Diving into Fossil Penguin Brains

Here is a wonderful reconstruction of the fossil penguin we analyzed in our recent penguin brain evolution study, created by artist Santiago Druetta. The fossil species is hunting down an icefish, a type of fish also known from the 34 million year old fossil La Meseta Formation deposits that yielded the fossil penguin skull.  In the background swims a modern Chinstrap Penguin, a species named for the party-hat-string-like band of black feathers across its chin. The brains of these penguins are shown in the upper right corner.

But what species is the fossil?  Actually, we are not sure. This is because the many extinct species have been named from the La Meseta Formation and each was described by scientists based on limb bones. Because the skull we studied was found in isolation with no traces of the rest of the skeleton, we can’t be sure which species it belongs to with certainty. The skull is roughly the same size as the skull of an Emperor Penguin, but we know that many extinct penguins had small heads relative to their overall body size. Thus, the skull could easily belong to a giant penguin like Anthropornis nordenskjoedli or Palaeeudyptes gunnari.

The situation is even more complex when you consider that we looked at two additional fossil skulls from the La Meseta Formation in the study, and found evidence that each belonged to a different species than the main skull.  One has a very different external morphology. The other looks the same on the outside, but had such a different brain shape that we concluded it must belong to a third species. Regardless of the precise species identifications, these skulls have provided excellent new data on early penguin brain structure.

Life restoration of a 34.2 million year old Antarctic fossil penguin and the extant chinstrap penguin, with virtual brain endocasts of their brains based on X-Ray CT scan data (top right). Original artwork by Santiago Druetta.

Police apprehend penguin in Peru

The title says it all!

Ancient Penguin Brains from Antarctica

Today, a new research article on fossil penguin brains is available at the Journal of Vertebrate Paleontology. I’m pleased to have been part of this study, led by Dr. Claudia Tambussi and Dr. Federico “Dino” Degrange. We looked at three Eocene fossil skulls from Antarctica, each belonging to a 34 million year old penguin. These fossils were recovered during expeditions by Dr. Tambussi and other scientists to Antarctica. In order to shed some light on the neuroanatomy morphology of the ancient penguins, we used CT scans of the skulls to create virtual endocasts – 3D models of the brain and  sensory organs.

Virtual endocasts of the brain (blue) and semicircular canals (pink) in (a) new Antarctic fossil, (b) Paraptenodytes antarcticus, (c) Emperor Penguin, (d) Black-footed Penguin, (e) Magellanic Penguin, (f) Little Blue Penguin, (G) Chinstrap Penguin, (H) Adélie Penguin. Not to scale. Image from Tambussi et al. (2015)

Penguins are considered flightless, but when it comes to wing-propelled diving they are essentially practicing underwater flight. The brain morphology reflects this as modern penguins retain an overall “flight-ready” brain. The new Antarctic fossils are important because they provide the oldest penguin endocasts available for study (they are more than ten million years older than the Paraptenodytes antarcticus endocast we studied in 2012). These fossils show that ancient penguin brains had several important differences from modern species.
One of the interesting features in the endocasts from the fossil species was the larger size of the olfactory bulbs. Modern penguins have very small olfactory bulbs compared to their closest relatives, the tubenose Procellariiformes (petrels and allies). This is likely related with their reduced reliance on smell to locate prey compared to sensitive-nosed petrels, which can sniff out stinky slicks of chum from miles away. Penguins tend to rely more on vision to locate prey. The early fossil species had relatively larger olfactory bulbs (though nowhere near as large as petrels), suggesting that reduction of olfactory capabilities was a slow trend in penguin evolution following the loss of flight.  Another interesting facet of the endocast data is that it provides more support for the hypothesis that the morphology of the Wulst, a brain structure associated with complex visual function, changed in similar ways in different groups of birds. Researchers like Dr. Stig Walsh have demonstrated that the Wulst is restricted to the front part of the brain surface in early members of many groups of birds, but expands backwards in modern species. Because we see the same pattern in different types of birds (including penguins) over the course of millions of years, it suggests they  evolved this feature independently. What advantage this may have conferred remains mysterious because of limits of the technology for studying fossil brains: our endocasts only provide the surface morphology, but cannot “see” the boundaries that would have existed between layers of cells associated with different brain functions because all that remains is the void where the brain was once housed.

Reference:

Tambussi, C.P., F.J. Degrange and D.T. Ksepka. 2015. Endocranial Anatomy of Antarctic Eocene stem penguins: implications for sensory system evolution in Sphenisciformes (Aves). Journal of Vertebrate Paleontology: e981635.