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.

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.

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

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.

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.