Vertebrate Paleontology: Mammals
Rates of evolution
Mammals are advantageous for microevolutionary studies because many have determinate growth and complex molar teeth that are distinctive in form and indicative of body size and diet. Paleocene and early Eocene mammals have an exceptional fossil record in the Bighorn Basin of Wyoming, where they can be collected in quantity, bed by bed, through millions of years of geological time starting some 60 Ma and ending about 50 Ma. Quantitative study enables comparison of rates of change on a range of time scales, showing that rates on the time scale that matters, from generation to generation, can be as fast as any rates found in laboratory experiments. This is remarkable information retrieved from evolutionary time series documented by fossils deep in the geological past.
PETM and other greenhouse warming events
The Bighorn Basin fossil record was the first to show the sudden appearance of mammalian artiodactyls, perissodactyls, and primates as evolutionary ‘innovants’ and immigrants during a light-carbon isotope excursion marking the beginning of Eocene time. This isotope excursion also marks what we now know as the Paleocene-Eocene Thermal Maximum or PETM, a major greenhouse warming event at 55.8 Ma recorded on continents and in oceans all over the world. Research with Dutch and German colleagues is ongoing, studying surface sections and well cores to understand the role planetary cycles play modulating Earth’s climate, perturbing environments, and motivating evolutionary change.
Two kinds of primates are evident when these first appear at the beginning of Eocene time. Most people agree that Teilhardina and close relatives are tarsioid and related to living Tarsius. The second kind, represented by Cantius and close relatives, is more controversial. The Cantius group has long been linked to lemurs because they retain many primitive characteristics seen in Lemur and relatives today. Advanced features of the Cantius group indicate a closer linkage to higher primates, Anthropoidea, including monkeys and apes. A century-old debate about the origin of anthropoids was rekindled recently by an exceptionally complete Eocene skeleton found in Germany and studied with Norwegian and German colleagues. The new find again combines primitive features of lemurs with derived features of monkeys, as should be expected in a transition form linking lemurs and tarsiers to higher primates. Surprisingly, the name Darwinius was still available for this fossil in 2009, two centuries after Darwin’s birth.
Origin and early evolution of whales
The origin and early evolution of whales is a remarkable example of macroevolution, documented skeleton by skeleton through freshwater and marine strata of middle and late Eocene age, first in Pakistan and more recently in Egypt. These countries have Eocene strata exposed at the surface in vast tracts of dry hills and deserts. The oldest archaic whale known from skeletal remains is Pakicetus from about 48 million years ago, but the best known is Maiacetus from 47 Ma highlighted by a mother whale with a near-term fetus preserved in utero. The mother is clearly female, and the complete skeleton of a male Maiacetus is known as well. Maiacetus is one of a group of early ‘protocetids’ that were found to have skeletons with distinctively artiodactyl-like ankle bones, linking whales to an artiodactyl origin. Middle Eocene ‘protocetids’ were semiaquatic foot-powered swimmers that lived much of their life in water but still came out on land to rest and give birth. By 37 Ma the archaic whale fauna was dominated by ‘basilosaurids’ like Dorudon that were fully aquatic, tail-powered swimmers. Basilosaurids are the connection to later toothed and baleen whales.
Maiacetus mother and calf
Use the color-coded guide to help you to understand what you're seeing -- which bones belong to the mother and which to the fetus.
Beige is the mother's skull.
Pink is the mother's body skeleton.
Light Blue shows the fetus skeleton and skull.
Brown highlights the teeth of the mother and fetus.