Dinosaur teeth can reveal history
Late in the Cretaceous Period, when tyrannosaurs and their kin were the ruling predators on land, large swimming reptiles ruled the seas. Among them were the plesiosaurs and the mosasaurs.
The suffix “saur” means “lizard.” The dinosaurs and plesiosaurs were not lizards, but mosasaurs were true lizards, close relatives of the largest lizard that lives today, the Komodo dragon.
Mosasaurs were essentially Komodo dragons that went to sea.
And well-equipped for the sea they were, with four large flippers and a large flat tail to propel them through the water. Some species had blunt teeth for crushing clams and other hard-shelled prey, but most had sharp teeth for eating fish, squid and whatever else they could catch.
One specimen contains remains of some of its last meals, including bones of fish, an aquatic bird and even another mosasaur. The largest species reached lengths of 56 feet, bigger than a bus.
Their remains are most common in the chalk deposits that are characteristic of that time — the Latin word for chalk is creta, basis for the Cretaceous Period’s name. Those chalk deposits formed in the oceans that covered many areas of the world then, including Kansas, where many mosasaur fossils have been found.
There is increasing evidence that they also frequented more near-shore environments such as lagoons and estuaries. A recent bit of research added to that evidence from a new direction, measuring oxygen isotopes in their teeth.
All atoms of oxygen have 8 protons. Some also have 8 neutrons in their nuclei, giving those atoms a mass of 16, while others have 10 neutrons, giving them a mass of 18. Those different “varieties” of the same element are called isotopes.
Ocean water has more of the heavier isotope, while freshwater contains more of the lighter oxygen. Animals ingest water, and the relative amounts of heavy versus light oxygen in that water is reflected in the composition of the enamel of their teeth.
Mosasaurs continually replaced their teeth, like modern lizards and crocodiles. New teeth were formed in the jaws by adding daily increments of enamel. Teeth took an average of about three months to form, but sampling sequential teeth allowed determination of isotope changes over longer periods.
The new research investigated mosasaur teeth from Kansas and Alabama, which at that time was also under ocean water. Two species were sampled, two adults and one juvenile.
When graphed, the isotopes had periodic sharp dips in the relative amount of heavier oxygen. That indicated that one of the adults swam to areas with more freshwater input every 12 to 20 days, the other adult did so every 4 to 7 days, while the juvenile did so every 6 to 7 days.
Deep ocean water also has more of the lighter oxygen isotopes, but being air breathers, the mosasaurs would not have spent enough time in those depths to incorporate it in their teeth.
So why did the mosasaurs make those trips? Modern sea snakes must drink fresher water periodically to regulate the salt content of their bodies. Now we suspect that mosasaurs did, too.
Mosasaurs continually replaced their teeth, like modern lizards and crocodiles.