Archive for April 2010
Madrynornis mirandus is one of the better known fossil penguins, represented by almost the entire skeleton (although only one individual has been found so far). This species lived about 10 million years ago in Argentina. It was actually found almost 100km away from the city of Puerto Madryn along Golfo San José, but is named for the Puerto Madryn Formation. Madrynornis mirandus was about the same size as a modern Adélie penguin, which should be familiar to many as it is a popular zoo animal. The species appears to have been a dietary generalist. From its based on bill shape, it probably ate fish and crustaceans. In fact, there seems to be a fish – and a large one at that – preserved right next to the Madrynornis holotype specimen. The disk-like bones near the feet of the penguin specimen are clearly not avian but instead belong to bony fish. They are still lined up in a row, but there does not seem to be any other parts of the fish preserved, at least in the picture. This particular specimen was of course much too big for Madrynornis to have tackled.
Madrynornis mirandus is important because it is a crown clade penguin. This means the species shares the most recent common ancestor of all living penguins. Penguins that branched off the evolutionary tree before this common ancestor evolved are called stem penguins. One can imagine these archaic penguins occupying the lower, “stem” part of a branching plant and the crown penguins occupying the tree top, or crown. All stem penguins are by definition extinct, while crown penguins may be either living or extinct – a mix of green and brown leaves in the canopy.
Phylogenetic studies show Madrynornis mirandus is closely related to the living Yellow-eyed Penguin (Megadyptes antipodes) and the crested penguins of the genus Eudyptes. As the name suggests, the Yellow-eyed Penguin has a bright yellow eye with a stripe of yellow feathers across it. Eudyptes penguins are characterized by a crest of yellow head feathers that gives them a somewhat fancy visage. One of the best known Eudyptes penguins, the Macaroni Penguin (Eudyptes chrysolophus) is actually named after a style of feathered hat fashionable the 1800s, not the noodle.
Knowing that Madrynornis mirandus is related to these living penguins enhances our inferences about the species. Because the fossil occupies a branch on the tree in between the Megadyptes and Eudyptes branches, we can say that it is bracketed by those two living species. This extant phylogenetic bracket concept was formalized by Dr. Larry Witmer and has been applied to many paleontological questions. Many soft parts of an animal, like skin and feathers, don’t typically fossilize. However, if we understand the evolutionary tree for a group and study the closest living relatives of the extinct species, we can make some extrapolations about these soft tissue features. The strongest basis for inferring the morphology of a non-fossilized feature is when both living taxa that bracket the extinct species possess a feature – then we can safely assume the feature was probably present in the fossil too. In the case of Madrynornis mirandus, we can be fairly confident the extinct species also had decorative yellow head feathers. The evolutionary tree helps us “flesh out” the appearance of this long extinct penguin without having to just guess.
Another important aspect of crown fossils is that they can help us reconstruct the timing of evolutionary events. Because Madrynornis mirandus occupies a branch in between the Eudyptes penguins and the Megadyptes penguins, we know these two groups must have separated off onto their own evolutionary pathways by at least 10 million years ago. This is an important piece of data that can unlock a lot of information about the pattern and timing of penguin evolution. We’ll cover how in a future post.
Acosta Hospitaleche, C., C. Tambussi, M. Donato, and M. Cozzuol. 2007. A new Miocene penguin from Patagonia and its phylogenetic relationships. Acta Palaeontologica Polonica 52: 299-314.
Witmer, L. M. 1995. The extant phylogenetic bracket and the importance of reconstructing soft tissues in fossils. In Functional morphology in vertebrate paleontology (ed. J. J. Thomason), pp. 19–33. Cambridge University Press