Reptile Fossil Tracks Trigger New Evolutionary Insights

The appearance of four-limbed animals, or tetrapods, marked a crucial milestone in the evolutionary path of numerous modern species, including humans.

Our latest study in Nature, reveals ancient fossil footprints from Australia that challenge the established timeline of early tetrapod evolution. The findings also indicate that significant chapters of this evolutionary story may have unfolded on the southern supercontinent Gondwana.

This fossil trackway suggests we may have been searching for the origins of modern tetrapods at the wrong point in time—and possibly in the wrong location.

The Earliest Steps On Land

Tetrapods first appeared long ago during the Devonian period, when unusual lobe-finned fish began dragging themselves onto land, likely around 390 million years ago.

These early pioneers eventually branched into two major evolutionary paths. One gave rise to today’s amphibians, like frogs and salamanders. The other evolved into amniotes—animals whose eggs contain amniotic membranes that protect the developing embryo.

Modern amniotes include reptiles, birds, and mammals. With over 27,000 species, they represent the most diverse and successful group of tetrapods. They’ve adapted to almost every land environment, conquered the skies, and many have thrived in aquatic life again.

Despite their dominance today, early amniotes were small and lizard-like in appearance. Their exact origins remain somewhat mysterious. Until now, the earliest tetrapods were believed to be primitive, fish-like creatures like Acanthostega, with limited land mobility.

Most scientists believe that amphibians and amniotes diverged at the beginning of the Carboniferous period, around 355 million years ago. Later, the amniote lineage split into the ancestors of mammals and the group that led to reptiles and birds. Now, this neat picture is unraveling.

An Intriguing Set of Tracks

Central to our discovery is a 35-centimeter-wide sandstone slab from Taungurung country, near Mansfield in eastern Victoria.

The slab is marked with the footprints of clawed feet, which can only belong to early amniotes, likely reptiles. This pushes back the origin of amniotes by at least 35 million years.

Despite significant differences in size and shape, all amniotes share certain characteristics. A common trait is that limbs with fingers and toes usually have claws, or nails in humans.

In other tetrapod groups, true claws are absent. Even claw-like, hardened toe tips found in some amphibians are very uncommon. Claws leave distinct imprints in footprints, helping identify if a fossil was made by an amniote.

The Earliest Tracks With Claws

The previous oldest fossil evidence of reptiles comes from footprints and bones found in North America and Europe, dating back to around 318 million years ago.

A new discovery in our study also reveals the oldest record of reptile-like tracks in Europe, from Silesia in Poland, which are approximately 328 million years old.

However, the Australian slab is significantly older, with a date range of 359 to 350 million years. It comes from the early Carboniferous period, found in rock formations along the Broken River (Berrepit in the Taungurung language of the local First Nations people).

This region has long been recognized for its rich collection of fossilized fish that lived in lakes and large rivers. Now, for the first time, we get a glimpse of life along the riverbank.

Two fossilized trackways cross the upper surface of the slab, with one path stepping over an isolated footprint facing the opposite direction. The surface is dotted with dimples left by raindrops, indicating a brief shower occurred just before the footprints were made. This confirms that the creatures were moving on dry land.

All the footprints display claw marks, with some showing long scratches where the foot appears to have been dragged along.

The foot shape closely resembles that of known early reptile tracks, leading us to confidently conclude that the footprints belong to an amniote. Our brief animation below offers a reconstruction of the ancient environment near Mansfield 355 million years ago and illustrates how the tracks were formed.

Revising The Timeline

This discovery significantly alters the timeline for the origin of all tetrapods.

If amniotes had already evolved by the early Carboniferous, as indicated by our fossil, the last common ancestor of amniotes and amphibians must have existed much earlier, in the Devonian period.

By comparing the relative lengths of different branches in DNA-based family trees of living tetrapods, we can estimate the timing of this split. It suggests that the divergence occurred in the late Devonian, potentially as far back as 380 million years ago.

This implies that the late Devonian world was inhabited not only by primitive, fish-like tetrapods and transitional “fishapods” like the well-known Tiktaalik, but also by more advanced forms, including close relatives of modern lineages. So, why haven’t we found their bones?

The location of our slab offers a potential clue.

Major Evolutionary Questions

All other records of Carboniferous amniotes have been found in the northern hemisphere’s ancient landmass, Euramerica, which included present-day North America and Europe. Euramerica also yielded most of the Devonian tetrapod fossils.

In contrast, the new Australian fossils come from Gondwana, a vast southern continent that once included Africa, South America, Antarctica, and India.

Across the vast landmass that stretched from the southern tropics to the South Pole, our small slab is currently the only tetrapod fossil from the earliest part of the Carboniferous.

The Devonian record is only slightly better. The Gondwana fossil record of early tetrapods is surprisingly incomplete, with vast gaps that could hide—well, almost anything.

This discovery raises a significant evolutionary question. Did the first modern tetrapods, our distant ancestors, originate in the temperate Devonian landscapes of southern Gondwana, long before they spread to the sun-drenched semi-deserts and humid swamps of equatorial Euramerica?

It’s very possible. Only further fieldwork, uncovering new Devonian and Carboniferous fossils from the ancient Gondwana continents, may eventually provide an answer to that question.

Read the original article on: Sciencealert

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