Saurolophus Skin Suggests Speciation

A long, long time ago, I wrote a post about dinosaur integument color, and talked about "Dakota" the Edmontosaurus.  She, like her cousin "Leonardo" the Brachylophosaurus, is an exceptionally preserved dinosaur "mummy".  While most body fossil remains are made up of the hard parts of an animal (in the case of dinosaurs, it is generally their bones), these two hadrosaurines have lots of preserved soft tissue material.  Specifically mineralized skin and muscles.  Which is pretty cool, if you ask me.  From preserved skin and muscle, we can get a much better picture of what an animal would have looked like in life.  Don't get me wrong, we can tell a lot from the bones.  But the more information we have the better; every little bit helps.

The holotype of Saurolophus osborni (AMNH 5220).  Panel mount photographed by Barnum Brown in 1913.  Image from Wikimedia Commons.

And now we're able to get a better picture of another hadrosaurine genus, Saurolophus.  In a paper recently published in PLoS One, Phil R. Bell has collected loads of data on skin impressions from two different species of Saurolophus: S. osborni from Canada and S. angustirostris from Mongolia.  These two animals have been clearly established as distinct, separate species by their osteological remains.  But Bell has shown, rather effectively I think, that the skin impressions collected from multiple specimens are also distinct enough to be able to differentiate the two species on the basis of their integument alone.  That's even cooler.

In order to compare the skin impressions on the two Saurolophus species, Bell had to first establish a terminology that could be used to describe the various types of scales present.  That terminology is, briefly, as follows: there are 'basement scales' which serve as a backdrop to 'feature-scales', the larger more distinct scales.  'Interstitial-tissues' lie between the scales, and allow the skin its flexibility.  There are 'midline feature-scales' that run down the backs of the animals, and 'polygonal scales' which are, as you may have guessed, polygonal in nature and can be either 'basement' or 'feature'.  On the other hand, 'pebbly scales' (also known as 'pebbles') are the smallest type of scales, and always form the 'basement'.  There are asymmetrical 'shell scales', circular ' shield scales', and 'irregular scales' with no clear geometric proportions.  Bell looked at skin impressions containing these types of integument from the skull and mandible, the forlimbs and hindlimbs, the tail, and the main body of the animals.  And based on the location of these scales on the body, their anatomical direction (in relation to the axial midline of the animal), and the scale shape and pattern, he was able to establish a scale morphology that can actually be used to show taxonomic differences between these two very closely related dinosaur species.  Did I mention how cool this is?

Figure 1. Hadrosaur scale morphology showing (A) polygonal basement scales, shield feature-scales, and interstitial-tissues on S. angustirostris, (B) pebbles on S. angustirostris, (C) irregular, radially oriented basement scales on Edmontosaurus annectens, and (D) shell basement scales on S. angustirostris, with a 1 cm scale bar.  Image from Bell, 2012.

This is the first time that dinosaur integument has been used to show a distinction between species.  And Bell's analysis not only shows difference in skin texture across the body, and between the different species.  The patterning and arrangement of integumentary structures can also be used as a potential source to infer additional information on dinosaur color.  This is most evident in the tails of the animals, where S. osborni exhibits a mottled integument, and S. angustirostris exhibits a banded pattern.  So in essence, he took integument from various locations on two different species of Saurolophus...

Figure 12. Regions of skin impressions (light grey) currently known from (A) S. osborni and (B) S. angustirostris.  Image from Bell, 2012.

... and was able to provide enough evidence to produce a pretty impressive, most accurate-to-date reconstruction of of the same two animals.

Figure 13. Soft tissue reconstructions of Saurolophus based on skin impressions from (A) S. osborni and (B) S. angustirostris.  Illustration by L. Xing and Y. Liu, from Bell, 2012.

Cool.  Cool, cool, cool.  There is nothing else to say.  This is great research, and can be applied to other skin impressions, both from other hadrosaurs, and possible dinosaurs at large.  I cant' wait to see what new studies come out of this one.

References
Bell, P. R. 2012. Standardied Terminology and Potential Taxonomic Utility for Hadrosaurid Skin Impressions: A Case Study for Saurolophus from Canada and Mongolia.  PLoS ONE 7(2): e31295. doi:10.1371/journal.pone.0031295