Masiakasaurus Proposed as a Fossorial Animal Hunting Specialist

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And now for a change of pace from serrated tongued terror birds and bonesaw theropods to a new look at Masiakasaurus knopfleri in terms of its lifestyle. This is an idea of mine that I have been kicking around for a while so time to dust it off and present it. It's a bit of a stop-gap until I wade back into talking about ziphodont cutlery.

Masiakaraurus knopfleri credit James St. Johns. CC2.0 

When I look at how an extinct animal likely lived I use a blending of three criteria:

1) Is there a reasonable analogue available? or at least a composite analogue?

2) Is the lifestyle feasible with regards to the local ecology/environment this animal likely lived in? i.e. maybe dedicated piscivory is not the best idea in a semi-arid habitat.

3) Are there any substantiating osteological/biomechanical clues in the skeleton? Trace fossils? Gut contents?

So with Masiakasaurus we have an interesting little character. It's actually the most well known noasaurid. In fact it is one of the better known theropods at this point. Now because Masiakasaurus knopfleri is a relatively recent addition to the theropod canon it gives us a good glimpse of how the suggestion of a lifestyle, then becomes repeated, and then becomes unquestioned. It is in fact a picture perfect view of how dogma arises.

used w/permission credit Luis Rey

And for Masiakasaurus the lifestyle that is most heavily repeated is the fishing one. Why is this?

Well first of all let's review why the dental design is so weird in this guy. The anterior end of the jaw is characterized by the procumbent dentition and lack of serrations which grade back into more typical laterally compressed serrated teeth towards the back.  Carrano, Sampson & Forster 2002 interpreted this heterodont pattern as indicative of the animal having the ability to grasp at small prey items which could be further minced in the rear teeth. They muse that "One possibility is that Masiakasaurus was insectivorous or piscivorous, using its anterior teeth for acquiring small, whole prey items and its posterior teeth for maceration."

Evolving from this simple suggestion you can clearly trace the insertion of this view as the dominant one with regards to how Masiakasaurus lived. But given that it has really never been fully tested (although Jaime Headden questioned it) is there any merit to dedicated piscivory in Masiakasaurus?

Is fishing Masiakasaurus really just a knee jerk reaction - "Ughhh it's doing something different with those teeth... ummm.... fish!!"

I am going to endorse theirs and others suggestions that the anterior teeth did indeed seize relatively small, single prey items and the posterior teeth did indeed help slice prey items too big to swallow whole. But I don't think that these prey items were commonly fish - which were likely not that common in semiarid Late Cretaceous Madagascar - but that its most common prey was the various likely fossorial animals it shared its island home with.

I hypothesis it was a fossorial animal hunting specialist.

Is there anything in the skeleton of this animal that could substantiate this claim?

from Carrano et al. 2011

If we start working our way back from the head we can see that the neck, at least for a theropod, is relatively straight. And there is an anatomical reason for this. From Carrano 2002: "The posterior cervical vertebrae of Masiakasaurus lack any elevation of the anterior face over the posterior face, suggesting that the base of the neck was relatively straight. They differ from those of coelophysids in being taller and wider relative to their length, bearing larger epipophyses, and having antero-posteriorily shorter neural spines."

To translate the vertebrae did not form the classic sigmoidal "S" shaped neck of theropods. Masiakasaurus was not optimized to achieve the kind of strike that herons do when fishing. Additionally  the relatively short vertebrae (taller and wider relative to length) is incongruent with the relatively long vertebrae of azdharchids, herons, and Tanystropheus - all likely strikers of small prey.

Quetlzalcoatlus sp. Jaime Headden CC3.0

Tanystropheus longobardicus. CC3.0 credit Ghedoghedo

Great Blue Heron. Ardea herodias

So for three tetrapod lineages that - certainly in the case of the heron but most likely in the other two -strike at small prey with their neck in a downward motion and all of which feature very elongated cervical vertebrae, Masiakasaurus does not line up well with.

Additionally from Carrano et al. 2002: "Like abelisaurids and most other basal theropods (but unlike coelurosaurs), the cervical centra zygapophyseal facets are not "flexed" and exposed anteriorly. The three cervicals from the Late Cretaceous Lameta Group of India ascribed to Laevisuchus indicus (Huene & Matley, 1933) are similar to those of Masiakasaurus in most respects, as is the cervical neural arch of Noasaurus. All three display short, anteriorly-placed neural spines, and postzygapophyses that are swept back strongly posteriorily."

What this means, in my shotgun blast translation, is that all that stuff that stuff that sticks out off the cervical centra - the neural spines, the zygapophyses - it is all swept back to streamline the whole contour of the cervical osteology. Almost as if the animal wanted to have any potential obstructions diminished that would impinge it from sticking its neck into tight spots - like deep into a burrow for instance. Furthermore this adaptation might be congruent in other noasaurids hinting at potential similar function and ecology across this group.

An interesting transition occurs when we move from the cervical vertebrae into the dorsal (i.e. torso) vertebrae. Carrano et al. (2011) take note of the interesting transition in this theropod and that it differs from the pattern in most theropods: "The tenth cervical vertebra (C10) lies at or near the cervicodrosal transition, although this can be difficult to define in theropods. Its unusually long proportions indicate that Masiakasaurus was characterized by antero-posteriorly lengthened centra thoughtout the presacral vertebra column. This is unlike the condition in most theropods, where the cervicodorsal transition is marked by one or two anteroposteriorly short vertebrae."

Masiakasaurus is breaking all the rules... but why?

Gone are the short vertebrae of the cervical region instead (from Carrano et al. 2002): "They are spool shaped, weakly amphicoelus, and lack foramina. Unlike the dorsals of Majungatholus, the centra are not shortened but remain anteroposteriorly elongate (approximately twice as long as either as either wide or tall), as in most coelophysids, smaller theropods, and Elaphrosaurus."

When vertebrae are amphicoelus that means both surfaces of a vertebrae are concave and there is good potential for mobility in several directions. This feature, combined with the relatively longer vertebrae, speak to a relatively mobile trunk region. Which is an interesting contrast to the relatively stiffer neck region. Its almost as if the animal needs the ability for its trunk to squeeze and squirm around in tight places - like into a burrow.

So if the anterior of the body is, as I am suggesting built to get into tight spaces i.e. burrows we should also expect to see some interesting adaptations in the pectoral girdle.

From Carrano et al. 2011: "The scapula and coracoid show an unusual morphology that finds some complacement among other members of Ceratosauria. When articulated, the scapulocoracoid is mediolaterally curved and presents an enormous area for muscle attachment anterior and ventral to the glenoid."


So we have some buffed arms... potentially for digging maybe? However despite the indication of a robust pectoral girdle the morphological theme of trying to "streamline" the bauplan - which I argue is to better access burrows - is persistent.

From Carrano et al. 2011:

"The scapula has a curved blade that reflects the shape of the underlying rib cage, and is broad relative to its length as in coelophysoids as well as other ceratoraurs."

Tranlation: the shoulder blade closely hugs to and mimics the contour of the rib cage. The ecological inference I argue is to diminish obstacles when pummeling into burrows.

"The glenoid has a pronounced anterodorsal rim, as in Ceratosaurus and Majungasarus. In posterior view the scapular portion of the glenoid is D-shaped, and substantially taller dorsoventrally than wide mediolaterally. Its ventral margin along the coracoid suture is oriented obliquely rather than horizontally."

Again the glenoid, or shoulder socket, is showing adaptations to diminish lateral projections and make the anterior of the animals body as flush and streamlined as possible. All useful traits to facilitate entry into tight spaces i.e. burrows.

from Carrano et al. 2011

From Carrano et al. 2011:

"The coracoid is expansive and oval, with long axis oriented anteroposteriorly (fig 18a above). It is much broader than the same element in basal theropods such as Dilophosaurus and Coelophysis, more closely resembling the condition in Elaphrosaurus. Limusaruus, and abelisaurids (although it is more anteroposteriorly elongate than in the latter). The posteroventral process is blunt, projecting only slightly beyond the posteriormost part of the glenoid. It is extremely thin mediolaterally, more so than in almost any other theropod, and lacks the characteristic medial concavity."

Did you pick up on that? The corarcoid, though large, is orientated in a front to back direction but the authors note that it is relatively more elongate than other ceratosaurs. The posteroventral process - a process implies a ridge of bone - is blunt. Not only is it blunted it it is extremely shallow mediolaterally, which means it is thin going from the midline of the body out away from the body - more so than any other theropod the authors mention. Both the bluntness and thiness speak to further diminishing any lateral projection that might diminish fitting the anterior of the body into tight spaces.

Let's look at the humerus now (from Carrono et al 2011):

"Overall it is concave curved medially but nearly straight in the anteroposterior plane. The deltapectoral crest is proportionally short, extending down only about one-third of the total shaft length. The distal condyles, although slightly damaged, are clearly flattened proximodistally as in abelisauruds, Ceratosaurus and Elaphrosaurus. Both the entepicondyle and ectepicondyle are located along the narrow margins close to the distal end, but are indistinctly developed in this specimen."

credit Carrano et al. 2011

Now, for our purposes here the most illustrative picture is B, the posterior view - which means you are looking at the humerus from the rear. What you want to notice is that the humerus curves medially- or inwards - from top to bottom. This exquisitely follows the trend that I have been noting all along of lateral projecting bits of anatomy are brought as close in line with the body to precent blocakage into tight spaces. Furthermore the authors note that the condyles - the rugose attachments at the distal end of the humerus - are undeveloped. This feature further streamlines and narrows the anterior aspect of this animal.

But despite these anatomical concessions to diminish lateral projections the pectoral girdle was robust, undiminished and likely very mobile From Carrano et al. 2011:

"No reduction in functionality is evident from the preserved remains. In contrast, the morphology of the humeral head and the expanded muscle origination areas on the ventral pectoral girdle suggest that mobility was significant and perhaps enhanced over the primitive theropod condition."

The morphology of the manual phalanges is interesting - might be useful to compare their morphology with the phalanges of animals that dig....

credit Carrano et al. 2011

Looking at the manual unguals we see they are relatively short and mediolaterally compressed. They do look a little blunt to be optimized for seizing prey. Could be indicative of some capability in digging.

credit Carrano et al. 2002

Moving towards the pelvic area this trend continues.

On the pubic boot (Carranno et al. 2002):

"The distal end is enlarged into a relatively small, rounded "boot" that projects posteriorly form the main shaft axis.... In most other theropods the boot is either small, lobular and unremarkable (coelophysisds), enlarged anteroposteriorly (allosauroids), or lacks an inset (most coelurosaurs)."

Admittedly not the most compelling piece of evidence so far but it is interesting to note the divergence from other theropods and that the "boot" projects posteriorly - again to diminish bony protuberances that might block passage into burrows.

credit Carrano et al. 2002 

Looking at the femur Carrano et al note:

"The femoral shaft is bowed strongly anteriorly and more subtly medially (fig 14)."

credit Carrano et al. 2002

This medial bowing is best seen in B - looking at the femur posteriorly (from the rear). It is quite evident that it bows medially and this feature is consistent with the trend I have been noting of potential laterally projecting bony features being brought close to the body medially. All of which is consistent with a body plan designed to exploit fossorial creatures.

Does this suggested lifestyle make sense within the paleoecology of where it lived? I must state that here things get a little equivocal as pinpointing true fossorial adaptations in the presumed prey base that I suggest Masiakasaurus exploited is currently lacking.

While evidence of dedicated fossorial specialists that lived with Masiakasaurus is equivocal (but that may change) there are a lot of promising candidates. These are animals that - even if they don't actively burrow themselves - they certainly fall within a size range of animals that frequently use other animals burrows, abandoned or otherwise. Especially in hot arid land with various terrestrial crocs and predatory theropods on the surface.

Simosuchus clarki, the pug nosed notosuchian. credit Gordon E. Robertson CC3.0

Vintana sertichi credit Lucille Betti-Nash (c)

Beelzebufo ampinga. credit Nobu Tamura(spinops.blogspot.com)  CC3.0

Araripesuchus patagonicus. remains of the same genus have
been recovered from Madagascar. credit Gabriel Lio. CC3.0

A diverse fauna of snakes also inhabited Madagascar in the Late Cretaceous. Madstoia madagascariensis is the largest at close to 8 meters and, although no skull is known, was likely a macrophagous predator. Kalyophis, which shows some weakly developed aquatic adaptations.  Menarana, which has been suggested to be fossorial. There is a paper online here if you want to read up on them.

Again I am not going to get into the nitty-gritty of were these guys really fossorial. Chances are in a hot arid climate, with predatory crocs and theropods roaming about, and at their size range they likely were. It is certainly a more tenable and viable food base than fish for which I can find no reference to in the Maeverano formation. Furthermore I am not excluding fish (or scavenging, insectivory) as part of their diet. Indeed exploitation of lungfish coiled up in their burrows during the dry season is a very viable option.

Canis simensis. credit Harri J. CC2.0

And finally I propose Masiakasaurus most analogous to: the Ethiopian Wolf (Canis simensis). This canid of the Ethiopian highlands primarily hunts burrowing rodents - especially big-headed mole rats. And it has specialization in its jaw indicative of this lifestyle with an elongated head, long jaw, widely spaced and slightly procumbent dentition.

Ethiopian Wolf. credti Paul Gervais

credit Jaime A. Headden The Strange Case of Dr. Masiaka & Mr. Vicious CC3.0

Duane Nash

Ethiopian Wolf credit Rod Waddington (lolz) CC2.0

So to wrap it up:

The unique and compelling skeletal morphology including; relatively straight neck due to short vertebrae; unique transition to long, flexible dorsal vertebrae from short, stiff neck vertebrae at cervical/dorsal juncture; streamlining of vertebral processes, pectoral and pelvic girdles; potential adaptations for digging in manual unguals/phalanges, mobile pectoral girdle & muscle attachments; medial bowing in of femur & humerus; and long noted procumbent dentition.

A possible diverse and abundant fossorial prey base.

A likely viable and useful modern analogy in the form of the Ethiopian wolf.

All speak to a tenable interpretation of Masiakasaurus knopfleri as a fossorial animal hunting specialist.

Further analysis and discovery of other noasaurid material may show similar adaptations potentially illuminating a Gondwanan radiation of fossorial hunting specialist. Fossorial notosuchians and mammals diversifying at the time may have fostered these specializations.


Can you dig it?

special thanks to Andrea Cau for providing pdfs of papers

Papers

Carrano, Mathew T., Loewen, Mark A., & Sertich, Joseph J.W. (2011) New materials of Masiakasaurus knopfleri, Sampson, Carrano, and Forster. 2001 and implications for the morphology of the noasauridae (theropoda: ceratosauria). Smithsonia Contributions to Paleobiology number 95

Carrano, Mathew T., Sampson, Scott D., & Forster, Catherine A. (2002) The osteology of Masiakasaurus Knopfleri, a small abelisaurid from the Late Cretaceous of Madagascar. Journal of Vertebrate Paleontology 22(3) Sep 2002 510-534

Sampson, Scott D., Carrano, Mathew T., & Forster, Catherine A. (2001) A bizarre predatory dinosaur from the Late Cretaceous of Madagascar. Nature vol 409. January

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Terror Bird Update

More madness in my book Dinosaur Enlightenment: Piercing the Veil on Kaiju Dinosaur in an Age of Disruption availabe on Amazon as an ebook and soft cover!!

I would be remiss if I didn't mention this study on the neck anatomy and implications for vigorous movement in the sagittal plane.

Tambussi CP, de Mendoza R, Degrange FJ, Picasso MB (2012) Flexibility along the Neck of the Neogene Terror Bird Andalgalornis steulleti (Aves Phorusrhacidae). PLoS ONE 7(5): e37701. doi:10.1371/journal.pone.0037701

Since both my scenario of bite, grind and pull and the hammer blow hypothesis both rely on extensive, dynamic, and robust neck musculature I can't really say that this work supports either notion more strongly.

What I did find of interest was the one comment someone left in the comment field for this PLoS ONE paper:

Well RobertoS, as you made the connection between Allosaurus/theropods way back on October 11, 2013 looks like you beat me to the punch a bit :^)  .... Just add the serrated tongue/choanal papillae and stress reducing headgear and it's complete!!

Furthermore the authors responded to this valid point and question with.... nothing!! Seriously what is with writers/researchers not fielding the questions that someone who took the time and energy to write on your very own published work - and probably had to go through the hastle of getting a PLoS ONE account. Say what you will about blogging but it is a living, breathing, responsive media.

Terror Birds Cometh: A New Hypothesis Unlocking Phorusrhacid Feeding Dynamics & Ecology

Ok freaks, get your groove on with my book Dinosaur Enlightenment: Piercing the Veil on Kaiju Dinosaur in an Age of Disruption available at Amazon in ebook and soft cover!! Get on!!

*Warning scenes and depictions of extreme animal violence will commence

If you are new to this blog or this series of posts you will get best utility by starting with this post and working forwards to follow the thrust of my thought and ideas since these posts build into one another.

All right time to talk about terror birds or, as I like to think of them, the most recent radiation of large bodied terrestrial ziphodont theropods. Yep they have all the hallmarks of a true ziphodont theropod (except for, you know, the teeth); long and muscular neck; strong and rugose orbital brow ridge/top of skull; tall and deep skull braced for strength in the dorso-ventral plane but still relatively lightweight with a central hollow cavity and mechanically weak in the lateral plane; and a relatively weak bite force. Sound like anybody we know?

Now wait a second... what about that whole study pointing to these birds using a unique punch and jab routine in which repeated hammer blows delivered from the tip of the beak killed prey (Degrange et al. 2010)? Well, I am going to be getting into that study on Andalgalornis, in fact the data from that study is what I am going to use to prop up my contention that not only were teratornithids true big game hunting ziphodont predators they were "toothed" but not in the normal sense of the word. Like modern carcass rendering birds (new & old world vultures, petrels) they were equipped with cutlery in the form of a serrated tongue and choanal papillae that interfaced together to commence "choanal grinding" and were true flesh rendering machines of the highest caliber.

Andalgalornis credit Degrange et al. 2010

Again, note the congruence in general form to other theropods. Deep reinforced skull but with weight saving pneumatic cavities, narrow laterally, vaulted and thick orbital ridge and upper bill to absorb stress and strain. Remind you of somebody else with similar features?

Allosaurus credit Witmer labs

Now Allosaurus had those serrated, mechically reinforced teeth that, I argued, when combined with rapid dorsoventral movements of the neck in a "shimmy" motion instigated a diabolical bone saw technique to saw through skin, muscle, cartilage, and bone. The jaw was weak because the driving power was outsourced to the neck musculature. Phorusrachids lacked serrated teeth but they did have that strong downcurved beak. It was the hooked tip of the bill that grabbed onto a plug of meat from a prey animal (alive or dead) which brought said plug of meat into the shearing section - the middle of the jaw - I contend. Here a combination of rapid dorsoventral shimmies of the neck and rapid serrated tongue oscillations against the choanal papillae worked to render the meat. After being mechanically compromised, the flesh was broken off with a strong leveraging hook and pull action powered by the entire head, neck, and legs.

If choanal grinding was the predominant feeding method of these birds we should expect to see the highest capacity for stress and strain along the middle section of the jaw.

Granted these various finite element tests (from Degrange et al. 2010) on Andalgalornis, a golden eagle, and cariama were calculated with the bite force (133 N for the terror bird) centered at the tip of the jaw. It is worth looking at where the stress is greatest though. In order to help read this the small chart in the upper left of the diagram goes from relatively weak Von Mises force in the blue upwards to progressively higher forces through the green, blue, yellow, and red colors. If the color is white, well that means the force is off the charts and could potentially imply catastrophic mechanical failure with increasing force. In A, D & G we see that the lateral shake is not looking to good for any of the birds. Note that although the middle of the jaw for Andalgalornis is running red the tip of the beak is off the charts in white realm. I will return to lateral forces later in this post btw. If we look at B, E & H which is a normal bite the whole jaw line is in the nice blue realm for Andalgalornis while the tip of the beak is running red. This suggests that the tip of the beak - which supposedly is meant to withstand the most harrowing pressures when it is slammed into prey - is actually starting to trend into inferior performance during a normal bite pressure. What would happen if it slammed into struggling prey at even higher pressures? And finally we get to the pull back and here again we see the middle of the jaw outperforming the tip of the beak which we can quite readily see is going from red into white color implying the potential for catastrophic failure.

credit Marcos Cenizo

If the tip of the beak really functioned in the battle axe capacity that has been suggested for it as a way to stun/kill prey then we should expect this portion of the skull to show lots of blue color during these strain tests. Except we don't. In all the tests the middle of the jaw - which is where the action is occurring via choanal grinding I suggest - is outperforming the tip of the jaw.

Prediction is met and the battle axe interpretation appears less valid than typical choanal grinding as performed by modern carcass rendering birds (new & old world vultures/petrels) which I first discussed here.

*It should also not go unnoticed, per my previous post theorizing on the mechanical stress relief of theropod head crests/ridges/lacrimal crests that the surparorbital ridges maintain some of the most "blue" areas confirming the mechanical advantage I ascribed them.

Here are some other problems with the battle axe interpretation:

1) Lack of a modern analogue. If this method of prey acquisition is adaptive we should expect to see at least a few of the 10,000 or so birds in the world evolve some type of analogous prey capture technique. But they are lacking.

2) Difficulty striking a moving, struggling prey item with precision. To kill an animal with a precise blow to the back of the skull or neck as suggested in this method is likely more difficult than assumed. Felids have to grapple and restrain prey in order to initiate their precise biting mechanism. Repeated blows to kill larger prey items seems problematic too as such hooked tips tend to gouge into and get caught in flesh rather than cut through items.  We know this due to recent functional tests on the "sickle" claws of dromaeosaurid theropods working more as hooks than slicers. Additionally if a phorusrhacid gets entangled with a large prey item in this manner it is now in danger of sustaining destructive lateral forces from said prey struggling.

3) Dubious depth perception capabilities. For the head to swing back and then into a prey item it is hard to imagine how the phorusrhacid's eyes can stay trained on the prey - especially to hit precise targets such as the back of the skull, vertebral articulation ete etc. If I were to play creator for a second and wanted to design a bird that could pull off this killing technique I would make a sort of composite heron/woodpecker design. Exquisite depth perception, reinforced skull, straight and pointed bill for precise powerful blows. Phorusrhachid skull design is more similar to other carcass rendering birds and extinct theropods and most likely suggests that it fed in a similar fashion.

4) Prey size question. While the battle axe interpretation does not outright dismiss large prey capture it certainly favors a more small/medium game emphasis. This interpretation is ecologically untenable because - with the exception of a few land crocs and marsupial predators - phorusrhacids were far and away the largest, most diverse, and longest lasting predators in South America. They were also the only large South American predators that penetrated into North America. It just makes more ecological sense for them to prey on the largest prey items available to them. No extant terrestrial predators grow to the size of Titanis or Kelenken to primarily hunt prey the size of rabbits or small antelopes.

5) Bite strength. The authors of the Degrange et al. study openly struggle with the relatively weak bite force of Andalgalornis (133 N) which is less than several small mammalian predators. This relatively weak bite force, especially when combined with an increasingly akinetic skull is a bit of an enigma.

Larry Witmer (from here) on bite force:

"We were surprised that the force of biting wasn't greater in Andalgalornis. As we say in the article, our estimate of bite force might be a bit low for some technical reasons, but in general we don't think this bird was delivering hugely powerful bites."

6) Battle Axe/Hammer blow model not consistent with pneumatic structure within skull. Although the skull was large, reinforced mechanically in the dorso-vental plane, and, due to it's size, was absolutely heavy it was made relatively light by a large central pneumatic cavity in the upper jaw. In the battle axe model such a cavity would diminish the impact of blows because of loss of mass. Although, in fairness, such loss of mass could be compensated for by increased acceleration.

Andalgalornis with special prominence of choanal papillae & serrated tongue (c) Duane Nash

However these problems seem to melt away if we invoke a biting adaptation analogous to modern  carcass rendering birds (new & old world vultures, giant petrels) and Mesozoic theropods.

1) Loads of modern analogues for this method. In terror birds what you are essentially looking at is a hopped up lappet-faced vulture or giant petrel. While typically castigated as weak billed scavengers these birds routinely dominate the "predatory" eagles/hawks/falcons at carcasses, engage in combative behavior amongst themselves, drive off and intimidate mammalian carnivores, and are well established hunters and killers in their own right. The absolutely more massive and robust bodied/billed phorusrhachids would be even more impressive in these regards.

2) No precision bite necessary in this model. Prey is bit, hooked into by the bill tip, grinded in the mouth against serrated tongue and choanal papillae, and finally yanked back by the beak/neck & leg musculature until massive trauma facilitates death. Consumption of prey likely began before death. Not pretty or quick but a time honored theropod tradition.

3) Depth perception to a high degree is uncalled for. Through comparison with the way giant petrels and various vultures kill prey - through prolonged and repeated biting, yanking, and grinding encounters - we see it is not necessary for precision bites as called for in the battle axe model i.e. precise depth perception not necessary.

4) Average prey size increases. Giant petrels kill birds almost as large as themselves. Vultures kill relatively large ungulate calves, especially in mobs. Single phorusrhacids were likely capable of killing animals as large, or slightly larger, as themselves and, in groups, targeting animals absolutely larger than themselves. This makes sense ecologically because although South America hosted other large carnivores, phorusrhacids were the most diverse, widespread, long-lasting, and common large predators on the continent. It makes ecological sense for them to be targeting the largest prey available to them. There are no large 50 kg plus terrestrial predators that concentrate or is limited to a prey base that is 1/4 or less of their own size. You might not agree with this assertion but I can't think of one predator that is so ecologically hemmed in. Comments section is open. Peer review has been outsourced to the readers of this post. Generally large carnivores take prey up to and marginally beyond their own size.

5) Strong bite force is not necessary for this model. The cutting of meat via choanal grinding is dependent on speed and friction - not static pressure. The driving musculature facilitating prey trauma and flesh rendering is the serrated tongue and choanal papillae followed by pulls from the neck and  legs to pull off bites. These were the two largest muscle masses in the bird's body and are where we should look for possible clues to feeding tactics.

6) Large pneumatic cavity in skull convergent with large foramen (especially antorbital) in ziphodont theropod skulls. Such a weight saving adaptation proves useful in the need for rapid dorsoventral movements of entire head to assist in choanal grinding and pull back motions for prey dismemberment.

credit Degrange et al., pneumatic cavity in Andalgalornis skull

There is no need to invoke a feeding method for which there is no modern proxy, and which has significant mechanical and practical problems. A method largely congruent with modern day large carcass rendering birds (new & old world vultures, giant petrels) provides an elegant analogy for phorusrhacid feeding behavior albeit not taken to the extremes that phorusrhacids took such behavior and adaptations. All things being equal, it is more parsimonious to seek modern analogues - especially within the same order - than assume novel and unparalleled feeding adaptations.

For physical confirmation of the likely presence of potentially robust and mechanically reinfornced choanal papillae below is a still taken from the video from this study. You can clearly see the two large ridges of bone - on which the choanal papillae would have sat - in this shot of the roof of the mouth in Andalgalornis.

Turkey vulture mouth closeup showing serrated tongue & choanal papillae.
credit Williston Conservation Bird Trust Blog

Now the above gif is not provided to provide simple shock value, shocking as it is, but used to argue a corollary in the feeding apparatus that I argue is largely congruent among phorusrhacids, Mesozoic ziphodont theropods, and modern flesh rendering birds like giant petrels. You should note the strong leveraging capability of both the neck and legs working in conjunction. As well as the rapid "chewing motions" of the beak where I have argued the serrated and stiff tongue is shredding the tissue against the choanal papillae. But more importantly you want to note how the petrel works to keep the fight in the vertical realm. As mentioned earlier and as supported by the Degrange et al. study the phorusrhacid skull is weak when confronted with stresses in the lateral realm. Allosaurus and Mesozoic ziphodont theropods likewise had skulls that were ill-equipped to handle substantial stress in this realm. Petrels and vultures too have skulls likely weak in this realm and their feeding behavior supports this notion. They don't shake their head side to side when interacting with prey or a large carcass. Instead all these animals have a vested interest in keeping the field of battle strictly limited to the vertical plane.

Gina Carano Ground & Pound CC2.0

To make a rough analogy to sports let us look at the two main contrasting styles of fighting in mixed martial arts. You have fighters that are most comfortable and effective as upright strikers. They would rather play to their strengths and keep the fight off the mat but upright where their ability for rapid and powerful strikes is most useful. And then you have the specialist floor grapplers. They want to take the fight to the floor where they can utilize their superior grappling skills and specialized submission hold technique. And of course there are fighters that blend the two such as in the pic above where Gina Carano blends grappling with striking to devastating effect in a technique known as "ground and pound". The take home message is that it is in the fighters vested interest to dictate the terms of in what realm the fight occurs based on his/hers respective abilities.

So if we look at the topic from this perspective we can test it to see if it holds any merit based on presumed prey base for these birds. If phorusrhacids were analogous to "strikers" and wanted to keep the fight upright and in the vertical plane we should expect their prey base to be on the whole more like "grapplers" with a lower center of gravity and more geared towards taking the fight to the lateral realm. Which when we look at the presumed prey base of phorusrhacids they are, as a whole, not striking me as especially swift/cursorial track & field superstars.

Notoungulates (including toxodonts): although there are some smaller and swifter varieties like Protypotherium that might have been about as quick as rabbits they are overwhelmingly pretty stolid, clunky bruisers when you get down to it. Certainly no indication that they were on their way to matching the cursorial abilities of derived horses or antelopes. *Update, spoke too soon. Thomas Holtz caught my mistake - there were cursorial herbivores in South America. (see comments below)

Homalodotherium cunninghami. Notoungulate. credit Smokeybjb. CC3.0

Nesodon imbricatus. Charles R. Knight. public domain

Xenarthrans - these are the weird armored guys - including ground/tree sloths, armadillos, glyptodonts, anteaters & tamanduas. Again these animals are not going to win any races.

Megatherium americanum. Hakan Svensson. CC3.0

Glyptodon & Doedicurus. credit Robert Bruce Horsfall. public domain

And later after the great American interchange Gomphotheres were very successful in South America as well.

Stegomastondon mirificus. Wolfman SF. public domain

But don't forget about Macrauchenia. These interesting animals - what is that a camel spliced with an elephant - appear a little leggier than the other critters discussed so far. Speed is always a good tactic but, just giving it an eyeball test I would probably most likely infer it on par with a camel. Probably a good fast trot - but not on par with advanced horses/antelopes.

Macrauchenia patachonica. credit Robert Bruce Horsfall. public domain

But, whatever cursorial ability this animal had, what is more important is the suggestion that via its flexible shin and ankle bones these animals had outstanding agility. They could twist and turn on a dime. Admittedly I can't track down an actual source for this ability - it is just repeated several times on the wikipedia page. If anyone has more info, the comments section is open.

All right, so a prey base not necessarily highly cursorial but with armor plated skin, striking hand claws or clubbed tails, large size/robust build, and potentially good maneuverability. Does this sound familar? It should because these characteristics describe the Mesozoic prey base for theropods pretty much tit for tat.

Thyreophoran dinosaurs - stegosaurs, nodosaurs, and ankylosaurs - were both squat, armored, and to varying degrees armed with counterattacking tail clubs, shoulder spikes etc etc. Very reminiscent of xenarthan mammals such as armadillos/glyptodonts.

Pelorplites & H. sapiens immature female

Sauropods/Saurolophine hadrosaurs - Absolutely larger than theropods but also armed to varying degrees with strong tails/tail clubs, foot claws, dermal armor, and lateral sweeps of neck and tail. Analogous to gomphotheres but also tail wielding glyptodonts and giant sloths to a point.

Saurophaganax, Apatosaurus ajax, & Matt Weddel CC4.0

Therizinosaurs/Giant Oviraptors/Ornithomimids - Hand claws and stout build have often been compared to giant sloths.

Nothronychus credit Taylor/Weddel 2013 CC4.0

Cerartopsids/Pachycephalosaurians aka Marginocephalia - Stout, sturdy build & counterattacking head armament & beaks. Some parallels to notoungulates.

Montanoceratops cerorychos. credit Barnum Brown. public domain

Hadrosaurs/Iguanodonts - Turning agility superior to theropods perhaps analogous to turning ability in Macrauchenia vs. phorusrhacids. Kicks and tails swipes also important.

Iguanocolossus. credit lukas Panzarin from PLOS ONE

To have such high congruence in anti-predator strategies in Mesozoic dinosaurs and South American mammals should not be dismissed. I am not the first person to note this congruence. But it is suggestive of a similar method of attack - and weakness - shared between phorusrhacids and Mesozoic predatory theropods.

Faced with theropod marauders you can out-grow, out-fight, out-manoeuvre, or out armor them. Or some combination there of. But none of these methods was likely completely foolproof and phorusrhacids, as their theropod antecedents did likewise, probably were crafty enough to find flaws to exploit in all of the above defenses.

An interesting comparative test is useful when we compare placental mammalian predators/herbivores. The dominant trend here I would argue is that mammalian herbivores have been characterized by an increasing cursorial adaptation. This makes sense when we look at their predators. Canids and hyaenids are pursuit pack hunters. Felids are stealth grapplers. If you are big and strong it is sometimes a good tactic to stand your ground and fight - but this is only viable if you are many orders of magnitude larger and stronger than your predator. Otherwise running is probably the best tactic that works against both felids and canid type predators. Which is taken to extreme levels in various ungulates but especially horses and gazelles. If marsupial predators in South America were the prime danger to the large prey species that lived there we should expect them to evolve primarily in a direction that negated the likely stealth/ambush/grapple strategies employed by borhyaenids / sebecid crocodylomorphs / sabre-toothed Thylacosmilus. Except we don't see that pattern. South American prey species were relatively slow - although sometimes agile - and were more often big, clunky, armored brutes. Exactly opposite the pattern we see in mammalian pack hunting/ambush predator faunas but exactly congruent with Mesozoic ziphodont theropod dominated ecosystems. And like Mesozoic prey species they developed anti-predator armament - tail clubs, large clawed forelimbs, large size - useful in dealing a strong lateral blow to the theropod glass jaw.

The eastern Shasta ground sloth (Nothrotheriops texanus) and the four Titanis walleri terror birds had been at an impasse for several hours. Although both species were recent immigrants to North America the long standing predator prey arms race between these two lineages was not a recent development and heralded back tens of millions of years to their cooevolution on South America. The ground sloth had backed itself into a thicket of palmetto palms and cactus where it turned and faced its attackers.

Scrub Palmetto credit Jud McCranie CC4.0

The terror birds could not attack from the rear here and, faced with the pummeling hand claws of the sloth, were disadvantaged. But the location was in the full hot Florida sun and the birds then commenced a waiting game. When the sloth - after several hours in the sun and facing heat exhaustion - saw several of the scurrilous birds engage in an actual physical fight made it's break for cover. Less than 50 meters away it had excavated an extensive burrow system in the Karst topography. But the sloth would not make it 10 meters.

Although surprisingly quick, especially in dense brush, the sloth had failed to account for the location of the dominant matriarch of the birds. She, the largest of the flock, stood over 2.5 meters tall and weighed over 170 kg. Having quietly crept down to lie behind some palmettos posterior and adjacent to the sloth she had simply waited for her opportunity. When the sloth made for cover her long legs covered the distance in less than a second. Her aim was deliberate and intentional. She quickly drove into and bit with her hooked beak the softest and most exposed area on the rear of the sloth under the tail - it's anus . And with a series of almost imperceptibly quick serrated tongue and neck oscillations she had established a grip on the sloth's rectum and large intestine. Now with the muscles in her neck and muscular legs working in conjuction she pulled back hard. In a macabre and gruesome tug of war,  the sloth pulling in the other direction from the bird helped facilitate its own fatal outcome - disembowelment.

The sloth realizing it's predicament switched from fleeing to fighting. Mortally wounded it's fate was already sealed. Dancing and lunging from all directions the terror birds harried and harassed the cornered sloth. They knew that exhaustion and blood loss would hasten the death. And with their superior aerobic capacity, speed, and heat threshold the advantage lay firmly with the birds now.

Perhaps there was good reason for ground sloths to excavate and seek shelter in their extensive tunnel systems....

bats in ground sloth paleoburrow Brazil. c/o GeorgiaBeforePeople

We can just be thankful for the advent of relatively quick killing predators like felids and constricting/poisonous snakes. The most common method of killing prey during the Mesozoic and in Cenozoic South America was most likely breaching of the body cavity via the anal/cloacal opening. They eat your asshole out of you while you are still alive.

P.S. Some, or even most, might strongly disagree with my self-publishing style via blog post. Indeed the pitfalls of such method are well documented. I choose this method because it is accessible to all, and - with the use of visuals, video, links, gifs - a superior platform to express ideas in my opinion. Furthermore published academic papers tend to suck the life out of such work, are often behind paywalls, insist on inane formatting rules, cumbersome citation regulations, and peer review is no fool-proof method anyways. To be perfectly transparent I am not going to go through all that. So this is what you get. I know that I have an intelligent and growing audience so I would rather out-source the peer review process to them. If something can survive the slings and arrows of the internet comment section then maybe there is some merit there.

And what does everyone ask as soon as a new paper is published? "Is it online? open access? Can someone send me a pdf?"

Finally going through such prescribed channels is against my D.I.Y. ethos. Why should I PAY some other agency for the privilege to publish MY idea? Especially when they are going to insist on such a dry, cumbersome format? And then THEY get to profit from it?

And to those who simply will not cite, mention, or acknowledge my work because it has not gone through this prescribed process I give you this. If you claim to be a curious, scientifically minded person you should be less concerned with "THROUGH WHAT VENUE I AM GETTING THIS INFORMATION?" but instead "DOES THIS INFORMATION HAVE MERIT?" I have no hidden specimens outside of scientific scrutiny. Whatever I use is from online information available to all.

BTW I have seen popular books, personal communication, unpublished data, anecdotal records all in one form or another cited in reference in published peer reviewed material. So why not blog posts?

Cheers!!

Papers

Degrange FJ, Tambussi CP, Moreno K, Witmer LM, Wroe S (2010) Mechanical Analysis of Feeding Behavior in the Extinct “Terror Bird” Andalgalornis steulleti (Gruiformes: Phorusrhacidae). PLoS ONE 5(8): e11856. doi:10.1371/journal.pone.0011856

Snively, Cotton, Ridgely & Witmer 2013 Multibody Dynamics Model of Head and Neck Function in Allosaurus (Dinosauria, Theropoda). Paleontologica Electronica May 2013

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