They were among the first creatures to invade the land during one of the great evolutionary transitions in Earth's history. Now, scientists believe the first four-limbed vertebrates, known as tetrapods, that lived 360 million years ago may have been more like modern crocodiles than previously thought. They were able to reconstruct the skull of one of these animals in 3D from fragments of fossils that have been found in East Greenland. Scientists have reconstructed a 360 million-year old tetrapod's skull from bone fragments, to reveal it was similarly shaped to a modern day crocodile's. The top image shows the articulated cranium and lower jaws, in the middle the image shows the right facial skeleton in an 'exploded view' and the inside of the sutures and the bottom image shows the lower jaw also in an 'exploded' view. Individual bones are shown in different colours . Acanthostega gunnari lived between 380 and 360 million years ago when animals made the transition from the sea to land. Scientists are fascinated by the species because it is thought to be crucial to understanding the anatomy and ecology of the earliest tetrapods. However, after hundreds of millions of years buried in the ground, fossils are often damaged and deformed. They found that the reconstructed skull had a longer postorbital region and a more strongly hooked lower jaw than previously thought, making it relatively similar to modern crocodiles. A saltwater crocodile is shown . The evolution of hind legs began as enhanced fins, according to a study publsihed last year. Researchers discovered a well-preserved pelvis and a partial pelvic fin from a 375 million-year-old fossil of a transitional species, Tiktaalik roseae, between fish and the first legged animals. The find challenges the existing theory that large, mobile hind appendages were developed only after vertebrates transitioned to land. Corresponding author of the study Doctor Neil Shubin, of the University of Chicago, said: 'Previous theories, based on the best available data, propose that a shift occurred from "front-wheel drive" locomotion in fish to more of a "four-wheel drive" in tetrapods. 'But it looks like this shift actually began to happen in fish, not in limbed animals.' The fossil was discovered in 2004 and represents the he best-known transitional species between fish and land-dwelling tetrapods. A lobe-finned fish with a broad flat head and sharp teeth, Tiktaalik looked like a cross between a fish and a crocodile, growing up to a length of nine feet as it hunted in shallow freshwater environments. To reconstruct the skull of the species from numerous pieces, scientists from the University of Bristol put fossil specimens of Acanthostega gunnari from East Greenland into a CT scanner, before using a computer programme to assemble it. They found that the reconstructed skull had a longer postorbital region and a more strongly hooked lower jaw than previously thought, making it relatively similar to a modern crocodile's skull. The reconstruction has also provided some clues as to how the species fed. The size and distribution of the reptile's teeth and the shape of contacts between individual bones of the skull, called sutures, suggest Acanthostega may have initially seized prey at the front of its jaws using its large front teeth and lower jaw. The researchers plan to apply these methods to other flattened fossils of the earliest tetrapods to better understand how modifications to these early animals' bones and teeth helped them meet the challenges of living on land. 'Because early tetrapods skulls are often "pancaked" during the fossilization process, these animals are usually reconstructed having very flat heads,' said Dr Laura Porro, who led the study published in Plos One. 'Our new reconstruction suggests the skull of Acanthostega was taller and somewhat narrower than previously interpreted - more similar to the skull of a modern crocodile.'
Bristol University researchers scanned bones of Acanthostega gunnari . Ancient tetrapod, or four-limbed vertebrate, lived 360 million years ago . Experts created a 3D computer model to reconstruct the skull's shape . It's more similar to a modern crocodile's skull than previously thought .