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Basilosaurus (a.k.a. Zeuglodon) is a genus of primitive whale from the Late Eocene. The typical species were around 18 meters/60 feet in length in life, and had serrated, triangular teeth with two roots, and had a second pair of small, possibly functional flippers. Together with other basilosaurid whales, Basilosaurus is frequently referred to as a transitional form between the four-legged, often semi-terrestrial primitive whales ("Archaeoceti") and "modern" whales, much to the dismay of creationists.
As a result, Basilosaurus actually comes up fairly frequently in debates with creationists. Notably, in debates between PZ Myers and religious dogmatists - Basilosaurus is one of his favorite examples of a transitional form.
The fossils are rather common, being found in Late Eocene strata of Egypt (in the Valley of the Whales), Pakistan, and Alabama and Mississippi. In 1845, one self-titled "Doctor" Albert Koch unveiled a 35 meter/114 foot long skeleton, titled "Hydrarchos," ("water ruler") built from the remains of 5 fossil whales, including some Basilosaurus. Hydrarchos "went extinct" by being one of the numerous casualties of the Great Chicago Fires of 1871.
 As a cryptid