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Mammal from the Age of Dinosaurs found in Maastricht
Maastricht, December 30, 2005

A diminutive, 66-million-year-old upper right molar represents the newest addition to faunas from Cretaceous rocks as exposed at the Maastricht ENCI quarry. For the first time these rocks, which date back to the Age of Dinosaurs in Europe, have produced remains of a marsupial. The new find is a molar is that of a so-called herpetotheriid marsupial, an extinct group related to the opossum.

Until now, palaeontologists assumed that these marsupials had not made the crossing from North America until the Eocene, some 10 million years after the extinction of the dinosaurs. The new fossil suggests that already near the end of the Cretaceous there were temporary, trans-Atlantic land bridges. The new find thus fundamentally changes our views on the youngest time slice of the Age of Dinosaurs.

Two amateur collectors, Roland Meuris and Frans Smet, discovered the tooth. The find of this new marsupial specie has just been published in the scientific journal, Journal of Mammalian Evolution. In this paper, the new species is also given a name, Maastrichtidelphys meurismeti, the ‘Maastricht marsupial of Meuris and Smet’. The diminutive tooth, less than two millimetres in size, is on exhibit (under magnification), next to a life-size model of Maastrichtidelphys, at the Natuurhistorisch Museum Maastricht.


Background information on the new Cretaceous mammal Maastrichtidelphys

On the discovery
In 2002, amateur collector Roland Meuris took a rock sample at the ENCI quarry near Maastricht, to analyse this for small teeth. Fellow collector Frans Smet subsequently recognised a mammal tooth in that same sample – the first mammal tooth from the Maastricht Cretaceous – upon which he contacted staff of the Natuurhistorisch Museum Maastricht.

Study
Only in scanning electron microscopy does the small tooth, measuring 1.36 x 1.85 mm, reveal its true identity, because it is the details that count in describing and identifying mammal teeth. The placement of the various cusps, protuberances on the chewing surface, their height and differences therein, as well as the structure and position of ‘troughs’ between the cusps are all characteristic features, allowing the identification of a species on the basis of a single tooth.

The first mammal from the Maastricht Cretaceous
Never before had mammal teeth been recorded from the Maastricht Cretaceous. Maybe that does not come as a surprise, because many non-specialist books the Cretaceous is referred to the ‘Age of Dinosaurs’, the Mesozoic. Although correct, the name ‘Age of Dinosaurs’ is slightly misleading: even in those days mammals were already around, although they played a far more modest role and were far less impressive than dinosaurs which often grew to gigantic size.

Terrestrial animals
During the Cretaceous, all mammals were land-based; in the oceans, while whales, which originate in the Eocene, could only come into existence after the demise of mosasaurs, and bats did not yet exist. All remains of terrestrial animals in the Maastricht Cretaceous rocks thus, in principle, come from dead bodies transported by rivers into the sea.

Age
The new find is about 66.1 million years old.

Mammals in the Age of Dinosaurs
During the Cretaceous, mammals did not grow larger than the average household pet (i.e. a small dog or cat); most species did not even reach that size, as they were only of mouse- to rat-size. Smaller mammals have diminutive bones, delicate and fragile. Only under special conditions does any mammal skeleton stand a chance of being fossilised. Of necessity, our current knowledge of primitive mammals mainly derives from the study of teeth. Teeth are small, but very durable thanks to the enamel, which is why they are often the sole remains to be found of Cretaceous species. Maastricht is no exception.

New species, formal publication
In the December issue of the Journal of Mammalian Evolution the new find is formally named, Maastrichtidelphys meurismeti, the ‘Maastricht didelphid of Meuris and Smet’. To describe the fossil, the Natuurhistorisch Museum Maastricht sought the help of two American experts of Mesozoic mammals, Dr James Martin of the South Dakota School of Mines and Technology (Rapid City), and Professor Dr Judd Case of St Mary’s College (Moraga, CA). During their visit to Maastricht in 2003, right after the discovery of the tooth, they recognized it as one of a herpetotheriid marsupial – an unexpected occurrence in the Maastricht Cretaceous.
Herpetotheriids are a group of opossum-like marsupials, which in all probability came into being in North America during the Cretaceous. A detailed comparison with the structure, position and height of cusps, and many other features of the Maastrichtidelphys molar with other Late Cretaceous and Early Paleogene mammals, suggests that Maastrichtidelphys is a close ally of the North American marsupial, Nortedelphys, described last year by Case and colleagues.

Land bridge with North America
A North American mammal present in the latest Cretaceous in Europe, suggests that Maastrichtidelphys, or its direct ancestor, had managed to cross the Northern Atlantic at that time. This sounds dramatic for a small marsupial, but the Atlantic Ocean in those days was half the width (at most) of today’s Atlantic Ocean, and further north there may have been a land bridge, via Greenland, during periods of low sea level. Around 71 million years ago, and again four million years later, there were two intervals worldwide where sea level was particularly low. Above 70 degrees N, the ‘Thule’ dispersal route may have been dry land; via islands in northern Canada, Baffin Island, Greenland, across the Faroes and Great Britain, Maastrichtidelphys may have reached the European mainland.

Greenhouse effect
But: were temperatures high enough to allow animals to cross? The best way to lower sea level is by turning water into ice. However, it is widely known that during the Cretaceous climate was much better than today’s climate, which makes the existence of polar ice caps unlikely. Except for land ice, we can also postulate movements along tectonic plates and activity of mid-ocean volcanic ridges that may have had an impact on sea level. More and more data on climate changes and sea level oscillations during the latest Cretaceous are becoming available; according to recent studies, the mean annual temperature at the poles was ca 6 degrees Celsius during the latest Cretaceous. If that scenario is correct, a crossing during summer months would have posed no problems whatsoever. The find of a few mammal hairs in Siberian amber, which oozed from trees some 70 million years ago, at around 70 degrees N then, make such estimates even more plausible.

Other crossings?
The discovery of this new mammal sheds more light on other finds of typically North American terrestrial animals in the European Cretaceous. After all, there are Hadrosaurus-like dinosaurs in the Maastricht area, animals otherwise well represented in North America. Some snake remains from the uppermost Creatceous in northwest Europe show similarities to North American boas. And, a single bone fragment of a carnivorous dinosaur from the Maastricht Cretaceous resembles the North American Dryptosaurus to some extent. Seen individually, these finds perhaps are not conclusive evidence to postulate a northerly land bridge during the latest Cretaceous, but taken together with the new mammal these finds become much more meaningful.

Original publication
The description of Maastrichtidelphys has appeared in the scientific journal Journal of Mammalian Evolution; The complete reference is: Martin, J.E., Case, J.A., Jagt, J.W.M., Schulp, A.S. & Mulder, E.W.A. (2005). A new European marsupial indicates a Late Cretaceous high-latitude transatlantic dispersal route. Journal of Mammalian Evolution 12(3/4): xx-xx.


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