Selection for the Plexus

Insulation and vascular heat-retention mechanisms allow penguins to forage for a prolonged time in water that is much cooler than core body temperature.  One of the key structures that penguins evolved long ago is the humeral plexus, a system of arteries and veins that form a counter current heat exchanger.  My colleagues Dr. Daniel Thomas and Dr. Ewan Fordyce have been collecting data on this fascinating structure from different penguin species and recently published the results of a comprehensive study. They dissected the flippers of two genera of penguins that had not been examined for plexus morphology before (learn more about the 6 genera here).  Don’t worry, no penguins were harmed – all specimens were birds that died in the wild and were salvaged from beaches.  One of the interesting results is that the plexus is not identical in every species – larger species tend to have more arteries in the plexus, even though they have the same number of basic arteries in other parts of the flipper. Little Blue Penguins have only 2 plexus arteries, while Emperor Penguins have 15 plexus arteries.

Relationship of the number of plexus arteries to body size and to environment. From Thomas and Fordyce (2012).

So why do larger penguins have more arteries in the plexus?  Well, this is a tricky question to answer.  One the one hand, larger penguins will have larger wing surface areas. Like elephant ears, penguin flippers have a high surface area to volume ratio, and so easily shed heat. Thus, larger penguins might need to increase the number of humeral arteries to compensate for heat loss.  On the other hand, penguins that live in colder areas would benefit more from additional humeral arteries because heat will be lost more rapidly in colder water.  The tricky part is that the largest penguins also tend to live in the coldest environments.  Thus, you can fit a nice curve to data showing a relationship between size and artery number, and can fit a similarly nice curve to data showing a relationship between environmental temperature and artery number.  The authors of the study tested the strength of these relationships and found that humeral arteries has a stronger correlation with wing surface area than with sea water temperature.  However, both probably play some roll – in the end its all about keeping warm whether its preventing heat shedding though a big flipper or fighting sub-zero temperatures.

 

Reference:

Thomas, D.B. and R.E. Forydce. In Press 2012. Biological Plasticity in Penguin Heat-Retention Structures. Anatomical Record.

Happy New Years from March of the Fossil Penguins

Thanks for reading and joining the discussion, and best wishes for 2012!  It will be a big year, and at least two new species of penguin will join the march. Stay tuned!

Heading Home

Today, the excursion comes to an end.  After pursuing penguins in the quarry, in the museum, and in the oceans (we saw some Little Blues on one return drive), Dr. Brinkman and I will return to North Carolina.  However, the work has only just begun.  Over the next few months, Dr. Fordyce and I will continue studying the fossils and seeing just how much they can tell us about how ancient penguins lived. Eventually the results will be announced – and you can read about it right here.  Thanks to everyone who followed along and posted comments.  I’ll pick up the thread again after the holidays – some very exciting New Zealand penguin news is on the horizon.

A Penguin Line Up

One of our last stops on this research trip is the Otago Museum.  This museum, located in Dunedin, has an important collection of penguin fossils, including many of the specimens that Dr. Brian Marples collected in the 1940s and 1950s.  One of the exhibits features a line up of penguin silhouettes indicating the size of different species.

Penguin line-up, from right to left: Korora, Platydyptes, Archaeospheniscus, Palaeeudyptes, and Pachydyptes. For scale, that’s me next to Pachydyptes.

This motley line-up helps bring to life the idea of an ancient penguin fauna, where these birds made up a bigger part of the ecosystem than today. In late Paleogene and early Neogene marine communities in New Zealand, there were a great variety of penguin species spanning large size ranges.  In these ancient times, the largest giant penguins may have played the role of sea lions, big penguins that of seals, and small penguins that of, well, penguins. Certainly there was room for different prey specialization between titans like Pachydyptes and small fellows like Korora.

Success!

Back in the lab, we have started preparing the penguin specimen collected from the Kokoamu Greensand.  As each grain of matrix is removed, more of the animal springs to life.  In the field, we noted that there were several wing bones exposed in the cliff face.  Preparation proved two of these bones to be the humerus (the main bone of the flipper) and the coracoid (part of the shoulder girdle).  But here was also a surprise.  One of the small pieces of bone we exposed in the field turned out to be part of the beak (upper right).  After preparing the area more extensively, we are now certain we have a spear-billed penguin.

It will take many more hours to reveal all the bones in the block.  Let’s hope there are more surprises in the block!

Some More from the Field

Here are a few more images from our prospecting and collecting forays into the Oligocene and Miocene. It has been a busy week and we will be wrapping things up soon.

A great exposure of Oteikake Limestone. In the background the Southern Alps beckon. Last time I visited, it was the southern winter and the mountains were cloaked in snow. Changing seasons, especially a good rain, can help expose new fossils by eroding any the rock.

 

Dr. Ewan Fordyce points out a fossil discovery at the base of an overhang.

 

One of those quiet, pleasant moments in life: a caterpillar has made its cocoon in a tiny cutting where a fossil shell was removed earlier.

Walking on Seashells (and Penguins)

The latest stop on our search for penguins takes us to the Otekaike Limestone.  This unit spans the Oligocene-Miocene boundary, with fossils dating to about 25 million years ago.  Shells abound in the Otekaike Limestone – it is almost like walking along a never-ending high tide line after a storm.

Walking on shells – it is literally impossible not to step on them!

Today’s trip yielded a few bits of penguin bone, including the base of a flipper and, more importantly from a scientific perspective, a whale skull bone.  Each piece brings us closer to understanding what these extinct species were like.  Even though the Otekaike Limestone is only a few million years younger than the Kokoamu Greensand, the penguins are much smaller and more modern looking.  Comparing fossils from the two Formations is like viewing two frames of a film on penguin evolution, one taken a few moments after the other.  Lots of other frames are missing, but the movie is still showing today in New Zealand, Antarctica, and everywhere else living penguins thrive.

Sample of the shelly fauna. A: Zealcolpus, a turret-shell gastropod, B: another gastropod, probably Alcithoe, C: Lentipecten, a scallop, D: Cucullaea, an ark shell bivalve, E: Waipairia, a brachiopod.

Penguin in Hand

Here is the end of the excavation.  The cutting in the rock is much larger than the fossil, but we need to keep some leeway in the form of rock around the bone, so none are damaged by removal or transportation.  The smaller opening is from another find, some fossil whale bones. Now, packed in burlap and cushioned by foam, the fossils will make the long journey back to the lab.

 

Collecting penguins in the Kokoamu Greensand

The Kokoamu Greensand is the final resting place of many archaic penguins and dolphins.  Formed about 26 million years ago when much of present day New Zealand was covered by the Pacific Ocean, these deposits are exposed along modern day river valleys.  Coarse grains of greenish to orange sand record the layering of sands and the burrowings of invertebrates over millions years.  Some of the most common fossils are pectins (scallops) and brachiopods.  The later are sometimes known as lampshells because of their vague resemblance to Roman oil lamps, and are very rare today.

 

An exposure of Kokoamu Greensand, marine rocks now stranded far inland in a grassy hillside.

Our main target on this trip is a fossil penguin that Dr. Fordyce found exposed along a cliff face. With the bones visible in cross-section a meter up, the only way to get the fossils back to the lab is to cut them out of the rock.  Using a chainsaw, air scribes (aided by a gas powered compressor), picks and crowbars, we will slowly extract the fossil intact within the rock. In the montage below, you can see the start of the excavation.

On the Hunt for Fossil Penguins

During this trip, we will search for some South Island penguins in a few of the richest fossil sites worldwide.  It takes a lot of tools and supplies.  Here, in early morning chill, the equipment is laid out. Wish us luck!