Hybodontiformes

The Rhaetian marine transgression, which occurred across Europe in the latest Triassic, 205.5 Ma, famously deposited one or more bone beds. Attention has generally focused on the basal bone bed alone,
but here we explore this bed, and a stratigraphically higher bone bed at the top of the Westbury Formation, and compare the faunas. The Rhaetian at Hampstead Farm Quarry, Chipping Sodbury, Gloucestershire, UK, has produced more than 26,000 identifiable microvertebrate remains, including teeth and scales of chondrichthyan and osteichthyan fishes, as well as vertebrae of sharks, bony fishes,
ichthyosaurs, and plesiosaurs. The higher bone bed (‘bed 9’) contains more small specimens than the basal bone bed, and they are also less abraded, suggesting less transport. Both bone beds yield largely
the same taxa, but Rhomphaiodon minor and rare Vallisia coppi and Sargodon tomicus are found only in the basal bone bed. Duffinselache is reported only from units above the basal bone bed, but low in the
Westbury Formation, and durophagous teeth only from two horizons. Four out of nine chondrichthyan species are common to both bone beds, whereas Rhomphaiodon minor and Ceratodus are absent, and
hybodonts in general are rarer, in bed 9. Bed 9 is the richer source of marine reptile remains, including ichthyosaur teeth, jaw fragments, vertebrae, rare plesiosaur teeth and vertebrae, and a few Pachystropheus vertebrae and limb bones. Whereas the basal bone bed represents considerable transport and possible storm bed deposition associated with the onset of the Rhaetian Transgression, bed 9 was deposited under a lower energy regime.

A new genus and species of hybodontoid elasmobranchs, Gansuselache tungshengi, is described on the basis of an articulated skeleton from the Fangshankou Formation (Late Permian) of the Mazongshan Mountain of Gansu Province, northwestern China. All dermal skeletons including dorsal fin-spines, cephalic spines, dermal denticles and one tooth are well preserved in their natural position. It is a shark of about 490 mm total length with a fusiform body. The braincase has large, downturned postorbital processes and otic capsules, no postorbital articulation, long palatoquadrate and large triangular Meckel’s cartilage. It bears two dorsal fins and two pairs of heteromorphic cephalic spines. The tooth is of low-crowned, multicuspid type; the elongate tooth crown has stout crown shoulder and well-developed, rounded labial peg; the cusp and cusplets are moderately tumid, and have a few strong and curved folds; the tooth root is higher on the labial face than the lingual face. The dermal denticle is placoid scale-like, with its crown surface ornamented with some parallel ridges. Gansuselache, the first articulated hybodontiform from Asia, also represents the first complete hybodontiform from the Permian.

Acrodus olsoni, Acrodus sweetlacruzensis, and Lissodus zideki, are all hybodontiform sharks from the early Permian Period (Artinskian Stage: 283.5-290.1 million years ago), whose fossils been found in rocks corresponding to nearshore ecosystems in Permian Basin of Texas. We also report these species in deeper marine, cephalopod rich, carbonates of the Leuders Formation in Shackleford County, Texas. These new fossils indicate that Hybodonts from this time routinely traveled between fully marine and nearshore marine environments, and may have helped connect the well-understood terrestrial/freshwater food webs of the early Permian to the marine realm. This behavior may also be the cause of increased Hybodont diversity from the Shackleford County site, compared to fossil communities from the same age elsewhere, in that it allowed for these sharks to diversify their diets in ways that similarly sized marine sharks and other fishes from the Leuders Formation could not.