Part of a now-extinct diversification of giant penguins, Inkayacu certainly would have stood out when compared to its living relatives. At an estimated mass of 54-59 kilograms, Inkayacu was roughly twice as heavy as today’s emperor penguin, and, like its giant cousin Icadyptes, it had an hyper-elongated bill which it used to snap up fish in the ancient equatorial sea. “If you could have stood on an Eocene beach,” lead author  and paleontologist Julia Clarke wrote by e-mail, “their silhouettes would have been striking – not quite a ‘Jaws’ film but as close as you get in the penguin world.” What truly makes the first-described specimen of Inkayacu remarkable, however, are its feathers.

No fossil penguin has ever been found with intact feathers before, and this has frustrated paleontologists who have attempted to work out the tempo of penguin evolution. Just as the skeletons of penguins became modified to life in the water, the feathers of penguins must have also become adapted to a semiaquatic lifestyle, but without fossil traces of penguin plumage it has been impossible to know the details of this soft-tissue change. Now, thanks to feathers preserved in place along the tip of its left forelimb, Clarke and colleagues have been able to determine that the wing feathers of Inkayacu – many of them “covert feathers” which laid on top of other feathers – were very similar to those of living penguins. The feathers along the wing tip of Inkayacu showed the same kind of tight, flattened, and stacked arrangement seen in living penguins, and, given that Inkayacu was not ancestral to extant species, this means that this type of feather arrangement was likely present in the common ancestor of both lineages during the early Eocene. By 36 million years ago, and possibly earlier, penguins already had streamlined flippers for propelling themselves through the water. Where Inkayacu was unique, however, was in its coloration.

The left wing of Inkayacu with preserved feathers. The red dots in the line drawing represent feather samples taken for SEM analysis.

A particularly popular paleontology trope is that we can’t tell what color extinct organisms were. Color is often treated as something which is entirely speculative and open to artistic license, but during the past several years paleobiologists have developed a technique which has allowed them to identify the ancient vestiges of color in bird and dinosaur feathers. At least some of the coloration of bird feathers comes from microscopic structures called melanosomes, and, as seen in living birds, different melanosome shapes correspond to different colors. By using the microanatomy of avian shades as a guide paleontologists can now narrow down the probable palette of fossil feathers, and this is precisely what Clarke and her co-authors did in their description of Inkayacu.

Putting the feather colors of Inkayacu in context required a knowledge of the melanosomes in other penguins. This had never been studied before, and when the scientists looked at the feathers of living penguins they found that their melanosomes were about as long as in other birds, but were significantly wider. Oddly, however, the melanosomes of Inkayacu did not match up with those of living penguins. Instead the shape and density of the sampled feathers of Inkayacu clustered with patterns seen in other birds and indicated that its feathers were grey and reddish brown in color. Viewed from an evolutionary perspective, this means that the wide type of melanosome seen in extant penguins was not an ancestral trait shared by all fossil forms; this characteristic melanosome type specifically evolved in the lineage containing all modern penguins after the split with Inkayacu and its kin about 50 million years ago. As put by one of the study’s co-authors, Jakob Vinther (who helped pioneer the use of melanosomes to detect coloration in fossils), “When we looked at Inkayacu we found melanosomes similar to other modern birds as well as the dinosaur Anchiornis that we studied. So even though feathers in this fossil penguin are rather similar to modern penguins, the melanosomes are still of the ancestral [i.e. archaic] condition.”

A life restoration of Inkayacu, by Katie Browne.

Just why Inkayacu differed from modern penguins in terms of its melanosomes, however, is unknown. Perhaps the disparity is attributable to some aspect of ecology, the authors propose, such as changes in the array of predators which fed upon ancient penguins. “These Eocene seas are a different kind of place,” Clarke explained. “There were no seals– they evolve later in the Cenozoic – likely sometime around the demise of the giant penguins. Are they related, I don’t know. Could color shifts be related to this rise of groups that today comprise primary penguin predators?”

Then again, the odd arrangement of melanosomes in living penguin feathers may be attributable to the need for increased structural support. Since wide, tightly-packed melanosomes would have made feathers resistant to breaking, the authors state in their description, then the change in melanosomes (and hence color) might have been driven by the pressures of flying underwater. “Why did these giant melanosomes evolve in living penguins – was it in response to some kind of evolutionary constraint or an unanticipated response to the extreme demands of what is essentially underwater flight– flight in a medium ~800 more dense than air?” Clarke wonders. That is a question for further study. As Clarke herself puts it, “Some aspects of penguin evolution are clearly not black and white.”

For more, see Not Exactly Rocket Science and March of the Fossil Penguins.

Images (1): Reconstruction of Inkayacu paracasensis in oblique anterior view showing recovered elements in white and photographs of the holotype specimen (MUSM 1444): skull and mandible in dorsal, ventral and lateral views; scapula in lateral view and humerus in posterior, ventral, anterior and distal views; femur
in dorsal, medial, ventral and distal views; patella in anterior view; tibiotarsus in lateral view and tarsometatarsus in proximal, distal, anterior and  plantar views (Caption adapted from Clarke et al., 2010).

References:

Julia A. Clarke, Daniel T. Ksepka, Rodolfo Salas-Gismondi, Ali J. Altamirano, Matthew D. Shawkey,, & Liliana D’Alba, Jakob Vinther, Thomas J. DeVries, Patrice Baby (2010). Fossil Evidence for Evolution of the Shape and Color of Penguin Feathers Science : 10.1126/science.1193604

Authors: Brian Switek

to know more click here