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imagine a whale running to catch a prey, instead of swimming in the ocean to gulp one? The picture may be unimaginable today, but it was a reality about 50 million years ago.
Scientists have found that the primitive hoofed mammals were actually predecessors to the whale family. The evidence they found linking the two were partial fossil skeletons of two species of whales with well-developed limbs, fingers and toes.
It has been known for long that whales, dolphins and porpoises -- the cetaceans -- descended from land mammals with four limbs. However, confusion prevailed till now over which branch of the animal kingdom had they split from. The new fossils -- the first ones to have combined sheep-like ankle bones and archaic whale skull bones -- could help resolve the debate over the evolutionary link.
The fossils were discovered last year in the Baluchistan province of Pakistan by a team comprising Philip D Gingerich, paleontologist at the University of Michigan, usa , Iyad Zalmout of the Michigan University, Hans Thewissen of the Northeastern Ohio Universities College of Medicine, Rootstown, usa , and researchers from the Geological Survey of Pakistan and the University of New Hampshire, uk. They have published their work in Nature (September 20, 2001).
The fossils take us a step forward in understanding the origins and evolutionary relationships of whales. The limbs of early cetaceans found till date reflected that the primitive whales were amphibious. But the findings indicate that they were fully terrestrial and in fact, good at running.
Two hypothesis have been propounded by academicians about the evolution of whales. Some researchers used morphological analysis (the study of an animal's structure and form) to suggest that whales had descended from mesonychians, an extinct group of meat-eating animals that resembled hyenas with hooves. The other group of researchers used dna molecular and genetic techniques and suggested that whales and hippopotamus are more closely related to one another than either of them is to any other species. The discovery of the fossils lends credence to the second theory: whales are related closely to hippopotamus and have descended from the group of animals known as artiodactyls, whose other members are the sheep, cow, pig, camel and deer.
Scientists believe that the artiodactyls developed a taste for fish, learned to swim and eventually took to the water altogether. Over the course of time, their descendants lost their limbs and became fully adapted to a marine environment. The newly discovered fossils' ankle bones have specialised features that place them in the artiodactyls group. Certain ankle bones show specialised features typically associated with adaptation to running. Such features are unique to artiodactyls, living and extinct.
"The presence of artiodactyl-like ankles in the primitive whales strongly suggests common heritage rather than convergent evolution," said Kenneth D Rose of the Programme for Functional Anatomy and Evolution at Johns Hopkins University School of Medicine in Baltimore, Maryland, usa . Convergent evolution is the process by which different groups of organisms may evolve similar characteristics in response to particular environmental requirements. "While ankles from primitive ancient whales have been discovered before, these are the first that are well-preserved enough to provide clues about whale ancestry," added Rose. In addition to helping resolve the confusion about whale evolution, the recent discoveries should also lend more credibility to molecular, genetic, and immunological approaches to understanding evolutionary relationships, Gingerich said.
The discovery is one of the most important events in the past century of vertebrate palaeontology. Only a very few fossils, such as these, reveal a link between two groups of vertebrates that are vastly different in terms of shape yet closely related in terms of evolution. When there is a drastic shift in habitat -- such as from land to water -- the morphology of the newly adapted animals is generally so greatly modified, because of the high selective pressure, that any resemblance to the original ancestor is quickly obliterated. But the new fossils superbly document the link between modern whales and their land-based forebears, and should take their place among other famous 'intermediates', such as the most primitive bird, Archaeopteryx , and the early hominid Australopithecus.