Tracing an Early Placental Mammal Ancestor

A small, furry-tailed, insect-eating creature was the earliest ancestor of placental mammals—a widely diverse group of animals ranging from bats to humans—according to a study by a team of international scientists, including a core group of Museum researchers.

In findings published on February 7, 2013, in the journal Science, the researchers analyzed the world’s largest dataset of mammalian genetic and physical traits to reconstruct our extinct placental mammal ancestor and to find that placental mammals diversified into present-day lineages after the extinction event 65 million years ago that eliminated non-avian dinosaurs, much later than suggested by some genetic studies. The placental mammal ancestor was a shrewlike animal with small eyes, small ears, and sharp teeth. It weighed less than 250 grams, or 9 ounces. 

 

"Combining our historical collections with 21st-century tools is the next wave in comparative and evolutionary biology."  

Michael Novacek Provost of Science

 


 

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Lead author Maureen O’Leary is a Museum research associate.


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Curator Nancy Simmons was also part of the research team.


For their analysis, researchers drew on two sources of data: phenomic data, which includes anatomical and behavioral observations gleaned from physical specimens, and genomic data from DNA analyses.

“Discovering the tree of life is like piecing together a crime scene—it is a story that happened in the past that you can’t repeat,” says lead author Maureen O’Leary, an associate professor in the Department of Anatomical Sciences in the School of Medicine at Stony Brook University and a Museum research associate. “Just like a crime scene, the new tools of DNA add important information, but so do other physical clues like a body or, in the scientific realm, fossils and anatomy. Combining all the evidence produces the most informed reconstruction of a past event.”

 

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Researchers used a powerful online database called MorphoBank.


For the phenomic piece of the puzzle, researchers harnessed a powerful cloud-based and publicly accessible online database called MorphoBank. Because phenomic datasets are built on physical objects like fossils that are limited in number and take time to excavate, prepare, and analyze, evolutionary trees based on anatomy usually don’t exceed several hundred traits. For this study, researchers recorded traits for 86 mammal species—mostly placentals, a subgroup of mammals that excludes egg-laying and marsupial species—including 40 species known only from fossils. The effort added more than 4,500 characteristics and more than 12,000 images, producing a dataset 10 times larger than any previously used to study mammal relationships.

“Phenomic data have a major role in the direct reconstruction of trees,” says Museum Provost for Science Michael Novacek, a curator of paleontology who was an author on the study. “Such data include features preserved in fossils where DNA recovery may be impossible. The mammalian record is notably enriched with well-preserved fossils, and we don’t want to build trees without using the direct evidence these fossils contribute.”

 

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Co-author Suzann Goldberg examines a specimen.


The tree of life produced in this study, funded by the National Science Foundation, shows that placental mammals arose 36 million years later than what has been projected using purely genomic data. Reconstructing the anatomy of the common ancestor required researchers to map traits onto the tree and compare features of placental mammals with those of their closest relatives to determine which characteristics appeared first and which were retained from more distant ancestors. The conclusions: the common ancestor had a two-horned uterus, a brain with a convoluted cortex, and a placenta in which maternal blood came in close contact with membranes surrounding the fetus, as in humans. 

In addition to Dr. O’Leary and Dr. Novacek, the research team included Museum Curators Nancy Simmons, John Flynn, and Jin Meng, and researchers Andrea Cirranello, Andres Giallombardo, Norberto Giannini, Suzann Goldberg, Paul Velazco, and Marcelo Weksler. 

The team also included colleagues from the Carnegie Museum of Natural History, Florida Museum of Natural History, Institute of Vertebrate Paleontology and Paleoanthropology in China, Minas Gerais Federal University in Brazil, Museu Nacional-UFRJ in Brazil, National University of Tucumán in Argentina, University of Chicago, University of Louisville, University of Tennessee at Chattanooga, University of Toronto Scarborough, Western University of Health Sciences, and Yale University’s Peabody Museum of Natural History.

Read more about this study here