The King Lizard: Basilosaurus cetoides

The evolutionary history of cetaceans is a long an fascinating one however it wasn’t until around 40 million years ago that the group really started to get huge. Case in point, Basilosaurus cetoides which at the time was the largest predator to exist since the Cretaceous-Paleogene Mass Extinction. Basilosaurus translates to “King Lizard” although it of course wasn’t actually a reptile but rather a primitive cetacean which lived across much of Egypt as well as Alabama and Mississippi during the Late Eocene between 40 and 35 million years ago. Basilosaurus belongs to the paraphyletic group, Archaeoceti or “ancient whales” a parvorder which includes the ancestors of modern whales, porpoises and dolphins but excludes all modern cetaceans hence why it’s paraphyletic. The genus contains two recognised species, B. cetoides from America, which was named in 1839 by Richard Owen and B. isis from Egypt, which was named in 1906 by Charles Andrews. Basilosaurus was a huge whale and it’s fortunately known from decently complete skeletons. Remington Kellogg described a mounted a skeleton of B. cetoides in 1936 which which measured 16.76 meters (55 ft) and possessed a 1.5 meter (5 ft) long skull. Kellogg reconstructed B. with 58 vertebrae however it would later be suggested that at least 3 more thoracic vertebrae were present, this adding a 25 percent increase in length indicating a total body length of 20.95 meters (68 ft). Philip Gingerich in 1990 suggested B. isis possessed up to 7 more vertebrae than B. cetoides but was evidently slightly smaller. The B. isis specimen CGM 42195, measures 15 meters (49 ft) and has a skull that measures 1.2 meters (4 ft). 

While superficially Basilosaurus may look similar to its modern relatives it actually possessed a collection of morphologies not seen in modern cetaceans and therefore represents a transitional form between the terrestrial ancestors of cetaceans and true modern whales. For instance the teeth of Basilosaurus are heterodont like those of other terrestrial mammalian carnivores rather than the conical homodont teeth of modern toothed whales which are better adapted for gripping slippery prey like fish. The nostrils are located in the middle of the skull, intermediate between their location in Pakicetus which are at the front of the skull and modern whales which have theirs at the very back. The forelimbs of Basilosaurus were still at least partially flexible at the elbow joint, unlike those of modern cetaceans. Basilosaurus was not able to echolocate like modern toothed whales (Odontoceti) can and most strikingly of all it still retained the external hindlimbs of its terrestrial ancestors which would become completely vestigial in later whales. Basilosaurus is also unusually slender for a whale, this is because it lived during the Eocene where temperatures were much higher and it didn’t need blubber to keep itself warm. Philip Gingerich hypothesised that because of the elevated vertebrae number Basilosaurus’ locomotion was anguilliform like an eel, it would undulate only vertically. This allowed it to navigate its environment because unlike modern whales Basilosaurus did not live in the open ocean, rather it was confined to shallow, marshy coastal habitats. It’s diet most likely consisted of large semi aquatic mammals which shared its environment like Moeritherium, large fish and the smaller contemporary cetacean Dorudon. A study published in 2019 analysed the stomach contents of a B. isis specimen and this indicated it did indeed feed on the smaller Dorudon. 

Design Notes: 

I based the overall proportions on the skeletal reconstruction of the B. isis specimen CGM 42195, which was included in the 2019 paper, “Stomach contents of the archaeocete Basilosaurus isis: Apex predator in oceans of the late Eocene”. I also used Kellogg’s 1936 skeletal reconstruction of B. cetoides. The overall life appearance is based off the Amazon River Dolphin as this is another cetacean which has a minimalistic blubber amount due to it living in a tropical climate. The snout is inspired by a Bottlenose Dolphin’s. The presence of Vibrissae or whiskers is based on the whiskers of hippos as well as the North Atlantic Right Whale. It’s likely Basilosaurus possessed whiskers because it was still a primitive cetacean and even modern dolphins still retain them as juveniles. 

You may notice the four small nails on my Basilosaurus’ front flippers. This is based on the front flippers of manatees which also retain four small vestigial nails. The evolution of manatees and cetaceans parallels each other very closely, both animals descended from terrestrial mammals and have since adapted their anatomy to better suit an aquatic lifestyle. Modern whales and dolphins have completely lost any trace of their nails but manatees still retain them as a vestigial structures. They are completely vestigial because manatees live a fully aquatic lifestyle and their nails serve no function, other than perhaps the occasional belly scratch. The reason their nails haven’t been completely lost yet is because they don’t hinder the animal’s chance of survival in anyway and so there’s no selective pressure to lose them. Cetaceans evidently did lose them completely at some point during their evolutionary history and this is likely because their anatomy is more extremely adapted to an open ocean lifestyle while manatees prefer shallow waters. What does this have to do with Basilosaurus? Well because Basilosaurus represents a very primitive stage in cetacean evolution it’s possible that at this point whales hadn’t yet fully lost their nails. Like manatees Basilosaurus also still had a flexible elbow joint, something which has been totally lost in modern whales. Basilosaurus and manatees also lived in similar shallow, marshy, coastal habitats so it’s certainly possible that Basilosaurids represented the last stage in cetacean evolution that retained their vestigial nails. Of course this is purely speculative and it’s also possible whales lost their nails a lot earlier on their evolutionary history. It’s been suggested that manatees use their nails to “walk” along the sea floor as they feed, something Basilosaurus wouldn’t have had to do however if the nails did offer an advantage then why would the Amazonian manatee lack it’s nails. Clearly they don’t offer any noticeable advantage, some species have 4 per flipper, some 3 and some none at all. 

References: 

Gingerich, Philip D.; Smith, B. Holly; Simons, Elwyn L. (1990). "Hind Limbs of Eocene Basilosaurus: Evidence of Feet in Whales". Science. 249 (4965): 154–157. 

Gidley, J. W. (1913). "A recently mounted Zeuglodon skeleton in the United States National Museum". Proceedings of the United States National Museum. 44 (1975): 649–654.

Zalmout, I. S.; Mustafa, H. A.; Gingerich, P.D. (2000). "Priabonian Basilosaurus isis (Cetacea) from the Wadi Esh-Shallala Formation: first marine mammal from the Eocene of Jordan". Journal of Vertebrate Paleontology. 20 (1): 201–204.

Owen, R. (1839). "Observations on the Basilosaurus of Dr. Harlan (Zeuglodon cetoides, Owen)". Transactions of the Geological Society of London. 6: 69–79.

Kellogg, R. (1936). A review of the Archaeoceti (PDF, 46 Mb). Washington: Carnegie Institution of Washington.

Andrews, C. W. (1906). A descriptive catalogue of the Tertiary Vertebrata of the Fayûm, Egypt. British Museum (Natural History)

Voss, M., Antar, M., Zalmout, I. and Gingerich, P. (2019). Stomach contents of the archaeocete Basilosaurus isis: Apex predator in oceans of the late Eocene. PLOS ONE, 14(1), p.e0209021.