Paleontologists use fossils to map the tree of life, trying to discover which creatures branched off from which and when. One of the most exciting mysteries has been the genesis of Placentalia, the division of mammals whose young grow in the womb, nourished with the mother’s blood by a placenta. This family includes 5,400 living species; almost all of the mammals that we are familiar with: mice, tigers, dogs, elephants, whales, bats and people. There are only two exceptions. The first are the Monotremes, which are mammals but lay eggs, such as the platypus and the echidna. The second are the Marsupials, which use a pouch instead of a placenta to nourish their young; such as the kangaroo, opossum, and koala.

In recent years a debate has grown among paleontologists and molecular gene researchers as to when the first placental mammal lived. Genetic clues put the first placental mammal on the earth relatively early; in the Mid-Cretaceous before the dinosaurs went extinct. Paleontologists who rely on fossils however, believe the first placental mammal lived much later, right around or after the Cretaceous extinction event, about 60 million years ago.

A report in the journal Morphobank, allows researchers to enter more than 4,500 different characteristics for a known mammal species, living or fossil; such as bone structure, teeth, internal organs, muscles, fur patterns, and particularly traits associated with animals who give live birth. The program takes both fossil and DNA information into account and matches it up with existing information in the database to determine a species’ identification and/or ancestry.

It took six years to sort through all the data, but with the help of MorphoBank, scientists have reconstructed what the ancestral placental mammal must have looked like. Protungulatum donnae was a small animal, the size of a small rat or a large mouse. It had a long, furry tail, and a fleshy nose. The fur on its belly was probably light colored and it lived by hunting insects.

Scientists are pleased with the MorphoBank discovery of Protungulatum donnae. The data-crunching capability of programs like MorphoBank will greatly influence the way paleontologists organize and draw up phylogenetic trees in the future. However, they are far from resolving the old disputes. MorphoBank estimates that Protungulatum Donnae must have come into existence 200,000 to 400,000 years after the end of the Cretaceous period. Scientists who work with molecular genetics aren’t fully convinced that amount of genetic change could have happened in such short amount of time. Dr. Anne D. Yoder, author of “Fossils vs. Clocks” and evolutionary biologist at Duke University is on the DNA side of the debate. She says paleontology researchers “devoted most of their analytical energy to scoring characteristics and estimating the shape of the tree rather than the length of its branches.” It seems the feud will rage on.


The Paleontological Research Institution, Ithaca, New York, is pleased to sponsor Paleontology content for This View of Life. Founded in 1932, PRI has outstanding programs in research, collections, and publications, and is a national leader in development of informal Earth science education resources for educators and the general public.


Published On: February 26, 2013

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