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| Utahraptor by Duane Nash |
You know its that time again folks, more dromie madness. Antediluvian salad has a bit of a controversial history with these most pernicious of stem birds. You can be sure that part is not going to change, the controversial bit, that is.
For a brief review:
Making Dromaeosaurids Nasty Again Part I: Wing Pummeling Abuse In which I discuss the current state of affairs with regards to killing claw use and hypothesize that prey restraint by the foot "killing claw" coupled with clawed wing pummeling may have been utilized. As you will see in this post I no longer consider such a method of restraint and killing as preeminent. That is - shades of grey here folks - I don't think that both the raptor prey restraint model and the wing pummeling hypothesis were chief strategies for prey dispatch in most dromies. Could both style of attack been used on occasion by some dromies? Yes, but as I will explain both theses methods become vanishingly tenable tactics especially at larger sizes (hello Utahraptor).
Making Dromaeosaurids Nasty Again Part II: No Shame in the Scavenging Game I start building the case for many dromies as excellent facultative scavengers. Not only is there fairly unequivocal fossil evidence for it, the size range of dromies slots in nicely to an ecological "sweet spot" where terrestrial scavenging is most utilitarian. Also Dakotaraptor as a large, cursorial carcass bully.
Making Dromaeosaurids Nasty Again Part III: Life Appearance - Dapper of Deranged Probably - at least based on the comments - the most controversial of the series. I would say it has held up nicely, especially in light of the compelling evidence that tyrant lizards "de-feathered" significantly. "Wut you mean that dromies did not all look like carbon copies of each other in the 100 million years of evolution they underwent? Get out!" Look folks it's not even that wild of a suggestion, in fact pretty common sense actually. Almost boringly so.
Making Dromaeosaurids Nasty Again Part IV: New Hypotheses on Dromaeosaurid Feeding Technique & Role of Tail in Movement For me this was the most fun and interesting post in the series as it combines unusual oral feeding styles with a novel idea on dromaeosaur locomotive strategy.
I introduced, for lack of a better term, the "woodpecker hypothesis" of dromie carcass feeding technique. In this scenario quick twitch muscles generated in the body vibrate culminating energy at the tooth tip where strange apically hooked serrations on the teeth allow the tooth to literally dig into carcasses. Several examples of worn dromie teeth and inexplicable bone damage on Tenontosaurus can be potentially explained by this hypothetical feeding style.
I then discussed how the pattern of caudal rods in dromie tails could potentially work as an elastic recoil allowing energy efficient, long distance travel, useful for reaching ephemeral food sources and carcasses. I make the comparison to wolverines which, despite their short legs, are remarkable and unstoppable moderate paced long distance cursors. I augmented this suggestion with a review of dromie ichnological traces - the footprint record shows that these animals had not only large foot and toe pads but that they most likely cruised at a fairly high pace.
Here I have to admit to having a bit of mud on face here folks as a frequent theme of my "Making Dromaeosaurids Nasty Again" series was a diminishing of the role of the killing claw in predatory endeavors, highlighting wing pummeling and biting as more preeminent tactics for dispatch while the feet grappled and held prey. So I'm gonna eat some crow here in coming around full circle in my opinion on that most famed of claws - foot ungual #2, aka the "killing claw" I now believe really was a killing claw just not in the way that we have been interpreting it.
Full disclosure, the genesis of this idea is not mine own although after mulling it over and especially with the Utahraptor reveals I think it should be the leading hypothesis. What I want to document is the transmutations and permutations an evolving hypothesis should go through and in this case I believe did go. New ideas rarely come fully realized and perfected into the world. They need refinement. In the case of this idea which I will dub the "pierced from within" hypothesis the first semblance of it to my knowledge was put forth by Kenneth Carpenter. However the man who improved - but didn't perfect it - is.... wait for it... a certain chap named John Jackson.
Upon writing that name I can almost hear the sound of mouses clicking on the close window for this page. But bear with me. To those uninitiated John Jackson is best known as a chief proponent of the "Birds Came First" BCF idea of theropod/dinosaur evolution. He also has a reputation online for a particularly prickly correspondence to put it mildly. You can do your own internet sleuthing on John Jackson if you are new to this idea or the man - here is a good place to start (read comments). But let me unequivocally state I don't subscribe to the BCF idea of dinosaur/bird evolution (ironically someone brought up a retooled version of this in the comments from my last post). I'll admit it had a certain intuitive appeal to it in the 90's and was fresh and original - but the evidence has not borne it out. John got in contact with me after reading several of my dromaeosaurid posts and we had a brief series of email exchanges (we don't correspond anymore, you can take a wild guess why). I try to keep an open mind on things and gave his self published book - The Secret Dinobird Story (free on kindle) - a read. The book is a bit of a slog to get through, and I told John this. I came away still unconvinced about BCF and this post is not about this topic nor do I want to discuss BCF in the comments. But nestled within John's writings on the philosophy of science, unorthodox family trees, "arboreal stem dinos", and complete eschewing of cladistics ( I have problems with cladistics too but don't think we should discount them), is hidden what I believe is an important and unrecognized broad stroke analysis of how dromaeosaurids actually used the famed killing claw. I can only assume that people who know of John or have perused his book glossed over his bit on the killing claw. It is in my opinion an unpolished gem and should see the light of day. With whatever light I can give this idea I will shine upon it.
The killing claw is not a tool used as a crampon to hang onto the sides of other dinosaurs, nor is it a tool used to pin and hold subequal sized prey items ala the raptor prey restraint hypothesis. It is a tool used with almost surgical precision to slice into and penetrate a prey item in select spots. Such a claw is not optimized to scythe style cut long gashes in the tough hides of prey but instead cuts a single entry hole into prey. A laterally compressed horny sheath with a cutting edge can, after the initial entry into said prey item, now repeatedly plunge into, explore, and cut into the underlying soft tissues. Such trauma will perforate vessels, arteries, lungs, and viscera. Although from the outside trauma will be evinced by a simple entry hole the interior damage will be substantial and often times fatal. Arms and/or jaws assist in stabilizing struggling prey in such a manner to allow entry of killing claws into prey for fatal dispatch. Such a tool is every bit the theropod equivalent to saber - toothed predators and allowed dromaeosaurids an ability to punch well above their weight class in predatory endeavors.
John called it the toe and tail, grab and stab method. He believed that the caudal tendons of the tail worked somehow to help "kick into" the prey item deeper but the (unpublished) revelation that Utahraptor dispensed with these caudal rods causes me to distance myself from that aspect of the idea. As I have discussed before I think that those caudal rods assisted in long range, mid paced efficient travel and it makes sense that Utahraptor dispensed with them as it likely was the >the most predatory< and least adapted to facultative scavenging among known dromaeosaurids due to its size, extreme robusticity, and heavy investment in weaponry. Furthermore the loss of such rods helped the tail in flexibility as it could both deliver and take a beating.
To better understand and see how we got to where we are today and where I think we will be going a quick review of the pertinent thought on the use of the killing claw in these animals dominated by two papers Manning et. al. (2005) and Fowler et. al. (2011) with necessary criticisms.
Not a Slicing Weapon But a Puncture and Pierce Weapon
The main death knell to the "ride the back of iguanodonts and slash at the sides with toe claws" hypothesis of dromaeosaurid killing technique came in the form of a mechanized Deinonychus leg built and utilized by the team of Manning et. al. back in 2005. They found that not only was the hole created by the claw very superficial but slicing through skin in order to create long gashes would be inefficient due to the skin tending to bunch up prevent said slicing. However, as John points out in his book such an analysis is lacking in terms of puncture depth because the experiment did not take into account the compliant nature of body tissue and that the claw can be pressed into the prey animal deeper via body weight and/or muscular force. Long story short the experimenters did not think with enough murderous intention. They did not think like prison inmates trying to achieve fatal blows with self made prison shanks. When body tissue is not put under pressure all the soft, delicate and gushy stuff is relatively safe due to the layer of integument, muscle, adipose tissue etc etc. But press into this tissue - with a prison shank or a dromie killing claw - and the margin of safety diminishes. Stick the knife in and twist.
Critical reception to Manning et. al. is not however new and not isolated to John Jackson. Both Dave Hone (archosaur musings) and Mike Taylor (SV-POW) express similar notions in this interesting back and forth from Ask a Biologist. Many of the points they raise, especially with regards to the problems of "puncture and hold" and the likely inference of a sharp cutting edge to the killing claw can be extended out to criticisms of the now dominant RPR model of Fowler et. al. (2011).
Not a Holding Claw But a Cutting Claw
After Manning et. al. prescribed their case against killing claws slicing meter long gashed into the side of prey items the next big chapter in this saga came in a paper by Fowler et. al. (2011) that brought us the now dominant hypothesis of the Raptor Prey Restraint model (RPR) that proposes a certain commonality with modern birds of prey that grab prey with foot talons, flap with wings to maintain dominant position, and eat/dispatch with the head. As I have said in the past there is much to like here and it is not surprising that many have become somewhat smitten with the RPR restraint model. But as the authors themselves concede the grasping ability of dromaeosaurids is not >as mechanically strong< as modern raptorial birds of prey. Add to this; dromaosaurids do not have truly opposable halluxes like raptors - they can't do a good strong vice grip; longer legs decrease mechanical advantage further diminishing the strength of the grip; and dromaeosaurids had big foot and toe pads which would diminish the tightness of the grip. Because of the big foot and toe pads that dromies had getting a firm grasp becomes problematic, sort of like trying to grasp things with your own toes to a lesser extent. Not impossible for us and not impossible for dromies, but issues arise suggesting a less than optimal performance.
But the final nail in the coffin for the dominance of the RPR model lies in the shape of the claw itself. It is not circular in cross section as we see in extant raptorial birds of prey but is laterally compressed like a knife is. It is just begging us to infer a sharpened cutting edges for the keratin sheath.
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| credit Robert DePalma killing claw Utahraptor (L) Dakotaraptor (R) |
*Update 8/3/17
However even in claws with a more circular cross section it is possible for not one but two cutting edges to develop as shown in this graphic provided to me by John Jackson of a claw from a hawk of the genus Buteo. The black cross section gives an idea of what is possible with a keratin growth over the bony core. Also might be useful to take a gander at your cats claws if you one has so adopted you...
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| Buteo claw credit John Jackson |
The Diminishing Utility of Stability Flapping in Larger Dromaeosaurids
Not to dismiss the RPR restraint model in its totality - I can picture small game and smallish dromies (troodontids especially) - pinning small with their #2 claw and even flapping with their arm wings a bit to maintain top dominance. However in larger and larger dromies this tactic becomes vanishingly feasible and outright ludicrous in Utahraptor sized dromies. It is, in essence the bio-functional equivalent to a fat guy in a little coat, two things combined that don't make sense.
Think about it, Utahraptor was big and robust, like polar bear sized. It was no light weight. The notion that arm wings on such an animal - if they even existed and were not functionally reduced - could generate enough power and lift to achieve any semblance of control and lift necessary for stability flapping should be tossed in the scrap heap of bad ideas. Again, not sail gliding Stegosaurus bad, and it is entirely possible that some small dromies/microraptorines/troodontids engaged in some stability flapping in choice circumstances. But pretty indefensible when we imagine what stability flapping really means - that dominant vertical position is maintained via wing-arm strength flapping strong enough to control not just one body but two struggling bodies - then the utility of stability flapping becomes vanishingly small in dromaeosaurids much larger than - ball parking here - turkey sized? I definitely would say stability flapping is pretty nonsensical in full grown Deinonychus and maybe even Velociraptor...
What Does the Evidence Actually Show?
Which is exactly what the fossil record tells us. Remember we do have a certain Velociraptor locked in mortal combat with a Protoceratops.
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| CC2.0 credit Yuya Tamai Protoceratops & Velociraptor fighting dinosaurs |
We shouldn't feel compelled to explain such a situation as abnormal or a very rare occurrence. It was common enough to enter the fossil record after all. And it is doing exactly what should be expected in the scenario John Jackson laid out in his book. Velociraptor is not hitching a ride on the side of the Protoceratops, not is it stability flapping or grasping the animal with all of its foot claw in some weird type of proto-raptor foot grasp. The head of the Velociraptor is poised to strike but may not be all that important for the killing. The arms and hand claws are very important in the predatory endeavor as they seek to restrain and stabilize the prey animal for ultimate dispatch by the killing claws. Given their increased range of motion and length compared to other theropods they performed a different role in predatory actions. They grappled with prey and helped to hold and stabilize prey which in turn allowed the killing claw to strike with better precision and accuracy. This is directly analogous to the manner in which large felids will grapple with prey using their forelimbs and secure the prey for dispatch via throat bite or nasal blockage. Finally the killing claws are doing exactly what they should be doing... killing!! They appear to be literally gouging into the neck region!! Poor Protoceratops!!
We actually had an earlier iteration of this idea laid out to us by Kenneth Carpenter in a review paper titled Evidence of Predatory Behavior by Carnivorous Dinosaurs (GAIA, 1998). Carpenter raises concerns with the idea of sickle clawed theropods disemboweling prey items but in his analysis of the fighting dinosaurs he makes specific mention of the likely killing style as evinced by this most remarkable of preserved interactions:
Carpenter also provides several diagrams of sickle claws with specific mention of a likely sharp cutting edge. Unfortunately Carpenter strangely backs off of the cutting edge aspect of the claw likening it to being "less sharp than a dull knife". Perhaps he was being a tad overly conservative, in any case Carpenter reasons that sickle clawed dinosaurs had such blunt claws because: "although we don't know how sharp the keratin sheath of a dromaeosaurid claw was, it was probably less sharp than a dull knife because there was no way for a dromaeosaur to hone an edge."
Let's reason this out. Dromaeosaurs revamped their entire hindlimb morphology, literally raising digit two off the ground in walking posture. And we are to presume all of these sweeping morphological changes were done merely to create a claw with a sharpness less than a dull knife?!? Come on now.
If we merely make the defensible assumption that the keratin sheath grew constantly then we can safely assume that the animal had to hone it down through use on prey items or why not simply hone its claw like a felid does? What's going to stop it?
So when we add it all together we see this first iteration of the stab and kill from the inside hypothesis from Carpenter. Unfortunately Carpenter downplays several aspects of it such as the likelihood of a sharp cutting edge and for whatever reason the idea doesn't get much traction, even though it better explains the evidence than what Ostrom suggested or even subsequent works by Manning (2005) & Fowler (2009). Then comes along John Jackson, whom I don't recall if he cited or read Carpenter, but suggests an improved, but still problematic hypothesis, of sickle claw killing. Jackson gets it right I think in assuming a sharp cutting edge and that once the claw had penetrated and pressed into tissue it was free to repeatedly stab and traumatize tissue from the inside. Jackson thinks that the caudal tail rods of dromaeosaurids were necessary for this motion but I have my doubts and the revelation that Utahraptor lost its tail caudal rods but otherwise shows hyper carnivorous attributes supports this. Finally it should come as none too great a shock that the burly iguanodonts, nodosaurs, and sauropods that Utahraptor shared its habit with and which formed its prey base all had a singular vulnerability - a vulnerable neck.
So that is where I stand with what I regard is the leading hypothesis on dromaesaur killing claw function. It is interesting I think not so much for what it says about dromaeosaur killing claws but what it says about us. It is a tale on the transmission of ideas and hypotheses - what counts as a good hypothesis and who is allowed to advance such hypotheses. It also shows how good ideas - though not necessarily perfect when advanced - sometimes get pushed aside or disregarded. Lost in the mix so to speak. But then later on they can get rediscovered, dusted off, and shown the light of day.
I do think that this idea - or collection of ideas, the evolution of a hypothesis via disjointed bits and pieces - will ultimately be one that is nursed back to health, retooled and refashioned.
The Raptor Fossils Project (Utahraptor) w/link to gofundmeUtahraptor
Refs
Carpenter, Kenneth Evidence of Predatory Behavior by Carnivorous Theropods. GAIA no 15 (1998) December pp 135-144 online pdf
Fowler DW, Freedman EA, Scannella JB, Kambic RE (2011) The Predatory Ecology of Deinonychus and the Origin of Flapping in Birds. PLoS ONE 6(12): e28964. doi:10.1371/journal.pone.0028964
Jackson, John. The Secret Dinobird Story December, 2013
Manning, PL, Payne, D, Pennicott, J, Barrett, PM, Ennos, RA (2006) Dinosaur killer claws or climbing crampons. Biology Letters (2006) 2 110-112 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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