Mahakala – Size Evolution Preceding??? Avian Flight

Today we’ll look at a flightless bird/dromaeosaurid: Mahakala.
While the paper by Turner et al. (2007), “A basal dromaeosaurid and size evolution preceding avian flight” (pdf). was ostensibly about the origin of flight in birds, it actually says nothing about taxa preceding Archaeopteryx and neither does their included cladogram. Mahakala (Fig. 1, the new basal dromaeosaurid, as everyone knows, is Late Cretaceous and demonstrates the reduction of the wings FOLLOWING Archaeopteryx, its Late Jurassic predecessor. Lots of birds reduce the wings when they are no longer necessary for flight. Makhala is only one more. If you want to read more about the taxa preceding Archaeopteryx, look here.

Mahakala dromaeosaurFig. 1. Artist’s reconstruction of Mahakala omnogovae, a two-foot-long dinosaur unearthed in the Gobi Desert. © Frank Ippolito This is a good-looking representation of an incomplete fossil.
Some thoughts from Xu Xing in NatGeo 
Xu said a combination of birds’ ability to fly and to evolve quickly might have helped them survive.

“Birds mature within one year, and that gives them the means to adapt very rapidly to big changes in the environment,” he said.

My research indicates some birds take more than one year to mature (ostrich = 3-4 years, crow=2 years). Others take less. Reporters may have reported what they wanted to, perhaps misquoting Xu.

Some thoughts from Alan Turner in NatGeo|

“Paleontologists have long thought that miniaturization occurred in the earliest birds, which then facilitated the origin of flight,” said Alan Turner, lead author on the study and a graduate student at the American Museum of Natural History and Columbia University in New York. “Now, the evidence shows that this decrease in body size occurred well before the origin of birds and that the dinosaurian ancestors of birds were, in a sense, pre-adapted for flight.” Not so. Where’s the logic here? Large wings in the Jurassic clearly preceded the small wings of Mahakala of the Late Cretaceous. And, at least some of the more birdy taxa that followed Archaeopteryx were much, much smaller, as we looked at here.

Although paleontologists have shown that birds evolved from bipedal carnivorous dinosaurs known as theropods, fossil evidence of miniaturization and other characteristics leading to flight have been sparse. Not really… just not studied much. As in pterosaur precursors, everyone is looking for what they think they should find and ignoring what evidence is already out there.

Now Mahakala is providing the first signs of some of these early evolutionary steps. In particular, while other dinosaurs of the Cretaceous Period were evolving in favor of increased body size, Mahakala represented a progressive step towards miniaturization of body forms that would be necessary for feathered dinosaurs to eventually take flight. Not really. Mahakala is about the same size as Archaeopteryx.

“Flight isn’t an easy thing, because you are, in effect, countering the force of gravity,” said Turner. “Being really small appears to be a necessary first step. Other groups that evolved flight, such as pterosaurs and bats, all evolved from small ancestors.” Really? And those ancestors are…????? (Really, I know pterosaurs and bats evolved from small ancestors, see bats here and pterosaurs here. But those specimens are not acknowledged by traditional paleontologists. So, I’m wondering which taxa Turner was referring to? Or hoping for?)

Traditionally it’s been thought that the earliest birds were the first theropods to become really small. With the discovery of Mahakala we were able to show that this miniaturization occurred much earlier.” Ahem. You mean extended much later to the Late Cretaceous. Who was editing/proofing this copy??

 Mahakala shows that dinosaur size decreased progressively as they evolved toward birds. While this is something that has long been expected, Mahakala provides the first empirical evidence of this phenomenon. Again, the charts included in the article say otherwise, both chronologically and phylogenetically.

But what about….?
Now, I realize the Early Cretaceous tritosaur lizard, Huehuecuetzpalli, chronologically succeeded Triassic fenestrasaurs and pterosaurs. However, phylogenetically a sister of Huehuecuetzpalli preceded Triassic fenestrasaurs and pterosaurs. Makhala does not phylogenetically precede Archaeopteryx, according to the published charts, which show  no pre-Archaeopteryx taxa. That’s just not right when other studies have found a series of taxa with a gradually accumulating list of bird traits.

What we know

The origin of birds cladogram.

Figure 2. Click to enlarge. The origin of birds cladogram. Large clades are colorized. Red arrow points to the place on the chart where we will someday place the closest known ancestors of Archaeopteryx.

At least this cladogram (Fig. 2) shows some Triassic and Jurassic taxa (not that that’s required). In any case, we’re still looking for the precursor to Archaeopteryx that is not a closer precursor to oviraptorids, alvarezsaurids, etc. One of the keys to figuring out flightless or flapless taxa that came before or after is the design of the coracoid, as we looked at earlier with the precursors to pterosaurs that were flapping long before they were flying. With a locked down coracoid flapping can progress. With a sliding coracoid (the plesiomorphic condition), not so much.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Turner AH, Pol D, Clarke, JA, Erickson GM and Norell M 2007. “A basal dromaeosaurid and size evolution preceding avian flight” (pdf). Science 317 (5843): 1378–1381. doi:10.1126/science.1144066PMID 17823350.

About these ads

One thought on “Mahakala – Size Evolution Preceding??? Avian Flight

  1. Sorry for not getting back to your second email asking what I would change on this cladogram (which is the Baysian analysis of Xu et al.’s 2011 matrix). Things have been very busy here. One of the many issues with the analysis is the absence of Mahakala itself, and nobody’s ever analyzed other Mesozoic coelurosaur matrices with Baysian assumptions, so how this one compares to others is unknown (e.g. in the maximum parsimony analysis, Archaeopteryx is a deinonychosaur and Haplocheirus is an alvarezsaur). In any case, if Mahakala is a basal dromaeosaurid as recovered by Turner et al. (2007), it adds evidence that the basal paravian (and thus the ancestor of Archaeopteryx) was small, since Mei, Microraptor and Rahonavis were too. And now we have Anchiornis and Xiaotingia to add to that trend.

Leave a Reply

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

WordPress.com Logo

You are commenting using your WordPress.com 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