How is continental drift an ongoing process

How the ratites came halfway around the world

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Adelaide - Those who can fly have an advantage when it comes to finding new living spaces. Birds have established themselves on numerous islands that mammals would never have reached without human help.

A group of birds seems to have managed this without being able to fly. The ratites are represented on all continents of the southern hemisphere with the exception of Antarctica: ostriches in Africa, rheas in South America, emus in Australia, cassowaries in New Guinea and finally kiwis in isolated New Zealand. Some common features - most notably the lack of a sternum crest - suggest that all of these species have common ancestors. But how could they spread over half the world?

Ride on the continents

The historical explanation for this is that the ancestors of today's ratites lived on the supercontinent of Gondwana. When this began to break up in the Jura - a process that lasted into the modern earth era - the primordial ratites also broke up into different lines of development. They owe their current widespread distribution solely to the continental drift.

Since DNA analysis found its way into evolutionary biology, some plausible-sounding hypotheses about animal relationships had to be thrown overboard. This is also the case with ratites: In the recent past, a number of studies have shown that the relationships between the individual ratite species do not correspond to the pattern in which today's land masses have separated from the rest of Gondwana.

Long-distance relationships

A particularly blatant example is described by researchers working with Kieren J. Mitchell from the University of Adelaide in the current issue of "Science". Their investigations into mitochondrial bird DNA came to the conclusion that the closest relative of the New Zealand kiwi was not the giant moa, which up until historical times lived in New Zealand as a direct neighbor of the kiwi. Instead, it was another giant who lived half a globe: the elephant bird of Madagascar, which was also only exterminated in historical times. And even in the days of Gondwana, there was a huge piece of land between New Zealand and Madagascar.

This upheaves the family tree of ratites in a similar way to the earlier discovery that the South American cockatiels are actually also ratites. At least today they are combined with the ratites in the group of Palaeognathae, which is compared to all other bird species. And these birds, vaguely chicken-like to non-ornithologists, may not be masters at flying, but they can.

Dissemination on its own

For Mitchell and his colleagues, both findings combine with fossil finds from Palaeognathae in the northern hemisphere to form a picture that contradicts the old Gondwana hypothesis. The Australian researchers believe that ratites' ancestors could fly and actively spread across the world.

Once in their new habitats, according to this theory, they would have lost their ability to fly several times and completely independently of one another. At the same time, the majority of ratite species - again independently of one another - developed gigantism: a case of evolutionary convergence. That sounds like an unlikely coincidence, but according to the researchers it could be a self-evident further development of diurnal herbivores.

Who is late ...

In any case, according to Mitchell and his colleagues, timing was crucial, and in several ways. The Palaeognathae used an important time window, namely the first ten million years after the extinction of their dinosaur kin, when the mammals had not yet occupied all ecological niches. In this way, giant flightless birds were able to establish themselves in some regions - in some cases up to the present day.

On the other hand, the timing aspect could explain why the Palaeognathae species each colonized a land mass on their own. And why in the two exceptional cases the neighbors were or are extremely different: Giant moas and small kiwis in New Zealand, tall rheas and small cocktails in South America. Neither here nor there are the immediate neighbors most closely related.

The researchers interpret this to mean that only the first-time arrivals on a continent or island encountered a niche for large non-flying herbivores. Palaeognathae relatives who arrived later found the niche already occupied and therefore remained small - and in the case of the cockroaches even airworthy. (jdo, derStandard.at, May 24, 2014)