Skull audit: Wedel responds
February 9, 2022
Left to right: alligator, beaver, black bear, armadillo, cat, ostrich. I know, the archosaurs aren’t mammals, and the alligator isn’t even a skull. But if you can’t have a lounge lizard crash your mammal skull party, what are you even doing with your life? Not pictured: about four rabbit skulls I forgot I had boxed up, plus a couple of turtles (yeah, yeah) sitting on a friend’s desk, in their locked office.
It warmed my crooked little heart to see Mike Taylor, noted sauropodologist and disdainer-of-mammal-heads, return mammal skulls to the blog’s front page yesterday. Naturally I had to support my friend and colleague in this difficult time, when he may be experiencing confusing feelings regarding nasal turbinates, multi-cusped teeth, and the dentary-squamosal jaw joint.
My skull collection is split across home and office, but I had to go in to campus this afternoon for a video recording thing, so I got most of the office set, shown above, on that jaunt.
After the workday ended, I had just enough time before the light faded to assemble and photograph the home collection:
Back row: peccary, pig, deer, sheep, dog. Middle row: opossum, rabbit. Front row: opossum, marten (both hemisected). Not pictured: emergency backup sheep, moar rabbits
I’ve blogged about the bear, the pig, and the hemisected skulls, but I think that’s it. I should do more skull blogging, most of these have a story:
- I prepped the armadillo, cat, rabbit, and sheep skulls myself (besides the bear and pig). The first two I found in the woods, the mostly-decomposed rabbit was a gift from my father-in-law, and the sheep head I obtained from the market down the street ($10, and I ate the meat).
- The alligator head and deer skull were gifts, from Vicki and from my brother Ryan, respectively.
- The rest I purchased here and there over the years, usually when they were on deep discount. The peccary is a memento of a trip to Big Bend back in 2007 (I bought it at a taxidermy shop a long way outside the national park), and the dog came from the seconds bin at the Museum of Osteology — I plan to saw off the top of the braincase to see the cranial nerve exits, just as in the preparation by Peter Dodson shown in this post.
I have more heads awaiting skull-ization in various freezers, too. Couple more pig heads at work, and at the house a strategic reserve sheep head, plus skunk, squirrel, and rat. Plus a partially-mummified but mostly defleshed armadillo whose saga deserves a detailed recounting:
NB: the stray bits toward the bottom of the image are from a cat. Mr. Armadillo’s limb bones and vertebrae are still in the armadillo kit.
In the first comment on Mike’s post yesterday, I expressed envy that he had the better skull collection. After pulling together all my critters, I think I just have a worse memory. In my defense, it’s been almost two years since I was in the office regularly, and about half the skulls in the home collection are recent-ish acquistions (~last three years), so a lot of stuff had either fallen out of memory or not gotten properly established yet. But Mike has definitely prepped more — and more exotic — skeletons, and it was his enthusiastic collecting and blogging of dead animal bits that inspired me to start my recent-ish spate of skull preparations. More to come on that front as time and opportunity allow, probably starting with this:
Afield in Oklahoma, part 3
July 2, 2018
It’s been a bit since my last update. That’s how things go on the road. We got in some time for exploration and a little prospecting.
We also had to close the quarry. Anne Weil, whose dig London and I were out there to assist on, brought a speaker on the last day and played us out with a hydration song while we shut everything down for another year. We found some great stuff, and I’m anxious for you to get to see it, but there’s work to do first. Rest assured, I’ll keep you posted when the time comes.
The extant wildlife continued to be a source of enjoyment and inspiration, especially this cottontail. Rabbits, baby!
Time goes on, and so does the road. My road leads back home, and then back out again. I’ll check in when I can. Hope your summer is half as fun.
The “Growth series of one” poster is published
September 22, 2017
This was an interesting exercise. It was my first time generating a poster to be delivered at a conference since 2006. Scientific communication has evolved a lot in the intervening decade, which spans a full half of my research career to date. So I had a chance to take the principles that I say that I admire and try to put them into practice.
It helped that I wasn’t working alone. Jann and Brian both provided strong, simple images to help tell the story, and Mike and I were batting ideas back and forth, deciding on what we could safely leave out of our posters. Abstracts were the first to go, literature cited and acknowledgments were next. We both had the ambition of cutting the text down to just figure captions. Mike nailed that goal, but my poster ended up being slightly more narrative. I’m cool with that – it’s hardly text-heavy, especially compared with most of my efforts from back when. Check out the text-zilla I presented at SVP back in 2006, which is available on FigShare here. I am happier to see, looking back, that I’d done an almost purely image-and-caption poster, with no abstract and no lit cited, as early as 1999, with Kent Sanders as coauthor and primary art-generator – that one is also on FigShare.
I took 8.5×11 color printouts of both my poster and Mike’s, and we ended up passing out most of them to people as we had conversations about our work. That turned out to be extremely useful – I had a 30-minute conversation about my poster at a coffee break the day before the posters even went up, precisely because I had a copy of it to hand to someone else. Like Mike, I found that presenting a poster resulted in more and better conversations than giving a talk. And it was the most personally relaxing SVPCA I’ve ever been to, because I wasn’t staying up late every night finishing or practicing my talk.
I have a lot of stuff to say about the conference, the field trip, the citability of abstracts and posters (TL;DR: I’m for it), and so on, but unfortunately no time right now. I’m just popping in to get this posted while it’s still fresh. Like Mike’s poster, this one is now published alongside my team’s abstract on PeerJ PrePrints.
I will hopefully have much more to say about the content in the future. This is a project that Jann, Brian, and I first dreamed up over a decade ago, when we were grad students at Berkeley. Mike provided the impetus for us to get it moving again, and kindly stepped aside when I basically hijacked his related but somewhat different take on ontogeny and serial homology. When my fall teaching is over, I’m hoping that the four of us can take all of this, along with additional examples found by Mike that didn’t make it into this presentation, and shape it into a manuscript. I’ll keep you posted on that. In the meantime, the comment field is open. For some related, previously-published posts, see this one for the baby sauropod verts, this one for CM 555, and this one for Plateosaurus.
Flying over Baffin Island on the way home.
And finally, since I didn’t put them into the poster itself, below are the full bibliographic references. Although we didn’t mention it in the poster, the shell apex theory for inferring the larval habits of snails was first articulated by G. Thorson in 1950, which is referenced in full here.
Literature Cited
- Bair, A.R. 2007. A model of wear in curved mammal teeth: controls on occlusal morphology and the evolution of hypsodonty in lagomorphs. Paleobiology 33(1):53-75.
- Gilmore, C.W. 1936. Osteology of Apatosaurus with special reference to specimens in the Carnegie Museum. Memoirs of the Carnegie Museum 11: 175-300.
- Hatcher, J.B. 1901. Diplodocus (Marsh): its osteology, taxonomy, and probable habits, with a restoration of the skeleton. Memoirs of the Carnegie Museum 1:1-63.
- Kraatz, B.P., Meng, J., Weksler, M. and Li, C. 2010. Evolutionary patterns in the dentition of Duplicidentata (Mammalia) and a novel trend in the molarization of premolars. PloS one, 5(9), p.e12838.
- Sych, L. 1975. Lagomorpha from the Oligocene of Mongolia. Palaeontogia Polonica 33:183-200.
- Thorson, G. 1950. Reproductive and larval ecology of marine bottom invertebrates. Biological Reviews 25(1):1-45.
- 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, M.J., Cifelli, R.L., and Sanders, R.K. 2000b. Osteology, paleobiology, and relationships of the sauropod dinosaur Sauroposeidon. Acta Palaeontologica Polonica 45:343-388.
Rabbit skulls strike back: Kraatz and Sherratt (2016)
September 22, 2016
Facial tilt in extant leporids is strongly correlated with locomotor mode – fast movers have more strongly tilted faces. There’s a lot of homoplasy, which is to be expected with characters that are strongly driven by current function. Kraatz and Sherratt (2016: fig. 5).
Judgmental readers will recall that I have dabbled in mammal skulls, thanks to the corrupting influence of my friend and colleague, Brian Kraatz. At the end of my last post on this sordid topic, I mentioned that Brian and Emma Sherratt were working on a version 2.0 based in 3D morphometrics. The first volley from that project was published today in PeerJ.
Happily for all of us, Brian and Em confirmed the relationship between facial tilt and locomotor mode that we first documented last year, using more taxa, more landmarks, and two more dimensions (Kraatz and Sherratt 2016: 12):
…in accordance with previous findings by Kraatz et al. (2015), facial tilt angle is correlated with locomotor mode (D-PGLS, F(2,17) = 11.13, P = 0.003), where lower facial tilt angle, meaning more pronounced cranial flexion, is found in cursorial species, and high angles are found in generalist species.
That’s just the most personally relevant tip of a very large, multifaceted iceberg, including a monster supplementary info package on FigShare with, among other things, 3D models of bunny skulls. It’s all free and awesome, so go have fun.
That homoplastic pattern shown in figure 5, above? It’s been going on for a while. I’m gonna go out on a limb and guess that Hypolagus was a rocket. Kraatz and Sherratt (2016: fig. 7).
References
You can predict how rabbits run by looking at their skulls (using this one weird trick!)
March 17, 2015
Meet some of my new friends: (A) Brachylagus idahoensis, (B) Lepus capensis, (C) Poelagus marjorita, (D) Pronolagus crassicaudatus, (E) Lepus americanus, (F) Oryctolagus cuniculus, (G) Nesolagus timminsi, (H) Bunolagus monticularis, and (I) Romerolagus diazi. Kraatz et al. (2015: fig. 1).
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:
Compare to Kraatz et al. (2015: fig. 2)
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, and 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.
Kraatz et al. (2015: fig. 5a)
Why would that be so? We don’t know 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).
Mara photo from Wikipedia
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).
Relevant to the next paragraph: DILU is ‘diastema length upper’ and BLD is ‘bulla diameter’. Kraatz et al. (2015: fig. 4).
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. (Update: pilot episode here.)
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!)
References
- 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.
Sauropod Vertebra Picture of the Week 
