Tuesday, May 21, 2013

Formation of reptilian head scales

Designers of dragons and other mythological creatures might wonder how to draw the scale patterns on the head and body. How would the scales form on such an animal?

Above: Briton Riviere (1840-1920), "Saint George and the Dragon."


Dr. Michel Milinkovitch from the University of Geneva studied Nile crocodiles to understand the origin of their head scales. His study was published this year in Science magazine.

It turns out that reptilian scale patterns form in two different ways: genetic patterning and stress fractures. In the first system, the scales are developmental units that begin to form in embryonic stages and are usually symmetrical from one side of the head to another. 

This is typical of snakes. The symmetrical pattern can be seen by overlaying the pattern from the right and left sides (the yellow and red line patterns at lower right).

By contrast, crocodile head scales form by randomized stress fracturing, like cracks in auto safety glass. The scales are non-overlapping polygonal shapes that are smaller in areas where there is more flexing. There's no symmetrical alignment from one side to another of a single individual, nor is there a consistent pattern from one individual to another. 

(Video link) Here's a video that explains the idea.

Scales on the rest of the body of the croc, unlike the head scales, follow the snake-like genetic patterning model. 

9 comments:

Drew said...

Fascinating! This makes me wonder though if non-feathered dinosaurs had stress fracture scales like their very distant ancestors.

Keith Parker said...

Interesting. This got me thinking what if the dinosaur or dragon had a skin that was somewhat less brittle? So I looked up pictures of baby crocs. That's also an idea for what imagined creatures might look like. So you could imagine the creature to have no discernable scales covered in a thick shiny hide with veins and possibly a few cracks under the surface!

Katy Hargrove said...
This comment has been removed by the author.
Katy Hargrove said...

There is a fantastic animal called Xenodermus jananicus, the dragon snake, that has a skin resembling a crocodile. Very good reference for dragons. I have wondered if this little snake has scutes along with its scales.

William B. Hand said...


I take issue with this declaration that crocodile head scales are all stress-fracture oriented. Although there's certainly gonna be some of that going on, the illustration seen up-close (bottom) doesn't allow one to notice what is actually a relatively consistent pattern on croc face scales. In the illustration 2nd from bottom one can see comparable patterns of distribution and shape (that suggest genetic instruction). although the patterns aren't as obvious as in the snake, they are indeed there. The claim of this article is in error.

James Gurney said...

Thanks, William. Those are reasonable objections. I hope I have stated the basic case of the authors accurately, and I also hope one of them will speak to your comments.

Drew, I think the article mentions that many feather patterns are genetically determined, though, as I understand it, there is an element of randomness even in the way those structures form, according to the laws of reaction-diffusion mechanisms (RDMs).

Katy, thanks for letting us know about Xenodermus jananicus

Cameron said...

This is interesting because I believe Crocodylians are fairly social animals, so I wonder if they use these asymmetrical patterns for facial recognition.

Brett Nelson said...

Yeah, I think it would be a crocodile scales that would be the closest match.

N'rai said...

These patterns made me think of this: http://www.youtube.com/watch?v=53kc8QoCohU