A dusty leg bone, pulled from the ground in 1963 and stored away in a museum drawer for decades, may now be helping scientists rethink the early days of dinosaurs.
Originally found in what’s now Zambia, this 225-million-year-old fossil was largely ignored for years.
However, new research suggests the fossil holds key information about an ancient group of reptiles known as silesaurs. The study may shift long-held assumptions about the size and role of the first dinosaurs.
Fossil challenges dinosaur size
Researchers have been re-examining the femur – first uncovered by British scientists more than 60 years ago – and what they’ve learned is surprising.
“Some fragmentary fossils from the silesaurids and a group of early dinosaurs called the herrerasaurids suggest that these animals could grow much bigger than more complete remains suggest,” said Jack Lovegrove, a Ph.D. student who led the research.
“As more large animals are found close to the origin of dinosaurs, it raises the possibility that the first dinosaur was bigger than we predicted.”
This idea runs counter to the long-standing belief that early dinosaurs started out small and gradually evolved into giants. If Lovegrove’s team is right, dinosaurs may have actually shrunk in size over time.
“If this is the case, then some groups of dinosaurs would have actually gotten smaller across the late Triassic,” Lovegrove said. “Finding more silesaur fossils could help us to figure out how they are related to early dinosaurs and confirm the existence of these size trends.”
Fossils of dinosaur relatives
Silesaurs are an extinct group of dinosaur-like reptiles that lived roughly between 240 and 200 million years ago. For a long time, scientists couldn’t quite figure out where they belonged on the reptile family tree. Were they dinosaurs? Close relatives? Something else entirely?
In 2010, silesaurs were officially recognized as their own group. The most famous example is Silesaurus, a two-meter-long animal with a beak-like jaw that may have helped it eat insects or plants.
Originally considered dinosaur cousins, silesaurs are now thought by some experts to actually be early dinosaurs. One clue is the toothless tip of their lower jaw, which links them to ornithischians – a major group of dinosaurs that would later include species like Triceratops and Stegosaurus.
But even now, placing silesaurs neatly into the dinosaur family tree is tricky. Most of the fossils found are incomplete, which makes identification difficult. That’s why researchers are taking a fresh look not just in the field, but in museum collections.
Old fossil comes back into focus
The femur at the center of this study had been sitting in the Natural History Museum in London for over 50 years before its importance was recognized.
“This fossil was discovered on an expedition to what are now Zambia and Tanzania in the early 1960s,” Lovegrove explains. “The researchers were mainly interested in studying mammal-like reptiles such as the dicynodonts and cynodonts but also found a variety of other fossils.”
“As the fossil wasn’t what they were focusing on, it hadn’t been studied until one of my co-authors, Brandon, came across it. This shows how important museum collections are at preserving specimens whose importance can be appreciated by future generations.”
Silesaurs: One species or many?
The rocks in Zambia where this bone was found are packed with silesaur fossils. Only one species from the region has been formally named – Lutungutali sitwesis – but researchers have found many leg bones that don’t seem to match that species.
Most are about 15 centimeters long. One, however, is more than double that size. This raises two possibilities. Either the large bones belonged to Lutungutali at a more mature stage of life, or multiple species were living side by side.
The team can’t say for sure yet, but the new study shows clear differences in growth patterns between the Natural History Museum bone and others from the area. That might suggest that more than one type of silesaur lived in the same ecosystem.
“It’s historically been assumed that there was just one silesaurid per area in the past,” said Lovegrove. “As a result, fossils from different species might have been lumped together.”
“This could explain the uncertainty we find when we try to understand how silesaurids relate to other animals. New datasets will be important to untangle these evolutionary relationships and work out what’s really going on.”
Bigger than we thought
There’s another piece to this puzzle: size. If silesaurs like the one in the study were as large as the leg bone suggests, they may not have been minor players in their environment. They might have been in charge.
“The size of the bone we’ve studied, as well as others from this formation, suggest that silesaurids might have been the largest herbivores in some parts of the world at this point in the Triassic,” noted Lovegrove.
“The biggest silesaurids were probably taller and longer than the dicynodonts, even if they were lighter. It suggests they probably had a much greater impact on the ecosystem than we’ve realized, especially as they are the most common archosaur found in this region.”
Rewriting the story of dinosaurs
The early evolution of dinosaurs is still full of questions, but fossils like this Zambian femur are helping scientists get closer to the answers.
Silesaurs may have been bigger, more diverse, and more influential than anyone thought. And if they turn out to be early dinosaurs themselves, we may need to rewrite the story of how dinosaurs came to be.
For now, one thing is clear: sometimes the most important discoveries are already sitting on a shelf – waiting for someone to take a second look.
The full study was published in the journal Royal Society Open Science.
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