Crown Penguins: Younger than Ever
Penguins as a total group are old. Waimanu manneringi is approximately 61 million years in age, placing the origin of flightless diving penguins close to the Cretaceous-Paleogene (often called the K-T) boundary. However, “modern” or crown clade penguins have a much more shallow fossil record. In fact, the oldest fossil penguins that fall within the modern radiation (or, to put it scientifically, share the most recent common ancestor of the 18 living species) are only about 10 million years in age. However, because we do not have fossil for every modern penguin lineage, it has been unclear how long ago modern penguins appeared. It is possible they have been around for much longer than 10 million years, but we have missed the evidence because rocks of the appropriate type and age are either inaccessible or non-existent from many key regions like Antarctica.
A new study attempts to get around this limitation by combining fossil ages and DNA sequences. A team led by Dr. Sankar Subramanian conducted a “molecular clock”, or divergence dating analysis. This essentially means that they set the age of certain branching events to a range of times based on the age of fossils from well-represented types of penguins, like the Spheniscus lineage which has lots of cool fossils. With these fossil ages as calibration points, the ages for branching events in the penguin tree where fossil representation is poor can be estimated based on the amount of DNA divergence observed in the living species. Using this method, the age estimate for the origin of modern penguins is roughly 20 million years ago. This is cool, because it reinforces a paleobiological pattern that has been getting stronger and stronger as more fossils are recovered: modern penguins replaced archaic species in the not-so-distant past.
One of the strengths of this study is that the team took advantage of the fossil record of penguins to help date the tree. Besides the dates, an interesting finding is that tree groups together the two most ice-loving genera, Aptenodytes (King and Emperor Penguins) with Pygoscelis (stiff-tailed penguins). This result has actually been supported by morphological data in the past, but has not been supported by previous molecular datasets.