Tacuadactylus: a Jurassic pterosaur from Uruguay

Tacuadactylus luciae is a Late Jurassic pterosaur from the Tacuarembó Formation of Uruguay. It is known from the front of an upper jaw, fragments of the lower jaws and isolated teeth rather than a complete skeleton. Its snout broadens into a shallow spoon-shaped tip, and its closely spaced teeth point forward and outward. A CT scan helped show that structures once mistaken for the teeth of a sawfish were sediment-filled pterosaur tooth sockets. The underside of the snout also preserves openings for blood vessels and nerves, suggesting a sensitive surface, although no soft receptors survive. The pterosaur catalogue places this South American gnathosaurine among other fragmentary species without treating a feeding model as direct observation.

Quick facts

How a supposed sawfish fossil was identified

The principal specimen, FC-DPV 2869, was recovered in the Tacuarembó region of Uruguay. Its narrow, flattened outline and a row of projections initially suggested the rostrum of a fossil sawfish. Closer examination showed a different arrangement. Computed tomography revealed deep tooth sockets oriented toward the front and sides; the tooth-like projections that had drawn attention were sediment filling those spaces. A preserved root and the shape of the jaw confirmed that the fragment belonged to a pterosaur.

The material was reported in 2018 as the first recognized pterosaur record from Uruguay. Additional discoveries, including lower-jaw fragments and isolated teeth, supplied a more useful basis for comparison. Matías Soto and colleagues formally named Tacuadactylus luciae in 2021. The generic name combines the Tacuarembó region with the Greek word commonly used for a finger in pterosaur names. The species honours Lucía, daughter of the first author.

A spoon-shaped snout and unusual tooth rows

The holotype preserves the anterior upper rostrum and an internal cast of its expanded tip. The front widens gradually into a spoon-like outline rather than meeting the narrow snout at a sharp shoulder. Eighteen tooth positions were counted on the preserved upper-jaw fragment. Across the measured region, published estimates give roughly 1.58 to 2 alveoli per centimetre. Raised borders around the sockets and deep hollows between them contribute to the wavy edge of the jaw.

The teeth were not all directed straight down. Those near the front projected forward, while more lateral teeth angled outward. Their spacing was broadly similar to the width of a crown. The preserved enamel has a fine, veined surface texture and no prominent cutting edges. These details allow comparison with other ctenochasmatids, but a fragmentary jaw cannot provide the full tooth count or the appearance of the entire skull.

The lower-jaw material assigned to the species is separate from the holotype. It helps researchers compare the two sides of the mouth, but the collection does not unite every piece into one articulated individual. Isolated teeth are still less informative about body proportions. Referrals rely on matching diagnostic features and geological context, and the evidence should not be described as a complete skeleton.

Relationship to gnathosaurines

The 2021 phylogenetic analysis recovered Tacuadactylus within Gnathosaurinae, a branch of Ctenochasmatidae, close to the European genus Gnathosaurus. The broad, spoon-like rostral tip, outward-facing teeth and deep spaces between alveoli contribute to that comparison. The Uruguayan species has its own combination of a gradual expansion, raised socket rims, tooth density and enamel texture. It is not simply a South American specimen of Gnathosaurus.

Other ctenochasmatids, including Ctenochasma and Pterodaustro, carried much denser arrays of slender teeth associated with specialized filtering. Tacuadactylus has more separated teeth. That difference makes continuous fine-particle filtration a less direct interpretation, though the jaw alone cannot reconstruct every movement of the mouth. Its classification remains a testable result of a character analysis rather than a certainty guaranteed by one resemblance.

What the snout may have sensed

Several openings and channels occur on the underside of the rostrum. Their distribution indicates a neurovascular network, structures that carried nerves and blood vessels through the bone. In living animals, comparable arrangements can accompany sensitive skin. The fossil therefore supports a hypothesis that the expanded tip helped the animal detect contact or movement near the bottom or surface of shallow water.

That functional interpretation is not the same as a preserved sensory organ. The receptors themselves were soft tissue and are absent. Researchers cannot tell from these holes exactly how sensitive the snout was, whether it swept side to side, or whether it touched sediment, water or prey. A tactile foraging model is plausible, but the fossil does not show a hunting episode.

The teeth could have helped seize small aquatic animals once detected. Fish, crustaceans and other invertebrates are reasonable candidates in the setting, but no stomach contents or prey remains are associated with the type. Calling the animal a filter feeder also requires qualification: its sockets are less densely packed than the fine tooth batteries of strongly specialized filter-feeding pterosaurs. The safest reading separates direct jaw anatomy from inferred feeding behaviour.

Size, missing anatomy and geological context

The preserved rostral fragment is about 63 millimetres long. A wingspan near 1.5 metres has sometimes been proposed by scaling from related gnathosaurines, but no wing bones, vertebrae, shoulder girdle or legs are known for Tacuadactylus. That figure is an estimate based on comparison, not a direct measurement. Body mass, adult size and proportions are likewise unavailable.

The fossils come from the Batoví Member of the Tacuarembó Formation, a succession of continental river and floodplain deposits. Its age has been debated around the Late Jurassic and earliest Cretaceous; the naming study treated the pterosaur as Late Jurassic, commonly discussed within a Kimmeridgian–Tithonian framework. Correlating particular layers remains less precise than the formation-level label.

As the oldest named South American ctenochasmatid reported in the original description, the animal broadened the known geographic record of the group. One occurrence cannot establish where ctenochasmatids originated or how they dispersed. A scientifically careful illustration can show the preserved spoon-tipped jaw and tooth directions, while the skull behind the rostrum, wing membranes, body covering, colour and precise foraging motion remain comparative reconstruction.