Someone on Facebook asked whether sauropods had subcutaneous fat, and by the time my answer hit five paragraphs I thought, “The merciful thing to do here is blog this and link to it.” So here are some things to keep in mind regarding the integumentary systems of sauropods.

Emu dissection at UC Santa Cruz back in 2004. Note the fat pad on the chest and how it abruptly comes to an end.

Sauropods may have had some subcutaneous fat – we can’t rule it out – but it probably wasn’t broadly distributed as it is in mammals. In the interaction of their air sac systems with connective tissue, sauropods were probably a lot like birds. Most birds don’t have subcutaneous fat all over their bodies. Instead, they have subcutaneous air sacs (or pneumatic diverticula) over parts or all of their bodies – in pelicans these are like bubble wrap under the skin, presumably for impact padding and insulation (Richardson 1939, 1943). The diverticula go everywhere and most places they go, they replace adipose tissue, even the harmless bits of fat between muscles that are basically the body’s packing peanuts (broiler chickens don’t count here, they’ve been artificially selected to be radically unhealthy). We suspect that sauropods had subcutaneous diverticula because so many other aspects of their pneumatic systems correspond to those of living birds (see the discussion in Wedel and Taylor 2013b for more on that).

Contrast the narrow line of adipose tissue down the ventral midline with the almost-completely-lean hindlimb.

That’s not to say that birds don’t have subcutaneous fat, just that it tends to be highly localized. Back in grad school I got to help dissect an emu (link) and a rhea (one, two), and in both cases the fat was concentrated in two places: huge paired fat pads over the pelvis, like big lozenges, and a concentration over the sternum with extensions along the ventral midline from the base of the neck to the cloaca. It was weird, the fat would be present and then it would just stop, like somebody flipped a switch. We pulled 18 lbs of fat off a 102-lb emu, so it wasn’t a trivial part of the body composition. IME, even relatively fatty birds like ducks tend to have the fat start and stop abruptly, and again, the fat deposits tend to be concentrated on the breast and tummy and over the hips.

Fat-tailed gecko, borrowed from here.

A lot of lizards and crocs and even some turtles carry fat deposits in their tails, and that is one aspect of sauropod anatomy that is definitely un-bird-like. So some sauropods might have had fat tails.

We can be pretty sure that at least some sauropods had thick skin. Osteoderms (armor plates) from Madagascar show that the bits that were embedded in the skin could be up to 7cm thick, so the surrounding skin was at least that thick and possibly even thicker (Dodson et al. 1998). And that was most likely on Rapetosaurus, which was not a huge sauropod. So giant sauropods might have had even thicker skin. Maybe. Remember that big-ass-ness (here arbitrarily defined as 40+ metric tons) evolved independently in:

They probably didn’t all get there looking the same way, beyond sharing the basic sauropod bauplan.

I’m too lazy to write about the fossil evidence for scaly skin and keratinous spines in sauropods – see this post and the references therein.

One final thing to think about is scar tissue. The scar tissue on the chest of a male elephant seal can be up to 5cm thick. Some sauropods might have had calluses or patches of scar tissue in predictable places, from combat, or habitually pushing down trees with their chests or tails, or doing whatever weird things real animals do when we’re not looking.

So in the toolbox of things to play with in reconstructing the integument of sauropods, we have thick skin, keratinous spines, osteoderms, fat pads (possibly concentrated over the hips and shoulders or on the tail), subcutaneous diverticula, calluses, and scar tissue. And that’s just the stuff we have found or reasonably inferred so far, barely 150 years into our exploration of animals we know mostly from bits and bobs, whose size means they mostly got buried in big sediment-dumping events that would not preserve delicate integumentary structures. Give us a millennium of Yixian Formations and Mahajanga Basins and Howe Quarries and the picture will probably change, and the arrow of history dictates that it will change for the weirder.

Likely? Probably not. But roll the evolutionary dice for 160 million years and you’ll get stranger things than this. Recycled from this post.

Finally, and related to my observation about big-ass-ness: sauropods were a globally-distributed radiation of animals from horse-sized to whale-sized that existed from the Late Triassic to the end of the Cretaceous. The chances that all of them had the same integumentary specializations, for display or combat or insulation or camouflage or whatever, are pretty darned low. Support weird sauropods – and vanilla ones, too.

Almost immediate update: I’ve just been reminded about Mark Witton’s excellent post on dinosaur fat from a couple of years ago. Go read that, and the rest of his blog. I’m sure I missed other relevant posts at other excellent blogs – feel free to remind me in the comments.



Out today: a new Turiasaurian sauropod, Mierasaurus bobyoungi, from the Early Cretaceous Cedar Mountain formation in Utah. This comes to us courtesy of a nice paper by Royo Torres et al. (2017),

Royo-Torres et al. 2017, fig. 3. The postcranial skeleton (UMNH.VP.26004) of Mierasaurus bobyoungi gen. nov, sp. nov. with the following elements: (a) middle cervical vertebra (DBGI 69 h) in right lateral view; (b) middle cervical vertebra (DBGI 69G1) in right lateral view; (c) anterior cervical vertebra (DBGI 165) in right lateral view; (d) anterior cervical vertebra (DBGI 69G2) in right lateral view; (e) atlas (DBGI 5I) in anterior view; (f) atlas (DBGI 5I) in right lateral view; (g) posterior cervical vertebra (DBGI 95) in right lateral view; (h) posterior cervical vertebra (DBGI 19 A) in right lateral view; (i) posterior cervical vertebra (DBGI 19 A) in ventral view; (j) middle cervical vertebra (DBGI 38) in right lateral view; (k) middle cervical vertebra (DBGI 38) in dorsal view; (l) middle cervical vertebra in posterior view; (m) middle cervical vertebra (DBGI 38) in left lateral view; (n) right anterior cervical rib (DBGI 5D) in medial view; (o) right anterior cervical rib (DBGI 28 A) in medial view; (p) right anterior-middle cervical rib (DBGI 95 C) in medial view; (q) right middle cervical rib (DBGI 45 F) in dorsal view; (r) right middle cervical rib (DBGI 95 A) in dorsal view; (s) left anterior cervical rib (DBGI 95B) in lateral view; (t) left middle cervical rib (DBGI 95 H) in lateral view; (u) left middle cervical rib (DBGI 95D) in dorsal view; (v) right posterior cervical rib (DBGI 10) in dorsal view. A plus sign (+) indicates a diagnostic character for Mierasaurus bobyoungi gen. et sp. nov. An asterisk (*) indicates an autapomorphy of Mierasaurus bobyoungi gen. et sp. nov. (© Fundación Conjunto Paleontológico de Teruel-Dinópolis) in Adobe Illustrator CS5 (

[Because this paper is in Nature’s Scientific Reports, it inexplicably has a big chunk of manuscript chopped out of the middle, supplied separately, not formatted properly, and for all we know not peer-reviewed. This includes such minor details as the specimen numbers of the elements that make up the holotype, and the measurements. Note to self: rant about how objectively inferior Scientific Reports is to PeerJ and PLOS ONE some time.]

Anyway, this is a nice specimen represented by lots of decent material, including plenty of presacral vertebrae, which is great.

But here’s where it gets weird. Until now, Turiasauria has been an exclusively European clade. Just like Diplodocidae used to be an exclusively North American clade until Tornieria turned up, and Dicraeosauridae used to be an exclusively Gondwanan clade until Suuwassea turned out to be a dicraeosaur, and so on.

I mentioned this in an email to Matt. His initial take was:

There is a semi-tongue-in-cheek biogeography “law” that states “Everything is everywhere, and the environment selects”.

It is kinda blowing my mind that so many taxa were shared between North America, Europe, and Africa in the Late Jurassic and yet we don’t see any turiasaurs in North America until the Cretaceous. I wonder if they are there in the Morrison and just not recognized — either some of the undescribed or undiscovered northern-Morrison weirdness, or currently lumped in with Camarasaurus.

I responded “That’s one read. Another is that we’re seeing convergence on similar eco-niches within widely different clades, and our analyses are not figuring this out.”

What I mean is this: what if our “Brachiosauridae” clade is really just a collection of not-closely-related taxa in the tall-shouldered very-high-browser ecological niche? And what if our “Dicraeosauridae” clade is just a collection of short-necked grazers, with independent evolutionary origins, but all converging on morphology that suits the same lifestyle?

And that is the thought that is currently freaking me out.

Royo-Torres et al. 2107, fig. 4. The postcranial skeleton (UMNH.VP.26004) of Mierasaurus bobyoungi gen. nov, sp. nov. with the following elements: (a) anterior dorsal vertebra (DBGI 54 A) in posterior view; (b) anterior dorsal vertebra (DBGI 54 A) in anteroventral view; (c) neural arch of a middle dorsal vertebra (DBGI 37) in right anterolateral view; (d) posterior neural arch of a dorsal vertebra (DBGI 19 A) in posterior view; (e) anterior dorsal vertebra (DBGI 16) in right lateral view; (f) anterior dorsal vertebra (DBGI 16) in posterior view; (g) posterior dorsal vertebra (DBGI 16) in anterior view; (h,i) posterior dorsal vertebra (DBGI 100NA 1) in anterior view; (j,k) posterior dorsal vertebra (DBGI 100NA 1) in posterior view; (l) posterior dorsal vertebra (DBGI 100NA 1) in left lateral view; (m) middle dorsal vertebra (DBGI 11) in anterior view; (n) centrum of a posterior dorsal vertebra (DBGI 24B) in ventral view; (o) centrum of a posterior dorsal vertebra (DBGI 24B) in anterior view; (p) centrum of a posterior dorsal vertebra (DBGI 192) in ventral view; (q) anterior-middle caudal vertebra (DBGI 23B) in anterior view; (r) anterior-middle caudal vertebra (DBGI 23B) in right lateral view; (s) posterior neural arch of a posterior caudal vertebra (DBGI 48) in left lateral view; (t) posterior caudal vertebra (DBGI 21) in anterior view; (u) posterior caudal vertebra (DBGI 21) in right lateral view; (v) distal caudal vertebra (DBI 37-34-529) in right lateral view; (W) anterior caudal vertebra (DBGI 192) in posterior view. For abbreviations see supplementary information. (i), (k) and (l) were drafted by R.R.T. (© Fundación Conjunto Paleontológico de Teruel-Dinópolis) in Adobe Illustrator CS5 (

When I mentioned this possibility to Matt, he shared my existential terror:

What haunts me is this: we know from mammals and extant reptiles that morphological analyses suck. Laurasian moles, African moles, and Australian moles all look the same, despite evolving from very different ancestors. Ditto wolves and thylacines, horses and litopterns, etc.

Matt reminded of a paper we’ve talked about before (Losos et al. 1998), showing that this is exactly what happens with Caribbean anole lizards. Each island has forms that live on the ground, on the trunks of trees, and on branches. Phylogenetic analyses based on morphology put all the ground-livers together, ditto for trunk-climbers, ditto for branch-climbers. But molecular analyses show that each island was colonized once and the ground, trunk, and branch forms evolved separately for each island.

What if “turiasaur”, “brachiosaur”, and “titanosaur” are the sauropod equivalents? For “Caribbean island” read “continent”; for “lizard species”, read “sauropod clade”.

Will we ever know?

Matt is hopeful that we will. He’s confident that in time, we’ll get molecular analyses of dinosaur relationships — that it’s just a matter of time and cleverness. When that happens, things could be upended bigtime.



A while back, I mentioned that I’d written and released VertFigure, a program for drawing schematic comparative diagrams of vertebral columns. Matt and I used it in our vertebral bifurcation paper to illustrate patterns of bifurcation in various Morrison-Formation sauropod specimens:

Figure 9. Degree of neural spine bifurcation of presacral vertebrae in well-preserved Morrison Formation sauropod specimens representing several taxonomic groups. In all taxa with deep bifurcations, these are concentrated around the cervico-dorsal transition. ‘No data’ markers may mean that the vertebrae are not preserved (e.g., posterior dorsals of Suuwassea emilieae ANS 21122), that the degree of bifurcation cannot be assessed (e.g., anterior cervicals of Barosaurus lentus AMNH 6341), or that the serial positions of the vertebrae are uncertain so they contribute no information on serial changes in bifurcation (e.g., the four cervical vertebrae known for Barosaurus lentus YPM 429). The Camarasaurus specimens are roughly in ontogenetic order: C. lentus CM 11338 is a juvenile, C. grandis YPM 1905 and GMNH-PV 101/WPL 1995, and C. supremus AMNH 5761 are adults, and C. lewisi BYU 9047 is geriatric. See text for sources of data.

Wedel and Taylor (2013a: figure 9). Degree of neural spine bifurcation of presacral vertebrae in well-preserved Morrison Formation sauropod specimens representing several taxonomic groups. In all taxa with deep bifurcations, these are concentrated around the cervico-dorsal transition. ‘No data’ markers may mean that the vertebrae are not preserved (e.g., posterior dorsals of Suuwassea emilieae ANS 21122), that the degree of bifurcation cannot be assessed (e.g., anterior cervicals of Barosaurus lentus AMNH 6341), or that the serial positions of the vertebrae are uncertain so they contribute no information on serial changes in bifurcation (e.g., the four cervical vertebrae known for Barosaurus lentus YPM 429). The Camarasaurus specimens are roughly in ontogenetic order: C. lentus CM 11338 is a juvenile, C. grandis YPM 1905 and GMNH-PV 101/WPL 1995, and C. supremus AMNH 5761 are adults, and C. lewisi BYU 9047 is geriatric. See text for sources of data.

But downloading, compiling and running Perl programs is not everyone’s cup of tea. So when Emanuel “Brontosaurus” Tschopp wanted to use it to illustrate the presence and absence of various laminae along the vertebral columns of lizards, I put a running copy online so that he — and anyone else — could play with it.

Now Emanuel’s paper is out (Tschopp 2016), and you can see the lamina diagrams in the nine supplementary tables. Here’s an example:


S9 Table. Postspinal lamina (POSL), serial variation in presacral vertebrae of Lacertini. Boxes represent the vertebrae in the column, including the atlas. Filled boxes indicate presence of the lamina in the respective vertebrae, whereas a dash stands for absence. Only the seven specimens with articulated vertebral column could be assessed.

I’m delighted that this program has been put to good use, and once again commend it to anyone who needs to produce similar diagrams. Free to download, free to use online. Have at it!


Murphy and Mitchell (1974: fig. 1)

Murphy and Mitchell (1974: fig. 1)

One thing that I’ve never understood is why some people are skeptical about sauropods using their tails defensively, when lizards do this all the time. I’ve been digging through the literature on this for a current project, and there are some really great accounts out there, and by ‘great’ I mean ‘scary’.

Here’s a key passage from Murphy and Mitchell (1974: p. 95):

V. salvator uses the tail to strike repeatedly in combination with biting for defense…Captive Varanus (varius, spenceri, mertensi, and salvadorii) use the tail for defense, but only salvadorii appears to aim directly for a handler’s eye. An adult male V. salvadorii accurately struck the senior author’s eye with the tip of the tail as he was attempting to maneuver the lizard. On many subsequent occasions, the monitor tried to strike the eye of the handler with accuracy.

Not being a monitor expert, I was initially thrown by the V. salvator/V. salvadorii issue. V. salvator is the water monitor, V. salvadorii is the crocodile monitor. Both get pretty darned big; Wikipedia lists 3.21 m (10.5 ft) for V. salvator and 2.44-3.23 m (8.0-10.6 ft) for V. salvadorii.

Anyway, I’d heard of lots of anecdotal reports of lizards from many clades using their tails to lash at rivals, predators, or handlers, but I’d never read about a lizard aiming directly for the target’s eyes. It immediately made me think about (1) sauropod tails, especially the whip-lash tails of flagellicaudan diplodocoids and at least some titanosaurs (Wilson et al. 1999), and (2) the supraorbital crests and ridges in many theropods, especially big Morrison forms like Allosaurus and Ceratosaurus. Of course, supraorbital crests in theropods could serve many functions, including shading the eyes and social and sexual display, but it’s interesting to speculate that they might have had a defensive function as well. Has anyone ever proposed that in print?

Diplodocus USNM 10865 - Gilmore 1932 pl 6 - cleaned up

Diplodocus longus USNM 10865, from Gilmore (1932: plate 6)


Most of the papers that pooh-pooh the use of whiplash tails in defense (e.g., Myhrvold and Currie 1997) argue that the tail-tip would be too small to do any serious damage to a multi-ton attacker, and too fragile to survive an impact. This seems wrong-headed to me, like arguing that unless you find putative animal weapons broken and caked in their adversaries’ blood, they aren’t used as weapons. A structure doesn’t have to do lethal damage or any damage at all to serve as a weapon, as long as it dissuades a predator from attacking. I’d think that getting hit in the eye by a 35-foot bullwhip might convince an allosaur to go have a look at Camptosaurus instead.

Now, one could argue that if the whip-lash doesn’t do any serious damage, predators will learn to blow them off as dishonest signals (we’re assuming here that having your eye possibly knocked out doesn’t count as ‘serious damage’ to an allosaur). But it’s not like the whiplash was the only weapon a diplodocid could bring to bear: the proximal tail could probably deliver a respectable clobberin’, and then there’s the zero fun of being stomped on by an adversary massing a dozen tons or more. In that sense, the whip-lash is writing checks the rest of the body can certainly cash. It’s saying, “Getting hit with this will be no fun, and if that isn’t enough, there’s plenty more coming.”

All of this is leaving aside more obvious defensive adaptations of the tail in Shunosaurus, maybe Omeisaurus and Mamenchisaurus, and probably Spinophorosaurus (although I’d feel better about Spinophorosaurus if the association of the spikes and the tail was more secure). I suspect that all sauropod tails were useful in defense, but only some sauropod taxa used that behavior enough for a morphological enhancement (club, spikes, whiplash) to have evolved. Similarly, common snapping turtles, Chelydra serpentina, will wiggle their unspecialized tongues to attract fish (I’ve witnessed this myself in captive specimens) but lack the worm-shaped tongue lure found in the more ambush-specialized alligator snappers, Macrochelys temminckii. On reflection, there are probably few morphological changes in evolution that aren’t preceded by behavior. Not in a Lamarckian sense, just that certain variations aren’t useful unless the organism is already (suboptimally) performing the relevant function.

Bonus observation: Mike noted back when that Shunosaurus and Varanus retain complex caudal vertebrae all the way out to the end. Since in this case ‘complex’ means ‘having processes that muscles can attach to’, maybe that has something to do with keeping up relatively fine motor control in your bad-guy-whomping organ. Would be interesting to compare caudal morphology between tail-whomping lizards and committed caudal pacifists (assuming we can find any of the latter that we’re certain about – maybe tail-whomping just doesn’t get used very often in some taxa, like those that have caudal autotomy). Anyone know anything about that?


  • Murphy, J. B., & Mitchell, L. A. (1974). Ritualized combat behavior of the pygmy mulga monitor lizard, Varanus gilleni (Sauria: Varanidae). Herpetologica, 90-97.
  • Myhrvold, N. P., & Currie, P. J. (1997). Supersonic sauropods? Tail dynamics in the diplodocids. Paleobiology, 23(4), 393-409.
  • Wilson, J. A., Martinez, R. N., & Alcober, O. (1999). Distal tail segment of a titanosaur (Dinosauria: Sauropoda) from the Upper Cretaceous of Mendoza, Argentina. Journal of Vertebrate Paleontology, 19(3), 591-594.

Image courtesy of Emma Schachner.

Gotta say, I did not see that coming.

Today sees the publication of a new paper by Emma Schachner and colleagues in Nature, documenting for the first time that unidirectional, flow-through breathing–previously only known in birds and crocodilians–happens in freakin’ monitor lizards. The image above, which is most of Figure 1, pretty much tells the tale.

Some quick background: until the early 1970s, no-one was quite sure how birds breathed. Everyone knew that birds breathe, and that the air sacs had something to do with it, and that the bird lungs are set up as a series of tubes instead of a big array of little sacs, like ours, but the airflow patterns had not been worked out. Then in a series of nifty experiments, Knut Schmidt-Nielsen and his students and colleagues showed that birds have unidirectional airflow through their lungs on both inspiration and expiration. Amazingly, there are no anatomical valves in the lungs or air sacs, and the complex flow patterns are all generated by aerodynamic valving. For loads more information on this, including some cool animations, please see this page (the diagram below is modified from versions on that page). For a short, eminently readable summary of how undirectional airflow in birds was first discovered (among many other fascinating things), I recommend Schmidt-Nielsen’s wonderful little book, How Animals Work.

Avian breathing

After 1972, biologists had almost four decades to get used to the idea that birds had this amazing miraculous lung thingy that was unique in the animal kingdom. Then in 2010, Colleen Farmer and Kent Sanders of the University of Utah blew our collective minds by demonstrating that alligators have unidirectional flow-through lungs, too. That means that far from being a birds-only thing, unidirectional flow-through lung ventilation was probably primitive for Archosauria, and was therefore the default state for non-avian dinosaurs, pterosaurs, the other ornithodirans and the hordes of croc-line archosaurs.

Crocodilian breathing - Schachner et al 2013a fig 10

Diagrammatic and highly simplified representation of airflow through the dorsobronchi and ventrobronchi during inspiration (A) and expiration (B) in the crocodilian lung, and inspiration (A) and expiration (D) in the avian lung. The avian model is a modification of the Hazelhoff loop (Hazelhoff, 1951). Arrows denote direction of airflow, white arrows show air flowing through the parabronchi, blue arrows show air entering the trachea, the red circled “X” demonstrates the location of the aerodynamic inspiratory valve (i.e., air does not flow through this location during inspiration). Colors represent hypothesized homologous regions of the lung in both groups. Abbreviations: d, dorsobronchi; P, parabronchi; Pb, primary bronchus; v, ventrobronchi. [Figure 10 and caption from Schachner et al. 2013a.]

The birdy-ness of crocodilian lungs was further cemented earlier this year when Schachner et al. described the lung morphology and airflow patterns in Nile crocs, which have lungs that are if anything even more birdlike than those of gators. I got to review that paper and blogged about it here.

Now…well, you read the headline. Monitor lizards have unidirectional airflow through their lungs, too. This falls at about the halfway point between “whatisthisIdonteven”–I mean, dude, unidirectional airflow in friggin’ lizards!–and “yeah, that makes a weird sort of sense”. Because to sum up a lot of science unscientifically, monitors just kick a little more ass than other squamates. They have crazy high aerobic capacities for animals that aren’t birds or mammals, they’re ecologically versatile and geographically widespread, they get waaay bigger than any other extant lizards (Komodo dragons) and until recently got even bigger than that (Megalania). Is it going too far to link the success of varanids with their totally pimpin’ flow-through lungs? Maybe, maybe not. But it seems like fertile ground for further study.


Phylogeny for Diapsida showing lungs of representative taxa.
Greyscale images are modified from Milani and transected. The coloured
three-dimensional images are the bronchial tree (right lateral view). Images are
not to scale. a, Diapsida. b, Sphenodon punctatus. c, Crocodile sp. (left) and
Alligator mississippiensis (right). d, Squamata. e, Iguana iguana (left) and
Polychrus marmoratus (right). f, Gekko gecko. g, Lacerta viridis. h, Python sp.
in dorsal view . i, Varanus bengalensis (left) and V. exanthematicus (right).
The blue regions of the phylogeny reflect the hypothesis that unidirectional
airflow evolved convergently; the green arrow shows the alternative hypothesis
of an ancestral origin. [Figure 3 and caption from Schachner et al. (2013b).]

Now, obviously the gigantic question looming over all of amniote biology like one of those monoliths from 2001 is: does this mean that unidirectional flow-through lung ventilation is primitive for all diapsids? That is a super-interesting possibility, and in the new paper Schachner et al. advance some evidence both for and against. On the “for” side, well, hey, there’s uniflow in monitors, crocs, and birds, and in all three cases, air flows down the primary bronchus into a sac at the caudal end, and then back cranially through series of interconnected sacs or tubes. On the “against” side, the patterns of airflow in varanids are similar to those in archosaurs but not identical: in archosaurs, the caudal-to-cranial flow goes through dorsal, tube-shaped secondary bronchi, whereas in varanids it goes through ventrolateral, sac-like bronchi. Also, varanids and archosaurs are phylogenetically distant, so if uniflow was primitive for diapsids, it would seem to have been lost in a lot of other lineages–potentially, all the non-varanid lepidosauromorphs.

On the gripping hand, uniflow would seem to have been lost in all those other lepidosauromorphs, but maybe it wasn’t. Maybe some of them are in the same state varanids were in until this year: they’ve had uniflow lungs forever and we don’t know because no-one has looked yet. And this is one of the concluding points in the new paper: we need to go look more at how living animals actually work.

A small sample of monitor lung diversity, from Becker et al. (1989).

A small sample of monitor lung diversity, from Becker et al. (1989).

In fact, we don’t just need to look at more critters in general, we specifically need to look at more monitors. I have been casually throwing around the terms “monitors” and “varanids” as if the findings of Schachner et al. (2013b) apply to all of them. They may not–the new paper is only about airflow in the savannah monitor, Varanus exanthematicus (same species as Mike’s “sauropod” Charlie), and monitor lungs are sufficiently diverse in form to have been used as taxonomic characters (Becker et al. 1989). So monitors may actually provide multiple windows into the evolution of unidirectional, flow-through lung ventilation. This is especially tantalizing because extant monitors cover a much wider range of body sizes and ecologies than extant crocs, so–just maybe–we can find out if and how diversity in lung structure and ventilation is related to body size and mode of life. Somebody get on that, stat.

Hypothetical bird lung intermediates - Perry 1992 fig 6

Figure 6 from Perry (1992).

My favorite part of all this? Something virtually identical to how monitor lungs work was proposed just over two decades ago by Steve Perry, as a hypothetical stage between saccular lungs and bird-like lungs. See the “Euparkerian grade” lung in the above figure, with perforations between adjacent chambers? Compare that to the diagram of the monitor lung in the image at the top of the post–they’re pretty darned similar. Now, two caveats. First, Steve was suggesting this as a plausible ancestral state for archosaurs, not monitors, and as mentioned above, monitors do things a little differently than archosaurs. Second, there are some things in this figure that are now known to be incorrect, primarily the lack of unidirectional airflow in the crocodilian lung. In fact, on the page opposite this figure, Steve explicitly discounted the possibility of unidirectional airflow in croc lungs. Still, he recognized that croc lungs and bird lungs share profound structural similarities, that they are really points on a spectrum of plausible intermediate conditions, and that crocs had the potential to shuttle air around their lungs because of the complex connections between chambers. So if Steve was not completely right, neither was he completely wrong; it might be most accurate to say that he was less wrong than anyone else at the time, and for about 20 more years after. Which is pretty darned good; I’ve had to rebut myself within the space of five years (Wedel 2007: prosauropod pneumaticity is equivocal. Yates et al. 2012: oh no it’s not!).

Here are the thoughts that have been tumbling through my head since I first learned about this. Obviously structures can be simplified or lost through evolution. Birds and turtles lost their teeth, numerous tetrapods have lost one or both pairs of limbs, and, heck, the platypus lost its stomach. But I rarely see hypotheses of derived simplification entertained for organs like hearts and lungs. There seems to be an unstated but widespread assumption that complex = better when it comes to core physiological processes like breathing.

Reptilian lung morphospace - Perry 1992 fig 2

Figure 2 from Perry (1992)

But it ain’t necessarily so. Following Steve Perry’s diapsid-lung-continuum diagrams, I have often wondered if croc lungs are derived from bird lungs instead of the reverse; maybe the ancestral archosaur had a fully bird-like lung/air-sac system and the non-diverticular, not-super-aerobic lungs of crocs represent a simplification of that system to suit their more sedate lifestyle as semiaquatic ambush predators. That’s pretty much what Seymour et al. (2004) suggested for crocodilian hearts, and it seems plausible given that so many early crocodylomorphs were long-legged, terrestrial, and possibly cursorial (e.g., sphenosuchians). In other words, maybe extant crocs are secondarily ectothermic, with secondarily and possibly paedomorphically reduced air sac systems.

Heck, maybe even bird lungs are simplified compared to their ancestral condition. Most birds have nine air sacs: paired cervical, anterior thoracic, posterior thoracic, and abdominal sacs, and an unpaired clavicular air sac. Some have reduced the number further through loss or fusion of adjacent air sacs. But they all start out with 12 embryonic air sacs (the extras fuse together, IIRC almost all of them becoming part of the clavicular sac), which suggests that the ancestors of birds might have had more than the standard nine.

If we assume that there was some diversity in respiratory anatomy in Mesozoic dinosaurs–which is not much of a stretch, given the diversity we see within (let alone among) monitors, crocs, and birds–it would be an awfully big coincidence if the only dinosaur clade to survive the end Cretaceous extinction just happened to have the fanciest lungs. As far as I know, no-one has proposed that birds survived because they out-breathed everyone else. If anything, the decent-to-high survival rates of mammals, crocs, and turtles across the K-Pg boundary, and the complete extinction of air-sac-equipped pterosaurs and non-avian saurischians, suggests that lung ventilation had nothing to do with survivorship. So what are the chances that crown birds have the most complex lungs among ornithodirans? (Don’t say “flight” because enantiornithines and pterosaurs had air sacs and died out, and bats don’t have air sacs and fly just fine.)

I’m not saying these “awesomeness came first” hypotheses are currently more parsimonious than the standard view. But they’re plausible, and at least potentially testable, and if nothing else an antidote to the idea that birds sit at the top of some physiological Great Chain of Being.

Back to the homology-vs-convergence question. If flow-through lungs are primitive for diapsids, maybe they’ll turn up in a few more critters. But maybe evolving undirectional airflow just isn’t that hard, and only requires poking some holes through the walls of adjacent lung chambers–as stated above, we need to go check more critters. But either way, the form and function of the lungs in V. exanthematicus are not only fascinating in their own right, they give us a window into what the early evolution of archosaurian–and maybe even early diapsid!–breathing might have been like. And that’s phenomenal.

I have some more thoughts on this, particularly the implications for sauropods and other dinosaurs, but those will have to wait for another post.

Images and figures from Schachner et al. (2013b) appear here courtesy of Emma Schachner (website), who kindly offered to let me look under the hood before the paper came out. She also created a cool video showing the 3D lung anatomy of V. exanthematicus. Thanks, Emma, and congratulations!


MoO 2013 - Tegu skull

Another shot from my visit last month to the Museum of Osteology in Oklahoma City: the business end of a tegu (Tupinambis). Lots of cool stuff in this pic: heterodont dentition, wacky sclerotic ossicles, and some sweet neurovascular foramina along the maxilla. Someone should knock out a shrink-wrapped life restoration, a la All Todays.