Monday, April 8, 2013

Out of Africa: The diversity and extinction of Africa's oldest penguins

Not many people realize how diverse penguins are. Usually associated with the frigid wastes of Antarctica, penguins are actually spread throughout all continents in the southern hemisphere, and range in habitat from pack ice to sandy beaches to fern forests. Out of all penguins, one sticks out from the rest, purely due to its range and habitat: the African penguin, Spheniscus demersus. This little penguin, about 2ft tall, nests among boulders on none other than the sunny, sandy shores of South Africa and Namibia.

Modern African penguins in their natural habitat.

It may seem odd to us that a penguin could find a suitable place to nest somewhere so warm, compared to the cooler climes of other species. However, penguins have been present in Africa for much longer than previously thought. New fossils from the Saldanha Steel formation near Cape Town are not only the oldest evidence of African penguins, they also show just how diverse the area was.

During the mid- to late Miocene, some 10-12Ma, sea levels were much higher than they are today: in the heyday of the African penguin, the sea was a full 90m higher than present levels. This not only provided much more swimming space, but it created a small archipelago. Penguins, being flightless birds and rather clumsy on land, prefer nesting in isolated areas such as islands; this eliminates the threat from large terrestrial predators. The ample nesting space could support a variety of species with no competition between them.

Modern sea level at Saldanha Steel (A) and during the early Pliocene (B), when the sea was 90m higher than today. The scattered islands in B would have been critical nesting grounds for penguins. From Roberts et. al, 2011.
Clearly, the higher sea levels were a blessing for the ancient penguins, for not one, not two, but four new species were uncovered from Saldanha Steel. The largest of these was as tall as the modern king penguin, the second-largest living species. The smallest was about as large as the smallest living species, the little blue penguin. The other two were comparable in size to the modern African penguin. The species were already very "modern" in appearance; penguins are among the oldest families of birds, and quickly evolved into recognizable species.

A brief history of African penguins. The four newly-discovered species are silhouetted in gray. From Thomas & Ksepka, 2013.

I imagine the scene at Saldanha Steel would have been very similar to modern nesting colonies of penguins, which would have been a site to behold, especially when you realize that scenes like this one would have taken place in Africa.

A colony of king penguins on the island of South Georgia. Perhaps Saldanha Steel would have boasted a much similar scene, with even more diversity, some 10Ma. Photograph by Arthur Morris.

As sea levels receded to their modern levels, the islands on which the penguins nested connected with the mainland, allowing predators to invade. Could predation have led to the extinction of these penguins? Or was it the disappearance of crucial nesting space? It's likely that the penguins were more and more pressed to find adequate nesting grounds, and the curtain was drawn with the arrival of large terrestrial predators. No matter what the cause of their extinction, it is fascinating to picture the diversity of penguins which once roamed South Africa, surely the last place we would readily associated with those clumsy, comical birds. 

1 comment:

  1. It's always great to see a post about fossil penguins! In my opinion, you never see enough of them, and I always make sure to write about them whenever I can! I'm really enjoying the blog, keep up the good work!