Pterodaustro

Description

Pterodaustro had a maximum adult wingspan of approximately 3 m (9 ft 10 in) and a maximum body mass of approximately 9.2 kg (20 lb). Its hindlimbs are rather robust and its feet large. Its tail is uniquely elongated for a pterodactyloid, containing twenty-two caudal vertebrae, whereas other members of this group have at most, sixteen.

Pterodaustro has a very elongated skull, up to 29 cm (11 in) long. The portion in front of the eye sockets comprises 85 percent of skull length. The long snout and lower jaws curve strongly upwards; the tangent at the point of the snout is perpendicular to that of the jaw joint. Pterodaustro has about a thousand bristle-like modified teeth in its lower jaws that might have been used to strain crustaceans, plankton, algae, and other small creatures from the water. These teeth stand for the most part not in separate alveoli but in two long grooves parallel to the edges of the jaw. They have a length of 3 centimeters (1.2 in) and are oval in cross-section, with a width of just 0.2 to 0.3 millimeters (0.0079 to 0.0118 in).

At first it was suspected these structures were not true teeth at all, but later research established they were built like normal teeth, including enamel, dentine and a pulp. Despite being made of very hard material, they might still have been flexible to some extent due to their extreme length-width ratio, a bend of up to 45 degrees being possible. The upper jaws also carried teeth, but these were very small with a flat conical base and a spatula-formed crown. These teeth also do not have separate tooth sockets but were apparently held by ligaments in a special tooth pad, that was also covered with small ossicles, or bone plates. It appears that they were not replaced, unlike the teeth of most other reptiles.

The back of the skull was also rather elongated and in a low position; there are some indications for a low parietal crest.

Paleobiology

Feeding

Pterodaustro probably strained food with its tooth comb, a method called "filter feeding", also practised by modern flamingos. Once it caught its food, Pterodaustro probably mashed it with the small, globular teeth present in its upper jaw.

Like other ctenochasmatoids, Pterodaustro has a long torso and proportionally massive and splayed hindfeet, adaptations for swimming. A recent study suggested that its ankle facilitated movements required for wading behavior.

Robert Bakker suggested that, like flamingos, this pterosaur's diet may have resulted in a pink hue.

At least two specimens of Pterodaustro have been found, MIC V263 and MIC V243, with gizzard stones in the stomach cavity, the first ever reported for any pterosaur. These clusters of small stones with angled edges support the idea that Pterodaustro ate mainly small, hard-shelled aquatic crustaceans using filter-feeding. Such invertebrates are abundant in the sediment of the fossil site.

Life history

A study of the growth stages of Pterodaustro concluded that juveniles grew relatively fast in their first two years, attaining about half of the adult size. Then they reached sexual maturity, growing at a slower rate for four to five years until there was a determinate growth stop.

Biomechanical modelling suggests that Pterodaustro juveniles were capable of powered flight, contradicting the ‘fly-late’ and ‘glide-early’ hypotheses about changes in their flight capabilities over ontogenetic development.

In 2004 a Pterodaustro embryo in an egg was reported, specimen MHIN-UNSL-GEO-V246. The egg was elongated, 6 centimeters (2.4 in) long and 22 millimeters (0.87 in) across, and its mainly flexible shell was covered with a thin layer of calcite, 0.3 millimeters thick. Three-dimensionally preserved eggs were reported in 2014.

In 2009, possible medullary bone tissue was reported from the largest known femur of Pterodaustro (V 382). Medullary bone tissues are previously only known from modern birds that are laying eggs in reproduction, and some non-avian dinosaurs such as Tyrannosaurus rex, so this record would make it the first reported occurrence of a non-dinosaurian medullary bone. In 2021, Padian and Woodward suggested that this tissue might instead represent a crushed endosteal tissue.

Comparison with waterfowls

Comparisons between the scleral rings of Pterodaustro and modern birds and reptiles suggest that it may have been nocturnal and similar in activity patterns to modern anseriform birds that feed at night, although the method of this research has been questioned by some researchers.

Because of its long torso and neck and comparatively short legs, Pterodaustro was unique among pterosaurs in having difficulties to launch. Even with the pterosaurian quadrupedal launching mechanism, it would have required frantic and fairly-low angled take-offs possible only in open areas, much like modern geese and swans.

Phylogeny

Bonaparte in 1970 assigned Pterodaustro to the Pterodactylidae; in 1971 to a Pterodaustriidae. However, from 1996 cladistic studies by Alexander Kellner and David Unwin have shown a position within the family Ctenochasmatidae, together with other filter feeders.

In 2018, a topology by Longrich, Martill and Andres recovered Pterodaustro within the family Ctenochasmatidae, more precisely within the tribe called Pterodaustrini, in a more basal position than Beipiaopterus and Gegepterus.