The Allen Formation is not the most talked-about or best-known of all Cretaceous fossil beds. It is located in Argentina's Rio Negro Province. Certainly the fauna found here are better-known than the rock formation itself.
It was probably a wet and forested lowland environment of southern beech, araucaria, palmetto and other common plants from the former supercontinent of Gondwana. South America had by this time, more or less split from Antarctica, which was drifting ever-southward. At the time though there still were no polar ice caps although winter did fall on the South Pole. These were still temperate forests this far south though. The site itself dates back to the Maastrichtian Stage of the Late Cretaceous, around 70 to 68 million years ago.
One of the main mid-level carnivores here was the pterosaur Aerotitan. It was not as large as its name suggests and was an azhdarchid, a terrestrial stalker that went after small animals.
The fauna here consists mostly of titanosaur sauropods, and not the largest of them either. By this time, the massive titans of the Middle Cretaceous were somewhat rarer, and certainly none were found at Allen. The erect-necked giants, which once weighed upwards of 80 tons and could browse 18 meters above ground were being replaced by a profusion of small and short-necked animals.
Arguably the best-known of them is Saltasaurus. This animal has for a while been a fixture in popular culture as the animal that truly made titanosaurs famous. It was a fairly standard animal, not as long-necked as its gigantic relatives. It still had the fat, double-wide body of titanosaurs proper, and the rather long tail and columnar legs that are the defining characteristic of all sauropods. Saltasaurus is also the first sauropod with signs of armor.
These are now known to have been common among the titanosaurs, with artists often exaggerating them to show off a side of the sauropods that was decidedly not soft and gentle. Saltasaurus was just 12 meters long and weighed a paltry 7 tons. However, most importantly it is known from extensive finds of fossilized eggs. The Auca Mahuevo site of Argentina has revealed the remains of thousands of Saltasaurus eggs, all deposited in small nests. Each egg was roughly 12 centimeters across, small by any standards.
These eggs also showed signs of having fossil embryos and even the skins of the embryos have been preserved with exceptional detail. The mothers covered their eggs in rotting vegetation to help incubate them, and probably stayed at the nesting ground until the eggs hatched. Sauropods may have nested communally but caring for their young was not high on their agenda. Instead the young separated from the nesting site as soon as they hatched and wandered around on their own, or at least in a group. In under 10 to 12 years though, they could at least reach three-fourths their adult size and keep growing slowly for the rest of their life.
Other titanosaurs from this formation include the markedly similar Aeolosaurus and Laplatasaurus. Aeolosaurus was a lightly-built animal, far more than Saltasaurus was. It too was an armored titanosaur. Most of these small titanosaurs were in the same size range, going from rhino-size upwards to being elephant-sized.
Austroraptor was probably one of the more intriguing aspects of the Allen fauna. It was a dromaeosaur, or raptor dinosaur. It was much larger than most, close to six meters in length and as tall as a man but it was not a fierce predator of larger animals. Instead Austroraptor was short-armed and its snout was long with small teeth. It was a fisherman, and it hunted somewhat like a heron. Austroraptor waded into streams and rivers and caught its prey with a rapid strike of its jaws.
All in all, it is one of the more interesting animals of the Allen Formation. It was not unique, though. All South American raptors seem to have taken up this specialized lifestyle. Elsewhere, raptors were ambush predators that either ate bite-sized snacks or leaped on animals similar in size to themselves. They would then eat their victims alive. Austroraptor, one of the largest of all raptors, was not of this ilk.
Quilmesaurus was the probable top predator here. It was an abelisaur, a member of a family of rather primitive theropods that ruled the Southern Hemisphere at the time.
Abelisaurs had short, small skulls and a pair of arms so tiny that they made T.rex's look large by comparison. But while Tyrannosaurus had incredibly dense muscle in its arms, abelisaur arms were skinny and practically useless. All an abelisaur could do with its arms was stick them out to the sides and wiggle them. Quilmesaurus is poorly known. It can easily be restored as a generic abelisaur with no special features. It was probably lean and fast, as many of them seem to be, and probably hunted sauropods and smaller animals.
Duckbill dinosaurs were not known from the Southern Hemisphere so the discovery of duckbills in the Allen Formation came as a surprise. Called Willinakaqe, this large creature was probably alone in the south as one of South America's sole non-sauropod big herbivores.
It seems out of place among the small titanosaurs that were more the norm here. In fact, it coexisted with several of these armored herbivores. Duckbill dinosaurs are able to switch between stances. They can rear up on their hind legs to run or eat from tall branches, and Willinakaqe was no exception. It is known from relatively complete remains, including jaw elements. In total, it was similar to the petite sauropods in size, around 9 meters long.
