Originally, this whale's was incorrectly thought to be a reptile because of its long and narrow skull [note: the suffix saurus has traditionally been used for reptile names]. It was referred to genus Zeuglodon by very early paleontologists; most of whom (all?) believed Basiolosaurus to have been a mosasaur (a reptile). The name Basilosaurus means “king lizard, but this animal was not a lizard! It has been wrongly said, in generalized context, that Basilosaurus was the whale that replaced mosasaurs.
Basilosaurus was a giant predatory whale (i.e., an apex predator) that lived during the latest middle Eocene (41.3 million years ago) to early Oligocene time (33.9 million years ago). It had a long, eel-like body, a long, narrow skull, large jaws, and a variety of different-shaped teeth. Its back flippers were very small. It was an unusual-looking whale that was up to 65 feet long and weighed 15 tons. It had a comparatively small brain. Its front flippers retained their elbow hinges. In sum, this genus represented “a very low branch on the whale evolutionary tree.” It is generally believed that modern whales did not evolve directly from Basilosaurus.
Figure 1. Basilosaurus
The vertebrate of Basilosaurus were filled with fluid rather than solid bone as is the case with all modern whales. Thus, Basilosaurus lived mostly near the water’s surface, otherwise its hollow backbone would have crumpled under intense water pressure in deep water.
Basilosaurus had a fearsome set of teeth: canine-shaped incisors in front and flattened, but strongly serrated teeth “molar” teeth in back. Thus, Basilosaurus had heterodont teeth (meaning different types of teeth for different purposes). It was a whale, therefore, not a reptile! It teeth makes Basiolsaurus unique in the world of whales!
Figure 2. Basilosaurus skull and teeth. Canine-shaped incisers in front and flattened, serrated triangular “molars” in back. Some workers have referred to the latter as “yoke teeth.”
Figure 3. A single Basilosaurus "yoke" molar tooth
The genus name Basilosaurus was once a “wastebasket taxon” and could still be! It has been used for all sorts of poorly known and poorly understood collection of middle to late Eocene whale fossils. It could still well be a “wastebasket” taxon. There are only two confirmed species of this early whale: a species from the southeastern United States (e.g., Mississippi and Alabama) and a different? species from Egypt and Jordon. In recent years, other “species” have been reported from every continent, including Antarctica (see Wikipedia, 2025). In my humble opinion, a detailed morphologic comparative study is needed for all of these whale fossils in order to positively ascertain their generic identification(s?) and to determine their relationship to one another.
To date, Basilosaurus is restricted to middle to upper Eocene deposits. There was a climate change at the end of the Eocene epoch. This change this was caused by global cooling and shifting of important current patterns (Squires, 2003).
References Cited:
Fenton, C.L. and M.A. Fenton. The Fossil book. Doubleday, New York. 740 pp.
Squires, R.L. 2003. Turnovers in marine gastropod faunas during the Eocene-Oligocene transition, west coast of the United States. In: From Greenhouse to Icehouse, the marine Eocene-Oligocene transition. Columbia University Press, New York, pp. 14-35.
Wikipedia. 2025.
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