Freshwater Mosasaur Stirs Marine Reptile Relationships
If ever there were marine creatures that could justifiably be called “sea monsters”, the fearsome mosasaurs would fit the bill. These extinct aquatic lizards – cousins of today’s monitors and Komodo dragons – were the apex predators of the world’s Late Cretaceous seas from the coast to the open ocean, and they gulped down prey with frightening, expandable jaws. As Samuel Wendell Williston, the early 20th century’s foremost authority on ancient marine reptiles, put it:
It is certain that the mosasaurs were much more predaceous and pugnacious in their habits than were any other truly aquatic backboned air-breathing animals of the past or present. They were the ‘land sharks’ of the ancient seas, and probably the only ones among water reptiles that would be dangerous and offensive to man, were they all living today.
From the time the remains of these aquatic lizards were discovered in the 18th century to the present, mosasaurs have been considered as wholly marine reptiles. They slid into the sea and, over time, natural selection molded them into streamlined, paddle-finned, fluke-tailed cruisers like Platecarpus and Plotosaurus. But a collection of over a hundred isolated bones found in a Hungarian bauxite mine complicates that evolutionary outline. These lizards weren’t restricted to the seas, and sleek, oceanic mosasaurs may have evolved more than once.
Between 85 and 83 million years ago, what is now western Hungary was an island – one of many in an archipelago created by the Tethys Ocean. Freshwater rivers flowed across these patches of land, nourishing habitats that boasted a variety of prehistoric life. The deposits preserved at the Iharkút mine, in particular, have yielded fish, amphibians, turtles, crocodiles, pterosaurs, plants, invertebrates, and dinosaurs, including the first ceratopsian known from Europe. Isolated vertebrae hinted that there was a relatively large monitor lizard in the same habitat, too, but, as paleontologist Laszlo Makadi and coauthors report today in PLoS One, skull elements found in the same quarry indicate that the lizard bones actually belonged to many, many mosasaurs.
Named Pannoniasaurus inexpectatus, the mosasaur is only known from a collection of individual bones and isolated fragments. No one has yet found any articulated parts of the skeleton. Nevertheless, as Makadi and colleagues argue, the various pieces bear distinctive characteristics that allow them to be attributed to just one species. The various parts fit the bauplan of an archaic mosasaur.
When assembled and brought to the same scale, the pieces collected so far comprise most of the skull, much of the vertebral column, aspects of the forelimb, and part of the hips. More than that, the collection represents a population of Pannoniasaurus – aquatic lizards that are estimated to have ranged between two and twenty feet long.
As shown by the geological context and other fossils found at the site, all the Pannoniasaurus bones were preserved in the remnants of a Cretaceous river. The newly-described species is not the first mosasaur to be found away from the sea, however. One previous find – a specimen of Plioplatecarpus found in Canada and described by Robert Holmes and collaborators in 1999 – was a mosasaur that had apparently swum up an ancient estuary into a freshwater river. Since all other Plioplatecarpus specimens are known from marine habitats, though, this animal seemed to be a wayward fluke. Paleontologists couldn’t be sure whether Plioplatecarpus made a regular habit of swimming upriver or whether this individual was doing something unusual. Pannoniasaurus, on the other hand, is represented by what must be multiple individuals of different ages that were buried together.
Was Pannoniasaurus a dedicated freshwater mosasaur? Makadi and colleagues acknowledge the possibility that the mosasaurs might have swum into freshwater rivers when food was abundant or for reproductive reasons. But, they argue, the mine contains remains of too many individuals for the aggregation to be dismissed as a rare jaunt into freshwater, and a previous study of geochemical data in Pannoniasaurus bones indicated that the lizards spent almost all their time in the river rather than moving back and forth from the sea.
Makadi and collaborators hypothesize that Pannoniasaurus primarily prowled the island’s rivers. The lizard might have been an ambush predator similar to the crocodiles it lived alongside. While fragmentary, the pieces of the reptile’s arms and pelvis hint that Pannoniasaurus had relatively robust limbs that it could use to launch itself after prey in the shallows.
In general form, Pannoniasaurus resembles early mosasaurs that had specialized skulls and bodies that were similar to their terrestrial precursors. While these lizards – such as Dallasaurus – were aquatic creatures often found in marine sediments, they were not as exquisitely-adapted for life in the water as Mosasaurus, Tylosaurus, and other iconic genera.
Strangely, though, Pannoniasaurus wasn’t an especially close relative of Dallasaurus and other mosasaurs at the base of the family tree. Instead, Makadi and colleagues found that Pannoniasaurus grouped most closely with Tethysaurus and similar genera in a new mosasaur subgroup the researchers call the Tethysaurinae. This group sits at the base of a radiation of marine mosasaurs such as Tylosaurus and Platecarpus.
The evolutionary distance between the two mosasaur groups – with Dallasaurus and kin at the base of one, and Pannoniasaurus and relatives at the base of the other – indicates that marine mosasaurs with paddle-fins and other seagoing specializations evolved at least twice during the Cretaceous. The story of the mosaurs was not a simple tale of lizards that became ever-more-adapted to life at sea in a linear fashion. Their history is one of multiple radiations and parallel evolution, a grand proliferation snuffed out by the catastrophic extinction that forever changed the world 66 million years ago.
Holmes, R., Caldwell, M., Cumbaa, S. 1999. A new specimen of Plioplatecarpus (Mosasauridae) from the lower Maastrichtian of Alberta: comments on allometry, functional morphology, and paleoecologyA new specimen of Plioplatecarpus (Mosasauridae) from the lower Maastrichtian of Alberta: comments on allometry, functional morphology, and paleoecologyA new specimen of Plioplatecarpus (Mosasauridae) from the lower Maastrichtian of Alberta: comments on allometry, functional morphology, and paleoecology. Canadian Journal of Earth Sciences. 26: 363-369
Makadi, L., Caldwell, M., Osi, A. 2012. The first freshwater mosasauroid (Upper Cretaceous, Hungary) and a new clade of basal mosasauroids. PLoS One. 7, 12: e51781
Williston, S. 1914. Water Reptiles of the Past and Present. Chicago: University of Chicago Press. pp. 158-159