Sunday 16 May 2021

Durnonovariaodus maiseyi: A new species of Hybodont Shark from the Upper Jurassic Kimmeridge Clay Formation of Dorset, England.

The Hybodonts were a distinctive group of Sharks, which first appeared in the Late Devonian and persisted until the end of the Cretaceous, reaching their maximum diversity in the Triassic and Jurassic. They were the sister group to the Elasmobranchs (the group which includes all modern Sharks and Rays), and had a distinctive morphology with two dorsal fins supported by heavily ornamented spines exhibiting numerous retrorse denticles arranged along the posterior midline, the males in addition having a single or double pair of cephalic spines each with a trifid base carrying a prominent hook-shaped spine on the skull posterior to the orbit. Like other Shark groups, living and extinct, Hybodonts continuously produced and shed teeth throughout their lives, but had a skeleton comprised entirely of cartilage, resulting in an extensive fossil record based upon disarticulated teeth, but relatively few body skeletons, making understanding the taxonomy of the group difficult.

In a paper published in the journal PeerJ on 11 May 2021, Sebastian Stumpf of the Department of Palaeontology at the University of Vienna, Steve Etches of the Museum of Jurassic Marine Life, Charlie Underwood of the School of Earth and Planetary Sciences, at Birkbeck College, University of London, and Jürgen Kriwet, also of the Department of Palaeontology at the University of Vienna, describe a new species of Hybodont Shark from the Upper Jurassic Kimmeridge Clay Formation of Dorset, England.

The new species is named Durnonovariaodus maiseyi, where 'Durnonovariaodus' is derived from Durnonovaria, the ancient name of the town of Dorchester from which the name Dorset derives, and the Greek noun odus (ὀδούς), meaning tooth, and 'maiseyi' honours palaeontologist John Maisey of the American Museum of Natural History, for his significant work on better understanding Hybodontiform taxonomy and systematics and his contribution to the field of palaeoichthyology in general.

The species is described from a single specimen, MJML K1624, which is preserved on a slab of rock of about 1785 mm maximum length and 700 mm maximum width preserving disarticulated elements of the splanchnocranium with associated teeth, a single fragmentary dorsal fin spine, the pelvic girdle, as well as abundant cartilage fragments of uncertain identity, plus countless dermal denticles scattered all across the slab. The specimen was collected by Steve Etches from early Tithonian beds referred to the Pectinatites pectinatus Ammonite zone accessible near Freshwater Steps, Encombe, Dorset.

Durnonovariaodus maiseyi, MJML K1624, holotype, from the Upper Jurassic Kimmeridge Clay Formation (early Tithonian) near Freshwater Steps, Encombe, Dorset, England. (A) Slab containing specimen. (B) Interpretative line drawing. Stumpf et al. (2021).

The endoskeletal remains are strongly compressed, but still show a certain degree of relief suitable for identifying morphological features. They are composed of well-mineralised, tessellated cartilage (cartilage covered in a mosaic of mineralised tiles called tesserae, a distinctive Shark feature), which gives them a rough and scratchy surface texture. The scattered, but closely arranged skeletal elements support Stumpf et al.'s interpretation that all belong to a single specimen.

The splanchnocranium (skull) is incomplete and highly disarticulated. It includes the mandibular arch (jaws) as well as part of the hyoid arch (which supports the tongue) and gill arches. The mandibular arch is disarticulated and includes the paired palatoquadrates (upper jaws) and Meckel’s cartilages (lower jaws). The right palatoquadrate and the right Meckel’s cartilage are complete, while their left counterparts are incomplete.

Durnonovariaodus maiseyi, MJML K1624, holotype, from the Upper Jurassic Kimmeridge Clay Formation (early Tithonian) near Freshwater Steps, Encombe, Dorset, England. (A) Splanchnocranium; (B) Interpretative line drawing. Anatomical abbreviations: af, adductor fossa; artcot, articular cotylus; br, branchial element; brr, branchial rays; ch, ceratohyal; dgr, dental groove; eart, ethmoidal articulation; hym, hyomandibular; l, left (in parentheses); Mc, Meckel’s cartilage; mqj, medial quadratomandibular joint; pbpr, palatobasal process; pq, palatoquadrate; qf, quadrate flange; r, right (in parentheses). Stumpf et al. (2021).

The right palatoquadrate is visible in lateral aspect, measuring 257 mm in maximum length and 89 mm in height. The left palatoquadrate is less well-preserved and exposed in lateral view. It is incomplete in its most-posterior portion and along its ventral margin. The right Meckel’s cartilage is exposed in medial view, measuring 261 mm in maximum length and 125 mm in maximum height. Its left counterpart is visible in lateral aspect and lacks its dorsal portion.

The palatoquadrate of Durnonovariaodus maiseyi is elongate and rather massive. It can be roughly divided into an anterior palatine and a posterior quadrate portion. The latter is formed into a large, well-developed quadrate flange, which anteriorly gives rise to a prominent, well-defined ridge that bounds a deep adductor fossa dorsally. The dorsal margin of the palatoquadrate exhibits a low and reduced palatobasal process, which is located at about one-third the total palatoquadrate length from the anterior tip. The palatoquadrate is widely convex antero-dorsally and slopes slightly downwards towards its anterior tip. A large articulation surface for the ethmoid process of the neurocranium extends along the antero-dorsal margin of the palatoquadrate. The anterior tip of the palatoquadrate is bluntly pointed and the ventral margin of the anterior palatine portion of the palatoquadrate is widely convex and reinforced by a narrow, slightly elevated ridge.

The Meckel’s cartilage is elongate, rather deep posteriorly and tapers slightly towards its anterior tip, which is bluntly pointed rather than sharply tipped. Medially, there is a deep, well-developed dental groove, which extends approximately one-half the length of the Meckel’s cartilage. Ventrally, the dental groove is delimited by a prominent ridge. The dorsal margin of the Meckel’s cartilage is straight for the length of the accompanying dental groove until it forms a wide and low indentation that is delimited posteriorly by a large, well-defined medial quadratomandibular joint. The articular cotylus for the articulation with the palatoquadrate is moderately well-developed and shallowly recessed. A lateral quadratomandibular joint could not be observed. The postero-ventral margin of the Meckel’s cartilage is widely convex and merges smoothly into the ventral margin, which is straight for most of its length and reinforced by a narrow, slightly elevated ridge. Laterally, the Meckel’s cartilage bears a similarly developed ridge extending along its ventral margin. Labial cartilages could not be identified.

The hyoid arch and gill arches of Durnonovariaodus maiseyi are incomplete and very badly preserved. There is a single hyomandibular and two slender, slightly curved cartilages that are here tentatively identified as the ceratohyals, plus numerous well-calcified cartilage fragments of uncertain identity. The preserved hyomandibular is broken distally and could not be identified as either left or right. The proximal end of the hyomandibular, which articulates with the neurocranium, is formed into an anteriorly directed hook-like process.

About 80 disarticulated teeth that are scattered on and around the right Meckel’s cartilage and right palatoquadrate, suggesting that they derive from both the upper and lower dentition. Morphologically, the teeth can be differentiated into those coming from tooth files of anterior, lateral and posterior positions, indicating a disjunct monognathic heterodonty. There is no indication for dignathic heterodonty in Durnonovariaodus maiseyi, but this must be considered as tentative due to the incomplete and disarticulated nature of the holotype specimen, pending the discovery of more complete material.

The dentition of Durnonovariaodus maiseyi encompasses relatively large, up to 18 mm wide and 12 mm high, symmetrical to slightly asymmetrical multicuspid teeth that are characterized by strongly labio-lingually flattened crowns displaying a high, fairly wide and pointed main cusp without a sigmoidal profile. The main cusp is usually flanked by two to three pairs of low but well-developed lateral cusplets, which diminish in size away from the main cusp and reach up to one-third its height. The cutting edges are slightly labially displaced, sharp and continuous, extending from the principal cusp across all lateral cusplets. There are no serrations on the cutting edges. The labial crown base is slightly incised above the crown-root junction and somewhat swollen. The tooth crown ornamentation is reduced and comprises very short, inconspicuous vertical folds that occur on both the lingual and labial bases of the crown above the crown-root junction. The distribution of these vertical folds slightly differs on the lingual and labial faces, with those occurring on the lingual face occasionally being restricted to the bases below the lateral cusplets only, or may even be absent entirely. The crown-root junction is straight lingually and labially.

Durnonovariaodus maiseyi, MJML K1624, holotype, from the Upper Jurassic Kimmeridge Clay Formation (early Tithonian) near Freshwater Steps, Encombe, Dorset, England. Close-up view of teeth. (A)–(D) Anterior teeth in labial views. (E) Antero-lateral tooth in lingual view. (F) Lateral tooth in labial view. (G)–(I) Lateral teeth in lingual aspects (J) Postero-lateral teeth in labial views (K) Postero-lateral tooth in lingual view. (L), (M) Extreme posterior teeth in (L) labial and (M) lingual aspect. Stumpf et al. (2021).

The tooth root is prominent, about as high apico-basally as deep labio-lingually, and slightly lingually displaced beneath the crown, forming a narrow, lingually sloping shelf. The basal root face is flat and bears a shallow depression that extends along the labial edge. The lingual and labial faces of the root are perforated by numerous tiny, densely arranged foramina, resulting in a somewhat trabecular appearance of the root. In addition, a series of larger, rather regularly arranged foramina occurs along both the lingual and labial base of the root.

The morphological variation that passes posteriorly through the dentition of Durnonovariaodus maiseyi mainly involves a distal inclination and reduction of the principal cusp. Teeth from anterior positions are symmetrical and display a moderately robust and erect principal cusp that is flanked by two pairs of low, slightly divergent lateral cusplets. These are, as measured from the crown-root junction, up to one-half the height of the crown. In addition, a third pair of very small to incipient lateral cusplets may be developed.

Lateral teeth exhibit a fairly wide, triangular-shaped and slightly distally inclined main cusp, which has a more or less straight mesial but a slightly concave distal cutting edge. The main cusp is usually flanked on each side by three pairs of lateral cusplets. These are up to one-half the height of the crown as in teeth of anterior positions. 

Posterior teeth have a wide and very low profile. The main cusp is wide, particularly low and distally inclined and has a long, slightly undulating mesial cutting edge. It is flanked on each side by two to three pairs of low, commonly reduced lateral cusplets.

The specimen includes a single dorsal fin spine only. The fin spine is incomplete and exposed in left lateral view, lacking its distal portion. It is ornamented with strong, non-bifurcating costae. The unornamented fin spine base, which includes the deeply inserted posterior slot that received the cartilaginous basal plate of the dorsal fin, appears to have been rather long. No further information can be retrieved due to the poor state of preservation of the dorsal fin spine. There is a cartilage fragment of roughly triangular shape, which may represent a dorsal basal fin plate.

Durnonovariaodus maiseyi, MJML K1624, holotype, from the Upper Jurassic Kimmeridge Clay Formation (early Tithonian) near Freshwater Steps, Encombe, Dorset, England. (A) Dorsal fin spine. (B) Interpretative line drawing of complete specimen. (C), (D) Pelvic girdle. (E) Close-up view of dermal denticles. (F, G) Simplified sketch drawing of dermal denticle in (F) lateral and (G) anterior view. Anatomical abbreviations: bp, basal plate; df, diazonal nerve foramina; pvg, pelvic half-girdle. Stumpf et al. (2021).

The pelvic girdle of Durnonovariaodus maiseyi is represented by two separate pelvic half-girdles, both displaying a series of diazonal nerve foramina aligned along the distal margin. There is an elongate, broken cartilage preserved in close proximity to one of the pelvic girdle halves, whose precise identity remains unknown due to preservation.

The specimen has countless very small, densely packed dermal denticles that occur all across the bedding plane. Morphologically, the dermal denticles correspond to the ‘non-growing’ (monodontode) type. They all have a thorn-like appearance, measuring less than 1 mm in maximum height, with a circular to oval base and an upright, slightly recurved cusp displaying a few strong vertical folds that extend from the apex to the base of the cusp. These folds usually merge apically to form a keel-like leading edge extending along the anterior face of the cusp.

Durnonovariaodus maiseyi was a large Hybodont, probably reaching about 2 m in length, which would have made it one of the largest Jurassic Sharks. The Jurassic was a time of high faunal turnover among Shark communities, with the modern Elasmobranchs appearing, undergoing a major evolutionary radiation, and becoming the dominant Chondrichthyan group by the end of the period. This presumably placed ecological stress on Hybodonts, which would have faced competition for resources in many niches. However, few Elasmobranch Sharks in the Jurassic reached anything like 2 m in length, while the largest Hybodonts reached around 3 m and lived in open marine environments, suggesting the larger Hybodonts were utilising food resources not yet available to Elasmobranchs. Smaller Hybodonts rapidly became restricted to marginal marine environments with reduced or fluctuating salinities, suggesting that these environments were also not yet colonised by Elasmobranchs.

Unusually for a large Hybodont, Durnonovariaodus maiseyi seems to have inhabited a deep-water environment on the edge of the continental shelf. This environment was also inhabited by Secarodus polyprion, which was dentally similar to Durnonovariaodus maiseyi, which is known from the Bathonian of England, suggesting a similar lifestyle. Strumpf et al. suggest that these Sharks may have had a similar lifestyle to modern Hexanchiforms (Frilled and Cow Sharks), which inhabit deep-water environments, but which occationally move into shallower coastal waters to feed.

Numerous Hybodont remains have been reported from the Kimmeridge Clay, predominantly teeth and fin spines, with a few partial skeletons predominantly attributable large-bodied taxa. The taxonomy of Hybodonts is still poorly understood, despite considerable recent effort, but at least five genera of large Hybodonts appear to have been present in the Kimmeridge Clay, to whit Durnonovariaodus, Planohybodus, Asteracanthus, Strophodus, and Meristodonoides. Meristodonoides has only recently been identified in the formation, extending the range of the genus from the Cretaceous into the Jurassic. A single species of small Hybodont, Hybodus lusitanicus, has also been identified.

The most commonly found Kimmerage Hybodont is Planohybodus, which is extremely widespread from the Middle Jurassic to the Middle Cretaceous. These Sharks were dentally similar to Meristodonoides, with both genera probably having adapted for clutching and tearing rather than cutting prey. The teeth of Durnonovariaodus are more specialised, with a heterodont dentition (dentition in which teeth with different shapes and functions are found in different parts of the mouth) in which symmetical, gracile teeth are found at the front of the mouth and asymmetrical teeth are found on the sides and back of the mouth, suggesting a feeding style in which prey was captured with the front teeth then moved further back in the mouth for processing by the back teeth.

Hybodont Sharks have an extensive fossil record in the Palaeozoic and Mesozoic, and have been studied by palaeontologists for almost two centuries. This has left us with a good understanding of their dental specialisations, and therefore feeding habits, but a poor knowledge of their taxonomy and systematics, due to the paucity of skeletal remains.

Unfused pelvic half-girdles are generally thought to be a primitive feature within Hybodonts, but are seen in Durnonovariaodus maiseyi and the related Hybodus hauffianus, from the Lower Jurassic Posidonia Shale of Germany, neither of which would otherwise be considered to be, suggesting that this trait might have been more widespread within the group, and that it is therefore of limited taxonomic value.

Durnonovariaodus maiseyi shows skeletal resemblances to members of the genera Hybodus and Egertonodus, which are grouped together in the family Hybodontidae, but its teeth are more similar to those of Secarodus, which also has distinctive multicuspid cutting teeth on the sides of its mouth (although this species was originally placed in the genus Hybodus). Strumpf et al. tentatively place Durnonovariaodus maiseyi within the family Hybodontidae, although they acknowledge that currently available phylogenies for Hybodonts are unsatisfactory. 

The Museum of Jurassic Marine Life in Kimmeridge, England, contains an extensive, but understudied, collection of Jurassic Hybodonts, including much skeletal material, with the potential to expand this knowledge. 

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