Some 252 million years ago, a series of volcanic eruptions in what is now Siberia is thought to have triggered the massive extinction that wiped out 90 percent of the planet’s living species. Just after the Permo-Triassic extinction, scientists say, a newly identified ancient reptile known as Teyujagua paradoxa, or "fierce lizard," roamed alongside the lakes of Brazil, feeding on fish, amphibians and smaller reptiles. By studying Teyujagua’s well-preserved fossilized skull, recently discovered in the Brazilian state of Rio Grande do Sul, scientists hope to fill the gap in their understanding between primitive reptiles and archosauriforms, a group that includes dinosaurs as well as modern crocodiles and birds.

The Permo-Triassic extinction, also known as the Great Dying, marked the boundary between two geologic periods (the Permian and the Triassic) and two eras (the Paleozoic and Mesozoic). The first archosauriforms, the diverse group that spawned dinosaurs, crocodiles and birds, emerged during the late Permian period, prior to the mass extinction. In its aftermath, at the dawn of the Triassic, archosauriforms would come to dominate the world’s ecosystems, maintaining that position for almost the entire Mesozoic era—some 175 million years.

How did this ancient group come to be so dominant? Scientists are trying to answer this question by studying the evolution of early archosauriforms, but they have been limited in their efforts by major holes in the fossil record dating to the era around the Permo-Triassic extinction. Recently, however, scientists from three Brazilian universities discovered a well-preserved fossil skull in an exposure of Triassic rock near the southern city of São Francisco de Assis. In a study published last week in the journal Scientific Reports, they explain how the new species—Teyujagua paradoxa—promises to open a window into the little-understood time just before the dinosaurs appeared.

According to the new study, the small, crocodile-like Teyujagua is a very close relative to archosauriforms, and lived in Brazil in the period just after the Permo-Triassic extinction, some 250 million years ago. From its skull, scientists know the species had curved teeth that were sharply pointed and serrated, suggesting it was carnivorous. They believe Teyujagua, which grew up to about five feet in length, lived alongside lakes and fed on fish, amphibians and the extinct, lizard-like reptiles known as procolophonids. Its nostrils were located on the upper part of the snout like many aquatic or semi-aquatic animals, including modern crocodiles.

Dr. Felipe Pinheiro of the São Gabriel campus of the Universidade Federal do Pampa in Rio Grande do Sul, one of the scientists who discovered the Teyujagua skull, told BBC News that “Ever since we saw that beautiful skull for the first time in the field, still mostly covered by rock, we knew we had something extraordinary in our hands.”

Pinheiro and his colleagues say Teyujagua is different from other fossils found of species that existed during the same era. Because its anatomy puts it somewhere in between more primitive reptiles and archosauriforms, the scientists believe Teyujagua fills a crucial evolutionary gap, and will help them understand dinosaurs and other early archosauriforms better. In particular, the newly discovered species may shed some much-needed light on the archosauriform skull—a key evolutionary advance that allowed dinosaurs to become so dominant in the Mesozoic era—and how it first developed.