[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Sit-and-wait predation (was RE: the maniraptoran wrist)




On Sunday, September 15, 2002, at 12:09 PM, Williams, Tim wrote:

Brian Lauret wrote:

The best explanation I've read so far.
(The idea that endotherms could be sit-&-wait predators is hard to
believe.

Well I'm sorry but aren't bird species like kingfishers,puffbirds and frogmouths sit-and-wait-predators?

Yes - and many other birds besides, including many birds of prey (Falconiformes). When the ambush attack is launched from a tree, the technique is called "perch hunting".

Certain primates also exhibit sit-and-wait preadtion.  I believe it's
well-attested in tarsiers, for example.

I'm not sure if this is what Greg is talking about, but it seems to me that if you look at predators which use rapid-extension mechanisms to catch prey, they aren't chasing, they're slowly stalking or absolutely still when they strike, maybe lunging with the body, but not running. Examples might include mantids, mantis shrimp, predatory inchworms, dragonfly nymphs, snakes, frogs, chameleons, and bolitoglossid salamanders. Fishing birds like herons are another example.
Anyhow dromaeosaurids are beside the point- the semilunate carpal block is not a dromaeosaurid synapomorphy, it's a synapomorphy of a group including oviraptorosaurs, therizinosaurs, dromaeosaurs, troodontids, and Aves. The context in which it evolved is a shared common ancestor of these animals, not Dromaeosauridae themselves. Whatever dromaeosaurids did with their carpus, it was either a retained primitive behavior or an exaptation of a preexisting structure.


re: flight adaptation, wing folding is perhaps just as useful for a glider as a flier; Draco can fold the wings alongside the body; flying squirrels retract the cartilage that supports the wingtip alongside the forearm. There are some pecularities to how the semilunate functions, though, which are consistent with it being an adaptation for either continuous vortex or ring vortex powered flight modes (I briefly described why at SVP last year) , but these are inconsistent with other hypotheses (e.g. Dial's incline-running hypothesis). These are, along with the presence of well developed remiges and retrices in oviraptorosaurs, perhaps the strongest evidence we have for adaptation for powered flight in these animals.
Wing assisted incline running has other flaws- the basic setup of the avian wing (the venetian-blind structure of the remiges) tends to let air through on the upstroke rather than maintain wing integrity, (see the fiigures in RA Norberg's paper in the Archaeopteryx volume for an example of this in action); while it's possible to generate lift on the underside of the wing its basic design is one specialized for producing lift on the upper surface of the wing. Perhaps the biggest problem is that flapping the wing up to generate a down-and-forward aerodynamic force predicts a well developed ability to raise and retract the humerus. Oviraptorosaurs and deinonychosaurs however had large, ventrally situated pectoral musculature attachment sides well designed to depress and retract the humerus (again, precisely what one would predict if their ancestors were fully volant powered fliers) and no obvious muscular specializations for humeral elevation. Given that the scapula isn't expanded for added deltoid musculature, and the coracoid seems incapable of supporting a supracoracoideus pulley, it seems more likely that the ability to generate thrust on the upstroke is a derived characteristic of advanced birds like enantiornithes and ornithurae, rather than the primitive condition from which flight was evolved, and that a well developed downstroke and weak upstroke is the primitive condition. So testing against the fossils, the hypothesis doesn't seem very consistent with what we see, in my opinion. Anyways, this whole "how did the flight stroke evolve" thing ignores that (a) Ulla Norberg has already provided a plausible, workable model for addressing this, and (b) the whole issue of where wings evolve (i mean ya gotta have something to flap in the first place, right), which is in arboreal climbers, and occasionally fish, but not in cursors. There are a number of other reasons (Rayner's paper in the Ostrom Symposium volume hits almost all of them I can think of) why it looks more feasible to evolve powered flight from gliding to a continuous vortex (or perhaps ring vortex) gait.


re: archaeopteryx remiges I went back and looked and I had misinterpreted the shadows, the ventral surface of the wing is the one preserved so it is possible to see the anterior and posterior of the vane in the first two or three feathers on the leading edge of the wing where they aren't overlapped by the more anterior feathers.