Happy feet: why a 61m-year-old penguin foot has researchers dancing for joy

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This article titled “Happy feet: why a 61m-year-old penguin foot has researchers dancing for joy” was written by Hanneke Meijer, for theguardian.com on Wednesday 5th April 2017 07.00 UTC

The theory that birds descended from bipedal dinosaurs, Coelurosaurs to be exact, is now well-established within the palaeontological community. With that one out of the way, bird palaeontologists can focus on more pressing issues, such as the origin and evolution of Neornithes, the group of birds that comprises all living birds. Several groups of extinct birds are known to have existed alongside the dinosaurs, such as the aquatic diving birds Hesperornithiformes, the large, toothed Ichthyornithiformes, and the “opposite birds” Enanthiornithidae, named after the distinct anatomy of their shoulder girdle. None of them gave rise to the birds we see in our backyard today.

The early evolution of modern birds is fuzzy, to say the least. Models based on molecular clocks place the origin of Neornithes as far back as the Early Cretaceous, whereas others suggest that modern birds did not diversify until the Late Cretaceous (see Brocklehurst et al., 2012 for a discussion). The sparse fossil record of Mesozoic Neornithes does little to clear things up.

Modern birds can be split into two major groups; the Palaeognaths (meaning “old jaws”) include the flightless ostriches and kiwis, whereas the Neognaths (“new jaws) contains all other birds. The earliest group of birds to split off within the Neognaths was that of the Galloanserae, the group containing the Anseriformes (ducks and allies) and the Galliformes (pheasants, grouse and allies). The earliest Neognath may be the galliform Austinornis lentus, dated to about 85 million years ago (Clarke, 2004), although some doubt about its presumed identity as a galliform remains. The description of Vegavis iaai as a Late Cretaceous (68-66 million years) member of the duck lineage suggests that Neognaths had already split into modern groups before the end of the Mesozoic era. However, whether the radiation of the remaining Neognaths occurred before or after the Cretaceous – Paleogene extinction remains a topic of discussion.

(How about the Palaeognaths, you ask? Well, that’s a story for another time.)

This is why bird paleontologists are looking with great interests at the period beyond the Cretaceous – Paleogene extinction, because this is where the ancestors of all our modern birds are to be found.

A recent paper by a team of bird paleontologists from the Canterbury Museum in Christchurch, New Zealand, and the Senckenberg Museum in Frankfurt, Germany, describes a penguin fossil foot from the mid-Paleocene Waipara Greensand location in New Zealand. This locality is dated to about 61 million years ago. The fossil foot belongs to a new species of penguin. The specimens were found at the same locality where another species of large penguin was found as well, Waimanu manneringi (Slack et al. 2006), which is considered the oldest penguin known to date.

The new giant penguin fossil foot bones (left) next to those of a modern penguin (right). Photograph: Gerald Mayr

Tarsometatarsi vary tremendously from one group of birds to another, and are therefore a very useful bone to identify birds. In penguins, the tarsometatarsi is very distinct; it is extremely short and broad, as if someone took a heron’s tarsometatarsus, cut out the long bit in the middle and glued the top and bottom together again. But even within penguins, the shape of the tarsometatarsus varies. The new fossil foot differs from Waimanu, and looks more like modern penguins, in its proportions and the shape of the hypotarsal canals, a set of ridges on the plantar side of the bone that keep the tendons for the foot in place during locomotion.

The finding of this new fossil penguin foot alongside more primitive species of penguins indicates that penguin diversity early after the Cretaceous – Palaeogene extinction event was larger than expected. Moreover, the penguin foot shows characteristics that we are used to seeing in much younger penguins. This shows that the typical penguin tarsometatarsus morphology evolved more rapidly than we thought. The difference in tarsometatarsus morphology between the new specimen and Waimanu likely reflects differences in locomotion both in the water and on land.

The new tarsometatarsus is much stouter than Waimanu and corresponds with small individuals of Anthropornis nordenskjoeldi, a species of giant penguin that lived 45-33 million years ago and could reach 165 cm in length. Thus, early penguins not only looked like modern penguins, they also attained very large sizes early on in their evolution, and kept it up for almost 30 million years. Why giant penguins disappeared around 20 million years ago remains unclear. Their disappearance roughly coincides with a rise in marine mammals, particularly the evolution of toothed whales (Odontoceti). Competition for food may have been a contributing factor in their demise (Ando and Fordyce 2014).

Artist reconstruction of the Waimanu giant penguin compared to a modern Emperor penguin and a gentleman. Illustration: Gerald Mayr

This unexpected diversity of penguins so soon after the end of the Cretaceous has bearing on hypotheses regarding the early evolution of Neornithes. A number of molecular analyses suggested that different penguin groups started to develop around 62 million years ago (Jarvis et al. 2014; Prum et al. 2015). However, according to this hypothesis, the presence of both primitive and more evolved types of penguins, as shown by the material from New Zealand suggests that early penguins evolved in an extremely short time period. A much more likely scenario would be that penguins started to diversify in the Late Cretaceous, well before the mass extinction event. Why penguins survived, and Tyrannosaurus rex did not, one can only wonder.

References:

Ando, T & Fordyce, RE, 2014. Evolutionary drivers for flightless, wingpropelled divers in the Northern and Southern Hemispheres. Palaeogeography Palaeoclimatology Palaeoecology 400:50–61

Brocklehurst N, Upchurch P, Mannion PD, O’Connor J, 2012. The Completeness of the Fossil Record of Mesozoic Birds: Implications for Early Avian Evolution. PLoS ONE 7(6): e39056

Mayr, G, et al., 2017. A new fossil from the mid-Paleocene of New Zealand reveals an unexpected diversity of world’s oldest penguins. The Science of Nature 104:9

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