[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: emu history



At 08:43 PM 1/13/2004, Nancy Cavanaugh wrote:

Hello,

My name is Nancy Cavanaugh. I am currently writing a book about the emu.
In doing my research I have found some discrepancies about when they
appeared on Earth.

The commonest answer is 80 million years ago, which from my reading was
determined in 1971 (or was it in 1973? the research I've done indicates
both years) by Joel Carcraft. Today, however, I tripped across a site
that said they couldn't have arrived before 45 million years ago, which
was backed by reasonably solid scientific reasoning.

Which is the correct date? Who is this date attributable to? What
evidence do they use for this date?

Of course there is no certain way to determine exactly when the modern emu, Dromaius novaehollandiae, first "appeared on earth" -- and, since it obviously evolved from an ancestor which was not quite identical to it, it is not even correct to say that it appeared (as opposed to achieving a certain level of differentiation from its ancestors). So the first question to ask is, are you interested in when the modern species arose, when the genus arose, the family, or the order?


There are three ways of getting some sort of answer to these questions: the obvious one, of course, is to come up with a fossil of known age. Another is to use genetic data to estimate how long ago a species (or higher category) diverged from the common ancestor it shared with its nearest living relative (in this case, with the cassowaries if you are referring to the emu family, as opposed to the living species -- bear in mind that there are two recently extinct emu species, from King Island and Kangaroo Island.) A third, perhaps more questionable, technique is to look at the history of continental drift. The argument would go, for example, that the emu lineage -- or at least the emu-cassowary lineage as separate from the lineage is that lead to other large flightless birds such as ostriches -- must be at least as old as the point at which Australia broke away from the land mass of Gondwanaland. That is where the 45 million-year-old figure comes from, because that is apparently the time at which Australia and Antarctica separated. However, this does not tell us when emus and cassowaries differentiated from each other, as they both occur in Australia. Presumably this happened later, but not necessarily as the separation could have occurred before the birds became isolated on the Australian landmass. Obviously there are none left in Antarctica!

That said, and noting that Joel's name is "Cracraft", the Handbook of the Birds of the World, Volume one, published in the 1992, notes that the oldest fossils that can be ascribed to the living genus of emus are only some 5000 to 10,000 years old, and belong to the King Island Emu which has since become extinct. Obviously, emus as a lineage are older than that. however, the most recent book I have on the subject, a detailed monograph called "Ratites and Tinamous" by SJJF Davies (Oxford, 2002), casts doubt on any of the attempts to determine precisely how old these groups of birds are, and does not suggest a date.

I would be inclined, given this, not to attempt to come up with an age for the emu family. All that I think you can really say is that emus and their closest relatives, the cassowaries, have been separated from their nearest relatives ever since Australia drifted away from Antarctica and so, together, must have been around in one form or another for at least 40 to 45 million years.


--
Ronald I. Orenstein Phone: (905) 820-7886
International Wildlife Coalition Fax/Modem: (905) 569-0116
1825 Shady Creek Court
Mississauga, Ontario, Canada L5L 3W2 mailto:ornstn@rogers.com