The evolution of the cetaceans: 50 million years underwater

Indohyus: the artiodactyl cousin

Indohyus, a small artiodactyl of the raoellid family (the size of a fox), is today regarded as one of the closest fossil relatives of the cetaceans on the terrestrial side. Described as early as the 1970s by A. Ranga Rao, it was Hans Thewissen who brought out its importance in 2007 (Nature): its ear already showed the structure characteristic of cetaceans, and its dense bones (osteosclerosis) indicated a semi-aquatic way of life, probably walking on the bottom as the hippopotamus does today. Indohyus was not a direct ancestor, but it illustrates what the close artiodactyl relative from which the cetacean lineage diverged looked like.

• Group: artiodactyl (raoellid), not yet a cetacean
• Diet: herbivorous / omnivorous, rather than exclusively carnivorous
• Key clue: cetacean-type ear plus dense bones (ballast for aquatic life)

Pakicetus: one of the oldest known cetaceans

Discovered in present-day Pakistan, Pakicetus (~50 Ma, with deposits dated at around 48.5 Ma) is one of the oldest recognised cetaceans and the best documented at this stage. Contrary to a widespread image, it was not a “sea dog”: it was a four-legged hoofed mammal the size of a wolf (~1 to 2 m), with long functional legs. It lived beside watercourses and is today regarded as semi-aquatic and tied to fresh water: Thewissen’s reconstruction (2001) describes it as “no more amphibious than a tapir”, that is, able to move about on land but strongly associated with shallow aquatic habitats. Its diagnostic character is acoustic: the tympanic bulla formed from the ectotympanic bone alone, and the involucrum, middle ear structures peculiar to cetaceans. But Pakicetus could not yet hear well underwater: it lacked the fatty pad in the jaw that carries waterborne sound in modern cetaceans, and it picked up sound through the external auditory canal, like a land mammal.

• Habitat: river banks and fresh water bodies of the Tethys
• Diet: carnivorous, probably piscivorous
• Key clue: ear (bulla / involucrum) already cetacean in type, but hearing still terrestrial

Ambulocetus: the “whale that walked”

Ambulocetus natans (“the walking whale that swims”) is one of the most emblematic fossils of this transition. With its short legs, its webbed hands and feet, its elongated snout and its body of about 3 metres, it suggests an amphibious predator lying in wait in shallow water, an ambush hunt comparable, in ecological terms, to that of a crocodile. For swimming, on the other hand, it is compared rather to a large otter: it propelled itself by dorsoventral undulations of the back and by powerful hind feet, and not by crocodile-like lateral movements.

• Size: about 3 metres
• Habitat: coasts and estuaries
• Way of life: amphibious, ambush in shallow water; swimming by dorsoventral undulation

Rodhocetus: the semi-aquatic form and its artiodactyl ankle

Rodhocetus, a protocetid of the Lutetian (~46-47 Ma), marks a major step towards marine life: reduced hind limbs, nostrils set further back, streamlined body. It is also in this animal that the double-pulley astragalus which confirmed artiodactyl ancestry was identified. The idea that it already possessed the “locomotor signature” of modern cetaceans has, however, now been revised: Gingerich (2003) considers that Rodhocetus probably had no tail fluke and swam mainly at the surface by alternating beats of the hind feet, the tail serving above all as a rudder, a mode close to that of the desman or the otter. True propulsion by undulation of the tail would appear later, in the basilosaurids.

• Age: ~46-47 Ma (Lutetian)
• Locomotion: swimming mainly with the feet, tail as rudder; probably without a fluke
• Key clue: double-pulley astragalus (evidence of the link with the artiodactyls)

Basilosaurus and Dorudon: the first fully marine cetaceans

Described as early as the 19^th century, Basilosaurus is a wholly marine cetacean, very elongated, 15 to 20 metres long, without functional hind limbs but retaining tiny vestigial legs. Its relative Dorudon, more compact (~5 m), is probably more representative of the lineage that gave rise to the present-day cetaceans. The basilosaurids propelled themselves with the tail and very probably had a horizontal tail fluke, an inference drawn from the proportions of their caudal vertebrae, since the fluke, having no bone, does not fossilise. The exact swimming mode of Basilosaurus, whose body is particularly serpentine, remains debated; recent analyses (Gingerich, Uhen) favour essentially dorsoventral undulations rather than lateral ones.

• Anatomy: tail with an inferred horizontal fluke, nostrils set further back
• Habitat: tropical oceans worldwide
• Significance: the first cetaceans with a fully accomplished marine life

The great split: mysticetes vs odontocetes

Towards the end of the Eocene, the Neoceti lineage divides into two groups with distinct ecological trajectories. Molecular estimates place this divergence about 34 to 39 million years ago (central value ~37 Ma according to Steeman et al., 2009), while the fossil record documents it from about 34 Ma onwards: the Oligocene record does not always support a radiation as early as the molecular clocks indicate, and this remains a matter of debate.

• The mysticetes: development of baleen and filter feeding.
• The odontocetes: development of echolocation by means of a fatty melon, active predation.

This divergence allowed the two groups to occupy complementary ecological niches (see the difference between whale, sperm whale and dolphin).

From teeth to baleen: the transitional mysticetes

Baleen did not appear all at once. The oldest known mysticetes still had teeth: Mystacodon, Coronodon and Janjucetus were toothed mysticetes without baleen, while Aetiocetus (Oligocene) shows both teeth and indications of baleen, an intermediate stage in the shift to filter feeding. In parallel, the first odontocetes capable of echolocation appear in the Oligocene. Acoustic specialisation and filter feeding therefore came into place gradually, and not instantaneously.

Emergence of the sperm whale lineage

The superfamily Physeteroidea diverges from the rest of the odontocetes about 25 million years ago. It brings together two living families: the Physeteridae (a single living species, the sperm whale, Physeter macrocephalus) and the Kogiidae, which differentiate later and comprise the pygmy sperm whale (Kogia breviceps) and the dwarf sperm whale (Kogia sima). Beware of a frequent error: the two Kogia are not Physeteridae but Kogiidae; it is the superfamily Physeteroidea that unites them with the sperm whale.

Diversification of the delphinids

The oceanic dolphins (Delphinidae) diversify, giving the most numerous family of living cetaceans: common bottlenose dolphin, common dolphin, killer whale, pilot whale, Risso’s dolphin… This is also the period of expansion of the porpoises and the river dolphins.

The shift towards rorqual gigantism

The blue whale (Balaenoptera musculus) is the largest animal known from any period. Its gigantism belongs to a shift in the evolutionary regime of body size in the Plio-Pleistocene, dated to about 4.5 million years ago (Slater, Goldbogen & Pyenson, 2017). This shift is linked to the seasonal intensification of cold-water upwelling, which concentrates krill into dense swarms that can be exploited by bulk filter feeding. The extreme gigantism of the rorquals is above all Pleistocene: speaking of the blue whale “appearing” at 4 Ma is therefore a convenient but approximate shorthand.