From Land To Sea: How Could A Land Mammal Turn Into A Whale?

From Land To Sea: How could a Land Mammal turn into a Whale?

by Tamara Schupp

San Diego State University

04/03/06

H100/M4

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From Land to Sea: How could a Land Mammal turn into a Whale?

The evolution of whales has long been a mystery to researchers. A whale has so many derived characteristics of a land mammal, such as a large brain, it breathes air, gives birth to live young and is warm blooded, and yet it still manages to live in the sea. Evolutionists believe that the ancestors of whales were land mammals that during the process of evolution lost their legs and became adapted to an aquatic lifestyle. The controversy is that evolution teaches that the first animals crawled out of the sea onto land and not reverse. How could this transformation of whales from land mammals to marine mammals happen? What group of land mammals gave rise to whales? Thanks to a profusion of intermediate fossils that were discovered within the past few years, these questions can be answered and the transformation of whales has become more clear.

This paper will discuss the existing evidence, that whales descended from terrestrial mammals, from a palentological point of view and will explain the transformation from terrestrial mammals-- through more and more whale-like forms-- until the appearance of modern whales.

The idea that whales are descendents from land mammals goes back to Darwin’s suggested theory that whales arose from bears. His theory proposed that selective pressure might cause bears to evolve into whales. Soon embarrassed by criticism of his theory of swimming bears he removed the idea from his edition of the Origin.1One of the

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first fossils that proved the transition of terrestrial mammal to whales were found by paleontologist Phil Gingerich in 1983. He discovered a 52-milion-year-old skull in Pakistan that is similar to the fossils of Mesonychids, which are wolf-sized carnivores that lived in the early Eocene period. Interesting about this discovery is that the skull also shares features with the Archaeocetes, which is the oldest known whale. The discovered new bones were named Pakicetus and they seem to have all the important features that were transitional between land mammals and whales. This transition could be seen best , while comparing the skulls. While the shape of the Pakicetus skull resembles more that of a whale, the hearing was more terrestrial than marine, and the teeth were between the Mesonychids and the Archaeocetes.2 Gingerich reported, “In time and in its morphology, Pakicetus is perfectly intermediate, a missing link between earlier land mammals and later, full-fledged whales.”3

Almost ten years later Thewissen and colleagues found a truly astonishing fossil called Ambulocetus in the same area that Pakicetus was found, but in deposits 120 meters higher. Dated 50 million years ago between the early to middle Eocene period the Ambulocetus is also called “the walking whale that swims”. The Ambulocetus resembles more that of an whale but having functional legs and forelimbs that were equipped with fingers and hooves the skeleton still allowed some terrestrial walking. Even though not an efficient walker the Ambulocetus could get around the water and land successfully, probably similar to an sea lion today. Interesting is also the spine movement while swimming. Looking at the anatomy of the spinal column one can

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assume that the spine is swaying up and down while swimming, which resembles much that of an terrestrial animal. It seems that the Ambulocetus was not quite yet disconnected from its terrestrial ancestors but already adapting to an aquatic environment. 4

Still retaining some terrestrial features the Rodhocetus took the changes to an aquatic life even further. Dated back 46 to 47 million years ago and living in the middle of the Eocene period, Rodhocetus shows evidence of an increasingly marine lifestyle. Having developed a powerful tail and a more stable, less flexible neck, the Rodhocetus has adapted to swimming. Also the ear region is more specialized for underwater hearing. Nevertheless the pelvis was smaller than its ancestors but still connected to the sacral vertebrae giving evidence that Rodhocetus could still walk on land. Rodhocetus was probably a worse efficient