Pakicetus

Pakicetus was a prehistoric mammal species that lived about fifty-three million years ago. Its name comes from the country of Pakistan, where the first fossilized remains of this animal were found. Although the Pakicetus was a four-footed land animal that most likely had at least a light coat of hair, scientists consider it an early ancestor to the whale.

Background

Whales are large mammals that can be found in all of the world's oceans. They are from a class of animals known as cetacean. Their body shape, fins, and tails make them resemble fish outwardly. However, because they give birth to live infants that are fed milk from the mother's body, have lungs and breathe air, and have some thin body hair before they are born, whales are classified as mammals.

Scientists have determined that whales and several other species of sea animals are evolved from animals that once lived on land. These animals developed characteristics that allowed them to function in water, such as a leg design that allowed them to propel themselves through water easily. Eventually, these animals changed from living on land to living in the sea.

Paleontologists were able to determine the evolutionary path by examining the bones of the ancient animals and comparing them to living species of whales. Two key areas that matched were the skull near the ear and the teeth. Contemporary whales have teeth or use a baleen, or plate that helps the animal filter small marine life out of the water to serve as food. In the late twentieth century, researchers discovered that bones on some animals found in Pakistan were very much like the teeth in contemporary whales. They also discovered that the skull structure near the ear was very similar. The oldest of these fossilized skeletons belonged to a species of animal scientists named Pakicetus.

Overview

The first remains of a Pakicetus were uncovered in 1978 along a shallow body of water known as the Tethys Sea by paleontologist Phil Gingerich. The species was named and unveiled in 1981. The first bone found was a skull, along with some fragments of a jaw. Together, they were just enough for Gingerich and his colleagues to determine that the bones were from an animal that lived on land but had key features that identified it as an early whale.

One of these features was the shape of the skull, which more closely resembled the long skull of a whale. Another feature was the structure of the skull near the animal's ears. Whales are among several types of mammals that have an auditory bulla. This bony dome inside the skull encases the ear bones. However, whales have a distinctive auditory bulla that exhibits a condition called pachyosteosclerosis, in which the bone in the middle of the dome is very thick and solid. In all of history, the only animals found to have this were whales, but the newly discovered skull also exhibited this feature. The animal also had teeth that were similar to those of contemporary whales, and the scientists identified it as an ancient whale ancestor.

The Pakicetus was about the size and overall shape of a large dog or small wolf. It had four legs and a tail that was thicker by the body and tapered to a point. It is believed to have had a light coating of fur. The animal's eyes were higher on the head and close together, giving it the ability to be submerged in water and still see above the water. The teeth of the Pakicetus were large and sharp, indicating it was a carnivore, or meat eater. However, the teeth show signs of wear that indicate it ate fish.

Paleontologists believe that Pakicetus may have lived in and around the water, which was plentiful during the rainy season but dried to shallow ponds and lakes other times of the year. Although the animal resembled a dog or wolf, it is believed it did not move quickly and hunted by ambushing its prey. It is thought to have spent more time in the water; however, this was freshwater, unlike the salt water where contemporary whales live.

Over time, paleontologists discovered more skeletal remains of these animals. Eventually, it was determined that there were multiple types of Pakicetus, based on the large number of different types of teeth found in the same region. The original fossils were dubbed Pakicetus inachus. Teeth that were found that were originally thought to belong to a different type of whale were later renamed Pakicetus attocki.Other teeth were determined to belong to two additional types of animals: Pakicetus chittas and Pakicetus calcis. Scientists also determined that several different types of Pakicetus existed at the same time, based on the age of the rock in which the remains were found.

Eventually, more fossilized remains of the Pakicetus were found. From these, paleontologists determined that the ancient animal had an anklebone that resembled that of a class of animals known as artiodactyls. Artiodactyls are ungulates, or hooved animals, with an even number of toes. Ungulates are usually regarded as animals such as pigs, sheep, deer, cows, giraffes, antelopes, and hippos. Whales fall into this classification because their ancestors were ungulates.

Pakicetus is not the only type of ancient whale that started on land. Although Pakicetus was the first species, others, including the Indohyus and Ambulocetus, also began as land animals and evolved into other species that lived full-time in the water. This type of reverse adaptation of land mammals to live in water is rare but has occurred in other species. About one hundred different animals from three classifications are mammals that lived full- or part-time in water. They include the cetaceans, such as whales and dolphins; carnivora animals such as sea lions, seals, and walruses; and animals such as manatees from the Sirenia order.

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