Newly published research in the journal “Nature” postulates that reptiles originated forty million years earlier than previously thought. Fossil tracks (trace fossils) created by a small tetrapod with clawed feet, most probably a reptile, from the Snowy Plains Formation of Victoria, Taungurung Country significantly changes our view on the timeline of early amniote evolution.
The known fossil record of crown-group amniotes (the group that includes mammals, birds and reptiles), begins in the Late Carboniferous with sauropsid trace fossils and the body fossils of Hylonomus. The earliest body fossils of crown-group tetrapods are mid-Carboniferous, and the oldest trackways are Early Carboniferous. Based on this evidence, it had been thought that the tetrapod crown group originated in the earliest Carboniferous (early Tournaisian), with the amniote crown group appearing in the early part of the Late Carboniferous. This new fossil discovery challenges this view.
Flinders University Professor John Long and colleagues identified the fossilised tracks of an amniote with clawed feet. The slab containing the fossil tracks is around 350 million years old.
Professor Long commented:
“Once we identified this, we realised this is the oldest evidence in the world of reptile-like animals walking around on land, and it pushes their evolution back by 35-to-40 million years older than the previous records in the Northern Hemisphere.”
Professor Long and his co-workers think the tracks were made by a small, lizard-like creature.
A reconstruction of an early amniote. The researchers believe the tracks were produced by an early reptile. This suggests that reptiles evolved some 40 million years earlier than previously thought. Picture credit: Martin Ambrozik.
Picture credit: Martin Ambrozik
Early Amniote Evolution
This discovery indicates that the first amniotes probably evolved in the Southern Hemisphere. They may have evolved on the ancient supercontinent Gondwana. Amniotes are vertebrates that undergo an embryonic developmental stage within an amnion. An amnion is a thin membrane that encloses the embryo and immerses the developing foetus in fluid. In simple terms, the amnion provides an environment in which the foetus can develop. In contrast, the eggs of amphibians do not have an amnion. They are dependent on a source of water in their environment in which to raise their young.
Professor Young added that the implications of this discovery for the early evolution of tetrapods are profound. All stem-tetrapod and stem-amniote lineages must have originated during the Devonian. It seems that tetrapod evolution proceeded much faster, and the Devonian tetrapod record is much less complete, than has been thought.
He explained:
“All stem-tetrapod and stem-amniote lineages must have originated during the Devonian period – but tetrapod evolution proceeded much faster, and the Devonian tetrapod record is much less complete than we have believed.”
Much remains uncertain regarding the evolution of early tetrapods and the rise of the amniotes. Scientists think that global warming may have led to the gradual dominance of the Reptilia and the demise of amphibians and their close relatives.
A dramatic change in climate may have aided the rise of the reptiles: Global Climate Change May Have Helped Reptile Evolution.
New Fossil Evidence
This new fossil evidence from Australia falsifies the previous, widely accepted timeline. For Professor Long, his involvement with this amazing research goes back some forty-five years. He did his PhD thesis on the fossils of the Mansfield district. However, it was only recently after organising palaeontology field trips with Flinders University students that locals joined the hunt for fossils.
Two locals, Craig Eury and John Eason (co-authors of the study), found the slab covered in trackways. At first, it was thought this trace fossil represented amphibian tracks. However, a closer examination revealed that one track has a hooked claw coming off the digits, evidence for a reptile-like animal.
Searching for fossils in the Mansfield district of northern Victoria. Picture credit: Flinders University.
Picture credit: Flinders University
Crystal Clear Trackways
The trackways are preserved in astonishing detail. Dr Alice Clement (Flinders University and co-author), helped scan the fossil tracks to create digital models that could be analysed in detail. Scientists from Uppsala University (Sweden) led by Professor Per Erik Ahlberg collaborated in this research. Digital heat maps were produced highlighting the depth and the topography of individual prints.
Dr Clement stated:
“We study rocks and fossils of the Carboniferous and Devonian age with specific interest to observe the very important fish-tetrapod transition. We’re trying to tease apart the details of how the bodies and lifestyles of these animals changed, as they moved from being fish that lived in water, to becoming tetrapods that moved about on land.”
Dr Aaron Camens (Flinders University), an expert in early tetrapod and amniote trackways explained that trackways record behaviour and tell scientists a lot about the animal’s locomotion.
He commented:
“A skeleton can tell us only so much about what an animal could do, but a trackway actually records its behaviour and tells us how this animal was moving.”
As Professor Long has been studying fossils from the Mansfield district since 1980, the team had a clear idea of the age of the rock deposits. He has produced an extensive catalogue of fish fossils from the Mansfield district. These fossils could then be compared with other fossil sites, with accurately dated rocks from around the world. This location has produced some amazing fish fossils. However, the quest had always been to find evidence of land animals (tetrapods).
Revising the Timeline of Early Amniote Evolution
The Mansfield district trace fossil slab dates from the Early Carboniferous (Tournaisian faunal stage). Ironically, it was a comparison of fish fossils that permitted the team to calculate the age of the specimen to within ten million years. This ground-breaking research has permitted the team to re-write the timeline of early amniote evolution. They conclude that these fossilised tracks show reptile-like animals appeared on Earth up to forty million years earlier than previously thought.
Revised timescale of early tetrapod evolution. Stratigraphic timescale representation of the Devonian and Carboniferous, showing the impact of the Snowy Plains Formation sauropsid tracks. The track record is shown as a pink rectangle, of double height to indicate possible age range. Picture credit: Long et al.
Picture credit: Long et al
Everything Dinosaur acknowledges the assistance of a media release from Flinders University in the compilation of this article.
The scientific paper: “Earliest amniote tracks recalibrate the timeline of tetrapod evolution” by John A. Long, Grzegorz Niedźwiedzki, Jillian Garvey, Alice M. Clement, Aaron B. Camens, Craig A. Eury, John Eason and Per E. Ahlberg published in the journal Nature.
The Everything Dinosaur website: Prehistoric Animal Models and Figures.
