Ichthyosaurs evolved a fish-shaped body like whales, but with unexpectedly dense, heavy ribs

Lungs are the problem both animals had to solve. After a breath at the surface, an air-breathing diver is buoyant in the wrong direction and has to work against itself on the way down ^[15]. Whales get help from their ancestry: a mammalian spine that flexes up and down, plus a horizontal tail, which makes tipping the front of the body down and driving for depth fairly easy ^[16]. Ichthyosaurs inherited the reptile arrangement, a spine that moves sideways like a lizard's and a vertical tail closer to a shark's, and Jorgen Kroglid says the same maneuver is not that simple for them ^[17]. Dense ribs are what the team points to on the other side of that ledger, having gone back to the two groups' separate origins to look for an explanation ^[20], origins that are genuinely separate: whales from artiodactyls, ichthyosaurs from reptiles still not identified ^[8].

The thing the thin sections do not tell you is whether that weight was working as ballast. The study read bone, not diving, and the buoyancy account is an interpretation the anatomy permits rather than a measured result ^[20].

The denominator is four animals: two ichthyosaur fossils and two toothed whales, a porpoise and a beluga ^[9][19]. Several of the ichthyosaur ribs came out nonporous ^[12], and what is reported is the direction of the difference, without compactness values or rib counts ^[21]. Ichthyosaurs were in the oceans for something on the order of 150 million years ^[1], which leaves room for one lineage to differ from another, and two specimens do not close that.

What makes the comparison worth the damage is resolution. In the fossils the cavities where bone cells once sat are still there, so growth from the inside can be read in a rib 150 million years old much as it can in a whale skeleton ^[10][11]. Lene Liebe Delsett is direct about the price: sectioned material cannot be glued back together, and permission to cut it requires showing that a lot of new physiology comes out the other side ^[18].

For the question the Oslo project was built around, whether there is more than one way to arrive at a fish-shaped body ^[6], the ribs point where Delsett says the comparison with living toothed whales points, which is yes ^[7]. Ichthyosaur skeletons are generally light and the ribs are the exception ^[3], the opposite of what a fast swimmer is expected to carry ^[2] and closer to what shows up in slow movers such as manatees ^[5]. My read: the direction of the difference in these four animals looks solid, the ballast explanation is a hypothesis with a testable shape rather than a finding, and anyone reading ichthyosaur swimming style off body outline now has a heavier rib cage to account for ^[13][14].