Mosasaur
Mosasaurs (from Latin "Mosa" meaning the 'Meuse river', and Greek σαύρος "sauros" meaning 'lizard') are an extinct group of large marine reptiles containing 38 genera in total. Their first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764. Mosasaurs probably evolved from an extinct group of aquatic lizards known as aigialosaurs in the Early Cretaceous. During the last 20 million years of the Cretaceous period (Turonian-Maastrichtian ages), with the extinction of the ichthyosaurs and pliosaurs, mosasaurs became the dominant marine predators. They became extinct as a result of the K-Pg event at the end of the Cretaceous period, about 66 million years ago.
Marcellus Formation
There are relatively sparse inclusions of fossilized marine fauna found in the Marcellus, but these fossils are still important to paleontology. For example, the Marcellus contains the oldest known diverse collection of thin-shelled mollusks still having well preserved shell microstructure.
It is also where goniatites, an extinct shelled swimmer similar to a squid, make their first appearance in the fossil record.
Life on land also enters the fossil record in the Marcellus, with the trunks of branchless conifer trees that floated out to sea to be preserved in the black shale.