Description and Classification

Avisaurus was a member of the Enantiornithes, a diverse group of early birds that thrived during the Mesozoic Era, often called “opposite birds” due to a difference in their shoulder joint structure compared to modern birds. These birds typically had teeth and clawed fingers on their wings. Avisaurus belonged to the family Avisauridae, a group of relatively large and robust enantiornithine birds also known from South America, including relatives like Soroavisaurus.

Since Avisaurus is primarily known from its tarsometatarsus (fused ankle and foot bones), much of its appearance is inferred. The known foot bones suggest it was a fairly large enantiornithine, perhaps the size of a modern raven or a small hawk, with strong legs and feet. The robust nature of these bones indicates it likely had powerful grasping capabilities.

Distinguishing Features

Avisaurus can be identified by several features, mostly related to its preserved foot bones:

• It was a relatively large enantiornithine bird for its time.
• The tarsometatarsus (fused foot bone) is robust and strong, indicating powerful feet, possibly used for perching securely or grasping prey.
• The second metatarsal (the bone leading to the innermost of the three main forward-pointing toes) is noticeably shorter and more slender than the third and fourth metatarsals.
• The articular surfaces at the lower end of the tarsometatarsus, where the toe bones (phalanges) would attach, have a distinct shape characteristic of avisaurids.

Paleoenvironment and Diet

Avisaurus fossils have been found in the Hell Creek Formation of Montana and the Lance Formation of Wyoming. During the Late Cretaceous, these areas were part of a coastal plain with rivers, floodplains, swamps, and forests. This environment supported a rich ecosystem, and Avisaurus lived alongside famous dinosaurs such as Tyrannosaurus rex, Triceratops, and Edmontosaurus, as well as various other birds, mammals, reptiles, and amphibians.

The strong, possibly raptor-like feet of Avisaurus suggest it might have been a predator, perhaps preying on small animals like lizards, mammals, or other birds. It could also have been an insectivore, using its strong feet to grip branches while foraging, or potentially an omnivore. Its exact diet is not confirmed due to the limited fossil material.

Significance and Ongoing Research

Avisaurus is significant because it provides evidence of the diversity and ecological roles of enantiornithine birds in North America just before the Cretaceous-Paleogene extinction event that wiped out the non-avian dinosaurs. Its initial misidentification as a small non-avian dinosaur also highlights the challenges paleontologists face when working with incomplete fossil remains and how scientific understanding evolves with new evidence and analysis.

Ongoing research continues to focus on finding more complete specimens of Avisaurus and other avisaurids. Such discoveries would help scientists better understand their full anatomy, how they flew, their specific ecological niche, and their relationships to other early birds. Studying birds like Avisaurus helps us piece together the evolutionary history of birds and how they survived the extinction event that other dinosaur groups did not.