Oryctodromeus

MEANING: Digging runner

PERIOD: Late Cretaceous

CONTINENT: North America

Oryctodromeus is a basal ornithischian dinosaur that lived in the Late Cretaceous of what is now North America. It was a small, fast running herbivore, and the first non-avian dinosaur discovered with direct evidence of burrowing behavior. Oryctodromeus was about 2 m in body length, and weighed around 30 kg.

Oryctodromeus is from the Late Cretaceous. The Cretaceous is the third and final geological period of the Mesozoic Era, with the Late Cretaceous making up roughly the second half of it, lasting from about 100 to 66 million years ago. It was a time of significant evolutionary change, with dinosaurs reaching their greatest diversity before the mass extinction at the end of the Cretaceous.

The Cretaceous was a period with a relatively warm climate, though the Late Cretaceous experienced a global cooling trend, caused by falling levels of carbon dioxide in the atmosphere. The continents were nearing their present positions, but high sea levels flooded low-lying regions, turning Europe into an archipelago, and forming the Western Interior Seaway in North America. These seas were home to a variety of marine reptiles, including mosasaurs and plesiosaurs, while pterosaurs and birds shared the skies.

On land, dinosaurs continued to thrive and diversify during the Late Cretaceous, producing many of the most well-known goups, including tyrannosaurs, hadrosaurs, and pachycephalosaurs. Established Cretaceous dinosaur clades like the ceratopsians, ankylosaurs, and dromaeosaurs continued to flourish. Sauropod species consisted almost exclusively of titanosaurs, which seemed to be confined to the Southern Hemisphere for much of the Late Cretaceous. Flowering plants and grasses diversified and spread, becoming the dominant flora similar to what we see today.

The Cretaceous (along with the Mesozoic) ended with the Cretaceous-Paleogene (K-Pg) extinction event, a large mass extinction in which many groups, including non-avian dinosaurs, pterosaurs, and large marine reptiles, died out. This event, likely triggered by an asteroid impact, is marked by the abrupt K-Pg boundary, a distinct geologic layer separating the Mesozoic and Cenozoic Eras. In its aftermath, mammals and avian dinosaurs rapidly diversified, becoming the dominant land animals of the Cenozoic Era.

Oryctodromeus is a basal ornithischian. Ornithischia is one of the two major clades of dinosaurs, sister to the saurischia, which includes theropods and sauropods. Basal ornithischians were small, lightly built herbivores or omnivores and were typically bipedal, with long hind limbs and relatively short forelimbs, well-suited for swift movement. Their body plans were often simple, lacking the extreme specializations seen in later ornithischians like ceratopsians or hadrosaurs. Many had simple, triangular skulls with leaf-shaped teeth, suitable for cropping low vegetation, and some evidence suggests they may have lived in groups. Though they form the base of the ornithischian family tree, they do not belong to any of the more derived subgroups and instead exhibit a mix of ancestral traits.

Basal ornithischians are mostly known from the Jurassic. The absence of definitively known ornithischians from the Triassic has puzzled paleontologists, though some propose that certain Triassic animals, like the silesaurs, may fill this gap. However, these relationships remain debated. As the Jurassic progressed, basal forms gave rise to a wide array of more specialized ornithischians, but these early dinosaurs remain a crucial and still somewhat mysterious chapter in the evolutionary history of the group.