Three skulls from animals belonging to the genus Hypsognathus were uncovered in Pennsylvania. Two were found in a mudstone deposit at a construction site in Exeter Township, and a third was found in the town of Pennsburg. These sites are located in the area between Allentown and Pottstown, Pennsylvania, and were part of the Newark rift basin millions of years ago, formed as the supercontinent Pangea began to break apart. The remains of these animals have been dated to just 500,000 years before the extinction at the end of the Triassic period, approximately 200 million years ago. They present paleontologists with a rare fossil record of animals that lived during this time.

The Hypsognathus reptiles were 1-foot-(0.3-meter-) long horned herbivores, somewhat analogous to modern-day groundhogs. They lived during the Triassic period among animals such as amphibians and reptiles, including the then-relatively new and small dinosaurs. Scientists believe something happened about 200 million years ago: a mass extinction that changed the course of evolution that allowed the dinosaurs to dominate and grow into larger and more diverse forms. Evidence for this mass extinction has been very rare, mainly due to a lack of skeletal remains from this time period. The discovery of the Hypsognathus skulls shows these animals lived right up to the boundary between the Triassic and Jurassic, thus lending support to the mass extinction theory.