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.
Mastodon
The first remnant of "Mammut", a tooth some in weight, was discovered in the village of Claverack, New York, in 1705. The mystery animal became known as the "incognitum". The first bones to be collected and studied scientifically were found in 1739 at Big Bone Lick State Park, Kentucky, by French soldiers, who carried them to the Mississippi River, from where they were transported to the National Museum of Natural History in Paris. Some time later, similar teeth were found in South Carolina, which, according to the slaves, looked remarkably similar to those of African elephants. This was soon followed by discoveries of complete bones and tusks in Ohio; people started referring to the "incognitum" as a mammoth, like the ones that were being dug out in Siberia. Anatomists noted that the teeth of mammoth and elephants were different from those of the "incognitum", which possessed rows of large conical cusps, indicating that they were dealing with a distinct species. In 1806 the French anatomist Georges Cuvier named the incognitum "mastodon".