Monsters from New Jersey

On my first trip to the Inversand marl pit in Sewell, New Jersey, I didn't find the wonderfully preserved Dryptosaurus skeleton I had been dreaming of. I come across a number of bivalve shells and geologically younger sponges, but other than a few scraps of "Chunkosaurus," my excavations didn't yield very much. Before my paleontology class left the site, though, we took a walk by the spoil piles, great green mounds of sediment that had already been mined for glauconite. It had recently rained, and little pillars revealed fragmentary fossils sitting on the surface. Out of the corner of my eye I spotted a roughly triangular object covered in a criss-cross network of bone and picked it up; I had found a mosasaur vertebra.

Although the Inversand pit is presently on dry land, just a stone's throw from Philadelphia, at the end of the Cretaceous the area was under about 100 feet of water. Dryptosaurus hunted two (maybe three) species of hadrosaur on the land, while marine crocodylians and mosasaurs patrolled the near-shore waters. During this time the mosasaurs were the top predators in the sea, and rather than there being just one species present, the marl pits of southern New Jersey have yielded a diversity of the aquatic predators. Mosasaurus hoffmanni, Mosasaurus dekayi, Mosasaurus conodon, Plioplatecarpus, Leiodon sectorius, Halisaurus platyspondylus, and
Prognathodon rapax are all known from material collected from New Jersey, but this is not the only location that they're known from. These same species are known from localities in Europe (and elsewhere), too, suggesting similar marine ecosystems across the ancient Atlantic (Mulder 1999).

With so many predators in one place, though, there must have been competition for resources, and different species may have occupied different marine zones. A study of mosasaurs in Alabama, for instance, seemed to show the presence of a near-shore Tylosaurus zone, an offshore Clidastes zone, and then a Mosasaurus zone eventually replacing the Clidastes zone in the same habitat (Kiernan 2002). This doesn't mean that the remains of other mosasaurs are not found in each of the zones, but rather that the remains from one genus dominate each assemblage, making up 57.7% (Tylosaurus), 78.2% (Clidastes), and 63.3% (Mosasaurus) of the remains found in each zone, respectively.

Why the mosasaurs were ecologically segregated, however, is still an open question. Habitat preference was probably closely connected to prey preference, although the presence of many species of predators also influenced the segregation (which areas were "open" for exploitation vs. those already occupied by other predators). The sort of resolution seem in Alabama is lacking in New Jersey, however, especially due to the fragmentary nature of many of the fossils and the fact that there is only one marl pit currently being excavated (although others likely have more material waiting to be exhumed). Still, it is stunning to see how successful mosasaurs were during the Cretaceous, and as is reconstructed in the superb artwork of Dan Varner (who, coincidentally, will be visiting New Jersey this summer to give a presentation at the state museum), the ancient seas must have been very terrifying, indeed.

References;

Kiernan, C.R. (2002) "Stratigraphic Distribution and Habitat Segregation of Mosasaurs in the Upper Cretaceous of Western and Central Alabama, With an Historical Review of Alabama Mosasaur Discoveries." Journal of Vertebrate Paleontology, Vol. 22 (1), pp. 91-103

Lingham-Soliar, T. (1995) "Anatomy and Functional Morphology of the Largest Marine Reptile Known, Mosasaurus hoffmanni (Mosasauridae, Reptilia) from the Upper Cretaceous, Upper Maastrichtian of the Netherlands." Philosophical Transactions: Biological Sciences, Vol. 347 (1320), pp.155-180.

Mulder, E.W.A. (1999) "Transatlantic latest Cretaceous mosasaurs (Reptilia, Lacertilia) from the Maastrichtian type area and New Jersey." Geologie en Mijnbouw Vol. 78, pp. 281-300.