Modern toothed whales (odontocetes) and baleen whales (mysticetes) can be traced backward in time to a group of ancient whales (archaeocetes) that lived during the Eocene from around 50 million years ago (mya) to around 39 mya. The group of terrestrial mammals thought to be the ancestors of whales are the mesonychians, which are first found in the fossil record around 63 mya. The origin of whales is marked by the first indication of a transition from terrestrial to aquatic life. Archaeocetes are known from many fossil localities in Eocene marginal marine and shallow marine rocks on six continents (North America, Europe, Asia, Africa, Australia-New Zealand, and Antarctica).
The time of origin of whales is best approximated by the geologic age of the first fossil whale. But, we might not have found the earliest fossil whale. With information on the distribution of archaeocetes in the rock record and a model sampling distribution of potential fossil whales, we can statistically compare different hypothesized times of origin of archaeocetes. The model sampling distribution is based on the area of sedimentary rocks in which we can look for archaeocetes through geological time and the number of archaeocete species during their diversification. Based on 5% likelihood of error, we estimate the time of origin of archaeocetes (and thus whales in general) to have been at or after about 51.6 mya, within the early Eocene. A similar statistical analysis was performed on mesonychians (the presumed ancestors of whales) found on the three northern continents. We estimate the time of origin of mesonychians to have been at or after about 66.7 mya (virtually at the Cretaceous-Paleocene boundary, the end of the age of dinosaurs). Artiodactyls (even-toed ungulates), the closest living relatives of whales, have a fossil record extending to the beginning of the Eocene, with a record of their ancestors extending into the latest Cretaceous. The fossil record as now known indicates evolutionary divergence of mesonychians and cetaceans from artiodactyls and their ancestors at the end of the age of dinosaurs or the very beginning of the age of mammals.
Artiodactyls - Members of the mammalian order Artiodactyla. These are the even-toed unglates (animals with hooves). Artiodactyls include cows, sheep, deer, camels, hippopotami, and many other animals.
Mesonychians - A group of primitive hooved mammals that are thought to be the closest relatives of, and possible ancestors to, cetaceans (whales and dolphins). Mesonychians had five hooved fingers on each hand (and toes on their feet). Mesonychians are thought to have been scavangers or carnivores.
Mysticetes - Members of the Mysticeti, one of the modern suborders of cetaceans. Mysticetes lack teeth and instead have a set of structures made out of baleen in their mouths. Mysticetes use the baleen to filter food from seawater.
Odontocetes - Members of the Odontoceti, one of the modern suborders of cetaceans. Odontocetes often have many similar peg-like teeth, or have only a few teeth in their mouths. Odontocetes capture individual prey items, such as fish or squid, and then swallow them whole.
Check out these web sites for more information on the following topics, or books on whale evolution.
More on whales and their history:
http://www.enchantedlearning.com/subjects/whales/classification/Whalefossils.shtmlThe earliest whale known, Pakicetus:
http://www.ucmp.berkeley.edu/mammal/cetacea/cetacean.htmlAlabama's state fossil, Basilosaurus:
http://alaweb.asc.edu/general/st_fosil.htmlC. Zimmer, At the Water's Edge: http://www.amazon.com/exec/obidos/ASIN/0821223100/qid=906498094/sr=1-2/002-9158165-2687000
H. Thewissen, The Emergence of Whales:
http://www.plenum.com/title.cgi?0306458535
Philip D. Gingerich, Department of Geological Sciences and
Museum of Paleontology, The University of Michigan, Ann Arbor,
Michigan 48109-1079, U.S.A.
Mark D. Uhen, Cranbrook Institute of Science, Bloomfield
Hills, Michigan 48303-0801, U.S.A.