"An uncommon forerunner discloses the means by which colossal flightless birds migrated to distant islands"
In the distant past, around 79.6 million years ago, a common ancestor of the peculiar group of birds known as paleognaths roamed the Earth. This fascinating family, which includes ostriches, emus, rheas, and others, has a history that spans millions of years and offers insights into the evolution of flightlessness.
A recent study, analysing the fossil of an ancient paleognath named Lithornis promiscuous, has shed light on this intriguing chapter of avian history. L. promiscuous lived around 59 to 56 million years ago, a time when the world was undergoing significant changes.
The break-up of the supercontinent Pangaea, which occurred approximately 195 million years ago, had created the continents we know today. This geographical shift, combined with the aftermath of the Cretaceous-Paleogene extinction event, cleared the world of many predators, allowing ground-feeding birds to evolve and become flightless.
L. promiscuous is believed to be the oldest fossil representative of the Paleognaths, and it was possibly capable of long oceanic flights. The shape of its sternum, the bone that supports the wings, is similar to that of living birds like great egrets and herons, which can travel long distances across oceans. This suggests that L. promiscuous could have handled a range of aerobic, flapping flight styles, enabling lengthy flights.
However, it's important to note that the finding about L. promiscuous suggests a case of convergent evolution in the evolution of flightless birds, meaning that similar adaptations occurred independently in different species, not as a result of coordination between species.
Interestingly, the majority of birds are residents found in one continent, island, or small area and don't cover long distances. Paleognaths, on the other hand, are found on six landmasses separated by oceans. In more recent times, flightless birds would have adapted by becoming big and intimidating (like the cassowary) or by becoming swift runners (like the ostrich).
The dodo (Raphus cucullatus), an example of a flightless bird that lived in a predatorless island environment, is perhaps the most famous of these adaptations. Sadly, like the great auk, another flightless bird species driven to extinction due to human activities, the dodo is now extinct.
It's worth noting that Peter Hosner, curator of birds at the Natural History Museum of Denmark, was not involved in the study about L. promiscuous.
Today, there are approximately 60 species of living paleognaths, including about 45 species of tinamou, up to five species of kiwi, one species of emu, three species of cassowary, two species of ostrich, and either one or two species of rhea.
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