The idea that emus evolved from dinosaurs does not mean to imply that Dromaiidae are direct descendants of Dromiceiomimus. They aren't. Avian evolution isn't that simple. Nothing in reality ever is. And it invites some of the most heated debates in paleontology or biology. OK, so in the turmoil of our socio-political environment and the stresses of our everyday lives, this may not be the hottest topic. But among scientists, pseudo-scientists, and interested laymen, such as myself, the origin of birds is often a highly controversial subject.
When I realized that I was going to have to care for a 6 foot tall, 120 lb. exotic bird in the presence of small children, I went to several websites of the people who knew best how to feed, house, and handle them. And I learned a good deal of useful things that have helped me out in the husbandry of that animal. But not much in the way of its history.
Most of the websites dedicated to emu farming indicate that these birds have been resident in Australia since well before the extinction of the dinosaurs. But that belief, however common among them, is based entirely on a handful of misperceptions. For one, many of these sites confuse the extant emu for the extinct emeus, which were a family of Moas in New Zealand. An understandable error.
Their numbers came from a single estimate made way back in 1971 by Joel Cracraft, a noted paleo-ornithologist who considered the fact that most of the largest ratites were on islands that he thought had separated from Gondwanaland in the mid-Cretaceous. So he concluded that they must have already been there before the continental separation, which geologists estimate to have begun about 80 million years ago and continues today, albeit slowly. Cracraft had no other evidence for his conclusion than the lack of any alternative explanation for how else these birds could have been on these remote shores in recent history if they couldn't have flown there at any time in the past.
60 to 100 million years ago or so, (give or take a whole long time either way) most all the land on Earth was crushed into one massive continent, called Pangea. The Southern portion was once another continent called Gondwana. By the close of the Mesozoic era, Madagascar was just separating from Africa, which was still partially connected to South America which was still connected to Antarctica, which itself was still joined to Australia and in turn, still attached to portions of New Guinea and New Zealand, if not by continuous land bridges, then by island chains. And while Antarctica was already very far to the South, it still had not centered on the South Pole where it is today.
But there were a few things Cracraft didn't (or couldn't) consider back then. Ratites are an ancient order among aves. In fact, they are the oldest of all living birds. But recent evaluation of Mitochondrial DNA indicates their divergence from the tinamou, (their closest flying cousins) at only 45 million years ago, in the Eocene period, many millions of years after the dinosaurs were already extinct. That was only half of Cracraft's estimate, but still some 39 million years before anything close to an aboriginal human.
So how did ratites find themselves in such remote locations as Australia, New Zealand, and Madagascar? Cracraft didn't realize that most ratites are surprisingly strong swimmers, for one thing. Very strong in fact, since their physiognomy allows them to hold their heads above water almost indefinitely. Its rather like the tiger syndrome. Unlike most cats, tigers actually like water and have been found miles out to sea more than once as they travel between the islands of Indonesia. Doubtless there were many islands between these land-masses for a very long time. And several events, (such as when some of these islands finally sank beneath the waves) that would have driven the birds to dryer shores elsewhere, whether they meant to do that or not.
Hurricanes and flotsam debris have accounted for more than one accidental migration as well, so we can't rule that out either. There are uncertainties, of course. That mitochondrial date could be off, for example. And not every continent split away from the others simultaneously. And separation took millions of years to do. But in each case of an island dwelling bird, there was a continental cousin on the nearest shore, (except Antarctica, so far as we know right now). The three species of cassowary exist both in New Guinea and in nearby Australia, the moas on the opposite shore from the emu, and the elephant bird lived on the next beach over from the ostrich, who was right across from the South American rhea. Thus their ancestors didn't have to have crossed impressive spans of land or water to get to their new homes. In many cases, they actually could have walked it. But when they did find themselves living in terrain that were separated from large predators, they often far exceeded their mainland cousins in size, as many island-dwellers tend to do. And just as visually similar, but unrelated animals often fulfill the same environmental role of similar animals of distant but similar climes, (rabbits and kangaroos for example) so did the ratites emu-late the Cretaceous dinosaurs, whose niche they came to fill. The giraffe-like moas and the mighty elephant bird both exceeded 10 feet in height in Liu of any predator until primitive human tribes came along and ate them all. Very sad.
This partial skeleton with only one claw on each arm is not that of an emu, ostrich or rhea.
Its a Cretacious dinosaur called Mononykus.
Click it to see it in life.
Cassowaries have a face that
only a mommasaur could love.
Wherever there are ratites, there are usually also tinamous. Ratites and tinamous (paleognaths) are very similar to each other, but lack many of the distinctions of the more familiar flying birds, (neognaths). Few of the more "modern" birds have any hint of a claw in their wings at all. When it is there, (such as in falcons, for example) it is barely detectable unless you're really looking for it with a fine-toothed comb. Its almost more of a cuticle spike and not as distinct as the claws of an emu and certainly not as distinct as those of an ostrich. Another distinction is that paleognaths, as with mammals, reptiles, etc. all have sutures in their skulls which join the major components together. But in neognaths, these sutures have fused into a single one-piece carapace. Another pronounced distinction is the "keeled" sternum, common to all familiar birds, but lacking in prehistoric aves, such as emus and elephant birds.
So one would expect there to have to been a common ancestor of all paleognaths that would link them to the earlier birds of the Eocene with sutured skulls and flat sternums. And there were large terrestrial walking birds wandering about at the right time, (50 million years ago) in North America and Europe. The American variant was Diatryma, a frightful wingless eagle-looking thing of about 7 feet tall. It was the top predator of its day, to be sure. There were no large carnivorous mammals yet, no dire wolves, nor cave bears, nor sabre-toothed cats. These wouldn't appear for a long long time to come. Nestled between the dinosaurs of the Cretaceous and the familiar beasties of Man's relatively recent early history, Diatryma wandered through possibly the least famous and least studied period of biological history, which was a very strange world indeed.
But Diatryma was not a likely candidate for ratite ancestry. One reason is that it had the classic keeled sternum (visible in the animation at left) that all flying birds, (except tinamous) share in common. Ratites don't have one, and neither to the most ancient aves of the Mesozoic era. Another reason to discount Diatryma is that the Tinamou, who share the ratite's ancestry, have fully functional wings. They're not very good at flying, but they can do it just well enough not to be considered flightless. So the grandfather of all paleognaths would have to have wings also, and not just the vestigial limbs of more modern moas. But Diatryma had no wings at all.
There was another bird around at that same time, bearing many of the right features that could have made it a likely common ancestor of the entire global paleognathic aviary. Gastornis lived in Europe from the upper Paleocene until about 50 million years ago. It looked very much like a collossal crane, standing up to 12 feet tall! He also had much larger wings (proportionately) than any of the ratites would ever sport. Big enough to hold a large eagle aloft perhaps, (with a running start) but not nearly broad enough to raise a 500 lb. Heron off the ground, even if it could run 50mph!.
Another intriguing feature to Gastornis, (which Diatryma didn't share) was its teeth! Emus have only the slightest hint of teeth, which are now mere serations in the end of its bill. Ducks and geese have ridges that count as teeth, but they aren't used in the same way. But Gastornis had actual teeth set into the bone of its bill, as did other, more ancient birds to come before it. Unfortunately, it too appears to have had a keeled sternum. There is some contraversy as to whether it was reconstructed properly the first time around and is currently being re-evaluated. But if the sternum was correct, as it appears it was, then Gastornis would not be a likely fore-funner to ratites, which exhibit traits much older than that. There were other very large walking birds in earlier times, even into the Cretaceous period of the dinosaurs. But ratites probably didn't start out all that big. 50 million years ago, they just hadn't yet grown up yet. And they could still fly, if only just a little bit.
Gastornis
Click here to continue Click the ratite
grandaddy to go back
This is an
illustration of
paleognath clades.
The common ancestor
of ratites and tinamous
looked more like the tinamou,
but it was more like a dinosaur than either of them.
