All over bar the Xiaotingia?

The title’s really to help remember the name of the new dinobird – the new find actually adds a little subtlety rather than clarity, so it isn’t all over bar the shouting thanks to this fossil – though actually much of the coarse-resolution picture was completed some time ago. (Not that you’d know it from reading the pundits.) But as usual the “old guard” need the subtleties pointed out for them. In their view the new Xiaotingia zhengi* has changed the shape of the tree so Archaeopteryx is no longer the “first” “bird”. Instead, it’s now the first of the deinonychosaurs – the tribe most agree includes Troodon and Velociraptor. (I’m not italicising Latin names today 🙂 .)

A number of points arise:

1: Deciding what is meant by “Bird” is actually less arbitrary than one might think, since it makes things a lot easier if you pin it to things that had powered flight. Or more precisely, those that could take off from the ground. (Some may say it’s ridiculous to try for this since how can you ever know – but it’s easier to know that, than to get insights through cladomising these particular creatures.) It’s likely rather than possible that just after the very first ability to develop wing thrust appeared, multiple lineages enhanced this to the point where ground launch became possible (following the inevitable tree-launch prior to this). My definition of ‘Bird’ would include all lineages that eventually acquired ground launch ability, along with their last common ancestor (likely to be Anchiornis or its ancestor). For those familiar with Confusiusornis/Sapeornis and such-like, I think I can see their lineage here alongside and somewhat separate from the enant birds/Archaeopteryx/deinonychosaur/arcto/ovis/modern birds lineage, though maybe the enant birds should also be slightly detached from that second group. The paper doesn’t say this new Xioatingia is the first ‘bird’. In fact it says it’s over in the deinonychosaur group, but also that the true birds are in a sister group. (See graphic at end for their tree.)

2: Most of the dinobird mob only have one tool – the cladogram: a family tree generated by an oversimplistic computer trick. It can be useful but it’s tricky to interpret at the best of times, and they have no skill with it and almost never any behaviour or relevant qualifications to suggest they might have. That is their hammer, and to them the whole world looks like a nail (this isn’t news anymore, but it’s still important). There are too many parallelisms in dinobird evolution and the skill lies in choosing which features to give weight to. (And, also if you can call it a skill, in only putting close contemporaries and relatives in to the cladogram together. Mickey Mortimer has had a stab at this but has some way to go before making it work.) And the cladogram has “poor support”, which means the metrics emerging from the cladogram give even its producers little grounds for confidence… but even when they’re a bit more encouraging those metrics can’t be trusted. It just so happens ‘the tree de jour’ says Archaeopteryx was ancestral to the dromaeosaurs, but with trees constantly appearing, one was bound to say it one day. Greg Paul seems pleased they’ve tipped their hat to him over this but when a bunch of drunks say they now like you, you don’t have much genuine cause to be proud. Any idiot could see the similarity between Archaeopteryx and dromaeosaurs like Deinonychus indicated a close relationship, and it’s only now after 40 years that the masses have noticed which came first. I had the original version of this early parlogram out about five years ago which explained the trees much better: (click image or open it in a new tab to see details.)

(My treelet positioning some recently discovered early forms appears below.)

Even though the cladists now have the Archaeopteryx/deinonychosaurs together at last it doesn’t mean they’ve got anything else right. The other group in their cladogram supposedly containing proper birds and their early relatives is rubbish. They’ve got things in there that descended from dromaeosaurs (ovis and modern birds) along with things that branched off just before Archaeopteryx such as the enants and Confuciusornis, and also things much further removed such as Epidexipteryx. In their paper they do warn of the danger of cladogram distortion:
“This low support is partly caused by various homoplasies, many of which are functionally significant, that are widely distributed across coelurosaurian phylogeny.”
They cite Holtz on this but need to go way beyond his appreciation of the problem.

3: Judging flapping power and hence flying ability. This refers back to 1. The main flight muscles have been poorly understood, and have led people to doubt whether certain things could flap the wings up well since the up-flap machinery was often not present – at least in the way they appear in today’s birds, but bats don’t have the version modern birds use either but they can still power the up-flap. But the key is the muscle attachment points on the actual upper arm bone (the humerus). Look for them, and if they were well developed, then wherever the other end of those muscles attached to, it must have had good down-flap ability (or down-flapability!). Those down-flap muscles didn’t attach to the breastbone in quite the same way as in modern birds but they probably attached to the wishbone or ligament sheets off it. And with a good down-flap, powered flight was indicated whether the up-flap mechanism was just different or even largely absent, since once you’re flying, the up-force can come largely from lift, at least in birds above a certain size.

Related to this is guessing how high the wings could be lifted above the back. Joint mobility is not understood as well as people like to think, and the uplift angles are often underestimated. We know this because features like well developed down-flap muscle attachment points, combined with huge and perfectly-evolved flight feathers to go with them, tell us powered flight must have been present in types like Microraptor, even though the shoulder joint might not look terribly promising for uplift. The down-flap muscle attachment points on both humerus of Xiaotingia seem to be largely obscured in the fossil, but what we can see suggests they weren’t as well developed as on Anchiornis, which itself didn’t have asymmetrical primary feathers suitable for producing thrust. But Anchiornis’ muscle attachments actually were beginning to closely approach the shape and size of Archaeopteryx’. In the end though, the new fossil doesn’t seem to have the muscles for powered flight.

What’s the sensible view on Xiaotingia then? The tail wasn’t preserved but actually that would probably only give clues as to the closeness to Archaeopteryx and Anchiornis. The hand seems much shorter than those two relatives’ but it’s hard to rule out the possibility that it had the power of flight in its ancestry but lost it. It’s exact origins are uncertain but if it’s Late Jurassic as people think, it will fit well with most people’s ideas. It’s not as interesting as Anchiornis which certainly does seem to be the stage prior to Archaeopteryx, and which suggests Archaeopteryx actually did evolve powered flight for itself. Could Xiaotingia have been the stage prior to Anchiornis? (click image or open it in a new tab to see details. Drawings by James Field, commissioned by me.)

Possibly. Could it have been a pre-flyer ancestral to the enants or the confs? You’d have to check out all it’s little details before guessing that one but if it wasn’t, there will have been things much like it that were. Was it a flightless descendant of Archaeopteryx on the way to becoming a troodont then? It isn’t troodontish enough for that and it certainly isn’t dromaeosaur enough to be an early ex-flying dromaeosaur. But it might conceivably have lost flight on the way to becoming a grounded lineage that died out.

My guess is that it was related to an ancestor of Anchiornis if it wasn’t one itself. However we knew such things existed already so we’re not much wiser. Anchiornis already had a surprisingly Protoavis-like head, and although Xiaotingia’s skull is rather fragmented the authors reconstruct it (in their illustrated cladogram) as being very Anchornis-like, so I have no problem with the new fossil X. being on or near the line from Protoavis to Anchiornis.

A final notelet is that if you take the deinonychosauria treelet (the lower half) from the author’s cladogram seen on the left, give it a new root in the arc leading to Xiaotingia, and dump their top half containing “avialae” and ovis, it would probably be right for the types it would now include. However their structure where Epidexipteryx seems to have lost modern feathers and evolved a very inferior design of them is unbelievable.

* Xing Xu, Hailu You, Kai Du and Fenglu Han (28 July 2011). “An Archaeopteryx-like theropod from China and the origin of Avialae”. Nature 475: 465–470.

This entry was posted in Bird evolution, dinosaur evolution, The Secret Dinobird Story and tagged , , , . Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s