This is the second in a series of posts in which I review the Sideshow Collectibles Apatosaurus maquette. The rest of the series:

• Part 1: introduction
• Part 3: the neck
• Part 4: body, tail, limbs, base, and skull
• Part 5: posture
• Part 6: texture and color
• Part 7: verdict

First, a note on the photos. There a few minute white flecks on the head in the pictures. These are near-microscopic pieces of styrofoam packing material, which I only discovered after I’d shot the photos–they are, seriously, too small to be noticed otherwise. Just be aware that they are not flaws in the paint. The whole head, from external ear to snout tip, is 35 mm long, which gives you some idea of the quality of the sculpting and painting. The entire maquette is detailed to the same degree.

The general form and proportions are a good match for the skull of Apatosaurus. In particular, the head is roughly rectangular in dorsal view, with a very squared-off snout. Among extant animals, square snouts are typically found among grazers, and grazing on low-growing vegetation has been suggested for diplodocids as well (Stevens and Parrish 1999, Whitlock 2011). It is worth keeping mind, however, that anatomy is not destiny (Smith and Redford 1990); the behavior of living animals is often more varied than their skeletal form might suggest, and in some cases morphological specialization can lead to ecological generalization.

In a paper with direct relevance to grazing and browsing, Feranec (2003: 230) analyzed the diet of the Pleistocene camel Hemiauchenia. He found that “hypsodonty is not strictly associated with obligate grazing; instead it may, in this case, represent an adaptation to widen niche breadth that allowed grazing as well as browsing.” In other words, the tall, long-wearing (hypsodont) teeth necessary for eating tough grass do not prevent hypsodont herbivores from browsing on softer vegetation as well, whereas committed browsers with lower tooth crowns would have a harder time dealing with tough, abrasive grasses.

Sauropods didn’t chew their food, so tall grinding molars are beside the point, but snout shape is not. It is possible that a broad snout widened the niche breadth of diplodocids to allow both grazing and browsing, whereas a narrow-snouted sauropod like Camarasaurus would probably have made a poor grazer. I’m not discounting the hypothesis that diplodocids were partially or even predominantly grazing animals–in particular, it would help make sense of Nigersaurus, which seems to have taken the grazing adaptations seen in other diplodocoids to an extreme. Just pointing out that certain kinds of morphological specializations broaden, rather than narrow, the ecological opportunities of the animals that bear them. I should also point out that Whitlock’s (2011) analysis did not rely on muzzle shape alone but also on some interesting tooth microwear data. That paper is well worth reading, and happily it’s free, so go read it if you haven’t.

Back to the maquette. Several issues of the soft tissues of the head deserve comment.

The nostrils are down near the end of the snout, as predicted by Witmer’s (2001) work on nostril position in extant vertebrates. I know that some people are skeptical about the nostril position in dinosaurs hypothesized by Witmer, but it makes good sense to me. First, in formulating the hypothesis, Witmer did something that none of his critics have done, which is actually establish the nostril position in a wide range of extant animals. By itself, this doesn’t show what the nostril position in dinosaurs must have been, but it establishes a null hypothesis, which should only be discarded if there is compelling evidence to the contrary. And the scarcity of counterexamples among extant vertebrates constitutes a second, normative argument: if the default nostril position was an easy constraint to break, we’d expect to see more taxa that have broken it. (Both of these arguments also apply to the alert neck posture of tetrapods, by the way.)

Second, Witmer’s hypothesis has explanatory power: it makes sense of the troughs and tracks in front of the nostrils in sauropods with retracted nares. These tracks are most clearly expressed in the skull of Giraffatitan (see, e.g., the images at the top of this post), but they are present in other sauropods as well, like the Denver museum Brachiosaurus sp. skull shown above. Witmer’s hypothesis of nostril position made good sense to me because of my experience working on postcranial pneumaticity in sauropods. External pneumatic traces on sauropod vertebrae often consist of pneumatic foramina set inside larger pneumatic fossae (see, for example, this, from here). Similarly, the bony nares of sauropods can be thought of as pneumatic foramina set at the posterior end of the pneumatic fossae formed by the troughs and tracks on the snouts.

One last thing on nostril position in dinosaurs. I’ve seen people argue that terminal nostrils would have been bad for dinosaurs (especially carnivores) because they would have gotten poked by vegetation or fouled with food. To which I can only say, good people, stop trying to figure out dinosaurs from first principles and just look at live animals.

Next item: the teeth are covered by fleshy lips. The hypothesis that some dinosaurs had lips is not new, but it hadn’t received much technical attention until recently. Enter Ashley Morhardt (research page, blog). For her MS work under Matt Bonnan (research page, blog) at Western Illinois University, she did something that no one had done before: she counted nutrient foramina (blood vessel holes) in the jawbones of extant vertebrates and related foramina counts to the kinds of soft tissues the jaws supported: marginal scales, muscular lips, beaks, and so on. Then she looked at dinosaurs and applied what she’d learned. That work is still on the road to publication, so I won’t give away the game. But I did ask Ashley specifically about the plausibility of the lips in the Apatosaurus maquette, and she was kind enough to share her thoughts. She writes (with permission to cite):

The foramina present at the margin of Apatosaurus‘ mouth are more similar in relative size, shape, and distribution to those of crocodylians than those of mammals. … A conservative EPB approach would shy away from reconstruction that might include any type of fleshy seal at the oral margin.  This would leave the teeth bare and the posterior margin of the mouth covered in skin without any overhanging scales. … The current maquette is gorgeous, but potentially incorrect.

Ashley is now working on her PhD with Larry Witmer (research page, blog) at the University of Ohio, and we can surely expect more cool science from her in the future. Please also note that the question of dinosaur lips was recently the subject of a long, thoughtful post by Jaime Headden.

Since this post involves soft tissues of sauropod heads, I’m contractually obligated to point out that, like the maquette, real sauropods didn’t have trunks.

Finally, I’m happy to say that the head avoids shrink-wrapped dinosaur syndrome. There’s enough underlying anatomy to show that It’s built up from the skull of Apatosaurus, but you can’t see every little ridge and divot in the skull (nor should you). And the soft tissues are plausible and detailed, so the head doesn’t just look like a smooth bullet of meat. And the sculpting itself is detailed enough to support close examination. All of these are big pluses, even if the lips are a (small) step beyond what our current understanding will support.