Thewissen discovered or worked on four missing links in the
evolution of cetaceans. In addition, he worked on the following.
Discovery of Ambulocetus Thewissen and Hussain discovered a partial skeleton of a new cetacean
Ambulocetus in 1992, working with and a team from the Geological Survey of Pakistan in the Kala Chitta Hills of
Punjab, Pakistan. When this new ancestral whale appeared in the magazine
Science in 1994,
Stephen Jay Gould dubbed it 'as the smoking gun of whale evolution.
Ambulocetus was recovered from Pakistan(, paleocoordinates ) in 1993 by Thewissen and Muhammed Arif, and was described by Thewissen, Hussain, and Mohammad Arif in 1994. "I sat on the porch of a Pakistani guesthouse, puzzling over the
sea lion-size skeleton that we had just dug up in the Kala Chitta Hills. I opened some of the packages containing fossil remains that I had wrapped earlier that day, and as I scraped with a dental tool, I realized that this was a whale—one that could walk around on the large hind legs that we had unearthed. It was the first such whale to be seen by a human, ever."
Discovery of Kutchicetus Sunil Bajpai and Thewissen collected fossils in District Kutch,
State of Gujarat, India, in the desert area close to the Pakistani border. Here, they found the skeleton of a small whale called
Kutchicetus minimus. The
holotype of
Kutchicetus consists of some skull fragments, many vertebrae, and ribs and the limb bones, although parts of fore- and hind feet were not found. A jaw fragment allowed several whale skulls and lower jaws to be from the same species. However, the Kala Chita Hills in Pakistan is a
bone bed where cetaceans and other animals were buried together, so anatomical association between different parts is lost, and West was unable to determine which skeleton bones were associated with the teeth. Thewissen excavated the site further, discovering hundreds of bones of different mammal species. Whale teeth were the most common teeth recovered, and there were no mammals of the same sizes. This allowed the researchers to identify bones of the
Pakicetus skeleton in a preliminary fashion. They later tested this identification by studying the stable isotopes of the bones, which matched the stable isotopes of the teeth and not those of the teeth of other mammals.
Research on Indohyus The Indian geologist A. Ranga Rao collected fossils in the 1960s and 1970s in Indian Kashmir, which he named
Indohyus. Upon his death, his widow, Dr. Friedlinde Obergfell, gave the rocks to Thewissen to study. During the extraction of the fossils, the fossil preparator accidentally broke one of the skulls. In the cracked specimen, Thewissen recognized the ear structure of the
auditory bulla which had a shape which is highly distinctive, found only in the skulls of living and extinct cetaceans, including
Pakicetus. This suggested that
Indohyus was related to cetaceans, and this was later confirmed by formal systematic analysis. Thewissen was able to extract many skeletal bones of
Indohyus, which showed that the species was similar in body shape to a modern mousedeer (also called
chevrotains). Thewissen postulates that the first steps whale ancestors took toward aquatic habitats may also have involved escaping predators. Thewissen's discovery of
Indohyus helped refine the connection between whales and hippos and suggested that
Indohyus was closely related to hippos too. Fred Spoor, an anthropologist at
University College London, said the significance of the latest find was comparable to
Archaeopteryx, the first fossils to show a clear transition between dinosaurs and birds. "For years, cetaceans were used by creationists to support their views because for a long time, the most primitive whales known had bodies that looked like modern whales, so there seemed to be this enormous gap in Evolution. But since the early 1990s, there's been a rapid succession of fossils from India and Pakistan that beautifully fill that gap," he said. '' '', with brain (green) and ear (yellow and red) showing through, as based on high-resolution CT scan. The large green canal on the right carries the olfactory nerve and indicates that these whales had a developed sense of smell.
Age estimation of modern whales Thewissen's lab was instrumental in estimating the age of Arctic whales. This data is crucial to gathering data on overall population rate of increase or decrease. Some cetaceans, such as
belugas, lay down layers in their teeth, analogous to tree rings. Thewissen's lab determined that there are several sets of finer repeated layers within the large-scale layers. One of these finer sets is linked to daily processes, and indeed, the thickness of 365 of these layers matches one large-scale layer, suggesting that the large-scale layers reflect annual intervals.
Bowhead whales do not have teeth, but their
baleen plates grow with age and can be used to estimate age in younger whales. It was already established that one of the bones of the ear, the
tympanic part of the temporal bone, grows annually by laying down a layer of bone. Thewissen's lab studied this for bowhead and determined that this bone may also be used for determining age in this species. Both dental aging and the temporal bone aging are effective methods for determining age in fossil whales. Thewissen also established that in some cases the presence of earwax in bowhead whales may be used to establish age. In some baleen whales, earwax grows in annual layers that are not expelled through the ear canal and this can be used in age estimation.
Brain evolution research Thewissen's current work involves counting neurons in
bowhead and
beluga whale brain samples, to assess brain function, in
Utqiaġvik—formerly Barrow—Alaska. Though the size of the brains of
sperm whales and
killer whales are larger than those of any other organism, including humans, a better measure of brain function is to determine how many neurons there are in the brain.
Suzana Herculano-Houzel has developed a method of counting of neurons of human and other animals' brains and the relation between the cerebral cortex area and thickness and number of cortical folds. Humans and other primates pack about twice the number of neurons in a cubed inch of brain as most other mammals. ==Appearances in science films and TV shows==