01 June 2013

Fossil Vertebrate of the Month: Tylosaurus ivoensis

Tylosaurus. ivoensis and other marine vertebrates from the Karlstad Basin (Sørensen et al 2013)
Many of you may be familiar with my long-standing love of all things Swedish as well as my more recent interest in Denmark (born out of a couple of trips to Copenhagen and the discovery that I am distantly related to the Danish Vikings that invaded Ireland).  Both countries celebrate their national holidays in June (Denmark on the 5th, Sweden on the 6th) and Midsummer, a major holiday throughout the Nordic countries, occurs late in the month, so I thought I'd revel in my Scandinavophilia (Scandinaviophilia?  Scandinaviaphilia?) by highlighting one of the more spectacular fossil vertebrates to have been discovered in Skåne, the historically Danish region of southern Sweden.  During the Late Cretaceous, sea levels worldwide were extremely high, flooding low-lying areas of land, including most of Europe and what are now the Great Plains of North America.  These shallow seas were home to organisms ranging from the plankton whose shells would eventually form the chalk beds that gave the Cretaceous its name (and the White Cliffs of Dover their characteristic color) to the first marine birds to fish of all shapes and sizes to large marine reptiles.  Remains of such reptiles are especially common in the Kristianstad Basin of Skåne, and in particular along the shores of Ivösjön, one of the large lakes dotting the landscape of southern Sweden.  Reptiles recovered from the area include crocodiles, turtles, plesiosaurs, and mosasaurs, relatives of snakes and monitor lizards and the group to which Tylosaurus ivoensis belongs.  Mosasaurs are by no means unusual in the fossil record of northern Europe, but they have an especially rich fossil record in the Kristianstad Basin.  The seas of Skåne played host to a complex food web (as demonstrated by a study published just last month) and while smaller mosasaurs likely preyed upon fish and ammonites, there is direct evidence that large species such as T. ivoensis fed on other marine reptiles.  Mosasaurs, of course, are absent from the world's oceans today, and marine rocks from Scandinavia give some indication as to why.  Not all extraterrestrial impacts are associated with widespread extinctions, as the fantastically-named Mjølnir Crater of Norway shows, but an impact at the end of the Cretaceous seems to have played a major role in sealing the fate of many animals, including mosasaurs.  Early evidence for this impact also came from Scandinavia: the cliffs of Stevns Klint, a Danish site south of Copenhagen, were among the localities studied by Walter Alvarez when he first proposed his now-famous impact hypothesis.