Description and Classification

Paraxenisaurus was a medium-sized theropod dinosaur, estimated to be about 5.7 meters (19 feet) long and weighing around 600 kilograms (1,300 pounds). Like other ornithomimosaurs, often called “ostrich-mimic” dinosaurs, Paraxenisaurus would have been bipedal, meaning it walked on two legs. It likely had a long neck, a small head possibly with a toothless beak, and long arms. Scientists believe many ornithomimosaurs were covered in feathers, so Paraxenisaurus might have been feathered as well. What makes Paraxenisaurus somewhat unusual for an ornithomimosaur is that its bones suggest it was more robustly built compared to many of its more slender relatives.

Paraxenisaurus belongs to the group Ornithomimosauria. The scientists who named it classified Paraxenisaurus within the family Deinocheiridae. This family includes the giant and unusual dinosaur Deinocheirus from Asia, which was known for its enormous arms. If this classification is correct, Paraxenisaurus would be one of the first well-documented deinocheirids from North America, making it a relative of other large ornithomimosaurs like Deinocheirus and Beishanlong.

Distinguishing Features

Paraxenisaurus can be recognized by a unique combination of features, some of which differ from other ornithomimosaurs:

• A more robust and heavily built body compared to the typically slender build of many other ornithomimosaurs like Struthiomimus or Gallimimus.
• Specific characteristics in its vertebrae (backbones), including low and sturdy neural spines on some of its back vertebrae.
• The first bone in its hand (metacarpal I) was more robust than the second and third metacarpals, which is distinctive.
• Unique features on its foot bones, including a particular triangular scar on the fourth metatarsal (a long bone in the foot) where a muscle likely attached.
• Its hand and foot claws (unguals) were relatively straight and not as strongly curved as seen in some other theropods, which might suggest differences in how it used its hands and feet.

Paleoenvironment and Diet

Paraxenisaurus lived in what is now Coahuila, Mexico, during the Late Cretaceous period. The environment at that time, preserved in the Cerro del Pueblo Formation, was a coastal plain with rivers, deltas, and likely a warm, humid climate. This area supported a rich variety of life. Other dinosaurs that shared this habitat with Paraxenisaurus included hadrosaurs (duck-billed dinosaurs) like Velafrons and Latirhinus, horned dinosaurs such as Coahuilaceratops, armored ankylosaurs, and other theropods like tyrannosaurids and dromaeosaurids.

The exact diet of Paraxenisaurus is not known for certain, but most ornithomimosaurs are thought to have been omnivores. This means they likely ate a mix of plants, insects, and small animals. Given its more robust build compared to other ornithomimosaurs, some scientists suggest Paraxenisaurus might have had a slightly different diet or way of gathering food than its more lightly built relatives, possibly eating tougher plants or different types of small prey. However, more research is needed to understand its specific eating habits.

Significance and Ongoing Research

The discovery of Paraxenisaurus is significant for several reasons. It adds to our knowledge of the diversity of ornithomimosaurian dinosaurs, particularly the deinocheirid group. Finding a deinocheirid like Paraxenisaurus in North America is important because most other known deinocheirids, such as Deinocheirus, come from Asia. This suggests that this group of dinosaurs was more widespread than previously thought and helps scientists understand how these animals spread across different continents during the Cretaceous period.

Ongoing research on Paraxenisaurus will likely focus on further studying its fossil remains to confirm its relationships with other ornithomimosaurs and to learn more about its unique features. Paleontologists will also be on the lookout for more complete skeletons, which could provide even more information about its appearance, biology, and how it lived. Understanding dinosaurs like Paraxenisaurus helps paint a clearer picture of the ecosystems in ancient Mexico and the broader story of dinosaur evolution before their extinction.