Fossil Evidence from Tanzania Provides New Insight into Dinosaur Evolution
The Triassic is regarded as a transitional period in the evolution of vertebrates, the Permian mass extinction event devastated the mega fauna of planet Earth, with something like 95% of all species becoming extinct. It was into this brave, new world of the Triassic, that the ancestors of the dinosaurs evolved and they, plus their archosaur relatives were soon to dominate life on land for many millions of years.
The first dinosaur fossils have been dated to approximately 230 million years ago, but a team of scientists working in Tanzania (Africa) have uncovered evidence of the closest relatives of the Dinosauria in much older rocks dating to the Early Triassic. This latest discovery, reported in the scientific publication “Nature” pushes back the origin of the dinosaurs by at least 10 million years.
Dinosaur Evolution
Although the exact evolutionary lineage of the dinosaurs is still debated, it is clear that by approximately 228-230 million years ago a number of different types of dinosaur had evolved. One of the better known early dinosaurs is Herrerasaurus. Herrerasaurus (H. ishigualestensis), was a large theropod dinosaur named and described in 1988 when skull material of this dinosaur species was found. Other elements of the skeleton had been discovered in 1959, but with the skull; a more positive description of this early, meat-eater could be made.
An Illustration of the Early Carnivore Herrerasaurus
Herrerasaurus drawing.
Picture credit: Everything Dinosaur
An Ancient Archosaur
The discovery of an ancient archosaur, a member of the archosaur group that gave rise to the dinosaurs indicates that the evolutionary line that led to Dinosauria was already established approximately 240 million years ago (Anisian faunal stage). The dinosaurs evolved from ornithodirans, a group of archosaurs that possessed a hinge-like ankle. The fossilised remains of a primitive ornithodiran (also known as an avian-line archosaur, as this lineage eventually gave rise to the birds), has been discovered in strata from Tanzania. The animal has been formerly named Asilisaurus kongwe, it has been classified as a member of the Silesauridae, a sister taxon to Dinosauria.
Asilisaurus kongwe
Interestingly, the teeth found in association with a number of the skeletons of these 1-metre-long animals indicates that they may have been at least in part, herbivorous. Prior to this new discovery, it had been thought that the closest relatives to the Dinosauria were small carnivores.
One of the authors of the report, palaeontologist Randy Irmis, of the University of California’s Palaeontology department commented:
“This shows that the lineage leading to dinosaurs goes a lot further back in time than we thought. The second thing is that it shows that there’s this real ecological diversity. No one thought that the closest relatives to dinosaurs were these four-legged, herbivorous animals, we thought they were small carnivores.”
The new findings indicate that the Dinosauria and the Silesauridae, diversified many millions of years earlier than previously thought. That means dinosaurs must have originated sometime before, putting back the origins of the Dinosaur by some ten million years or so.
Partial Skeletons
The team found more than a dozen partial skeletons of Asilisaurus in the rocky outcrops of the Tanzanian grasslands. During the Triassic, this part of Gondwanaland looked very different from today’s day savannah. The area was a large river system, with extensive woodlands. The climate would have been warm and humid for much of the year.
Lead author or the paper, palaeontologist Sterling Nesbitt of the University of Texas described the early Triassic environment of Tanzania as being like “the Mississippi today.”
Asilisaurus was primarily a quadruped with a long whip-like tail. Although closely related to the Dinosauria, the hind leg and hip bones show that it lacked the fully open acetabulum to receive the femur, a characteristic of the Dinosauria.
The beak-like jaws and leaf-shaped teeth helped these animals eat the soft, fibrous leaves of the ferns, Sphenopsids (horsetails) and conifers that were prevalent during the Triassic. The fossil evidence indicates that these animals may have had a mainly vegetarian diet.
Studying Teeth
The research team comment on the similarity between the teeth of this member of the Silesauridae and the later saurischian dinosaurs the sauropodomorphs and the ornithischian dinosaurs. They suggest that teeth designed for accommodating a diet of tough, course vegetation developed independently in these three types of reptile. All these evolutionary changes took place within 10 million years of each other.
Commenting on this aspect of their study, Irmis said:
“We were really surprised. These are three different groups that are really closely related to each other, so you’d expect that maybe their common ancestor had this tooth form. And no, it evolved independently in these three groups.”
That suggests that each of these lineages evolved separately to take advantage of a large, untapped food source, he went onto add. There may have been a sort of evolutionary arms race to exploit the rapidly diversifying new forms of vegetation that were evolving around this time, or indeed a throw back to the Permian mass extinction when many large herbivorous parareptiles had become extinct. These different types of avian-line archosaurs each exploiting a vegetarian niche in Triassic eco-systems.
Everything Dinosaur stocks a wide variety of Triassic archosaur figures: Dinosaur Models and Archosaur Figures.
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