March 17, 2015
I have a new paper out today in PeerJ: “Ecological correlates to cranial morphology in leporids (Mammalia, Lagomorpha)”, with coauthors Brian Kraatz, Emma Sherratt, and Nick Bumacod. Get it free here.
I know, I know, I have fallen from grace. First Aquilops, now rabbits. And, and…skulls! I know what you’re thinking: that maybe I’m not just experimenting with the non-vertebrae of non-sauropods anymore – maybe I have an actual problem. But I don’t. I can quit anytime! You’ll see.
Actually rabbits are the freakiest of all mammals and their skulls are wicked cool. They have double incisors, with the second set right behind the first, hence the name Duplicidentata for rabbits and their close relatives. They have weird fenestrations in their maxillae (pretty much all taxa) and parietal and occipital bones (some more than others) – I’ll come back to that in a bit. And, as we discuss in our new paper, you can tell something about how a rabbit runs by looking at its skull. I thought it would be fun to relate how we figured that out, and why.
A long time ago in a graduate seminar far, far away…
1950: DuBrul, Laskin, and Moss
I met Brian Kraatz at Berkeley, where he and I were part of the cohort of students that came into the Integrative Biology Department in the fall of 2001 (faithful readers may remember Brian from his work tracking oliphaunts from, gosh, three years ago already). We took a lot of classes together, including a seminar by Marvalee Wake on evolutionary morphology. I’m pretty sure that seminar was the first time I’d actually read DuBrul and Laskin (1961), “Preadaptive potentialities of the mammalian skull: an experiment in growth and form”, or as I think of it, “How to turn a rat skull into a pika skull for fun and profit.”
Pikas (Ochotonidae) are the sister group to rabbits (Leporidae) and together these groups make up crown Lagomorpha. If you’re not familiar with pikas, Brian describes them as starting with bunny rabbits and then making them even cuter and cuddlier. Seriously, go do an image search for ‘pika’ and try not to die of cute overload.
Pikas are interesting because in many ways their skulls are intermediate between those of rodents, especially rats, and rabbits. This is maybe not surprising since rodents are the sister group to lagomorphs and are united with them in the clade Glires. E. Lloyd DuBrul was all over this rat-pika-rabbit thing back in the mid-twentieth century. Here’s an illustration from DuBrul (1950: plate 2; labels added by me):
So DuBrul knew from pikas and in particular he had the idea that you could maybe just tweak a rat skull – say, by knocking out the basicranial sutures in a baby rat to limit the growth of the skull base – and produce a gently domed skull like that of a pika. That’s what DuBrul and Laskin (1961) is all about. They did that experiment and here are their results (DuBrul and Laskin (1961: plate 3). Normal rat skull on the right, and dotted in the bottom diagram; experimental “pika-morph” rat skull on the left, and solidly outlined below.
What’s going on here morphogenetically is that the facial skeleton is getting tilted down and away from the back end of the skull. DuBrul was hip to that, too – here’s a relevant image from his 1950 paper (plate 4; labels added by me):
The common reference point against which these skulls are registered is the cranial base (the floor of the braincase just forward of the foramen magnum). Again, the pika is a pretty good intermediate between the rat and a ‘normal’ rabbit, and the dang-near-dog-sized Flemish Giant rabbit takes the lagomorph face-tilting thing to its extreme. (‘Flemish Giant rabbit’ is another entertaining image search that I will leave you as homework.)
Turns out there’s another way you can get rat skulls with different geometries: you can cut off their legs and make them walk on two feet. In an experiment that you might have trouble getting past an Institutional Animal Care and Use Committee today, Moss (1961) lopped off the forefeet or hindfeet in two experimental batches of rats, to see what effect this would have on their skulls. I’ll let Moss speak for himself on this one (Moss, 1961: pp. 301-303, emphasis in the original):
Circumnatal amputation of the forelimbs has successfully produced what are in essence “bipedal rats,” i.e., rats whose habitual mode of kinetic and static posture is permanently altered. […] The animals never became bipedal in the exact sense; that is, they never walked erect on two limbs at all times. […] Nevertheless, bipedal posture and motion were more frequently observed than in controls. […]
Animals whose hind limbs were removed represented another picture. They most certainly did not walk about on their intact forelimbs. Neither did they seem able to use their hind limb stumps as satisfactory substitutes. Their gait was not uniform and seemed to consist in a series of short pushes or hops. The most noticeable thing about them was, among other things, apparent accentuation of their cervical vertebral curvature. The sum of these changes was an upward rotation of the skull.
He wasn’t kidding: when the two groups of bipedal rats grew up, their facial skeletons were tilted relative to the control group, but in different directions (Moss, 1961: fig 3; ‘fore’ and ‘hind’ refer to which limbs the animals had left to locomote with):
Brian and I read Moss back at Berkeley, too. In fact, we were minor Moss junkies. If you’re interested in how living forms come into being, you owe it to yourself to read Moss (1968), “A theoretical analysis of the functional matrix”.
The upshot of all of this is that although neither Brian nor I had done anything with our deep (and, okay, deeply weird) knowledge of how to experimentally jack up rat skulls by the time we graduated from Berkeley, we were also primed to be thinking about how skulls attain their shapes – especially the skulls of rodents and rabbits.
2009: American Museum of Natural History
I went to the AMNH in February, 2009, to visit Brian, who was on a postdoc there at the time, and to spend one day looking at sauropods with Mike, who was over from England for a conference. What Brian and I planned to work on was the fenestration of rabbit skulls, because I’m always interested in the strategic loss of bone from skeletal structures. We spent probably half a day talking about that, and I filled a whole page in my notebook with related noodlings:
But as the sketch on the right shows, it didn’t take us long to figure out that there was something even more interesting to do with rabbit skulls. Brian had a whole shedload of rabbit skulls from different taxa sitting on his desk, and we noticed pretty quickly that one of the primary ways they varied was in the tilt of the facial skeleton relative to the back of the skull. Here’s the very next page of my notes from that trip:
The skull up top belongs to Caprolagus, the Hispid hare, which I tend to think of as the “bulldozer hare”. Seriously, it looks like a tank. It doesn’t bound or even hop, it scrambles. Here, stare into the abyss:
That rabbit will cut you, man. And just look at how flat its skull is. Even in life Caprolagus looks more rodent-y than rabbit-y. Or, more precisely, more Ochotona-y.
At the the other extreme are taxa like Bunolagus and Pronolagus, which really push the “I’m going to cute you to death by dint of my incredible bunnosity” thing:
As Brian and I started going through skulls of as many extant rabbits as we could, we noticed that the flatter-skulled taxa, with less pronounced facial tilt, tended to be the stolid, foursquare scramblers like Caprolagus, whereas the speed demons tended to have more strongly tilted skulls. It also seemed like the latter group were achieving that pronounced facial tilt by changing the geometry of the occipital region of the skull. Look back up at the red quadrilaterals I drew on the Caprolagus and Bunolagus skulls in my notebook – those mark the basioccipital ventrally and the dorsal exposure of the supraoccipital. Perhaps unsurprisingly, supraoccipital length is not the whole story; it turns out that some face-tilters get that way by having longer or more strongly arched parietals, BUT it remains true that if you find a rabbit skull with a long dorsal exposure of the supraoccipital, it will also have pronounced facial tilt.
ANYWAY, by my last night in New York, Brian and I decided that the best way to attack this would be to go down to the basement and stay up most of the night drinking beer and measuring rabbit skulls. We then tried to correlate the various measurements and angles with information on the locomotor and burrowing habits of each species. That was a big job, after a couple of years with little forward progress (to be fair, Brian was moving across the country and taking his first tenure-track job in this interval, and I was helping birth a sauropod) we brought in Brian’s graduate student, Nick Bumacod, to do most of it. Later on the three of us were forced to acknowledge that we knew enough statistics to get ourselves into trouble but not enough to get back out. Brian had taken a geometric morphometrics course for which Emma Sherratt was a TA, and he started bugging her for help with the stats. Emma has been involved in writing new software packages for R, and we realized that the paper would be a lot stronger if we just brought her on as an author and gave her free rein with the data. Along the way Brian and Nick were giving presentations on the project everywhere from the local Western Area Vert Paleo meeting to the World Lagomorph Conference in Vienna. I got my name on four abstracts along the way, which I think is record abstract-to-paper ratio for me (especially considering that 90% of my effort on the paper was invested in a single evening in 2009 over a couple of six-packs).
But enough navel-gazing, what did we find?
2015: Rabbit skulls reveal their mode of locomotion
Our results, which you can read for free, support the hunch that Brian and I had back in 2009: slow-moving rabbits that locomote by scrambling or scampering instead of hopping tend to have less facial tilt, and faster-moving saltatorial (hopping) and cursorial (leaping and bounding) rabbits have more facial tilt. Interestingly, facial tilt does not distinguish the saltators from the cursors. So the break here is between scrambling and any kind of hopping or leaping, but not between hoppers and leapers.
Why would that be so? We don’t for sure yet, but our top hypothesis is that if you’re moving fast, it pays to see the ground in front of you more clearly, and getting your nose down out of the way probably helps with that. This is pretty similar to the hypothesis that tyrannosaurs have pinched nasals for better binocular vision (Stevens, 2006). Rabbits are prey animals and probably can’t afford to point their eyes forward, and they may need wide nasal airways as air intakes while they’re sprinting. Tilting the nose down may be the next best thing.
Some circumstantial support for this comes from the Caviidae, the family of South American rodents that includes guinea pigs, cavies, maras, and capybaras. Here’s another plate from DuBrul (1950: plate 6) contrasting the flatter skull of the guinea pig (Cavia porcellus, top) with the decidedly arched skull of the mara or Patagonian hare (Dolichotis magellanica, bottom). Compare the mara skull to the sectioned rabbit skull in the other DuBrul plate, above – there aren’t a lot of obvious characters to separate the two (beyond the lack of double incisors in the mara).
Despite being commonly referred to as ‘hares’ and looking a lot like short-eared rabbits, maras are rodents that evolved their rabbit-like form independently. The acquisition of pronounced facial tilt in two separate lineages of small fast-moving herbivorous mammals is further evidence for the influence of locomotor mode on skull form. Irritatingly, I think we neglected to mention the guinea pig : mara :: pika : rabbit correspondence in the paper. Oh well, it wasn’t our novel observation, and we did cite DuBrul (1950).
We found lots of other interesting things, too. The PCA plots we produced from our data separate the living rabbits in unexpected ways. The length of the diastema (the toothless portion of the upper jaw) and the diameter of the auditory bulla seem to be particularly important. Diastema length isn’t too hard to figure out – most of the face-tilters have long diastemas, and the flat-heads tend to have short ones. We have no idea what bulla diameter means yet. I mean, obviously something to do with hearing, but we don’t have any ecological variables in our analysis to address that because we didn’t see it coming. So there’s a chunk of new science waiting to be done there.
Speaking of new science, or at least a relatively new thing in science, we published the full peer-review history alongside the paper, just as Mike and I did back in 2013 and as Mike did with his stand-alone paper last December. More than 80% of PeerJ authors elect to publish the peer review histories for their papers. I can’t wait until it’s 100%. PeerJ reviews are citeable – each one gets a DOI and instructions on how to cite it – and I’m tired of having my effort as a peer reviewer used once and then thrown away forever.
If you’ve been reading this whole post with gritted teeth, wondering why we were using linear measurements instead of geometric morphometrics, chillax. Brian and Emma are on that. They’ve been CT scanning the skulls of as many extant rabbits as possible and plotting landmarks for 3D morphometrics – if you were at SVP last fall, you may have seen their talk (Kraatz and Sherratt, 2014). So stay tuned for what will soon be a new ongoing series, Rabbit Skulls: The Next Generation.
I probably won’t be on that voyage. I’ve had fun getting acquainted with a completely different part of the tree of life, but there are an awful lot of shards of excellence – busted-up sauropod vertebrae, that is – crying out for my attention, and I need to stop neglecting them. I’m done with rabbit skulls, I promise. I’m going clean. (Wish me luck!)
- DuBrul, E. L. (1950). Posture, locomotion and the skull in Lagomorpha. American Journal of Anatomy, 87(2), 277-313.
- DuBrul, E. L., & Laskin, D. M. (1961). Preadaptive potentialities of the mammalian skull: an experiment in growth and form. American Journal of Anatomy, 109(2), 117-132.
- Kraatz, B., and Sherratt, E. (2014). Evolution, ecology, and modularity of the lagomorph skull. Journal of Vertebrate Paleontology, 35(3, Supplement), 162A.
- Kraatz, B.P., Sherratt, E., Bumacod, N., and Wedel, M.J. 2015. Ecological correlates to cranial morphology in leporids (Mammalia, Lagomorpha). PeerJ, 3:e844. https://dx.doi.org/10.7717/peerj.844
- Moss, M. L. (1961). Rotation of the otic capsule in bipedal rats. American Journal of Physical Anthropology, 19(3), 301-307.
- Moss, M. L. (1968). A theoretical analysis of the functional matrix. Acta Biotheoretica, 18(1), 195-202.
- Stevens, K. A. (2006). Binocular vision in theropod dinosaurs. Journal of Vertebrate Paleontology, 26(2), 321-330.
December 23, 2014
Arriving as an early Christmas present, and coming in just a week before the end of what would otherwise have been a barren 2014, my paper Quantifying the effect of intervertebral cartilage on neutral posture in the necks of sauropod dinosaurs is out! You can read it on PeerJ (or download the PDF).
Yes, that posture is ludicrous — but the best data we currently have says that something like this would have been neutral for Diplodocus once cartilage is taken into account. (Remember of course that animals do not hold their necks in neutral posture.)
The great news here is that PeerJ moved quickly. In fact here’s how the time breaks down since I submitted the manuscript (and made it available as a preprint) on 4 November:
28 days from submission to first decision
3 days to revise and resubmit
3 days to accept
15 days to publication
TOTAL 49 days
Which of course is how it ought to be! Great work here from handling editor Chris Noto and all three reviewers: Matt Bonnan, Heinrich Mallison and Eric Snively. They all elected not to be anonymous, and all gave really useful feedback — as you can see for yourself in the published peer-review history. When editors and reviewers do a job this good, they deserve credit, and it’s great that PeerJ’s (optional) open review lets the world see what they contributed. Note that you can cite, or link to, individual reviews. The reviews themselves are now first-class objects, as they should be.
At the time of writing, my paper is top of the PeerJ home-page — presumably just because it’s the most recent published paper, but it’s a nice feeling anyway!
A little further down the front-page there’s some great stuff about limb function in ratites — a whole slew of papers.
Well, I’m off to relax over Christmas. Have a good one, y’all!
November 20, 2014
Back in 2013, when we were in the last stages of preparing our paper Caudal pneumaticity and pneumatic hiatuses in the sauropod dinosaurs Giraffatitan and Apatosaurus (Wedel and Taylor 2013b), I noticed that, purely by chance, all ten of the illustrations shared much the same limited colour palette: pale brows and blues (and of course black and white). I’ve always found this strangely appealing. Here’s a composite:
I’m really happy with this coincidence. In fact I think I might get it printed up as a poster for my office.
(Thought: if I did, would anyone else be interested in buying it?)
Update (a couple of hours later)
At Matt’s suggestion, I switched the order of figures 7 and 8 (the last two on the third row) to get the following version of the image. It breaks the canonical order of the figures, but it’s visually more pleasing.
Now we should write an updated version of the paper that reverses the order in which we refer to figures 7 and 8 :-)
- Wedel, Mathew J., and Michael P. Taylor. 2013. Caudal pneumaticity and pneumatic hiatuses in the sauropod dinosaurs Giraffatitan and Apatosaurus. PLOS ONE 8(10):e78213. 14 pages. doi:10.1371/journal.pone.0078213
November 5, 2014
Last night, I submitted a paper for publication — for the first time since April 2013. I’d almost forgotten what it felt like. But, because we’re living in the Shiny Digital Future, you don’t have to wait till it’s been through review and formal publication to read it. I submitted to PeerJ, and at the same time, made it available as a preprint (Taylor 2014).
It’s called “Quantifying the effect of intervertebral cartilage on neutral posture in the necks of sauropod dinosaurs”, and frankly the results are weird. Here’s a taste:
A year back, as I was composing a blog-post about our neck-cartilage paper in PLOS ONE (Taylor and Wedel 2013c), I found myself writing down the rather trivial formula for the additional angle of extension at an intervertebral joint once the cartilage is taken into account. In that post, I finished with the promise “I guess that will have to go in a followup now”. Amazingly it’s taken me a year to get that one-pager written and submitted. (Although in the usual way of things, the manuscript ended up being 13 pages long.)
To summarise the main point of the paper: when you insert cartilage of thickness t between two vertebrae whose zygapophyses articulate at height h above the centra, the more anterior vertebra is forced upwards by t/h radians. Our best guess for how much cartilage is between the adjacent vertebrae in an Apatosaurus neck is about 10% of centrum length: the image above shows the effect of inserting that much cartilage at each joint.
And yes, it’s weird. But it’s where the data leads me, so I think it would be dishonest not to publish it.
I’ll be interested to see what the reviewers make of this. You are all of course welcome to leave comments on the preprint itself; but because this is going through conventional peer-review straight away (unlike our Barosaurus preprint), there’s no need to offer the kind of detailed and comprehensive comment that several people did with the previous one. Of course feel free if you wish, but I’m not depending on it.
Gilmore Charles W. 1936. Osteology of Apatosaurus, with special reference to specimens in the Carnegie Museum. Memoirs of the Carnegie Museum 11:175–300 and plates XXI–XXXIV.
Stevens, Kent A., and J. Michael Parrish. 1999. Neck posture and feeding habits of two Jurassic sauropod dinosaurs. Science 284(5415):798–800. doi:10.1126/science.284.5415.798
Taylor, Michael P., and Mathew J. Wedel. 2013c. The effect of intervertebral cartilage on neutral posture and range of motion in the necks of sauropod dinosaurs. PLOS ONE 8(10):e78214. 17 pages. doi:10.1371/journal.pone.0078214
July 29, 2014
I wrote this paper in 2003-4, and it was rejected without review when I submitted it back then. (For, I think, specious reasons, but that’s a whole nother discussion. Forget I mentioned it.)
I haven’t touched the manuscript since then (except to single-space it for submission as a preprint). It’s ten years old. That’s a problem because it’s an analysis of a database of dinosaur diversity, and as everyone knows, the rate of recognising new dinosaurs has gone through the roof. That’s the reason I never made any attempt to update and resubmit it: dinosaur diversity is a fast-moving target, and each time through the submit-reject cycle takes long enough for the data to be outdated.
So much for the history. Now the question: how should I cite this paper? Specifically, what date should I give it? If I cite it as from 2004, it will give the misleading impression that the paper has been available for ten years; but if I cite it as from 2014, it will imply that it’s been worked on at some point in the last ten years. Both approaches seem misleading to me.
At the moment, I am citing it as “Taylor (2014 for 2004)”, which seems to more or less capture what’s meant, but I don’t know whether it’s an established convention. Is there an established convention?
Releated: where in mv publications list should it appear? At present I am sorting it under 2014, since that’s when it came out; but should it be under 2004, when it was written? I guess publication date is the one to go far — after all, it’s not unusual even now for papers to spend a year or more in press, and it’s the later (publication) date that’s cited.
Help me out. How should this be done?
- Taylor, Michael P. 2014 (written in 2004). A survey of dinosaur diversity by clade, age, place of discovery and year of description. PeerJ PrePrints 2:e434v1. doi: 10.7287/peerj.preprints.434v1
New (but very old) preprint: A survey of dinosaur diversity by clade, age, place of discovery and year of description
July 11, 2014
Today, available for the first time, you can read my 2004 paper A survey of dinosaur diversity by clade, age, place of discovery and year of description. It’s freely available (CC By 4.0) as a PeerJ Preprint. It’s one of those papers that does exactly what it says on the tin — you should be able to find some interesting patterns in the diversity of your own favourite dinosaur group.
“But Mike”, you say, “you wrote this thing ten years ago?”
Yes. It’s actually the first scientific paper I ever wrote (bar some scraps of computer science) beginning in 2003. It’s so old that all the illustrations are grey-scale. I submitted it to Acta Palaeontologica Polonica way back on on 24 October 2004 (three double-spaced hard-copies in the post!) , but it was rejected without review. I was subsequently able to publish a greatly truncated version (Taylor 2006) in the proceedings of the 2006 Symposium on Mesozoic Terrestrial Ecosystems, but that was only one tenth the length of the full manuscript — much potentially valuable information was lost.
My finally posting this comes (as so many things seem to) from a conversation with Matt. Off work sick, he’d been amusing himself by re-reading old SV-POW! posts (yes, we do this). He was struck by my exhortation in Tutorial 14: “do not ever give a conference talk without immediately transcribing your slides into a manuscript”. He bemoaned how bad he’s been at following that advice, and I had to admit I’ve done no better, listing a sequence of old my SVPCA talks that have still never been published as papers.
The oldest of these was my 2004 presentation on dinosaur diversity. Commenting on this, I wrote in email: “OK, I got the MTE four-pager out of this, but the talk was distilled from a 40ish-page manuscript that was never published and never will be.” Quick as a flash, Matt replied:
If I had written this and sent it to you, you’d tell me to put it online and blog about how I went from idea to long paper to talk to short paper, to illuminate the process of science.
And of course he was right — hence this preprint.
I will never update this manuscript, as it’s based on a now wildly outdated database and I have too much else happening. (For one thing, I really ought to get around to finishing up the paper based on my 2005 SVPCA talk!) So in a sense it’s odd to call it a “pre-print” — it’s not pre anything.
Despite the data being well out of date, this manuscript still contains much that is (I think) of interest, and my sense is that the ratios of taxon counts, if not the absolute numbers, are still pretty accurate.
I don’t expect ever to submit a version of this to a journal, so this can be considered the final and definitive version.
- Taylor, Michael P. 2006. Dinosaur diversity analysed by clade, age, place and year of description. pp. 134-138 in Paul M. Barrett and Susan E. Evans (eds.), Ninth international symposium on Mesozoic terrestrial ecosystems and biota, Manchester, UK. Cambridge Publications. Natural History Museum, London, UK. 187 pp.
- Taylor, Michael P. 2014 (written in 2004). A survey of dinosaur diversity by clade, age, place of discovery and year of description. PeerJ PrePrints 2:e434v1. doi:10.7287/peerj.preprints.434v1
I think it’s fair to say that this “bifurcation heat-map”, from Wedel and Taylor (2013a: figure 9), has been one of the best-received illustrations that we’ve prepared:
Back when the paper came out, Matt rashly said “Stand by for a post by Mike explaining how it came it be” — a post which has not materialised. Until now!
This illustration was (apart from some minor tweaking) produced by a program that I wrote for that purpose, snappily named “vcd2svg“. That name is because it converts a vertebral column description (VCD) into a scalable vector graphics (SVG) file, which you can look at with a web-browser or load into an image editor for further processing.
The vertebral column description is in a format designed for this purpose, and I think it’s fairly intuitive. Here, for example, is the fragment describing the first three lines of the figure above:
Taxon: Apatosaurus louisae
Specimen: CM 3018
Taxon: Apatosaurus parvus
Specimen: UWGM 155556/CM 563
Taxon: Apatosaurus ajax
Specimen: NMST-PV 20375
Basically, you draw little ASCII pictures of the vertebral column. Other directives in the file explain how to draw the various glyphs represented by (in this case) “Y”, “V”, “u”, and “n”.
It’s pretty flexible. We used the same program to generate the right-hand side (though not the phylogenetic tree) of Wedel and Taylor (2013b: figure 2):
The reason I mention this is because I released the software today under the GNU General Public Licence v3.0, which is kind of like CC By-SA. It’s free for anyone to download, use, modify and redistribute either verbatim or in modified form, subject only to attribution and the requirement that the same licence be used for modified versions.
vcd2svg is written in Perl, and implemented in part by the SVG::VCD module, which is included in the package. It’s available as a CPAN module and on GitHub. There’s documentation of the command-line vcd2svg program, and of the VCD file format. Also included in the distribution are two documented examples: the bifurcation heat-map and the caudal pneumaticity diagram.
Folks, please use it! And feel free to contribute, too: as the change-log notes, there’s work still to be done, and I’ll be happy to take pull requests from those of you who are programmers. And whether you’re a programmer or not, if you find a bug, or want a new feature, feel free to file an issue.
A final thought: in academia, you don’t really get credit for writing software. So to convert the work that went into this release into some kind of coin, I’ll probably have to write a short paper describing it, and let that stand as a proxy for the actual program. Hopefully people will cite that paper when they generate a figure using the software, the way we all reflexively cite Swofford every time we use PAUP*.
Update (12 April 2014)
- Swofford, D. L. 2002. PAUP*: phylogenetic analysis using parsimony (* and other methods). Sinauer Associates, Sunderland, MA.
- Wedel, M.J., and Taylor, M.P. 2013. Neural spine bifurcation in sauropod dinosaurs of the Morrison Formation: ontogenetic and phylogenetic implications. Palarch’s Journal of Vertebrate Palaeontology 10(1): 1-34. ISSN 1567-2158.
- Wedel, Mathew J., and Michael P. Taylor. 2013. Caudal pneumaticity and pneumatic hiatuses in the sauropod dinosaurs Giraffatitan and Apatosaurus.PLOS ONE 8(10):e78213. 14 pages. doi:10.1371/journal.pone.0078213 [PDF]