|Skiorpovenator by Duane Nash|
I told you that this was going to be a weird one, that I will be going out on that speculative branch and perhaps sawing it off. But I believe there is a sliver of a suggestion of a possibility that those bedeviled architects of weirdness the abelisaurids are even stranger than we are already allowing them. That they may in fact hail from a primarily omnivorous antecedent - with a special dietary emphasis on various gymnosperm "fruitifications" from cycads, ginkgoes, podocarps - and that even the more derived carnivorous abelisaurids carried on with a significant component of "frugivory" and were important seed dispersers and maintainers of sauropod gardens. Long story short, that abelisaurids may in fact be secondarily carnivorous.
Was Ken Ham inadvertently prescient?!?
|accidentally prescient? Ken Hams Garden of Eden Carnotaurus|
What the hell you smoking Nash? How do you get a herbivore from this animal? Look at that mug?!?
Bear with me as I make the case. I don't necessarily plan to or expect to convince you… merely open your mind to the idea… maybe.
|Aucasaurus credit FunkMonk CC2.0|
Clue #1 Those Puny Vestigial Arms
Abelisaurids have arms that make T. rex look like Arnold Schwarzenegger. If you asked an abelisaur "which way to the gun show?" it could not even point you in the direction. While tyrannosaurids and other predatory theropods like carcharodontosaurids had somewhat reduced arms they still had functional arms. We might debate what if any role they played in prey capture, if they were used in ritualized combat, mating, or primarily hauled dinosaur brisket back to feeding layers, but it is very evident that predatory theropods - with the exception of abelisaurids - could do stuff with them.
I think that point should be iterated again become I notice some conflation of the two notions of vestigial and reduced in theropods - reduced is not the same as vestigial.
Abelisaurids arms were infinitely useless in any sort of combat - although I do hold out the possibility that they were useful in dinosaurian foreplay heavy petting sessions - and most assuredly represent a genetic defect that became established in this lineage of theropod. According to this article by Brian Switek the work of Alexander Vargas (Switek cites an unreferenced paper from 2002 although I have faith in this idea as I have heard it referenced before) suggests that abelisaurids underwent a mutation in two genes that regulate the formation of the forelimb: HOXa11 and HOXd11.
This mutation, I presume (correct me if I'm wrong in the comments), is the same or similar to the genetic defect that gave diminutive, vestigial forearms to; Limusaurus; kiwis; cassowaries; Aepyornis (elephant bird); and moas. The proliferation and transmission of a genetic defect is of no real evolutionary consequence flightless birds. In the case of Limusaurus it is trending ontogentically into a herbivorous/omnivorous lifestyle and loss of a raptorial forearm grasp is of no special consequence either.
Moving from this observation it is patently obvious that predatory derived abelisaurids are very distinct ecologically from all other theropods and derived flightless avians that underwent mutations resulting in vestigial arms or wings . It is also very probable that this mutation - or ones that result in similar outcomes - arose in many lineages of theropods. But it is striking that in only one lineage of arch-predator theropod did this mutation take root - abelisaurids.
One of these things is not like the other, one of these things is something else…
With all of those diverse lineages of predatory theropods rummaging around during the 140 million years of the Mesozoic why did solely abelisaurids take on and proliferate a mutation that, it seems reasonable enough to presume, other predatory theropods were routinely exposed to?
I know evolution is not perfect and that sometimes negative mutations do get root and proliferate. But I'm still left with the stumbling block in this scenario of other theropods reducing (keep in mind reducing is different than vestigial) forearms like tyrannosaurids and even some coelurosaurs and maniraptorans, but only abelisaurids threw the arms away to the evolutionary scrap pile.
It seems logical to me to suggest that abelisaurids accepted the conditions of this genetic defect not when they were mighty titanosaur slayers but when they were piddly omnivores yet to be kings, more prone to feast on cycad fruits than sauropod entrails. If deeper in their ecological history abelisaurids were of a more omnivorous bent that could potentially explain how such a mutation could take root in animals that did not require strong raptorial forearms for their lifestyle and is more in line with the pattern of other vestigial arm reductions in theropods that have no special use for their forearms (i.e. kiwi, cassowary, moas, Limusaurus, Aepyornis). It also explains why no other predatory theropods got vestigial arms - as primarily predators this deleterious mutation would always be eliminated from the gene pool.
Clue #2: Signals of an Ecologically Adventurous Ceratosauria Being Received From the Jurassic
|Limusaurus credit Levi Bernardo CC3.0|
After the jarring sensation from the initial wtf of the idea has washed over your brain cells what should immediately come to your mind is that ceratosaurs, it has become increasingly evident, were up to some pretty weird stuff getting into the Jurassic. Limusaurus is, for me, the gift that keeps on giving. Not only does it readily encapsulate in one tidy ontogenetic sequence the transition from a predatory lifestyle to a herbivorous one complete with parallel loss of teeth, growth of beak, and arguably loss of extra-oral "lippage" but it cries out loud and clear: "We all were not scarfing on bronto guts! You should expect more of us!!"
There is every reason to suspect that, largely obscured by the passage of deep time and lack of early-middle Jurassic exposures, there was quite the adaptive radiation of ecologically diverse ceratosaurs in that period of time. Many of which were experimenting and exploring non-traditional theropod ecologies. Could the putative ancestral abelisaurid been such a dietary deviant? Other members of elaphrosaurinae should also get a nod here as they too have been suggested to be deviating from the norm in terms of lifestyle but alas no heads have been found.
Although elaphrosaurinae are currently understood to be a sister group to abelisaurids the possibility is there that somewhere deep in the pedigree of abelisaurids is perhaps an animal more prone to frequent the salad bar than the sauropod prime rib carving table.
But wait a second we already have this animal, we have Eoabelisaurus and it is just another good ol' meat chomping abelisaurid….
Or is it?
Clue # 3 Eoabelisaurus May Not Be the Meat Chomper We All Assume It to Be
When Eoabelisaurus was first revealed it was hailed as a lazarus taxon, evidence of a cryptic lineage of abelisaurids going back 40 million years earlier than the next oldest member. Personally I was always suspicious of such a long tenure of seemingly unchanged abelisaurids. Save for the shrinking forearm nothing much seems to change from Eoabelisaurus to Rugops. Of course such stasis is not beyond the pale, perhaps that is just what happened. But I have my doubts about Eoabelisaurus. I am not in doubt that it was an early abelisaurid, I am in dount that it was just another good ol' meat chomper. The reason I say this is because of some characteristics of what we have of the skull. Or what we have left of the skull.
Like any good mystery we don't have any of the tooth row from the dentary or maxillae recovered from Eoabelisaurus. What we do have are some elements from the roof and back of the skull. And why they are interesting to me is not for what they have but for what they don't have. Missing are the thickened lacrimals, postorbitals, and overall thickening and rugosity of upper skull elements so characteristic of large predatory theropods and especially every other abelisaurid yet known. In Eoabelisaurus these elements are not thickened nor is notably ornamental:
From the paper (Pol & Rauhut, 2012):
"The skull roof is not notably thickened and no cranial ornamentation is present."
It's long been a contention of mine that the thickened and gnarly skull roofs of large carnivorous theropods primarily serve as sinks for stresses and strains incurred along the tooth row. I have wrote about this idea here and there are some converging lines of evidence that point in this direction. Such thickened skull elements are not found in theropods that transition into omnivory/herbivory from carnivory. I don't take the suggestion that thickened skull roofs were primarily used for head butting competitions seriously. No studies have been conducted pointing in this direction.
Display was a secondary function I contend. The recent paper on theropod skull ornamentation (Gates et al., 2016) linked cranial ornamentation with increasing giantism in those species that had cranial adornments. What the paper failed to do was consider that biomechanical stresses may have increased the likelihood of thickened skull roofs and consummate adornments. In fact the pattern that they elucidate - cranial ornamentation linked to increased giantism - is essentially what should be predicted if these same structures served a primarily bio-mechanical function. If the function of such skull ornamentations is primarily for display we should expect such a positive feedback loop to create the largest and most elaborate osseous display features to occur in the most derived theropods. In fact the opposite seems to have occurred. The osseous displays of tyrannosaurids, abelisaurids, and carcharodontosaurids are relatively subdued compared to the earlier and more dramatic osseous display features of guys like Dilophosaurus, Crylophosaurus, Sinosaurus, and Ceratosaurus. Guanlong, an early tyrannosauroid, has much more elaborate head crests than later tyrannosauroids. Of course I think later and larger theropods largely replaced osseous display features with more dynamic, striking, and communicative soft tissue features (hello flesh antlers) but that is for another time. The paper also made the error in positing that Acrocanthosaurus lacked osseous cranial adornments & thickened skull roof when it most obviously does. In fact all large, macro-predatory ziphodont skulled theropods have thickened skull roofs often times parlayed into cranial adornments. Certain tyrannosaurids and abelisauruids even fused some of the skull roof bones together for better stress absorption. All known abelisaurids had such gnarly, thickened skull roofs. All except for Eoabelisaurus that is.
Now you may take this reasoning with a grain of salt, the jury is still out on what was the reason for thickening of the roof of theropod skulls. But what can't be disputed is that Eoabelisaurus differs fundamentally in this aspect from not only all other abelisaurids but all other large predatory theropods as well. Eoabelisaurus was perhaps not the rugged and strong biter that later abelisaurids were. It was possibly not wrestling large sauropods with that jaw. This is the best why? that I can currently come up with.
Clue # 4 Those High, Short Snouts
It has long been recognized that abelisaurids have some freakin' weird heads going on. The contrast in skulls is striking against other predatory theropods. Other large predatory theropods have narrow, long, and low snouts while abelisaurids have high, wide, and short snouts. Traditionally this difference has been explained as an adaptation on the part of abelisaurids to bite on and hold to prey as opposed to bite and slash of other theropods. My question is what is the best exaptation that would set this trend in motion?
A potential explanation that may have initially shifted abelisaurids to this shape is that it was not selected for under the auspices of a hyper-carnivorous ecology but a selective omnivore. Such a short blunted skull could better pick, pluck and select choice bits of plant matter (especially reproductive propagules), small animals, eggs etc etc. Sort of like the skull of Avimimus but still toothy and retaining some predatory ability. Later on in their evolutionary tenure when abelisaurids ramped up their carnivorous inclinations the short high skull was an exaptation towards their divergent jaw and biting style among theropods. Omnivory offers a potential explanation for why abelisaurids jaws and biting style are so different from other carnivorous theropods.
|credit GhedoGhedo CC3.0|
Clue #5 Gymnosperm - Titanosaur - Abelisaurid Ecology
|educational use. Podocarpus nakaii credit Ming Weng|
Of course I should stipulate that a putative omnivorous ancestral abelisaurid was always a herbivore with second thoughts. They never committed full hog to herbivory; probably did not take up gastroliths; did not digest cellulose; they obviously did not push back the pubic bone and expand the guts to ferment large piles of roughage. Their primary exploration of herbivory I suggest would have consisted in the high quality reproductive propagules, the "fruit" if you will of such gymnosperms as cycads, podocarps, and ginkgoes. Plants that were very emblematic of Gondwana and that in a diffuse coevolutionary feedback loop may have enlisted the help of abelisaurds and other tetrapods to swallow, distribute, and germinate their seeds. This coevolutionary relationship may have persisted in the younger, more derived, carnivorous abelisaurids. While Laurasia was exploding in theropod and ornithischian diversity it always seemed like Gondwana was a bit of an ecological throwback to late Jurassic times. Indeed the stasis of the gymnosperm - titanosaur - abelisaur ecological relationship that seems to have stuck around for a long time in Gondwana may in part be due to abelisaurids helping maintain sauropod gardens. Indeed where better than the rich phosphorous dung of a carnivore - even better than herbivore dung - to germinate from.
Here is a good review of the "false fruits" of the Mesozoic.
|credit Brewbooks flickr. Lepidozamia peroffskyiana cycad|
This omnivorous heritage and potential maintenance would have potentially caused a receding "lip" at least on the upper jaw to allow better fine tune biting and grasping of small reproductive propagules. The lower jaw may have retained a bit more of the "meat-curtain" look, especially as large lower lips would have aided in tactile input while engaging with large prey via a neural net.
What would such a putative ancestral omnivorous abelisaurid have looked like? Probably not too different than derived abelisaurids. Actually I would take the general body plan of abelisaurids and plop on a head a little smaller, less muscular, no thick skull roof, little cranial ornaments, teeth fairly small and homodont although not necessarily non-serrated, and give it good running legs as it is now a prey animal. Such an animal was probably gorging on gymnosperm propagules when available but during the offseason dining on eggs, small game, hatchling dinosaurs often swallowed hole, carrion and just taking it easy living the easy life. We do know that abelisaurids grew pretty slow and possibly had a moderate paced mesotherm lifestyle (Ratsimbaholison, 2016). Indeed this slow paced, boom and bust lifestyle may have served these animals well. The ecology of cycads which are likewise slow metabolism plants (yes plants have metabolisms) may in fact be mirrored in one of their chief dispersal agents. Just lounging around, soaking up rays, waiting for either the next crop of cycad fruits to come into mast, the next egg laying season, or the next titanosaur to drop dead. You know, living the bear necessities.
|hypothetical ancestral omnivorous abelisaur by Duane Nash|
The notion of secondarily carnivorous abelisaurids - and other "carnivorous" theropods - that augmented their mainly carnivorous diet with various high quality plant propagules is adventurous but not really beyond the pale when we put the idea in context. Ecological slosh between carnivory and frugivory is certainly a trend and frugivory seems like an ideal transitional stage to more dedicated realms of herbivory. We now know of crocodiles eating fruit and who knows how often this goes on in the wild? Specialist fruit eating monitor lizards are definitely a thing. There are several vultures that augment their diet with fruit and specialists like the palm nut vulture. Mammalian carnivorans flip flop between eating fruit and animals all the time, some becoming ultimately more tied to vegetable resources. Maned wolves are now known to be particularly fond of fruits. On the other hand polar bears are a good example of a secondary reversion to a mainly carnivorous diet from an omnivorous brown bear antecedent.
I also should at least mention the often noted rotten flesh/cheese smell put off by ginkgo fruits. The mind intuitively drifts to ideas of these Mesozoic relicts enlisting carrion glutton theropods to help consume and disperse seeds.
Rugops The Inflatable Face Monster
For having such a cool sounding name Rugops is one of the more vanilla looking abelisaurids on first impression. I mean don't get me wrong, it still had the rugged textured skull and strange proportions of abelisaurids but it was no Carnotaurus. First impressions can be deceptive however and if we peer closer at the skull roof - which compared to other abelisaurids is pretty bereft of ornaments or gnarly rocky outgrowths - there is evidence of potential for soft tissue ornamentations.
This evidence comes in the form of parallel rows of foramina on either side of the dorsal aspect of the skull. These foramina, quite apparent on the top picture, could have fed display structures. I'm not the first to notice this, Paul Sereno has speculated as such and taken note of them, although if I recall he thinks they might be keratin outgrowths. Additionally the dorsotemporal fenestra show adequate size and shelving to support additional soft tissue structures ala flesh antlers.
While many in the paleo-community (at all levels) have been generally recalcitrant to acknowledge and entertain the possibility of such features, this conservatism is not surprising to me. And you know I'm not waiting for them either or asking for their permission either. Give that Rugops a crazy looking engorged facial tissue nightmare rape face.
|Rugops Tentacle Face by Duane Nash|
As I mentioned earlier when discussing the paper on osseous display features linked to theropod giantism (Gatres, 2016) why would such osseous display features - if they were so significant in spurring theropod evolution and giantism - seemingly diminish in size over the course of theropod evolution? The answer, I posit, is that such features did not so much as diminish in size but become replaced by more useful flesh and skin derived display features. Such features have the advantage over osseous display features in that they can; become engorged with blood thus changing size, shape and color and highlighting changes in mood and intent; can become damaged and re-heal, unlike osseous crests that - if bitten into - can open up the skeletal system to infection BAD!!; and serve an enhanced thermal function.
You Don't Want To Get Bit By an Abelisaur
I have to admit to developing a heavy dose of skepticism with regards to any study that gives "teh absolute bite force" of an extinct animal. I consider this akin to studies that purport to give "teh absolute speed" of an extinct animal. What I do consider such studies useful for is to give us a good range of powers/speeds but to posit an absolute number is untenable in my estimation and somewhat presumptuous. First of all let us remind ourselves that coming up with really good bite force numbers and speed estimates for living animals is notoriously hard and often changes. That should be mentioned. Secondly when we get past all the metrics, fancy math, moment arms, and finite analysis we are always left with a basic physiological question - are we talking about slow twitch muscle or fast twitch muscle? More to the point what is the relative abundance of either type of muscle - slow twitch giving more of an endurance type benefit and fast twitch more of an explosive burst of power type benefit. There is no way to decipher the relative abundance of those two muscle types from the fossil record. It is their interplay has a profound influence on questions of power, speed, endurance etc. etc.
Discussion on abelisaurid biting style are all over the place; they are called quick weak biters; sometimes Rugops (whom always seems to get picked on) is called an obligate scavenger; some call them tyrannosaurid mimics; Carnotaurus is called a weak and quick biter of only small prey. What and how were those maws operating?
I actually think that the wikipedia page on Majungasaurus summarizes the basic abelisaurid biting style quite nicely (seriously some dino-wiki pages are becoming fantastically informative kudos to the people putting in work). They were bite and hold predators. A strange paradox of the skull is featured in these animals - unlike any other predator. A thickened and stout upper jaw, a pug nosed crocodile if you will, combined with a lower jaw somewhat more slender looking and kinetic, like a snake. While struggling with prey the upper jaw could withstand extreme stresses while the lower jaw would bend and flex to prevent catastrophic damage and fracture. A rather neat and ingenious combination that differs from all other theropods and all other tetrapods that I can think of. That's my thinking as of now. Not my idea, I just happen to agree with it.
What about that small prey specialist niche for Carnotaurus? I think the idea should get some attention because the teeth in lower jaw look fairly slender, however they are not really that bad and could have been strengthened by being partially enmeshed in thick gums. More so than that the neck is just tremendously powerful looking and it has no other attributes of a small game specialist. On an ecological note where are all the modern large terrestrial small game specialist predators? Seems to me once you get much bigger than a coyote you tend to go after animals as big or even bigger than yourself - at least terrestrial predators. I mean there are no lion or bear sized mammalian predators that eschew hunting ungulates and only chase rabbits? Reminds me of the story of phorusrhacidae evolving two foot long skulls and growing eight feet high just to chase big rodents and somehow at high speeds have the skill and preternatural accuracy to conveniently slam their beak tips down on the back of the skull of their small, more agile, zig-zagging fleeing prey. Yup, I think the notion of giant terrestrial small game specialists is exactly what it looks like - a fairy tale - for both Carnotaurus and terror birds. Gigantic small game specialists are a thing in the water, but due to the unforgiving nature of moving around at 1 G on land large sized small game terrestrial specialists I would put forth as a fairy tail that, unfortunately, a lot of teh professional believe in (he-he-he). It's just not a physically and ecologically tenable existence.
|credit Canale et al. 2008|
Check out the skull of my favorite abelisaurid, the little heralded Skiorpovenator. Not only is it fairly complete, it has that gnarly bony buttress above the orbits and it has just a weird looking freakish jaw geometry. It also has what looks like some attachments for some fairly large pterygoideus muscles on the lower jaw which what I gave it in my depiction. I would not want to get bit by an abelisaurid.
|Skiorpovenator by Duane Nash|
Well that about wraps it up for this one. I can't help but remember when first learned about abelisaurids as a young lad reading Gregory S. Pauls Predatory Dinosaurs of the World. So little was known about them then and it is interesting to see just how many specimens have came to light since then, and what still awaits.
The end… for now.
Gates, TA, Organ, C & Zanno, L. (2016) Bony cranial ornamentation linked to rapid evolution of gigantic theropod dinosaurs. Nature Communications 7. September 27, 2016. online
Pol, Diego & Rauhut, Oliver W.M. (2012) A middle Jurassic abelisaurid from Patagonia and the early diversification of theropod dinosaurs. Proc. Biol. Sciences August 2012 279(1741) 3170-3175 online
Senter, P. (2010). Vestigial skeletal structures in dinosaurs (1), 60-71 DOI: 10.1111/j.1469-7998.2009.00640.x
"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