March of the Fossil Penguins

A few posts back, we talked about the penguin rete mirabile, or arterial plexus.  This network of blood vessels helps living penguins keep their core body temperature warm by shutting down heat flow to the tip of the flipper when they are in foraging in cold water.  A new paper published today in Biology Letters tracks the origin of this feature deep into the fossil record.  How is this possible?  After all, blood vessels, don’t usually fossilize. As a co-author of the study, I can explain how we drew our conclusions.

This research project started all the way back in 2006, when lead author Dr. Daniel Thomas and I were both still graduate students.  Daniel was studying the evolution of penguin counter current heat exchange systems through dissections and isotope studies, while I was focusing on the evolution of the penguin skeleton and trying to understand what each bump, groove, and ridge on the flipper bones actually meant.  In many cases these structures mark the places where muscles attach, but there were a few “mystery grooves”.  As Daniel and I shared penguin dissection data, we realized one deep groove across the humerus, the main bone of the flipper, was particularly important.  This groove sat right below the vessels of the arterial plexus.  In fact, the groove was formed by the vessels being pressed against the bone.  While this might seem like a mere “fun fact”, it had big implications for us – this bony mark (or osteological correlate in paleontology jargon) provided a way to determine whether fossil penguins might have had the an arterial plexus as well.  Over the next few years (in between finishing graduate school many other projects) we surveyed nearly every penguin humerus in museums worldwide.  Together with Dr. R. Ewan Fordyce, a cetacean expert who has also discovered many important penguin fossils, we used our data to map where the plexus first appeared on the penguin evolutionary tree.

The bony groove on the penguin humuerus (sulcus) houses the arterial plexus. This groove lets us infer whether the plexus was present in fossil penguins, even though the vessels themselves don't fossilize. Image from Thomas et al. 2010.

Now, if we only knew about today’s penguins, we might naturally assume the plexus arose in association with penguins moving into cold climates.  Fascinatingly, the fossil record tells us the opposite.  As we surveyed fossil penguin wing bones to check whether they preserved the groove for the blood vessels, we found that the oldest penguins lacked it.  These species lived in New Zealand about 60 million years ago.  The first penguins that show signs of the sulcus start popping up all over the Southern Hemisphere at about 45 million years ago.  The twist is that they appear during one of the hottest times in Earth history, during an interval when there were no permanent polar ice sheets.

Of course, the plexus is also very useful for penguins in Antarctic environments.  In fact, they might not even be able to survive without it due to the intense physiological demands of swimming in  nearly freezing water.   However, the fossil record shows that the plexus did not evolve in response to global cooling, but instead probably  appeared to help penguins spend longer periods feeding at sea, where heat leaves the body faster than in air.  Having inherited a feature evolved by warm-weather ancestors tens of millions of years in the past, modern penguins were more than ready to invade sea ice shelves over their more recent history.  This is an example of exaptation – a feature that evolved for one purpose and then came to serve another.  In this case, the plexus evolved to allow longer periods of foraging and was later perfect for preserving warmth during long marches across ice sheets and weeks of huddling in gale force winds at nest time.

Reference:

Thomas, D.B., D.T. Ksepka and R.E. Fordyce. 2010. Penguin heat-retention structures evolved in a greenhouse Earth. Biology Letters (published online before print December 22, 2010, doi:10.1098/rsbl.2010.0993)