Archive for June 2012
Penguins are part of the marine food chain. Today, they dine on fish, squid and krill. Different species have different preferences, and diets may also change between breeding and non-breeding months when some penguins change the average distance they travel from shore. One focus of penguin paleontology is figuring out what extinct species ate. However, it is also interesting to know what organisms ate penguins. Sharks and sea lions are two major penguin predators today, but the remains of a blubbery penguin get utilized by all sorts of small critters as well, once they hit the seafloor. A paper on trace fossils in penguin bones recently looked at what markings on fossil can say about the fate of penguin remains back in the Miocene, about 20 million years ago.
Argentina is home to several penguin species today, and in the Miocene the most common type of penguin was Palaeospheniscus, an average sized penguin taxon with a fairly modern skeleton. One of the most common styles of markings I have seen on penguin bones is a series of slashes, which are interpreted by paleontologists as shark bite marks. Because bones with these types of markings always seem to show up isolated, it is possible that the shark caught the penguin, but also possible that only scavenged the remains. More unusually, the team that studied the Argentine penguin fossils found bite marks that appear to have been made by mammals. The cool part is that they may have been made by now-extinct marsupial predators, who either stumbled upon a dead penguin that washed ashore or caught a bird at a colony.
Smaller organisms occupy the “clean-up” sectors of the marine food chain, and it is apparent that a variety of invertebrates feasted on penguin bones once large predators had there fill, and even used some for homes. The markings that these animals make are formally classified by paleontologists as trace fossils, and can be given official scientific names just like skeletal fossils. An interesting trace fossil called Entobia represents the workings of a drilling sponge. These simple creatures excavate holes in surfaces to live in, and sometimes do so with penguin bones littering the seafloor. Gnathichnus is another type of trace fossil, represented by an excavation believed to be made by drilling echinoderms (sea urchins) in their quest to eat algae or worms living on bones. Both types of trace fossils were identified by the team. Together, all these seemingly insignificant pits and scratches paint a vivid picture of life on the seafloor, and the community in which these ancient penguins lived.
Cione, A.L., Hospitaleche, C.A., Perez, L.M., Laza, J.H. & Cesar, I. 2010. Trace fossils on penguin bones from the Miocene of Chubut, southern Argentina. Alcheringa 34, 433–454.
In this installment of our tour of the penguin skeleton, we will take a look at the phalanges. Phalanx (plural phalanges) is the word anatomists use for a finger bone. We have 14 total phalanges in our own hands, 3 in most fingers but just 2 in the thumb. You can tell where two of these bones meet because the connection forms a knuckle joint. Birds, of course, are not running around with visible fingers (not since the Cretaceous Period anyway), but they do have phalanges embedded within the wing. Most birds have one phalanx remaining in their first digit (equivalent to our “thumb”), two phalanges in their second digit (equivalent to our “pointer finger”), and one phalanx in their third digit (equivalent to our “middle finger”). These are of no use for manipulating objects, but do serve a purpose by anchoring some feathers. In particular, the phalanx of the “thumb”, in birds called the alular phalanx, is important because it anchors the alula, a special feather that is important in controlling flight speed during landings.
In penguins, the phalanges are weird. They look a bit like a normal birds phalanges got run over – this is part of a general pattern of flattening seen in the penguin wing skeleton, which makes it more flipper-like. The third digit is typically tiny in birds, but in penguins it is huge. The phalanx is long and tapers to a pointed tip, and has a sharp backward pointing projection. One of the really cool things about this bone is that you can trace its evolution in the fossil record. In more basal fossils penguins like Icadyptes, the phalanx is large compared to flying birds, but still much smaller than in living penguins, and also lacks the projection. Extending this bone to modern lengths happens about 30 million years into penguin evolution, and results in a slight decrease in aspect ratio of the flipper. Most importantly, penguins lack an alula altogether. This may be related to the lack of differentiation in penguin feathers. Certainly, they don’t need to “land” anymore so a special alula feather is probably superfluous. In the image below, you can see the major differences in shape between the wing of a shearwater (a flying relative of penguins) and an Emperor Penguin.