An Awesome Whale Tale

When I was a little kid, almost nothing was known about evolution of whales. They were huge, they were marine and they were mammals, but their evolutionary ancestry was open to speculation. Some (like Darwin himself) hypothesized that the terrestrial ancestor of whales looked like a bear. Others favored the idea of a hippo-like or even a pig-like ancestor.

Over the decades, two things happened. First, the revolution in molecular biology and computing power allowed scientists to compare many genes of many mammals and thus infer genealogical relationships between whales and other groups of mammals. Second, some smart palaeontologists decided that a good place to look for fossil whales would be Pakistan. The rest is, as they say, history. Digs in Pakistan unearthed a wealth of whale fossils over the years, so many of them, in fact, that the fossil record of prehistoric whales is now one of the best examples of the bushy tree of mammalian evolution in any lineage.

We now know that there were several gradual changes in whales over their evolutionary history from terrestrial animals to a number of branches of aquatic animals - some of these branches went extinct over time, while others have living descendants today. They tended to increase in size. Their front legs evolved into flippers. They gradually lost their hind legs: the large, strong hind legs of early whales were used for swimming by paddling, but later decreased in size as the undulating mode of swimming (and the evolution of the flat horizontal tail) took over. Today's whales have remnants of hind legs still hidden deep inside their large bodies, in the form of two smalish bones.

While genetics discovers evolutionary relationships, and fossils can tell us about evolution of morphology, it is much more rare that a fossil find allows us to infer much about an extinct animal's physiology, behavior or ecology. And the discovery of one such fossil was just published in PLoS ONE today: New Protocetid Whale from the Middle Eocene of Pakistan: Birth on Land, Precocial Development, and Sexual Dimorphism (also watch the accompanying video of the fossil). Here is the abstract:

Background

Protocetidae are middle Eocene (49-37 Ma) archaeocete predators ancestral to later whales. They are found in marine sedimentary rocks, but retain four legs and were not yet fully aquatic. Protocetids have been interpreted as amphibious, feeding in the sea but returning to land to rest.

Methodology/Principal Findings

Two adult skeletons of a new 2.6 meter long protocetid, Maiacetus inuus, are described from the early middle Eocene Habib Rahi Formation of Pakistan. M. inuus differs from contemporary archaic whales in having a fused mandibular symphysis, distinctive astragalus bones in the ankle, and a less hind-limb dominated postcranial skeleton. One adult skeleton is female and bears the skull and partial skeleton of a single large near-term fetus. The fetal skeleton is positioned for head-first delivery, which typifies land mammals but not extant whales, evidence that birth took place on land. The fetal skeleton has permanent first molars well mineralized, which indicates precocial development at birth. Precocial development, with attendant size and mobility, were as critical for survival of a neonate at the land-sea interface in the Eocene as they are today. The second adult skeleton is the most complete known for a protocetid. The vertebral column, preserved in articulation, has 7 cervicals, 13 thoracics, 6 lumbars, 4 sacrals, and 21 caudals. All four limbs are preserved with hands and feet. This adult is 12% larger in linear dimensions than the female skeleton, on average, has canine teeth that are 20% larger, and is interpreted as male. Moderate sexual dimorphism indicates limited male-male competition during breeding, which in turn suggests little aggregation of food or shelter in the environment inhabited by protocetids.

Conclusions/Significance

Discovery of a near-term fetus positioned for head-first delivery provides important evidence that early protocetid whales gave birth on land. This is consistent with skeletal morphology enabling Maiacetus to support its weight on land and corroborates previous ideas that protocetids were amphibious. Specimens this complete are virtual 'Rosetta stones' providing insight into functional capabilities and life history of extinct animals that cannot be gained any other way.

What does this all mean? Unlike the gradual loss of hind legs, graudal increase in size, gradual evolution of front legs into flippers and gradual evolution of the horizontal tail, we did not have information about the way the prehistoric whales gave birth. We know that all large terrestrial mammals give birth head-first, while all aquatic mammals (not just whales, but also manatees and such) give birth tail-first. But we did not know when did this switch occur.

This paper shows that some early whales, already well along the way of evolving into creatures recognizable as whales but still possessing sizeable hind legs they used for swimming, gave birth head-first. This indicates that these animals, at least on those rare occasions when they were giving birth to their young, had to go up onto dry land. Thus, we now have the timing a little better on the question of when exatly did the whales become completely aquatic, i.e., never coming to land at all, and it is somewhat later than we thought until now.

I tried to explain this in as simple language as possible - suitable for complete laymen or middle-school students, but if you want a little more detail and some better expert opinion on this fossil find and the paper that describes it, please read what others have written about it:

Carl Zimmer
Mike Dunford
Ed Yong
Brian Switek
Greg Laden
Cosmos Magazine
WIRED Science

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