Sulcusuchus: a grooved-jawed plesiosaur from Patagonia

Sulcusuchus erraini was a short-necked plesiosaur of the polycotylid family from Late Cretaceous Patagonia. Its most unusual feature is a set of deep, broad grooves running along the snout and lower jaw. The grooves distinguish the genus, but their original soft contents are not fossilised. The case belongs in the marine reptile catalogue because it shows how an anatomical feature can be securely observed while its biological role remains open to competing tests.

Fossils come from the La Colonia and Los Alamitos formations of Argentina and are dated to the upper Campanian–lower Maastrichtian interval. No complete skeleton is known, so the overall body outline must be compared with other polycotylids rather than presented as a direct copy of a single specimen.

Quick facts

From crocodile fragment to polycotylid

When the animal was first named in 1990, the available snout fragment was interpreted as belonging to a dyrosaurid crocodile-line reptile. An elongated rostrum is not enough to diagnose a group: similar outlines evolved repeatedly in aquatic predators. Later discoveries added parts of the jaws, palate and skull base. Their character combination led researchers to recognise a plesiosaur, and the 2013 revision by José O’Gorman and Zulma Gasparini emended the diagnosis of the genus and species.

The 2013 work identified two especially distinctive features: deep, broad grooves on both rostrum and mandible, and a broad plate formed by the posterior pterygoid notch together with part of the basioccipital. The grooves contain numerous openings and have a smoother inner surface than the adjacent bone, evidence that soft structures occupied them. The identity of those structures is a separate question.

What could the grooves have contained?

O’Gorman and Gasparini considered several explanations rather than selecting one from appearance alone. Oral glands, including supralabial and sublabial glands, are one possibility, although their biological role could not be reconstructed. A second hypothesis invokes specialised electrosensory or mechanosensory tissue, perhaps useful when detecting prey in or near soft substrate. The comparison with living aquatic animals provides a model for testing, not proof that the same organ existed in this plesiosaur.

The grooves therefore support a narrow but important conclusion: a substantial soft-tissue system was associated with the jaws. They do not show what the animal sensed, whether it searched the seafloor, or which prey it captured. The long jaws and numerous teeth are consistent with seizing fish and other mobile prey, but a stomach-content record has not established a precise menu.

Fragmentary remains and late Cretaceous habitat

The known skull material includes pieces of the cranial roof, palate, rostrum and lower jaw rather than one complete head-and-body skeleton. Postcranial elements are also fragmentary. Consequently, the short neck and compact profile shown in reconstructions are based on the wider anatomy of polycotylids, including relatives such as Pantosaurus, not on an intact Sulcusuchus skeleton. A confident total length cannot be calculated from the available pieces.

The La Colonia and Los Alamitos deposits record coastal and marginal-marine environments in Patagonia near the end of the Cretaceous. The region contained shifting shorelines and water settings rather than a single uniform open-ocean habitat. A fossil's burial location narrows the environmental context, but does not prove the exact place where an individual usually fed or spent most of its life.

Stones in the fossil record

A 2025 study documented 95 gastroliths from Maastrichtian levels of the La Colonia Formation and reported the first evidence of lithophagy in Sulcusuchus erraini. This adds a new observation to the older, fragment-based picture of the animal. It supports the conclusion that the animal swallowed stones, while the specific function remains harder to establish. Gastroliths in aquatic reptiles have been discussed as aids to food processing or buoyancy, but neither role should be assigned automatically to every cluster.

Preservation and collection practices also affect whether stones are noticed and recovered with a fossil. The new evidence makes swallowing stones a documented behaviour for this taxon; it does not turn every loose stone found near a plesiosaur into a gastrolith. In the plesiosaur record, Sulcusuchus stands out for combining a distinctive jaw anatomy with unusually direct evidence that invites new questions about feeding.