Fossil footprints give glimpse of how reptiles became dominant on land

A project led by third-year Palaeobiology and Palaeoenvironments MSci student Luke Meade, has revealed a new glimpse of the world when reptiles began taking over from amphibians as the dominant land animals. 

Luke worked with Dr Richard Butler and PhD student Andrew Jones as part of a Palaeontological Association funded project on the unique collection of footprints from Carboniferous rocks in the Hamstead area of northwest Birmingham. Using cutting-edge technology, he was been able to identify the creatures that probably made the footprints around 310 million years ago. This gives us a unique snapshot of life in the Birmingham area at the time that reptiles started diversifying and began to overtake amphibians as the dominant group of vertebrates on land. 

The results of the summer research project on fossil footprints from the Lapworth Museum of Geology collections has now been published in an international peer-reviewed journal. 

Pelycosaurs. Wikipedia Commons

Pelycosaurs. (Wikipedia Commons)

Carboniferous Birmingham

Birmingham in Carboniferous was a very different world. Lush tropical rainforests covered the land, and primitive plants like horsetails and club mosses grew into towering 30 metre tall trees. A great diversity of early amphibians, insects, arachnids and other invertebrates lived among the lush vegetation. As the plants died, their remains formed thick layers of peat that over time were compressed to form rich deposits of coal that today underlie many parts of Britain. These coal deposits would later power the industrial revolution that would make Birmingham a world-leader in manufacturing. 

But towards the end of the Carboniferous period (299 million years ago), the world began to change. The global climate became increasingly dry and the rainforests of Europe began to contract and vanish. This rainforest collapse is believed to have caused a mass extinction among plants, while many groups of amphibians disappeared and were replaced by early groups of reptiles. 

The fossil footprints from Hamstead provide a unique window into this changing world. A schoolteacher, Walter Henry Hardaker, discovered the footprints in the early 20th century, but, since his work, they have lain largely ignored in the collections of the Lapworth Museum. The footprints are preserved on about 20 red sandstone slabs. They formed as animals walked over muddy areas next to river channels, and were preserved by a covering of sand in subsequent floods. Trackways show how the animals that lived there moved and skittered across the floodplain. Exquisite details of the environment are preserved, including raindrops and mudcracks that formed during drier intervals. 

Amphibian tracks

Amphibian tracks

Luke used cutting-edge photogrammetric technology to study the footprints. Each trackway was photographed from numerous different angles, and special software was then used to turn these photos into high-resolution 3D models. These models are freely available to download, and can be used for educational purposes. Colour-coding the models allowed Luke to produce topographic maps of each specimen that illustrate them in three dimensions. By comparing these models to Carboniferous tracks from other parts of the world, he was able to identify the types of animals that probably made the tracks. 

Most common were amphibians, ranging from a few centimetres up to a metre or more in length. Alongside these amphibians lived two other vertebrate groups. Rare tracks show that large pelycosaurs were present, probably superficially remembling modern day Komodo dragons. Pelycosaurs were part of the great evolutionary lineage that would eventually lead to mammals, including humans. 

Smaller tracks document early representatives of another great branch of the vertebrate evolutionary tree, the sauropsid reptiles, which were small and lizard-like. These would ultimately diversify into everything from dinosaurs to lizards, turtles and birds. 

This snapshot of geological time captures an important evolutionary moment. Although amphibians were still abundant, their dependence on moist environments and bodies of water for spawning may have put them at a disadvantage compared to the rapidly diversifying reptiles as the climate became increasingly arid. 

This story has also featured as an article on The Conversation

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