects of redfieldiid snout tubercles based on a recently collected, exceptionally well-preserved specimen (VMNH 129827) of †Redfieldius gracilis from lacustrine deposits of the Early Jurassic (Hettangian, ~201 million years old) Waterfall Formation of the Culpeper Basin (Newark Supergroup) of Virginia. We determine that the body and skull of †Redfieldius is covered with dermal odontodes, which are extended into bristle-like tubercles on the snout and orbit. The phylogenetic position of †Redfieldius outside of the teleostean total group indicates that redfieldiid snout tubercles evolved convergently from those in living fishes, such as the bristlenose catfish (Ancistrus) and the denticle herring (Denticeps). †Redfieldius does not have strong morphological overlap with extant snout tubercle-bearing ray-finned fishes, limiting inference of their ecological function. However, we posit that the snout tubercles of †Redfieldius, in concert with a covering of pointed dermal odontodes on the scales, may have served a defensive function. The enlarged snout tubercles in †Redfieldius are part of an emerging pattern of increasingly specialized tooth morphology in Late Triassic–Early Jurassic ray-finned fishes that mirrors living ray-finned fishes.