A Fossil Fetus and what it can tell us about the life history of early whales.

An article published tonight in the journal PLoS ONE is forcing scientists to rethink everything they thought they knew about whale evolution.

OK. That's not actually true. But I've got a bet going that "someone" is going to use the phrase "rethink everything" in their story about this find, so better safe than sorry. Plus, it's a way cooler lede than "new whale fossil discovery matches predictions beautifully", even if the mundane description is the one that's just the tiniest bit more accurate.

Seriously, though, a multinational team of authors led by University of Michigan rock star Philip Gingerich is reporting a huge paleontological find - a fossilized pregnant early whale. It's the first time that an early cetacean has been found with a fetus. They also report finding the remains of a male of the same species. This combination of fossils - male, female, and fetus - is absolutely fantastic, because it provides information about so may different parts of the life history of this species.

Since one of the fossils found was a pregnant female, the researchers have named the new species Maiacetus inuus. The Maiacetus part of the name comes from the Greek for "mother whale". The inuus comes from the ancient Roman god Inuus, who was apparently the deity who handled (so to speak) sexual intercourse. That conjugation of names might be a bit unfortunate. It isn't as bad as some I've seen (Amorphophallus geei immediately springs to mind), but it does look like one possible translation of this whale's name would be "holy mother whale...

But anyway.

The fossils in question were found in 2000 and 2004, in an area of Pakistan that's already taught us a great deal of what we know about early whales. The newly-reported remains date to the early middle Eocene epoch (about 47.5 million years ago). This was a really interesting period in whale evolution, because it was a time when whales had not yet become fully marine organisms.

The skeleton of the male is the most complete of the three, and provides the best view of what the animal would have looked like while alive:

Artist's conception of male Maiacetus inuus with transparent overlay of skeleton. (Click to enlarge)

Credit: John Klausmeyer and Bonnie Miljour, University of Michigan Museums of Natural History

image courtesy of PLoS

So what does this guy tell us about the species lifestyle?

For starters, this is clearly the skeleton of an animal that was semi-aquatic. As Gingerich et al. point out:

While the hind limbs were capable of bearing the weight of the body on land, the proportions of the limbs and the long phalanges of both hands and feet would have limited terrestrial locomotion and prevented Maiacetus from traveling any substantial distance from water.

As cool a find as the nearly-complete male was, it's blown completely out of the water by the other discovery:

Fossils of female Maiacetus inuus with near-term fetus in utero, as found in the field. The female's skull is shaded white (teeth brown), and other parts of her skeleton are shaded red. The single fetus, in birth position inside the mother whale, is shaded blue (teeth orange). The specimen was collected in three plaster jackets (blue dashed lines), and additional bones were picked up separately. The red dashed line indicates the edge exposed by erosion.

Copyright: University of Michigan Museum of Paleontology.

The blue shaded bones in the figure above belong to a fetus. We know it's a fetus (as opposed to a meal) because it's just a bit too intact to have been eaten by someone with the big sharp pointy teeth that you see in the attached skull. It's also clearly the same species, and clearly a very young individual.

Since we know that there's a fetus associated with the skeleton, it's reasonably safe to assume that the larger individual was a female. The other adult is larger and has canine teeth that are proportionally larger than the female's teeth. It also has a pelvis that is what you'd expect to see on a male. That combination gives the researchers the confidence to identify him as a male. Because he's not all that different in size from the female, the researchers conclude that this probably wasn't a species where the males controlled individual breeding territories or harems of females.

As I already mentioned, the male skeleton is intact enough to demonstrate that the animal was semi-aquatic. It pretty clearly fed in the water, but the orientation of the fetus shows us that giving birth was one of the things that it still did on land:

The fetal skeleton is positioned for head-first birth, a universal birthing posture in large-bodied land mammals, but one that is anomalous in fully-aquatic marine mammals. A near-term fetus in an ungulate or whale may rotate about its long axis as it passes through the birth canal, but it cannot turn head-to-tail Discovery that early whales delivered calves like land mammals indicates that birth in semiaquatic protocetids still took place on land.

The level of development seen in the fetus tells us even more about these animals. The fetus had partially formed permanent molars. That's an indication that it was near-term. It's also an indication that the species gave birth to precocial young - infants that are able to move around on their own from the moment of birth. This pattern is seen in all semi-aquatic and aquatic mammals today, and it's also very common in the artiodactyls, which is the group believed to contain the last common ancestor of whales and extant land mammals.

All in all, not a bad take of knowledge from three sets of remains.

Philip D. Gingerich, Munir ul-Haq, Wighart von Koenigswald, William J. Sanders, B. Holly Smith, Iyad S. Zalmout (2009). New Protocetid Whale from the Middle Eocene of Pakistan: Birth on Land, Precocial Development, and Sexual Dimorphism PLoS ONE, 4 (2) DOI: 10.1371/journal.pone.0004366