Life as a stegosaur: the SJG stegosaur special, part III


Time to wrap up on the SJG special - make sure you see part I and part II first. Wow, I never thought I'd end up writing three long articles on this series of papers (hmm, a familiar theme). In the previous articles we looked at stegosaur systematics, and at Heinrich Mallison's work on the ranges of movement and posture of Kentrosaurus. This time round, we look at the papers that discuss preserved soft tissues, the possible role of the plates, and on feeding behaviour. Oh, and we finish up by looking at some other stuff too...

As I said before, the papers I'm discussing here appeared last year in a special issue of Swiss Journal of Geosciences (SJG from hereon). These papers resulted from a meeting held at the Sauriermuseum Aathal, Switzerland, in June 2009 (Billon-Bruyat & Marty 2010). Thanks to Daniel Marty for his help.

Stegosaur skin and 'plate sheaths'

New data on stegosaurian soft tissues are described by Nicolai Christiansen and Emanuel Tschopp in their report of integument preserved with a Hesperosaurus specimen (see the Stegosaur Wars article for a discussion of Hesperosaurus and whether it warrants distinction relative to Stegosaurus).


These several skin impression patches show that small, non-imbricating tuberculate scales - of the sort widespread in archosaurs - covered the animal's body; the scales are approximately hexagonal [one of the skin impressions is shown here, from Christiansen & Tschopp (2010)]. Larger, domed scales (only two were preserved) are present near the top of the animal's back. These larger scales were surrounded by smaller ones to form rosette-like patterns: as Christiansen & Tschopp (2010) describe, rosettes of this sort seem to have been widespread in dinosaurs. These are not, by the way, the only skin impressions known for a stegosaur: some were also described for the Chinese taxon Gigantspinosaurus sichuanensis* (you can see a photo of them here), and an unpublished Morrison Formation specimen identified as cf. Stegosaurus also preserves some.

* Yeah, I hate the generic name too. It's one of the dumbest names ever given to an animal.

While it has (so far as I know) always been assumed that stegosaur spines were sheathed in horn, matters have been less clear when it comes to plates. The surface texture of these structures has led some workers to propose that they were covered only with skin (I really should find out when and where this idea originated). The idea that the plates were sheathed in horn seems more likely, mostly because big bony things that stick out of tetrapod bodies are virtually always encased in this material (the antlers of deer and ossicones of giraffids are remarkable exceptions). Christiansen & Tschopp (2010) report a continuous, presumably keratinous covering on one of the plates of their Hesperosaurus specimens. Interestingly, this covering is decorated with fine, closely spaced ridges. Because the fossil is a mold, these would have been grooves in life. This would seem to confirm once and for all that horn did, indeed, sheath the bony cores of the plates.


Stegosaur plates for thermoregulation, revisited, again

And having mentioned plates... one of those things that everyone seems to know about stegosaurs - or, rather, about Stegosaurus in particular - is that its remarkable plates may have functioned in thermoregulation. Given that most organs that stick out from the body can absorb and radiate heat (the most oft-mentioned 'facultative thermoregulatory structures' are the horns of bovids), we can be confident that stegosaur plates and spines had some thermoregulatory role. But were they specialised organs whose distinctive shape had evolved under selection for such a function?

I admit to being sceptical of such explanations because it seems to me that people try too hard to find 'functional' explanations when confronted with flamboyant biological structures. The latter could just as well be flamboyant because they've evolved under selection for, well, being flamboyant. On the other hand, you could make the argument that - limited as we are, without time machines - we should seek to test the possible 'functions' of flamboyant structures to see if they might be good for something other than display before concluding that this was their probable role.

Farlow et al. (1976) suggested that the plates could have been efficiently deployed as "forced convective heat loss fins, [constituting] a physiologically effective thermoregulatory adaptation" (p. 1124) [that article made the cover of Science, as you can see from the image above]. De Buffrénil et al. (1984) drew attention to the presence of vertical 'pipes' within the plates and implied that a thermoregulatory role for the plates was more robustly supported than were other roles. However, Carpenter (1998) drew attention to the fact that, were the plates really specialised thermoregulatory organs, they would most likely be ubiquitous across Stegosauria. They aren't: Stegosaurus-sized stegosaurs inhabiting similar environments to Stegosaurus typically had smaller, differently shaped plates. Carpenter's argument is suggestive and certainly not conclusive, but it's in agreement with the hypothesis that the plates functioned primarily in visual display. Main et al. (2005) argued that the gross and microstructural features of the plates were merely typical growth features of thyreophoran scutes (internal 'pipes' like those of Stegosaurus plates are present in ankylosaur armour), and they concluded that there was no clear evidence for a special thermoregulatory role. Rather, they argued that the plates were 'extreme' display structures used in species recognition (a hypothesis supported by the elaboration of scutes across thyreophoran phylogeny).


The new paper - by James Farlow and colleagues - reports CT scanning of Stegosaurus plates and alligatoroid osteoderms, and uses this data to look anew at a possible thermoregulatory role for stegosaur plates (Farlow et al. 2010). The new scans show that those internal 'pipes' formed a branching structure, connected by a 'main channel' that extends along each plate's base [shown here, from Farlow et al. (2010)]. It's suggested that, whatever its homology or origin, this network could well have worked as a "vascular distributary system" (p. 179). Some work (including thermal imaging reported in the paper) shows that crocodilians use their scutes to transfer heat and warm themselves, and sub-vertical canals in the osteoderms (reminiscent of those 'pipes' in the stegosaur plates) may well play a distributary role in transferring this heat.

In the end, the conclusion is that the plates were likely multi-functional and had the potential to play a thermoregulatory role (Farlow et al. (2010) mention bovid horns here). So, if the plates worked well as heat-shedding fins, this was plausibly facultative rather than adaptive. This seems absolutely reasonable, but note that saying this is not the same as actually finding support for a thermoregulatory function. If you want to be cynical, you could say that (1) people are only trying to find evidence for a thermoregulatory role because, historically, this is the role that's often been favoured, (2) the fact that crocodilians apparently use their scutes to help collect solar heat doesn't tell you much about stegosaurs, especially since the main thermoregulatory role for stegosaur plates has been that they were good at shedding heat, not collecting it, and (3) no direct evidence supports the thermoregulatory hypothesis [image below - showing Stegosaurus battling Ceratosaurus - by John Conway].


Stegosaurus bites

Stegosaur palaeobiology is also examined by Miriam Reichel in another of the special issue's papers. Bite forces were estimated by digitally constructing 3D Stegosaurus teeth and subjecting them to simulated loading. Reichel (2010) concludes that Stegosaurus teeth and jaws were strong enough to bite through small branches and leaves, but that the animal was not able to crush branches more than 12 mm in diameter. This leads to the suggestion that Stegosaurus perhaps "took advantage of the abundance of smaller, fast growing plants" (p. 239) in the Morrison ecosystem.

Given that the relatively slim muzzle and small teeth of Stegosaurus are already suggestive of selective browsing, one would probably not predict the biting and ingestion of twigs and/or branches anyway. Of course there is the caveat that modelling techniques such as those employed here may not necessarily do an accurate job of replicating in-vivo behaviour. Sorry for the cryptic statement, but I cannot elaborate on that at the moment (this article is already too long).

Final musings on behaviour and such

This compilation of papers represents a significant addition to our knowledge of stegosaurs. Combined with Susannah Maidment's (2010) historical review and phylogenetic overview of the group, it means that we have a nice collection of papers on stegosaur systematics, biomechanics and anatomy.

We still know essentially nothing about stegosaur social behaviour, though the close association of several Kentrosaurus specimens at Tendaguru has led to the suggestion that this taxon at least was a herding animal. Conversely, the fact that most stegosaurs are discovered on their own has caused some to think of them as mostly solitary animals, but arguments like this are hardly conclusive given that even the most social of animals often die alone. Mortality in some Stegosaurus specimens seems to have been drought-related (Carpenter 1998): Robert Bakker has suggested that Stegosaurus was non-migratory and that it stayed, year-round, in close proximity to big rivers and lakes (Psihoyos & Knoeber 1994, pp. 101-102). However, it's doubtful whether the data is good enough to show which dinosaurs were seasonal residents and which were migrants. The dinosaur fossil record - even that from the Morrison - is not on par with that of animals where migratory habits can be confidently asserted or denied, like Pleistocene bison.

Baby stegosaurs are known (and some eggs have also been identified as stegosaurian), but nothing is known about parental care. Clusters of juvenile ankylosaurs found preserved together suggest that young thyreophorans lived together in pods (a behaviour that may well have been widespread across extinct archosaurs).


And what about mating behaviour? Stegosaur plates and spines were not mobile (no matter what Bakker said in The Dinosaur Heresies), and it seems very difficult to imagine that males could mount females in the manner conventional for quadrupeds. It's been seriously suggested that stegosaur mated ventro-ventrally (as shown in the adjacent cartoon) or that males had an enormous, laterally flexible phallus, perhaps deployed transversely when the animals stood close to one another. Another idea is that females lay on their sides and were straddled from behind [see illustration below, by Patrick Redman]. Most recently, Isles (2009) suggested that a mating pair might approach end-to-end. I would love it if stegosaurs were parthenogenetic and didn't mate at all, but this is contradicted by claims of sexual dimorphism in members of the group.


Incidentally, the frequent complaint that palaeontologists never ask awkward questions about dinosaur sex (here's an example from the Ask A Biologist site) is another one of those generalisations that is simply and absolutely not true. If anything, palaeontologists have done rather too much speculating on dinosaur mating poses and such, and there is a large literature on this subject. Unsurprisingly, most of it has appeared in popular and semi-popular sources... hell, there's even a whole book called Dinosaur Sex* [hence image at very top].

* Hats off to Michael Brookfield for writing Dinosaur Sex, but I'm afraid it's not the dinosaur sex book I would have written. It includes hardly any discussion of or speculation on Mesozoic dinosaurs, features really terrible diagrams, and has peculiar and fairly pointless sections on such topics as masturbation, oral sex and fetishism.

Anyway, I sort of went off on a tangent there at the end. With the completion of this little section on palaeobiology, I'm hereby done in writing up my thoughts on the SJG stegosaur special issue. I hope you enjoyed this look at Stegosauria and have a renewed appreciation for this remarkable, enigmatic group of flamboyant dinosaurs.

For previous Tet Zoo articles on stegosaurs, please see...

For articles on other thyreophorans, see...

Refs - -

Billon-Bruyat, J.-P. & Marty, D. 2010. Preface: Symposium on Stegosauria proceedings. An international conference on stegosaur finds of the world organized by the Sauriermuseum Aathal (8th and 9th June 2009, Aathal, Switzerland). Swiss Journal of Geosciences 103, 139-141.

Buffrénil, V. de, Farlow, J. O. & de Ricqlès, A. 1984. Histological data on structure, growth and possible functions of Stegosaurus plates. In Reif, W.-E. & Westphal, F. (eds) Third Symposium on Mesozoic Terrestrial Ecosystems, Short Papers. Attempto Verlag (Tübingen), pp. 31-36.

Carpenter, K. 1998. Armor of Stegosaurus stenops, and the taphonomic history of a new specimen from Garden Park, Colorado. Modern Geology 23, 127-144.

Christiansen, N. A. & Tschopp, E. 2010. Exceptional stegosaur integument impression from the Upper Jurassic Morrison Formation of Wyoming. Swiss Journal of Geosciences 103, 163-171.

Farlow, J. O., Hayashi, S. & Tattersall, G, J. 2010. Internal vascularity of the dermal plates of Stegosaurus (Ornithischia, Thyreophora). Swiss Journal of Geoscience 103, 173-185.

FARLOW, J., THOMPSON, C., & ROSNER, D. (1976). Plates of the Dinosaur Stegosaurus: Forced Convection Heat Loss Fins? Science, 192 (4244), 1123-1125 DOI: 10.1126/science.192.4244.1123

Isles, T. E. 2009. The socio-sexual behaviour of extant archosaurs: implications for understanding dinosaur behaviour. Historical Biology 21, 139-214.

Maidment, S. C. R. 2010. Stegosauria: a historical review of the body fossil record and phylogenetic relationships. Swiss Journal of Geosciences 103, 199-210.

Main, R. P., de Ricqles, A., Horner, J. R. & Padian, K. 2005. The evolution and function of thyreophoran dinosaur scutes: implications for plate function in stegosaurs. Paleobiology 31, 291-314.

Psihoyos, L. & Knoebber, J. 1994. Hunting Dinosaurs: On the Trail of Prehistoric Monsters. Cassell, London.

Reichel, M. 2010. A model for the bite mechanics in the herbivorous dinosaur Stegosaurus (Ornithischia, Stegosauridae). Swiss Journal of Geosciences 103, 235-240.


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What stegosaur is that in the picture at the top? I can't identify it??

By Tim Morris (not verified) on 10 Jan 2011 #permalink

Drat, I was hoping no-one would ask. It's a hypothetical generic stegosaur, illustrated for a publication that died on the operating table.

It occurs to me that one fairly easy way to decide if the plates of Stegosaurs were thermoregulatory might be to see if the ratio of the surface area of the plates to the volume of the animal's entire body was consistent across the entire size range of Stegosaurs. Since surface area increases by the square and volume by the cube, if the plates had thermoregulatory functions you would expect the plates of bigger Stegosaurs to be proportionately bigger than those of smaller ones, to keep the amount of heat-shedding ability per unit volume of body the same.

If however the plates were not greatly used in thermoregulation, then the ratio would not hold out; if used mostly for display then you would expect the plates to have some form of weight-reduction system in them; pneumaticity of some sort, say?

By Dan Holdsworth (not verified) on 10 Jan 2011 #permalink

If the plates were vascularized, then they were used for thermoregulation. All amniotes can control rates of heating and cooling by regulating peripheral blood flow. It can work, especially with a breeze, in either direction, depending on the temperature gradient and preferred body temperature. A huge exposed surface area like that could speed up or slow down heating or cooling quite a bit and (if they were well vascularized) it's only reasonable to accept that use as a given.
Which doesn't mean that's what they evolved for in the first place.
They look energetically costly to grow and maintain, so there's bound to be a benefit. Species recognition? There are way cheaper ways. Sexual selection? You'd expect them to be dimorphic. Effectiveness against a particular predator?

By Sven DiMilo (not verified) on 10 Jan 2011 #permalink

Dan - nice idea, but as usual we are let down by a patchy fossil record. There just aren't enough good specimens for this sort of thing to be reliably worked out, and those specimens with more than one or two complete plates (referring here to Stegosaurus alone) are about similar in size. Having said that, seems to me that some specimens screw with any possible body-size correlation: e.g., I'm looking at reconstructions of a S. ungulatus specimen with relatively small plates that is 7 m and 3.8 tonnes and a S. stenops with relatively huge plates that is 6.5 m and 3.5 tonnes.

As for weight reduction, the plates are primarily composed of trabecular medullary bone, contain large 'erosion rooms', and have but thin layers of external compact bone: in other words, about as reduced in weight as you could get (in an apneumatic skeleton).

Sven - good call. Ok, the plates (and spines) were thermoregulatory in the same way that horns are. Part of the argument for a 'primary' sociosexual role for the plates comes from the diversity of scute anatomy seen across Stegosauria and the rest of Thyreophora, as it's not as if Stegosaurus is a singleton. On sexual selection: as per above, we don't have enough specimens to confirm or deny, but note that sexual selection does not require the presence of dimorphism (thanks to the phenomenon of mutual sexual selection). And do check out Main et al. (2005).

Not all Stegosaurus members had the plates. Instead, some had spikes with smaller surface areas.
Scientists looked to modern animals such as deer, whose antlers also have large blood vessels. In this example, those blood vessels have nothing to do with heat transfer; rather, they enable rapid growth of the antlers. Dinosaurs, too, are thought to have been fast-growing animals

By Traduceri (not verified) on 10 Jan 2011 #permalink

The text from comment 6 is a cut-and-paste job from this article by Rachel Beavins. Accordingly, I am banning this commenter: does anyone know if it's a spambot, or is there a person behind this? The 'Traduceri' site (url now deleted) is a pay-to-get-stuff-translated site.

Hats off to Michael Brookfield for writing Dinosaur Sex, but I'm afraid it's not the dinosaur sex book I would have written.

I look forward to the dinosaur sex book you will write. :-)

Seriously. It seems like the kind of thing that would suit you, since you do a good job of explaining why we know what we know and don't know what we don't. And, just think, it might do for you what 'Dr Tatiana's Sex Advice to All Creation' has done for Olvia Judson. Maybe when the kids are old enough not to be too scandalized.

It's a hypothetical generic stegosaur,

You mean Hypothestegosaurus genericus.

By Mike from Ottawa (not verified) on 10 Jan 2011 #permalink

I admit I have often wondered how stegosaurs (really, all thyreophorans) had teh_sex. Mounting was clearly not feasible. How do modern porcupines and echidnas do it?

I'm surprised that no one has mentioned the most preposterous use of stegosaur plates. If I recall correctly, Edgar Rice Burroughs (the *cough* eminent paleontologist) had a stegosaur tilting the plates flat and using them to glide across a canyon in one of the Pellucidar novels. It was also a carnivore, chasing the (scantily clad) heroine...

More seriously, I'm thinking about Jurassic environments again. If one is going to posit that Stegosaurs used their plates for display, they needed to live in an environment where the plates were visible, needed eyes good enough to see them, and lived in an environment where the plates wouldn't substantially hinder their movement (through low-hanging branches or similar barriers). The thagomizer has similar limitations, as it only works where there's room to swing it freely. These strongly suggest that Stegosaurus at least was a creature of the open savanna.

By heteromeles (not verified) on 10 Jan 2011 #permalink

I admit I have often wondered how stegosaurs (really, all thyreophorans) had teh_sex. Mounting was clearly not feasible. How do modern porcupines and echidnas do it?

Very carefully.... !!

(/Obvious straight line is obvious)

By Owlmirror (not verified) on 10 Jan 2011 #permalink

How do modern porcupines and echidnas do it?

I think that porcupines lay their quills flat when mating. That's certainly how hedgehogs manage it. But, as Darren said in the article, this isn't an option for stegosaurs. Echidnas, which have sturdier (and I presume less mobile) quills, mate face-to-face while lying on their sides. I find this a little harder to imagine for stegosaurs because of their proportionally longer legs.

With the full admission that I know nothing on the subject, I'd be inclined to favour the idea of a long intromittent organ. But I don't see how one could actually tell from the fossil record. Soft-body preservation of stegosaurs that died while mating seems a little much to hope for.

Great post! Stegosaurs are so cool. I found the paper on thermal function of osteoderms very useful, because of the crocodilian data. I have heard of osteoderms as "heat colectors" in crocodilians lots of times on tv. But this is the first time I see something published on this. I wonder if there are more papers on this subject.

The issue of a skin vs horny covering for Stegoaurus plates was mentioned by Buffrenil, et al (1986, page 467). Perhaps it was where the idea of skin-covered plates was originated:

"...If a thick, highly developed horny covering was indeed there, it should have followed the growth of the subjacent bony plate, though the activity of the basal cell layer of the (Malpighian) epidermis. Among living mammalsa and reptiles, the activity of such epithelia in producing keratin is generally very ciclical, the horny material being deposited in a succesion of thick sheets or strata, which themselves leave a clear imprint on the subjacent bone itself. This can be observed on the horn cores of bovids and on the bony carapaces of tortoises. The apparent complete lack of such imprints on Stegosaurus plates suggests, although it does not prove, that a thick horny sheath was not there."

V. de Buffrenil, J. O. Farlow, A. de Ricqles (1986). Growth and Function of Stegosaurus Plates: Evidence from Bone Histology. Paleobiology, 12(4):459-473


I am banning this commenter: does anyone know if it's a spambot, or is there a person behind this? The 'Traduceri' site (url now deleted) is a pay-to-get-stuff-translated site.

'Traduceri' also left a similar cut-and-paste comment in the 'Lal the chicken-eating cow' comments thread; the text (typos and all) in that comment is stolen from Google Answers.

By the way, was that really the Ptr Mhld who left a now-deleted comment here yesterday? Is he still craving for your attention?

Thanks for comments. Some brief responses.

Dinosaur sex book: I'd love to write one. Tim Isles and I were talking about that sort of thing, but when I pitched a book on dinosaur behaviour some time last year I didn't get much interest. Getting a publisher interested is the biggest problem in producing books - or, it is if you need to get any money out of it, anyway. As Chris says, porcupines (and hedgehogs) have highly mobile quills and the female can flatten them when mounted. Everything about echidna sex is weird. They have group sex where numerous males pursue the same female, males sometimes mate with females while the latter are still semi-dormant during the winter, and echidna penises have four heads.

Thanks heteromeles about the mention of ERB and the gliding stegosaur... wow. It reminds me that I forgot to mention Halstead's idea that the plates were not held erect, but flopped sideways over the flanks. I must write about that some time. On habitat, it seems reasonable to think that most (or all?) stegosaurs inhabited parkland-like habitats where there were indeed open areas. Certainly this is true for the Morrison and Tendaguru.

Jorge: yes, I too have often heard about crocodilian scutes being used as 'heat collectors', typically in passing. Both Seidel (1979) and Richardson et al. (2002) discussed the possibility of a thermoregulatory role, but didn't actually test it, so this might be a first. The thermal imaging pics featured in the paper are pretty neat. Buffrenil et al. (1986) were not the first to suggest coverage of the plates by skin alone, as there are definitely reconstructions pre-dating 1986 where the plates are shown as skin covered and horn-less.

Finally.... yes, Mihalda is still craving my attention as just a couple of days ago he sent me a long email about therizinosaurs and how they can't be theropods. Despite being cced to many people, I was mentioned by name, so he obviously is obsessed with me in particular. However, it wasn't him who left that comment here yesterday... that was someone else, trying to be funny.

Refs - -

de Buffrenil, V., Farlow. J. O. & de Ricqles, A. 1986. Growth and function of Stegosaurus plates: evidence from bone histology. Paleobiology 12, 459-473.

Richardson, K. C., Webb, G. J. W., & Manolis, S. C. 2002. Crocodiles: Inside Out. A Guide to the Crocodilians and Their Functional Morphology. Surrey Beatty & Sons, Chipping Norton.

Seidel, M. R. 1979. The osteoderms of the American alligator and their functional significance. Herpetologica 35, 375â380.

note that sexual selection does not require the presence of dimorphism (thanks to the phenomenon of mutual sexual selection)

Never heard of that. What is it? Is it different from species recognition?

Is he still craving for your attention?

Mine too. Sends me about two e-mails a week about how segnosaurs must, must, must be sauropodomorphs because their bones show sauropodomorph autapomorphies. (I will leave the discovery of the logical fallacy to the esteemed reader.) Also still sends screeds about how all of archosaur phylogeny is obviously, obviously wrong.

echidna penises have four heads

That may be less unusual than it sounds. I was recently told that marsupial penises have two heads. Is that true?

By David MarjanoviÄ (not verified) on 11 Jan 2011 #permalink

Mutual sexual selection: I and colleagues currently have two manuscripts in review on this subject and on how it relates to fossil archosaurs, one of which has been in review for months and months and months. Let's forget this subject was mentioned for the time being, thanks :)

Yes, many marsupials do indeed have a forked penis (and the vagina is bifurcated). In opossums, kangaroos and possums at least, the forking is transverse, but I've heard that in dasyures the two appendages are stacked dorsoventrally... which, err, kinda makes you wonder what they're doing with at least one of them. Remember also that marsupials often have pre-penile scrotums (yup, scrotum is anterior to penis). I should definitely write more on genitalia.

Apart from thermoregulation and intra-specific display, the only other use for the plates I can envisage is an anti-predator display, either of fitness in a way similar to the exaggerated jumping that a lot of deer do when startled, to show off how good they are at running away, or to simply make the animal look really, really big from the side.

Exaggeration of size is only going to work with pretty stupid predators; toads inflate themselves to appear too big for a predatory snake to eat (which works at least some of the time), but how smart allosaurs and similar big predators were is open to debate. Certainly evidence from predator trap tar-pits tends to suggest that they weren't intelligently cautious about boggy ground, but whether a nice big display of plates making a subadult animal look much bigger would put off predators or not is up for debate.

The other problem with this idea is that we now know that a lot of dinosaurs were able to breed whilst not of full adult size (actually, the notion of adulthood probably needs revisiting with dinosaurs) so the impression of a population where the dinosaurs are either subadult and concentrating on growing, or adult and breeding is likely to be false; one of juveniles growing fast, breeding but not full-size individuals growing more slowly and a few really big animals which hog most of the breeding opportunities is probably nearer the truth.

By Dan Holdsworth (not verified) on 11 Jan 2011 #permalink

Regarding parthogenesis, that is an interesting idea. However, I would question why stegosaurs would have such prominent display structures if they did not reproduce sexually.

Perhaps they were like whiptail lizards in that they still had to stimulate each other for parthogenesis to actually occur. There's an interesting thought: All stegosaurs were lesbians?

By Jamie Stearns (not verified) on 11 Jan 2011 #permalink

Ummm, most parthenogenetic whiptails evolved from bisexual species. Therefore, I think the proper answer is that extraordinary claims require extraordinary proof.

Given that a) stegosaurs could apparently stand bipedally, and b) they could be their tails over 45 degrees off-line, I don't think that a conventional intromission would be that hard. The hard task, for the males, is to avoid getting thagomized by unreceptive females. Come to think of it, tiny brains+nasty tail weapons might be the reason that they evolved elaborate sexual displays. The message of the fins might be "don't swat me, I'm a male!"

Since not that many birds have phalli, I'm also surprised that no one is speculating on whether dinosaurs had phalluses like those of modern ducks. Anseriforms are a rather ancient group, after all. Imagine a stegosaur with a phallus like a Meller's duck. Or a sauropod, for that matter.

By heteromeles (not verified) on 11 Jan 2011 #permalink

heteromeles | January 12, 2011 12:35 AM:

The hard task, for the males, is to avoid getting thagomized by unreceptive females. Come to think of it, tiny brains+nasty tail weapons might be the reason that they evolved elaborate sexual displays. The message of the fins might be "don't swat me, I'm a male!"
Since not that many birds have phalli, I'm also surprised that no one is speculating on whether dinosaurs had phalluses like those of modern ducks. Anseriforms are a rather ancient group, after all. Imagine a stegosaur with a phallus like a Meller's duck. Or a sauropod, for that matter.

I had thought there was reasonably well accepted theory that a longer phallus was correlated across species with a higher frequency of attempts to mate with unreceptive females. So your suggestion of a duck-like phallus seems to run counter to your previous suggestion that a male's primary problem might be getting thagomized by an unreceptive female.

I wasn't being entirely serious on the parthenogenesis idea - as I said, it's contradicted by some possible sexual dimorphism: individuals of Kentrosaurus, Dacentrurus and Stegosaurus differ in how many sacral ribs they have. As for sexual organs and the possibility of polygyny, polygynandry and so on, do see the discussion in Isles (2009) [cited above].

True, I was simply thinking outside the box on phallus structure. While I think that a conventional coupling posture is the simplest hypothesis, I'd bet that Stegosaurs had phalluses, simply because vent-to-vent coupling seems a bit inefficient.

We also haven't touched on the question of how far muscles can throw ejaculate, for that matter. Vent-to-vent copulation might only work in a big dinosaur if the sperm can fly up to a meter, and/or swim several meters to an oviduct. Moving sperm through a phallus cuts the travel distance and/or ejaculation pressures substantially.

But, as others noted, I do have to be careful in speculating. I wouldn't want to be caught promulgating a phallusy.

By heteromeles (not verified) on 12 Jan 2011 #permalink

Ha ha :) We should _assume_ the presence of phalluses in non-avialian dinosaurs given their presence in squamates, crocodilians and crown-birds (palaeognaths, galliforms and anseriforms all have them: they only seem to have been lost in neoavians). And, if you don't like phylogenetic bracketing, I think we can agree that their presence makes biomechanical sense :)

The "Not all Stegosaurus members had the plates" post may have been a cut-and-paste but it makes a point I have always wondered about. With all the farmers out there why has the idea that highly vascularized bone always means that the bone was covered in skin lasted so long in popular books?

As a kid I grew up near a farm and knew that cattle horns bleed when removed (a nasty and bloody buisiness by the way, you can see straight into the steer's nasal passages through the hole in the head where the horn was)and with the skin removed the horn-base is covered with the same sort of grooves found on stegosaur plates.

I always wondered how stegosaurs mated too, most popular books just compared them to porcupines. Unfortunately American porcupine females cover their spikes with their flattened tail to give the male a nice furry cushion to lie on and Old world porkies have movable spines - so as you pointed out this is nothing but a red herring. Nice to see some more realistic hypothesis.

Oh, Congratulations on getting a new Giant Panda at the London Zoo.

Darren: I was hoping it would be some unpublished Craterosaurus stuff, oh well.

By Tim Morris (not verified) on 21 Jan 2011 #permalink

What about the plates as armour? If heat absorption and/or deflection was so important to a dinosaur of that size (which was probably an endotherm anyway right?) why don't Stegga's contemporaries have such structures. I like the sexual selection hypothesis too. Parthenogensis, um? Maybe they could fold their plates down to faciliate copulation? Do you suspect the plates were mobile?

By Lindsey J Gray (not verified) on 03 Feb 2011 #permalink