By James Ashworth
An ancient leg bone uncovered in Africa might provide a step forward in understanding how dinosaurs and their close relatives evolved.
The 225-million-year-old fossil adds to growing evidence that an enigmatic group of reptiles known as the silesaurs played a bigger-than-expected role during the Triassic Period.
The overlooked fossil of an extinct reptile suggest that the size of the first dinosaurs might have been underestimated.
Back in 1963, a prehistoric leg bone was uncovered by a team of British scientists in what is now Zambia. While it wasn’t known at the time, the fossil femur they found was one of the first silesaur bones known to science.
Debate on the exact identity of silesaurs continues, but they’re believed to be either an early group of dinosaurs or the dinosaurs’ closest relatives. This means that the ancient reptiles can be used to offer new clues about the ancestors of dinosaurs.
Jack Lovegrove, a PhD student who led new research into the Zambian femur, says that it provides additional evidence suggesting that early dinosaurs might have shrunk over time rather than starting out as small as was previously thought.
“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,” Jack says. “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.”
“If this is the case, then some groups of dinosaurs would have actually gotten smaller across the late Triassic. 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.”
The findings of the study were published in the journal Royal Society Open Science.
What are silesaurs?
Silesaurs are a group of dinosaur-like reptiles that lived during the Triassic between 240 to 200 million years ago. Despite the ancient age of these animals, they were only recognised as a distinct group in 2010.
The best-known species is Silesaurus, which gives the larger group its name. This animal was around two metres long and had beak-like jaws that might have allowed it to eat insects or plants.
At the time of their discovery, silesaurs were initially assumed to be a sister group to dinosaurs. But subsequent research now suggests that silesaurs are actually early dinosaurs. The toothless tip of their lower jaw has been used as evidence to link them to the ornithischians, a major dinosaur group which later included animals like Stegosaurus and Triceratops.
Settling this debate has been difficult because the remains of these animals are generally fragmentary and so difficult to place on a family tree. Scientists are now hunting for new fossils to improve their understanding of these animals.
As well as searching for new discoveries out in the field, researchers have also been looking at fossils in museums. The leg bone in the current study had been kept at the Natural History Museum for more than 50 years before its significance was realised in the 2010s.
“This fossil was discovered on an expedition to what are now Zambia and Tanzania in the early 1960s,” Jack 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.”
A step change for the silesaurids
The Triassic rocks of Zambia are rich in the remains of silesaurs. To date, there is a single named species from the country, Lutungutali sitwesis, but dozens of different unidentified silesaur femur fossils have also been discovered.
Many of these fossils are around 15 centimetres in length, but the largest measures more than twice that. The different sizes of leg bone could represent Lutungutali at different ages or multiple species all living alongside each other.
While the study can’t conclusively rule one way or the other, the growth patterns recorded in the bone cared for by the Natural History Museum are different from other specimens discovered in the same region. This hints that a variety of silesaur species were all living at the same time, but scientists will need more fossils before they can confidently name them.
“It’s historically been assumed that there was just one silesaurid per area in the past,” adds Jack. “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.”
The presence of a large silesaur in the region changes our understanding of ecosystems during the Triassic. While these animals were thought to be a small part of dicynodont-dominated landscapes, it’s possible silesaurids might actually have ruled the roost.
“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,” Jack explains. “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 realised, especially as they are the most common archosaur found in this region.”
While silesaurids remain poorly understood for now, our knowledge of these reptiles is rapidly expanding. As palaeontologists dig deeper into these animals, they come closer to settling the origins of the silesaurids and the dinosaurs once and for all.