How amphibious whales returned to the sea

Protocetids were likely amphibious, with large fore- and hindlimbs that could support the body on land. Maiacetus inuus is a member of the Protocetid family.

Pakicetus, one of the first Archaeocetes, or ancient whales. Pakicetids lived in the early Eocene, about 53 million years ago.

Millions of years ago, the first animals emerged from their watery habitat to live on dry land. After becoming fully adapted to a terrestrial environment, however, some animals, such as whales, ultimately returned to the ocean. But the evolutionary steps involved in that watery return have long been a mystery. Now, some exceptional fossils — and one really old baby — are shedding some light on how whales went back to the sea.

Phillip Gingerich, a vertebrate paleontologist at the University of Michigan in Ann Arbor, and his colleagues discovered two 47 million-year-old whale specimens in Pakistan, in 2000 and 2004. The first whale was a female of a new species the team called Maiacetus inuus. It is a member of the primitive group of ancient whales known as Archaeoceti, of which many members have hind limbs, indicating that they were probably fully terrestrial. The female was found to have a well-developed fetus inside of her, positioned for head-first delivery. “We don’t know of another fossil whale with a baby inside. It should be common; it’s odd to me now that we haven’t found more,” Gingerich says.

Four years later, the team uncovered another exceptional fossil in the same area: a male of the same species, measuring 8.5 meters in length. The male specimen was absolutely complete, Gingerich said, down to its digits and, significantly, its tail — a segment that was lacking in the other specimens.

Gingerich and his team hypothesized that this species was amphibious, feeding in the sea but still emerging onto the land to rest, mate and give birth. One piece of evidence for this amphibious lifestyle is the rear limbs, which were modified for swimming but could still be used on land to travel short distances. But one of the best indicators that this species dwelled at least partially on land is the orientation of the fetus.

Unlike land mammals, marine mammals are generally born tail-first. And, although there’s some intermediate ambiguity (seals, sea lions and hippopotamus are born both tail- and head-first), there has to be some explanation behind such a clear pattern, Gingerich says. He hypothesizes that tail-first births might be better for marine mammals because marine mammal babies may drown if the birth is difficult and they have no air to breathe, or perhaps because, since the mother is swimming while giving birth, it makes more sense to have the baby facing the same direction.

At this point the reason behind this tendency is not yet clear. But thanks to these specimens, Gingerich and his team were able to narrow down the time when whales first gave birth in the sea, eliminating their dependence on land. They project that this transition would have taken place between 47.5 and 36.5 million years ago — a gap that shrank by several million years after the analysis of these two Maiacetus inuus specimens.

Gingerich isn’t holding his breath for more mother-baby specimens to nail down the date of this amphibious-to-aquatic transition — there are other ways to make the date more precise, he says. Amphibious whale ancestors, such as Archaeoceti, propelled themselves through the water primarily with their fins, which could also be used as feet when they emerged from the water. But fully aquatic whales use their tails to move them through the water, marking a transition that is clearly marked in the fossil record by the disappearance of the back limbs. All of this is an important part of documenting the evolution from one type of whale to another, Gingerich says, which is his ultimate goal.

The Maiacetus inuus specimens date from the middle of this transition, but to truly understand what evolutionary changes took place and when, scientists need to uncover more well-preserved specimens like these, says Mark Uhen, a paleontologist at the University of Alabama in Tuscaloosa. “[Gingerich] is finding some astonishingly good things from this pivotal period,” Uhen says. “Now we need some slightly younger fossils.”

Early whales gave birth head-first

Early whales delivered calves head-first like land mammals (birth process, A-C) rather than tail-first, like modern marine mammals such as a harbor porpoise (D). That suggests that these amphibious whales still gave birth on land.

 

Alexandra Ossola
Thursday, February 19, 2009 - 10:30