Mosasaurs terrorized Cretaceous rivers
29 July 2009, by Sara Coelho
With T-rexes on land and 15-metre long reptiles in the sea, the age of dinosaurs was not an easy time for small prey. But it gets worse: palaeontologists have found that mosasaurs, top marine predators of the Cretaceous, lurked in rivers too.
The findings come from an old open-cast aluminium mine at Iharkút in Hungary. In the Late Cretaceous, 84 million years ago, the area was covered by a wide river delta, five to six metres deep, and surrounded by swamps and tropical forests.
The six-metre-long mosasaur reptiles living in this environment were 'by far the largest animals in the river', says Dr László Kocsis, from the National Oceanography Centre Southampton, and lead author of the research. 'They probably hunted small crocodiles, turtles and fish.'
'We were really surprised to find so many mosasaur fossils in these rocks, because they were generally known only from marine environments,' he adds. Mosasaurs were well adapted to hunt in coastal areas and shallow oceans. The largest known species was 17 metres long.
To make sure that the mosasaurs discovered in Hungary were living permanently in the river, and were not just one-time visitors or stranded animals, Kocsis and his team decided to look at the isotope composition of the fossil bones and teeth.
Not every oxygen atom is the same. Some are slightly heavier than others, and 'the proportion between heavy and light oxygen isotopes can give clues to the environment where these animals lived,' Kocsis explains. 'Similarly, strontium isotopes can give further hints about environmental conditions,' he adds.
The results, published last week in the journal Palaeogeography, show that the mosasaurs were permanent residents of the river: 'the oxygen and strontium isotopic compositions of the fossils are incompatible with Cretaceous marine environments,' says Kocsis.
Mosasaurs first appeared in the oceans about 90 million years ago and died out at the end of the Cretaceous, together with the dinosaurs. The fossils recovered from the Hungarian mine have some primitive anatomical features, showing that they were early representatives of the group.
'This means that the Hungarian species evolved very quickly maybe from marine mosasaurid ancestors to freshwater environments,' says Kocsis.
But the Iharkút mine held yet one more surprise: the team found fossils of an extinct group of fish - the Pycnodontiformes - which was also believed for long to be exclusive to marine environments. Pycnodont fish have laterally compressed bodies and their special jaws are well adapted to crush molluscs, crustaceans or even eat corals.
Freshwater pycnodonts are known from only a few localities around the world. Kocsis chemical analyses reveal that these fish were at home in the Hungarian Cretaceous river as well and hence the result broadens the known occurrences of the rare freshwater pycnodonts.
The detailed paleontological description of the freshwater mosasaurs will be published in the near future by László Makádi, a PhD student at Eötvös Loránd University, Budapest.
L. Kocsis, A. Ösi, T. Vennemann, C.N. Trueman and M.R. Palmer. Geochemical study of vertebrate fossils from the Upper Cretaceous (Santonian) Csehbánya Formation (Hungary): evidence for a freshwater habitat of mosasaurs and pycnodont fish, Palaeogeography (2009), doi:10.1016/j.palaeo.2009.07.009
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