New Species of Fossil Hybodontiform Shark Identified in Japan

Paleontologists in Japan have described a new species of shark-like cartilaginous fish based on the fossilized teeth from the Late Triassic Momonoki Formation.

Named Parvodus ominechonensis, the newly-identified shark species lived during the Late Triassic epoch, between 237 and 227 million years ago.

It belongs to Parvodus, a small genus of extinct hybodontiform sharks known from the Mesozoic Era.

“Hybodontiformes is an extinct clade of sharks generally considered to be the sister clade to Neoselachii (i.e., skates, rays, and modern sharks) within the chondrichthyan clade Euselachia,” said Dr. Benjamin T. Breeden III from the National Museum of Nature and Science in Tsukuba and his colleagues.

“The earliest known unambiguous hybodontiform body fossils are from the Mississippian, but isolated teeth extend the earliest records of the group to the Late Devonian.”

“Hybodontiform fossils are known primarily from coastal and lagoonal strata throughout the Late Paleozoic, suggesting that the ancestral paleoecology of the clade was shallow marine.”

“However, some hybodontiforms apparently became euryhaline early in the evolution of the clade, and from the Mississippian onward, hybodontiforms repeatedly invaded freshwater environments.”

“Hybodontiforms were the most abundant sharks in both marine and non-marine vertebrate assemblages throughout the early Mesozoic, dwindling in diversity after the Early Jurassic until the extinction of the clade at or near the end of the Cretaceous.”

Several isolated teeth of Parvodus ominechonensis were collected from the non-marine Momonoki Formation in the town of Omine-chō (romanized Ominecho) in western Mine, Yamaguchi, Japan.

“A diversity of Triassic hybodontiforms is known from Japan, but of these, Parvodus ominechonensis is the only species known from non-marine deposits and the first reported occurrence of the family Lonchidiidae,” the paleontologists said.

According to the study, Parvodus ominechonensis fills a gap in the stratigraphic record of Parvodus between occurrences in Middle Triassic and Middle Jurassic layers.

“The global Triassic record of Parvodus comprises Parvodus huizodus from the Olenekian of China, Parvodus sp. from the Anisian of China, and Parvodus ominechonensis from the Carnian of Japan, of which Parvodus ominechonensis and Parvodus huizodus are known from non-marine deposits,” the authors said.

“After the Triassic, Parvodus is known from marine and non-marine layers across Laurasia and in South America until its extinction during the Early Cretaceous.”

“This suggests that Parvodus may have originated in freshwater habitats in the South China region of Pangea following the end-Permian mass extinction and diversified throughout the Triassic in present day East Asia before attaining a global distribution later in the Mesozoic.”

The discovery of Parvodus ominechonensis is reported in a paper in the Journal of Vertebrate Paleontology.

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Benjamin T. Breeden III et al. 2023. A new freshwater lonchidiid hybodontiform shark (Chondrichthyes, Elasmobranchii) from the Upper Triassic Momonoki Formation in Yamaguchi, Japan. Journal of Vertebrate Paleontology 43 (5); doi: 10.1080/02724634.2024.2322749