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

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.

The Phylogeny of Rays and Skates (Chondrichthyes: Elasmobranchii) Based on Morphological Characters Revisited

“…Pelvic girdle: (0) Separated; (1) fused. Modified from Maisey [ 13 ] (char, 37), based on observations by Klug et al [ 20 ] ( Figure 2 ), Stumpf et al [ 108 ] and Coates et al [ 109 ]. Current fossil evidence suggests that the separation of two halves, or at least a not very well-mineralized mid-bar of the pelvic girdle, is the basal state across hybodontiform-like sharks ( Figure 42A,B ) (SMNS 10062) (NHMUK PV P 339).…”

A Synoptic Review of the Cartilaginous Fishes (Chondrichthyes: Holocephali, Elasmobranchii) from the Upper Jurassic Konservat-Lagerstätten of Southern Germany: Taxonomy, Diversity, and Faunal Relationships

“…Maisey [ 153 ] and Maisey et al [ 137 ] proposed another character that unites this group, which is the fusion between paired halves of the pelvic girdle to form a puboischiadic bar. However, in recent years, increasing evidence suggests that this is a misinterpretation in fossil remains, and both males and females of several hybodontiforms, present and unfused pelvic girdle (see [ 154 ]. A preliminary re-examination of the late Pennsylvanian hybodontiform, † Hamiltonichthys , using UV light seems to support this, but a more detailed study is needed (E.V.S.…”

“…Hybodontiforms are the extinct sister group of neoselachians (modern sharks and rays) and form a speciose clade of Palaeozoic to Mesozoic shark-like chondrichthyans characterized by distinct cranial and postcranial traits [ 136 , 155 ]. However, even after almost two centuries of extensive research, there still is no reliable phylogenetic framework for hybodontiforms yet available, thus leaving their interrelationships dubious and unresolved (e.g., [ 60 , 154 , 156 ]).…”

“…The genus is represented by a single, almost complete skeleton from the lower Tithonian of Solnhofen representing a female individual ( Figure 11 ). The specimen was first figured by [ 19 ], who assigned it to † Hybodus obtusus Agassiz, 1839 [ 171 ], a species now considered a junior synonym of † Asteracanthus ornatissimus [ 154 , 171 ].…”

“…Later, and the genus †Strophodus Agassiz, 1838 [ 171 ], was established on the basis of low-crowned durophagous crushing teeth, and synonymized with † Asteracanthus following the discovery of associated dental and skeletal material from the Middle Jurassic of England [ 178 – 180 ]. This long accepted taxonomic scheme has recently been challenged by Stumpf et al [ 154 ], who proposed an amended diagnosis for † Asteracanthus , particularly based on the female skeleton from Solnhofen. Key morphological features displayed by this specimen include the presence of tuberculate dorsal fin spines and high-crowned multicuspid teeth that markedly differ from the prominent durophagous crushing teeth that have traditionally been assigned to † Asteracanthus .…”