1868 - Elasmosaurus - plesiosaur

From wikipedia:
Elasmosaurus from Greek elasmos 'thin plate' (referring to thin plates in its pelvic girdle) + sauros 'lizard') is a genus of plesiosaur with an extremely long neck that lived in the Late Cretaceous.

It was about 14 m (46 ft) in length and weighed over 2,000 kg (2.2 tons), making it the second longest plesiosaur. It had a large body and four flippers for limbs. More than half of its length was neck, which had more than 70 vertebrae, more than any other animal. It had a relatively small head with sharp teeth.

Discovery (finally! Wikipedia goes into details about how a prehistoric creature was discovered. Never fear - I'll be doing the necessary research and re-posting all posts that do not have such detailed info - after I find it out!)
Elasmosaurus platyurus was described in March, 1868 by Edward Drinker Cope from a fossil discovered and collected by Dr. Theophilus Turner, a military doctor, in western Kansas, USA. Although other specimens of elasmosaurs have been found in various locations in North America, Carpenter (1999) determined that Elasmosaurus platyurus was the only representative of the genus.

Cope did not include the hind paddles in this figure in part due to his erroneous belief that Elasmosaurus was propelled by its extremely long “tail ”When E. D. Cope received the specimen in early March, 1868, he had a pre-conceived idea of what it should look like, and mistakenly placed the head on the wrong end (e.g. the tail). In his defense, at the time he was an expert on lizards, which have a short neck and a long tail, and no one had ever seen a plesiosaur the size of Elasmosaurus.

Although popular legend notes that it was Othniel Charles Marsh who pointed out the error, there is no factual justification for this account . However, this event is often cited as one of the causes of their long-lasting and acrimonious rivalry, known as the "Bone Wars". In fact, although Marsh personally collected at least one plesiosaur from Kansas, and had several more from Kansas in the Yale Peabody collection, he never published a single paper on them (Everhart, 2005).

Although Cope verbally announced the discovery of Elasmosaurus platyurus in March 1868, he did not publish the "preprint" of his erroneous reconstruction of Elasmosaurus until August 1869. While much smaller, long-necked plesiosaurs from the Jurassic of England were well known at the time, this was the first time anyone had ever seen a Cretaceous elasmosaur. Cope's reconstruction showed it to have a long sinuous tail like a lizard or a mosasaur.

Note that while O.C. Marsh claimed to have pointed out Cope's error "20 years after the fact" in an 1890 newspaper article, it was actually Joseph Leidy who pointed out the problem in his Remarks on Elasmosaurus platyurus address at the Academy of Natural Sciences of Philadelphia meeting on March 8, 1870.