A dog-size reptile slipped through fern-choked forests 237 million years ago in what is today Paraíso do Sul, Brazil. The animal had the body of a greyhound, a long neck and tail, and a small, nipping beak. Running on four, upright legs, the reptile looked remarkably like an early dinosaur. Yet it lived almost 10 million years before any other known dinosaurs.
A study published last month in the journal Gondwana Research argues that this animal, Gondwanax paraisensis, in fact is one of the oldest dinosaurs ever found. Further support for this finding could solve the mystery of how and where the largest family of plant-eating dinosaurs emerged.
Since the 1800s, most paleontologists have agreed that all dinosaurs share a common ancestor that emerged in the Triassic Period about 200-250 million years ago, said Steve Brusatte, who is a paleontologist at the University of Edinburgh and was not involved in the study. They were reptiles called archosaurs, and some had shared anatomical traits.
Those ancestral dinosaurs split into three major subgroups: predatory theropods, long-necked sauropods and the ornithischians. The last group included diverse and well-known beaked herbivores like Triceratops, Stegosaurus and Iguanodon.
Yet while some early theropods and sauropods are known from 230-million-year-old Triassic rocks, there are “just a few scrappy, fragmentary road kill fossils” that could possibly — “if you squint”— be Triassic ornithischians, Dr. Brusatte said. That absence of clear evidence caused some researchers to wonder whether the ancestral ornithischians they were looking for were hiding in plain sight.
Enter the silesaurids, members of an enigmatic family of small- to medium-sized omnivores. The silesaurids have traditionally been considered either direct forerunners of dinosaurs, or extremely close relatives, said Rodrigo Temp Müller, who is a paleontologist at the Universidade Federal de Santa Maria in Brazil and an author of the new study.
Some paleontologists have argued that silesaurids — rather than being a singular family of close dinosaur relatives — were an artificial grouping containing some of those ornithischian ancestors. Many of them share specific anatomical features with later ornithischian dinosaurs, like ankle bones and beaked jaws.
“The problem is that we have a lot of material of early silesaurids, but the forms that are probably most related to ornithischians are rare,” Dr. Müller said.
Early silesaurids also tend to have several primitive features in their skeletons that make establishing their lineage tricky, he added, and material from later members of the family is often missing the most useful pieces of anatomy, like skulls and forelimbs.
In January, Dr. Müller received a fossil donation from Pedro Lucas Porcela Aurélio, a doctor and amateur fossil hunter. In the chunk of rock, Dr. Müller identified the hips, femur and vertebrae of a silesaurid.
As always with the family, not everything was advanced: The femur lacked specific traits connecting the muscles of the leg to the tail, suggesting it walked less efficiently than other silesaurids or dinosaurs, Dr. Müller said. He named it Gondwanax.
But unlike other reptiles and early members of its family, Gondwanax had three vertebrae in its hips rather than two, part of a trend toward an increase in the number of hip vertebrae that looks distinctively dinosaurlike. That suggests Gondwanax and its kin are either Triassic ornithischians or their direct ancestors, and thus dinosaurs in their own right.
If that’s true, Dr. Brusatte said, it will add as many as seven to 10 million years to the history of ornithischians. It also suggests that dinosaurs were much more common and widespread earlier than had been previously guessed.
However, it’s too early to call the case closed.
“I wish I could say that this new silesaurid fossil solves the riddle once and for all, but for me, it is still an open question,” Dr. Brusatte said.
Dr. Müller agrees. “At this point it’s really messy,” he said. “The origin of dinosaurs is so obscure right now that there’s no consensus — both hypotheses are well supported. If we get more material, probably we will solve it.”
More material is likely on the way. In early October, Dr. Müller and his colleagues discovered a new, more complete silesaurid specimen — possibly also Gondwanax. He plans to begin preparing the fossils soon — and contribute more evidence for specialists seeking a definitive answer to the biggest mystery in dinosaur history.
“I believe that in the next few years, there will be far more specimens of silesaurids and dinosaur relatives,” Dr. Müller said.
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