Terror Birds!

The terror bird Titanis

This will be the Synapsida post that is current as of 1st April this year. Now, I'm not going to do an April Fool's post, not least because this won't actually go up on the 1st. But, nonetheless, as a sort of reversal-of-expectations, why not, just this once, do something a little bit different?

So here we go: a post about a bird! Gasp!

Birds, despite being warm-blooded, air-breathing vertebrates, are, of course, not mammals, or even synapsids. Instead, they are diapsids, part of an evolutionary line that parted company from the synapsid/mammalian line over 300 million years ago, and which today also includes most, perhaps even all, of the reptiles. (The position of turtles is, to the best of my knowledge, still slightly contentious - they don't look anatomically like diapsids, but there are reasons to think that might be misleading. This is, however, a subject well beyond the scope of this post).

As a result of this long evolutionary separation, birds and mammals are substantially different in a number of ways. The most obvious is perhaps that birds have feathers, and mammals (usually) have hair, but there are many more differences 'under the skin'. For example, the wings of bats and those of birds, while performing the same function, have quite a different skeletal structure to them.

There are, in fact, more living species of bird, arranged in more families, than there are of mammals. For all that, there is a greater degree of similarity among living birds in terms of their general body plan, than there is among mammals. All living birds have feathers, wings, a short tail bone, feet of the same general shape, toothless beaks, and so on - there's nothing like the difference between, say, dolphins, bats, horses, and tigers. Which isn't to say that there isn't still a heck of a lot of diversity there.

Although there are a few others, two main groups of birds today occupy the land predator niche: the owls and the raptors (eagles, hawks, and so on). These are birds large enough to regularly eat small mammals, and if you're say, a mouse, you have as much to fear from an owl as you do from a fox. Larger mammals, however, really aren't that bothered, and birds today aren't really in quite the same league that animals like wolves and lions are.

But it wasn't always so.

Enter the terror birds. More technically known as "phorusrhacids", terror birds are by no means the only large predatory birds to have existed, but they are, perhaps the most significant. For millions of years, South America was an island continent, just as Australia is today, evolving its own unique creatures in isolation. Significantly, it didn't have any dogs, bears, or sabretooth cats until it eventually collided with its northern counterpart. Instead, the top predators on the continent, the most deadly land-based hunters you could find there, were the terror birds.

The exact position of phorusrhacids within the bird family tree isn't certain. However, it's generally agreed that their closest living relatives are probably the seriemas, two species of ground-dwelling bird still found on that continent. Seriemas today largely feed on things like insects and small vertebrates, but, while that undoubtedly makes them carnivores, they are as nothing compared to the terror birds.

While seriemas can fly if they really have to (though they aren't particularly keen on it), terror birds were entirely flightless, and had a heavily-built, long legged and long-necked, body form not unlike that of an ostrich. Unlike ostriches, though, terror birds had large head with a wicked curved beak ideal for tearing flesh. Although some were as small as living seriemas, the very biggest were also somewhat heavier than ostriches, making them really quite fearsome predators. As best we can tell, they attacked with repeated strikes from the beak, having a skull reinforced to resist the stresses caused by that sort of motion, or else just swallowed smaller prey whole.

A newly described fossil, however, may give us even more insight into the biology of these unusual birds. I've often talked before about how most fossils are rather less complete than the articulated reconstructions you see in museums might lead you to believe, but this one is something of an exception. Certainly, there are a few bits missing - some of the toes, part of the wings, and most of the tail-bone - but what remains is remarkably well preserved, and includes parts of the animal never seen before in a terror bird fossil.

Dating from the late Pliocene of Argentina, this isn't one of the really large, bigger-than-a-human, terror birds. Medium-sized, as terror birds go, it stood around 120 cm (4 feet) tall, and probably weighed something like 20 kg (45 lbs). Which, to be fair, is still kind of big for a bird.

Because of its preservation, the fossil, of a newly identified species named Llallawavis scagliai, includes features rarely seen in terror bird fossils, and, in some cases, never seen before at all. One of these is the presence of a long tendon along the back of the lowermost part of the leg. The tendon is visible because it is ossified, and at least partially turned into bone. This ossification is an unusual process in birds, but by no means unknown. It gives the tendon greater strength and rigidity than it would otherwise have, and it has been suggested that it helps with the energetic cost of running, something that would obviously be helpful to a predator.

Also never seen before in a terror bird fossil is part of the syrinx. This an organ not found in mammals, but it's functionally equivalent to our "voicebox" or larynx. Unlike the larynx, it's at the bottom end of the trachea (or "windpipe"), rather than the top, and it has a vibrating wall rather than the vocal cords that we have. Nonetheless, this is the sound-producing organ for birds, and is responsible for birdsong in those birds that can do that sort of thing.

In this case, it is apparent that, as in most birds, much of the syrinx was surrounded by rings of cartilage. Unfortunately, these, not being bone, have not preserved well enough to give us a clear indication of what sort of sounds the terror bird could have made, although we can be confident it wouldn't have been any more like "song" than the sounds of, say, a heron or a duck are. But it's a fair bet that the bird made sounds of some sort, most likely for communication with others of its kind. (As I've mentioned before, deadly predators do not roar or screech at animals they attack for food; the last thing they want to do is give them a warning they're coming).

But, while we're on the subject of sound, the researchers were also able to take CT scans of the skull that showed, for what is apparently the first time, the detailed structure of a terror bird's inner ear. Birds don't really have an outer ear, in the mammalian sense, and their middle ear is also much simpler than ours, basically just being a bony rod that transmits sound from the eardrum to the inner ear. But their inner ear is remarkably similar to our own, and performs the same two basic functions.

Firstly, via a set of structures collectively called the vestibular system, the inner ear creates our sense of balance and acceleration - the reason that you know which way up you are without having to open your eyes, and the sense that makes you feel dizzy when it goes wrong. In birds, as in mammals, and, indeed, all air-breathing vertebrates, the crucial part of this is a set of three semicircular canals, arranged at right angles to one another in 3-dimensional space, in order to detect forward-backward, right-left, and up-down movements.

In seriemas, this structure is relatively small, and the canals have a flatter shape than is usual in birds. It might be that this is because they do a lot less flying than other birds do, since balance is likely something that's quite important in flight. Indeed, the same modifications are found in ostriches... although also, for some unknown reason, in geese, so clearly there are other possibilities.

It turns out, though, that the vestibular system of terror birds actually looks just as well developed as those of flying birds, and therefore unlike that of their closest living relatives. While we know that they didn't fly, the shape of the canals does suggest that these would have been agile birds, and if they really darted their heads back and forth when they attacked, a good sense of balance might well have helped with that.

The other function of the inner ear, of course, is that of hearing. The inner ear includes a structure called the cochlea, which translates the vibrations picked up and transmitted by the rest of the ear, and turns them into nervous impulses that run down the auditory nerve to the brain for interpretation. The cochlea of birds is a relatively straight, simple structure, unlike the lengthy coil found in most mammals, but it fulfils the same basic function.

Analysis of a number of living species provides a method for determining the hearing abilities of birds and reptiles from the length of their cochlea, and this can be applied, at least in theory, to any extinct animal for which we have these dimensions. In the case of terror birds, the dimensions of the cochlea predict that their hearing would not have been very sensitive to high-pitched sounds, and certainly much less so than would be true of, say, owls or parrots today. On the other hand, it would have been broadly in the same range as ostriches or penguins, so it isn't ridiculously low.

This might indicate that terror birds made relatively low-pitched noises when communicating with one another (compared with smaller birds, that is), although, presumably, they used hearing to find prey too, at least to some extent. Like most birds, though, it's a fair bet they had good eyesight, too, which may well have been more important in hunting.

Not all terror birds were necessarily like Llallorwavis. It apparently represents an early branch within the terror bird family tree, albeit one that survived for just as long as the branch containing the truly large species. On the other hand, how exactly different terror birds related to one another is still a matter of some debate, and while the authors of this new study propose a system they're happy with, it's sufficiently early days that we can't say whether or not it will be the final word. But nonetheless, we know a lot more than we did before, and this shows how apparently minor details of a bird's skeleton can tell us something about how it lived.

Next week: mammals.

[Photo by "Amanda", from Wikimedia Commons]