Archive for May 2010
South Africa, host of the next World Cup, is one of the best places on Earth to see penguins in the wild. Only one species of penguin lives there today (the Jackass Penguin Spheniscus demersus), but it is possible to visit quite up close with these charming birds. The Jackass Penguin earns its name for its abrasive braying call rather than any foolhardy behavior. Historically, this species has place of pride as the first penguin to be encountered by European explorers. This momentous occasion occurred as Vasco da Gama’s voyage around the Cape of Good Hope landed for supplies near present day Mossel Bay in 1497 (of course, native peoples in South America, Africa, New Zealand and Australia were certainly well aware of penguins for hundreds if not thousands of years prior to this encounter). It is not recorded how the penguins greeted their strange visitors, but the humans behaved as customary when encountering wondrous new life forms by plundering a good number of them to eat. Relations have since improved between our species and theirs, and Jackass Penguins are a fixture at many aquariums and zoos worldwide.
While Spheniscus demersus is the only species surviving on the coast of Africa today, several fossil species have been described. Interestingly enough, all South African penguins are from the Miocene-Pleistocene Epochs – substantially younger than the oldest fossils from other continents. Whether penguins arrived to Africa late, or we simply have not searched diligently enough for them in older rocks, remains to be seen. One interesting and fortuitous fossil discovery from South Africa was the unearthing of Nucleornis insolitus during excavations for the Koeberg nuclear power station. Geroge Gaylord Simpson, featured in the last post, named the species in 1979. The etymology of the genus name should be obvious. Unfortunately only two foot bones – tarsometatarsi in avian anatomical lingo – were found. Because the purpose of the excavation was to sink a foundation for the power plant, not search for penguin fossils, little effort was devoted to deciphering the age of the rocks in which the penguin bones were found. They are thought to be Miocene in age – roughly between 5 and 23 million years old, and probably closer to the lower end of that range. Very little is known about Nucleornis insolitus because we have so little of the skeleton. Based on the foot bones, the species was comparable to the living Jackass Penguin in size. In all of the preserved morphological details, the tarsometatarsus resembles that of living penguins. While it is not possible to definitively say whether Nucleornis shared the most recent common ancestor as the living penguin species, it was at least very closely related. These scraps of fossil represent one piece of the puzzle of how penguins arrived and radiated in Africa, but much more work remains to be done before we will understand the whole story.
Simpson, G.G. 1979. A new genus of Late Tertiary penguin from Langebaanweg, South Africa. Annals of the South African Museum 78: 1–9.
In 1933 the famous paleontologist George Gaylord Simpson led an expedition to collect fossils around the town of Trelew in Patagonia. At this time, Simpson was still a young man. Later he would become one of the “four horseman” of the New Synthesis of evolutionary theory, bringing the deep time perspective of paleontology into a new perspective on evolution unifying natural selection and genetics. In 1933, though, he was focused just on the excitement of collecting fossils. Near Trelew, the Chubut River meets the Atlantic today. It seems that this area also comprised a rich estuarine ecosystem in the past and both terrestrial and aquatic animals gathered, lived, and died here, quite often making it into the fossil record. During the trip, the team collected many mammal fossils but also repeatedly came upon penguin bones. These were not the focus of the trip, but no good paleontologist would leave well-preserved fossils in the field regardless of what type of animal they belong too. More than a hundred scattered bones of average sized penguins were gathered up, but one find in particular changed the face of penguin paleontology. This specimen was a roughly 20-25 million year old, nearly complete skeleton of a single bird – a rather large one by modern standards, approaching King Penguin size. Most of the leg, part of the flipper, many vertebrae were intact. Most importantly, the skull was there too – the first time a skull had ever been found for a fossil penguin.
At the conclusion of the successful field season, the team returned to the US with a bounty of fossils to prepare and study. Simpson was, as mentioned, a mammal paleontologist, more interested in marsupials and such than in birds. Thus, he attempted to pass the fossil penguin skeleton to one of the American Museum of Natural History’s many ornithologists. None, however, took him up on the offer. At the time, ornithologist’s were absorbed in details of the feathers and beaks of birds and had little interest in the bones of a penguin. Collections of stuffed skins were emphasized over osteological collections at the time (and still are in many museums) and so most ornithologists probably had scant appreciation for skeletal remains of any kind of bird.
Around this time, World War II interrupted George Simpson’s pleasant work on fossils and he served several years in the Army as a staff officer to Patton. By his own account, this was a low in his career and he longed to get back to scientific pursuits. With the conclusion of the war, he happily returned to the American Museum of Natural History. Simpson found the penguin still unstudied, and tired of the poor bird languishing set about studying it himself. This resulted in a monumental 1946 paper titled simply “Fossil Penguins”. The skeleton was identified as belonging to the species Paraptenodytes antarcticus, previously known only from a few bones. Besides describing the skeleton, Simpson’s monograph revised the dozens of fossil species that had been named by this time (discarding many ill-founded ones) and definitively traced the ancestry of penguins to a flighted bird, laying to rest some bizarre theories about flightless terrestrial birds or even reptiles as penguin ancestors.
Simpson’s Paraptenodytes specimen was the key that opened the door to the modern study of penguin evolution. Up to this time, almost all the penguin fossils that had been found were isolated bones. This made it difficult to reconstruct what these extinct species might have looked like and how similar or different their lifestyles were from living penguins. Paraptenodytes antarcticus gave us the first good look at an extinct penguin species. The species certainly had a strong bite compared to living penguins, based on the insertions areas on the skull for the muscles that work the jaw. The flipper is rather slender and intermediate between Eocene penguins and modern species in most aspects of the underwater flight muscle placements. The leg is pretty standard, with the typical short stubby feet of today’s penguins. Overall, compared to the older species known from Antarctica and New Zealand, Paraptenodytes was closer to having a modern skeletal plan. Morphologies of the skull were interpreted by Simpson as evidence of a close relationship between the Sphenisciformes (the penguin clade) and the Procellariiformes (albatrosses and allies), a hypothesis that is well-supported by DNA evidence today.
This work was not to be Simpson’s only venture in to the world of fossils penguins. Penguin are addicting, you see, and Simpson subsequently wrote around 20 additional scientific papers on penguins, named a dozen new species and visited every one of the living penguin species in the wild. So by chance discovery (and the recalcitrance of the ornithologist’s at the museum), one of paleontology’s greatest minds was brought to bear on the evolution of this wonderful group.
Today I would like to introduce the penguin evolutionary tree. This tree serves as the framework to almost every avenue of research in fossil penguin evolution, because we need to know where a species belongs on the tree before we can quantitatively study patterns of changes in features like bone density, flipper shape, geographic range, and body size.
Systematists construct these trees by searching for shared evolutionary novelties (synapomorphies) that support recent common ancestry between groups of species. These features may be morphological – for example the shared novel feature of having flippers (instead of wings, the primitive state for birds) indicates all penguins share a more recent common ancestor with one another than they do with any other species of bird. At a finer scale, the King and Emperor penguins both have an opening in the mandible (lower jaw bone) that indicates these two species share a more recent common ancestor with each other than either does with any other penguin species. Although morphological features are key to deciphering the evolutionary relationships of extinct species, molecular data (DNA) provide another line of evidence for grouping living species (and in some cases, extinct species too – we’ll explore that in a later post).
So with that brief introduction, here is the penguin tree:
Some of the names on the branches may be familiar from previous posts. The pattern of the branches represents the evolutionary relationships of the species. This is somewhat like a family tree, except that branches can only split (as two taxa split off from their common ancestor) but not reticulate – i.e., they never join back together.
Rooting the Tree: It is intuitive to use tree terms to talk about the parts and shape of these evolutionary trees, and that is just what we do. At the very base of the tree is the root. This is the portion of the tree that represents the earliest part of the evolutionary history of a group. We root trees so that the bottom of the tree represents the evolutionary split between the group pf interest and its relatives – in the case of penguins, we would root the tree to the point where penguins split off from the procellariform seabirds (albatrosses and petrels).
Here is what the tree would look like if we turned it on its side and “planted” it by turning it on its side.
Stem Taxa: Fossil species that are lower down the tree are called stem taxa. Essentially, these species represent dead branches sticking off the stem or trunk of the tree. They split off before the most recent common ancestor shared by the living species evolved. Each branch from the stem part of the tree represents a lineage of penguins that evolved along their own path for some amount of time, and then died out, leaving no descendents. Stem taxa are by definition extinct – each can be thought of as a short, leafless twig from a once growing, but now dead, part of the tree.
Crown Taxa: At the top of the tree are the crown taxa, or “modern” penguins. This is analogous to the crown of a tree – the leafy part at the top. Each living species can be thought of as a green leaf sitting on the end of its own branch. The crown clade includes all the living species of penguins plus all fossil relatives that share their most recent common area. Thus, while stem taxa are always extinct, crown taxa may be living or extinct. These fossil crown taxa can be envisioned as brown leaves on live branches.
The trees for different clades may have very different shapes. Those shapes depend both on the number of fossils and living species, and their pattern of evolutionary relationships. Long, sparse branches characterize the trees for groups where deep evolutionary splits have left a handful of highly unique species – like the monotremes (the platypus, echidnas and their handful of fossil relatives). Dense, leafy bushes represent clades where recent explosive radiations led to hundreds or thousands of closely species – groups like bats.
To provide a visual example, the penguin family tree is most similar to certain types of pine. There is a very long stem leading to the crown. That is, there is a very long series of now completely extinct penguins leading from the root of the tree (where penguins first split off from related birds) to the modern radiation of the 19 living penguin species. The stem of the tree is “tall” because so many extinct species are represented. In fact, in penguins the fossil species outnumber the living species.
Building the evolutionary tree, and updating it as new fossils are found is a major endeavor. How we collect the data that yields the shape of the tree will be the focus of a future post.