Abstract
A partial skeleton of a hybodontiform shark-like chondrichthyan from the Upper Jurassic Kimmeridge Clay Formation of Dorset, England, is described and designated as a new genus and species, Durnonovariaodus maiseyi gen. et sp. nov. The holotype and only known specimen, which is represented by disarticulated splanchnocranial elements with associated teeth, a single dorsal fin spine, the pelvic girdle, as well as unidentifiable cartilage fragments, plus countless dermal denticles, exhibits a puzzling combination of dental and skeletal features, providing important new insights into the morphological and ecological diversity of hybodontiforms. Durnonovariaodus gen. nov. displays a unique set of dental characters, showing close morphological resemblance to Secarodus from the Middle Jurassic of England, which was erected for distinctive, strongly labio-lingually compressed multicuspid cutting teeth originally described as Hybodus polyprion. Skeletally, Durnonovariaodus gen. nov. resembles Hybodus and Egertonodus in having a palatoquadrate with a palatobasal process and an ethmoidal articular surface, combined with the possession of dorsal fin spines ornamented with costae. Therefore, and given the absence of any conclusive phylogenetic framework, Durnonovariaodus maiseyi gen. et sp. nov. is here tentatively referred to Hybodontidae until more complete material becomes available in order to enable a more reliable suprageneric identification. The holotype of Durnonovariaodus maiseyi gen. et sp. nov. contains two separate pelvic half-girdles, a feature previously considered as evolutionarily primitive among hybodontiforms. However, unfused pelvic half-girdles also occur in the supposedly closely related species Hybodus hauffianus and may in fact have been more widely distributed among hybodontiforms than previously thought, thus rendering the phylogenetic utility of separated pelvic half-girdles for inferring hybodontiform interrelationships difficult and unresolved.
Highlights
Specimen Museum of Jurassic Marine Life (MJML) K1624, which is here designated as new genus and species, Durnonovariaodus maiseyi gen. et sp. nov., was briefly described by Underwood (2020), who referred it to Planohybodus peterboroughensis Rees & Underwood, 2008, which is known from rare dental and fragmentary skeletal material from the Callovian–Oxfordian of England
Hybodontiforms have an extensive fossil record elucidating a speciose clade of Palaeozoic to Mesozoic shark-like chondrichthyans that have developed diverse dental adaptations in relation to prey and feeding
The Etches Collection, which is housed and curated in the Museum of Jurassic Marine Life of Kimmeridge, England, contains well-preserved but largely unstudied hybodontiform skeletal material from the Upper Jurassic Kimmeridge Clay Formation of southern England, including a partial skeleton of a comparably large-bodied hybodontiform, which is here described and designated as a new genus and species, Durnonovariaodus maiseyi gen. et sp. nov., and which significantly adds to our limited understanding of the diversity, ecology and distribution of Late Jurassic hybodontiforms
Summary
Hybodontiformes, which forms a supposed extinct sister group to the elasmobranch crown comprising modern sharks, skates and rays (= Neoselachii sensu Compagno, 1973), represents a speciose clade of Palaeozoic to Mesozoic shark-like chondrichthyans characterized by distinct cranial and dental morphologies, and two dorsal fins supported by heavily ornamented spines exhibiting numerous retrorse denticles arranged along the posterior midline (Maisey, 1978, 1982; Maisey, Naylor & Ward, 2004; Ginter, Hampe & Duffin, 2010; Cappetta, 2012). 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, 1982, 1983, 1986, 1987, 1989; Maisey, Naylor & Ward, 2004; Lane & Maisey, 2009, 2012; Stumpf et al, 2021). The incomplete nature of the hybodontiform skeletal fossil record precludes deeper insights into their taxonomy and systematics in many cases, and any new information about their skeletal morphology potentially increases our knowledge about their evolutionary history and ecological diversity
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