Thursday, December 20, 2012

The First Known Freshwater Mosasauroid

An artist's drawing of the Pannoniasaurus inexpectatus 
that lived 84 million years ago in freshwater floodplains.
 (Image: Tibor Pecsics)
Until now, mosasauroids have been considered exclusively marine. However, László Makádi from the Hungarian Natural History Museum, and colleagues from the University of Alberta, Canada and MTA-ELTE Lendület Dinosaur Research Group, in Hungary report the discovery and describe Pannoniasaurus a new species having unsuspected adaptations to freshwater ecosystems.

The new mosasaur discovered in Hungary is the first known example of this group of squamate reptiles to have lived in freshwater river environments similar to modern freshwater dolphins.

The species lived about 84 million years ago, the largest specimens reached about 20 feet in length, and belongs to a family called 'mosasaurs', conventionally thought of as gigantic finned marine lizards, similar and perhaps even related to present day monitor lizards. The researchers discovered several fossils of the new species, ranging from small juveniles to large adults that suggest that this species had limbs like a terrestrial lizard, a flattened, crocodile-like skull, and a tail unlike other known members of the mosasaur family.

The fossils were recovered from an open-pit mine in the Bakony Hills of Western Hungary, which were once flood-plains. According to the study, this is the first known mosasaur that lived in freshwater, and only the second specimen of a mosasaur to have been found in rocks that were not once deposited in the ocean. Makadi says, "The evidence we provide here makes it clear that similar to some lineages of cetaceans, mosasaurs quickly adapted to a variety of aquatic environments, with some groups re- invading available niches in freshwater habitats. The size of Pannoniasaurus makes it the largest known predator in the waters of this paleo-environment."

Whether or not Pannoniasaurus was restricted to freshwater environments, or perhaps instead was a seasonal, opportunistic migrant and consumer in these habitats, remains uncertain. Sedimentological, taphonomical, morphological and geochemical evidences suggest the former. In association with the facies analysis and depositional environment interpretations, the collected evidence indicates that Pannoniasaurus is best interpreted as an inhabitant of freshwater ecosystems. Currently, among derived pythonomorphs, Pannoniasaurus, whether being an obligatory freshwater animal or a seasonal or opportunistic migrant, remains the first and only know river-dwelling member of the clade including aigialosaurs and mosasaurs.

Even in the modern world, squamate reptiles in the aquatic world are extremely rare. Only a few species live in the water, and even fewer, like marine iguanas and sea kraits, live in the oceans. The new species described here probably adapted to freshwater environments similarly to river dolphins, such as those now inhabiting the Amazon, Ganges and Yangtze rivers.

The evidence we provide here makes it clear that similar to some lineages of cetaceans, mosasauroids quickly radiated into a variety of aquatic environments, with some groups reinvading available niches in freshwater habitats, and becoming highly specialized within those ecosystems.

László Makádi, Michael W. Caldwell, Attila Ősi. The First Freshwater Mosasauroid (Upper Cretaceous, Hungary) and a New Clade of Basal Mosasauroids. PLoS ONE, 2012; 7 (12): e51781 DOI: 10.1371/journal.pone.0051781

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