Theropod trackways as indirect evidence of pre-avian aerial behavior (free pdf)

[Ben Creisler]

Ben Creisler

bcre...@gmail.com

A new paper:

Free pdf:

T. Alexander Dececchi, Kyung Soo Kim, Martin G. Lockley, Hans C.E. Larsson, Thomas R. Holtz Jr., James O. Farlow, and Michael Pittman (2024)
Theropod trackways as indirect evidence of pre-avian aerial behavior
Proceedings of the National Academy of Sciences 121(44): e2413810121
doi: https://doi.org/10.1073/pnas.2413810121
https://www.pnas.org/doi/10.1073/pnas.2413810121

Free pdf:
https://www.pnas.org/doi/epub/10.1073/pnas.2413810121

Body fossils set limits on feasible reconstructions of functional capacity and behavior in theropod dinosaurs, but do not document in-life behaviors. In contrast, trace fossils such as footprints preserve in-life behaviors that can potentially test and enhance existing reconstructions. Here, we demonstrate how theropod trackways can be used as indirect evidence of pre-avian aerial behavior, expanding the approaches available to study vertebrate flight origins. This involved exploring the behavioral implications of a two-toed Cretaceous-aged theropod trackway produced by a small, bird-like microraptorine moving at high speed. Applying first principle running biomechanics, we were able to conclude that the trackway is atypical, indirectly evidencing pre-avian aerial behavior. This trackway documents the evidence of wing-assisted aerodynamic force production during locomotion, supporting a broader distribution of this behavior than currently known. These findings support previously proposed aerial behavior in early bird-like theropods, showing how trackways will help to deepen our understanding of theropod flight origins.

***
News:

Researchers Solve Mystery of ‘Dinky’ Dinosaur’s Unusually Long Stride
UMD paleontologist Thomas R. Holtz Jr. helped recreate the movements of a bird-sized raptor, shedding new light on the origin of flight.

https://cmns.umd.edu/news-events/news/thomas-holtz-helps-solve-mystery-microraptorine-footprints


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[Tim Williams]

There's been some pushback on this work, including from some footprint specialists.  For me the findings are entirely consistent with the anatomy (musculoskeletal and plumage) of _Microraptor_, especially smaller individuals.  

The authors write: "we cannot make any claim linking a specific aerial behavior to the trackway (e.g., launch, landing, accelerated downward glides, or wing-assisted stride extension)".  But if the trackway records a launch, this makes me wonder whether _Microraptor_ needed a running take-off - I thought a stationary takeoff would be possible for _Microraptor_, initiated by a vertical leap (no running required).  So if the wings were engaged during running (such as to extend stride length), then any takeoff would be a separate behavior.  The wings could have multiple aerodynamic functions (wing-assisted cursorial locomotion; powered flight), as well as non-aerodynamic functions (such as display).

More broadly, the findings are consistent with the aerial behaviors inferred for small pennaptorans in general: Small, terrestrial/cursorial pennaraptorans with large wings were capable of an array of aerial behaviors, with a certain subset of these crossing the threshold into true powered flight.  As the authors put it: "Thus, the origin of flight may not be a simply binary of “can or cannot” but a spectrum with different lineages utilizing aerial locomotion differently to suit their own needs".  

Those folks arguing for a scansorial/arboreal origin of avian flight might feel cheated by the fossil record, in that footprints of small theropods climbing up trees don't get preserved.  But considering that there's no anatomical support for _Microraptor_, _Archaeopteryx_ etc being capable of climbing trees, it's a moot point.

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[Mike Habib]

I helped the authors with some early drafts of the manuscript. I can say that they don’t really think it’s a launch, but they didn’t want to get into a reviewer battle about it, as it’s not critical to the paper. As such, they just covered all the bases.

You’re absolutely right that the data and analyses to date support leaping launch in Microraptor. That could change with new data, of course, but that’s the current best estimate. 

The thing is, if someone were to recover a result in which Microraptor was not capable of leaping launch, then the correct conclusion would be that it was flightless, not that it was a running launcher. This is also true if one holds to a gliding flight model for Microraptor: arboreal, obligate gliders are all leaping launchers.

The myth seems to persist that when animals are “weak” flyers, or very large, that they have to “resort” to running launch. The reality is that running launch is a *specialization* for dealing with an extremely compliant launch surface: water. For some species, running launch is facultative: they leap from land and run on water (gulls, for example). For others, they are so specialized for swimming that they run to launch from all surfaces or simply cannot launch from land at all. And of course, there are other ways of launching from water (see: mallards, osprey, etc), but running takeoffs are the more common specialization. Probably because it simultaneously solves the compliance problem and the takeoff-with-diving-hind limbs problem (birds with stout hind limbs placed far back on the body essentially cannot jump).

Cheers,

—Mike

On Oct 22, 2024, at 11:13 PM, Tim Williams <tij...@gmail.com> wrote:



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[Tim Williams]

Thanks Mike.  Wow, you certainly know your birds.

Yes, rather than being an ancestral 'pre-avian' behavior, running takeoffs would be a derived avian behavior, limited to certain birds (typically semiaquatic/aquatic) on water.  

Similarly, although used for very different reasons, WAIR would be another derived/specialized avian behavior (used by juvenile galliforms in this case), rather than an evolutionary precursor to true powered flight.

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[Ethan Schoales]

So how do you think birds originally evolved flight? Something must be an ancestral behaviour.

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[Tim Williams]

Ethan Schoales <ethan.s...@gmail.com> wrote:

> So how do you think birds originally evolved flight? Something must be an ancestral behaviour.

In general, I favor a terrestrial/cursorial origin of flight (as opposed to arboreal/gliding) - because as I see it, this is what's supported by the current weight of evidence.  Rather than nominating an individual behavior that led directly to flight, I'll defer to the published literature on this point:

"Specifically, feathered forelimbs, coupled with a nascent flight stroke, may have contributed subtle, but evolutionarily advantageous performance benefits to high speed maneuvering and braking and balancing during prey capture. Even slight performance enhancements to vertical and horizontal leaping may have had highly positive adaptive effects."

Dececchi TA, Larsson HCE, Habib MB. 2016. The wings before the bird: an evaluation of flapping-based locomotory hypotheses in bird antecedents. PeerJ 4:e2159 doi.org/10.7717/peerj.2159

[Gregory Paul]

OK, there are a number of issues with this situation that appear to be being missed. 

For starters, why is this being identified as a microraptorine? This dates from the Albian, 10-20 million years later than the middle-late Barremian and maybe earliest Aptian Jehol microraptors with big foot feathers. These issues were not discussed. It could be any sort of a two toed deinonychosaur. Probably a juvenile considering its extremely small size. May not be able to achieve adult flight. May be partly secondarily flightless. 

The trackway is very peculiar. The apparent speed if via purely ground locomotion appears much too high for something its size. But if the creature had large wings it should having been moving fast enough to be in full flight rather than still running. It's a head scratcher. 

Too many auto assumptions have been made as to the identity of the trackmaker, it's characteristics, and what it was doing. May have little or nothing to do with the much earlier foot feathered microraptorines and cannot be relied on to tell us little about what they were up to. 

GSPaul