Overview

Anodontosaurus (AN-oh-DONT-oh-SAWR-us; “toothless lizard”) is a genus of armored dinosaur belonging to the ankylosaurid family. It roamed what is now Alberta, Canada, during the Late Cretaceous period, approximately 72.8 to 66 million years ago, making it one of the later non-avian dinosaurs. The name “toothless lizard” arose because the first skull discovered was damaged, and its teeth were not immediately apparent. However, Anodontosaurus did possess teeth. First named by Charles M. Sternberg in 1929, it was long considered a species of the more widely known Euoplocephalus, but more recent research has established it as a distinct genus.

Description and Classification

Anodontosaurus was a heavily built, quadrupedal herbivore, characteristic of the Ankylosauridae group. Scientists estimate it reached lengths of about 4 to 6 meters (13 to 20 feet) and may have weighed around 2 to 3 metric tons. Its body was covered in an extensive armor of bony plates and knobs, called osteoderms, embedded in its skin, providing significant protection from predators. Like other advanced ankylosaurids, Anodontosaurus possessed a formidable tail club, formed from fused osteoderms at the end of its stiff tail, which it likely swung as a defensive weapon.

The skull of Anodontosaurus was broad, roughly triangular when viewed from above, and also heavily armored with fused bony plates. Its teeth were small and leaf-shaped, suited for cropping low-lying vegetation. Anodontosaurus is classified within the family Ankylosauridae, and more specifically within the subfamily Ankylosaurinae. It is considered a close relative of other North American ankylosaurs such as Euoplocephalus tutus and the giant Ankylosaurus magniventris.

Distinguishing Features

Anodontosaurus can be distinguished from its close relatives by a combination of features, particularly concerning its skull and armor:

• The skull roof tends to be relatively flat.
• The squamosal horns, which are the large bony projections at the back corners of the skull, are typically triangular, point mostly backward and slightly outward, and often possess a distinct keel or ridge.
• The pattern and shape of the caputegulae (small armor plates) on the snout, especially on the nasal bones, can be diagnostic.
• Its tail club is generally rounded and proportionally wide, sometimes as wide as or wider than it is long.
• The cervical half-rings, which are bony collars protecting the neck, had a specific arrangement and morphology of their fused osteoderms.

Paleoenvironment and Diet

Fossils of Anodontosaurus have been primarily recovered from the Horseshoe Canyon Formation in Alberta, Canada. During the Late Cretaceous, this region was a dynamic coastal lowland plain, featuring extensive river systems, swamps, estuaries, and forests. The climate was generally warm and humid, with seasonal variations. Anodontosaurus shared this environment with a diverse community of other dinosaurs, including hadrosaurs like Edmontosaurus, ceratopsians such as Pachyrhinosaurus, other ankylosaurs, and theropods like the tyrannosaurid Albertosaurus, which was a potential predator.

As an herbivore, Anodontosaurus likely fed on low-growing plants. Its broad muzzle and relatively small teeth suggest it was a non-selective browser, consuming ferns, cycads, and early flowering plants available in the undergrowth of the Late Cretaceous forests and floodplains.

Significance and Ongoing Research

The re-evaluation and recognition of Anodontosaurus as a distinct genus have significantly contributed to a better understanding of ankylosaur diversity in North America during the final stages of the Cretaceous period. For many years, most ankylosaur material from formations like the Horseshoe Canyon was attributed to Euoplocephalus, but it’s now clear that several distinct types of ankylosaurs co-existed or succeeded each other.

Ongoing research continues to refine the anatomical differences between Anodontosaurus and its relatives, helping to clarify their evolutionary relationships. Studies also focus on the paleoecology of these armored dinosaurs, including their defensive strategies involving their tail clubs, their locomotion, and their role within their ecosystems. The precise stratigraphic positions of Anodontosaurus fossils within the Horseshoe Canyon Formation are also crucial for understanding faunal turnover and changes in dinosaur communities leading up to the Cretaceous-Paleogene extinction event, which Anodontosaurus lived very close to in time.