To cut right to the conclusion: We now have reason to believe that the proto-whale Maiacetus inuus, a true transitional form, gave birth on land, not in the water.
Artist’s conception of male Maiacetus inuus with opaque skeleton overlay. Credit: John Klausmeyer and Bonnie Miljour, University of Michigan Museums of Natural History
Maiacetus inuus is a newly described member of a larger group of proto-cetids (proto-whales) that are believed to be largely aquatic. However, the absolute degree to which these four-legged not quite-whales really was aquatic is subject to further study and analysis. In this paper, an adult female Maiacetus inuus wiht a nearly full term (and rather large) baby Maiacetus inuus still inside it provides a very useful clue that birth was on land. The position of the baby is not whale like (tail first) but rather, land-mammal like (head first).
The fossils are from sediments in Pakistan, and date to about 47.5 million years ago. At that time, the mountainous region of Pakistan was a flat low area, often covered in a sea. This sea was raised up (and thus the fossil bearing sediments exposed) by tectonic forces associated with India shoving into Asia over the last several tens of millions of years.
The stratigraphy of this region is very well worked out. The right side of this diagram shows layers that are tilted and thus exposed over a large area … these are the layers that the Pakistani mountains are made out of. There are very few anomalies in the stratigraphy here. The left column shows radiometric dates; the next column shows paleomagnetic stratigraphy, the next shows standard chronometric units, and the next shows the biostratigraphy based on comparison of faunal materials. The column with the three color bands (“Sequence stratigraphy”) shows various characteristics of the sediments, which are discussed at length in other publications.
The red box shows the layer of limestone, deposited in an ancient sea, in which the fossils of concern here were found. There can be no questions about the date of the fossils.
(Here is the creationist quote mine of the above paragraphs, ready for use on creationist web sites, etc.: According to Harvard trained Palaeoanthropologist, Greg Laden, speaking of Gingerich’s find, “The stratigraphy of this region is … well … tilted and thus … There are … anomalies in the stratigraphy here. The left column shows radiometric dates… which are … of concern… There can be no … date of the fossils. “)
U-M paleontologist Philip Gingerich, who led the team that made the discoveries, was at first perplexed by the assortment of adult female and fetal bones found together. “When I first saw the small teeth in the field, I thought we were dealing with a small adult whale, but then we continued to expose the specimen and found ribs that seemed to too large to go with those teeth,” he said. “By the end of the day, I realized we had found a female whale with a fetus.”
This is the very first case of a fetal skeleton of an Archaeoceti (extinct proto whale). The name of the new species, by the way, derives from “Mother” and Innus, the Roman god of fertility. So it means “Mother of all whales, Apparently Quite Fertile.” What will they think of next…
The fetus was positioned head first which probably means delivery on land, though this is somewhat conjectural as it could have been an intermediate system. In addition, the fetus has a full set of teeth, indicating a precocious offspring, ready to fend for itself to at least some degree, was in the offing.
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.
In the same deposits as this female was found a male, somewhat larger in size and also well preserved. The male also has larger canines than the female, scaled to body size. The total degree of dimorphism (size/shape difference) in this species is moderate. This says something about the whale’s social system. Almost certainly, the males were not territorial, and there was moderate inter-male competition, possibly indicating mixed sex groups.
Gingerich interprets the large teeth, seemingly adapted for catching and eating fish, as indicative of a life at sea, and he believes that they came to land only for resting and giving birth, and possibly mating. These creatures had four limbs modified partly into flippers for swimming.
Maiacetus is clearly a transitional form between sea-dwelling but possibly near-shore mostly aquatic animals and dedicated sea-dwellers. Also, these particular specimens are very very complete and well preserved. You may recall that some years ago, creationists chided evolutionists because some guy named “Gingerich” had reconstructed all of whale evolution on the basis of a single ‘ankle bone’ found in a box in some dusty museum. Well, that was never really true. But now, with yet another transitional form and one that is so well preserved to boot, the creationists will be scraping thick egg off their chagrined faces for the rest of the year. According to Gingerich, “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.”
On a final note: This is a major article, published by an internationally recognized dream team of palaeoanthropologists. Since this is published in the Open Access journal PLoS ONE, this publication is a significant marker in the history of Open Access publishing. This is roughly like having a very famous food critic pick your resturant to eat in because she likes it.
The article can be downloaded and enjoyed by anyone with access to the internet here.
Other blog posts on this paper:
Maiacetus at Pharyngula
An Awesome Whale Tale at A Blog Around the Clock
Maiacetus, the good mother whale at Laelaps.
Fossil foetus shows that early whales gave birth on land at Not Exactly Rocket Science
A Fossil Fetus and what it can tell us about the life history of early whales. at The Questionable Authority
Ancient Whales Gave Birth On Land at Palaeoblog
Early whales gave birth on land, fossil find reveals at Science centric
Whale ancestors gave birth on land at ScienceNews
Primitive Proto-Whales May Have Clambered Ashore to Give Birth at Discover 80beats
Gingerich PD, ul-Haq M, von Koenigswald W, Sanders WJ, Smith BH, et al (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