There is a recurring sentiment that I keep coming across in theropod research - that overemphasizes niche partitioning to an extreme. "The young of such and such tyrannosaur species had different jaw mechanics and therefore ate different prey than the adults." Or the other corollary that "multiple coexisting large theropods had some almost benevolent driving force admonishing them to concentrate on different food stuffs to avoid too much competition etc. etc." And this is probably true to a certain extent I have to concede. However too often we assume that extreme partitioning has to occur. Modern examples beg to differ. For example lions and spotted hyenas are both social, live in the exact same habitats, and have significant dietary overlap.
When the shit hits the fan environmentally - when all the small prey has went into torpor, when all the fish have retreated with receding water or aestivated in the mud, when all the young dinos have migrated out of the neighborhood - that is when the true crucible of theropod competitive fires is stoked. That cauldron I speak of is who gets to eat what, how much, and when at carcass gatherings. This really should come at no surprise when we look at how modern theropods i.e. birds partition carcasses especially in times of environmental stress. Let's forgo the vulture example because it is too obvious and instead pay some credence to the importance of large carcasses to many passerines, corvids, and raptors in areas of extreme winters.
What I want to attract your attention to is a sentence towards the end: "As snow depth increased, jays and great tits increased scavenging. We suggest that carrion use by scavengers is not random, but a complex process mediated by extrinsic factors and by behavioural adaptations of scavengers."
Most readers of this blog are in know well enough to be aware of carcass utilization by such birds but I think it bears a little reinforcement that such usage is often linked to environmental stress - in this case heavy snow. In the Mesozoic it could as well have been a devastating drought. In such circumstances partitioning for theropod dinosaurs as it is often invoked goes out the window and truly weird menageries of scavenging theropod guilds potentially gathered at carcasses. Yes I am even looking at theropods traditionally interpreted as herbivorous, insectivorous, and omnivorous such as ornithomimids, oviraptorids, alvarezaurids and therizinosaurids potentially joining the cue to exploit carcasses in such circumstances.
|Chickadee scavenging elk carcass credit Jacob W. Frank
As interesting an aside as the thought of therizinosaurids moving in to scavenge among more traditional predatory theropods is, the take home message I am trying to convey is that such feeding bouts at carcasses likely shaped theropod social and behavioral ecology. Furthermore it is also entirely possible that some theropods developed intimidation and defense mechanisms to gain leverage at such dinner parties.
Among mammalian carnivores we don't often think of them having defensive mechanisms at hand or combative/intimidation arsenals to dominate carcasses - aside from their teeth and claws - and growling/hissing. Social carnivores can dominate carcasses in relatively cohesive groups but, especially among solitary felids, retreat is the usual option - even when faced with throngs of scavenging birds. Several bears - armed with bulk, a thick skin, and a fat layer - are well provisioned for carcass monopolization and spotted hyenas with size, thick skin, muscular forequarters, strength, and group tactics also come to mind.
However among several extant diapsids better analogies exist that offer lessons in Mesozoic theropod table manners. Crocodiles and komodo dragons have dermal ossifications embedded in their skin which offer utility in rugged feeding encounters. It is also little appreciated that nile crocodiles will swarm en masse to drive off land based predators from carcasses. Which suggests some semblance of, dare we say it, "group social tactics"
Despite the attractiveness of looking towards big toothy lizards and crocs when it comes to how Mesozoic theropods monopolized carcasses we are literally spoiled with analogies among modern theropods that regularly feed on such foodstuffs: giant petrels and vultures. No need to look so far away phylogenetically when we have, you know, actual dinosaurs that do this stuff today. Furthermore not only are carcass feeding vultures and giant petrels better analogies phylogenetically, they are, let's be honest here, a lot cooler to watch around carcasses than big crocs, monitor lizards, or mammalian mega-predators anyways!!
If there is one word I can use to describe these birds around carcasses it is theatrical. I really love this video of giant petrels gesticulating around a pinniped carcass. Not only does the video have a nice, grainy 1980's VHS gore movie quality to it the birds put on quite a dramatic show. The racket raises some questions - at least for me - about what these skirmishes really mean. There appears to be enough food there for all the birds yet the battles, both mock and actual, never let up. Are these confrontations really about setting up a pecking order when food is actually not so easy to come by? Another observation I want to draw attention to which I will return to later is the use of outstretched wings to maintain a "zone of influence" on the carcass that keeps other birds at bay.
Of course I would be remiss to not pay heed to one of the great Grande Guignol Theropoda performers of all time - the always on point lappet-faced vulture. Such presence. Such gravitas.
Note also in this portrait I am painting of intensely theatrical, gesticulating, and combative carcass skirmishing theropods, the last post I made on face biting theropods plays directly into this one. Grotesquely adorned, carnuncled, and gnarly faced/necked/forequarters would have all come into beautiful display and usage in such barbaric feeding bouts.
With these disturbing thoughts and images swimming inside your head let us revisit the paradoxical situation of the ulnar quill knobs in Convavenator. I refer to the situation as paradoxical because - although the trend of increasing "birdiness" in theropods is certainly a thing (or is it that increasing theropodness in birds is a better way to put it?) Concavenator was truly and squarely a carcharodontosaurid. These "land shark" megapredatorial theropods were not becoming birds nor were there antecedents on the way to or from flightedness. If anything they seem to be going in the opposite direction with smaller and smaller forelimbs and pretty much dedicated to hyper-carnivorous ways with out even the slightest hints of even an omnivore among the ranks. It is weird that quill knobs implying some sort of feather quill in these theropods appears, almost like evolution by proximity?
|from Ortega 2010
"indicates the presence of skin appendages in Convavenator, preceding the wing feathers present in Maniraptora".
Note the careful language that they use calling them"skin appendages" instead of feathers and "preceding the wing feathers in Maniraptora". I think that Cuesta et al. were very intelligent in making these semantic distinctions. What was most likely there - and what is most likely the most basal integument in dinosaurs - is literally a simple, shaft "quill" type feature.
The important word to keep in mind here is "quill" and here I want to revisit the observation I drew your attention to earlier of giant petrels establishing a "zone of influence" with outstretched wings at carcasses. If we imagine sharp and robust "quill" type appendages attached to these notably unusually placed "ulnar knobs" then it is not too hard to imagine their obvious utility at feeding bouts. Like the giant petrels earlier with wings outstretched these posteriolaterally projecting quills would help Concavenator "elbow in at the dinner table" among other theropods.
|Concavenator "Mouth For War... Last Fix2" by Duane Nash
Such devices remind me of the ludicrous spiked arm gauntlets worn by some black metal artists. I do recall going to several punk/metal shows and how studded shoulder spikes and wrist spikes were a definite and dubious threat in the mosh pit and became banned at many venues for their danger!!
Here I wanted to depict three very different predatory theropods Concavenator and a mega-dromaesaurid and baronychine converging on a Pelecanimimus carcass. Normally there is some dietary partitioning between these predators but a drought has forced them into proximity and competition over carcasses. With it's spiked and uncouth countenance the Concavenator is able to monopolize the carcass.
|Theropod Table Manners Gone Bad by Duane Nash
It's also worth noting that the outstretched wings of maniraptorine theropods could serve a similar function at carcass disputes - serving to both intimidate and establish a zone of influence at carcasses.
Referencia: Cuesta, E., Ortega, F., Sanz, J. Ulnar bumps of Concavenator: Quill Knobs or Muscular scar? Myological Reconstruction of the forelimb of Concavenator corcovatus (Lower Cretaceous, Las Hoyas, Spain). Abstracts of papers of the 75th Anuual Meeting of the Society of Vertebrate Paleontology: 111-112.
Naish, D. (2010). Concavenator: an incredible allosauroid with a weird sail (or hump)... and proto-feathers?. Tetrapod Zoology, September 9, 2010.
Ortega, F.; Escaso, F.; Sanz, J.L. (2010). "A bizarre, humped Carcharodontosauria (Theropoda) from the Lower Cretaceous of Spain" (PDF). Nature 467 (7312): 203–206. doi:10.1038/nature09181.PMID 20829793.
Selva, N.; Jedrzejewska, B.; Jedrzejewska W.; Wajrak, A. Factors affecting carcass use by a guild of scavengers in European temperate woodland. Canadian Journal of Zoology December 2005 83(12): 1590-1601 pdf