A Fossil Footprint Shakes the Tetrapod Family Tree
The prevailing narrative of tetrapod evolution, detailing the journey of four-limbed animals from aquatic existence to terrestrial dominance, has been significantly challenged by the discovery of fossilized footprints in southeastern Australia. This seemingly insignificant slab of sandstone, a relic from approximately 355 million years ago, contains evidence of clawed tetrapods predating previously understood timelines. The find compels a re-evaluation of established evolutionary relationships and pushes back the origin of reptiles, and consequently, the amniote lineage that includes mammals and birds, by millions of years.
The traditional understanding depicted a relatively linear progression. Fish, venturing onto land during the Devonian period, gradually evolved into tetrapods. These early tetrapods, initially possessing fish-like characteristics, adapted to terrestrial environments, eventually diversifying into the diverse array of reptiles, birds, mammals, and amphibians that populate the modern world. However, the discovery of these ancient footprints disrupts this orderly sequence, suggesting a more complex and interwoven evolutionary history.
The fossil slab, unearthed by amateur paleontologists, displays distinct long-toed impressions bearing unmistakable claw marks. These traces are crucial because claws are considered a hallmark of early amniotes – the group encompassing reptiles, birds, and mammals. Prior to this discovery, it was generally accepted that other tetrapods, particularly early amphibians and "fishapods" like Tiktaalik, lacked claws. The presence of claws in these newly discovered footprints implies the existence of primitive reptiles much earlier than previously thought.
Per Ahlberg, a researcher at Uppsala University and the lead author of the study detailing the find, underscores the significance of the discovery. According to Ahlberg, the fossil pushes back the timeline for a substantial portion of the tetrapod evolutionary tree, effectively shifting the origins of all lineages branching off before the reptile-mammal split. This necessitates a revision of our understanding regarding the timing and manner in which tetrapods adapted to terrestrial life. While scientists have long acknowledged the Devonian period as the birthplace of tetrapods, the conventional view held that these early forms were primarily aquatic, only gradually transitioning to land. The Australian footprints, however, suggest a more rapid diversification, with reptiles emerging much earlier than expected.
Grzegorz Niedźwiedzki, another researcher at Uppsala University and co-author of the study, expressed his surprise upon examining the specimen for the first time. The clearly preserved claw marks were instantly apparent, indicating the presence of advanced tetrapods capable of navigating terrestrial environments.
The existence of reptiles at the onset of the Carboniferous period, as suggested by the Australian tracks, directly challenges the established narrative. It forces a reassessment of the evolutionary relationships between different tetrapod groups and their respective timelines.
The discovery casts doubt on the widely held belief that Tiktaalik and similar "fishapods" were direct ancestors of tetrapods. While these creatures were undoubtedly closely related to tetrapods, the new evidence suggests that they lived much later than the earliest tetrapods and might even have been contemporary with the tetrapod crown-group node – the point where the major tetrapod lineages diverged.
This implies that the evolutionary line leading to reptiles, and consequently to mammals and humans, stretches back further into the past than previously estimated, pushing the appearance of reptiles back by roughly 35 million years. Consequently, the split between amniotes and amphibians must have occurred even earlier, deep within the Devonian period.
By integrating DNA-based family trees with the newly discovered fossil dates, the research team concludes that the crown-group node likely resided in the Devonian, potentially coexisting with creatures like Tiktaalik. This implies that more advanced tetrapods were already present while Tiktaalik was still adapting to a semi-aquatic existence.
The fossil claw print slab currently represents the entire known fossil record of tetrapods from the earliest Carboniferous of Gondwana, the ancient supercontinent encompassing Africa, South America, Antarctica, Australia, and India. The discovery prompts further questions about the biodiversity of this era and the potential for uncovering additional fossil evidence that could further illuminate the early evolution of tetrapods.
The research team now emphasizes the critical need for further fieldwork, particularly in Australia and other regions of Gondwana, to unearth more clues about the early amniotes that inhabited this vast landmass. While the discovery of more footprints would be valuable, the ultimate prize would be the discovery of body fossils, which would provide a more comprehensive understanding of the anatomy and evolutionary relationships of these ancient creatures. Such discoveries could potentially rewrite major chapters of tetrapod evolutionary history and offer profound insights into the origins of reptiles, mammals, and ultimately, humanity itself. The sandstone slab serves as a stark reminder that our understanding of the past is constantly evolving, and that even the most established narratives can be challenged by new discoveries, forcing us to rethink the intricate and fascinating story of life on Earth. The ancient footprints prompt speculation about the creatures that made them, about their environment, and about the future discoveries that might further refine our understanding of the early tetrapods of Gondwana.
