Fossil Trucks Redefine the Story of Terrestrial Animals, Departing from Aquatic Origins

A discovery in Victoria, Australia has unearthed evidence of some of the earliest reptile-like ancestors, which are the forerunners to many modern creatures including birds, reptiles, and mammals. This finding could potentially alter the timeline of when such animals transitioned from aquatic environments to terrestrial habitats, marking a pivotal evolutionary milestone, though opinions remain divided.

In 2021, two amateur fossil hunters stumbled upon a sandstone slab while exploring the banks of a river near Mansfield, Victoria. This slab bore three distinct sets of tracks, believed to belong to the same type of tetrapod—four-legged creatures. Notably, two of these sets exhibited five-digit impressions with curved claws.

Fossils caught the attention of John Long from Flinders University in Adelaide, who, along with his team, dated the slab to around 356 million years ago. This predates the previously known oldest nail fossils by over 35 million years, which were discovered in Nova Scotia, Canada, according to research findings.

“These new trackways clearly illustrate beautiful five-fingered hands and hooked claws,” Long states. He describes these as “remarkable finds,” representing creatures that developed young in amniotic fluid or via egg-laying—this includes reptiles, mammals, and notably, humans.

Moreover, Long eliminates the possibility that these animals were amphibians since early amphibians were characterized by a larval stage of development. “None of the early amphibians possessed well-formed claws.” he affirms.

This implies that the fossilized tracks are likely the earliest known examples of land-dwelling reptiles. “This marks a profound change in evolution, as it suggests that hard-shelled eggs allowed these creatures to move onto dry land and populate new territories,” he explains.

Additional evidence indicating that the tracks were made on land, rather than in water, includes the presence of raindrop impressions within the surrounding slabs, according to Long. While researchers remain cautious, he expresses a strong belief that “99% certainty points to early reptiles.”

Blake Dixon from the University of New South Wales in Sydney adds that both images and analyses suggest confidence that the fossils belong to clawed animals. “This represents the earliest evidence we have of advanced terrestrial movement,” he asserts.

However, trackway researchers Stephen Salisbury and Anthony Romilio from the University of Queensland acknowledge the significance of the new fossils but raise questions regarding whether the impressions are true nails or merely pointed digits.

“The discovery is on the cusp of understanding claws,” remarks Romilio. “If they are indeed nail impressions, that positions them closer to reptiles.” Long maintains that he believes these footprints do represent nails.

Salisbury also points out that the presence of raindrop impressions does not definitively indicate when the tracks were made; rather, it reflects conditions of the surface at the time. Long counters, stating, “The fact that raindrops left impressions signifies that they were present before the creature made its tracks, thus ruling out underwater creation.” The implications of these findings are extensive for understanding early terrestrial life.