The skull of Mosasaurus hoffmani. From Lingham-Soliar 1995.
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 picked up a number of Cretaceous bivalve shells and Paleocene 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 of the sandy green stuff supported 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. It was a mosasaur vertebra.
Although the Inversand pit presently sits on dry land just behind a newly-constructed mini-mall, at the end of the Cretaceous the site was under about 100 feet of water. The tyrannosauroid Dryptosaurus hunted two (maybe three) species of hadrosaur on the nearby shore, while marine crocodylians and mosasaurs patrolled the coastal waters. During this time the mosasaurs were the top predators in the sea, and 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 the numerous mosasaur 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.
According to fossil finds in Alabama, Tylosaurus occupied the near-shore zone and Clidastes lived further out. From Kiernan 2002.
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 versus those already occupied by other predators). Unfortunately, the sort of resolution seem in Alabama is lacking in New Jersey, especially due to the fragmentary nature of many of the marl-bound 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 restored in the superb artwork of Dan Varner, the ancient seas must have been very terrifying, indeed.
KIERNAN, C. (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, 22 (1), 91-103 DOI: 10.1671/0272-4634(2002)022[0091:SDAHSO]2.0.CO;2
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 of the Royal Society B: Biological Sciences, 347 (1320), 155-172 DOI: 10.1098/rstb.1995.0019
Mulder, E. (1999). Transatlantic latest Cretaceous mosasaurs (Reptilia, Lacertilia) from the Maastrichtian type area and New Jersey Geologie en Mijnbouw, 78 (3/4), 281-300 DOI: 10.1023/A:1003838929257