Ancient footprints from Australia reveal earliest-known reptile

Seventeen footprints preserved in a slab of sandstone discovered in southeastern Australia dating to about 355 million years ago are rewriting the history of the evolution of land vertebrates, showing that reptiles arose much earlier than previously known.

The fossilized footprints, apparently made on a muddy ancient river bank, include two trackways plus one isolated print, all displaying hallmark features of reptile tracks including overall shape, toe length and associated claw marks, researchers said. They appear to have been left by a reptile with body dimensions similar to those of a lizard, they said.

The footprints reveal that reptiles existed about 35 million years earlier than previously known, showing that the evolution of land vertebrates occurred more rapidly than had been thought.

"So this is all quite radical stuff," said paleontologist Per Ahlberg of the University of Uppsala in Sweden, who led the study published on Wednesday in the journal Nature.

The Australian footprints were preserved in a sandstone slab measuring about 14 inches (35 cm) across that was found on the banks of the Broken River near the town of Barjarg in the state of Victoria.

The story of land vertebrates started with fish leaving the water, a milestone in the evolution of life on Earth. These animals were the first tetrapods - meaning "four feet" - and they were the forerunners of today's terrestrial vertebrates: amphibians, reptiles, mammals and birds. Footprints in Poland dating to about 390 million years ago represent the oldest fossil evidence for these first tetrapods, which lived an amphibious lifestyle.

These creatures were the ancestors of all later land vertebrates. Their descendants split into two major lineages - one leading to today's amphibians and the other to the amniotes, a group spanning reptiles, mammals and birds. The amniotes, the first vertebrates to lay eggs on land and thus finally break free of the water, cleaved into two lineages, one leading to reptiles and the other to mammals. Birds evolved much later from reptile ancestors.

The Australian footprints each are approximately 1-1.5 inches (3-4 cm) long. They appear to have been left by three individuals of the same reptile species, with no tail drag or body drag marks. No skeletal remains were found but the footprints offer some idea of what the reptile that made them looked like.

"The feet are rather lizard-like in shape, and the distance between hip and shoulder appears to have been about 17 cm (6.7 inches). Of course we don't know anything about the shape of the head, the length of the neck or the length of the tail, but if we imagine lizard-like proportions the total length could have been in the region of 60 cm to 80 cm (24 to 32 inches)," Ahlberg said.

"In terms of its overall appearance, 'lizard-like' is probably the best guess, because lizards are the group of living reptiles that have retained the closest approximation to the ancestral body form," Ahlberg added.

The modest size of the earliest reptiles stands in contrast to some of their later descendants like the dinosaurs.

This reptile probably was a predator because plant-eating did not appear until later in reptilian evolution. The bodies of herbivorous reptiles tend to be big and clunky, whereas this one evidently was lithe with long, slender toes, Ahlberg said.

The researchers also described newly identified fossilized reptile footprints from Poland dating to 327 million years ago that broadly resemble those from Australia. Those also are older than the previous earliest-known evidence for reptiles - skeletal fossils from Canada of a lizard-like creature named Hylonomus dating to around 320 million years ago, as well as fossil footprints from about the same time.

The reptile that left the Australian footprints lived during the Carboniferous Period, a time when global temperatures were similar to today's, with ice at Earth's poles but a warm equatorial region. Australia at the time formed part of the ancient supercontinent Gondwana and lay at the southern edge of the tropics. There were forests, partly composed of giant clubmoss trees.

"The tracks were left near the water's edge of what was probably quite a large river, inhabited by a diversity of big fishes," Ahlberg said.