Penguins are amazing creatures. They belong to a group called Sphenisciformes, and are one of the few flightless groups of bird. They are beautifully adapted to life in the water, where they spend around half of their lives playing and hunting.
The first penguins are known from around the time the non-avian dinosaurs went extinct, 66 million years ago at the Cretaceous-Paleogene boundary. The first fossils are known from around Antarctica and New Zealand, which were much closer to each other at the time, and have since drifted apart since their tectonic life turned plutonic.
These days, southern Australasia and Antarctica are isolated from each other and around 4000 kilometres apart. This plate reconfiguration has had a distinct impact on penguin evolution in the southern hemisphere, due to its impact on possible dispersal pathways and species isolation.
These days, there is just a single species of penguin living in Australia, the little penguin, Eudyptula minor. And yes, they are totally as cute as the name suggests! But time travel back millions of years, and you have a very different story!
Travis Park, who also recently discovered that 26 million year old whales could echolocate, investigated the evolutionary history of penguins in Australia based on some poorly known fossil remains as part of his undergraduate research. The fossils were collected back in 2006, and come from marine rocks in Portland, Australia, and are around 7.2-7.9 million years old – a period known as the late Miocene. This research is important, as pre-Quaternary fossils of penguins from Australia are relatively rare, and therefore represents a major gap in our knowledge of penguin evolution and biogeography.
“The past 20 years has seen a renaissance in our understanding of penguin evolution,” explains Park, “However, Australian fossil penguins have, until now, been left out of this global picture and no-one has known where they fit into the story. Our paper is the first attempt at putting the missing piece of this puzzle in place.”
To assess the evolutionary history of penguins, they conducted a phylogenetic analysis using the new specimens and a previously published dataset on fossil and living penguins. By combining resulting evolutionary trees with geographic information, it becomes possible to make biogeographical inferences about the group, such as when and where species dispersed from one continent to another.
The evolutionary relationships of penguins mapped onto geography! Note the distribution of Australian species/specimens throughout the tree. (Park et al., 2016)
What Travis and his colleagues found is that the fossil penguins are distantly related from the little penguins still cooped in Australia, and overall Australian penguins are quite dispersed throughout the tree. What this means is that instead of a single lineage persisting in Australia, as looking at just the modern penguins would suggest, there have actually been multiple lineages colonising the Australian shores through geological history. This overall pattern is the same as what we see when looking at penguin colonisation in South Africa too, despite a fairly different geological history.
Travis also found an interesting clue about the geographical pattern of extinction in penguins. Around 23 million years ago, the giant penguins went extinct almost across the globe, but not in Australia, where the species Anthropodyptes gilli hung on beak and claw for another 3-7 million years. When this happens, surviving populations are described as persisting in a ‘refugium’ until their ultimate extinction, where some combination of ecological factors different from other regions promotes their evolutionary longevity.
Overall, the new information from fossil penguins in Australia ties in to several key events in their evolutionary history. In the late Eocene, there was the widespread dispersal of archaic penguins across the southern hemisphere. These ancient penguins went extinct at the Oligocene-Miocene boundary, and appear to have been replaced by smaller-bodied species in the early Miocene. Following this, all crown-group penguins dispersed again throughout the southern continents. However, in Australia, archaic giant penguins, and other earlier species, persisted much later than everywhere else until the early Miocene, and crown group penguins don’t arrive on the scene until later.
Bringing this back to geology (everything always comes back to the rocks..), the reason for this multiple colonisation and penguin longevity seems to be due to the drifting of Australia away from the Antarctic plate. This actually started 100 million years ago, back in the Cretaceous, as part of a late stage rifting phase and continuous break-up of the super-continent, Pangaea.
This means that slowly but surely, the seaway between Antarctica and Australia has got wider and wider over millions of years. The poor penguins in Antarctica would have found it increasingly difficult to move to Australia, and vice versa, effectively creating two isolated sets of populations on either continent.
The penguins did catch a break though. Around 10-15 million years ago, oceanic circulation patterns shifted and opened up a dispersal pathway through the strengthening of the Antarctic Circumpolar Current for penguins to re-colonise Australia. They would have been able to use this current to paddle from South America or South Africa across the Indian and Southern Oceans towards Australia.
In spite of this, it seems that modern penguins, the little penguins, didn’t appear to have reached Australia until the last million years or so. Were they just late to the party? And why does penguin evolution in Australia seem so different to that in the other southern continents?
As Dr Erich Fitzgerald, Museum Victoria’s Senior Curator of Vertebrate Paleontology and study co-author explains: “What happened to penguins in this part of the world during global warming 15 million years ago, and then when the Earth cooled dramatically about 3 million years ago, are chapters of the penguin story we want to know more about. Australian fossils may be the key to understanding these millions of ‘missing years’.”
Only by using fossils to connect the dots can paleontologists begin deciphering the evolutionary history of penguins down under.
The new paper is published open access in PLOS ONE at: http://dx.plos.org/10.1371/journal.pone.0153915