Fossils of a 46-million-year-old whale that walked on four legs on land, but swam with the undulating, up-and-down tail motion of a modern whale, have been discovered in Pakistan by paleontologist Philip D. Gingerich and researchers from the Geological Survey of Pakistan.
The discovery was announced in the April 28 issue of Nature.
Named Rodhocetus kasrani, the whale provides important information about structural and behavioral changes that occurred 40 to 50 million years ago as whales made the transition from land-dwelling to ocean-dwelling mammals.
Rodhocetus is the first early whale found with a complete thoracic, lumbar and sacral vertebral column, said Gingerich, director of the Museum of Paleontology and professor of geological sciences. It retains primitive features seen in land mammals, but also exhibits derived characteristics found only in later ocean-dwelling species. It is an excellent candidate for a direct ancestor of modern whales.
With its pointed snout, sharp teeth, short legs and robust tail, Rodhocetus may have looked something like a 10-foot-long crocodile with fur. According to Gingerich, it is the oldest whale ever found with the flexible back and heavily muscled tail needed for efficient swimming. This shows that tail swimming similar to that of modern whales evolved early in cetacean history, Gingerich said.
Rodhocetus fossilized remains were found during a 1992 excavation in northern Pakistan. The excavation site is now a rocky, mountainous desert, but 50 million years ago, it was located beneath the southern edge of an immense, ancient ocean called the Tethys Sea.
In 1979, Gingerich and his colleagues unearthed fossils from the same general area of an older whale named Pakicetus inachus. Living about four million years before Rodhocetus, Pakicetus was a small land mammal that probably walked on four legs and ate fish caught in the shallow edges of the Tethys Sea, according to Gingerich.
In 1989 while working in Egypt, Gingerich found new specimens of Basilosaurus isisa different whale species that was first discovered in 1904. Basilosaurus lived six million years after Rodhocetus. Because its tiny hind limbs were too weak to lift or support its body on land, Gingerich believes Basilosaurus spent its entire lifetime in the ocean. Rodhocetus is intermediate, both in time and evolutionary development, to Pakicetus and Basilosaurus, according to Gingerich.
Whales are a classic example of fundamental changes being driven by evolution, Gingerich said. Over a 10-million-year period, whales gradually adapted in ways that allowed them to spend longer periods in the water and catch more fish. Since good swimmers were more likely to live and reproduce, physical changes that improved swimming ability were more likely to be passed on to later generations.
Gingerichs co-researchers on the Rodhocetus expedition included S. Mahmood Raza, director, and Muhammad Arif and Mohammad Anwar, assistant directorsall of the Paleontology and Stratigraphy Branch of the Geological Survey of Pakistanand U-M graduate student Xiaoyuan Zhou.
Field research funding was provided by the Museum of Paleontology, Office of the Vice-President for Research, and the Geological Survey of Pakistan.