Cimoliasaurus magnus Leidy
Description: The long necked plesiosaur is represented uncommonly at Big Brook by its teeth, vertebrae, and sometimes other
skeletal material. It is the only plesiosaur identified from this locality, so any plesiosaur teeth are readily assignable to
The teeth of this plesiosaur are conical with an oval cross section at the base becoming more flattened toward the tip. They are
long and slender but somewhat flattened without carinae. Their teeth are coated with enamel and are usually smooth with some
growth cracks. Plesiosaur teeth have a conical indent in the bottom of the root to fit into the socket of the jaw. These teeth are
usually about 2 to 5 cm (about .75 to 2 in) long.
Their vertebrae are somewhat uncommon but when found are very distinctive. The centrum, or the main cylindrical to ovate part
of the vertebrae, is usually the only part that is recovered. They are more than likely lacking any of the other structures of the
vertebrae other than the centrum because they are easily broken. They have a cylindrical shape with flat ends resembling a soda
can. The ends may have a small divot in the center. The sides of the vertebrae may be smooth but more than likely the thin
coating of bone around the sides is warn off due to stream action, exposing the very spongy bone structure underneath. There
are three holes in the side of the vertebrae; one on the dorsal side while the ventral side has two. Most vertebrae are between 5
to 8 cm (about 2 to 3 in) and about 5 cm (about 2 in) in diameter but can get to about 10 cm (about 4 in) long and wide.
Other plesiosaur bones are not easily distinguishable and are usually very scrappy. The scrappy reptilian bone of mosasaurs, sea
turtles, "crocodiles", and plesiosaurs are similar and are all found at this locality. Unless the bones are complete, it is difficult to
attribute them to plesiosaurs.
Commonality: Teeth and vertebrae are uncommon. Other identifiable material is rare.
Similar fossils: Plesiosaur teeth are distinguishable from area mosasaur teeth in that they lack the carinae that the mosasaur
teeth have. They are also longer and more slender than mosasaur teeth. The teeth of the plesiosaur can also be confused with
the teeth from the gavil like "crocodile" which are smaller, usually less than 2 cm (about .75 in) long. Their vertebrae can
resemble crocodile or the mosasaur vertebrae but are lacking the convex and concaved ends.
Size: This plesiosaur reached a maximum length of 13 meters (about 40 feet) but commonly reached only 6 meters (about 20
Notes: The plesiosaurs were a group of large marine reptiles that possessed extremely long necks and tails. They roamed the
open surface waters of the sea feeding on the abundant schools of fish and small reptiles. They competed for this food supply
with the mosasaurs, "crocodiles", and sharks. Interestingly, plesiosaurs become very scarce in the fossil record before the end
of the Late Cretaceous while the mosasaurs (temporarily) and crocodiles become more abundant. This may suggest the more
ancient plesiosaurs, that where around since the Jurassic, were becoming out classed by the newer types of mosasaurs, modern
types of crocodiles, and sharks.
a. End b. Top c. Bottom showing two holes