Tuesday, April 29, 2008

A Nagging Thought

How come there are no modern reptiles with giant freaking sails? If the huge vertebral sails of Dimetrodon, Edaphosaurus, and various other critters through the millenia were strictly for absorbing heat, then why don't you see more examples in both the fossil and extent zoological record of sails? As it stands, sails are relatively rare. Off the top of my head:

  • Platyhystrix

  • Dimetrodon

  • Edaphosaurus

  • Arizonasaurus

  • Amargasaurus

  • Ouranosaurus

  • Spinosaurus

  • Of those animals, two are virtually sister taxa, one might not actually have sails, and the last two have interesting sail shapes. Dimetrodon and Edaphosaurus have wimpy rounded neural spines, while all of the archosaurs on the list (save Amargasaurus) have flat (side to side) but wide (front to back) neural spines. Surely there's a reason for these differences.

    But that's not even the point. If sails evolved to soak up heat, why didn't they evolve several times? Isn't it more parsimonious (as they say) to conclude that sails have a display function first, and a heat-soaking function second?

    Bah--I probably don't know what I'm talking about.

    Thanks to fellow NWR colleague Aaron Kaluszka for teaching me the HTML tags for bullet points. Now I feel smarter!


    Christopher said...

    Im going to shoot the next person to seriously consider the "hump" hypothesis. The fact is that reptiles, and dinosaurs are reptiles no matter how much fringe palaeontologists make them out to be similar to mammals, store fat in their rumps and tails.

    And there are modern reptiles that have freakishly huge sails (proportionally of course!) that are derived from elongate neural spines, the Chameleons. The neural spines are thin side to side and wide front to back, and thus are the perfect analog for sail backed theropods and ornithopods. Their primary function is display but they do give the animals a greater surface area with which they can orient towards the sun.

    Chamaeleo cristatus:

    Zach said...

    Whoa. That chameleon is sportin' some sail! I'll modify the post to remove the reference to a fat hump. I've never taken the theory very seriously--reptiles don't really have fat stores like mammals do.

    Christopher said...

    All rantings aside, sauropds have rather differently shaped neural spines, and can be kinda similar to the vertebrae of mammals like Bison. So some of them may have had humps.

    Oh, and thats not the only sailbacked chameleon species, just one of the more spectacular examples.

    Sean Craven said...

    What I find curious is the way that sails seem to be some kind of fad -- Dimetrodon and Edaphasaurus, Ouranosaurus and Spinosaurus...

    The frustrating thing is that if it happened once I'd be scratching my head. Three times and I'd strongly suspect a cause.

    But twice? That leaves me hanging and itchy. It's right in my discomfort zone.

    Still. What's up with that?

    Glendon Mellow said...

    I agree Sean, it is uncomfortable. It feels to me like there must have been a cause in the environment that necessitated the sail.

    Zach, this nags. You have spread around a little mystery-sail nagging-meme.

    I tend to think that they'd likely be for display first, regulaton second, but then again, many things like a peacock's tail or Lion's ruff don't have the major energy investment that bones do for the organism! That's a huge investment.

    Am I underestimating how far sexual selection will go in an organism? Am I somehow failing in the sexual selection game by not closing my eyes real tight and spontaneously mutating a sail out my spine?

    Ivan said...

    Platyhystrix is a temnospondyl.

    The following are a few other sailbacked archosaurs and pelycosaurs:





    The tall vertebrae of Rebbachisaurus have also been argued to support a sail of some sort.

    There's another photo of Chamaeleo cristatus that really shows off the sail:

    It's interesting that AFAIK, sails in chameleons are often a sexually dimorphic feature, with the males having larger, more pronounced sails. And AFAIK, all of the chameleon species sporting sails belong to the subgenus (genus?) Trioceros, which also has many species where the males possess horns (and in some cases, sails AND horns).

    Other lizards with sails include the semiaquatic Hydrosaurus (Agamidae) and Basiliscus (Iguanidae).

    Zach said...

    Thanks, Ivan! I've wondered if the sails of extinct tetrapods had a display function like chameleons. It would be difficult NOT to, at least secondarily, turn the sail into a display organ.

    corneredbadger said...

    dinosaurs had less surface area to mass than modern lizards (proportialy. just think about how much faster volume increases than surface area. they needed that sail to give them increased surface area to compensate for this. so yeah sailed dinosaurs were compensating