Did our own ancient ancestors, the earliest modern tetrapods, originally appear in the moderate Devonian environments of southern Gondwana before migrating to the boiling swamps and sun-baked semi-deserts of equatorial Euramerica? It’s really likely.

A significant turning point in the evolution of many modern species, including humans, was the appearance of four-legged creatures known as tetrapods.

Our recent finding, which was published in Nature, describes ancient fossil footprints discovered in Australia that contradict the timeframe of all tetrapods’ early history. It also implies that significant plot points could have taken place on the southern supercontinent Gondwana. This fossil trackway suggests that we may have been searching in the wrong location and at the wrong time for the genesis of contemporary tetrapods.

The first steps on the ground The first known tetrapods appeared during the Devonian era, most likely 390 million years ago, when odd lobe-finned fishes started pulling themselves out of the water. Two major evolutionary lineages subsequently emerged from this original population. One gave rise to amniotes, whose eggs include amniotic membranes that shield the growing fetus, and another to contemporary amphibians like frogs and salamanders. All reptiles, birds, and mammals are now considered amniotes. They are by far the most successful group of tetrapods, with over 27,000 species of birds, mammals, and reptiles. They have taken over every habitat on land, dominated the skies, and many made a magnificent comeback to the ocean.

However, the fossil record reveals that this amniote group’s early members were tiny and resembled lizards.

The beginnings

It has long been believed that the first tetrapods were rudimentary fish-like creatures like Acanthostega that could hardly move on land. The majority of experts agree that around 355 million years ago, at the beginning of the Carboniferous epoch, amphibians and amniotes split apart. The progenitors of mammals, reptiles, and birds were further divided from the amniote branch later in time. This neat image now unravels.

An odd trackway

A 35-cm-wide piece of sandstone from the Taungurung area, close to Mansfield in eastern Victoria, is the key to our finding. Only early amniotes, most likely reptiles, could have left the clawed foot tracks that cover the slab. It delays the amniotes’ emergence by at least 35 million years.

All amniotes have several characteristics, despite their extreme differences in size and structure. For starters, claws—or, in the case of humans, nails—always adorn the tips of limbs that include fingers and toes. There are no actual claws in other tetrapod groups. It is quite uncommon for some amphibians to have stiffened toe tips that resemble claws. Claws often create noticeable imprints that may be used to determine if an amniote left a fossil footprint.

The earliest trails made by claws

Footprints and bones from North America and Europe from around 318 million years ago provide the basis of the previous oldest reptilian fossil record. Our article also uncovered a new finding: Silesia in Poland has the earliest known record of reptilian-like footprints in Europe. Their age is around 328 million years. The Australian slab, which is estimated to be between 359 and 350 million years old, is far older than that. It is derived from the oldest portion of the Carboniferous rock outcropping along the Broken River, which the local First Nations people call Berrepit in their Taungurung language.

It has long been known that this region produced a wide variety of amazing fossil fishes that inhabited lakes and big rivers. Life on the riverside is now visible to us for the first time. The top surface of the slab is traversed by two fossil footprint trackways, one of which crosses over a solitary footprint pointing in the other direction. Raindrops have left dimples on the surface, indicating a quick shower just before the footprints were created. This demonstrates that the animals were roaming over arid terrain. Every footprint features claw impressions, some of which are lengthy scratches from the foot being pulled along. We are certain that the footprints belong to an amniote since the form of the feet resembles known early reptilian tracks. Our little video below demonstrates how the tracks were formed and reconstructs the prehistoric landscape around Mansfield 355 million years ago.

Changing the chronology

The chronology of all tetrapods’ origins is significantly altered by this discovery. The last common ancestor of amphibians and amniotes must have existed considerably earlier, in the Devonian period, if amniotes had already developed by the early Carboniferous, as our fossil indicates.

By comparing the relative lengths of various branches in DNA-based family trees of extant tetrapods, we may determine when the split occurred. It implies that the separation occurred as long ago as 380 million years ago, in the late Devonian. This suggests that the Devonian world was inhabited by sophisticated species, such as near ancestors of the extant lineages, in addition to primordial fish-like tetrapods and intermediate “fishapods,” such as the well-known Tiktaalik. Why, therefore, haven’t we discovered their bones?

Our slab’s placement offers a hint. Important problems in evolution The ancient northern hemisphere continent known as Euramerica, which included modern-day North America and Europe, is where all additional traces of Carboniferous amniotes originate. The vast bulk of Devonian tetrapod fossils were also found in Euramerica. Gondwana, a massive southern continent that included Africa, South America, Antarctica, and India, is where the new Australian fossils originated.