SYDNEY: Two newly described fossil whales; a pregnant female and a male have revealed how whales once gave birth on land and provide insights into how they made the transition from land to sea.
Uncovering a layer of 48-million-year-old marine sediments in Pakistan, palaeontologist Philip Gingerich and his team were perplexed when they found an assortment of adult female and foetal bones 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 too large to go with those teeth,” said Gingerich, who leads a team from the University of Michigan, in the USA. “By the end of the day, I realised we had found a female whale with a foetus.”
As detailed today in the journal PloS One this is the first discovery of a fossil whale with a foetus in utero. The team fittingly named the new species Maiacetus innus; after ‘mother whale’ and Innus, a roman fertility god.
The foetal skeleton was found in a head-first birthing position as is the case for land mammals, but not for modern whales. The researchers believe this shows these whales were semi-aquatic or amphibious and could still come onto land to give birth, which may have been a safer environment than the prehistoric seas, said Gingerich.
Another interesting clue to the whale’s life history is the well-developed set of true molars in the foetus, indicating that Maiacetus newborns were born well-developed, ready to eat solid food, possibly with their eyes open and equipped to fend for themselves at an early age.
The 2.6-metre male skeleton found at the same site was surprisingly complete, from the skull down to the tip of the tail and toes, and has offered several new clues about the amphibious lives of these early whales.
Measurements of the male’s body size revealed it to be 12 per cent larger and its teeth 20 per cent larger than the adult female. For a mammal, this size difference between the sexes (or ‘sexual dimorphism’) is relatively small, hinting that there was little competition among males for territories and females.
Furthermore, the fossil revealed that the shearing teeth of Maiacetus were suited to catching and eating fish and the powerful fore- and hind-limbs were designed for swimming powered by the feet rather than the body or tail.
Although the hind limbs would have been capable of bearing the weight of the animals body on land, said Gingerich, the proportions of its limbs to its toes would have made walking difficult, preventing Maiacetus from venturing too far from shore.
Like other early whales it fed at sea coming ashore to rest, mate and give birth, said Gingerich. “They were clearly tied to the shore… they were living on the land-sea interface and going back and forth.”
Compared to known fossil whales, Maiacetus occupies a more intermediate stage in the transition between land and sea dwelling. It offers invaluable new information on structural and behavioural changes that accompanied that transition, said the researchers.
“Whale evolution was already one of the best documented evolutionary transitions in the fossil record. This adds another beautiful step in the ladder” said Hans Thewissen, an evolutionary biologist at the Northeastern Ohio Universities in Rootstown, USA.
“All this is rather amazing fifteen years ago, none of these transitional whales where known from skeletons, and creationists were making fun of scientists for claiming whales were derived from land mammals because there was not a single fossil showing that,” added Thewissen, who was not involved in the study.
Specimens this complete are a virtual “Rosetta Stone,” said Gingerich. “They provide insight into the functional capabilities and life history of extinct animals that cannot be gained any other way.”
The study in PLoS One
Whale Origins – The Thewissen Lab
Research on the Origin and Early Evolution of Whales – University of Michigan