February 4, 2009 — Two newly described fossil
whales—a pregnant female and a male of the same species–reveal how
primitive whales gave birth and provide new insights into how whales
made the transition from land to sea.
The 47.5 million-year-old fossils, discovered in Pakistan in 2000
and 2004 and studied at the University of Michigan, are described in a
paper in the online journal PLoS One.
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 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."
In fact, it is the first discovery of a fetal skeleton of an extinct
whale in the group known as Archaeoceti, and the find represents a new
species dubbed Maiacetus inuus. (Maiacetus means "mother whale," and
Inuus was a Roman fertility god.) The fetus is positioned for
head-first delivery, like land mammals but unlike modern whales,
indicating that these whales still gave birth on land.
Another clue to the whales’ lifestyle is the well-developed set of
teeth in the fetus, suggesting that Maiacetus newborns were equipped to
fend for themselves, rather than being helpless in early life.
The 8.5-foot-long male specimen, collected four years later from the
same fossil beds, shares characteristic anatomical features with the
female of the species, but its virtually complete skeleton is 12
percent larger overall, and its canine teeth or fangs 20 percent
larger. Such size discrepancies are not uncommon among whales and their
kin; in some species the females are larger, while in others the males
are slightly to considerably bigger. The size difference of male and
female Maiacetus is only moderate, hinting that the males didn’t
control territories or command harems of females.
The whales’ big teeth, well-suited for catching and eating fish,
suggest the animals made their livings in the sea, probably coming onto
land only to rest, mate and give birth, said Gingerich, who is the
Ermine Cowles Case Collegiate Professor of Paleontology and director of
the U-M Museum of Paleontology. Like other primitive archaeocetes,
Maiacetus had four legs modified for foot-powered swimming, and
although these whales could support their weight on their flipper-like
limbs, they probably couldn’t travel far on land.
"They clearly were tied to the shore," Gingerich said. "They were living at the land-sea interface and going back and forth."
Compared with previous fossil whale finds, Maiacetus occupies an
intermediate position on the evolutionary path that whales traversed as
they made the transition from full-time land dwellers to dedicated
denizens of the deep. As such, it offers invaluable, new information on
structural and behavioral changes that accompanied that transition.
"Specimens this complete are virtual ‘Rosetta stones’," Gingerich
said, "providing insight into functional capabilities and life history
of extinct animals that cannot be gained any other way."
Gingerich’s coauthors on the PLoS paper are Munir ul-Haq of the
Geological Survey of Pakistan; Wighart von Koenigswald of the
University of Bonn in Germany; chief vertebrate preparator William
Sanders of the U-M Museum of Paleontology; associate research scientist
B. Holly Smith of the U-M Museum of Anthropology; and U-M postdoctoral
scholar Iyad Zalmout.
The researchers received funding from the Geological Survey of
Pakistan, National Geographic Society, National Science Foundation, and
Alexander von Humboldt Foundation.
Source : University of Michigan