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Beefy Bones and a Big Bite for the Ancient Whale Basilosaurus

Basilosaurus life restoration by Dmitry Bogdanov, CC-BY.
Basilosaurus life restoration by Dmitry Bogdanov, CC-BY.

Although its name sounds rather dinosaurian, Basilosaurus was in fact one of the first extinct whales to make a splash in humanity’s perception of the past.

When its bones were first described back in the early 19th century, the anatomy of the backbone was thought to most closely resemble that of some Mesozoic marine reptiles. Later work, and examination of other parts of the skeleton, revealed Basilosaurus to be an early whale that lived around 34 million years ago. Of course, more recent discoveries have since produced older whales with even more “primitive” features, but Basilosaurus and its close relatives remain important for piecing together the details of whale evolution. This past week, two papers in PLOS ONE added more information on the internal anatomy of the bones of Basilosaurus as well as its strong bite.

The first paper, by Alexandra Houssaye and colleagues, used computed tomography (CT) scans to peek inside the limb bones, vertebrae, and ribs of a number of early whales. For the purposes of this post, I’ll just focus on Basilosauruscheck out the paper for the story with the other species, which is pretty interesting in its own right.

Rib of Basilosaurus--note its high density, a common feature of whale ribs. Modified from Houssaye et al., 2015. CC-BY.
Rib of Basilosaurus–note its high density (gray is bone tissue, black is empty space), a common feature of whale ribs. Modified from Houssaye et al., 2015. Scale bar equals 10 mm. CC-BY.

Two distinct adaptations are common in the bones of aquatic amniotes (birds, reptiles, and mammals) that forage underwater. The first is Bone Mass Increase (BMI); this is achieved by making bone denser (getting rid of spongy bits) as well as by adding more bone tissue to an individual bone. The second adaptation is, by contrast, creating spongy bones with internal struts to hold up to the forces of swimming. Presumably, both of these adaptations evolved in order to maintain optimal buoyancy while enabling effective locomotion.

Basilosaurus had ribs largely made of compact bone, typical of whales (even those today). Its arm and thigh bones were also quite compact and dense, unlike the condition in some other early whales. This is actually a bit of a puzzle, because it is fairly different from close relatives of Basilosaurus that have less dense arm bones (presumably tied to swimming style), and doesn’t match with the overall picture of Basilosaurus as a predatory animal that lived in open waters. Because the hind limbs were fairly useless, it doesn’t make a lot of sense that they would have such dense bone (no need to stand up to big muscular forces). The basic conclusion: more research is needed. The study by Houssaye and colleagues provides some important data points to launch future investigations.

Blue Skull of Death. Computer reconstruction of the skull of Basilosaurus, with jaw muscles added. Modified from Snively et al. 2015. CC-BY.
Blue Skull of Death. Computer reconstruction of the skull of Basilosaurus, with jaw muscles added. Modified from Snively et al. 2015. CC-BY.

The next paper under consideration turned to the business end of Basilosaurus–its skull. Fossilized stomach contents showed that Basilosaurus chowed down on fish, wear patterns on the teeth suggested at least some shellfish in the diet, and tooth marks on the bones of other whales indicate a habit of chomping anything willing (or unwilling) to be eaten. Presumably Basilosaurus had the bite force to dent bone–could this be confirmed?

Eric Snively and colleagues constructed a computer model of the skull of Basilosaurus, complete with simulated musculature. After crunching all of the numbers, they estimated that this long-dead whale packed over 16,000 N of force in its bite (and potentially even more, depending upon the conditions invoked). This blows the bone-crushing hyena out of the water–our poor terrestrial pal weighs in at a “paltry” 3,500 N for measurements of bite force in captive animals.

The bite of Basilosaurus sounds impressive–and it certainly is–but context is everything. With a skull measuring over a meter long, Basilosaurus was far larger than hyenas. Big head, big bite. Modern crocodilians have a bite force well beyond that of Basilosaurus, and ancient marine reptiles and the “land shark” Tyrannosaurus also would have won in the cranial equivalent of an arm wrestling contest. Differences in skull size as well as muscular configuration are probably in play here. As Snively and colleagues suggest, it would also be nice to see how the bites of modern whales stack up (e.g., orcas). We’re only beginning to understand the evolution of whale bites.

So…we have a lot more information on Basilosaurus, but not much in the way of explanations or answers. Perhaps that is frustrating for some. As for me, I like these new mysteries. Science is at its best when it has questions to investigate!

Citations
Houssaye A, Tafforeau P, de Muizon C, Gingerich PD (2015) Transition of Eocene whales from land to sea: evidence from bone microstructure. PLOS ONE 10(2): e0118409. doi:10.1371/journal.pone.0118409

Snively E, Fahlke JM, Welsh RC (2015) Bone-breaking bite force of Basilosaurus isis (Mammalia, Cetacea) from the Late Eocene of Egypt estimated by finite element analysis. PLOS ONE 10(2): e0118380. doi:10.1371/journal.pone.0118380

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