A unique hybodontiform skeleton provides novel insights into Mesozoic chondrichthyan life

Abstract

Asteracanthus was 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. Asteracanthus was erected by Louis Agassiz for Late Jurassic fin spines with stellate tubercles. Later, Arthur Smith Woodward synonymized Strophodus, originally introduced by Agassiz for distinctive crushing teeth of Triassic-Cretaceous age, with Asteracanthus based 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 of Asteracanthus, 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's Strophodus teeth, providing evidence that Asteracanthus and Strophodus in fact represent two valid genera distinct from all other hybodontiforms. Morphological features that distinguish fin spines of Strophodus from those of all other hybodontiforms include a straight anterior border and distally distributed posterior denticles. These observations led us to present an emended diagnosis for Asteracanthus. Dentally, Asteracanthus shows strong resemblance to Hybodus, but it otherwise lacks a palatobasal process on the palatoquadrate. Therefore, and in the absence of any reliable phylogenetic framework, we recommend treating Asteracanthus as incertae familiae until hybodontiform interrelationships are resolved.

Durnonovariaodus maiseyi gen. et sp. nov., a new hybodontiform shark-like chondrichthyan from the Upper Jurassic Kimmeridge Clay Formation of England

“…The jaw suspension of Durnonovariaodus gen. nov. is therefore likely to have been hyostylic (sensu Lane & Maisey, 2012). This type of jaw suspension may have also been present in Asteracanthus, whose palatoquadrates otherwise lack a palatobasal process and instead display a deeply recessed dorso-medial articulation facet, presumably for the articulation with the ectethmoid process of the neurocranium (Stumpf et al, 2021). Tribodus is unique among hybodontiforms in having a jaw suspension reminiscent of the euhyostylic condition present in modern batomorphs (Maisey & de Carvalho, 1997;Lane & Maisey, 2012).…”

“…nov. with that of other hybodontiforms, particularly those that are known to have developed teeth of similar morphologies, the second section addresses similarities and differences in skeletal anatomy between the new taxon and better known hybodontiforms. Note that the genus Asteracanthus Agassiz, 1837 is here considered distinct from Strophodus Agassiz, 1838, following Stumpf et al (2021). In addition, the systematic position of Durnonovariaodus maiseyi gen. et sp.…”

“…Durnonovariaodus gen. nov. has costate dorsal fin spines, a condition shared with most other hybodontiforms (Maisey, 1978), except for Asteracanthus and Strophodus, which currently are the only known hybodontiforms for which dorsal fin spines with an ornamentation consisting of small to moderately well-developed, more or less regularly arranged tubercles can be unambiguously be confirmed (Stumpf et al, 2021), although this feature certainly was more widely distributed among hybodontiforms (cf. e.g., Werner, 1989;Case & Cappetta, 2004;Underwood & Cumbaa, 2010;Cicimurri, Ciampalgio & Runyon, 2014).…”

“…The shagreen covering the body of Tribodus limae Brito & Ferreira, 1989 is composed of similarly developed denticles, but also includes smaller denticles of different morphology, which co-occur intercalated between the larger ones, resulting in a unique two-size squamation pattern otherwise unknown in hybodontiforms (Maisey & Denton, 2016). Dermal denticles of Asteracanthus ornatissimus have a circular base carrying an upright, cone-like cusp that exhibits numerous vertical folds that radiate from the apex to the base of the cusp (Stumpf et al, 2021), resembling those covering the head of Planohybodus peterboroughensis (Rees & Underwood, 2008). In addition, similarly developed denticles have been observed in the snout region of Hamiltonichthys (Maisey, 1989).…”

“…Nevertheless, much uncertainty still surrounds the genus-and higher-level classification of many species, which resulted in the production of a series of different taxonomic and systematic schemes (e.g., Maisey, 1989;Rees, 2008;Cappetta, 2012). This is mainly because our knowledge of the taxonomy and systematics of hybodontiforms is strongly biased towards isolated teeth rather than those found associated with articulated or disarticulated skeletons, which otherwise remain extremely rare and limited to a few localities only, but commonly display important morphological features for inferring phylogenetic interrelationships (e.g., Maisey, 1982Maisey, , 1983Maisey, , 1986Maisey, , 1987Maisey, , 1989Maisey, Naylor & Ward, 2004;Lane & Maisey, 2009Stumpf et al, 2021). The incomplete nature of the hybodontiform skeletal fossil record consequently precludes deeper insights into their taxonomy and systematics in many cases, and therefore any new information about their skeletal morphology potentially increases our knowledge about their evolutionary history and ecological diversity.…”