Unique fossil of marine lizard discovered

10 September 2013

Illustration: Stefan Sřlberg

An ancient marine lizard in remarkable condition has been uncovered by a Lund University-led research team in Jordan. Its fish-like tail fin tells an evolutionary story about the species previously unknown to scientists.

One of history’s most fearsome predators, the mosasaur Prognathodon, lived during the Cretaceous Period some 70 million years ago. The most spectacular aspect of the fossil find is the tail fin, which is reminiscent of a shark’s fin turned upside down – despite the species being categorised as a lizard.

“After trying to reconstruct the tail fin for years, with only the skeleton to go by, I was suddenly standing in front of the definitive answer”, said Johan Lindgren, who lead the international group of Jordanian and American researchers involved in the discovery. “It was a fantastic feeling of euphoria.”

The fossil was initially discovered in 2008, but it was only when Johan Lindgren travelled to Jordan in 2011 that he discovered that soft tissue had been preserved around the tail fin, which ultimately led to its exposure. The fossil proved to be the first structure revealing the tail fin contour entirely, as it only lacked the head and the tip of the tail.

The mosasaur was previously thought to have a long, rectilinear body and a straight, elongate tail. Instead, it has a streamlined, fish-like body with a fluked tail. This is more in line with other big marine creatures, such as the extinct ichthyosaurs (250-94 million years ago) and today’s sharks and whales.

“These characteristics demonstrate in an outstanding way how organisms living in similar environments develop similar features, in a process known as convergent evolution”, said Johan Lindgren.

 He points out that the mosasaur’s evolutionary history – not just this discovery but in general – is one of the best overarching examples of large-scale evolution and how animals change appearance in order to adapt to a new environment. In this case, mosasaurs adapted to a marine life following life on land.

“This highlights the potential for new discoveries to challenge received wisdom about ancient animals, this time hypotheses and interpretations that have been around for nearly 200 years. It also shows what an exceptional capacity for adaptation reptiles have”, concluded Johan Lindgren.

 About the fossil:
The specimen of the mosasaur Prognathodon was discovered in Harrana in central Jordan, and is currently part of the collections housed at the Eternal River Museum of Natural History in Amman, Jordan. While individuals of Prognathodon sometimes grew to become 15 metres long, the fossil is of a young animal and measures only 1.5 metres.
It comprises two slabs and a counter slab, which together make up an exquisitely preserved and largely articulated skeleton, complete save for the head and distal fourth of the tail.

Nature Communications

Dr Johan Lindgren, Associate Professor, Lund University
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