In 1899, Henry Fairfield Osborn described the first complete skeleton of "Tylosaurus", a specimen measuring long that resides in the American Museum of Natural History as AMNH FR 221. Discovered in Kansas by a local named W. O. Bourne and initially identified as "T. dyspelor", the fossil has been considered remarkable in that it contained not only every bone apart from a few in the tail's tip, but also had exceptional preservation of the animal's soft tissue, including the larynx, trachea, bronchi, and cartilage in the throat, chest, and shoulders. This was due to calcification, a condition in a calcium-rich environment like the Kansas chalk that causes soft tissue to harden, allowing it to better fossilize. The completeness of AMNH FR 221 allowed Osborn to accurately measure the proportions of each body part; these ratios remain in use by modern paleontologists when extrapolating "Tylosaurus" fossils. Osborn (1899) included a life impression of AMNH FR 221 by paleoartist Charles R. Knight. The restoration carried a number of erroneous features, such as a baggy throat, bloated belly, and inaccurate paddles and dorsal fin. But a seminal feature was the addition of a dorsal crest (known as a fringe) lining the mosasaur's back. This was based on another 1898 description of a mosasaur fossil by Williston that erroneously identified tracheal rings as remains of a dorsal fringe, which he corrected in 1902. Knight updated his "Tylosaurus" restorations with the dorsal fringes removed to reflect this correction. However, his original depiction of a fringe was picked up as a meme, and many subsequent illustrations of the mosasaur continued to portray this inaccuracy.