North America’s earliest dinosaur is a 230-million-year-old, chicken-sized Wyomingite

The earliest dinosaur in North America is a Wyomingite. It’s not big and there isn’t much of it, but the importance of the 230-million-year-old Ahvaytum bahndooiveche can hardly be overstated.

Paleontologists from the University of Wisconsin-Madison published a January 8 article revealing North America’s newest and earliest dinosaur. The chicken-sized, bipedal omnivore was found in the Popo-Agie Formation, a Late Triassic rock deposit in western Wyoming.

The specimen is far from complete – just two tiny bones from an animal that would have been just 30cm tall. But they definitely come from a dinosaur.

“It’s funny to talk about how we got this exciting new dinosaur, and all we have is a tiny ankle bone the size of our last little finger bone,” says David Lovelace, paleontologist and lead author of the Ahvaytum study, said Cowboy State Daily. “It’s small but mighty.”

Definitely dinosaurs

Wyoming is known worldwide for the quality and quantity of Jurassic and Cretaceous fossils preserved in its rocks. That’s why Lovelace’s research has focused on the much earlier rocks from the Triassic period.

“Wyoming’s Triassic deposits just haven’t had much work done,” he said. “This is one of the things our lab has focused on over the last decade, and the reason it took over ten years to publish this dinosaur.”

The tiny fossils from Ahvaytum were found in 2013. The signature bone of this dinosaur, the holotype specimen, is one of the bones of the dinosaur’s left ankle, called the astragalus.

A second bone, the upper end of a femur, was also identified as belonging to Ahvaytum. Lovelace admits that there is a possibility that the two bones come from two different dinosaurs, but they are undoubtedly dinosaur fossils.

“The ankle bone and femoral head have diagnostic features that define Dinosauria,” he said. “We are very lucky to have these two bones in particular because that is how we knew we had a dinosaur.”

The shape and structure of the bones differ significantly from those of the amphibians and other reptiles that Lovelace studied and described in the Popo-Agie Formation. They show that Ahvaytum walked on two legs, while most of his larger contemporaries were quadrupeds.

Lovelace also said that the shape of the ankle and femur provides insight into the later lineage of the early dinosaur. Ahvaytum was a dinosaur or lizard-hipped dinosaur, the ancestor of the Wyoming sauropods such as Brontosaurus and Diplodocus and the theropods such as Tyrannosaurus.

“We believe it is a sauropodomorph, specifically like the Eoraptor from Argentina,” he said. “But we’re talking about one bone out of hundreds in his skeleton,” he said. “This can certainly change as more information is collected or new specimens are found.”

Small but mighty

Not much more can be said about Ahvaytum by examining the only two bones currently known. However, Lovelace said that by examining his relatives they would make some educated guesses.

“We can use phylogenetic inference to gain some insights,” he said. “It was probably an omnivore with sharp teeth in its mouth, which is strange to imagine. The ancestors of the giant herbivorous sauropods still had sharp, pointed teeth.”

Lovelace said a look into the femur revealed that the dinosaur was adult but not fully grown when it died, walked on two legs and was “almost certainly feathered.”

“We published a paper in 2020 describing the heat capacity of these animals,” he said. “From the metabolism based on the internal structures of the bones, we know that they were warm-blooded and feathered because of the environment. This is what they needed to live.”

In life, Ahvaytum would have been about 30 cm tall and 90 cm long – a chicken with tiny teeth and a long tail. This makes it one of the smallest dinosaurs in Wyoming and possibly the smallest ever found.

It is also Wyoming’s earliest dinosaur and the first Triassic dinosaur in the Cowboy State to be identified from skeletal material. Wyoming’s only other Triassic dinosaur, Agialopus, was identified in 1933 from a single footprint also found in the Popo-Agie Formation.

Northern aggression

In addition to its importance as Wyoming’s earliest dinosaur, Ahvaytum is also North America’s earliest known dinosaur. Therefore, the significance of this discovery can hardly be overestimated.

During the Late Triassic, the supercontinent Pangea split in two, forming the continents Laurasia and Gondwana. Modern-day Wyoming was in the middle of Laurasia and was thought to have been completely free of dinosaurs 230 million years ago.

Lovelace explained that while fossils of 230-million-year-old dinosaurs are common in the rocks of former Gondwana (the present-day continents of South America, Africa, Australia and Antarctica), they were previously unknown in the rocks of former Laurasia.

The absence of these dinosaurs in the Northern Hemisphere led to the hypothesis that dinosaurs originated in Gondwana and that something prevented them from moving to Laurasia.

“The oldest dinosaur specimens come from Argentina, Brazil and other layers of the southern hemisphere,” he said. “There were almost 9 to 10 million years between the first appearance of dinosaurs in the southern hemisphere and the northern hemisphere. So there was a hypothesis that climatic barriers prevented the expansion of dinosaurs into the Northern Hemisphere until much, much later.”

The discovery of Ahvaytum refutes this hypothesis. Dinosaurs were just as common and evolved in Laurasia as they did in Gondwana 230 million years ago.

Lovelace is convinced of this claim, thanks to the zircon dating of the rocks of the Popo-Agie Formation. High-precision dating using uranium preserved in the rock shows that the youngest rocks in the formation are at least 229 million years old.

“Ahvaytum is below the top of the formation, giving us an approximate age of 230 million years,” he said.

However, Lovelace does not completely rule out the possibility that climatic barriers prevented the spread of dinosaurs in the Late Triassic. At that time, Wyoming was at a latitude between 5 and 10 degrees, just above the equator.

“Maybe they just made it to the equator,” he said, “but the fact is that at a northern latitude we have a dinosaur that is the same age as the dinosaurs in the southern hemisphere.” That tells us that dinosaurs were too were widespread all over the world at that time.”

More than a partnership

Most dinosaur names come from Greek and Latin. Ahvaytum is the first dinosaur named in the Eastern Shoshone language.

Lovelace said the naming is part of an ongoing relationship between his team at the University of Wisconsin-Madison and the Eastern Shoshone Tribe on the nearby Wind River Reservation. Since the dinosaur was found on the tribe’s ancestral land, the paleontology team wanted to honor this and reflect it in its scientific name.

“Traditionally, it was exclusively white European men who engaged in all of science,” he said. “Researchers are beginning to open up to the idea of ​​representing the cultures and communities of the areas in which they work in the taxa they describe.”

Eastern Shoshone tribal elders and middle school students helped Lovelace’s team name their important discovery. Ahvaytum bahndooiveche roughly translates to “dinosaurs a long time ago.”

Several new species recently discovered in the Popo-Agie Formation have names derived from the Eastern Shoshone language. Lovelace regularly invites elders and students to his ongoing excavations during the summer and learns from their cultural traditions.

“We wanted to go one step further and create a partnership that will last for many years to come,” he said. “As Western scientists, we have learned how to approach the land with more respect from the perspective of local traditions. It was very special for us.”

Assembling a paleoenvironment

Work continues on the Popo-Agie Formation in Wyoming. Lovelace said they are looking for more fossils from Ahvaytum, but there are a variety of new animals from the Late Triassic waiting to be described and named.

“We are excavating some fantastic new sites that definitely contain new taxa,” he said. “I don’t know if we have more dinosaurs, but I wouldn’t be shocked if we found more.”

During the Late Triassic, the Popo-Agie Formation resembled the coastal plains of present-day Texas. Ahvaytum would have scurried through the undergrowth, dodging the jaws of much larger reptiles such as the 17-foot-long Heptasuchus, crocodile-like phytosaurs, and four-foot-long burrowing amphibians.

In addition to being largely overlooked, early paleontologists had a “collecting bias” in their work in the Popo-Agie Formation. Large fossils, which were easier to find, identify and excavate, took precedence over fragmentary specimens.

Lovelace and his team study every fossil they find as they slowly restore Wyoming to the way it was in the Late Triassic. In the same paper, the team described a previously unknown silesaur, another tiny reptile that is closely related but not entirely a dinosaur.

Ahvaytum shows how significant a tiny bone can be.

“It’s a pretty poor specimen, but it’s a dinosaur,” he said. “It’s less about this particular taxon and more about the fact that we have a dinosaur in this place and time that changed our view of dinosaur evolution.”

Andrew Rossi available at [email protected].