Wednesday, June 21, 2017

An Issue of Scale

Depending on how you deeply you read into the title of this post it can be not only a double but a triple entendre.

 "How many angels can dance on the head of a pin?"

Can be rephrased as "how many T. rex scales can fit on the head of a pin?"  That is the size of a millimeter and that is a little more than the average size of the T. rex "scale" documented in the recent Bell et al. (2017) paper that has got everyone flummoxed from here to kingdom come. Another visual is that more than 20 T. rex scales can be lined up across a penny. Take home point: these "scales" were small.



From Bell (2017) Note the scale bars in C & E are 5 mm while the scale bars in G & H are 10 mm.
These scales were minuscule!!


I think such dramatic issues of scale have been a bit washed over in the more hyperbolic proclamations  of the scale loyalists: "Yes, we have won!! Good riddance feather nazi elves!!" versus the recalcitrant permutations of the suddenly defensive feather crowd; "This tells us nothing new"; "Taphonomy washed away the feathers"; "Maybe T. rex had a feathered petty-coat?" Now I don't mean to mock such ideas and criticisms on the part of the pro-feather contingent but one can't deny there has been a palpable shift in sentiment from mainly feathered to mainly scaled in tyrant lizards (and by extension other largish theropods). Ironically many of the charges leveled at "awesomebros" or JP stylized scale loyalists are now being charged at the pro-feather faction; clinging to an ideal; putting an aesthetic before data; emotional attachment etc. etc.

Things are definitely a bit more unsettled and chaotic than many are comfortable with. Which is something I have been highlighting for some time in this blog; things are going to be getting more testy; a bit more non-consenus; a bit more uncomfortable for some time before things start to simmer down. Most of all things are going to be getting a bit weirder.

And those Wyrex scales are awfully weird. As I highlighted at the top of the post they are ridiculously small. Mark Witton called it the "Revenge of the Scaly Tyrannosaurus" in his blog post on the topic. If this is revenge via sub-millimetere "scales" it is perhaps the most underwhelming revenge story ever told. Sorry, I've had pimples bigger than those scales. I mean they are freaking small?! What is up with them? To put the size of these scales in perspective they are approximately equal to gila monster scales - komodo dragon scales would dwarf them. For all intents and purposes T. rex adorned in these minuscule scales would look pretty much nekkid. Especially so considering the absolute lack of any larger feature scales, larger keeled scales, osteoderms etc etc. Now, jokes about the puny size of T. rex scales aside, I do agree with Mark Witton on major points - and I'm glad he made this jump - that naked hided tyrannosaurids are not only a defensible option, but a likely one. He also raises the pertinent question - again something that I've been clamoring for here for a while - that many larger traditionally feathered dinosaurs may have lost significant amounts of fluff upon attaining larger size  (I do have some reservations on the osteological correlates for large facial scales that Mark seems to abide by but that is for another time). Another pertinent read is Andrea Cau's post on the subject. He makes some very valid and strong arguments, as he has for some time now, that taphonomy is key to a more detailed understanding of the nuances for feather, scale, "integument" preservation. Cau also raises issue with the strange shape and morphology of these scales - an observation I share and if you read the comments on his blog and know my history you can already probably guess where I am going in this piece … C-----A-----R-----U-----N------C…..

I don't know of any concrete studies linking scale size to body size but intuitively such a connection makes sense. I mean larger crocs have larger scales, larger monitor lizards have larger scales, larger boas have larger scales. That is just my off the cuff observations and I'm sure there are exceptions. But, generally speaking, scale size in a related group should get larger as the animals get larger both ontogenetically and in between related species. Did tyrant lizards break this pattern? Should smaller specimens have even smaller scales? Or are they all the same size regardless of size/species? The connection between scale size and body size both ontogenetically and between related species is probably a topic rife for potential studies.

*Turns out there are some studies linking scale size to body size. I went back and read the chapter of hadrosaur integument, authored by a certain guy named Bell, from the epic Hadrosaurs book from Indiana University Press and came across this gem which I will put here:

from A Review of Hadrosaurid Skin Impressions

"Scale morphology (especially scale counts) in modern squamates and crocodilians is typically conserved intraspecifically and is highly important in species identification (Spearman 1973; Brazitis, 1987; Hall, 1989; Cox et al., 1993; Charette, 1995; Branch, 1998). However, scale size (inferred by the inverse relation to number of scales) has been shown to vary positively with body size in some lizard species (e.g. Scleroporus, Ouferio et al., 2011). Similarly, body size (and hence, scale count) is variable both intra- and interspecifically among some squamates in relationship (both positive and negative) to Bergmann's Rule (i.e. body size decreases at lower latitudes or in warmer climates), which is correlated with minimum annual temperature and aridity (Sears and Angilleta, 2004; Oufiero et al., 2011). Despite these variations, scale morphology is one of the most reliable in the identification of extant squamates and crocodilians, especially in closely related species (Brazaitis, 1987; Charette, 1995). It is notable also that tarsal- and toe-scale patterns have been used successfully to identify individual species of extant avians, particularly raptors (Clark, 1972; Stauber, 1984, 1985; Palma, 1996)."

It is useful to note that the above paragraph is couched in a general discussion asking "can scales be used to identify individual hadrosaur species?". The promise for this sort of inquiry is great for hadrosaurs. For tyrannosaurids - as I will get into below - the promise for this sort of species identification based on scales is not looking as promising. But just for starters, who wants to venture a count on how many sub-millimeter sized scales covered a T. rex? What is the morphology of these scales? Any sort of repeating pattern? Architecture?

Are retro '90s Tyrannosaurs making a comeback? is a recent post by Paleo-King (Nima) on deviantart. While the general thrust of his piece is useful he makes a claim in the comments section that the various tyrannosaurid scale impressions are exactly the same as the well documented hadrosaur scales/mummifications. I do have to take issue with this comparison.

1) Extent of preservation. In hadrosaurids we have full on mummies that give us real life confident appraisals of where and how far scales occurred. So far in tryannosaurids we just have bits and pieces. Together these pieces suggest that "scaly" type integument is our most parsimonious appraisal for most of the body, if not all of it. But keep in mind that we as of yet don't have proof of "scaly" integument on the face and those persistent rumors of non-scaly neck displays in Tarbosaurus. Remember both crocs and birds have dispensed with scales on the face. Why? I'm not sure. It is interesting that both crocs and birds both have highly sensitive, tactile faces as well.

2) Size of scale and variety. While some hadrosaurs show scales on the millimeter range such as the Osborn "Trachodon Mummy" which are 1-5 mm and are described as "pavement scales" these patches of small scale are interspersed with regular patches of "feature scales" which are much larger at 5-10 mm.  What we see in terms of scale variety in tyrannosaurids so far is quite limited to very small "basement" type scales of a size smaller than in hadrosaurs. Hadrosaurs both at a distance and up close would have appeared much more textured than tyrannosaurids based on what we know so far. Unless you have some super X-men levels of visual acuity tyrant lizards would have looked basically nekkid.

3) Architecture. Nima points to an obvious architecture in the scales of tyrannosaurids. I have to admit that I can't see it. Carnotaurus shows architecture in the regularity of larger 4-5 cm tubercles arising every 10 cm or so from a more "basement" size scale of 5 mm tubercles. Sauropod and hadrosaur scales show a more obviously repeating scale architecture than tyrannosaurid tubercles. What I see in the tyranosaurid tubercles is something of a much more random, haphazard pattern. Apart from a size threshold of about 1 mm for the tubercles I can't discern any consistency in size; some appear more than twice as big as others and shape - apart from being generally polygonal there is a wide variety of shapes. Some of the tubercles appear rounder; others ovoid; some taper to a point; some wrap around others; and some almost to appear to bud off into other tubercles. This is a far cry from the more standardized scale pattern of hadrosaurs, which are often in repeated hexagonal patterns. Additionally given the 30 square cm patch of ilium scales we can be pretty confident that there was no larger patches of feature scales or repeating rows of larger keeled scales/osteoderms as in Carnotaurus, titanosaurs etc etc.

*Update. looks like a spoke too hastily, Nick Fonsesca says that Tarbosaurus has some feature scales form the supplementary info and then there is this from the paper:


Although I could not see the feature scales pictured in the supplementary there is indeed some evidence for a more orderly pattern of tubercles on some tyrannosaurid specimens, although not from the Wyrex material. It is of note that these feature scales are from the abdomen. Can they in fact reflect a more basal state retained? (i.e. never became completely feathered). Things are always aflux!!


So are 90's tyrannosaurs making a comeback? Sort of, I would say, but not exactly. The lack of discernible rows of feature scales or keeled scales is an obvious departure from GSP tyrannosaurids or any sort of organized row of larger scales so prominent in 90's style GSp tyrannosaurids.

And here I have to admit a weakness of not trusting my own gut and capitulating to the whole shaggy T. rex visage that has come into vogue in recent years. I blame Saurian. No, just kidding, I jest just a bit. Some have construed some of my posts as an attack on Saurian game design or that saurian should be changed or whatever. I can't disagree more. In fact I feel a bit sorry for the development team at Saurian, they must get pestered by young dino obsessives all the time. Just leave 'em alone at this point!! If Saurian is wrong fundamentally in some ways I think - at this point - these potential errors should be preserved for posterity. It's always interesting and iluminating to look back on paleoart to see how thought and ideas have changed. Saurian can be a great time capsule for what the general thought was of this period.

I do feel a bit of edification in getting back to my gut feeling for my latest tyrannosaurid art, in which I went with mainly nekkid skinned tyrant lizards with some manes and petty-coats of filaments. Proud to say this was completed before the new data came in. Boo - ya!! No need for revisions here, folks! Who says intuition, gut feelings, and following your muse have no place in science!! First published March 23, 2017 Gaslighting the Dinosaur: Just How Weird Can Dinosaurs Get?

Revelations by Duane Nash


Another argument that I would like to weigh in on is why Yutyrannus would go fully feathered while similarly sized northern tyrants like Albertosaurus went nekkid in a climate that was presumably not too different in temperature. Not to discount some of the other suggestions but merely augment, let's keep in mind that Yutyrannus was a normal slab chested theropod. Tyrannosaurids came wit da thikkness, they were barrel chested beefcakes, much more better heat retention in the torso. Big bois. Throw in some nice counter current heat exchange for the extremities, maybe a bit of a fat layer too, and you have a pretty good heat retaining system - much better than slimmer, earlier theropods at least. Perhaps a seasonal coat was a thing in northern variants, perhaps the young sported downy coat.

Back to the topic, again, my argument is not one of denying the overall message - that tyrannosaurids had a mainly "scaly" integument - but that the tyrannosaurid "scale" in both size, shape, architecture and pattern is quite distinct from those qualities of scales discerned in Carnotaurus, sauropods, and hadrosaurids. The differences in tyrannosaurid scales between these, presumably more basal integumentary patterns, very well might indicate secondarily derived "scale" patterns. In short that tyrannosaurids - like modern birds - had to reinvent the scale from a mainly feathered ancestral state.

This of course is not a novel concept, indeed the authors of the paper (Bell et al. 2017) seem to lean this way themselves. What I want to offer is that this T. rex "scale" - as suggested by the patterns discerned already - is best approximated by looking not at the exposed skin states of crocodilians with their regularly repeating, consistent, large, and architecturally sound scale designs. No the exposed skin state of tyrannosaurids is best approximated by comparison with the other archosaurs (i.e. birds) that have had to "reinvent" scales after losing them initially upon evolving a completely feathered countenance. When we look at both the small turberculate "scales" on the legs of birds and… wait for it... the minute "scaly" pattens discerned on some examples of carunculate skin of birds we can imagine a sort of novel tyrannosaurid integument that splits the difference between bird leg scales and carunculate skin. This novel skin morphology would be a lot more thicker and durable than carunculate skin but offer adequate blood flow for thermoregulatory and color flushes. Such a novel skin type combining elements of carunculate skin and bird leg tubercles would be consistent with the preposterously small tubercles of tyrannosaurids, their haphazard shapes, and offer immediate benefits for thermoregulatory and display functions.

Carunculate skin answers the question of why - when compared to other scaled dinosaurs - T. rex scales are diminuitive, non-architectural, no consistent shape, sort of globular, and devoid of rows, repeating patterns etc etc.

Oh yeah, and before you go "there goes Duane and his caruncles again" know that others have been pointing out this similarity between tyrannosaurid "scales" and carunculate skin as well…

credit Marco Muscioni
If you go back and review the T. rex skin impressions there are some very bumpy, textures reminiscent of carunculate skin. You will also notice the skin infoldings and the small size of the tubercles in carunculate skin is consistent with the unparalleled small size of T. rex "scales" and the skin creases shown in preservation.

Bell, 2017



You can take a wild guess how often the term caruncle comes up in the Bell paper (or the Carr paper for that matter). A big fat nada, zero, zilch, ninguno. I guess we don't have to talk about carunculate skin if we pretend it does not exist. Robert Bakker, I'm disappointed in you, especially since you have worked a lot with Luis Rey who gave us one of our first caruncle ridden theropods. Robert Bakker, you should read more antediluvian salad. Yes, the hate, disdain, disapproval, and ignorance for carunculate skin knows no bounds both from professionals and lay enthusiasts. And so perpetuates the false dichotomy of feather vs. scale, leaving out the retarded stepchild of carunculate skin… always the bridesmaid, never the bride.

Ye shall know it when ye sees it…

What was once scaled becomes feathered then becomes something new…

A quasi scaled, nekkid skinned, caruncled beast. Not completely serpent or fowl.

Ladies and gentlemen this is what modern theropods (i.e. birds) do when they get rid of their feathers. If we assume feathers and scales are more than a little competitive developmentally, when feathers diminish a nekkid skin is left. In order to toughen up this nekkid skin faux scales can be reinvented as they have been on the legs of modern birds. In other areas, especially of the head, nape, and neck a more motley growth pattern of various carunculate skin devices can commence. Carunculate skin can also have a superficially scaly facade. In large and gigantic theropods that mostly or completely lost feathers carunculate skin and tuberculate scales analogous to modern bird leg scales can potentially combine and envelope the torso as well, creating new motley and outlandish textures.

The dark little secret of carunculate skin is that it is found in not just one modern bird family, but many. We are left to consider two possibilities. Carunculate skin is basal to birds and quite possibly goes way back into theropods or: that it evolved independently in many different bird lineages. Both scenarios bode well for such epidermal growth to be quite common and expected in theropods that dispensed with feathers or possibly even some dinosaurs that never evolved feathers?!? (I'm looking at you Edmontosaurus)

Dang Tao Chicken "Do these caruncles make my feet look fat?"
And finally I leave you with a soft tissue preservation of another extinct giant theropod, a Moa. Do you detect some integumentary patterns that might in fact be construed as "scales"?



And finally, for the haters, social media harassers, stalkers, chastisers, discounters, "don't listen to Duane he is not teh true paleontologist", and all around perpetrators. You inspire me. Keep it coming. I stole the show with Spinosaurus and I'm doing it again right here. This C-walk is for you.
CARUNCLES BIATCH!! Up in Your Grill!!




Refs
  • Bell, P. R., Campione, N. E., Persons, W. S., Currie, P. J., Larson, P. L., Tanke, D. H., & Bakker, R. T. (2017). Tyrannosauroid integument reveals conflicting patterns of gigantism and feather evolution. Biology Letters, 13(6), 20170092.
  • Bell, P.R. A Review of Hadrosaur Skin Impressions. Hadrosaurs. Indiana University Press 2014
  • Carr, T. D., Varricchio, D. J., Sedlmayr, J. C., Roberts, E. M., & Moore, J. R. (2017). A new tyrannosaur with evidence for anagenesis and crocodile-like facial sensory system. Scientific Reports, 7.
  • Cau, Andrea. Those Scales Are Scales? Theropoda. June 7, 2017 webpage
  • Nima (Paleo-King) Are retro 90's Tyrannosaurs making a comeback? 2017 webpage
  • Witton, Mark. Revenge of the Scaly Tyrannosaurus. Mark-Witton.blogspot June 16, 2017. webpage


"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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Thursday, June 1, 2017

Making Abelisaurids Weird(er) Again: Secondarily Carnivorous Abelisaurids & Other Strange Tales

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.

Senter 2010

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 DilophosaurusCrylophosaurus, 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.



refs

Agnolin, F., & Chiarelli, P. (2009). The position of the claws in Noasauridae (Dinosauria: Abelisauroidea) and its implications for abelisauroid manus evolution Paläontologische Zeitschrift, 84 (2), 293-300 DOI: 10.1007/s12542-009-0044-2 

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 Journal of Zoology, 280 (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


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Wednesday, May 24, 2017

Lips Before Beaks Part II: Croc Faced Killers & Fleshy Lipped Genetic Monsters

I just can't get away from fleshy lipped theropods...

Recently I went out for a hike in the local Santa Monica mountains. Spurred on by the recent heavy winter rains in California I was expecting a bouquet of abundant wild-flowers. The flowers were there of course and it was breathtaking. However as the strange has an unusual affinity for me (and I for it) what got my attention as soon as I opened my truck door at La Jolla canyon was not the flowers, but a dead hatchling theropod on the ground. Not sure what species - probably a larger corvid due to size - but as I peered closer I noted a strange bundle of flesh at the crease of the mouth.




Some genetic aberration? The avian version of a cleft palate? Or perhaps a link to a more primal ancient pedigree of lipped theropodian monster faces? There is a saying in evolutionary theory: ontogeny recapitulates phylogeny. Is this a case of that? I looked at lots of pics of hatchling birds online and can't find any with that degree of bunched up oral tissue at the commissure.  Whatever your thoughts on what this might represent - probably is some genetic defect - you have to admit the weirdness of it all… a dead beaked bird with a fleshy extension of the oral margin directly reminiscent of the very same depictions I have been advocating.

The picture below is a screen shot from a youtube post by C.M. Kosemen detailing his thoughts on the chickenosaurus. The pic is actually taken from a book titled Incredible Life: A Handbook of Biological Mysteries by William Corliss, purportedly a rare treasure trove of arcane biological curiosities described by Kosemen as " the Necronomicom of biological weirdness". According to Kosemen the book documents a chicken from 1878 born without a beak and having strange "mammalian like head features". The oral tissue surrounding the mouth described as a quasi gummy but tough tissue.


Thanks for commenter Casielles for bringing this pic to my attention and noting how much it resembles some of my lippy theropod designs.

Wherever you fall on the chickenosaurus debate, the Dr. Moreau in me is still curious about what sort of oral tissue would come about on a chicken reverse engineered and stripped of its more derived facial attributes into something well… not quite a dinosaur... but not a typical beaked bird either. Could we garner a glimpse at the primordial oral margin of theropods prior to the evolution of beaks?

The photo below is credited to Bhart-Anjan Bhullar and shows how through suppression of certain proteins the premaxillary bone of a chicken (labelled experimental) can be suppressed giving arise to a more basal configuration.


While the fusion and expansion of the two premaxillary bones into the "beak" is certainly interesting,   all of this does beg the question: if beaks can be reverse engineered into snouts can we also reverse engineer the oral tissue that lines the mouth of birds into what once was?

Bhullar et al. (2015) isolated specific proteins that stimulate the development of beaks (i.e. the premaxillary bone) in modern birds. Although I can find no pictures of the creature that they came up with - from what I gather the birds were not allowed to hatch - they were reportedly able to control these proteins and in doing so reportedly come up with a chicken closer to the ancestral state in terms of facial bones and lack of beak. What they said - and what caught my interest - is that this manipulated chicken skull looked superficially normal from the outside but something obscured a clear look at where the beak should have been. What was obscuring the beak area, you ask? A flap of skin was. Was this flap of skin a reversion to a more primordial "lip" condition? I don't know. What is interesting in this study is that another unexpected osteological change occurred - the palatine bones reverted to a more ancestral condition. Manipulation of one aspect - the beak - can have unexpected correlates in another osteological condition and I'm asking maybe even supporting soft tissue components as well…

credit Bhullar. Showcases loss of beak creates ancestral palatine condition

Take what you will from these studies, they of course have been met with loads of skepticism. But I do have to wonder if they hint at a pathway to a closer approximation of soft tissue oral margins in theropods.


Croc Faced Killers

Croc-Faced Lungfish Fighting Mud Frolicking Spinosaurus by Duane Nash

Of course for those keeping abreast of things it is all about crocodile faced theropods right now. Yep, rugged,  tooth exposed, thin lipped, croc faced killers. And let's give credit where credit is due: Tracy L. Ford has been sounding off on this look for years before the Carr paper hit. You know some might think I am against the notion of croc faced theropods - quite the contrary actually.  I do want to go on record saying that the croc faced idea and lack of lips notion in theropods is a good one and a very important concept and likely prevalent in a great many theropods - just not for all of the theropods that this look is normally attributed to.

A necessary prequel to the ideas I will be elaborating on is my last post Behind Your Bony Mask of Face. In this post I made the argument that the texture of the skull of crocodiles has more to do with ecology than it does skin integument, that in fact a similar skull texture is found on many amphibious and aquatic stealth predators - some that have tightly adhering skin and some that do not. What tyrannosaurids like Daspletosaurus horneri and indeed most carnivorous theropods have is indeed a rough and textured skull that, I suggested, helps anchor a particularly tough and textured epidermis. This dermis - possibly even somewhat keratinized and/or cornfield - is not necessarily covered in scales.

Is there a test of this notion of mine? Can we locate an animal - preferably somewhat close ecologically, behaviorally and phylogenetically to tyrannosaurids - that shares such a rugose and textured skull BUT that does not have a scaled facial epidermis?

By golly, yes there is and it comes in the form of a lineage of theropods that are extinct, were arch hunter-scavengers, did have nasty biting abilities, most certainly face bit, were not aquatic, and were most certainly not scaled in the facial region. The answer is phorusrhachoid terror birds.


Phorusrhacos longissimus Royal Ontario Museum, credit Captmondo CC3.0
Phorusrhacos from here


AMNH 5027 public domain

What I am suggesting is that the facial osteological correlate of these animals is not so much a phylogenetic signal, but an ecological one. Large carnivorous derived theropods (i.e. phorusrhacoids) having a textured skull convergent with their ecological antecedents in large carnivorous extinct theropods. A necessary caveat of this is that, as I mentioned, those theropods that most converged closely with crocodilians ecologically likely had a croc - like oral margin and skin texture. These theropods would most notably include spinosaurids. Other small game theropods especially of the "kink - snouted" gestalt may have been similar or split the difference: lack of extra-oral tissue towards the front of the jaw but retaining some "lip" towards the rear of the jaw. Noasaurids which were always scrounging into the dirt after fossorial prey may have diminished lips as well. Many smallish coelurosaurs, dromies, troodontids, and compsognathids that were small prey scroungers may have followed this pattern of diminishing, receding lips and a more croc-like exposed oral countenance.

Toothed Birds, Pseudo-toothed Birds and Small Game Foraging To Lose Lips 

Sinosaurus credit Duane Nash
Sinosaurus has always been a favorite of mine. It looks basically like a more beefed up version of Dilophosaurus. But while Dilophosaurus' more slender teeth and jaws denote a theropod very much engaged with a prey base of smallish stuff and quite possibly dedicated piscivory - Sinosaurus looks like an animals unsatisfied with small fry and aiming at bigger game. However it still has that odd kinked snout in the front which suggests that it could optimally pluck at small game or perhaps evolved from something of a more Dilophosaurus gestalt, but ultimately was more turf than surf in its diet. To reflect this dichotomy of kinked snouted theropods I diminshed the extra-oral "lip" towards the anterior kink of the jaw but retained more fleshy, salivating meat curtains towards the rear.

Sinosaurus credit Ghedoghedo
Returning to Dilophosaurus we are presented with a conundrum similar to saber-toothed predators and extra-oral margins. It has slender, but very long teeth. Actually this is a case where the teeth from the upper jaw penetrate below the level of the neuromuscular foramen of the lower jaw - and actually might dip below the lower jaw itself. Unless of course the teeth in this lower pic by Jaime Headden are actually protruding too much ( I suspect they are). This creates a very dastardly situation for lower lips and looking at Dilophosaurus it might in fact be a true croc-faced killer!!

Jaime A. Headden (User:Qilong) - http://qilong.deviantart.com/art/The-Dilophosaurus-Yawn-252396964

Again with Dilophosaurus it is simple to formulate an ecological reason for it dispensing with lips. Because it shows tendencies towards small game, large lips in the anterior of the jaw would provide a cumbersome block to intricate poking and plucking of small critters whether in the water, in burrows, or whatever. Better to dispense with such lips, evolve a keratinized and/or cornified oral epidermis epidermis and retain sensitivity from the neurovascular foramina. Note how this parallels the argument in my last piece in this series where I suggested that the evolution of beaks and loss of teeth were coincident with an increasing emphasis of omnivorous/herbivorous diets. Where as the constant pecking, plucking, and cropping of seeds, fruitifications, foliage, insects would encourage a beak type oral margin to replace non-muscular fleshy "meat curtain" type oral margins in  theropods: in croc-faced killer theropods the lips would likely recede for the same basic reasons. In these small game hunting theropods the constant abrasion against substrate, the drag imposed on lips in water, the delicate plucking of small game would encourage a loss of extra-oral tissue. Like theropods that evolved beaks, theropods that moved towards specializing in small game, burrowing animals, "piscivory" would lose lips but they would not lose their teeth.

A constant argument I put forth for large lippy extra-oral tissue in large game theropods is that this tissue provides an extensive "neural net" that allows real time sensing of the movement of large prey in the jaw and corresponding bite adjustments. For theropods foraging for smaller prey in the water such a "neural net" is not needed. Water itself provides the medium for sensing prey movement and as the lips would hinder small game plucking, encourage drag, and are altogether redundant they would soon diminish and take on a "croc-like" oral gestalt. The sensitivity remains however, it is just the lips that recede.



It is also possible a very tough gummy type tissue covered up a lot of the tooth in Dilophosaurus. Such gummy tissue would of course be a lot more rugose and resistant to drying than typical gingiva but still anchor and strengthen the teeth.

Teeth, or better yet pseudo-teeth are important for both extinct and extant proper aves and near birds like the often toothed enantiornithines. There are many examples of the evolutionary utility of teeth, choannal papillae, and pseudo teeth in these animals showing us that not only are such projections useful and whenever possible retained BUT more importantly, I would argue, that there is no universal a piori directive, guidance, or eventuality of an edentulous "beak" in theropods. These examples also fundamentally address the importance of ecological consideration when thinking about extra -
oral margins and "toothiness".

Pelagonis mauretanicus crédit Didier Descouens CC4.0
I have to start off with the most obvious choice: the giant pteranodontid mimics, the pelagornithids. Their non-serrated toothlike extensions of the skull bone and rhampotheca just beg for us to consider the ecological utility of teeth in such marine, soaring predators. They also compel us to wonder if toothless pterodontids were not quite so "toothless" and that maybe they evolved a soft tissue choannal  papillae pseudotoooth and/or sharp tongue tooth replacement of their own. If birds were not winnowed down to the toothless beaked species through the K/T extinction one has to wonder if legit toothed birds would not dominate the oceans, like they did in the Mesozoic?

Pseudo teeth penguin credit: http://i.imgur.com/o3vze.jpg
Pseudo-teeth on penguins are a thing. Remember, penguins stem from a fairly early branch of avians and possibly even arose in the Cretaceous (although I doubt it due to competition from hespernornithids) but there are no truly toothed penguins that we know about from the fossil record. This reinforces the idea that only edentulous beaked avians made it through the K/T and had to reinvent teeth when they occupied niches where teeth come in handy.

To bolster this pattern simply look towards the dominant fish eating birds around in the Cretaceous: hespernornithiformes and ichthyornids both have toothed jaws. They had basically lipless keratinized beaks - as should occur given their predilection for aquatic prey. Yet they retained teeth. And there is no reason to suspect that they would lose their teeth if the K/T event did not occur.

 Hespernornis regalis YPM 1206 credit Heilmann 1926

It is worth mentioning that in the above species it has partially lost teeth in the upper jaw, replaced by sockets that the lower teeth slot into. Ichthyornis shows a similar pattern: tooth loss partially in the upper jaw but a continuous row of teeth in the lower jaw. One has to wonder if and why we should expect complete tooth loss in these groups given the propensity for truly toothless birds to evolve pseudo-teeth.

Ichthyornis public domain credit O.C. Marsh
I would be remiss not to put a word in for enantornithines, the true champions of tooth retention in birdy types. They count in their number a fair bit of smallish, predatory type toothed opportunists and specialists. They also, as should be expected, probably had extremely reduced and receding lips to better pluck and peck at small game. But alas they did not make it through the K/T as successfully as true avians perhaps because they did not fully commit to seeds and the ubiquitous "pecking" foraging method so emblematic of modern birds?

credit Stephanie Abramowizc
Indeed, far from showing a tendency to lose or diminish teeth many enantiornithines were going full hog with toothiness. Sulcavis geeorum had some pretty gnarly, crazy teeth seemingly well adapted for chewing up tough crustaceans and insects.

Sulcavis geeorum credit Stephanie Abromowicz

Jingmai O'Connor:

"While other birds were losing their teeth, enantiornithines were evolving new morphologies and dental specializations. We still don't know why enantiornithines were so successful in the Cretaceous but then died out - maybe differences in diet played a part."

So where does this leave us? Admittedly this piece is a little all over the place, and readers might be dumbfounded in how quickly I switch from say a critter like Sinosaurus to Pelagornithis. But there are some general themes and trends that stand out.

- The transition of toothed birds to toothless bird is not a direct line nor is it necessarily inevitable. The prevalence of pseudo-teeth in many modern birds (especially piscivorous) that correspond to toothed Cretaceous analogs should give us moment to pause. Perhaps the Cretaceous toothed ichthyornids and hesperornithiformes with their basal retention of teeth were not the inferior models compared to modern penguins and "gull" type seabirds but actually the optimal model due to their teeth which should outperform the pseudo-teeth of modern birds.

- Enantiornithines provide us with an alternative universe where toothy birds are not only common but the most successful "norm". We might be partially conditioned to think of modern derived toothless birds as the optimal model for what a bird should be, but what if this is not the case? Enantiormithines seemed to dominate toothless birds in the Cretaceous. Did the K/T boundary provide the ecological filter that allowed toothless seed eating birds to get through but prohibiting the more toothed - and carnivorous - enantiornithines from getting through as has been suggested in a recent paper?

- In both the toothed fish eating birds and the carnivorous enantiornithines I would put forth a condition of highly reduced lips - perhaps just a trailing edge of tissue at the commissure. This is also similar - albeit without a beak - to the near lipless condition I would expect in small game hunting, fossorial foraging, and piscivorous theropods. The realities of aquatic foraging, constant abrasion, and intricate plucking of small game would create a less than ideal situation for any substantial, non-muscular extra-oral tissue in these animals. This provides an alternative pathway to losing lips to what I discussed in my Lips Before Beaks part I where herbivory would diminish lips and create beaks in theropods.

In all the above examples I am actually explicitly arguing for an exposed toothed, primarily lipless, and even croc-like oral margin. Wait a second here… wasn't I the guy who argued for lippy sabertooth cats,  beneficial activity of saliva on tooth health and what not? Have I done an about face?

Let's just say that my thoughts have… evolved and become a bit more nuanced. Two things can be simultaneously true and seemingly at odds but still coexist. Sheathed teeth under the auspices of calcium and phosphorous buffered salivary broth I still believe is a great thing to have… just not mandatory in all cases. Pterosaurs, crocs, plesiosaurs they all break that "rule" and a good many theropods probably did too… just not all of them. I would still consider most largish big game, serrated toothed macro predatory theropods to maintain - and even re-evolve - large "meat curtain" type lips. This would include most largish tetanuran type theropods, allos, carchs, predatory ceratosaurus, tyrannosauroids, dromies, megaraptors - generally most serrated toothed zyphodont theropods.

You will note I left out abelisaurids, I think something neat was going on with them which I will save for another post. Yep, some have been asking for it and I finally have enough new ideas to make a post on abelisaurids. Trust me, they will be weird AF when I'm done with them. Weirder than they already are.

And finally I leave you with Austroraptor. It was a South American unenlagine potentially flightless spinosaurid mimic.  Remember spinosaurids themselves are crocodile mimics and share with them a skull texture (spinosaur skull texture is more similar to crocs than tyrannosaur skull texture is to crocs imo) and likely a lipless oral margin. Guess what the skull texture of Austroraptor looks a bit like?




Austroraptor cabezii by Duane Nash

Austroraptor is that unique beast that potentially tells us a lot. Because it was a maniraptoran and quite possibly secondarily flightless it may indeed hail from fully feathered ancestors. If it lost feathers on the face it may have indeed followed the trend of modern naked faced birds and eschewed feathers and scales on the face. After converging with spinosaurids (which converged with crocodiles) it evolved a highly, rugose, textured, and pitted skull. Not because it had a scaly face but because such a skull texture allows - as I suggested in Behind Your Bony Mask of Face - better tactile and neurosensory capability as a sensitive snouted aquatic predator. Hence the convergence of skull features as I highlighted in such disparate groups as temnospondyl metoposaurs, phytosaurs, snapping turtles, crocodilians, various fish, and others. These textured skulls in aquatic predators I hypothesize allow more surface area to "catch" vibrational frequency in the water medium like a gnarled, old used catchers mitt.





Refs

Bhullar, B.-A. S., Morris, Z. S., Sefton, E. M., Tok, A., Tokita, M., Namkoong, B., Camacho, J., Burnham, D. A. and Abzhanov, A. (2015), A molecular mechanism for the origin of a key evolutionary innovation, the bird beak and palate, revealed by an integrative approach to major transitions in vertebrate history. Evolution, 69: 1665–1677. doi:10.1111/evo.12684

Carr, T. D. et al. A new tyrannosaur with evidence for anagenesis and crocodile-like facial sensory system. Sci. Rep. 7, 44942; doi: 10.1038/srep44942 (2017).online
Ford, T. L., 2015, Tactile faced Theropods: Journal of 
Vertebrate Paleontology, SVP 75th annual meeting, Meeting Program & Abstracts, 
p. 125.


O’Connor, J., Zhang, Y., Chiappe, L., Meng, Q., Quanguo, L., Di, L. 2013. A new enantiornithine from the Yixian Formation with the first recognized avian enamel specializationJournal of Vertebrate Paleontology. 33, 1: 1-12

Weeks O, Bhullar B-AS, Abzhanov A. 2013. Molecular characterization of dentadevelopment in a toothed archosaur, the American alligator Alligator mississippiensis. Evolution and Development 15(6): 393-405. 

"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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