What To Do With Crests – Updated


Yesterday — on April 1st, which is about as warning bells as they come — I uploaded a post with a host of new art. This post is as much an admission that that post, somewhat subtly, is a joke. Well, sorta.

You see, it was my intention to use a little parody to poke at how fossils become reconstructed into living animals, the degrees to which some will validate a theory no matter how tenuous the claim is. Comparisons to this include, but aren’t limited to, the “always snarling” theropod, pterosaurs whose wing membranes look exactly as they had died, that the body outline of a flattened animal reflects that animal’s true outline, etc. (I could also say it extends to putting cheeks on dinosaurs or giving theropods croc faces, but I do that so often.) So I’ve annotated the post, shown below, to expose what I’m really thinking here.

[Original post begins, comments in brackets and in bold.]

Over the course of looking at the new specimen of Edmontosaurus (Bell et al., 2014) sporting an odd accretion, I had an epiphany [This should be your first clue this post was going somewhere strange -- aside from the date -- as I don't self-aggrandize myself this way.]: The “cock’s comb” was, in fact, merely a segment of the neck, falsely and purposely isolated by the vagaries of taphonomy. There used to be more of it. But it occurs to me that I was wrong, that there is, in fact, an eboshi-shaped soft-tissue crest, or a “cock’s comb.” (Also, the shape of many eboshi is exactly the shape of the bony crest in several oviraptorids, a useful comparison as they also adorn the head.)

Cock-a-doodle-CowQuite the priestly sort, this one, not the plebian I drew earlier.

[I went with a far more pebbly hadrosaur appearance, to contrast with the first one I did of Edmontosaurus annectens. I wanted to copy some of the reconstructions a bit, and didn't copy the shape of the "comb" as it appears in the fossil, which is rather larger than many artists have shown:

Partial skull and extensive soft-tissue preservation of UALVP 53722, referred to Edmontosaurus regalis, from Bell et al. (2014). Image is a link to a blog published in 2013, when the paper was digitally published the month before.

As you can see, it’s preserved as a sort of bilobed, heart-shaped structure, and really does sort of look like a cock’s comb. Unfortunately, few artists have drawn it this way, choosing to interpret the preservation as irregular, not true to its original form. This begs the question: What degree of interpretation is going on here, and how much of it is being made to support the reconstruction? I rendered this bit of skin and likely flesh as being present, but smoothly flowing into the neck, though I may be wrong. And if the margins of the structure are indistinct to permit a reconstruction that doesn’t bear much similarity to the fossil’s implied shape, if they are incomplete, why restrict the comb to just this portion of skull? If real (and I don’t discount this idea at all) the structure may have more variable distribution or a naturally far more extensive one at the least.]

But as I looked at the analogues for these soft-tissue crest, I came across some interesting dinosaur-like animals. Pterosaurs, as it turns out, often sport these ridiculous head pieces. I’d drawn a few before, and some key features are noticeable:

Part of my [Shown is the short-faced Tapejara wellnhoferi, as you'd know if you clicked the image, which is a link to my dA post about it. What I don't say is that there is a long strut of bone that supports the back end of the crest in tapejarids, one also found in dsungaripterids, that has been argued to present a posterior limit on crest position.]

These skulls have a big block of bone, but only at the front of the head. And along the top of the skull is a thin ridge, running down the length. Some other pterosaurs have one, too, but without the block of bone at the front, and examination under UV light [a technique becoming more and more important when fossils are preserved flat in lagertstätten] reveals that there was a decent amount of soft-tissue forming a parabolic curve up to the back of the head in Pterodactylus and Germanodactylus (Bennett, 2002, 2012).

It's not the size that counts, but how your mate mutually selects to you how to use it.[The above reconstruction is fairly conforming. There's one small catch: I made the crest more bulbous. There is literally no evidence that the actual crest is some thin sheet of whatever.]

But by all accounts, these crests seem fairly thin [in tapejarids, at least]. And I’m not sure why. Are they held up by nothing? Pterosaur wing membranes are stiffened by aktinofibers and by use of the wing finger as a bony strut. But in the tapejarids above, there’s no real clue what the internal structure is. A skin membrane? Keratin plates? Those can get pretty heavy, actually, and as such a softer, more pliable tissue might be more useful. But in Germanodactylus and Pterodactylus, only the bony basal core is present … and in these pterosaurs it is pretty short.

[Bennett (2012) also overturns an argument made by Wellnhofer (1970) that in Pterodactylus there was a long, dense, conical "lappet" or "occipital cone," which had been subsequently supported by UV examination of a private specimen (Frey & Tischlinger, 2000). Subsequent study has supported the presence of a soft-tissue crest extended backwards and above the orbit, and Frey and Tischlinger, in various papers, supported the idea that this "lappet" formed the lower base of the crest, extending backwards from the occiput. They hypothesized this would form the unossified "spur" that in some azhdarchoid pterosaurs would ossify and also form the base of the crest, as in Tapejara above.

However, in no specimen of Pterodactylus does the "occipital cone" or "lappet" contact the remains of the soft-tissue crest above the orbits, and in Germanodactylus (Bennett, 2002) the crest doesn't extend behind the skull at all, so the shape seen above approximates an implied natural shape. More peculiarly is what the "lappet" actually comprises: both Bennett (2012) and Frey and Tischlinger (2000) note the presence of a spiraling series of "fibers" which may have comprised some structure within the "cone." Such a structure seems like its function would be liable to changing shape, bending and unbending, extension or compression, without anything at the other end. Similar spiralling structures formed of collagen are found in erectile tissues (duh, the penis) and impart elastic properties without sacrificing structure, but Bennett (2012) suggests that these "lappets" are formed as tifts of extra-integumental structures, such as filaments like pterofuzz (or pycnofibers).]

But if the bony core at the base of the crest is useful as a support device for a cock’s comb in pterosaurs — as it is clearly analogous to — what about other taxa? So I started digging.

Turns out there’s a few other taxa with relatively high bony crests on their heads, some thin and short, some broad, some very, very high. First, I looked back at hadrosaurs, and it turns out hadrosaurs have a thin bony crest that arches up a bit between the supratemporal fossae. That seems a slam dunk: hadrosaurs with more distinct inter-temporal ridges would sport larger cock’s combs! [Exclamation point? More like rattlesnake point.] Then I looked around and realized I was excluded the “actual” crested hadrosaurs, the lambeosaurs. Turns out they sport the thin ridges, too. [These are between the supratemporal fossae, and relate to the adductor muscles. I'll go deeper into this below.] But not on the backs of their heads — instead, it’s between their noses, and extends all the way backwards. [All true.] Like Pterodactylus and Germanodactylus, the thin ridge is formed almost entirely from the premaxilla! This seems too good to be true. [But it is. What I do with that information, though, is another matter.]

Maybe the ridge helps the hinks resonate ... or whatever.Maybe in Saurolophus here, the rhamphotheca of the snout extended along the whole face and formed a sort of facial plate, as in swamphens (Porphyrio spp.). Some hadrosaurs may have combined the two, a harder facial structure and a softer, floppier comb-like structure at the back. [No, I don't actually think this is likely. I discussed what some, if not all, hadrosaurs were doing with their noses here. Some hadrosaurs are a little trickier, especially the lambeosaurines with the extremely small narial fossae like Parasaurolophus. What they were doing with their fleshy nostrils is {likely} more normal in comparison to the hadrosaurines.]The head fin helps it swim better.The prehistoric world has definitely had some truly monstrous creatures on land and in the sky, but my next trip was into the waters. Dolichorhynchops is a member of the plesiosaurian subgroup of Polycotylidae, all of which sport these tall crests in between their supratemporal fossae. Certainly, these must have been aquatic ebosi-bearers? [I recall a discussion on one of Darren Naish's articles on plesiosaurs (or perhaps some similar forum) where this topic was broached, but I didn't want to dig too deeply as it would ruin the tone, needless to say, these plesiosaurs have inordinately high intertoemporal ridges, so sport high muscle attachment sites, which seems fitting their long, narrow-ish jaws somewhat.] The relatively featureless faces of these plesiosaurs must have needed the added pizzazz [another shake of the rattle: I never use that word] of a crest, for they lack the tremendous neck of the elasmosaurs or the great gaping maws of the pliosaurs (the rhomaleosaurs, which also lacked these things, did not develop the head crest, a result that can only be supported by the presence of another, perhaps flashier ornament [I'm begging you to go along with me here: flashing billboard lights, like the Goodyear blimp, displayed across its sides]).

[Not just polycotylids, either; also, in some parvipelvian ichthyosaurs, the exceptionally short-necked and arched-back kind, including the potential complex of diverse ichthyosaurs named Temnodontosaurus. This feature of the back of the skull, as in many mammals, is generally an attachment site for the temporalis muscles; in sauropsidans, it's the where mandibular adductor muscles attach to the skull, and the height of this crest is both a relation to the increased moment for the muscle, which improves its leverage and lengthens contraction of the muscle, but also helps increase its volume. Muscles can serve more purpose than  merely close or open levers like jaws, and in some animals can be storehouses of mitochondria, improving warming of the body. Useful in some cold-water aquatic animals. Nonetheless, this ridge is found in theropod dinosaurs as well, and in hadrosaurs, but its function as a base for a crest is used here as an extended premise. Some have argued that the thin ridges atop some skulls are the bases of crests, often without soft-tissue to back it up, which is certainly reasonable given where these ridges can appear: at the back of the skull, along the top of the snout, or at the front, all examples appearing in pterosaurs. Applying this logic to other vertebrates suggests homology, but that is clearly not the case.

The tall ridge in ichthyosaurs and plesiosaurs serve an additional function, and this is one you also see in various vertebrates. The high leverage of the occipital muscles attaching to the neck. The ridge in ichthyosaurs increases when the back becomes extremely arched and the neck quite short, which implies the ridge is helping streamline the body and shorten the shoulder-to-head musculature. Save space and energy developing muscles you don't need. Similar structures occur in some fish for very similar reasons.]

Back on land, I realized that there were some lizards with these crests, and they certainly attested to my hypothesis! Not only did chameleons develop these enormous ridges (especially the veiled chameleon, Chameleo calyptratus), but some other iguanians such as Basiliscus as well.

Skulls of various Corytophanidae, © Dr. Raul Diaz, used with permission.

Skulls of various Corytophanidae, © Dr. Raul Diaz, used with permission.

This seems true on the mammal size of tetrapods as well, as many of them — from tiny Oligokyphus to dire wolves (not the George [of R. R. fame, who clearly stole those initials from Tolkien -- that's a joke, by the way, even though we're in bracketland] Martin kind) to gorillas! — have just enormous crests as to leave one assured there is something funky going on. I took a few liberties with the Gorilla, but think I pulled this off:

Do not tell me these horrible things are still around?!

[Just in case you needed me to tell you how ridiculous this argument is. If you had gone through the post and reached this point and still felt I was being serious, this image should have warned you off. Indeed, comments seemed to suggest I may have been believable ... up until this point. Also, I need to work on my mammal drawing.]

Ultimately, this makes me hopeful for reconstructing some of my more favorite animals, such as oviraptorosaurs. [I always know how to bring a subject around to these guys. I can play Six Degrees of Oviraptorosaur's Bacon better than anyone else alive.] As it is, it seems the sky’s the limit. There’s little reason for me to restrict the crest to slight extensions of the borders of bone. Not only does the keratinous casque of cassowaries barely follow the shape of the underlying bone, narrow ridges support wonderfully enlarged crests in tetrapods! So I present as my coup de grâce, a new reconstruction of Anzu wyliei, the oviraptorosaur with the largest known crest.

No mere duck, this -- nor meer duck, really.

[Yes, that's based on a duck. Not the crest, of course. Maybe I can achieve something with this post and get people to call Anzu the "Duck from Hell." Not that ducks from hell isn't already a thing. The bony crest is there alright, but did it support a much, much larger structure? Here is the problem with the implication for crests in a variety of taxa: None of it is based on observations derived from the structure of the bones or homologous tissues in living animals.

When workers refer to "fibrous bone" texture for pterosaur crests, or the nature of the tissues that make it up, they aren't necessarily looking at the direct homology involved when comparing to living animals with soft-tissue crests. I posted the image of the corytophanid lizards for a reason, and the same types of features can be seen in many chameleontids: The bone tissue arises, forms a very thin structure, and is surrounded by a variety of soft-tissues. The nature of these tissues appears to relate to what kind of crest you get. The keratinous casques of many hornbills aren't formed with an entirely bony framework, and the dense cornified tissue that makes up the base of such things in many birds, including cassowaries, have the same structure as the tissues that form the base of rhino horns (Hieronymous et al., 2009). While in some birds the underlying bone can reflect the shape of the protuberances that overly it, in those cases where it doesn't the surface of the bone often tells you that it won't: it will be porous, poorly defined as a "surface," and its composition less solid or smooth as the rest of the skull, especially those parts that only get covered in muscle and skin. The bony crest in Anzu wyliei doesn't look like this and we have relatively little of it, yet it's possible the crest was a little off.

However: Sometimes, a dinner plate is just a dinner plate.]

Bell, P. R., Fanti, D., Currie, P. J. & Arbour, V. A. 2014. A mummified duck-billed dinosaur with a soft-tissue cock’s comb. Current Biology 42 (1): 70-76. (Published online 12 Dec, 2013)
Bennett, S. C. 2002. Soft tissue preservation of the cranial crest of the pterosaur Germanodactylus from Solnhofen. Journal of Vertebrate Paleontology 22 (1): 43-48.
Bennett, S. C. 2013. New information on body size and cranial display structures of Pterodactylus antiquus, with a revision of the genus. Paläontologische Zeitschrift 87 (2): 269-289. doi:10.1007/s12542-012-0159-8
Frey, E. & Tischlinger, H. 2000. Weichteilanatomie der Flugsaurierfüße und Bau der Scheitalkämme: Neue Pterosaurierfunde aus der Solnhofener Schichten (Bayern) und der CratoFormation (Brasilien). Archaeopteryx 18: 1-16. (in German)
Frey, E., Tischlinger, H. Buchy, M.-C. & Martill, D. M. 2003. New specimens of Pterosauria (Reptilia) with soft parts with implications for pterosaurian anatomy and locomotion. pp.233-266 in Buffetaut, E. & Mazin, J.-M. (eds.) Evolution and Palaeobiology of Pterosaurs.
Hieronymus, T. L., Witmer, L. M., Tanke, D. H. & Currie, P. J. 2009. The facial integument of centrosaurine ceratopsids: morphological and histological correlates of novel skin structures. The Anatomical Record 292: 1370-1396. [PDF link]
Wellnhofer, P. 1970. Die Pterodactyloidea (Pterosauria) der Oberjura-Plattenkalke Süddeutschlands [The Pterodactyloidea (Pterosauria) from the Upper Jurassic chalks of southern Germany]. Bayerische Akademie der Wissenschaften, Mathematisch-Wissenschaftlichen Klasse, Abhandlungen 141: 1-133. (in German)

About these ads
This entry was posted in Art, Biology, Paleobiology, Paleontology, Philosophy, Reconstruction and tagged , , . Bookmark the permalink.

2 Responses to What To Do With Crests – Updated

  1. Miss Chili says:

    Great laughs at your entry yesterday, and I’m so glad to be able to tell people about it! Especially about the crests on gorilla skulls — yes, drawing more mammals in future might be a good plan! At any rate, I could tell where the entry was going well before the poor Gorilla gorilla had a large bone inserted under his scalp! :-D

    Oh, on today’s little jab at your readers — “comments seemed to suggest I may have been believable … up until this point” — either you’re not giving us enough credit or we’re giving you too much! Either way, with a laugh at the R.R. initial theft or loan, today’s entry was a treat! :-D

  2. Warren B. says:

    “Indeed, comments seemed to suggest I may have been believable … up until this point.”

    Well, to be fair, half of your previous giveaways needed to be checked against an official list of ‘words Jaime doesn’t use’. :P

    I started wavering after the sudden conformation with ‘mainstream’ soft tissue reconstruction. That was easily checked against the more obvious list of ‘things Jaime doesn’t do’. ;)

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s