Birds

Kindergarten teachers would find Crox Minor (then aged 4) an exasperating child. The reason was her habit of ignoring any instruction or direction but running around in mad circles with her arms stretched out and screaming “I’m Archaeopteryx! The First Bird!” When Mrs Crox collected our little feathered theropod manqué from school, she’d try to get away quick before she was cornered by a teacher who’d note, in a reproving voice, that Crox Minor “is not an Archaeopteryx – she’s a Little Girl.”

Such is the iconic status of Archaeopteryx, that even small children know its identity as the First Bird (you can just hear the Capital Letters, can’t you?)

Archaeopteryx was discovered in 1861, and could hardly have timed its entry any more finely: just two years after the publication of My Favourite Bathtime Gurgles On The Origin of Species by Mr C. D. of Downe, Archaeopteryx provided proof that missing links transitional fossils existed. With the feathers and wing structure, quite clearly, of a bird, but the long, bony tail and toothy jaws of a reptile, Archaeopteryx was everything it promised – an intermediate between reptiles and birds. Evolution, quite literally, caught on the wing. 150 years have passed since the discovery of Archaeopteryx, and, heavens to Betsy, another fossil arrives to knock it off its perch, as will be evident from a casual perusal of this paper in Your Favourite Weekly Professional Etcetera (accompanied by this characteristically pellucid commentary from Professor L. W. of Ohio.) In its sesquicentennial year, The Taxon Also Known As ‘Ancient Wing’ has been demoted. It is no longer The First Bird, but Just Another Feathered Dinosaur.

Small children the world over will now have to say “I’m Archaeopteryx, Just Another Feathered Dinosaur!” But if they clove to the moniker of First Bird, hanging on to it as if gripping a departing trouser leg, what fossil would stand in its place?

Technically, of course, none.

As you all know by now, because you’ve all read my book Deep Time, you can’t pin a rosette of First Anything on a fossil, because fossils aren’t buried with their birth certificates or prognostications on their evolutionary significance. Not that one could do such scholastic splits with a four-year-old – Crox Minor might however have been more accurate to have said that Archaeopteryx was “the earliest known bird,” implying that there might be earlier ones, but we don’t know about them. But — yes, yes, you’re way ahead of me — this implies that we allow for the fact that no fossil you can find is certainly the ancestor of anything (rather than a cousin in some degree), and, moreover, that you’d know what a ‘bird’ is, such that you can recognize it were it to poo on your head. As Crox Minor reminded me just the other day, freely adapting from Song of Solomon 2:12:

The sound of the turtle-dove is heard in our land.
And the sound goes ‘splat’.

Recognizing a bird nowadays is so easy that even cell biologists can do it, even those who can’t tell a toy of a rifle-wielding penguin from the real thing. How so? Because all birds, from ostriches to orioles to oropendolas to … to …. other birds beginning with ‘o’, are united by a fairly discrete subset of anatomical features. Yes, feathers. And a beak. And a lot of other specialisms of anatomy and physiology, often related to the commission of flight, whether now or (if flightless) in the Town Hall their immediate ancestry.

This wasn’t always the case. Archaeopteryx has been recognized as the First Earliest Known Bird, and not just by four-year-olds, ever since its discovery, even though it doesn’t have all those anatomical features we use to unite birds of more modern aspect. It doesn’t have the short, stubby tail of modern birds. It doesn’t have the big sternal keel. It has teeth, rather than a beak. It has fingers and claws on its wings. Sure, it has feathers – but it’s perhaps not a drastic oversimplification to say that this is practically all it shares with modern birds.

In hindsight, which is a wonderful thing, we can see that Archaeopteryx clung to its perch because it had been glued there largely by default. Until recently, fossils of birds have been so rare that its kindergarten claim has endured. As time went on, more fossil birds were found, but none nearly as ancient, nor as primitive. The fact that some of these latter-day fossil birds retained teeth, and wing-claws, gave retrospective credence to a trend — the view that Archaeopteryx really was a fossil in transition between reptiles and birds, rather than reptiles and something else, or nothing in particular.

The discovery of the first feathered dinosaurs from China was – once can now see with lovely hindsight – the beginning of the end for Archaeopteryx as the First Earliest Known Bird.

Sinosauropteryx, the first of many feathered dinosaurs from China (picture by me, at the Nanjing Institute of Geology and Palaeontology)

This wasn’t really apparent at the time – we were all distracted by a small but vociferous minority insistent that birds and dinosaurs were not closely related. And all the while, more feathered dinosaurs accumulated. By the truckload. Although a wide variety of dinosaurs appear to have had feathers of some sort, most of the featheriness happens in theropods (a large group of bipedal, mainly carnivorous dinosaurs), and in particular among four groups – the oviraptorosaurs, the dromaeosaurs, the troodonts and the avialae.

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Oviraptorosaurs are long, leggy dinosaurs with peculiar parrot-shaped heads, mainly with toothless beaks. Examples include Caudipteryx and Protarchaeopteryx, the second and third feathered dinosaurs to be described.

Dromaeosaurs are mainly small predators with toothy, wedge-shaped heads and prominent sickle-shaped claws on the second digit of the hindlimb. Examples include Deinonychus and Velociraptor.

Troodonts are more small predators with toothy, wedge-shaped heads. Examples include Troodon. Troodontids are renowned for having had relatively large brains for their body size. They had intelligence. Problem-solving intelligence. You can see them working it out.

Avialae includes the birds, and all those dinosaurs more closely related to modern birds than to other dinosaurs. Conventionally, Archaeopteryx has been included in the Avialae, in the family Archaeopterygidae.

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Nobody now doubts that these four groups are closely related, but the pace of discovery has been so great that opinions about their interrelationships have changed rapidly as new discoveries are slotted into the picture. Specifically, each new fossil taxon has a unique combination of features, and when these are taken into account, the overall topology of the evolutionary family tree is liable to change. All the while, though, Archaeopteryx has been safe in the Archaeopterygidae, which has stood as the most basal offshoot of the Avialae.

Which brings us to the latest paper in Your Favourite Weekly Etcetera. This isn’t actually about Archaeopteryx at all, but a hitherto unknown feathered dinosaur named Xiaotingia. When the anatomical features of Xiaotingia were totted up, it perched quite securely with Archaeopteryx, in the family Archaeopterygidae.

So, Xiaotingia is in the Avialae, then?

Well, no. The effect of adding Xiaotingia to the phylogeny is extremely strange – it removes the whole of Archaeopterygidae, including Archaeopteryx, from the Avialae, placing the Archaeopterygidae next to the dromaeosaurs and troodonts. The Avialae (sans Archaeopterygidae) are cousins to these groups together, with the oviraptorosaurs at one further remove. Although the authors are at pains to say that the result is yet fragile and tentative, it is significant, as it is the first ever result to remove Archaeopteryx from the ancestry of birds.

Now, the flurry of discovery is so intense that the next result could well place Archaeopteryx back in the Avialae again. But the damage has been done. The genie is out of the bottle. The cat is out of the bag. Pandora has opened her box. The cliché girrafe has fallen off its unicycle. We are now compelled to acknowledge that Archaeopteryx was only seen as a bird because for almost 150 years it was the best we had. Convention, rather than reason, placed it as the earliest known bird, when more discovery has shown it to be one feathered dinosaur among many.

But enough of such japery – what will be the consequences for our understanding of the ancestry of birds, and the home life of our own dear Queen dinosaurs?

One is that investigations of the origins of flight in birds will not concentrate so much on the (still much disputed) flight capabilities of Archaeopteryx, but will look more broadly at the aerodynamics of a whole range of feathered creatures, whether in the Avialae or not, some of which – but by no means all – would have been able to have flown, or glided. This is all to the good, as having a range of creatures at our disposal will allow us to test various hypotheses about the origin of flight.

A long-standing theme in the studies of the origin of flight is whether flight started from the ground and went upwards, or began in the trees and fell off. Given the close relationship between Archaeopteryx and fast-moving predators such as Deinonychus, the ‘ground up’ view was popular. The scenario was that fast runners would flap their wings and get airborne. The ‘trees down’ view suggests that smaller dinosaurs living in trees would have leapt from branch to branch, gliding, and, eventually, flying. The tiny ‘four winged’ dromaeosaur Microraptor seems an exemplar for this style of locomotion.

But now we suspect that Archaeopteryx is even more of a dromaeosaur than anyone thought – so much that it is no longer any more closely related to birds than Deinonychus – how did flight originate in Avialae? If Archaeopteryx is no longer in the picture, what is?

Enter Epidexipteryx, and, moreover, Epidendrosaurus. These creatures, the scansoriopterygids, represent a relatively newly found group of tiny theropods, feathered but almost certainly non-flying, distinguished by having very long arms and long digits, and, presumably, a tree-living habit. From the first, they were seen as very closely related to Avialae. But, because Archaeopteryx was also still in the Avialae, they were seen as bizarre offshoots with nothing much to do with the origin of flight.

From now on, all that changes.

Scansoriopterygids wouldn’t have looked or behaved like birds – more like feathered squirrels, perhaps. The long fingers of Epidendrosaurus have invited comparison with the insectivorous lemur, the aye-aye. But these strange creatures, according to the latest research, were more closely related to birds than Archaeopteryx, which looked more like a bird, but, it now seems, wasn’t a bird at all.

All of a sudden, 150 years of presumption and assumption about the origin of birds, and the origin of their most distinctive feature – flight – have dropped away beneath our feet. Now, I can just see creationists quote-mining the above (I have highlighted it in helpful red, just to make it easier for them to find) and doing what they usually do, which is to say that ‘prominent evolutionist’ Henry Gee ‘admits’ that evolution doesn’t work. Well, knickers to the creationists, who, I regret to say, will always be with us, like herpes. To me, and to anyone open-minded to discovery, the sentence I have highlighted in red represents the thrill of discovery, of stout Cortez and all his men, silent upon a peak in Darien, on their first look at the Pacific Ocean.

Science.

Ain’t it great?