A 220-million-year-old pond in Arizona held 26 new species and the oldest known caecilian

At Thunderstorm Ridge in Petrified Forest National Park, Arizona, fossil hunters have found a Late Triassic pond community of about 220 million years ago that holds fossils of 26 species new to science. They include the oldest caecilian in the global fossil record and the oldest frogs and possible salamanders known from North America. Ben Kligman, a physical science technician at the park and a doctoral candidate at Virginia Tech, reported the finds in the Fall 2021 issue of Park Paleontology News. Park paleontologists, interns and collaborators dug there in the summers of 2019 and 2021.

Why the Triassic matters

After the end-Permian mass extinction, about 250 million years ago, life recovered in a changing world. The Triassic, 251 to 201 million years ago, saw the Manicouagan asteroid impact (about 214 million years ago) and the start of the breakup of the supercontinent Pangaea (about 235 million years ago), which triggered the massive eruptions behind the end-Triassic mass extinction about 201 million years ago. In this interval the earliest members of every living group of four-limbed animals appear: caecilians, salamanders, frogs, lizards, relatives of the tuatara, turtles, crocodile relatives and dinosaurs, a group that includes living birds. Yet the Triassic record for the small groups is extremely scant, perhaps because their small, delicate skeletons rarely fossilize or because they are hard to find compared with a dinosaur’s big bones. The earliest reported communities that contained all these groups together came from Early Cretaceous rocks in Morocco, about 145 million years old, so a Triassic community like that had not been seen.

Thunderstorm Ridge

The site lies in the upper Blue Mesa Member of the Chinle Formation, on a former ranch the park recently acquired. Then, the park was in humid tropical lowlands of central Pangaea, where a large river and lake system flowed north toward the ancestral Pacific Ocean in what is now Nevada. The fossils were probably laid down at the edge of a lake or pond, where the remains of at least 55 vertebrate species piled up with plants, crustaceans and mollusks. They include freshwater sharks, lungfish, coelacanths, bony fish, amphibians, mammal relatives and many reptiles.

Stratigraphic setting of Thunderstorm Ridge, a map of the park showing its location, and a reconstruction of Earth in the Late Triassic. Image from the National Park Service’s page.

A fossil collection and preparation method developed by the park’s paleontologists preserved even small, delicate bones, giving a rare view of a whole Late Triassic land vertebrate community. It revealed a diversity of small tetrapods, including 26 species new to science, among them eight new species of tuatara relatives and stem lizards, as well as stem turtles, crocodile relatives, dinosaurs and stem mammals. The community is the earliest known to include all members of today’s main tetrapod groups, which suggests a largely hidden history of such communities awaits discovery in Upper Triassic and Jurassic rocks worldwide.

A selection of Thunderstorm Ridge species and a diagnostic bone for each: a stem caecilian (lower jaw), a stem frog (hip bone), a stem salamander (lower jaw and vertebra), a stem turtle (upper jaw bone), a crocodile relative (thigh bone and a bony plate), a theropod dinosaur (foot bone) and a stem mammal (lower jaw). The two jaws of the caecilian and salamander are shown as 3-D reconstructions from CT scans, and the other bones as photographs. Image from the National Park Service’s page.

Ten kinds of tuatara-relative and stem-lizard lower jaws from Thunderstorm Ridge; all but one are new species. Image from the National Park Service’s page.

What it suggests

The Thunderstorm Ridge fossils suggest these communities may have stayed relatively stable through the Manicouagan impact and the end-Triassic mass extinction. They also raise a question: could this very diverse fauna signal the richness of humid tropical settings in Pangaea, which may be preserved elsewhere? Present-day tropical ecosystems hold most of living diversity, and the pattern may reach back to the Triassic, but Triassic rocks from such settings are rare and poorly studied outside this park, which makes studying similar rocks worldwide important for understanding the origins of today’s life.

Sources

• Ben Kligman, Physical Science Technician at Petrified Forest National Park and PhD candidate in the Department of Geosciences at Virginia Tech, “NPS Paleontologists Discover Hidden Cradle of Tetrapod Evolution at Petrified Forest National Park,” Park Paleontology News, Fall 2021, https://www.nps.gov/articles/000/tetrapod-evolution-at-petrified-forest.htm; rewritten in hubnx’s own words. The page’s field photograph naming crew members and an artist’s reconstruction credited to private individuals are left out.