Friday, August 26, 2016

Microraptor Shenanigans Part I: Hind Wings & Stealth

Boy I really stepped in it now...  Microraptor. For a long time I avoided paraves, Microraptor, the whole dino-bird subject because; there is a lot of talk about maniraptorans on the web already; I enjoy focusing on "meatosaurs" (and still do); and, well, quite literally this whole backwater of quasi dinosaur, quasi bird conundrums literally ruins people. I mean it is a war zone out there folks. Seriously now, academic career types - not just free spirited blogger/idea spinners like yours truly - have gotten their heels so dug in on various sides of the debate it is quite astonishing. The stakes are high in this subject of birdy origins and history will see winners and losers... So why not dive in with Microraptor probably one of the most celebrated, controversial, and perplexing dinosaurs and certainly maniraptorans of the last 25 years? I would definitely put this animal in an unholy triumvirate with Deinonychus and Archaeopteryx in terms of paradigm shifting, scratching your head, game changing iconoclastic status. It really is an odd bird and one we still have loads of unanswered questions about.

closeup cast credit Hiroshi Nishimoto CC 2.0

So for this piece I am mainly going to side step questions about the aerodynamic efficiency of Microraptor. I don't have a wind tunnel, nor a model to test it in, nor an advanced understanding of aerodynamics. To summarize my current opinion on Microraptor's flight capability I am hedging towards rather limited flight capability - possibly more of a burst flyer of short distances, great maneuverability, with some gliding ability though far from ideal due to all that drag it incurs. I think that the biplane model with the legs spread wide is far from established as I still have questions about ,well, could it anatomically splay its legs? So for now I am going to go with a base line of fairly limited flight capability with pretty good maneuverability and pitch control via the tail and hind wings. I am not wedded to this interpretation and am open to new insight.

I will be working from this point in my attempt at a reasonable lifestyle analysis of this critter. I think it fairly conservative and in line with much of the research into this animal.

William Beebes' prescient 1915 "Tetrapteryx" 1915 public domain

Now onto another very contentious issue: terrestrial or arboreal? In order to untangle why this issue is so contentious - I don't think it need be actually - it is worth looking at this question through another lens. The issue of terrestrial versus arboreal in Microraptor has double implications because whether or not this animal was terrestrial or arboreal provides a robust layer of argument to the issue of the competing ground up or trees down hypotheses in the origin of flight in aves proper. And we all know how contentious that issue is and how various factions have their heels dug in on either side of the debate.

The answer to the question of arboreal versus terrestrial habits in Microraptor is quite literally that there is no question of arboreal versus terrestrial in Microraptor's head. And it is in the headspace of Microraptor that we need to get into here - not the head of a tenured professor with a life times' academic stake in the question of ground up or trees down origin of flight. That is quite literally the last head we need to get into in this question. We need to get into the head of a 1 to 2 kilogram predator of small things. An animal that goes recklessly and relentlessly where its prey is. If that prey is roosting 20 meters up a giant old growth ginkgo tree by golly that is where Microraptor is going. If prey is swimming and swarming in the shallows of a lake Microraptor is having a go at that prey. If prey is scurrying from the safety of a burrow in the still of the night Microraptor is right there to greet them. If prey is secreted away deep inside the hollow of fallen tree trunk, Microraptor is sniffing out that quarry and going in that rotten log and making their life hell. Microraptor is kicking ass and taking names and if you are a small swarming, swimming, flying, or scurrying thing and living in the same neighborhood as Microraptor you better watch your back because Microraptor is coming.



"Lock up your children the axeman is coming"

Yes in the trees, in the shallows, in the little cracks and crevices of the environment where the small things hide out Microraptor is going there. If Microraptor could evolve a human intelligence and humour and knew of our debates concerning it Microraptor would be laughing at the pedantic back & forth of terrestrial versus arboreal because Microraptor goes where Microraptor wants to go. Microraptor does not see a sharp division of terrestrial versus arboreal realms. Microraptor sees a 3 dimensional world where the subterranean, aquatic, terrestrial, arboreal, and aerial bleed into one another and Microraptor bleeds into them and exploits them all as it sees fit.

Seeing the world in such a 3 dimensional matrix of positional vortexes is not at all dissonant with how modern supple fingered, clawed, predatory, and small bodied tetrapods exploit and utilize their environment. We see echoes of this multi-functionality in mustelids, varanids, viverids, and probably most intuitively felids, especially our own house cats. I think it was Thomas Holtz who compared microraptors to small felids and I think the comparison is very apt. If you have ever wondered why your house cat has a penchant for seeking high ground - both to hide from threats and survey potential prey - you are seeing echoes of Microraptor in fluffy. I can also recall Darren Naish mentioning in several blog posts that although microraptorines (and other dromies) did not have highly specialized arboreal capabilities that does not mean we should exclude them from scrambling around in the trees, bushes, i.e. exploiting the >shrubbery<.

Of course the bauplan Microraptor started with is different from mammals and lizards. Lacking the low body plan as well as the supple and flexible limbs and torso that allow other tetrapod predators such as lizards and mammals to excel at this particular eco-niche microraptorines evolved alternative solutions to the problems of navigating a cluttered, three dimensional eco-space. The wings on the forearms served to break falls that occurred at elevation, aide in terrestrial balance and pursuit, and provide gliding and flight. Note that I am not concerned with the question of >what< stimulated the evolution of the wings in microraptorines merely how such wings would be useful for a theropod navigating three dimensional space in the pursuit of small game. The >what< of this question (i.e. what caused evolution of wings on the forelimbs) may never be boiled down into a single causal factor but a multiplicity.

Getting back down to earth, literally, I want to address a bit of non-truth that has consistently swirled around microraptorines for a while now. That piece of misinformation - a truism that has always been asserted but never actually demonstrated - is that their "hind-wings" severely hampered terrestrial movement. I see this assertion upheld again and again in both scientific and popular discussions. The idea being presumably that such flight feathers would be a hindrance in locomotion, get caught up in vegetation , and generally cause a kerflunkle for any microraptor trying to just... walk around.  That animals with relatively long cursorial looking rear legs; that plot out very much like ground birds morpho-metrically; that this animal could not run or even walk well points to a very mal-adapted and cumbersome beast. Caught in sort of an evolutionary purgatory - not yet an accomplished flyer, glider or climber as well as a cumbersome terrestrial mover - such an animal begs for a quick extinction. Seriously this notion of a terrestrial limited Microraptor should have always have been a non-starter. Kind of like sauropods that could not walk on land. Or giant pterosaurs that could not fly.  The kind of "evolutionary experiment" that paleontologists like to posit but can't find any examples of in the real world. Because in the real world their overly cumbersome traits would select them for Darwinian elimination before they even had a statistical chance of entering the fossil record.

Again, we need to get back to what it means to be a predatory small bodied, hot blooded terrestrial tetrapod. You are not just an OKish hunter that sort of blunders through the environment hoping to get lucky enough to snag a meal or two. Such an animal would be selected for extinction quite readily. Nope you are an efficient, ruthless, and relentless hunter. Just look at the pedigree of hunting prowess in small felids and especially small mustelids (i.e. "weasel" types). The true acme of hunting prowess in mammals is not to be found in large canids or felids - they fail all the time. Small bodied predators have to catch a lot more due to their relatively higher metabolism and the relatively small size of their prey. They can't afford to only have a 5% success rate. They are hungry and they are coming. No reason to assume among predatory theropods microraptorines were not the acme of theropod hunting prowess. In fact evolutionary pressure and comparison to modern small bodied tetrapod hunters would dictate that they are.

I mean, come on now, just look at those legs.

Specimens by Jaime Headden CC3.0
 (User:Qilong) - http://qilong.deviantart.com/art/Microraptor-Skeletons-27326378

Microraptorines could move and move around well on the ground. Assertions that the feathers on the legs would cause undue hardship and toil are simply not meted out when viewed against the panoply of abuse modern birds put their flight feathers through. I was recently watching the flock of California condors that are maintained at the Santa Barbara Zoological Gardens. Splendid birds and you really get a sense for the nuanced social dynamics at play when you watch them for some amount of time. One of the birds sort of crash landed into the branches of a large oak tree in the enclosure after a small skirmish. Mind you this was a California oak (not sure the species) and it had the harsh, spiky leaves of a oak adapted to the drought prone, semi-arid climate of California quite unlike the soft rounded leaves you might encounter on oak leaves in more verdant climes. Anyways, this was no soft landing that the condor made into the tree and it was totally caught up in the brittle, spiky canopy. The bird (second heaviest flying bird in North America) then proceeded to use its wings (e.g. its flight feathers) to literally "swim fly" through the brambly and spiky oak foliage until it could get up to a solid enough limb to perch upon. I got the feeling this was no extraordinary ordeal for the bird and had probably done it a number of times. Not a flight feather lost or even seemingly damaged. Long story short flight feathers - whether on the arm or leg - are not the wimpy structures sometimes assumed to be and are  deeply anchored within the dermis or even into the bone and can both receive and deliver substantial abuse and punishment. Microraptorine leg feathers were doubtless anchored well into the leg . Furthermore there is no reason that the feathers on the leg would not have been able to fold up against one another as the leg moved or crouched. It is also entirely possible that such feathers could be moved so as not to scrape against the ground. This is no different than what the flight feathers on the arms of birds and maniraptorans presumably are able to do.

Anyone who argues that bird feathers - especially the pennaceous "flight" feathers whether on the legs or arms - are prone to falling off, getting damaged, or just inhibiting to violent, physical interactions just has not been paying attention to what birds actually do.



Now that we put that whole notion of "could not walk well" to bed for maniraptorines I want to dive a bit deeper into potential uses for the leg feathers. I want to stipulate that these ideas are not mutually exclusive from any of the flight scenarios that people have posited for these animals. Remember that I am not concerned with could or how well Microraptor flew but creating a lifestyle portrait of a small animal killer. Being a specialized small animal killer is an easy inference given the diversity of, well, small animals found in the guts of Microraptor.

To make my case I first want to bracket Microraptor between several other theropods that sported leg feathers. Yes leg feathers were quite en vogue look for several theropods back in the day and not all of them were microraptorines.

Pedopenna daohugouensis. Remember this guy? Yeah I don't either. Just another in the long and confusing line of maniraptorines getting pulled out of the ground in China, I would be lying to you if I don't constantly have to 2x check the names of these guys. Except this species is one that you should take note of. First of all it predates Microraptor and the other Liaoning/Jehol fauna as it dates to somewhere in the mid-late Jurassic (age of the Daohugou beds are debated) but more importantly for our purposes here the legs are feathered - they had hind wings!! The legs are all we really know of this animal and according to wiki: "the long pennaceous feathers of the foot... differ from those of animals like Microraptor. Pedopenna hind wings were smaller and more rounded in shape. The longest feathers were slightly shorter than the metatarsus at about 55 mm (2 in.) long. Additionally, the feathers of Pedopenna were symmetrical, unlike the asymmetrical feathers of some deinonychosaurs (including Microraptor) and modern birds."

The symmetrical feathers of the hind wings in Pedopenna (and Anchiornis btw) are interesting as this suggests that they did not have a use aerodynamically. The hind wings are potentially vestigial and point to a flighted or at least gliding ancestor for Pedopenna (Xu & Zhang, 2005). This invokes the strong potential for "microraptorine" style flyer/gliders going back into the mid-Jurassic - very cool. This also suggests that flight could have evolved, got lost, got regained, and evolved in multiple parallel branches over a fairly long span of time. However I am not quite so sold on Pedopenna and Anchiornis being secondarily flightless. If this was true why would they so quickly lose  asymmetrical feathers? Would not asymmetrical feathers still be of use in short glides and other such aerial ventures? I mean, these were not large animals so retention of some aerial ability would still be of use in gliding one would presume?

Time to highlight the seldom mentioned fact that the giant compsognathid Sinocalliopteryx also is noted for non-pennaceous feathers along the back of the metarsus and longer ones on the back of the thigh. These feathers did not form a hind wing nor are compsognathids considered to be on their way to flightedness or secondarily flightless. Yet there was some congruence in evolving feathers along the trailing edge of the lower legs in these disparate animals.

To add a further layer to the enigma of hind wings and lower leg feathers is the amazing microraptor on steroids, Changyuraptor a turkey sized double winged and exceptionally long tailed microraptorine that, despite its size, still points towards being somewhat flight capable or at least gliding.

turkey sized microraptor Changyuraraptor credit Emily Willoughby CC4.0
Although I have my doubts about how long this animal could sustain powered flight, it is always good to remember that wild turkeys and peacocks do fly and they also go into trees as well.



I do think it pays to take a look at these flying large galliforme videos. Notice the seemless transition from an arboreal start in the trees to a short distance flighted phase effortlessly transitioning to an efficient terrestrial phase. Even at this size a turkey sees, interprets, and inhabits its world in a 3-dimensional matrix. I guess this "ground bird" with no obvious arboreal adaptations never got the message that it could not use the trees as it sees fit. Not impressed with flying turkey how about flying peacocks - another "ground bird" that goes where it wants to.


And watch these peacocks in China seemingly sail down en mass from elevation. Reminds me a lot of those turkeys sailing in from the tree tops in the earlier video. Very cool.


We can look at Microraptor not as an isolated hind wing animal but one that slots, or is bracketed in a sense not just phylogenetically but behaviorally,  between a number of hind wing or at least feathered leg/metatarsal having theropods. And I think it important to speak towards ecological and behavioral congruity in these theropods because, irrespective of phylogeny, similar lifestyles often confer similar adaptations by convergence.

And the unifying behavioral characteristic that unified microraptorines, Anchiornis, Pedopenna, and Sinocalliopteryx is that they were all terrorists of small game. I have actually discussed Sinocalliopteryx gigas before here. Sinocalliopteryx was obviously not arboreal and it was not flighted, near flighted, or secondarily flightless. But it was really good at snacking on animals that were flighted and/or more inclined towards aerial/arboreal capabilities than it was hence the belly full of dromie leg & enantiornithines. Because of this the authors suggest that it was a stealth hunter, a notion that is hard to dismiss.

Unpacking what it means to be a stealth hunter with a theropod bauplan I believe is key to illuminating what stimulated the evolution of hind wings.

When you are a hunter of small things that means that you have to go to where the small things are at. That implies negotiating tight spaces and deep, dense vegetation. If you are negotiating this eco-morphospace as a small mammal, lizard, or better yet snake - no problem you are already low to the ground with supple legs and torso. But if you have the stiff torso and the tall stiff action limbs of a theropod there is less of an exaptation for maneuvering in such tight places which means there is even more impetus on stealth. A theropod stealthy stalker of small game has to get as close as it can as quietly as it can because whatever you are stalking - chances are that once it detects you it has the better capabilities in navigating tight spaces (or just flying or swimming away). And so imagine the stealthy stalking ability of modern herons but imagine it occurring not while wading but while in deep vegetation. One of the problems such an animal would encounter is curtailing or muffling the disturbance and noise that would occur by walking on and brushing against vegetation/leaf litter/brush with those long, stiff action hind legs. A soft, padded foot would help with muffling footfalls. Likewise a trailing line of feathers along the back of the leg and going down the metatarsus to the foot would potentially muffle or curtail the audible "snapback" of brush that occurs when a leg moves through vegetation - especially dried and brittle twigs, stems etc. etc. Anyone who has done any significant off trail bushwhacking knows what I am talking about. As a tall biped we humans are especially noisy, cumbersome, and just maladapted for stealthy stalking through dense brush. In fact if you are going through dense brush in a group one will often find it necessary to grab a branch or stem that might in fact "snapback" and hit the person following you!! Theropods - which share with us a relatively tall, bipedal build - would face a similar dilemma of "noise pollution" as they moved through dense brush. Having a rearward guard of feathers along the back of the leg and foot would allow vegetation to be "caught" and eased back as the leg stealthily moved through it.

That modern ground birds lack such hind wings or even feathers on the back of their legs may in fact have more to do with a lack of birds engaging in cryptic, stealthy stalking of small game in dense brush. An eco-morph today dominated by mammals, snakes, and lizards.

So here we can draw a rough outline for the impetus and evolution of hind wings - appreciating their import in stealth - that is in fact congruent with what is seen in the fossil record.

A Rough Outline on the Evolution of Hind Wings

I Sinocalliopteryx gigas shows what a theropod with only the barest, incipient stages of this evolution would have. Just simple and small trailing lines of non-pennaceous feathers that assisted in stealthy movement through thick brush while stalking small and agile prey. Coelurosaurs had bigger and more audacious plans in their future so the hind-wing evolution and flight never became more realized in this group.

II Pedopenna, Xiaotingia & Anchiornis. From something similar to Sinocallipoteryx ecologically (albeit dromaeosaurine & smaller) animals like Pedopenna & Anchiornis would arise. The hind wing feathers are now pennaceous although they are not yet flight adapted as they are symmetrical (on both arm & leg wings). However these are true small game hunters of the underbrush as such they capably explored a more three dimensional realization of their environment, certainly more so than the relatively giant Sinocalliopteryx.  Although not exquisitely adapted for arboreal life they certainly found themselves up in the trees on occasion. The occasional fall was not an issue as both the arm wings and hind wings would serve as safety nets.

Credit Jaime Headden CC3.0 http://qilong.deviantart.com/art/The-Essence-of-Huxley-160904197

Despite consistent assertions to the contrary Anchironis and other hind-winged microraptorines were excellent terrestrial runners. Arguments against efficient terrestrial running have centered on the lack of leg feathers on modern ground birds. However these arguments have not accounted for a scientific control in terms of behavioral ecology. Big herbivorous ground birds are simply not ecologically comparable to stealthy, small game paravian hunters of dense vegetation. When stealthy hunting in deep vegetation is at a premium a trailing edge of feathers along the back of the leg will often develop in small theropods.


The merit of a strong hypothesis, or theory, is that predictions can be made and then tested. One such prediction of hind wings being intimately linked to a behavioral ecology of predatory stealth is that such hind wings should not occur in paravians with diminished predatory behavior i.e. omnivory/frugivory/herbivory. So far the prediction I can make holds up in light of the fossil evidence that we have accrued. Hind wings are only found on predaceous paravians and are completely lacking in the groups suspected of more omnivorous/herbivorous inclinations such as caudipterids/enantiornithines/therizinosaurids/oviraptorids etc. etc. 

That we have a diverse and well sampled inventory of lifestyles in paravians and it is only the predatory ones that show up with hind wings is an observation that one should keep in their back pockets.


Is there a test for my argument that hind-wings & leg feathers are intimately associated with both stealth and a small game carnivorous lifestyle? Such a paravian would have to be showing a roughly comparable level of flight efficiency to microraptorines with one exception in that it was not a hunter of small game but an omnivore/insectivore/herbivore i.e. stealth was not a critical aspect of its lifestyle.

Turns out the fossil record does provide such a test and it's name Eosinopteryx brevipenna. Eosinopteryx  comes from the same time and place as Anchiornis and Brian Switek gave it an excellent summary article here. Personally I think Switek was very prescient in observing how the glut of feathered dino-bird things coming down the pike in last couple of decades has caused a bit of a "numbing" effect. We are not as excited about them as we once were. Switek maintains that specimens like Eosinopteryx might yet have special stories to tell. Eosinopteryx does indeed have a special story to tell I suggest. Eosinopteryx does not have hind wings and it shows a trend indicating decreasing predatory habits. Loss of predatory features includes the lack of a toe claw or even highly recurved foot  claws at all and very foreshortened snout with reduced dentition.


Now granted Eosinopteryx is not especially flighted, in fact the arms have been argued to be especially lacking in the mobility needed for flight. But that does not imply it could not have clambered into trees - as other contemporary paravians that did have hind wings likely did - and that it could not have used the arm-wings as a safety net for falls or gliding. Yet it lacked the hind wings of the contemporary but decidedly more predatory animals like Anchiornis. Anchiornis lacked asymmetrical flight feathers itself and, if flighted, was hardly a good pilot. The main deciding trait that separated Eosinopteryx from hind winged paravians I suggest is diminished predatory habits.

*Update 8/26 I had it pointed out to me via Matt Martyniuk that Sapeornis has some hind wing action going on as well as several other enantiornithines such as those from the new Crato formation: link
Although the hypothesis is not as robust as I constructed it do to these exceptions, it is still possible that a use in stealthy predation was still coopted into the ability to fly with better maneuverability with enhanced hind wings...

III Microraptor. From animals similar to Anchiornis Microraptor likely evolved. Microraptor kept the long hind legs and strong cursorial ability of these animals but improved upon their condition by improving utilization of 3-dimensional space. This revolution was chiefly executed via more efficient flighted ability and true asymmetrical flight feathers on the fore & hind wings. The hind wing feathers still allowed for stealth movement through thick vegetation but were now co opted for aerodynamic purposes.

Hind wings in Microraptor allowed two very important facets of predatory behavior. 1) They allowed for increased maneuverability in the air. 2) They allowed for a more stealth approach in the air.

Microraptor 4 winged flight credit David Krentz
Point 1 is strongly supported via the studies of Hall & Habib et al. (2012) A strong take home message of the work of Hall & Habib is the concept of a "drag tax" in Microraptor's flight. All of the surface areas that increased maneuverability in Microraptor incurred a consequence in terms of speed. As Microraptor negotiated a very cluttered and complex forest ecosystem such compromise was necessary.   And as I will argue further in a future post speed was not at the crux of Microraptor's predatory arsenal - maneuverability and stealth were its chief aims.

Point 2 is a new aspect I want to highlight that will be further embellished in my next post. The hind wing of  Microraptor - which I argue is an exaptation co opted by small dromaeosaurines (like Anchiornis) that enabled stealth stalking through thick vegetation - now in the flighted form allowed for an appreciable degree of sound muffling while the leg moved through air.

Swing any long cylindrical object through the air with speed - a bat, a twig, or a leg - and you will get an audible "whoosh" as the air rushes around the cylindrically shaped object and collides together on the rear side. But put a trailing edge of feathers on such an object you will appreciably diminish this said "whooshing" effect. Owls have taken this aspect of stealth approach to the extreme via the sound damping effects of their feathers. I am not suggesting that Microraptor had such capabilities as an owl but the concept remains true to point out. And any such advantage a predator can use, even it only incrementally increases its odds, is still a useful and tactical one.

To restate the angle I am taking - how to figure out how a small theropod bauplan most effiectively operated as a small game hunting animal - let me get back to brass tacks. Microraptor hunted small enantironithines - flighted "birds" that were well ahead of Micraptor in terms of flight adaptations. The superiority of enantiornithine flight speed was no hindrance to the success of Microraptor's predation on them. Because Microraptor was a predator and predators use every advantage that they can garner, Microraptor simply honored the time honored tradition of predators immemorial. It struck in the dark.

Why Microraptor was in fact a nocturnal stealth predator of the utmost capability will be addressed on my next post...

Night Hunting Microraptor credit Robin Liesens


papers

Godefroit, P., Demuynck, H., Dyke, G., Hu, D., Escuillie, F., Claeys, P. 2013.Reduced plumage and flight ability of a new Jurassic paravian theropod from ChinaNature Communications. 4, 1394. doi: 10.1038/ncomms2389

Hall, JT, Habib, MB, Hone, DWE, Chiappe, LM. Hindwing function in four winged feathered dinosaurs. (2012) online

Ji, S., Ji, Q., Lu J., and Yuan, C. (2007). "A new giant compsognathid dinosaur with long filamentous integuments from Lower Cretaceous of Northeastern China." Acta Geologica Sinica81(1): 8-15


Xing L, Bell PR, Persons WS IV, Ji S, Miyashita T, et al. (2012) Abdominal Contents from Two Large Early Cretaceous Compsognathids (Dinosauria: Theropoda) Demonstrate Feeding on Confuciusornithids and Dromaeosaurids. PLoS ONE 7(8): e44012.doi:10.1371/journal.pone.0044012



Xu, X. & Zhang, F. (2005). "A new maniraptoran dinosaur from China with long feathers on the metatarsus". Naturwissenschaften92 (4): 173–177. Bibcode:2005NW.....92..173Xdoi:10.1007/s00114-004-0604-yPMID 15685441.


"A Long habit of not thinking a thing wrong, gives it a superficial appearance of being right, and raises at first a formidable outcry in defense of custom". Thomas Paine

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Tuesday, August 9, 2016

A New Wrinkle in the Megaraptor Mega-Mystery

credit Coria & Currie. Murusraptor CC 4.0

Definitely shaping up to be the year of clan megaraptoridae with new species and information seeming to come down the pike on a weekly basis. When such a glut of data comes at us at such in such a breakneck pace and when - in the case of megaraptorans - such large questions remains such as "what the hell are they actually?" it does pay to sometimes take a breather and digest things a bit. By slowing down and picking things over details that might otherwise be glossed over might see the light of day. It is one such detail that I want to highlight in this post. It does take me into territory that I certainly don't specialize in nor have the inclination or background to really get into - cladistics.

Yup, in life you gotta know your strengths and weaknesses and, no disrespect to the hard work others put into this aspect of paleontology, it's simply not my bag folks. So no, don't expect any data matrix from me folks or some prolonged digression into some obscure processes or foramen. I like the "softer" aspects such as functional stuff, appearance, behavior, and ecology. That being said the one feature I want to highlight in this short post is a feature that bridges the adaptational approach which influences my thinking and the - no disrespect to practitioners - the bean counting of cladistics with its emphasis on character traits, data matrices, parsimony analysis... yawn I can almost feel myself getting sleepy just talking about it.

Megaraptorans have some pretty neat teeth to talk about, not so much for the features that they have, but for the features that they lack.

Megaraptoran teeth lack interdenticular sulci (White et al., 2015).

On lateral teeth:

"There are no interdenticular sulci between any of the denticles on the distal carina"

From the discussion:



As I keep saying (and feel free to say it along with me) : It really is all about the teeth.



Readers who have been following me for a while will no doubt recognize the importance of this dental feature in tracing my evolving line of thinking on the range of functional ability and carcass utilization in theropods that featured such adaptations. My first real exposure to this dental adaptation in theropods by a paper by Brink et al. (2015) Developmental and evolutionary novelty in the serrated teeth of theropods which I discussed in my post Death Comes Ripping: Bonesaw Theropods. Basically interdenticular sulci are recesses between the denticles that arise developmentally. They serve to alleviate stress and overall strengthen and prolong the life of the denticle and therefore cutting proficiency of the tooth over its lifespan and we see rough analogy in expansion slots built into bone saws and cutting blades. Such features are not found in sharks, monitors, and sabertooth cats therefore creating the argument that serrated theropod teeth are on a functional level superior to the the teeth of these animals in cutting longevity. Which makes perfect sense because it is theropods that had to carve up and butcher the largest, thickest skinned, bone plate armored, cartilaginous and tendinous food base the world has ever seen - their herbivorous brethren. Bone is just another tissue in this regard just as likely to be sawed through as armor plated skin or thick tendons and joints as enamel trumps all these tissues in hardness scale. It's not a mistake that a great many theropods had a head narrow side to side but thick and strong from top to bottom that bears some uncanny resemblance to a blade or hatchet. Bonesaw theropods are not likely right because they are "awesome-bro" but because from an adaptationist approach animals with such tough rinds were what they had to cut through on a day to day basis and we should expect their enamel covered (and therefore likely lip covered) teeth to do the task that was set out before them.

credit Brink et al. 2015 interdenticular sulci in theropods


An inference I am going to make is that megraptorans - as the most common large carnivore in their ecosystem in many places (but especially Australia) likely fed on titanosaurs (alive or dead it don't matter). Titanosaurs certainly had a tough rind and lots of evidence of osteoderms in that family.

From discussion White et al., 2015:


The logical question arises that if megaraptorans evolved from some putative tyrannosauroid or carcharodontosaurid why would they lose their interdenticular sulci with such a food base? The answer of course is that they would not lose such a feature that benefitted hypercarnivory and that they did not evolve from a hypercarnicorous carcharodontosaurid or tyrannosauroid. That still leaves open the potential of evolving from a tyrannosauroid that did not have interdenticular sulci and which was a small game scrounger - a possibilty White et al. allude to:


While evolving from a basal tyrannosauroid that lacked interdenticular sulci is still possible it might be more promising to look at even more basal common ancestors as a distinct possibility - a putative small game hunting coelurosaur. Something like Compsagnathus, Juravenator, or Scipionyx? Brink et al. (2015) suggested interdenticular sulci as a synapomorphy of theropods secondarily lost by troodontids and spinosaurids. However in their study they did not investigate basal coelurosaurs which might lack sulci due to their small prey diet. I don't know for certain if some coelurosaurs lack sulci? Anybody have any info on this question out there?

If some coelurosaurs lacked sulci a putative basal small prey coeulurosaur might just be the subject we are looking for. Such a culprit might produce the blending of features that have caused various analyses to suggest spinosaurid, carcharodontosaurid, and tyrannosauroid affinities. Such a culprit might make a good island hopper/rafter and colonizer (e.g. Japan/Australia) as several compsagnathid species do seem to have excelled at colonizing islands. Evolving from a small game hunter that lacked interdenticular sulci is consistent with the strange anomalous lack of interdenticular sulci in megraptorids given a likely "brontophagist" niche. Given enough time megaraptorids may have independently evolved interdenticual sulci but as they were possibly just recently patriated to brontophagy from (potentially) a small game hunting ancestor they only had simple serrations.

And if megaraptorids did indeed arise from a generalized, island hopping, small prey eating putative coelurosaur this sort of makes megaraptorids their own thang right? Not some obscure offshoot of carcharodontosaurids or tyrannosauroids but their own rightful clan of unique hypercarnivorous theropods. Not claiming this idea as unique to myself as I think several other bloggers/researchers have put forth the same idea of megaptorids being their own thing. But I think looking at the tooth adaptation adds another layer of evidence in favor of megaraptors being their own clan.

A final caveat is that just because megraptorids lacked interdenticula sulci does not suggest that they were inferior carcass renderers than theropods that had them. It merely means that their denticles did not last as long and something as simple as higher tooth replacement rates could have kept them equipped for efficient brontophagist shenanigans.

a very "coelurosaur looking" Megaraptor credit Tom Parker, 2015 CC 4.0


papers

Coria RA, Currie PJ (2016) A New Megaraptoran Dinosaur (Dinosauria, Theropoda, Megaraptoridae) from the Late Cretaceous of Patagonia. PLoS ONE 11(7): e0157973. doi:10.1371/journal.pone.0157973

Brink, K.S.  et. al. (2015)  Developmental and evolutionary novelty in the serrated teeth of theropod dinosaurs.
Scientific Reports 5, article no. 12338, July 2015


White, MA, Bell, PR, Cook, AG, Poropat, SF, Elliot, DA, (2015) The dentary of Australovenator wintonensis (Theropoda, Megaraptoridae); implications for megaraptorid dentition PeerJ Dec 2015 online


"A Long habit of not thinking a thing wrong, gives it a superficial appearance of being right, and raises at first a formidable outcry in defense of custom". Thomas Paine

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Tuesday, August 2, 2016

Making Dromaeosaurids Nasty Again Part IV: New Hypotheses on Dromaeosaurid Feeding Technique & Role of Tail in Movement

Hey now... I really thought about splitting this post into two posts for both respective hypotheses, but for the sake of brevity and wrapping this series up I decided to combine them. Additionally, as I will elaborate on further, the two aspects I will focus in on here - biting & locomotion - are not mutually exclusive and one dovetails into the other. So I gave this article a really long title and hope you get something out of it!!

Readers of this series may have detected a slight yet pervasive diminution of the import of the famed "killing claw" over the course of these posts. In my first post I documented the shift in scientific thought on these claws from scythes that cut meter long slashes in prey to crampons that allowed hitching rides on the hides of dinosaurs to ultimately the prevalent modern interpretation of raptor prey restraint (RPR) model of Fowler et al. in which prey subequal in size is grasped by all four digits. I reiterated a point seldom mentioned from the Fowler et al. paper on the dromaeosaurid RPR hypothesis: relative to accipterids, the ungual grasping ability of dromaeosaurids was >not as strong< as these birds in that arena i.e. they were not simply scaled up hawks. Later in that post I suggested a role for the arm/wings for pummeling prey/combatants as the feet grasped and pinned the animal. In my next post focusing on aggressive/combative scavenging in these animals I focused in on digit II as a useful tool in pinning large carcasses down as the head, neck and teeth pulled back on flesh - an idea supported by the unique morphology of the denticles on these theropods and the presence of enlarged digit II claws in several birds that work in a similar fashion.

My contention is that the import of digit II - so highly regarded that it is referred to as the "killing claw" - has both culturally and scientifically influenced these animals to the point that other aspects have been enshrouded. But was the "killing claw" really the most pivotal aspect of these animals behavior and ecology? I think not, or at least >not always<.

If the use of the "killing claw" digit II was indeed the be all and end all of dromaeosaurid prey capture and feeding technique we should be able to make some predictions to test that assertion. That over the course of the 100 million year evolutionary trajectory of these animals an increasing reliance on ungual prey capture will 1) show a trend towards shorter and therefore stronger legs i.e. less cursorial adaptations 2) as firepower is concentrated in the feet for killing the robustness of the skull and teeth should hold steady or potentially diminish.  In the early Cretaceous Deinonychus we have a relatively sub-cursorial but highly adept foot grasper - again there is a bit of an inverse relationship between foot grasping strength and cursorial ability as I discussed in the first post and which Fowler et al. highlighted in their paper. I will cut and paste the source of this observation from the Fowler et al. paper:


As evidence for the purported trend in increasing foot strength Fowler et al. cite Deinonychus (early Cretaceous), and Velociraptor & Saurornitholestes (late Cretaceous). While Deinonychus and Velciraptor have relatively short metatarsi I can't see how they interpret the leggy Saurornitholestes as an example of this trend. Additionally there are some notable omissions, most obviously the name sake for the whole family Dromaeosaurus!!

In Dromaeosaurus albertensis cursorial adaptations are highlighted, the killing claw is relatively atrophied, and the skull is relatively massive and robust (almost tyrannosaurid like as GSP has commented). I mean just check out the skull of this animal, there is nothing slight, superficial, or atrophied about it at all:

robust head of Dromaeosaurus albertensis. credit LadyofHats. public domain
relatively diminished claw size/strength D. albertensis. credit LadyofHats. public domain
On the other hand digit II is not especially robust in Dromaeosaurus and the remaining unguals look more adapted towards a cursorial lifestyle than grasping. A "ground hawk" this was not.

Several of these trends towards diminished ungual strength and/or increasing skull robustness also play out in Dakotaraptor (cursorial w/diminished foot grasping ability) and the very robust skulled Atrociraptor.

Atrociraptor credit Ferahgo the assassin (Emily Willoughby) CC3.0
What am I getting at here? If anything the trend is towards increasing tooth and skull reliance over time >not always< towards increasing foot grasping & "killing claw" importance. I say >not always< because there were and likely always were dromaeosaurids that highlighted foot grasping ability. Sometimes foot grasping became diminished, sometimes it was very important. But what was always important and what was always highlighted in these animals was the jaws and teeth. They are the feature that always stayed pat or, if anything, increased in prominence.

No dromaeosaurids were not evolving protobeaks or going edentulous despite the persistent artistic meme and no they were not diminishing emphasis on teeth and jaws.

It really is all about the teeth....

To drive home this contention I want to revisit a famed piece of data that has caused quite a stir in terms of whom and how it was done - the famed Tenontosaurus bite marks and the case for Deinonychus "bite strength". A technical paper by Gignac et al. (2010), a blog post by Mark Witton, a blog post by central coast paleontologist, and an internet article/summary from The World of Animals all highlight the attention and thought these remains have attracted.

That these bite marks have evolved into a bit of a paleontological "who done it" has always irked me. Not because of a lack of data or some systemic problem with the analysis - but because of the pervasive "explaining away" of data that most parsimoniously points to Deinonychus as the perpetrator. Several  ideas have been bandied about in an attempt to account for these bite marks by Deinonychus, a predator that appears to not have an especially high bite force.

Let's unpack them:

1) An undescribed and undiscovered tyrannosauroid dinosaur did this damage.

We have seen this story before... tremendous damage to bone - no way a "blade toothed" theropod did it much less a puny dromie. Let's just imagine a stout toothed, bone crunching tyrannosauroid existed at a time when such animals were basically all blade toothed anyways, and make this essentially fictional animal the perpetrator. Made up tyrant lizards did it!! Pesky blade toothed theropods just stay in your lane - you guys can't bite through bone the way tyrant lizards can!!

As you can tell (snark alert) I am not so much a fan of this idea. We have evidence of Deinonychus being the most ubiquitous theropod in the area; the tooth arcade matches; broken teeth in the area; the well established Tentontosaurus - Deinonychus relationship - the whole tide of evidence points to Deinonychus. If a cryptic lineage of stout toothed, bony crunching tyrannosauroids existed at this time I will be happy to be proven wrong - as of now I know such evidence and of the tyrannosauroids from this time period they are blade toothed predators without expanded jaw musculature - although I have heard murmuring of tyrannosauroid teeth from the same formation (but blade toothed not lethal bananas).

2) Deinonychus could bite hard, but it did so extremely rarely.

I mean really? Remember when you kept hearing how humans only use 10% of their brain? Yeah, this explanation sounds a lot like that. Over designed with a bite force exceeding modern American alligators yet barely ever uses this strength? I can't really go with this thought.

3) Stronger bite than predicted from studies.

I don't think that this animal had much of a stronger bite than studies indicate. I believe that we have been a little bit more than led astray by always looking at static bite strength as opposed to other methods of cutting that highlight speed, friction, and getting those darn denticles to do the work for you. It really is all about the teeth and it really is all about getting the denticles to work in a way that maximizes cutting efficiency with minimal effort and wear & tear of the tooth.

Its high time we start looking at hypotheses that invoke Deinonychus as the prime perpetrator. I will  put out a hypothesis that highlights an unorthodox feeding mechanism in these animals, that is consistent with the data, and offers much explanatory power for the observed data.

To prime you for it I want to look at birds a bit (as usual). To really confound the situation the obvious choice is flamingoes - because what better to compare dromaeosaurids to than flamingoes, amirite?!?



I mean, excuse the poor video quality, but just look at those tongues go!! It is the tongue just pumping back and forth causing the whole neck to just vibrate. I have no idea why these flamingoes engage in this lingual vibration? Anyone ever see wild flamingoes do this? I would have to assume that they pump their tongues back and forth to filter food but in my observations of these captive Chilean flamingoes they just do it while walking around... probably just bored.

No I am not suggesting that Deinonychus had some sort of lingual vibrational apparatus set up - just pointing out how one muscular organ - the tongue - can move with such speed and power in this bird that it vibrates the whole head and neck of these animals. I mean can your tongue move with such speed and power that it causes the whole body to hummm and vibrate... ummm never mind. The message I am trying to convey here is that when we look at avian feeding mechanics - and by extension many dinosaurs and especially paravaians/maniraptorans/dromaeosaurids - there is a lot of potential for quick twitch muscle, full body and/or neck movement involved in the feding apparatus.  To drive home this point, literally, what would woodpeckers be without their exceptionally quick and rapid - fire neck movements? Yes, it is the skull of woodpeckers that is wonderfully equipped to handle the blows and stresses incurred but without the power and speed provided by the neck the woodpecker would, essentially, not peck. It would just be a bird with a strong skull.


An often overlooked aspect of feeding mechanics is elaborating on how parts of or the whole of the body is engaged in feeding mechanics - the head need not be looked at as an isolated aspect of the process. Regular readers should note that I have made this point before on antediluvian salad especially with regards to twist or torsional feeding (death rolls) in plesiosaurs and in my bonesaw shimmy hypothesis on Allosaurus in which it is rapid neck movement in both the fore and aft direction that allows the denticles on the front and back end of the tooth to saw right through tissue. Bite force was not especially important in that hypothesis, in fact tight clamping would work against free movement of the denticles over the tissue.

This hypothesis does take some inspiration from the bonesaw shimmy model but it does deviate from it in several ways.

I propose that fast twitch muscular contractions of the neck, torso, and even tail would pulse out vibrational waves of energy towards the head. As bipeds that do not have their front feet on the ground these pulses of vibrational energy would travel unhindered through the neck, head, teeth, and ultimately into the food item they are cutting into. As the vibrational energy literally vibrated the tooth back and forth into the food item the peculiar denticle pattern of dromaeosaurids comes into fruition as an optimized adaptation to literally bore and auger into tissue.


The most striking and unique feature about the denticles on Deinonychus is that they are fairly reduced on the front of the tooth but very pronounced on the rear. But even stranger is the manner in which they are curved on the rear side which is towards the tip of the tooth, referred to as apical hooking. Fowler et al. suggested that this unique denticle design would optimize cutting into tissue as the prey animal was held in the RPR model and the head of the dromaeosaurs was sub-vertical with the nose facing down and biting between the legs. However this suggestion by Fowler fails to address the issue that many other theropods likely held prey/food down with their feet and wrenched off bites in a sub-vertical manner yet these theropods did not evolve such weird denticles as seen in many dromaeosaurids.

But if we imagine each denticle as a "tooth" and each tooth having a respective duty in food processing a potentially new perspective emerges that could explain the unique bone damage ascribed to Deinonychus.


As the piece of food is grasped a strong bite is first established. The slight and reduced serrations on the front of the tooth are useful here in establishing a piercing bite - not very deep as their bite force was modest but merely a small indentation into the article of food. Once a purchase is made then the body commences vibrations - potentially a combination of head, neck, torso, and tail rapid fire twitches - which allow the tooth to bore and auger into the food particle i.e. bone. As the "bore hole" phase commences the utility of the weird apically hooked denticles comes into play as each denticle literally chips and shreds away at tissue like individual teeth. As the tooth works its way into the material it leaves a remarkably accurate impression of the tooth - a literal bore hole that for all intents and purpose can be read as a puncture. Once the integrity of the material is weakened substantially the item can be pinned with the arms and/or feet and the head and neck are pulled back strongly incurring further and more drastic damage as the tooth is dragged back through the (weakened) material literally leaving deep bone raking marks and furrows. It is also potentially possible that vibrations of the body were not emphasized or were in fact used in concert with multiple quick bites - essentially chattering of the jaw - in which micro - abrasions from the denticles work to carve into tissue.

This "vibrational feeding" hypothesis could potentially explain the two types of feeding traces recorded on the bones of Tentontosaurus which include longer gouges and simple punctures.



Above you see the type of "bone rakings" I mentioned earlier. An initial puncture is established and with the teeth embedded now the neck and body can pull back and rake through tissue.

What I suggest was occurring here is that these were investigative bites into bone. The theropods were gouging into the bones to see if there was ample nutritional value in them to justify the effort and potential wear of teeth. There would always be a three-way tradeoff between nutritional value versus the effort and wear on the animals feeding apparatus all of which is tempered by the relative health of the animal i.e. how desperate for food is it. Ultimately it looks like the theropods abandoned the bone consumption in this case.





That dromaeosaurid teeth occasionally show extreme wear - especially on the tips as should be predicted in this model - is very interesting.

worn tooth "Dromaoesauroides" wiki
Private "dromaeosaur" tooth Montana .84"
Judith River "dromaeosaur" tooth 
Clearly these animals were putting some heavy wear on their chompers, especially when we account for the fact that they were not keeping their teeth for life. An interesting test would be to see if komodo dragon teeth ever show equal levels of wear. But again, not the best test because theropod teeth were actually superbly designed to withstand stress more than any other ziphodont predators (Brink, 2015) (including komodos), yet they were still showing drastic wear... these animals were not getting this type of wear from just eating small animals and delicately nipping carcasses I'll tell you that much.

Of course it is worth mentioning that there is a lot of room for deviation in this model and we need not assume that all dromies employed vibrational feeding to the same extent. Indeed Dromaeosaurus could have employed a lot more emphasis on traditional "power chomps" than what I suggested for Deinonychus.

In theropods, being both ziphodont toothed and bipedal, there is no go to analogy among modern tetrapods - birds don't quite tell the whole story and neither do monitor lizards. So maybe we should expect some unothodox feeding mechanics.

Lifestyles of not only the quick and cursorial but the slow and persistent as well...

And now for the tail. Probably the aspect >least likely< to be assumed to be involved in "making dromaeosaurids nasty again". But it is the tail that is the most important aspect of these animals I will argue. The tail is what really pulls together all the disparate attributes of these animals and makes them what they were. And what they were was quite literally the most successful in tenure small to medium sized terrestrial hunter - scavengers that have ever existed. A unique blend; of accipterid "raptor"; combative scavenging vulture; bone chomping hyena ; a dash of felid; and, yes, highly efficient cursors similar to kangaroos, hyenas, humans, wolverines, and Arctodus.

One of the persistent ideas that has gained popular recognition in recent years is that dromaeosaurids were sub-cursorial - that they were slow. A chief argument put forth to support this notion is that the ankle bones - the metarsii - were rather short. And this is true for many species - Deinonychus and Velociraptor in particular - that were gaining mechanical advantage of foot claw strength at the expense of speed. But this was not so true in several other species - Dakotaraptor and Dromaeosaurus for instance - that were leggy to an exceptional degree. I am just not at ease with suggestions that species at the lower end of the spectrum were heavy footed clunkers - they could probably all put on a decent burst of speed if need be. Ursids (da' bears) have all the hallmarks of real clunkers but put on good speed with their short ankles. Keep in mind that dromies were competing with larger - and in the case of tyrannosaurids likely larger and quicker - theropods as well as azhdarchids. It is not always about being the fastest - but about being more agile when fleeing a larger threat. With their arm - wings and long tails doubtless many dromies frustrated an angry tyrannosaurid back in the day with their superior agility.

The dromie tail, just like the dromie "killing claw" has gone through a twisted and convoluted history of interpretations and revisions. A brief recap. Ostrom interpreted the tail as an intricate balancing rod that facilitated use of the "killing claw" for kicking and hanging onto prey. Each subsequent interpretation of dromie killing technique from hanging onto the side of prey and biting to the RPR method invoked the tail as intricate balancing organ for their respective prime foraging technique.

To add further context to the strange saga of dromie tails I want to revisit a post from Pterosaur.net Blog  (remember that great site?) Dragon Tails: What Pterosaurs Teach Us About Velociraptor that made the strange and startling comparison between dromaeosaurid tails and rhamphorynchid tails... wtf? Well there is a comparison to be made there and it is not soooo strange when we work from the starting point that dromaeosaurids likely had flighted ancestors... so that they inherited a tail that - presumably - shared a convergence in form and function with rhamphorynchid pterosaurs.

credit Scott Hartman used w/permission . blog Scott Hartman's Skeletal Drawing

credit Scott Hartman used w/permission . blog Scott Hartman's Skeletal Drawing


So if dromaeosaurids inherited the weird morphology of their tails from flighted ancestors - full of chevrons, diminished musculature, partially ossified dual tendons (i.e. caudal rods) there becomes two rather interesting questions: 1) what adaptive benefit did these features incur in flighted dromaeosaurids and tailed pterosaurs? and 2) how was this morphology coopted into terrestrial based dromaeosaurids? Question number #1 I am going to leave alone but I think it is a long overdue question that needs analysis but question number #2 is what I am going to approach here.

credit Scott Persons

What I am going to suggest is that dromaeosaurids across all ranges of absolute speed and leg length - were highly efficient long distance pacers. They could and did just keep going for miles at a time at a relatively moderate pace. The whole lot of 'em could just run you to death. And the key to this long distance efficiency was the tail. The tail - the whole organ - served as an elastic recoil that allowed these animals to store, redistribute, and recoup energy for efficient, long distance traveling. I have seen scant attention to the tail as an aide in terrestrial efficiency in dromies. Despite the fact that these animals were terrestrial and the tail of dinosaurs is intimately linked with movement - especially per the caudemofemoralis muscle. Darren Naish raised the question of dromie tails back in 2008 (What the hell is going on with dromaeosaur tails?) in light of Norell & Makovicky (1999) describing an articulated and sinuous Velociraptor tail. The comment section is interesting. I do note in it a pervasive sentiment of trying to "explain away" the sinuous tail - the presupposition being that stiff tails is the better supported null in dromies to start with. But is a stiff tail the better supported null or is it just how we grew up expecting dromie tails to behave? In either case lateral flexibility shown in both Velociraptor and Bambiraptor seems to have prevailed. But there is one comment by Alan #19 that I believe was very prescient and which received literally no attention in the discussion.




I think Alan was on the right track as goes energy efficiency although I doubt the hopping dromie scenario has much merit - indeed trackways have proven otherwise.

I will be working from the assumption that dromaeosaurids - whatever abilities they had for arboreal behavior or even some amount of gliding or even "flight" in small ones - that they were basically terrestrial animals and that the tide of evolutionary impetus should create a better and more efficient terrestrially adapted animal. Not an evolutionary experiment, and not a maladapted kinda-climber, kinda-jumper, kinda-walker but a reasonably well equipped and efficient animal that could do all the things that we should expect a small to medium sized hunter - scavenger to do in a highly competitive ecosystem. In short they could climb, they could potentially even swoop, they could swim, but what they did the most was walk and run around. Namely that means that they could move fairly quickly and efficiently to highly localized food sources -   carcasses, hatching dinosaurs, large concentrations of prey. Especially given their long tenure, efficient terrestrial movement should almost be expected. Contra the "ground hawk" image we need not assume that these animals were >always< sit and wait ambushers or would swoop down from a perch. Indeed sit and wait ambushing is more of an ectothermic strategy and even when warm blooded predators do ambush from trees or from cover they choose spots that have a high degree of certainty that prey will be there fairly regularly. Dromies possibly could have utilized this tactic to some degree but I hardly think it was their dominant foraging strategy given that several species developed obvious cursorial adaptations and that some species lived in areas with little tree cover or sparse vegetation in general (i.e. dune fields).

From my own experimental paleontology in which I strapped on a huge tail to my butt at SVP Los Angeles and commenced to simultaneously entertain and annoy attendees I noted several patterns. What was really interesting to me is how much that darned tail moved around. Literally the smallest movement I made would thoroughly send the tail in motion. And what was most notable was the dramatic up and down oscillations that the tail went through as I walked. Each foot fall would create a simultaneous rise and fall of the tail - even the smallest and daintiest step. Don't believe me strap one on yourself and be a dinosaur for a day - you'll see what I am talking about.



These up and down movements of the tail that occurred simultaneously with each footfall likely occurred in all dinosaurs to some degree.  What is interesting is that dromaeosaurid tails - because of their "caudal rods" - were designed to diminish this up and down movement of the tail as thoroughly explained by Scott Persons on his post on dromie/rhamphorynchus tail convergence. Note in the pic below how the caudal rods are neatly stacked against one another on the vertical plane to limit movement dorso - ventrally.

Caudal rods in Deinonychus prevent up and down movement of tail
credit Scott Persons
As has recently been illuminated by discoveries of articulated tails of Velociraptor and Bambiraptor these tails could still bend quite sinuously in the lateral realm.

Bambiraptor tail credit Scott Persons
So if the caudal rods of Deinonychus and other dromies diminish the up and down movement of the tails - which is a natural consequence of bipedal movement - we have some missing kinetic energy to account for. Energy is neither created nor destroyed. Something has happened to the energy otherwise absorbed and dissipated by the tail through up and down movement with each footfall in dromaeosaurids... where does it go? I suspect that this energy is recouped into the legs and aides in giving these animals just a little extra "bounce" to their step. The tail may work as a wonderful elastic rebound organ. We should potentially imagine dromies being very bouncy and springy as they paced along.

This model of locomotory efficiency is not without parallel in animals that have to move across vast expanses to find and locate rare and ephemeral food resources. A leading hypothesis concerning Arctodus is that it was a highly efficient long distance pacer that scavenged and usurped carcasses (Matheus, 2003) utilizing long legs and elastic recoil to travel at a moderate pace over long distances. Hyenas have long been noted for their efficient loping pace that allows large scale movements and carcass retrieval. Kangaroos and wallabies are well noted for their ability to travel long distance at an extremely energy efficient pace owing a lot to the elastic recoil in their leg tendons. Indeed a robotic kangaroo has been designed that utilizes such elastic recoil in the tail to recoup energy for movement.


I would be remiss not to mention the endurance running hypothesis has been invoked as a strategy for both scavenging and pursuit hunting in our own genus aided by the achilles tendon. To further quell the notion that short legs - such as in Deinonychus or Velociraptor - imply a suboptimal terrestrial movement capability let us not forget about wolverines which are notorious long range hunter -scavengers despite being very short limbed. I don't know if there have been any studies on the locomotory efficiency of these animals but I suspect there is something to 'em in those regards. There are at least loads of references to the marathon travels of these facultative scavengers.



"It is absolutely impossible for any human to keep up with a wolverine. What wolverine can do is just beyond human."

"A wolverine crosses a topo maps like we cross a street."

"They devour the landscape at a constant 4 mph regardless of terrain."

An energetic bundle of tooth, claw, and attitude? Switching from small game foraging to large carcass acquisition as the seasons dictate? Bone consumption? Able to outpace, outcompete, and outwork competitors that are several orders of magnitude larger in size? Thriving in areas and desolate habitats that other predators eschew (snowfields analogous to dune fields in these regards)? A little bit of the Gulo gulo in your dromie? You bet.

Making dromaeosaurids nasty again... Invoking the wolverine as a likely analogue for many dromies, it doesn't get much nastier than the demon of the north.

Ichnology: What Does It Tell Us?

Xing et. al. (2013) document a variety of dromaeosaurid trackways from the lower Cretaceouls Hekou group in China. The pace was not very high at about .75 meters/second which is about 1.7 mph or 2.7 km/hour. Average human walking speed is said to be about 3.1 mph or 5.0 km/hour. Let me just cut and paste the discussion:

So although these particular dromies seem to be moving along at slowish pace - perhaps they had full bellies or were just walking down for a drink. It is noteworthy that they mention several dromie ichno-species in the last paragraph that seem to be cruising along at quite brisk paces and one zipping along pretty good.

Dromaeosauripus from Korea at 4.86 m/s (Kim et al. 2008) which is 10.9 mph / 17.5 kmh

Paravipus  (Murdoch et al. 2010) at 1.67 m/s and 3.61 m/s which is 3.6 mph / 5.8 kmh and 8.1 mph / 13.0 kmh

Dromaeopodus at 1.63 m/s (Li et al. 2007; Kim et al. 2008) which is 3.6 mph / 5.8 kmh

Considering that to document an actual predatory chase in the footprint record is exceptionally rare and that there is no evidence that a chase was in progress in any of these instances the ichnological data is very interesting. We see a range of speeds here from the more leisurely .75 m/s to a quite hectic 4.86 m/s. If we assume that these are reasonable cruising speeds then the small sample size we have does point to a relatively fast paced "cruiser" similar to humans, wolverines, coyotes, and hyenas that can cover vaste expanses of land at an efficient pace as the penultimate terrestrial hunter - scavengers of their time.

The Seldom Mentioned Fact of Dromie Toe & Heel Pads

The trackways from this study demonstrate that dromies had big ol' foot pads like two toed ostriches but also large heel pads! So pay attention to this aspect paleo-artists  >at least some< dromies had big fat derpy looking foot/heel pads that are universally never depicted at all or large enough in paleo-art depictions (including my own). Why has this well documented aspect of dromaeosaur foot anatomy never penetrated into popular depictions? I mean no one - literally nobody - including world renowned paleoartists or more obscure/enthusiast artists depicts dromies with large heel pads. Yup the toes had big padding but the heel pad would have been very apparent in life. And this is from a peer reviewed paper with several notable authors including most notably to my western biased eyeballs, Phil Currie (who is btw the last author).





Such fleshy and large toe/heel pads would assist in stalking behavior by muffling sounds, stability, absorb stress from cursorial activity but I also have to wonder if such fleshy structures would diminish grasping effectiveness?



Also check out the base of digit II often reveals a bit of a fleshy toe pad. Dromaeosauripus yongjingensis represents a fairly large "Utahraptor" size dromie but other dromie footprints reveal fleshy toe pads and heel pads.




Kim et al. did a paper with reference to a speedy little dromie in the above discussion (Kim et al. 2008) from Korea of Dromaeosauripus moving along at about 4.86 meters per second (10.9 mph or 17.5 kmph).

Abstract:

The question is though does this represent a cruising speed or were we in fact lucky enough to document one of the rare instances that a theropod was actually "on the hunt"? Or neither? Could it be that dromies would normally walk at a fairly leisurely pace of less than 2 mph but when spurred into action (i.e. carcass or prey that have been detected via sensory cues but still require covering large terrain) that they then shift gears into a relatively higher pace 3 - 4 mph or even up to 8 - 10 mph / 16 -18 kmph? That is pretty fast but I hardly think it represent the top speed of these animals.

I also should give some space to the ichnological data pointing to at least six large dromies traveling in parallel and the special emphasis the authors give to the toe and heel pads in the footprints ( Li, 2007).






So when depicting the average large terrestrial dromie foot think more about ostrich feet than harpy eagle feet. Except that unlike ostriches dromies often had big ol' heel pads in addition to toe pads that would have further cushioned the foot and added a degree of stability normally not ascribed to these animals. The increased surface area would have facilitated greater efficacy and stability of movement in dubious terrain such as dune fields and mud flats.

ostrich foot credit Masteraah CC 2.0
Again it does beg the question that - at least among the dromies that sported such large heel & toe pads - how efficient a grip could have been enacted with the claws in the RPR model? I mean having such big, cushy organs between your claws and the animal you are gripping does pose some practical questions as goes the efficiency of such a grip.

A lot of questions to be answered but I do think that a fresh appraisal of these animals as primarily terrestrial long distance hunter - scavengers that have to cover a lot of ground efficiently is needed. Optimal walking versus optimal cruising speed can be addressed with larger sample size of ichnological data and computational methods... What I can say is that the anatomy of the tail likely has something to do with terrestrial locomotion and efficiency of gait is as good of a hypothesis to investigate as any...

These animals had to have been able to move and move well. They had to have traversed wide distances to secure meals in often times inhospitable terrain. They had to have competed against larger and aggressively hungry and growing youngsters of  tyrannosauroids, carcharodontosaurids, and other theropods. They had to get to carcasses before large pterosaurs got all the good stuff. They had to have been at least reasonably competent in these realms to have persisted as... I don't know... the longest tenured group of small - medium sized tetrapod terrestrial hunter - scavengers that ever existed ( I know I said it before but it bears repeating). Speedy thieves indeed.

Earlier in this article I suggested that there was a link between the tail and biting apparatus in these animals - that their functions dovetail together. At the risk of piling one hypothesis on top of another let me put it out there that the diminished dorso-ventral movement of the tail as dictated by the caudal rods would have shunted more of the potential energy towards the anterior of the body - essentially towards the head, jaws, and teeth - during vibrational feeding.

credit Duane Nash Tsaagan & Velociraptor

Final Thoughts

Both a scientific and cultural emphasis on the "killing claw" in dromaesaurids has obscured a more nuanced, multifaceted, and holistic approach to these animals; that the "ground hawk" model has so embedded itself into our conscious; that the potential role of arm-wings as brutal spiked clobbering devices analogous to wing pummeling in modern aves has been overlooked; that the teeth were highly specialized and brutal weapons in their own right capable of extreme insults to carcass integrity (including bones) and perhaps full body "vibrational feeding"; that the importance of head and tooth weaponry did not diminish over the evolutionary history of this group but sometimes increased while emphasis on "killing claw" and foot grasping capability did in fact sometimes diminish; that cursorial ability did often times increase in capability and that all dromaeosaurids may have benefited from elastic rebound provided by caudal rods in the tail enhancing long distance, mid-paced terrestrial efficiency of movement as well as large fleshy toe & heel pads; that life appearance may have been more varied than simply "grounded hawks" with "dapper" haircuts but imbued with much of the panoply of life appearance we see in ratites, predatory and scavenging accipterids, cathartidae, bucerotidae, galliformes, and other large/terrestrial aves including but not limited to large exposed fleshy areas including caruncles, wattles, frills, dewlaps, and other tough - elastic - and fleshy skin derived outgrowths for thermorgulation and sexo-social signaling; that these attributes when generously applied to an outstanding and long lasted dynasty - in fact the longest tenure of small to medium sized tetrapod terrestrial hunter - scavengers to have ever existed - create a strikingly original, efficient and for lack of a better term "nasty" eco-morphological package that punched above their own weights in many categories.

They were above all else... awesome... bro.

And finally... can we please stop calling them raptors? That name is already taken!! You may have noticed through the course of these articles that I have bounced a lot between dromaeosaurid and dromie... I probably in retrospect should have used the term eudromaeosaurid through out as they are what I am principally talking about here not microraptorines or unenlagines.

I vote for calling these guys "dromies"and am fully favor of eschewing the befuddled term "raptor".





Cheers!!

Papers

Scientific Reports 5, article no. 12338, July 2015

Fowler, D. W., Freedman, E. A., Scannella, J. B., & Kambic, R. E. (2011). The predatory ecology of Deinonychus and the origin of flapping in birds. PLoS One, 6(12), e28964.

Gignac, P. M., Makovicky, P. J., Erickson, G. M., & Walsh, R. P. (2010). A description ofDeinonychus antirrhopus bite marks and estimates of bite force using tooth indentation simulations. Journal of Vertebrate Paleontology, 30(4), 1169-1177.

Kim, J.Y., Kim, K.S. and Lockley, M.G. 2008. New didactyl dinosaurs footprints (Dromaeosauripus hamanesnsi ichnogen. et ichnosp. nov.) from the Early Cretaceous Haman Formation, south coast of Korea. Palaeogeography, Paleoclimatology, Palaeoecology 262: 72-78


Li, Rihui., Lockley, M.G., Makovicky, P.J., Matsukawa, M., Norell, M.A., Harris. J.D., Liu, M., (2007) Behavioral and faunal implications of Early Cretaceous deinonychosaurian trackways from China. Naturwissenschaften (2008) 95: 185-191 online 

Xing, L., Li, D., Harris, J.D., Bell, P.R., Azuma, Y., Fujita, M., Lee, Y.−N., and Currie, P.J. 2013. A new deinonycho−
723–730.http://www.xinglida.net/pdf/XING_et_al_2012_Deinonychosaurians_Tracks.pdf



"A Long habit of not thinking a thing wrong, gives it a superficial appearance of being right, and raises at first a formidable outcry in defense of custom". Thomas Paine

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