Friday, June 24, 2016

Making Dromaeosaurids Nasty Again Part II - No Shame In the Scavenging Game



Scavenging gets no respect. There is a reason you don't see hyenas and vultures put on coats of arms or have sports teams named after them. Lions and eagles yes, but never vultures and hyenas - ironic that all of these animals do scavenge (and hunt to various degrees). Doubly ironic is that in carcass disputes it is often vultures and hyenas driving the eagles and lions off of carcasses. It is a pet theory of mine that  what makes an animal an exquisite and refined hunter often works against them in carcass disputes. Because the refined hunter depends heavily on preserving the physical armaments that allow it to do its job it is at a disadvantage compared to more generalized opportunistic competitors who can be more reckless in battle. To make the sports analogy it is the difference between the tactical and technical boxer whom excels in the specific and controlled environment of the boxing ring and the unrefined street brawler. The highly skilled and nuanced technical abilities of the boxer will win the day over the street brawler in the ring for sure. But take this tactician out of the ring and into the street against the brawling street fighter - who fights dirty, uses bluff and swagger, and can simply take chances with regards to life and limb that the professional boxer can't - and the outcome swings in favor of the brawler. What makes a good street fighter work well is a different skill set than what a boxer has and what makes a good scavenger work well is a different skill set than what a specialized hunter has.

Cue youtube videos:


Note how the tawny eagle just can't hang with the vulture hordes. A better hunter the eagle definitely is - but a better brawler than the vultures? Sorry the eagle just can't bang with the vultures.


Of course I have to include a clip of my hometown hero the California condor displacing the largest eagle of North America the golden eagle. "Step up to me Mr. Eagle?!? Pwwwfff my ancestors used to go to toe to toe with teratornids over mammoth entrails! Go hunt some rabbits!!"





Don't forget to include mention of the epic dominance of cinerous vultures (sometimes called Eurasian black vulture not to be confused with the New World black vulture) Aegypius monachus which often usurps the golden eagle.



Were dromaeosaurids more the vulture or eagle in carcass disputes? Given that dromaeosaurids did not rely on flight to hunt and may have been afforded wing pummeling; that they were literally armed from head to toe in terms of combative weaponry; that they were good sniffers; that we have evidence very suggestive of scavenging; and the ecological imperative to do so; dromaeosaurids were likely very good facultative scavengers. Yeah, so I lean heavily towards more of a vulture than an eagle in these animals. Both in appearance and demeanor.



Taking the theme that I started with on my last post - an over reliance of the import of the "killing claw" in all endeavors dromaeosaurid and refining the dominant RPR model of a "ground hawk" dromaeosaurid - I am going to expand on that in terms of how and why dromaeosaurids were very good, capable, and successful facultative scavengers. The caveat being, I should just explicitly state, that this does not imply a lack to or unwillingness to hunt. But one more thing about that hypertrophied second digit - the "killing claw". It seems several vultures sports such an enlarged second digit - the magnificent cinerous vulture (Aegipius monachus) and the red-headed vulture (Sarcogyps calvus) pictured below.

red-headed vulture (Sarcogyps calvus) credit Dibyendu Ash CC3.0


cinerous vulture (Aegypius monachus) CC3.0 credit Mistvan

Enlarging the toe on the second digit serves a useful purpose in pinning meat down which allows the beak and head to get leverage in pulling bites off. The "killing claw' in maniraptorans might have been used just as much if not more in leveraging good bites while scavenging/feeding as opposed to outright "killing". Additionally the line of action on the dental serrations suggests that the denticles on the back end of the tooth - the lingual side - were doing most of the cutting work as the animal pulled back on food items. It really is all about the teeth and jaws anyways...

Achillobatar scavenging ankylosaur Talarurus by Duane Nash

Ecological practicality of terrestrial scavenging; optimal size; factultative vs. obligate terrestrial scavenging

First things first. The issue of ground based scavenging. We have all heard the argument "you can't have an obligate ground based scavenger because there simply isn't enough carcasses to sustain such an animal". Sounds reasonable enough and this was one of the trump arguments used to disavow the scavenging T. rex hypothesis. Is there a way to test it?

Some researchers recently did just that in a particularly ingenious and creative manner: Body size as a driver of scavenging in theropod dinosaurs (Kane et. al., 2016). Under various conditions of carcass detectability, carcass size, and competition they performed basically a SIMS version of let different sized theropods find dead stuff and see what body size pans out to be the most efficient in terms of cost/benefit analysis. Basically if you are looking for carcasses to supplement your diet the results are congruent with the assertion that only soaring scavengers can make a total living off of scavenging. Both small theropods and large ones had difficulty earning enough calories from facultative scavenging to make the activity worthwhile. If we think about the issues faced by both very small and very large theropods in terms of garnering benefit from scavenging this makes sense. Small scavengers can't cover as much ground and are easily displaced from carcasses. Large theropods - although they can dominate a carcass - have to move all that weight around and by the time that they detect and reach a carcass much of the available calories might already be consumed by smaller and more numerous scavengers.

What the researchers came up with is very interesting in terms of optimal body size for a land based scavenging theropod. There appears to be a mid-sized sweet spot. Small fry like microraptorines didn't do so well and at the other extreme multi ton giants like tyrannosaurids and carcharodontosaurids did not fair too well as facultative scavengers. Facultative scavenging is most optimal in terrestrial theropods in a size range of between (they get pretty specific) 27 and 1,044 kilograms.

Dromaeosaurids slot into that size range very nicely you should note.

While I have read of several people poo-pooing this paper and its conclusions I personally find a lot of merit (with some caveats of course) in this methodology and the conclusions reached. At the lower end of the size scale I would think that Velociraptor and Dromaeosaurus would still  fair pretty well even though they are lower than 27 kilograms. Jackals and coyotes do pretty well as factulative scavengers and they are a bit smaller than 27 kilograms. Additionally at the higher size range let us keep some perspective. Imagine just >one< 50 ton sauropod died in an ecosystem with several multi-ton theropods and loads of smaller and immature theropods. Chances are the big theropods could locate, dominate, and feast on that sauropod for a while. Their large size and relatively slower metabolism might allow them to scarf down a load of meat that sated them for several weeks or even months. So the big boys might scavenge less frequently but when they do dominate large carcasses that glut of food might form the bulk of their caloric intake for quite a while.

Not to mention this paper completely jives with the conclusions of a similar paper that came out in 2011 that has gone a little overlooked... when two independent studies reach similar conclusions that should perk your interest. The 2011 paper is called Intra-guild competition and its implications for one of the biggest terrestrial predators, Tyrannosaurus rex (Carbone et. al. 2011).

Here is my favorite excerpt from the results:



Physical evidence suggestive of scavenging

Dave Hone has written a bit on Velociraptor consuming and - in his interpretation - likely scavenging azhdarchid pterosaurs  here and here. Make sure to go back and read the commentary it is hardly a close and shut case of scavenging. Hone makes a more compelling case for Velociraptor scavenging a hefty size Protoceratops "fighting dinosaurs", part II. Hone also has some papers on the topics but they are not open access, shucks. The blog posts and commentary get the point across though.

credit Brett Booth
Phil Currie and colleague (1995) also describe another azhdarchid consumed by a velociraptorine theropod but don't conclude it was scavenged.

I don't fully agree that the idea of a velociraptor taking down a 2-3 meter wingspan pterosaurs is nonsensical so for me that is equivocal proof of scavenging. However the scraping of flesh off the jaw of the large Protoceratops is a more compelling argument for scavenging.

Additionally you have the famous Yale Deinonychus and Tenontosaurus quarry that might indicate not only scavenging but aggressive intraspecific killing and cannibalism among Deinonychus. And who knows what surprises await us with the new Utahraptor block James Kirkland and co. are working upon...

Roach and Brinkman (2007) advocate a "diapsid like" foraging strategy in Deinonychus and other theropods as the best and most parsimonious null hypothesis. They eschew the notion of cooperative pack hunting posited to bring down herbivores that are several orders of magnitude larger than the theropods but instead highlight scavenging, combat, and cannibalism as the likely culprits that resulted in the taphonomic signal from the Yale quarry. Chief among these is the presence of articulated tails in the quarry the argument being that the bony and tendonous tails were eschewed in favor of more meaty pieces that were hauled off. I do agree with a lot of what they are selling. They do leave the opening for opportunistic group foraging where a congruence of factors might lead to a "mobbing" type foraging scenario. We see this with sharks gathering escorting ailing whales along and literally eating them alive. Or crocodiles gathering at choice feeding situations or latching onto the same prey animal or loosely "cooperating" to shoal prey together. I would also extend this to the manner in which vultures gather around a carcass and use strength in numbers to harass and intimidate other predators off a carcass.

The blog post Raptors: Do They Live up to the Hype (part 2)? Goes a bit further into the Deinonychus saga if you want to go further...

My purpose here is not too weigh too heavily on whether each of the above scenarios are unequivocally evidence of scavenging but merely highlight that there is a body of evidence that points heavily in the direction of scavenging in dromaeosaurids.



On Hell (Creek) Patrol With Dakotaraptor

Few stories in dinosaur paleontology were as exciting and celebrated as the revelation of an honest to goodness mega-dromaeosaur in Dakotaraptor (DePalma et. al., 2015) that inhabited the latest Cretaceous of North America right alongside good ol' sexy rexy. Met with a flurry of fanfare and fan art I decided to let the dust settle a bit before giving this beast the antediluvian salad treatment.

Any epic beast needs a suitably epic soundtrack to go along with it - Judas Priest's Hell Patrol:



Like wild fire comes roaring
mad whirlwind burning the road

Black thunder white lightning
Speed demons cry the hell patrol

There are a few points from the paper on Dakotaraptor that have not received enough attention in my opinion. Specificities that really speak to the adaptations and ecology of an animal that should garner much attention.

Dakotaraptor Is Cursorial

Interpretations of Dakotaraptor that highlight the ecology of this animal as a cryptic, stalker of closed habitats - the "ghost of the forest" scenario - are missing the story that the bones are telling us. This animal was leggy, it was cursorial, and in life it would have departed strongly from the more low slung and slower Utahraptor or Deinonychus.

credit taphonomy CC4.0

I mean just check out those legs!! Keep in mind that this critter was having to go toe to toe with ridiculously leggy young T.  rex and you can imagine the evolutionary imperative to get speedy. Dakotaraptor appears to have made the necessary concessions needed to increase cursorial ability.

As I discussed in my last post there are certain compromises to be made in terms of raptorial grasping abilities in these animals versus cursorial ability as highlighted by Fowler. To review as grasping ability increases the metatarsals shorten for better leverage and strength but running ability is compromised; conversely as cursorial ability increases grasping strength decreases. Also bear in mind that all dromaeosaurids/maniraptorans were relatively weaker than modern raptors in terms of grasping power of the feet - they were not simply overblown red-tailed hawks.

An Interesting Incongruity

If you followed my articles on Spinosaurus and my argument for underwater punting in that animal you  know that I do have a fondness and attraction for anatomical incongruities; two things combined that seem to make no sense but when analyzed in a different light actually offer a more refined animal. In Dakotaraptor we have such an incongruity.

Digit II is hypertrophied. The "killing claw" in this animal lives up to all the hype. Not only is the claw relatively large, the attachment for the muscle and tendons that drive it - the flexor tubercle - is robust. Whatever way in which this animal was using its digit II it is obvious that it has invested heavily in it.

However the incongruity is seen when we move out to digit III and IV where the claws on those digits are not highly recurved nor is there a significant flexor tubercle present. In fact the flexor tubercle is so reduced in those digits to be pretty much non-existent!!

credit Taphonomy CC4.0 arrows point to flexor tubercle in foot claws Dakotaraptor

I mean just look at the contrast in the flexor tubercle attachment in these two claws. For me this is a compelling argument that the classic raptor prey restraint model posited for these animals is lacking. Dakotaraptor was moving further away from grasping things with its foot claws yet at the same time digit II was still large and strong. What is going on here?

Going back to my earlier bit on the cursorial aspects of this animal and resolving its place in the ecology of the environment in which it lived proves useful I contend. Several inferences are useful in giving better precision as to how this animal likely operated.

Inference 1 Dakotaraptor was highly cursorial.

Inference 2 Dakotaraptor - at least in the adult morph - was fairly limited in arboreal capacity. At best they were probably very clumsy in trees due to large size, long legs, and limited foot strength.

Inference 3 Lesser dromaeosaurids, azhdarchids, and tyrant lizards - especially juvenile and rapidly growing teenage rexes - created an especially quarellsome predatory and scavenger ecology. All of these animals would have been usurping, competing, and fighting over carcasses. Azhdarchids, juveniles rexes, dromaeosaurids, and Dakotaraptor all slot into the ideal size niche to be successful facultative scavengers.

Inference 4 Dakotaraptor could dominate pterosaurs, dromaeosaurids, and smaller rexes. However because it could not dominate larger immature rexes - which were still very cursorial - Dakotaraptor itself needed both speed and maneuverability to evade these animals as it was likely not highly arboreal.

Inference 5 The enlarged and robust ungual and claw in digit II primarily aided in combat - especially in competitive skirmishes over carcasses - and assisted in pinning meat down as the teeth, jaws, and neck pulled bites away from the animal (alive or dead). The denticles on the teeth of Dakotaraptor are most defined on the rear (lingual) side as they are in other dromaeosaurids supporting this notion, The RPR restraint model appears less important in this animal relative to other dromaeosaurids in prey capture. Instead digit II could primarily be used in combative endeavors while wing pummeling and the teeth and jaws did the actual killing.

HellPatrolDakotaraptor by Duane Nash

Dakotaraptor may just be a very specialized and rare component of the Hell Creek hunter/scavenger guild. It was large enough and well armed enough to drive off everything but the larger rexes from a carcass. The cursorial adaptations speak to an animal that could cover a lot of ground to find carcasses, prey, and evade larger competitors if need be. In short we see a lot of compromises that facilitated the existence of a sometimes hunter often times scavenger. It was big, but not so big that scavenging became impractical. It was well armed with a massive and powerful digit II but compromised the raptorial abilities of the other digits in favor of cursorial adaptations. The forelimbs were large, feathered, and powerful - great assets for combat, display, and intimidation. In many ways the large and rare Dakotaraptor was possibly like the lappet faced vulture of Africa. Large and strong enough to dominate most other scavengers at a carcass but would step aside for the larger tyrant lizards just as lappet faced vultures dominate other vultures, marabou storks, and jackals but move aside when hyenas show up.

Wings Suggested as Dominance Display

As in my last post on wing pummeling I will get behind another spotential behavior for dromaeosaurid wings that gets surprisingly scant attention - dominance display.

credit Christine Lamberth blog




Such wings could be spread wide over carcasses in order to appear large and more dominant over competitors. Indeed this is the classic show of dominance modern carcass rendering birds (vultures, raptors, petrels) display at carcass disputes. Wing display for intimidation is so painfully obvious I can't believe that such suggestions don't get more attention. I have heard of it before so I can't claim to be the first to suggest it but I am definitely in favor of it. When we look at what modern carcass disputing theropods do with their wings... you have heard that story before though.

Such a shame that vultures do not get the recognition and respect they deserve for their role in modern competitive ecosystems; that they are spiralling into extinction; that they are not posited as the go to analogy for how extinct theropods looked, behaved, and operated when rendering carcasses - despite the fact that, quite simply, they are the modern theropod that does such things most often. Instead grumble, grumble flipping venomous lizards for crying out loud. Venomous lizards.

When you are watching a scrum of vultures feeding you are witnessing a direct portal into to the feeding activities of dromaeosaurids and other theropods in the Mesozoic's past. The theatrical displays; the spread wings; the fast jerky bites; the constant fight for dominance and hierarchy; and, most importantly, the constant and non-stop flurry of motion, action, and violence. Such scenes would be enacted on a much larger and grander scale in the Mesozoic. A difference of degree but not type. A sight both startling and jaw dropping.

lappet-faced vulture and white backed vulture scavenge elephant carcass. credit Chris Fallows CC2.5


Coming up: restoring dromaeosaurids, ugly vs. pretty, and a new hypothesis on dromaeosaurid biting technique!! 

P.S. I will talk of the weird "tail plume" I depicted on the Dakotaraptor in my next post too so don't even ask about it.

References

Carbone, C., Turvey, S.T., Bielby, J. (2011) Intraguild competition and its implications for one of the biggest terrestrial predators. Proceeding of Biological Sciences. Sep 7(278) 1718 2682-2690. online here

Currie, P., & Jacobsen, A. (1995). An azhdarchid pterosaur eaten by a velociraptorine theropod Canadian Journal of Earth Sciences, 32 (7), 922-925 DOI: 10.1139/e95-077

DePalma, Robert A.; Burnham, David A.; Martin, Larry D.; Larson, Peter L.; Bakker, Robert T. (2015). "The First Giant Raptor (Theropoda: Dromaeosauridae) from the Hell Creek Formation."Paleontological Contributions (14).
Fowler, D., Freedman, E., Scannella, J., & Kambic, R. (2011). The Predatory Ecology of Deinonychus and the Origin of Flapping in Birds PLoS ONE, 6 (12) DOI: 10.1371/journal.pone.0028964
Hone, D., Choiniere, J., Sullivan, C., Xu, X., Pittman, M., & Tan, Q. (2010). New evidence for a trophic relationship between the dinosaurs Velociraptor and Protoceratops Palaeogeography, Palaeoclimatology, Palaeoecology, 291 (3-4), 488-492 DOI: 10.1016/j.palaeo.2010.03.028
Hone, D., Tsuihiji, T., Watabe, M., Tsogtbaatr, K. (2012). Pterosaurs as a food source for small dromaeosaurs Palaeogeography, Palaeoclimatology, Palaeoecology : 10.1016/j.palaeo.2012.02.021
Kane, A., Healy, K., Ruxton, G.D., Jackson, A.L., (2016) Body size as a driver of scavenging in theropod dinosaursAmerican Naturalist June 2016, V.187, No. 6

Roach, B.T., Brinkman, D.L. (2006) A reevalutation of cooperative pack hunting in Deinonychus antirrhopus and other non-avian theropod dinosaurs. Bulletin of the Peabody Museum of Natural History 48(1): 103-138





Thursday, June 9, 2016

Making Dromaeosaurids Nasty Again Part I - Wing Pummeling Abuse

"Night Terrors" Sinornithosaurus by Duane Nash

Making dromaeosaurids nasty again... Yes, there is a bit of a straw man argument in there because many might say they never stopped being nasty. But it is my straw man to make, tear apart, refashion, and burn to smithereens as I will... so while reading these posts on dromaeosaurids always keep in mind the tug of war between past and present interpretations scientific, artistic and popular. Don't forget as well these animals had a long tenure as small to medium sized predators so there is a lot of room for variation in terms of behavior, physiology, and appearance.

Something has happened to dromaeosaurids. They have went from jumping on the back of giant ornithopods to jumping on the back of opossums. They no longer look like the scaly, crazed, methed out gang overlords of the Mesozoic bestiary but instead dapper, attractive fashionistas that glided out of some Mesozoic audobon photo shoot. The psycho reptoid wolf-lizard of the Mesozoic is now the camera friendly fashion model of the Mesozoic.

Probably my favorite ol' school Deinonychus image credit William Stout
New school Deinonychus taking on smaller game in RPR model credit Emily Willoughby

A very attractive, dapper Deinonychus by Emily Willoughby. CC4.0


While the ol' school raptor I grew up with is now nothing more than a nostaligic memory ready to join the ranks of swamp bound brontosaurus I am not entirely at ease with all aspects of this new dapper "ground hawk" model for dromaeosaurids. Something just does not sit well with me.

First things first, the claw. Dat claw. Second to ol' sexy rexy and his chiseled and ruggedly masculine good looks the "killing claw" of dromaeosaurids and especially Deinonychus is probably one of the most emblematic and iconic elements of the theropod predatory arsenal. Are we focusing too much on the "killing claw" in terms of prey dispatch but negating other aspects such as the hand claws and teeth?

A brief review of several of the more pertinent works that have brought us to where we are now.

The Slashing Claw Denied...

While the youngins today have grown up knowing that the killing claw of these animals did not cut scythe like slashes through the hides of dinosaurian megaherbivores I do have to admit to feeling a pinge of let down in my inner fanboi when this was first revealed via the study "Dinosaur killer claws or climbing crampons" way back in 2005. Essentially what Manning et. al. did was build a robotic hydraulic Deinonychus antirrhopus claw and lower leg and attempted to drive it through a pig carcass. Instead of the meter long slashes of lore the results were a little underwhelming. The claw did puncture the carcass but as they attempted to drive it through the flesh tissue simply bunched together below the entry preventing a long slash wound.



In comparing the morphology of the claw they found it most lined up with the claws of climbing animals. Manning et. al. did not totally eschew the notion of dromaosaurids leaping onto the sides of megaherbivores but offered an alternative; the killing claw was now a crampon which allowed the predators to jump onto the side of prey and deliver slashing bites with the jaws.

Jumping on the Flanks of Giant Herbivores Denied...

Seems pretty legit right?

Well the next paper seemingly expunges the use of the claw in climbing onto the back of megarherbivores. Enter the Raptor Prey Restraint model (RPR) which probably needs little introduction to most readers here. Fowler et. al. reject the notion of dromaeosaurids latching onto the flanks of large herbivores and slashing at prey with foot claws or teeth. Instead prey subequal to the size of the dromaeosaur is pinned down beneath the weight of the predator; stability flaps of the "wings" and movement of the beam like tail help maintain an upright position; and prey is essentially eaten alive if it is not killed outright by the feet.



And you know... I like this a lot. It makes sense - there is a good analogy with modern accipitridae and it has a nice little thematic thing going on with the flight exaptation given that dromaosaurids are likely secondarily flightless. The authors also make a very striking contrast between dromaeosaurids and troodontids that suggests a degree of partitioning between the two groups. Dromaeosaurids had shorter but more powerful metatarsals suggesting a larger prey size seized by the feet than troodontids which compromised strength for cursorial ability and a quicker grip on smaller prey.

So ground hawk Deinonychus... what is not to love?

There are some caveats to this model that the authors address that I think warrant repeating:


So, despite the arguments in favor the RPR model for dromaeosaurids and the inherent attractiveness of the model it appears that dromaeosaurids were not >as good at it< as their modern avian counterparts. I am not saying RPR is not a thing and, again, this is from their own paper but it appears that a 50 kg Deinonychus is not equal to a scaled up 50 kg red-tailed hawk in terms of relative grasping power of the feet. Furthermore if you look at the vice like grip that modern raptors can enact in which digit I is rotated completely opposite the other three digits it appears that Deinonychus and other dromaesaurids do not fully rotate the digit opposite the other three for a truly powerful vice like grip.


credit eaglesohio
Again, not trying to imply that RPR is not a thing for dromaeosaurids or that they did not pin and even kill prey with their feet but let us be clear with what the science shows as of now. In dromaeosaurids the RPR model is a relatively less powerful and less efficient version than modern raptors essentially due to the anatomical concessions of retaining some cursorial ability. But I am sure it was still unpleasant...

Actually what dromaeosaur foot graspers remind me of is cats claws and paws (gasp!! mammals). No really... anyone who has had a cat "knead" on them (i.e. breadmaking) knows what I am talking about. As the cat's claws clench shut in a semi-opposable fashion the claws actually pin stuff against the lower arm. In dromaesaurids grasping things would be pinned against the bottom of the metatarsus. This is still a pretty good grip and with both legs working together likely very efficient.

Dino Kitty
The Role of the...
ArmsWings?

I am obviously not the first to ponder this and it is a perplexing issue in many, if not most, predatory theropods - but what were the clawed wing/arms of dromaeosaurids good for anyway? Fowler et. al. posit them as dynamic stabilizers that combined with the fully feathered tails (like Archaeopteryx not Caudipteryx plumes) allowed these predators to maintain vertical superiority via flapping over prey caught in the foot claws. I like this a lot. So please don't misquote me when I argue that there is something to augment this method not completely replace it.

Those big hand claws seem like such a waste if all they did with their arms is use them in stability flapping.

Issues have been raised as far as the practicality in using the hand claws to grasp small prey dexterously or even bring the hands together to grasp small prey with precision. I honestly don't know what the current thinking on this issue is - or if there is any consensus at all? I would love to hear thoughts and input in the comments section... From what I gather on my cursory research things seem a little equivocal. The wikipedia page on Deinonychus mentions papers that support grasping and others that bring forth some practical questions. Some observations that cause me to question fine tune grasping of small prey items include the potential issue of the "wings" getting in the way of each other when brought together. Also and this is my general observation of patterns in clawed grasping animals: it seems reasonable that in order to achieve a powerful and concise grip that the digits and claws work better aligned in a similar plane. Essentially when you look at the grasping claws of a felid, modern avian raptor, or hell... even our own hands what you see is the digits not varying tremendously in terms of length and that they line up together relatively closely when clenched.

This was simply not the case in maniraptorans with extremely divergent digit length. Such spindly, long claws just seem a little less than ideal for enacting a powerful and concise grip on something small that needs precision. Gripping a tree trunk or "bear hugging" an animal that is fairly large yes... but dextrous grabbing and manipulation of small stuff, I don't buy it. Especially with all those feathers in the way. Could hands like the ones below deftly grab and pick up say a scurrying lizard or mammal? 

credit John Conway. Deinoncyhus (L) Archaeopteryx (R) CC3.0
The digit lengths are just different from the pattern we see in other grasping predators even other theropods. For example in Allosaurus fragilis:

Allosaurus hand. credit Domser CC3.0

Another point is that if a dromaeosaurid wanted to reach out and grab a small prey item doing so with the head and mobile neck or even the feet seems more ideal. The arms - like all theropod arms - had limited mobility in the forward plane. Several of the problems in forelimb usage are summarized in a paper by Phil Senter comparing the forelimbs of Bambiraptor and Deinonychus:



For these and other reasons I find  "fine tune grasping of prey" hypothesis more than wanting. Enough so that other hypotheses warrant exploration.

The hypothesis I will offer - not sure if this idea has been explored yet anywhere to tell you the truth - is going to highlight exaptation of the flight stroke and musculature of the maniraptoran arm to a high degree. This is consistent with the strong hypothesis put forth by Gregory S. Paul that dromaeosaurids are secondarily flightless.

The clawed wing arms of dromaeosaurids could potentially act as clawed battering tools that would further bludgeon, wound, and traumatize prey and/or competitors especially that have been pinned by the feet.

Why not? The arms were strong, long, and heavily clawed after all. More so modern birds just love to bludgeon and smack other things around with their wings. Made famous in a series of posts at Tet Zoo(here 1, part 2part 3wrestling birds) by Darren Naish, some wings even have weaponized claws, clubs, and spurs. Dromaeosaurids - likely being secondarily flightless - already had the exaptation to use their wings as bludgeoning tools. All the musculature was already set up for it.

Two Deinonychus have a disagreement. Provided by Robin Liesens (Dontknowwhattodraw94)

As I mentioned earlier the study by Fowler argued quite well that dromaeosaurids were not >as good< as modern raptors in terms of prey dispatch via the foot claws. Other tools might be needed for prey dispatch... Why not use those nice hand claws powered by the incipient flight stroke to further gouge, pummel, and weaken prey that is being grasped by the footclaws and jaws? Not saying stability flapping did not happen just that stability flapping used in conjunction with with wing pummeling might have some merit. Furthermore the need to maintain vertical position over prey might be just a tad overstated - don't forget the fighting dinosaurs!! After all dromaeosaurids had a little bit more liberty in terms of getting down and dirty on the mat as opposed to accipterids which always have to be mindful of getting grounded with a serious wing injury.

CC 2.0 credit Yuya Tamai. Protoceratops & Velociraptor
And modern birds do love to fight and 'rastle!!


Did you check out that eye gouge at about 1:03? Note how right above the eye is nice a ridge of brightly colored, caruncled tissue... remind you of a suggestion I made before?


Or the above video which actually doesn't feature a stork eagle fight but loads of domestic breeds battling one another (uuurgh there Nash goes with domestics again). Loads of fleshy caruncled faces, face biting, wing pummeling, talon thrusting. In short very awesomebro!! but not necessarily without merit just because it is awesomebro!! You see the pattern - lunges and strikes are made with the head and/or feet. When brought into the line of fire of the wings, pummeling commences.

Tsaagan dispatching Velociraptor. work in progress Duane Nash

This methodology of prey capture/combat is essentially a bit of an inverse of a common tactic used by felids. Anyone who has a pet cat  (or who has a cat that has them?) should be familiar with it. If not simply stroke the vulnerable belly of a said felid and the forelimbs and/or jaw will lock into your arm and the back legs commence clawed kicks. In dromaeosaurids it would be the hindlimbs and jaws locking prey into place and the forelimbs delivering blows and trauma via wing pummeling.



Deinonychus wing pummeling Zephyrosaurus. credit Robin Liesens (Dontknowwhattodraw94)

In addition to the analogy to modern bird wing pummeling the analysis of theropod stress fractures and forelimb avulsions (Rothschild et. al., 2001) came up with some interesting results with regards to Deinonychus limb use: 43 hand bones and 52 foot bones were examined for signs of stress fracture - none were found. However the second phalanx in the second toe (the killing claw) has a healed fracture (YPM 5205). Why is this important? Well it suggests that the hand claws are not hooking into things and holding them tightly such as appears to be the case with Allosaurus which shows multiple manual pathologies - but the healed killing claw suggests that digit II is hooking into and holding struggling prey/combatants to a higher degree. On the other hand it is worth asking why wing pummeling would not create stress fractures? Perhaps with the force being distributed across the whole surface of the wing (including the feathers) stress fractures would not be such a problem? Are stress fractures a problem in modern birds that wing pummel? Could be an avenue of exploration...



Really I am quite surprised that wing pummeling in dromaeosaurids (and other winged dinos/maniraptorans) has not been proposed before... I mean has it? I dunno, can't find any mention and it seems like a pretty logical inference from what I have gathered.

Two potential criticisms I want to address:

"Yes but modern birds use this wing pummeling in antagonistic disputes not predatorial. Birds of prey do not batter their prey with their wings."

True. Remember Fowler quite convincingly argued that dromaeosaurids were not  as relatively >strong< in grasping as accipterids. Does this imply that they were small game specialists? I think not. My contention is bolstered by the "fighting dinosaur" specimen. You will hear some researchers try and explain this situation away as a "rare" or "aberrant" exception to the "baby killer specialist" or "small prey only" model. With all due respect I think that they are mistaken. Not that loads of baby dinosaurs were not munched on, merely that dromaeosaurids and most theropods were not "specialized" for that task.

Given that dromaeosaurid foot grasping was meh compared to modern birds of prey but they were still getting into the thick of things with some pretty rugged combat other lines of attack should be invoked. Those big hand claws seem awfully put to waste in mere stability flapping. Especially when attacking strong retaliatory prey like protoceratopsids.

"Why don't birds of prey pummel prey with their wings?" As I alluded to earlier accipterids differ from dromaeosaurids in that they are dependent on flight. Wing pummeling for them may be selected against as a predatory strategy because they risk an injury, not to mention their feet do the job just fine.

"What about the claws getting stuck in the flesh and skin of the prey during pummeling. Could that be a problem?"

I don't think so. If the downstroke can enmesh the claw in the animal then the upstroke can pull them back out. If claws getting stuck in stuff was so much a problem I guess that implies these animals could not use their claws to climb and clamber in trees as well, because their claws would get stuck in wood, amirite?

Given that; dromaeosaurids are likely secondarily flightless; that their digit morphology is aberrant from other predatory "graspers"; that wings may have got in the way of grasping, especially of objects on the ground; that the fingers remain spread during flexion; that one handed clutching of objects to the chest is just weird; that the elbow can not be fully extended and forelimb mobility is limited;  that extant aves often use their wings - sometimes coupled with knobs, spines, and claws - to pummel other animals; for these reasons I posit the hypothesis that clawed wing pummeling is a promising tactic used in dromaeosaurid predatory, combative, and defensive endeavors.

At the moment I see no reason that the idea of wing pummeling can not be extended to other winged dinosaurs and maniraptorans. Hello wing pummeling GallimimusGigantoraptor, Therizinosaurus, and Deinocheirus!!



Next up: biting, scavenging, scrumming, and bone cracking dromaeosaurids.



References

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

Manning, PL, Payne, D, Pennicott, J, Barrett, PM, Ennos, RA (2006) Dinosaur killer claws or climbing crampons. Biology Letters (2006) 2 110-112 pdf

Rothschild, B. Tanke, D, Ford, TL (2001) Theropod stress fractures and tendon avulsions as a clue to activity. Mesozoic Vertebrate Life. editor Tanke, D & Carptenter, K. Indiana University Press pp331-336

Senter, Phil (2006) Comparison of forelimb function between Deinonychus and Bambiraptor (Theropoda: Dromaeosauridae). JVP Volume 26 Issue 4 2006


"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

"It is not the critic who counts; not the man who points out how the strong man stumbles, or where the doer of deeds could have done them better. The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood; who strives valiantly; who errs, who comes short again and again, because there is no effort without error and shortcoming; but who does actually strive to do the deed; who knows great enthusiasms, the great devotions; who spends himself in a worthy cause; who at the best knows in the triumph of high achievement, and who at the worst, if he fails, at least fails while daring greatly, so that his place will never be with those cold and timid souls who neither know victory nor defeat." Theodore Roosevelt

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As I have seen an uptick in traffic and subsequent comments that lower the standard of conversation on this blog I will be moderating the comments section from here on out. I don't have a comment policy other than it is my blog and I will do whatever the hell I want to with it and ban whoever and whatever comments I want. Disagree with my ideas - fine, disagree with me so strongly that you launch smear campaigns and rants against me... fine... go make something original on your own. You will still be banned here.  As several recent commentators now are - banned for life. It should not be too hard to find out who those are from recent posts as I will leave their comments up as fair warning to others.

There is a big ol' internet out there and I don't need you.

Over all though I am happy with the input, differences of opinion, and general intelligence of the commentators. You guys are my "peer review" as much as I can muster at least and have helped me change and refine my own thoughts and perspectives numerous times. Keep it up.

Sorry if you thought that theropod lip post was happening now, a little bait and switch hahaha... some other stuff beforehand that is very interesting to build suspense...  but I am still coming for ya' lizard lipped theropods.













Thursday, May 26, 2016

Your Puny Lipped Sabertooth Kitty is SMASHED!!

I knew in my last post that some might take exception to my tone a bit... ahem... maybe the 'rastling analogy went over their heads. I went with my gut and my gut said take the fight to 'em which is what I did knowing full well that the blowback would be real and palpable. However the blows I delivered came in the form of valid, scientific arguments.  The majority of blows I received were anything but.  Again some might take exception to my approach calling it "antagonistic " or "childlike" but these critics did not get attacked like I did. No one enjoys getting called "inane", "ludicrous", or a "jealous idiot" (Ok I might be jealous some times and an idiot at other times but never the two at once). But this is all the majority of the arguments really amount to; name calling; dismissive; and clinging to custom. While there were some intelligent questions and criticisms I did not find any of them damning to such a degree that they warrant scrapping the hypothesis. Indeed the multiple lines of evidence I offered in that post will be further augmented in this post to such a degree that any attempt to invalidate this hypothesis need to disassemble each individual argument one by one not merely poke holes in individual arguments to cast shade. Because as much as I pleaded and prodded for scientific reasons why a small lipped sabertooth predator is the better and more supported option than what I suggested I received nada. Nothing. Nunca. There is literally nothing that puny lipped sabertooths give advantage to over big lipped ones. Indeed big lipped sabertoothed predators should have always been the null hypothesis but cultural inertia, dogma, and "awesombro" exposed toothiness got in the way.

Smilodon populator. used w/permission credit deviantartist Dontknowwhattodraw94 (Robin Liesens)


Another possibility, more classically felid. credit deviantartist Unloborgis. used w/permission (c) Sergio Perez


All the while I knew that if I shook the tree hard enough something might just fall out... which it did and it splattered everywhere.

Time to crack this thing wide open...



And crack is the word of choice here because as traditionally depicted cracking is what sabertooth canines would be doing a lot of. Not just because of being exposed to blows from prey/competitors. Not just because of grit working away at the denticles. But because of chemistry.


Special kudos to Brad McFeeters for turning me onto the abstract of an as yet to be published work on the relationship between enamel health, saliva, and closed oral lips. Something any good dentist could tell you. Indeed it was a man who said he teaches students taking the dental admission test that presaged this very fact to me in a comment from my last post:


Please excuse the "appeal to authority" but it is none other than the American Dental Association that refers to saliva as "the bloodstream of the mouth" :



Intuitively this makes complete sense. Ever fall asleep with a stuffy nose and then wake up with a mouth that feels like a crypt? The bad breath and general oral malaise of just spending one night with your mouth exposed and dried out is but a preview of the dental horrors that would befall a sabertooth predator spending a lifetime with its physiologically expensive canines unsheathed.

Here is an excellent, easy to follow, and well referenced summary of the benefits of saliva by the European Food Information Council:  Saliva - more than just water in your mouth

Of special relevance is the chemistry of saliva which is crucial in maintaining the mineral balance of tooth enamel and dentine and preventing the loss of calcium and phosphorous. I am not going to pretend to be a chemist or that I can explain it any better than they do so let me just outsource their summary below:


Essentially not having saliva - which is buffered with loads of calcium and phosphorous - would open up the crystal structure of hydroxyapatite (the building block of dentine and enamel) to chemical attack.   The attacking molecule that would strip calcium and phosphorous out of the crystal structure is... water "in water the crystal would steadily lose ions form the surface and shrink". Best keep your cutlery sheathed and lubricated with saliva otherwise you are looking at some brittle canines because water is pretty much everywhere!!

Time To Stomp the "Tusk" Argument to Smithereens


Clearly I did not state my case strongly enough in the previous post that the last thing you want to compare sabertooth canines to is tusks which differ in just about every fundamental metric from sabertooth canines. Never the less you still have people chiming in "but what about fanged deer"? Please don't insult these proud and magnificent predators by comparing them to..... deer ( I jest, just a little). To be fair musk deer have continual growth of their canines and as I will explain shortly tusked deer will most likely share histological features of other tusked animals. Let me reiterate; tusks are sexo-social symbols and hence need to be displayed while sabertooth canines are not; tusks are used for not only combat but coarse gouging of sediment, bark, roots, and other gritty abrasives - sabertooth canines do none of these activities, in fact to the contrary, the fine serrations on many sabertooth canines would be damaged through excess grit; tusks grow constantly, sabertooth canines do not. These are all patently good arguments that are also essentially adaptationist in their approach. However paleontology and evolutionary theory have fallen out of love with adaptationism to some degree dismissing such arguments as "just so stories". I blame Stephen Jay Gould for this pervasive sentiment. I don't mean to kick a man when is down (literally six feet under in this case) but Gould was a little less than precise in giving us good examples of evolutionary spandrels after all. But I digress.

This just means I got to look a little bit harder and deeper than "just so stories" to persuade people that tusks are the last thing that you want to compare to sabertooth canines. Luckily enough for us there is quite a bit of information out there on tusks because ivory - the cultural trade and carvings of large animal teeth - has been a pretty important thing in human economics that people have looked at. When we investigate the histology of  tusks (i.e. large exposed teeth) there are some interesting surprises...

credit USFW services

Notice that in the schematic above there is just a bit of enamel at the tip but the bulk of the external layer is actually cementum, followed by dentine. Now in the hierarchy of hardness scale enamel is hardest, then dentine, and finally cementum. However given that enamel occurs only at the tip where it is actually worn away during the life of the tooth it is a bit of a paradox. Why would tusks - which are subject to all sorts of rough and tumble activity - only have such a paltry layer of the hardest structure which is enamel? Not only that - it is counterintuitively cementum that covers the bulk of the exterior of tusks which is the softest of the three materials? It's a bit of a paradox that tusks disinvest in enamel but invest heavily in the softer tissues of dentine and cementum (also btw which is why ivory can be carved). The reasons I would suggest are manifold:

1) Enamel once it is formed can not keep growing, only maintained. Dentine on the other hand can be grown continuously via odontoblastic cells in the pulp cavity - which is why tusks keep growing. Cementum too can repair itself and grow continuously via cementoblasts.

2) Enamel is the hardest structure of the three but also the most expensive physiologically to grow, form, and maintain. Minerals are very precious things after all and investing so much mineral wealth in a structure that will be winnowed away is maladaptive.

3) Because the enamel in tusks is not constantly bathed in a buffering solution of saliva that maintains calcium and phosphorous balance of the crystal structure it is prone to not only physical attack but chemical. The investment of enamel in the tip of the tusk is a throw away investment. Over time it will be lost. From USFW on elephants/mammoth tusks: "Enamel is only present in the tusk tip in young animals. It is soon worn off and not replaced."

4) I will suggest that cementum and to a lesser extent dentine - having less of a solid crystalline structure than enamel and a higher percentage of other protein tissue that form it (e.g. collagen) - are less subject to chemical attack than enamel in the exposed state and offer an ideal compromise: just enough hardness to get the job done without the excess chemical attack that enamel suffers. Again, both dentine and cementum can grow constantly while enamel can not.

If this is a robust and true pattern we should see it again and again in animals that leave their teeth (i.e. tusks) exposed to the air. Through convergence this prediction should be met and these animals will disinvest in enamel on the exterior layer of the tooth and have relatively higher investment of the less mineralized and softer dentine and cementum.

And gee willikers look what we have here:

Walrus (Odobenus rosmarus)

From the USFWS: "The tip of a walrus tusk has an enamel coating which is worn away during the animal's youth."

cross section walrus tusk. C=cementum, PR=primary dentine, SD=secondary dentine

Also of note cementum is softer than dentine. This follows the somewhat counterintuitive pattern in having the least mineralized (softest) part of the tooth exposed to the environment as to not become demineralized. The cementum literally shielding the dentine from exposure.

Sperm and Killer Whales (Physeter catodon & Orcinus orca)

From the USFWS: "Both species display conically shaped teeth with a small amount of enamel at the tip. The rest of the tooth is covered by cementum."

killer whale
Which of course reminds me of the population of offshore killer whales that specialize in feeding on deep water sharks. This population pay a heavy toll in the form of declining dental health with old adults in such populations showing severe tooth wear reduced to gumming shark liver and the young doing most of the tearing up. Not too surprising that captive killer whales have horrible dental issues as well.

Narwhal (Monodon monoceros)

From USFWS: The narwhal horn (actually a modified upper incisor): "Enamel may be present at the tip of the tusk."

Narwhal
Wart hog (Phacochoerus aethiopicus)

From the USFWS: "a longitudinal enamel band with approximately one-half to two-thirds coverage mark the tusk's surface in the raw unpolished state."

Wart hog
Hippopotamus (Hippopotamus amphibius)

From the USFWS: "A broad longitudinal band of enamel covers approximately two-thirds of the surface area of the tooth... The surface which is not coated with enamel displays a very thin layer of cementum... Hippo incisor crowns are covered with enamel."

Hippopotamus
Wait a second here, the hippo seems to be breaking the pattern of tusked animals that have minimal enamel coating on the exterior of the tooth. What is going on here? All is not as it seems with this tusked animal. The hippo is a tusked animal that keeps its tusks and dentition covered with big lips that don't quit. Therefore it gains the protective benefits of saliva and can afford to have enamel rich dentition. By the way why does no one consider the massive lips of hippos preposterous or ridiculous but big lips on a smilodon? that is just ridiculous.

credti Quartl CC3.0 Huge Lips to Cover Huge Tips
The Croc Question

"Ok then, what about crocs? They leave their teeth exposed to the elements - including water - which can itself strip away calcium and phosphorous?"

All is not as it seems as well with our smiling crocodilian friends. Their sinister smile betrays a sneaky way around the problem of exposed teeth. First of all - analogous to ever growing tusks of mammals  - crocodilian teeth have the ability to get replaced. Second of all crocodilian teeth show a histology more congruent with tusked mammals than the enamel rich dentition of mammalian carnivorans or predatory theropods. Their teeth actually act more like miniature tusks in that they too share the pattern of a thrifty use of enamel on the exterior.

A recent study of this very issue (Enax, 2013) provides some telling information:


Some things attract my attention here.

"Virtual sections through the tooth and scanning electron micrographs showed that the enamel layer is comparably thin."

To beat that drum again that is exactly the pattern in tusked animals with exposed teeth and opposite the pattern in animals with closed lips and sealed mouths that allow teeth to bathe in saliva and remain enamel enriched.

"The crystallites in the enamel are oriented perpendicularly to the tooth surface."

Very interesting here in that the crystal structure - being perpendicular to the tooth surface - is oriented in a manner that minimizes exposure to the outside environment. Could a very logical (and adaptationist) hypothesis be that this orientation is not haphazard but in fact that it has selective benefit in minimizing surface area exposure to chemical attack from the environment?

From the discussion section and conclusion:




The authors explain the thin layer of enamel in crocodiles which contrasts to the thick layer of enamel in mammals due to "the enamel of crocodile teeth is very thin.. because crocodiles do not use their teeth for cutting and chewing." I don't really understand why people say crocodile teeth do not cut, I mean tell that to anyone or anything that actually who has got bit by a croc. Those teeth do cut. Point taken they are >mainly< graspers but they can inflict some pretty nasty damage as well.

Never the less it is readily apparent that the authors failed to compare croc teeth to exposed mammalian teeth (i.e. tusks) which they do share a convergence of composition, microstructure, and hardness. By the way also interesting that croc teeth compare in hardness with human teeth given the high disparity in bite strength. Probably helps in preventing brittle deformation in croc teeth but still strange to think about.

I would venture to say that in the worn teeth of old crocs all of the enamel has been worn off through either mechanical or chemical wear and you are in fact looking at the dentine after the thin enamel tip has been worn away.

any enamel left in those tooth crowns? I doubt it. American crocodile. credit Daderot public domain

 Shark teeth... well I don't even want to confuse the situation further.

The main point is that croc teeth and mammalian exposed teeth (i.e. tusks) show congruence in composition and structure that suggests a possible selective advantage in being thrifty with the amount of enamel coating the tooth. Tusk bearing animals that maintain an oral seal via lips such as hippo can coat their teeth with relatively more enamel due to the salivary benefits to enamel health.

So the million dollar question is: if sabertooth predators indeed left their canines exposed to the environment then they should show histological tooth characteristics similar to crocodilians and exposed tooth bearing (i.e. tusked) mammals such as minimal enamel coating of the tip and potentially a layer of cementum covering the coat of most of the tooth. If they were just good ol' mammals that kept their lips shut and their teeth sheathed and bathed in saliva then we should see bountiful amounts of enamel especially on the exterior of the tooth.

So which is it?

Unfortunately I can not find adequate and thorough histological work on the microstructure of all sabertooth predators. But I can find such work on the creme de la creme, el utlimo hombre of sabertooth predators Smilodon fatalis. If this predator keeps the pattern of thick enamel forming the exterior of the working tooth and not just the tip it squarely lines up with animals that cover their enamel rich teeth in big luscious lips and bathes them in a protective broth of saliva. And if this penultimate of sabertooth predators can cover its extreme canines with lips there is all the reason in the world to expect all the other sabertooth predators to fall in line.


Answer: Smilodon fatalis is a good ol' mammal that maintains lots of enamel... ergo it had big ol' lips draping over said enamel.

*Update thylacosmilids had more of a tusk like growth pattern with only a shallow enamel layer








From the paper Cementum on Smilodon sabers (Riviere & Wheeler, 2005) these depictions clearly label the cementoenamel juction. Everything on the exterior of the tooth towards the tip from this junction is enamel. Smilodon is not like a tusked mammal or crocodile but congruent with every other enamel rich mammal that sheathes its teeth in lips and saliva.

Interestingly the Riviere & Wheeler paper came to the conclusion that gingiva - in other words gum tissue - covered the root of the tooth all the way to the cementoenamel junction.

From (Riviere & Wheeler, 2005):


This interior soft tissue protection would inhibit infection, alveolar bone resorption, periodontal disease, additional tactile capability, and tooth stability. In short very consistent with the soft tissue benefits of large lips and a sealed oral cavity for tactile ability, protection, lubrication, and enamel health. It all meshes together to create a very gummy, lippy, and infinitely more adaptive vision of these predators than is classically portrayed.

used w/permission credit LWALTERS. deviantart page LWPaleoArt

Here is a more classically felid look that could work too.

This is for the haters - if your mind changes as the facts change this need not apply to you, move along and let it not apply to you - but if you attacked me with name calling or just dismissed my arguments as "ludicrous" then yeah, this is aimed directly at you. Can you feel that?



Call me childish, but hey, talk to me after you take a chance on something and take the shots I do. Gonna rub it in their face just a little bit.

And finally back to the yet-as-unpublished work by Robert R. Reisz that Brad McFeeters brought to my attention after the last post was published. Presented at the 2016 Canadian Society of Vertebrate Paleontology. Unpublished stuff is always "iffy" but hey if CNN is gonna cover it and the press is all over it then fair game for me I say. So far the press has put out the stock line you see in all theropod related news and somehow T. rex has to be the star of the show... all the while (predictably) ignoring the more obvious implications for sabertooth predators that this study also hints at.




I am very bolstered by the observations by Reisz and Larson on the histology of tusked mammals differing from theropods which converge with my argument regarding tusked mammals falling down as an analog to sabertoothed predators. However I would certainly encourage Reisz and Larson to look more closely at their statement "we propose that this requirement of hydration is not possible to maintain if the tooth is exposed permanently". As I discussed earlier saliva's benefit has much to do with maintaining chemical equilibrium of enamel while water that is not buffered with calcium and phosphorous can strip it away. Water has some pretty unique chemical properties (dipole moment, net negative charge) that can overtime strip enamel of positively charged calcium and phosphorous. Not saying that hydration does not play a role in all of this - it likely does - but chemical equilibria is another important aspect of enamel health. Again, remember hippos were the only "tusked" animals that have a lots of enamel on the exterior of the tusk but only because they keep them bathed in saliva.

To drive the point home one last time:

Facial tissue completely or mostly sheathing the upper canines as is corroborated by ALL extant terrestrial mammalian carnivorans and is the best null hypothesis; exposed and constantly growing tusked mammals use their non-serrated tusks for coarse hacking, chopping, digging, combat and display and are an inferior analog to sabertooth canines and need not be considered as they fail in comparison along nearly every metric; exposed toothed mammals (e.g. tusked) and exposed toothed crocodilians have differing tooth histology from sabertoothed predators*; unlike smilodon these animals have minimal enamel laid down at the tip of the tooth often worn away during the animal's lifetime; exposed toothed animals have an exterior tooth largely covered in cementum or dentine, both of which can grow constantly unlike enamel; saliva plays a crucial role in the dynamic chemical health of enamel; saliva provides a buffering role in hydroxyapatite crystal as saliva is enriched with calcium and phosphorous preventing demineralization of enamel; this salivary coating is only maintained in closed mouth (e.g. lipped) animals; the clouded leopard which has long canines comparable to many sabertoothed predators covers its canines completely; all five radiations of sabertooth predators display osteological evidence of protective sheathing on the lingual inferior aspect of the canine via a mandibular flange - a logical evolutionary inference is that protection for the superior labial aspect of the canine was also selected for in the presence of a large fleshy upper lip; the presence of this large fleshy upper lip is corroborated osteologically by the relatively large infraorbital foramen found in all sabertooth predators; this large infraorbital foramen supplies the blood and nerve supply to an extremely large and sensitive "nerve pad"; the extremely innervated nerve pad provides tactile support to make precise and crucial placement of canine entry for bite as well as early warning for violent torsional twisting of prey that could damage/snap canines; such tactile support would be diminished or non-existent in sabertooths depicted with modest sized upper lip region as this area would be scrunched away from the bite area when the mouth is opened and it would be the vulnerable canines that would "feel out" where to bite; extensive gingiva possibly reaching up to the cementoenamel juction protected, sheathed, and provided tactile support to the canines; large lips, gingiva, and supporting nerve pad evolved in lock step with increasingly large canines and forequarter strength for maximum safety and efficiency in these highly precise yet vulnerable predators.

Your puny lipped sabertooth kitty is not only smashed...  it is curb stomped!!

Destroy all those lines of evidence systematically and I will disavow this hypothesis. Come at me bro.



No we don't need a sabertooth mummy, we already have more than enough to sensibly conclude LARGE LIPS FOR THE WIN!!






*Update thylacosmilids had more of a tusk like growth pattern with only a shallow enamel layer


Special thanks to the work of Jaime Headden and of course this post has the spirit of "All Yesterdays" (Naish, Witton, Kosemen) smeared all over it. Whether or not these researchers agree with my conclusions this work would not have been compiled without their existing works, thoughts, and efforts.

Also special thanks to the deviantartists who took a chance on depicting big lipped sabertoothed cats. I fully believe that your risk taking will end up on the right side of history.

P.S. This was just the warm up. I'm coming for ya' next lizard lipped theropods...

References

Ivory Identification Guide. USFWS. https://www.fws.gov/lab/ivory_defined.php Natural Ivory. https://www.fws.gov/lab/ivory_natural.php

Saliva - More Than Just Water in Your Mouth. EUFIC. http://www.eufic.org/article/en/artid/Saliva-more-than-just-water-in-your-mouth/

Enax, J., Fabrittus, H.O., Rack, A., Prymak, O., Raabe, D., Epple, M., 2013. Characterization of crocodile teeth: correlation of composition, microstructure, and hardness. Journal of Structural Biology 184 (2013) 155-163 link

Reisz, RR, Larson D. (2016) Dental anatomy and skull length to tooth size rations support the hypothesis that theropod dinosaurs had lips. unpublished abstract 2016 Canadian Society of Vertebrate Paleontology

Riviere, HL, Wheeler, HT. 2005. Cementum on Smilodon sabers. The Anatomical Record. 7 June
2005. 285A 634 - 642. link


Wallace63. CC3.0


"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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