The two terms of the title are often used somewhat interchangeably (and I’ve been guilty of that myself in the past) but in fact they have rather different meanings, despite their close relationship. Here is a series of fossilised crocodile osteoderms that I picked up a few years back. The term osteoderm literally means bones in the skin and that’s exactly what these are, piece of bony armour that sit in the skin.
Scutes on the other hand are the keratinous sheathes that cover the osteoderms. You can see some of these here in this croc with those bigger chunky scale-like pieces running along the back. That of course means all those fossil ‘scutes’ from things like crocs and ankylosaurs and titanosaurs should really be called osteoderms. In fact while of course claws and similar things do preserve from time to time, and there are bits of horn sheathes etc. out there, I’m not aware of any scutes being known from the Mesozoic (though not working on any of the more obvious groups, I could easily have missed them).
Dave Hone's Archosaur Musings 
“Scutes on the other hand are the keratinous sheathes that cover the scutes.”
err… do you mean “that cover the osteoderms”?
I told you I get them mixed up too! 😉 I’ll go back and edit that!
And what is the difference between scutes and the varies types of epidermal appendages subsumed under the term “scale”?
Well they are bigger and thicker, but whether they truly come under scales or not I’m not sure. Sadly I’ve not got hold of one to dissect and checking up on scute definitions, it was never made clear if they were ‘just’ large robust scales that covered the osteoderms, or a specific additional structure attached to the scale. I think the former, but I can’t be entirely sure.
Didn’t know about this distinction before. Interesting!
Scutes on the other hand are the keratinous sheathes that cover the osteoderms.
Um…what’s the source on that? Not that I’m necessarily disagreeing; I just want to know where this definition is set down.
I can’t tell you off the top of my head Jerry (not on a Sunday anyway). Corwin Sullivan put me onto it, and I did look into it and found a couple of places that did explicitly state this. I’ll have a dig back through my stuff next week.
Ha! I’m as guilty as the next man for using both terms for the same thing. Thanks for clearing that up Dave.
This is why I’ve gotten away from using the word scutes for aetosaur osteoderms in my last few papers.
Thanks for bringing this to light Dave.
I looked it up in
Francillon-Vieillot, H., de Buffrénil, V., Castanet, J., Géraudie, J., Meunier, F. J., Sire, J. Y., Zylberberg, L. and de Ricqlès, A. 1990. Microstructure and mineralization of vertebrate skeletal tissues. In: Carter, J. G. (Ed.): Skeletal Biomineralization: Patterns, Process and Evolutionary Trends, pp. 471–548. Van Nostrand Reinhold, New York.
a standard work on vertebrate skeletal tissues, see p. 486:
“Scutes (= bony plates, transformed scales”) are dermal sclerifications composed of osseous tissue which shows a great diversity of shape and organization [… various examples for fish scutes follow …] Precise homologies between scutes and elasmoid scales are generally unknown […].
Among tetrapods, ‘scutes’ or ‘osteoscutes’ occur as bony plates in the skin of reptiles and mammals. Osteoscutes are thick bony plates in the dermis, and this term is to some extent used as an antonym of ‘corneoscutes’ (keratinous thick epidermal scales). See also osteoderms.”
Ergo osteoderms = (osteo)scutes and thick keratinous scales = corneoscutes (more explicit term for the type of skin covering referred to as ‘scutes’ in the blog post).
So, according to the ref supplied by Michael, you could use “scutes” to refer to both osteoderms and their keratinous coverings. Just prefix bony/osteo- or keratinous/corneo- if you need to be specific.
That’s interesting. I will have to dig through my books again and see where I did find this. At the lest this sort-of confrims that the two are no quite synonymous and that ‘osteoderm’ is more specific and less ambiguous.
Might osteoderm be restricted to the fossilized remains of the structure while osteoscute is the entire piece of integument, bone plus keratin (plus connections to the rest of the body…)?
Or just call ’em skin bones, skin bones, dem skin bones.
My understanding–and use–has always been that “osteoderm” was a category encompassing any sort of bone growing in the skin, and “scute” was simply one of the subsets: any of the generally flat or arched, occasionally keeled osteoderms. “Scutes” are a kind of osteoderm, along with “plates,” “spikes,” and “ossicles.”
Sorry I was late getting back to this. In my copy of Kardong’s ‘Vertebrates’ it says “If the epidermal scale is large and platelike it is sometimes termed a scute” and an osteoderm is “a dermal bone under and supporting an epidermal scale” which pretty much fits my above definition (though I’m really sure that wasn’t the book I check this in earlier).
Still Michael’s comment does allow for something similar (if with qualifications to the use of ‘scute’ on it’s own). So there might be other variations out there, though of course common use is not necessarily the same as correct use.
‘Osteoderm’ does certainly not refer to any kind of bone growing in the skin:
Dermal bones of the skull roof and shoulder girdle, which may show an ostoderm-like outer sculpture/ornamentation in basal tetrapods (see e.g. F. Witzmann, H. Scholz, J. Müller & N. Kardjilov. 2010. Sculpture and vascularization of dermal bones, and the implications for the physiology of basal tetrapods. Zoological Journal of the Linnean Society 160: 302-340.) are not considered as osteoderms.
Turtle plastron and carapace, whose derivation from osteoderms is controversial (see W. G. Joyce, S. G. Lucas, T. M. Scheyer, A. B. Heckert, A. P. Hunt. 2009. A thin-shelled reptile from the Late Triassic of North America and the origin of the turtle shell. Proceedings of the Royal Society B 276: 507–513
versus
S. F. Gilbert, G. A. Loredo, A. Brukman, A. C. Burke. 2001. Morphogenesis of the turtle shell: the development of a novel structure in tetrapod evolution. Evolution & Development 3: 47–58) are usually not regarded as osteoderms or consisting of osteoderms.
It is unclear to what degree the cranial “epi-ossifications” of ceratopsids are similar to osteoderms (see J. H. Horner, M. B. Goodwin. 2008. Ontogeny of cranial epi-ossifications in Triceratops. Journal of Vertebrate Paleontology 28: 134–144;
T. L. Hieronymus, L. M. Witmer, D. H. Tanke, P. J. Currie. 2009. The facial integument of centrosaurine ceratopsids: morphological and histological correlates of novel skin structures. The anatomical record 292: 1370–1396)
and thus they are usually not referred to as osteoderms.
There is another type of dermal ossification in basal tetrapods which is referred to as ‘scales’ and considered to be homologous to similar scales of tetrapodomorph fish (see F. Witzmann. 2007. The evolution of the scalation pattern in temnospondyl amphibians. Zoological Journal of the Linnean Society 150: 815–834). These scales (which have nothing to do with keratinous epidermal scales) are usually not considered as osteoderms (but see M. K. Vickaryous & J.-Y. Sire. 2009. The integumentary skeleton of tetrapods: origin, evolution, and development. Journal of Anatomy 214: 441–464).
“‘Osteoderm’ does certainly not refer to any kind of bone growing in the skin:”
But I don’t think I ever actually said that. I said they grown in the skin yes, but not ‘any’ bone in the skin. Dermal bones are obviously derived as you describe, but that doesn’t make them an osteoderm (though an osteoderm is a type of dermal bone). I’ve yet to see the term used for something that isn’t a piece of armour or armour-like structure. This can fuse to other bones, or each other, but that’s not the same as dermal bone.
My comment was meant as a reply to J. D. Harris. (I wrote this comment before I saw your answer.) Best Regards.
Ah right, sorry! Such is the problem with comment threads!
Sorry, that was my bad! I should have been more specific, too. I of course didn’t mean dermal bone. The way it’s always worked in my mind–not necessarily correctly, mind you, in the absence of any solid definition–is that osteoderms are bones that grow in the skin but that (a) don’t have much of a surface expression (they’re not covered by muscle), and (b) do not have direct articulations to other, underlying bones. That is, the underlying bones don’t have distinct facets on them that receive the osteoderms. Hence, ankylosaur scutes and spikes, stegosaur plates, spikes, and ossicles, crocodylian scutes, the ossicles of several lizards, etc., are all osteoderms. Of course, things like turtle carapaces are problematic because they do seem to articulate with the ribs, but as Michael noted, they’re problematic anyway!
…yes, the term ‘osteoderm’ appears to be defined by what it is not: “Osteoderms are bones that form in the skin but not … “.
I don’t know whether your point “(b) do not have direct articulations to other, underlying bones” is true for all kinds of archosaur osteoderm systems, but the osteoderms of some basal tetrapods, namely dissorophid temnospondyls, the plagiosaurid temnospondyl Plagiosuchus and chroniosuchian reptiliomorphs have either a joint or a suture or are co-ossified with adjacent trunk vertebrae (in Plagiosuchus and chroniosuchians this connection is realized by a deep-reaching osteoderm keel).
I found this thread through Google after searching for the term “osteoscute” which I only recently came across through a wikipedia entry for crocodilia. I understood osteoderms are covered by keratinous scutes (corneoscutes). But the wikipedia entry implies there is a distinction between osteoderms and osteoscutes… confusing.
Here is the wikipedia excerpt: “The rows of scutes cover the crocodilian’s body from head to tail, forming a tough protective armor. Beneath the scales and osteoderms is another layer of armor, both strong and flexible and built of rows of bony, overlapping shingles called osteoscutes, which are embedded in the animal’s back tissue.”
I’ve searched for a reference to uncover whether there are two (osteoderm + corneoscutes) or three (osteoscute, osteoderm, corneoscute) layers, but no results as yet.