The mass extinction 66 million years ago at the end of the Cretaceous Period marked the end of the Mesozoic Era, the Age of Dinosaurs, and the beginning of the Cenozoic Era, the Age of Mammals. More correctly, the Cenozoic is the Age of Placental Mammals as more than 6,000 of the 6,500 living species are placentals (including humans), the others being marsupials and egg-laying monotremes. The first ten million years of the Cenozoic are the Paleocene Epoch, a time when placentals were entering new ecological niches vacated by the recently extinct dinosaurs, and many placental lineages were increasing dramatically in body size. The continents were also changing, North America was still connected to Europe, and global temperatures increased 5 to 8°C to their high in the late Paleocene. Most Paleocene placentals are considered to be archaic mammals, that is, not belonging to any modern placental lineages, which appeared later during the Eocene Epoch (56 to 34 million years ago). The goal of this multi-institutional collaborative grant was to document and explain evolutionary changes to placentals during the Paleocene that set the stage for their later dominance of terrestrial and aquatic ecosystems.

Our international scientific team studied Paleocene fossils from North and South America, Europe, Asia, and Africa, making comparisons with older and younger fossils and living mammals. A key innovation of our project was incorporating new techniques into the study of Paleocene placentals. We used computer tomography (CT) scan data, which allowed us to see details on the insides of skulls and other bones of fossil and living mammals, through which we evaluated anatomical systems previously not accessible to scientists. We also used paleohistology, scanning electron microscopy, and geochemical analyses on fossils in order to reconstruct life history data, such as diets, age, and growth patterns.

A major result from our CT studies concerns the evolution of the mammalian brain. Because the inner surface of the braincase mirrors the outer contour of the brain, we were able to reconstruct the brains of Paleocene placentals and other fossils and make comparisons to living mammals. We showed that contrary to previous research stating that mammal brains have increased over time, Paleocene placentals initially showed a reduction of their relative brain sizes because of a higher rate in body mass increase. Later, during the Eocene, several modern placental lineages independently acquired relatively large brains through the growth of specific sensory regions, the neocortex, and the cerebellum, associated with senses such as vision. Paleocene placentals initially increased in body size as the survivors from the end-Cretaceous extinction filled vacant niches, but then the brain became larger as the different ecosystems started to saturate and competition intensified among archaic and modern placentals in the Eocene.

A major result from our paleohistological and geochemical analyses was the reconstruction of the life history of a ~62 million-year-old Paleocene placental from New Mexico, Pantolambda. This study utilized histological analysis of bones and teeth combined with high-resolution analysis of dental trace element concentrations of zinc and barium that are correlated with birth and suckling in living mammals to reconstruct the life history of Pantolambda, on a daily basis. This study extended the viability of trace element analysis in fossil teeth by an order of magnitude and showed that Pantolambda had a placental-like reproduction with a long gestation period (7 months) followed by a short suckling period (31-75 days). This is the oldest evidence for a placental-style reproduction in any mammal, and we hypothesized that this pattern of reproduction enabled some Paleocene placental mammals to reach large body sizes quickly after the end-Cretaceous extinction.

A major effort for this grant was the construction of the largest database to date of anatomical information about Paleocene placentals in comparison with older and younger fossils and living mammals, sampling more than 2,500 morphological features. This required tackling many questions of homology in order to make meaningful comparisons with mammals as diverse as elephants, whales, armadillos, and bats. We intend to publish this database to make it accessible to the scientific community. Ongoing studies by our scientific team will incorporate DNA and other molecular data from living mammals with this anatomical database into producing a family tree for Paleocene placentals that will address whether or not they are truly archaic mammals or closer relatives of modern lineages. The resulting family tree will provide a better understanding of what happened and why in the transition from the end-Cretaceous mass extinction 66 million years ago to the origin of modern placental lineages 10 million years later.

Last Modified: 01/14/2024
Modified by: Thomas E Williamson