Description and Classification

Labrosaurus is known primarily from a fragment of a lower jaw. If it was similar to its close relative Allosaurus, it would have been a large, two-legged predator, possibly reaching lengths of 8 to 10 meters (26 to 33 feet) and weighing one to two tons. Like other theropods, it would have had a large head with sharp, saw-edged teeth, powerful legs for running, and short arms with three-fingered hands equipped with sharp claws. Its long tail would have helped it balance while moving or hunting.

Labrosaurus belongs to the group Theropoda, which includes most carnivorous dinosaurs. More specifically, it is often placed within the Allosauridae family or considered closely related. The original fossil showed an unusual gap between teeth, which was once thought to be a defining feature. However, this gap is now widely believed to be the result of an injury or disease in that particular individual, rather than a characteristic of a unique type of dinosaur. Because the original fossil evidence is so limited and potentially misleading, many paleontologists consider Labrosaurus a nomen dubium, which means “doubtful name,” suggesting it might not be a valid, separate genus.

Distinguishing Features

The main feature initially used to identify Labrosaurus was a noticeable gap or notch in the tooth row of the lower jaw bone discovered by Marsh. This was thought to make it different from other theropods of its time.

However, here are some points to consider about this feature:

• Many scientists now think this gap was not a natural feature of a healthy Labrosaurus. Instead, it might have been caused by an injury, such as a lost tooth that didn’t grow back properly, or a bone disease in that one particular dinosaur.
• If Labrosaurus is actually the same as Allosaurus, then an Allosaurus individual simply had a damaged jaw.
• If it were a distinct genus, other features would be needed to separate it from Allosaurus, but the fossil evidence is very scarce.

Therefore, instead of unique features, the discussion around Labrosaurus often focuses on whether its supposed characteristics are real or misinterpreted from limited evidence. If it were an Allosaurus, its distinguishing features would be shared with that genus, such as a relatively lightweight but strong skull, small horns or crests above its eyes, and a robust build.

Paleoenvironment and Diet

Labrosaurus lived in what is now western North America during the Late Jurassic period. This region, known as the Morrison Formation, was a vast plain with rivers, lakes, and open woodlands. The climate was likely warm and seasonally dry, with wetter periods that supported lush vegetation like ferns, cycads, and conifers along waterways.

This environment was home to a wide variety of dinosaurs. Labrosaurus would have shared its habitat with giant long-necked sauropods like Diplodocus, Apatosaurus, and Brachiosaurus, as well as armored dinosaurs like Stegosaurus. Other predatory dinosaurs in the Morrison Formation included Ceratosaurus and the very common Allosaurus. As a large theropod, Labrosaurus would have been a carnivore, hunting other dinosaurs or possibly scavenging their remains. Its diet likely consisted of the herbivorous dinosaurs available in its ecosystem.

Significance and Ongoing Research

Labrosaurus is significant in paleontology mainly as an example of how difficult it can be to classify dinosaurs from very limited fossil remains. Its story highlights the challenges early paleontologists faced and how scientific understanding can change over time as new evidence is found or existing fossils are re-examined.

Ongoing research and discussion about Labrosaurus primarily revolve around its validity as a distinct genus. Most modern paleontologists lean towards the view that Labrosaurus is not a separate type of dinosaur but is likely a synonym of Allosaurus, meaning the jawbone belonged to an Allosaurus, possibly one with an injury. Some consider it a nomen dubium due to the insufficient and potentially misleading nature of its original fossil. Further discoveries of similar jawbones with the same “distinctive” features would be needed to reconsider it as a valid genus, but such finds have not occurred. Thus, Labrosaurus serves as an important case study in the rules and practices of naming and classifying extinct animals.