Author: zarmstrong1990

Freelance paleoartist and skeptic, interested in paleontology, evolution, mathematics, physics, biology, zoology, music, atheism, religion-science conflict, etc.

Reports of the death of lipless theropods are greatly exaggerated (part 1)

A while ago (over 4 years! Where does the time go?), I wrote a journal on why I didn’t think theropods had lips, at my deviantArt website (http://palaeozoologist.deviantart.com/journal/Theropod-lips-I-don-t-think-so-288659080).  That post generated a number of comments, both in favor of my hypothesis and against.

My attention had been directed to a new abstract was recently published by a commenter on my journal regarding this and I wanted to discuss it. Presented at the 4th Annual Meeting, 2016, Canadian Society of Vertebrate Palaeontology, in it the authors favor giving theropod lips. When I first read it, I thought, “They have a pretty good case, looks like I am wrong, and will need to start drawing theropods with lips again.” However, upon re-reading it, I am not so sure.

I am copying the text of the abstract since it is relatively short:

(Oral Presentation)
Dental anatomy and skull length to tooth size ratios support the hypothesis that theropod dinosaurs had lips
Robert.R. Reisz1, D. Larson2
1Department of Biology, University of Toronto Mississauga, 3359 Mississauga Road,
Mississauga, Ontario, L5L 1C6, robert.reisz@utoronto.ca
2Department of Ecology and Evolutionary Biology, University of Toronto, 100 Queen’s
Park, Toronto, Ontario, M5S 2C6, Canada, derek.larson@mail.utoronto.ca

Two competing hypotheses, whether the large teeth of theropod dinosaurs were
exposed or covered by peri-oral tissues or lips when the mouth was closed, are tested
using phylogenetic bracketing, dental anatomy and development, and regression analysesusing skull length to tooth size ratios.
Two different anatomical patterns can be discerned in reptiles. In crocodiles, the
closest extant, toothed relatives of theropod dinosaurs, about ¼ of the tooth is covered by gingiva, but there are no lips, and the crowns are exposed permanently. In contrast, in extant squamates, a more distant reptilian relative to dinosaurs, teeth are covered by lips when the mouth is closed, and there is extensive gingiva. Phylogenetic bracketing, in the absence of evidence from birds and from fossils would tend to support the hypothesis that the large teeth of theropod dinosaurs would be exposed when the mouth is closed, although there is little reason to suggest that the same was the case for the small teeth of other dinosaurs, like the cheek teeth of ornithischians.

Dental anatomy and development offer a different perspective. As the hardest
vertebrate tissue, enamel has a low water content (Zheng et al. 2013), and is hydrated and maintained by glandular secretions in the mouth. We propose that this requirement of hydration is not possible to maintain if the tooth is exposed permanently. We tested this by examining the exposed teeth of terrestrial mammals (tusks), modified teeth that evolved independently in several mammal clades.

Histological thin sections show that tusks in mammals do not have enamel. At the initial stages of development, some enamel may be formed, but soon after eruption the enamel is worn away, and may be replaced by cementum. This suggests that the large teeth of theropod dinosaurs, all known to have well preserved and maintained enamel, even with specialized ziphodonty (Brink et al. 2015), were not exposed permanently, but covered by reptilian lips similar to those found in squamates.

Similarly, ordinary least squares regression analyses of skull lengths to tooth sizes
in varanid lizards and theropod dinosaurs of various sizes indicate that the teeth of
theropod dinosaurs conform to the same pattern as varanid lizards. This provides strong added support to the hypothesis that theropod dinosaurs had lip-covered teeth, as teeth in theropod dinosaurs are no larger than would be expected in a similarly sized varanid lizard. This conclusion has wider implications, suggesting that this may be the primitive condition for all terrestrial vertebrates, allowing us to test whether the large, tusk-like structures of some basal ornithischians (Weishample and Witmer 1990), or the large canine-like teeth of terrestrial vertebrates (Brink et al. 2015) were exposed or not.

Finally, we propose that the lip-covered dental pattern is primitive for terrestrial
vertebrates, and that of crocodilians is a derived condition related to their secondary
aquatic or semiaquatic adaptations. It should be noted that terrestrial stem crocodilians (Clark et al. 2001) have a dental anatomy very similar to that of theropod dinosaurs, and likely had lips too.

 

Since it is an abstract only, and the actual data hasn’t been published, I can only comment on problems with I see with how they frame the hypothesis and test it.

In the third paragraph they posit that enamel needs to be adequately hydrated to be maintained, and that this would not be possible with permanently exposed lips. They appeal to histological evidence in mammalian tusks that show that the teeth are covered in cementum, not enamel, and thus dinosaurs must have had lips.

There are several problems with this.

(A) Mammalian tusks are very specialized structures and are not used like other teeth. Mammals that have tusks tend to use them in specialized tasks such as rooting through dirt, fighting (as in elephants) or digging (see warthog image below).20378063-mud-shovel-warthog-boar-digging-a-hole-to-sleep-in-photographed-in-namibia-stock-photo

Why would we expect animals engaged in such specialized behavoiors to be histologically similar to theropods? I am not aware of any theropod that is hypothesized to have used it jaws (and thus teeth) in similar behaviors. Of course if an animals is using it’s teeth in ways that are being constantly abraded by rough dirt and clashing, I would expect the enamel to be replaced by cementum. I don’t think this is a good behavioral analogy to theropods at all, and thus is a very weak argument in support of lips.

(B) Having teeth that are partially exposed some of the time (for extended periods) and then completely submerged in water on a repeated basis would actually be worse.  Crocodiles spend a significant time sunbathing in order to modulate their internal temperature as they are ectothermic:

130888216-american-alligator-adult-sunbathing-with-gettyimages

If hydration of the enamel were a problem, one would actually expect crocodiles to retain their lips. Anyone who has had to repeatedly wash their hands over an extended period of time knows how repeated washing and drying chaps the skin. For animals that are semiaquatic, one would think if lips were needed in order to protect the enamel, lips would be retained in order to avoid extremes of being dehydrated to being completely saturated.

(C) In addition to the problems with analogous behavior between theropods and tusked mamals, there is another problem: Mammals are, by-and-large, diphyodont, whereas theropods (as well as crocodiles) are polyphyodont. What this means is that most mammals only have two sets of teeth, the deciduous set (aka primary or “baby teeth” in humans), followed by the permanent set. Theropods, on the other hand, continuously shed their teeth throughout their lives. This rate varies, but appears to have been equivalent to modern day crocodiles (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC26184/), about 1 to 2 a year. In at least some sauropods, they replaced their teeth every one to two months (http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0069235).

Unlike squamates whose teeth are directly fused implanted to the jawbone (i.e., no tooth sockets; this can take the form of acrodonty or pleurodonty – the latter in which they are fused or ankylosed to the jaw bone; see Jaime’s comments below on my choice of terminology, as well as my response), theropods and crocodiles also share in having in their dental anatomy tooth sockets (alveoli). What this means is that there is already a replacement tooth right behind when a tooth is lost and can quickly be replaced.

So theropods have the best of both worlds, continuous (see comment below) repeated replacement of their teeth as in squamates, as well as having tooth sockets like mammals.

I would hypothesize that this would enable theropods to overcome any concerns of eroding enamel.

(D) Other semi-aquatic and aquatic toothed animals have not lost their lips, or extraoral seals. If you looked at aquatic and semi-aquatic squamates, such as sea snakes, marine iguanas and and some lizards (such as the Chinese Alligator Lizard, see below), none have lost their lips:

100_26042

chinese-crocodile-lizard-in-water

 

Sea snake:

Marine iguana:

17189

Anaconda:

And a nice short video of a water monitor (Varanus mertensi):

 

So I don’t understand what the evolutionary reasons would be for crocodiles to lose their lips, since all other extant toothed reptiles that are aquatic or semi-aquatic have not lost their lips.

Nor have aquatic and semi aquatic mammals:

Elephant seal (female):

Otter:

Orca:

(Quick side note on whales: while whales no longer have muscular movable lips, they have retained the oral seal, and when the mouth is closed, the teeth are invisible).

It’s therefore not clear why crocodiles would lose their lips,  and stating that is because of their aquatic or predatory habits is falsified by the above examples. It is also interesting to note that some of the most derived members of the Trionychia (soft-shelled turtles) have lost (or nearly lost) their beaks (external premaxillary, maxillary and dentary rhampotheca, see Jaime’s comments and my response below; they have retained an internal palatine and tomian rhampotheca) and have actually independently evolved fleshy lip-like structures, and are aquatic:

60141-adapt-768-1

Note that this really is an independent evolution, as the most basal members of crown Trionychia, such as Carettochelys , still have beaks, whereas derived members, such as Carettochelys Pelodiscus, have lost or nearly lost their beaks and replaced with lips (see cladogram from Wikipedia below). Turtles have interesting implications with regards to phylogenetic bracketing and the biological restrictions on lips, teeth and beaks. In the second part of this article, I want to discuss these evolutionary developmental reasons that, in my view, also suggest no lips – and not just for theropods, but likely for all dinosaurs.

Trionychia cladogram

The Incredibly Shrinking Dreadnoughtus

Dreadnoughtus was just described this past September by Lacovara et al. (2014). As per usual, both the paper and the media played up the size of Dreadnoughtus, with CNN in particular declaring on its website that it was “perhaps the biggest creature to ever walk the planet.” *Doublefacepalm* This should not be surprising for those who have followed science journalism covering paleontology, as nearly all gigantic sauropods that have been published in recent memory have had initial size estimates far higher than they likely were. For instance, “Seismosaurus”, Supersaurus, Argentinosaurus, Futalognksaurus, Puertasaurus – to name a few – were all declared to be the “biggest” at the time of their discovery but are now considered to be far smaller than initially reported. Only one of these, Argentinosaurus, may still be considered a contender for the biggest, although it too has had its size estimates reduced. Dreadnoughtus, however, is a particularly egregious example. In the paper, the mass was estimated at 59.3 tonnes, with the length estimated at 26 meters. However, doing my own skeletal reconstruction (see above) based on the 3D model in the supplementary material, I get a significantly different estimate.

My own mass estimate is about 29.2 tonnes, and the length estimate of 21.2 meters. In terms of mass, that is nearly a 50% difference! And the length estimate is still a difference of 18.5%. To get some perspective, that is like saying a person who is actually 5 ft 11 and 180 lbs is 7 ft tall and 360 lbs – a mighty big difference. Now, how did I get my estimate, and why is it so different? I got my estimate in a different way than the authors. I used a technique called Graphic Double Integration (GDI). I will go into detail this method and defend my reconstruction in future posts. For now, I want discuss my skepticism of the methods they used.

They used a mass regression model to predict the size based on methods in Campione & Evans (2012). Campione & Evans (2012) went to great lengths to rebut the slew of criticisms that have been made against these models in the past. Interestingly enough, the mass estimates they get for Giraffatitan and Diplodocus are quite close to mass estimates that I get using the GDI technique for both taxa. They estimate a mass of 26.8 tonnes – 44.7 tonnes for Giraffatitan, with a mean of 35.7 tonnes. A GDI estimate I did for Giraffatitan off of a figure by Greg Paul gave me an estimated mass of approximately 32.8 tonnes, which is quite close to the mean and right within the predicted range. Similarly, for Diplodocus I get a mass of about 10.5 tonnes, and they get a mean mass of 10.9 tonnes, with a range of 8.2 – 13.6 tonnes. Note that these mass ranges for both taxa are +/- ~25%. Applying the same range to Dreadnoughtus gives a range of 40.5 – 67.2 tonnes. The lower end of this range is closer to my estimate, but it is still off by about 25% – a significant difference.

So why is my estimate in this case so much lower than the equations used by Campione & Evans (2012)? One thing to note before I go on is that I think the similarity between my previous GDI estimates and how close they are to the regression method is possibly misleading. We are only working from a sample of 2, meaning the sample size is too small to draw any conclusions from this, it would be good to compare GDIs of the other taxa and compare them. For now, we are not able to do this. The real reason I suspect for this discrepancy is twofold: (1) the regression model is hit-and-miss when it comes to predicting masses in living taxa for which we already know the masses and (2) is that there is good reason to think that Dreadnoughtus is an outlier, and therefore would not be accurately predicted by the regression equation model.

How accurate are the regression equations when applied to living taxa?

In regards to the first point, I took the 2nd regression equation from Campione and Evans (2012) and used it to predict the masses of the living taxa. Doing this, I found that for over a third of the taxa the predicted mean mass that was +/-30% or greater from the observed living mass of the taxa. This is a significant problem, because it shows that for a significant portion of living taxa, the regression model produces a poor fit when predicting masses. In addition, 35 of the 255 taxa had predicted mean masses that were +/-50% from the observed mass, which is nearly 1 in 7 taxa. From this data, it should be clear that while the majority of the taxa have reasonable mass predictions, enough fall out of a range that I would consider reasonable to suggest that, used alone, the regression equations may not be sufficiently robust to predict masses of extinct animals. At the very least, another independent method should be used to give a check on mass estimates. I would suggest that a relatively easy method to do this would be GDI techniques. Screen shot of Campione and Evans 2012 mass file Screen shot of Campione and Evans mass file_2

Is Dreadnoughtus an outlier?

In regards to the second point, I suspect Dreadnoughtus may be an outlier (being in the range of masses of by more than +/- 50%) because the referred limb bones of Futalognkosaurus are very similar in size to Dreadnoughtus in terms of length. While femur length is a poor indicator of overall size and mass, in other comparable measurements (cervical vertebra size, pelvis size, etc.), it appears that Futalognkosaurus is quite similar in size in general to Dreadnoughtus. Futalognkosaurus was found by Benson et al. (2014) to have a mass of about 38.1 tonnes, which is not too different than my GDI estimate for Dreadnoughtus (although it should be noted that my GDI mass estimate for Futalognkosaurus is considerably lower than these estimates – that said, I consider my Futalognkosaurus reconstruction to be out of date in a number ways). Another reason why Dreadnoughtus is probably an outlier, and therefore not predicted well by the regression equations, is that in statistics there is the danger of extrapolation. This refers to using statistical models to predict values out of the range of the sample data. In this case, while Campione & Evans (2012) do an admirable job of trying to rebut criticism of this, I don’t think they do a very effective job.

For instance, in their data set the largest animal they have data for is Loxodonta africana, at about 6.4 tonnes. This is in comparison to masses of many sauropods which easily out classing this, as even the rather light Diplodocus is at least 50% heavier than this. The predicted mass of Giraffatitan being over 450% heavier than the heaviest taxon in the sample. Therefore, there is good reason to be skeptical of extrapolating animals much larger than the sample the regression equations are based on.

For fun, here is what a 52 tonne Dreadnoughtus looks like with the soft-tissues of my skeletal increased by 50% on the torso and 25% on the neck:

Dreadnoughtus_fat_small_version

And here is a screen shot of the calculation:

Fat Dreadnoughtus_2Me thinks that is not very likely.

Up next: A GDI mass estimate for Dreadnoughtus.

References:

Benson, R. B. J., Campione, N. E., Carrano, M. T., Mannion, P. D., Sullivan, C., Upchurch, P. & Evans, D. C. Rates of dinosaur body mass evolution indicate 170 million years of sustained ecological innovation on the avian stem lineage. PLoS Biol. 12, e1001853 (2014).

Campione, N. E. & Evans, D. C. A universal scaling relationship between body mass and proximal limb bone dimensions in quadrupedal terrestrial tetrapods. BMC Biol. 10, 1–22 (2012).

Lacovara, K.J. et al. A Gigantic, Exceptionally Complete Titanosaurian Sauropod Dinosaur from Southern Patagonia, Argentina. Sci. Rep. 4, 6196; DOI:10.1038/srep06196 (2014).

Introduction

First of all, welcome to my blog. I’m Zach Armstrong, “The Palaeozoographer”. I got tired of trying to “blog” via DeviantArt’s journal feature (see my DeviantArt page here: http://palaeozoologist.deviantart.com), so I will probably move all of my longer thoughts here. Hopefully, it will be easier to blog, and I can do it more often. Plus, here, anybody can comment, whereas only people signed up on DeviantArt can comment there. Therefore, the discussions will be more open and included a greater diversity of commenters. I will tend to blog about dinosaurs and my own palaeozoography (aka, “paleoart”), however I will likely cover more broad topics than that from time to time.

What is a “Palaeozoographer”?

Simply put, a paleozoographer is someone who illustrates fossil animals, either as life restorations or as reconstructions of the actual fossil material. I derive it from the term “zoography”, the primary definition of which is “the branch of zoology concerned with animal description.” However, the secondary usage is more analogous to what I have in mind, “pictorial art in general, but especially that which shows animals.” So, in this case, “palaeozoography” is the “branch of zoology concerned with the description of extinct animals in pictorial art in general, but especially that which shows extinct animals.”

This word is not technically a neologism of my own invention, as a quick Google search as of today (January 17, 2015), shows only 45 results (“paleozoography” showing 44 results), so it is not in common usage. The actual term “paleozoographer” or “palaeozoographer” shows only 8 and 24 results in a Google search as of today. Therefore, since it is not in common usage, I feel is OK to appropriate it for my own use.

Why not “paleoartist”?

For sure, “paleoartist” is a much better known term, as is the term “paleoart”, but as has been pointed out by others, it is not actually the best formed word. As constructed, it should mean “ancient art”, presumably referring to art produced by ancient people, such as cave art, etc. I also toyed with the idea of “paleobiographer” (which would be more inclusive of all ancient life, including plants), but that sounds like someone who does ancient biographies.

John Conway has attempted to co-opt the word “paleontography” in place of “paleoart”, however the definition and current usage of that is primarily the “description of fossil remains”, or more simply as “descriptive paleontology”. Also, “paleontography” is slightly more used, which Google showing about 8,300 hits. That said, paleontography has not yet caught on either. I doubt the term “palaeozoographer” and “palaeozoography” will catch on either (too long and clunky), but no matter because I like, being more unique than the other terms available. I am acutely aware that I am being animal-centric here, leaving out those who portray paleobotany, etc. I suppose they will have to stick with the more generic “paleontography”.

So, what will this blog be about?

While palaeozoography encompasses a vast number of extinct animals, I will largely focus on dinosaurs in my own art. Therefore, most of the topics I cover will be dinosaur-related. That said, I don’t intend this to be simply a dinosaur art blog, or even simply a dinosaur science blog, but I hope to cover a wide range of topics of all sorts. That said, both dinosaurs and dinosaur art will be a major if not primary focus of this blog. I am not sure how often I will blog, considering I have been very fickle with posting anything on my DeviantArt page, I can’t promise I will be very prolific. My goal, however, is to blog at least once a week, even if just a short post.

Who am I?

I’m a hobbyist artist, with little formal artistic or scientific training. I, of course, took art and science classes in college, and am deeply interested in both, but do not have an art or science degree. I have an Associate of Arts degree in Liberal Arts with a Mathematics Emphasis from Normandale Community College and an Associate of Arts degree in Accounting from Hennepin Technical College. By day, I work as a pension benefits analyst. My previous work experience includes being a college math tutor for 2 and 1/2 years as well as an IT/Accounting Technician for a year. I have been drawing dinosaurs for a long time, 15+ years, and have had a presence on DeviantArt with my current account for about 4 years as of present, with a shorter stint on there of about 2 years with another account.

Ending remarks

My hope is that this blog will bring further attention and add a new perspective to the intersection of science and art. Reactions, in-depth comments, and criticism are very much encouraged, but my hope is to always keep the discourse intelligent and civil. I reserve the right to censor or, in extreme cases, remove comments that do not adhere to a reasonable person’s definition of civil discourse, especially if comments include graphic language, threats or hate speech. Please strive to be kind to others, even if you intensely disagree with their views.

Hope to see you here!

-Zach

2013-12-22 17-52 Alamosaurus drawing small scales painting_6

A piece of palaeozoography – an Alamosaurus strolling along, by yours truly.