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Here’s what you’ll learn in this story:
The common ancestor of all tetrapods (including humans) was previously thought to have emerged at the dawn of the Carboniferous period.
Fossilized tracks from an early reptile are now the oldest known reptilian tracks, meaning the tetrapod ancestor most likely appeared earlier, during the Devonian period.
These tracks were made by clawed feet—a characteristic of amniotes. Their appearance pushes back amniotes evolution by 35-40 million years.
Between 359 and 350 million years ago, it rained. Lizard-like creatures crawled through the mud in what was once Gondwana (but is now Australia), leaving behind footprints that became frozen in time, fossilizing as mud turned to stone over the aeons. These tracks would later be unearthed in an excavation that questioned how far back in time our tetrapod ancestors walked on land.
Tetrapods (meaning “four legs” in Greek) include all amphibians, reptiles, birds, and mammals, and are thought to originate from lobe-finned fish that made their way out of primeval seas on fins that functioned as primitive legs. Humans are tetrapods, and like all tetrapods (except amphibians), we are also amniotes, with eggs that protect developing embryos in amniotic sacs. Amniotes are thought to have diverged from amphibians at the dawn of the Carboniferous period, about 355 million years ago. Mammals would diverge from reptiles and birds only 30 million years later.
The fossil footprints were discovered at the edge of an paleontological site in eastern Victoria known as Broken River (or Berrepit in Taungurung, the language spoken by local indigenous people). Whatever creature left imprints of its feet on the riverbank provides the first evidence of terrestrial life in this area, and claw marks from the footprints suggest it was an amniote—except that amniotes weren’t supposed to have evolved so early in the Carboniferous period.
“This pushes back the likely origin of crown-group amniotes by at least 35-40 million years,” the Australian and Swedish team of researchers who excavated at the Berrepit site said in a study recently published in the journal Nature. “[Amniotes] cannot be much younger than the Devonian/Carboniferous boundary, and [the origin of tetrapods] must be located deep within the Devonian.”
Before this find, the oldest known amniote fossils were tracks from Notalacerta and the bones of Hylonomus. Both species were sauropsids—part of a larger group of extant and extinct reptiles and birds that presumably lived during the late Carboniferous. The common ancestor of all tetrapods was thought to have emerged in the earliest years of the Carboniferous, but that changed when this team of experts came upon the mysterious tetrapod footsteps from Berrepit. They now think that the tetrapod ancestor appeared during the Devonian, and that amniotes began to diverge from them about 395 million years ago, 35 to 40 million years earlier than previously thought.
It is evident that the footsteps came not just from a tetrapod, but from an amniote because almost all amniotes have claws or nails. Claw marks scratched the wet earth after a short rain shower, and there is no evidence of a body or tail dragged across the ground. While it is impossible to know what this animal actually looked like, the spacing between forefeet and hind feet indicates that it was about 17 cm (about 6.7 inches) from shoulder to hip, with neck, head, and tail lengths unknown. Using a modern water monitor as a proxy, the researchers determined it must have been about 80 cm (about 31.5 inches) total in length.
Something else could possibly be demystified by the footprints—the end-Devonian mass extinction was thought to have such a catastrophic impact, it could explain why tetrapods don’t appear in the fossil record for another 20 million years. Tetrapods dating to after the gap are much more diverse and advanced than their pre-gap predecessors. Early Carboniferous sauropsid tracks mean that tetrapods must have been branching out from their common ancestor sometime during the Devonian, meaning that the mass extinction had little effect on the evolution of tetrapods.
“The [fossil footprints] have a disproportionate impact on our understanding of early tetrapod evolution because of their combination of diagnostic amniote characteristics and early, securely constrained date,” the researchers said. “They demonstrate, once more, the extraordinary importance of happenstance and serendipity in the study of severely under-sampled parts of the fossil record.”
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