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| Agriodontosaurus helsbypetrae Marke, Whiteside, Sethapanichsakul, Coram, Fernandez, Liptak, Newham & Benton, 2025 |
Abstract
The Lepidosauria is the most species-rich group of land-dwelling vertebrates. The group includes around 12,000 species of lizards and snakes (Squamata) and one species of Rhynchocephalia, the tuatara Sphenodon punctatus from New Zealand. Squamates owe their success to their generally small size, but also to their highly mobile skull that enables them to manipulate large prey. These key features of lizard and snake skulls are not seen in Sphenodon, which makes it important to understand the nature of their common ancestor. Lepidosaurs originated in the Triassic 252–201 million years ago, but confusion has arisen because of incomplete fossils, many of which are generalized lepidosauromorphs, neither squamates nor rhynchocephalians. Here we report a reasonably complete skull and skeleton of a definitive rhynchocephalian from the Middle Triassic (Anisian) Helsby Sandstone Formation of Devon, UK that is around 3–7 million years older than the oldest currently known lepidosaur. The new species shows, as predicted, a non-mobile skull but an open lower temporal bar and no large palatine teeth, and it seems to have been a specialized feeder on insects. This specimen helps us understand the initial diversification of Lepidosauria as part of the Triassic Revolution, when modern-style terrestrial ecosystems emerged.
Systematic palaeontology
Lepidosauria Haeckel, 1866
Rhynchocephalia Günther, 1867
Sphenodontia Williston, 1925
Agriodontosaurus gen. nov.
Agriodontosaurus helsbypetrae gen. et sp. nov.
Etymology. Agrio from the ancient Greek epithet of Dionysus, Agrionius, meaning ‘fierce’ and donto for ‘tooth’, which refers to the remarkably large teeth on parts of the dentary and maxilla, and saurus for ‘lizard’. Therefore, ‘fierce-toothed lizard’. The specific term ‘helsbypetrae’ refers to the Helsby Sandstone Formation (locally called the Otter Sandstone), the deposit in which the fossil was found; petrae is the genitive of petra, the latinized form of the ancient Greek word for rock.
Locality and age. The specimen is from the Helsby Sandstone Formation of Sidmouth, Devon, UK. It was excavated as a block by R.A.C. in 2015 from a temporarily exposed foreshore exposure beneath Peak Hill (UK National Grid Reference SY 109865), from the upper half of the formation, perhaps upper Anisian (244–241.5 Ma)15.
Diagnosis. Small rhynchocephalian with a body length of about 100 mm with a unique combination of the following 13 features: dentary and maxillary anterior teeth are simple and conical but robust; posterior teeth are more triangular with broad bases and set slightly en echelon; anterior maxillary and dentary teeth are acrodont and posterior teeth are pleuracrodont with a residual subdental shelf; maxilla with a pronounced anterior process and high facial process; lateral tooth row on the palatine absent; broad, flat parietal table composed of paired bones; ventral region of orbit bounded mainly by the jugal, which provides about 90% of the boundary and the maxilla the rest; jugal with a prominent, but short, posteroventral process that does not reach halfway in the ventral region of the lower temporal opening; quadrate with conch and large foramen; dentary extends posteriorly to underlie the glenoid of the lower jaw; fused prearticular, articular and surangular in the lower jaw; bicapitate ribs in the cervical and trunk regions; gastralia present; and bulb-shaped expansion of the posteriormost part of interclavicle.
Daniel Marke, David I. Whiteside, Thitiwoot Sethapanichsakul, Robert A. Coram, Vincent Fernandez, Alexander Liptak, Elis Newham and Michael J. Benton. 2025. The Oldest Known lepidosaur and Origins of lepidosaur feeding adaptations. Nature. DOI: doi.org/10.1038/s41586-025-09496-9 [10 September 2025]
