The evolutionary tree of penguins is the key to reconstructing the timing and pattern of the penguin radiation. To truly understand the evolution of penguins, one must consider the fossil record. While there are 18 species of penguins alive today, more than 50 extinct species existed in the past. Beyond sheer numbers, many of the fossil species reveal unexpected morphologies, such as the spear-like beak of Icadyptes salasi, the tall slender stature of Kairuku waitaki, or the rufous and grey feathers of Inkayacu paracasensis. The fossil record also reveals that penguins once played broader roles in ancient marine ecosystems: at many localities we can observe penguin faunas with triple the number of species we see in the same region today, and two or more times the range of body size.
One major focus of my work on penguins is to reconstructing the relationships of living and fossil penguins. This work involves combining several types of evidence: molecular sequence data, morphological data from the skeleton, and temporal data from the fossil record. Together, these lines of evidence provide a large scale picture of penguin evolution. Recent results have reinforced our understanding that penguins originated deep in the Paleocene, but that the last common ancestor of modern penguins originated very recently, about 13 million years ago. Biogeographic analyses have also shown that dispersal linked to major ocean currents have helped shape the distribution of both in the deep past and in the modern day penguins.
This work is supported by NSF DEB award 1556615 "Advancing Bayesian Methods for Synthesizing Paleontological and Neontological Data", a collaborative project between Dr. Tracy Heath, Dr. Rob Meredith, and myself.
Major changes in bone histology accompany the secondary adaptation to life in the water, and this is particularly dramatic in penguins which evolved from a hollow-bone ancestor. High bone density in penguins is achieved via compaction of the internal cortical tissues, resulting in the densest bone structure seen in living birds. Thin-sections, polished slices of bone ~100um thick, allow us to study the microstructure of bone in modern and fossil penguins. Recently, I studied Miocene and Eocene penguin histology with Sarah Werning, Michelle Sclafani and Zack Boles.
Surprisingly, the fossil record indicates that penguins continued to experiment with flipper bone histology at least 25 million years after the loss of flight, major differences in humeral structure were observed between these Eocene stem taxa and extant taxa. This indicates that the modification of flipper bone microstructure continued long after the initial loss of flight in penguins. We hypothesize that there was an initial reduction of the medullary cavity of the humerus due to a decrease in the amount of perimedullary osteoclastic activity early in penguin evolution followed by an increase in compaction resulting in a more solid cortex. We found that in extant penguins and the Miocene species †Palaeospheniscus from Argentina, most of the inner cortex is formed by rapid osteogenesis, resulting an initial latticework of woven-fibered bone that is later filled by centripetal deposition of parallel-fibered bone. Eocene stem penguins formed the initial latticework, but the subsequent round of compaction was less complete, and thus open spaces remained in the adult bone. Although cortical lines of arrested growth (LAGs) have been observed in extant penguins, we did not observe LAGs in any of the current sampled specimens. Therefore, it is likely that even ‘giant’ penguin taxa completed their growth cycle without a major pause in bone deposition.
Penguins are considered flightless, but when it comes to wing-propelled diving they are essentially practicing underwater flight. The brain morphology of modern penguins reflects this as they retain an overall “flight-ready” brain. With a great set of collaborators (Amy Balanoff, Stig Walsh, Amy Ho, Ariel Raven, Claudia Tambussi, and Federico Degrange), I have started to scratch the surface of penguin paleoneuroanatomy by using CT scan data to create virtual endocasts (models of the brain, nerves, and blood vessels based on the spaces occupied by these structures in the braincase). In 2012, we completed the first virtual endocast from a fossil penguin, the ~24 million year old species Paraptenodytes antarcticus from Antarctica. This enocast differed from modern penguins in several intriguing ways, including the enlarged semicircular canals and absence an interaural pathway.
More recently, we examined three new Antarctic fossils that provide the oldest penguin endocasts available for study. Pushing back deeper into the past, we observed several provocative features. The ancient Antarctic penguins had relatively large olfactory bulbs compared to other penguins (though nowhere near as large as birds like 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.