Another evolutionary mystery – the Stegosaur’s plates

by Matthew Cobb

Stegosaurs were large herbivorous dinosaurs that flourished in the Jurassic (roughly 170-145 MY ago). Only one taxon – Wuerhosaurus – made it through the end-Jurassic mass extinction. These were the dinos with the great big plates on their back, and vicious spikes at the end of their tail. And no, they never fought with T. rex, which was around much later.

stegos

Common myths are that they had “two brains” (one in the tail) and were intensely dim (see below). They only had one brain, which was about the size of a cat’s, although differently organized (they were reptiles, not mammals). And they weighed anywhere up to 2 tons, which is pretty hefty for any animal.

larson1

The tail spikes – now officially called thagomizers (honest! Paleontologists do have a sense of humour!) – were presumably used in defence, and some species from China, like Kentrosaurus, had funky spikes on their shoulders (i.e. at the top of their front legs – see above). Of course, these shoulder spikes explain why Kentrosaurus and its fellows went extinct – they couldn’t get through the door of Noah’s Ark…

The most striking thing about Stegosaurs, however, were the plates, or osteoderms.

What exactly were they for?

Maybe they weren’t ‘for’ any one thing – different species had different-shaped plates, as can seen above. But in the classic Stegosaurus, the plates are large and flat.

The most obvious suggestion is defense, although if the plates were upright, as on most reconstructions (see above), they would not protect the legs or the head, which is where any self-respecting predator would attack first. Even if the plates were flat against the body, they would not cover any of  the key parts of the animal’s anatomy.

In the 1970s, it was suggested that the plates may have had a thermoregulatory function – if stegosaurs were “cold blooded” like their reptile relatives, they would have to use the sun to warm up and get moving in the morning. By having lots of blood vessels running through the plates, they could be used as a kind of solar panel to gather heat.

This hypothesis conjours up the image of herds of stegosaurs stumbling bleary-eyed into a red Jurassic dawn, climbing slowly to the top of the nearest hill and standing in a north-south direction, their plates exposed to the rising sun. Alternatively, the animals might have had to dissipate heat generated by fermentation in their vast stomachs, and therefore used the plates to radiate excess heat, creating columns of rising warm air that would attract dancing midges.

However, in 2005 a group of researchers looked carefully at the anatomy of the plates and found that the internal canals were not connected with those to be found on the outside, suggesting that there as no way for blood to circulate around them, thereby arguing against the thermoregulatory hypothesis. Instead, they suggested, the plates were used in species or herd recognition, although there was no clear evidence presented to support this idea.

Often when evolutionary biologists encounter an inexplicably bizarre structure, they lay the blame at the door of sexual selection, normally by females. For sexual selection to explain the stegosaur plates, however, there would need to be some sexual dimorphism present. Sexing Stego skeletons is a tricky business, but there is no suggestion that the sexes had different-shaped or -sized plates. Maybe they were different colors?

Last month, doctoral student Shoji Hayashi of Hokkaido University kindly sent me a reprint of an article he has just published in the Journal of Vertebrate Paleontology, which looks at the relative growth rates of the plates and the main skeleton in a number of Stegosaur skeletons of different ages. He concludes that the plates carried on growing faster, later, than the rest of the skeleton, as shown in this figure:

f06_123

Shoji and his colleagues conclude: “It is probable that the plates and spikes have an intraspecific display function and reflect social status and/or sexual attraction. Individuals having the largest osteoderms would have appeared much larger to rivals and/or potential mates. Judging from the morphological variations of plates within Stegosauria, it is reasonable to suppose that plates have a species recognition function as previously suggested by Carpenter (1998) and Main et al. (2005). Stegosaurus plates may also have a function as thermoregulation by virtue of their arrangement and the flat and tall morphology, although this function is doubtful in other stegosaurian dinosaurs possessing only small plates and spikes.”

In other words, we still don’t know! And to be honest, it is hard to see how we will find decisive evidence one way or another. That is partly the joy of paleontology: trying to reconstruct something that can never be fully known.

Citations:

Carpenter, K. 1998Armor of Stegosaurus stenops, and the taphonomic history of a new specimen from Garden Park, Colorado.Modern Geology 23:127144.

Farlow, J. O., C. V. Thompson, and D. E. Rosner1976Plates of Stegosaurus: Forced convection heat loss fins? Science 192:11231125.

Main, R. P., A. de Ricqlès, Horner, J. R., and K. Padian2005The evolution and function of thyreophoran dinosaur scutes: implications for plate function in stegosaurs. Paleobiology 31:291314.

Hayashi, S., Kenneth Carpenter, K, and Suzuki, D. 2009. Different growth patterns between the skeleton and osteoderms of Stegosaurus (Ornithischia: Thyreophora). Journal of Vertebrate Paleontology 29:123-131.

9 Comments

  1. Posted April 10, 2009 at 7:08 am | Permalink

    The thagomizer term has made my day. That’s awesome, I had no idea!

  2. Sili
    Posted April 10, 2009 at 8:52 am | Permalink

    This is the second time in the last coupla weeks I’ve been informed that T-reges aren’t from the Jurassic.

    http://www.irregularwebcomic.net/comic.php?current=2257

  3. NMcC
    Posted April 10, 2009 at 2:45 pm | Permalink

    I’ll tell you who did fight with T. Rex though, David Bowie.

    Sorry. I do actually read your posts in a serious way – and learn something each time.

  4. whyevolutionistrue
    Posted April 10, 2009 at 2:58 pm | Permalink

    NMcC @3
    I was *there*, so that was a thought that whizzed through my mind as I was typing that sentence, but then it whizzed out again…

  5. Sili
    Posted April 10, 2009 at 3:20 pm | Permalink

    Oh. And I thought it was brontosaurs that were supposed to have an extra brain on account of their humongous size.

  6. whyevolutionistrue
    Posted April 10, 2009 at 4:22 pm | Permalink

    The “extra brain” thing is in fact a ganglion of neurons down at the back end. Mammals have them too in the spinal column. Not sure why stegos and brontos (renamed Apatosaurus for reasons that are too tedious to go into) get such a bad press when whales and elephants do not. I suppose because we *know* that modern mammals are smart. Plus, to be honest, the dinos probably were a bit dim.

  7. Richard
    Posted April 11, 2009 at 6:15 am | Permalink

    Why do people think that sexually selected traits NEED to be sexually dimorphic? They sometimes are but certainly not always. For example, reindeer males and females both grow antlers every year. There is no general trend for genetic correlations of sexually selected or even sexually dimorphic morphological traits to be low. In fact they are are usually about the same as other traits. I think part of it is that people interpret sexual dimorphism as sex limited (i.e. females don’t have it) rather than sex influenced (differences in relative size of a trait). So, I see the shoulder spikes on the stegosaurs and I want to know the relative sizes of them in males and females. I’ll bet they are different.

    I’m not sure about the evidence from the last paper you look at indicating that plates are faster growing and longer growing and thus important in social display. Does this mean human ears are important in social display?

  8. Sili
    Posted April 11, 2009 at 7:43 am | Permalink

    I didn’t think of them as stupid. If they had two brains that ought to make them twice as smart.

    But I did honestly think that there was an issue of controlling such a big body if the signals had to travel too far to reach the brain. I’m obviously not that knowledgable about biology. Stupid that I never considered whales as a counterexample.

    Thank you.

  9. caveping
    Posted July 7, 2013 at 3:16 pm | Permalink

    perhaps plates are faster and longer growing to correlate with increasing volume of the growing indvidual, supporting the traditional view of plates as structures for dissipate excess heat?? “some” vessels should be enough, not necessarily to be crowded with them…


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