Hungary's finest dinosaur site is the Csehbanya Formation, and reveals a strange and somewhat atypical ecosystem ruled by small animals. It dates back to about 85 million years ago, the Santonian Stage of the Late Cretaceous.
These beds show an environment of floodplains and swamps that were sometimes subject to flash flood events. The remains of frogs and fish have been found here, and these small animals provided food for much bigger creatures.
The strangest component of the Csehbanya fauna is Pannoniasaurus inexpectatus. It was a mosasaur, part of a family of very large and mainly marine-adapted whale-like lizards. Pannoniasaurus though – named after the Roman province of Pannonia – was a freshwater animal. It was part of a basal line of mosasaurs called tethysaurines.
Tethysaurine mosasaurs existed mostly during the Mid and Early Late Cretaceous, with their largest member being Pannoniasaurus. At 6 meters long, it is the biggest predator here but nowhere near as large as the monsters further out to sea.
As a basal tethysaurine, Pannoniasaurus also had very well-developed finger bones. The researchers who described the animal envisioned it as being able to haul itself up on land and actually move around. It was somewhat similar in habits to a reptilian seal, a design not seen in any other mosasaur.
Bakonydraco was an omnivorous or frugivorous tapejarid. It was an efficient walker on land and used its huge crest for display. It is the only named pterosaur in the Csehbanya ecosystem.
There are no large theropods in the Csehbanya ecosystem. Birds are known from this formation, including the genus Bauxitornis. This bird gets its name from the fact that Csehbanya is an open-pit bauxite mine.
The only named non-avian theropod from the ecosystem is Pneumatoraptor fodori. It is some kind of maniraptoran, the type of theropod related to birds, troodonts and dromaeosaurs. Beyond this, not much more is known about Pneumatoraptor.
It would have been either an omnivore or a hunter of small animals along the riverbanks. It is thought to have been just 70 centimeters long when estimating from the only bone found (a partial shoulder girdle), and can be reconstructed as a basic feathered maniraptor.
The main ornithopod and possibly most common herbivore in the Csehbanya rocks is Mochlodon. Mochlodon is a rhabdodontid iguanodont, a member of a family that was very common in Europe at this time. These animals were all rather small and bipedal, looking less like the more advanced duckbills and more like basal iguanodontians.
The most famous examples are Zalmoxes and Rhabdodon, both dinosaurs from parts of Europe (France, Spain and Romania). Mostly these herbivores grew from dog-sized to horse-sized at best. They were all very successful throughout the islands that then made up Europe.
Mochlodon was no exception. It was even named as a species of Iguanodon, called Iguanodon suessii by Emmanuel Bunzel in 1871. The remains were named so in honor of Eduard Suess, who discovered the dinosaur's remains in rocks in Austria. Finally, Harry Govier Seeley renamed it as Mocholodon suessi in 1881. In life, Mochlodon used its keratinous beak to selectively eat low-growing vegetation just like any other ornithopod its size.
Only one horned dinosaur is known from this rock formation. Named Ajkaceratops, it is also one of Europe's only horned dinosaurs. More specifically it is a basal member of the family, called a bagaceratopsid. Ajkaceratops kozmai is only known from a few skull fragments. Like its relatives it was probably a tiny runner that lived and foraged in the undergrowth. It was roughly a meter long and lacked any sign of horns.
The armored Hungarosaurus is the best-known armored dinosaur from Europe and the most well-protected dinosaur from Csehbanya. It is also one of Hungary's most completely known dinosaurs. Measuring 4 meters long, it was a nodosaur, a type of armored dinosaur related to the famous Edmontonia from North America.
It did not have a club on its tail but instead kept itself safe from attack via a series of sharp spines and plates. An adult might have been too much even for the predatory Pannoniasaurus to handle, but a careless youngster might have been an easier target.
Hungarosaurus tormai roamed the floodplains of Csehbanya, eating low-growing plants like ferns, cycads and deciduous plants.
