A clue has been found that the evolutionary timeline of terrestrial vertebrates is tens of millions of years earlier than previously known.
An international research team, including Uppsala University in Sweden, noted on the 15th that "the amniote footprints discovered in Australia have pushed back the emergence of terrestrial animals by about 40 million years." The research results were published in the international journal "Nature" on the same day.
Amniotes are the first vertebrates to lay eggs on land rather than in water. They are named because the embryo is enveloped in a membrane. They are considered the starting point for terrestrial animals such as reptiles, birds, and mammals. Based on fossil and genetic analyses, it has been estimated that amniotes first appeared about 320 million years ago.
The research team discovered what appears to be amniote footprints in rocks dating back about 356 million years in Victoria, Australia. This is 36 million years earlier than previous records. The footprints have five toes, and the traces of sharp and curved claws are prominently visible. This is a characteristic unique to amniotes, distinguishing them from early amphibians, which had no claws. Unlike early vertebrates that primarily lived in water, this creature walked on land, providing strong evidence.
The research team explained, "The animal that left the clawed footprints was likely a primitive amniote similar to reptiles," adding, "Considering that the footprint shape is similar to that of modern lizards, it was likely about 80 cm in size."
It was previously estimated that it took about 90 million years for four-legged animals to evolve from water to land as amniotes. However, this discovery revealed that this period was much shorter. It indicates that the transition from aquatic animals walking on land with four limbs and evolving into completely terrestrial animals capable of breeding on land may have occurred much earlier than expected. Amniote embryos can grow on land without going through an aquatic larval stage because they are in amniotic fluid.
This discovery also provides clues to solving the paleontological dilemma known as "Romer's Gap." Romer's Gap refers to a period in the fossil record where vertebrate fossils are absent from about 360 million years ago to about 345 million years ago. This phenomenon appeared to indicate that evolution had halted or slowed significantly for millions of years after the late Devonian mass extinction.
The research team concluded, "Romer's Gap is likely due to a lack of fossil records rather than a break in evolution," stating that "the fossils analyzed this time provide evidence that significant evolution of four-legged animals and amniotes was actively occurring before the mass extinction."
References
Nature (2025), DOI: https://doi.org/10.1038/s41586-025-08884-5