A unique hybodontiform skeleton provides novel insights into Mesozoic chondrichthyan life

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Abstract
Asteracanthuswas one of the most common Mesozoic hybodontiform chondrichthyans, given that remains traditionally referred to this genus have been reported almost worldwide from Middle Triassic to Late Cretaceous strata so far.Asteracanthuswas erected by Louis Agassiz for Late Jurassic fin spines with stellate tubercles. Later, Arthur Smith Woodward synonymizedStrophodus, originally introduced by Agassiz for distinctive crushing teeth of Triassic–Cretaceous age, withAsteracanthusbased on associated teeth and spines from the English Middle Jurassic. This taxonomic scheme has been accepted for more than 130 years until now, although articulated material has never been found. Here, we present a unique hybodontiform skeleton from the German Late Jurassic, displaying a striking combination of characters: tuberculate dorsal fin spines reminiscent ofAsteracanthus, and multicuspid teeth that markedly differ from the crushing teeth previously referred to this genus. Using qualitative and quantitative approaches, we compared its fin spines with those that were found in association with Agassiz'sStrophodusteeth, providing evidence thatAsteracanthusandStrophodusin fact represent two valid genera distinct from all other hybodontiforms. Morphological features that distinguish fin spines ofStrophodusfrom those of all other hybodontiforms include a straight anterior border and distally distributed posterior denticles. These observations led us to present an emended diagnosis forAsteracanthus. Dentally,Asteracanthusshows strong resemblance toHybodus, but it otherwise lacks a palatobasal process on the palatoquadrate. Therefore, and in the absence of any reliable phylogenetic framework, we recommend treatingAsteracanthusasincertae familiaeuntil hybodontiform interrelationships are resolved.

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