One of the most fundamental aspects of taphonomy is that of "taphonomic loss" - the loss of information through taphonomic processes. Perhaps the most common form of taphonomic loss is loss of the actual fossils, including organisms whose remains become fossilized but never found, and those organisms whose remains are never even fossilized. A recurring theme in vertebrate taphonomy is that not all bones are equal, even within the same animal. It's not a particularly insightful rule, but it has plenty of consequences.
Sauropods are an especially popular taxa of dinosaurs, famous for attaining body sizes unparalleled among other land animals. Not surprisingly, this leads to an extreme variety in bone size. As the image on the left illustrates (from here), it is not uncommon for sauropod limb bones to reach or exceed the size of an average truck driver. But also worth noting is the skull of the bizarre sauropod Nigersaurus to the left (from here), with the head of discoverer Paul Sereno to scale. For their size, sauropods had small heads, and some of their cranial bones were no larger or stronger than ours (non-homologously, of course). In fact, some of the cranial bones of the pictured Nigersaurus are "as thin as a paper plate." For the same reason you don't wash your lingerie with your denim coveralls, it's rare for a taphonomic mode to preserve both delicate cranial material and limb bones the size of tree trunks. Consequently, sauropod skulls remain a relatively rare find - the "recent" discovery of two complete sauropod skulls in Utah, for instance, was significant enough to make headlines.
Most fossil bone, as it turns out, hasn't just been converted into a bone-shaped rock. Original bone and cellular structure is often preserved, and cutting the bone into thin sections (thin: 20-120 micrometers) can make these features visible, as seen to the left, above (image from here). This is the basis behind the field of paleohistology. One such structure is "Sharpey's fibers": these are the numerous dark, sub-horizontal "dashes" to the left, below (titanosaur scute, from Chinsamy-Turan, 2005). Sharpey's fibers are basically the remnants of collagen fibers where some structure (tendon, skin, horn, etc) was anchored to the bone. If an animal had, for instance, a trunk, there would be abundant Sharpey's fibers in the bones where the "trunk muscles" were anchored.
After all, Sharpey's fibers are fairly easy to identify - even a grad student could handle this type of hypothesis testing. Unfortunately, my proposal has its own flaws. Even after making the thin sections, there's no guarantee the histological features won't have been obliterated by other processes (bacterial invasion, permineralization). And as it turns out, bacterial invasion is particularly common in cranial bones (Horner, personal comm). You may also remember my comment about how sauropod skull material is "rare," and how making thin sections involves "cutting." Not surprisingly, those repositories lucky enough to house sauropod cranial material are reluctant enough to allow access to it, let alone hand it out to be sliced up like deli meat, with the chance that there will be no appreciable data gained in the end.
If it weren't attached...
I've heard or overheard comments from several invertebrate paleontologists, and even an invertebrate taphonomist or two, on the tendency of dinosaur fossils to be found without any cranial material. It's a understandable impression to develop, but it actually sells many dinosaur taxa short. Among ceratopsians ("horned dinosaurs") and pachycephalosaurs ("dome-headed dinosaurs"), cranial material can be the most common material found, and according to Horner et al (2004), "[c]omplete skulls are known for nearly all of the genera and species of hadrosaurids [ed-"duckbilled dinosaurs"], and virtually every established hadrosaurid taxon is based on at least some cranial material." (one catch to the latter is the fact that many hadrosaurid taxa are identified predominantly by cranial features, but regardless...) One dinosaur taxa that is known for a lack of cranial material, however, is the sauropods.
Sauropods are an especially popular taxa of dinosaurs, famous for attaining body sizes unparalleled among other land animals. Not surprisingly, this leads to an extreme variety in bone size. As the image on the left illustrates (from here), it is not uncommon for sauropod limb bones to reach or exceed the size of an average truck driver. But also worth noting is the skull of the bizarre sauropod Nigersaurus to the left (from here), with the head of discoverer Paul Sereno to scale. For their size, sauropods had small heads, and some of their cranial bones were no larger or stronger than ours (non-homologously, of course). In fact, some of the cranial bones of the pictured Nigersaurus are "as thin as a paper plate." For the same reason you don't wash your lingerie with your denim coveralls, it's rare for a taphonomic mode to preserve both delicate cranial material and limb bones the size of tree trunks. Consequently, sauropod skulls remain a relatively rare find - the "recent" discovery of two complete sauropod skulls in Utah, for instance, was significant enough to make headlines.Definitely not ready to travel
Sauropods are remarkable animals for a variety of reasons, some of which are more infamous than famous. One reason which certainly qualifies as one of the former is the hypothesis that sauropods had trunks like modern elephants. There are many reasons not to believe this notion, and accomplished Scienceblogger Darren Naish has done as excellent job summarizing these reasons here. However, I would like to expand on one line of attack Naish presents. In discussing the cranial musculature associated with trunks, Naish points out:
[B]ig, strong muscles like those involved in any hypothetical trunk leave visible attachment sites, such as crests, scars, or fossae. These sorts of structures are obvious in extant trunked mammals... They are entirely absent in the skulls of sauropods.
Which is entirely true, but...these features might be lost or difficult to identify if the bones are poorly preserved or exhibit significant surface alteration. Fortunately, there is also associated evidence inside the bone.
Most fossil bone, as it turns out, hasn't just been converted into a bone-shaped rock. Original bone and cellular structure is often preserved, and cutting the bone into thin sections (thin: 20-120 micrometers) can make these features visible, as seen to the left, above (image from here). This is the basis behind the field of paleohistology. One such structure is "Sharpey's fibers": these are the numerous dark, sub-horizontal "dashes" to the left, below (titanosaur scute, from Chinsamy-Turan, 2005). Sharpey's fibers are basically the remnants of collagen fibers where some structure (tendon, skin, horn, etc) was anchored to the bone. If an animal had, for instance, a trunk, there would be abundant Sharpey's fibers in the bones where the "trunk muscles" were anchored.What are we waiting for?
After all, Sharpey's fibers are fairly easy to identify - even a grad student could handle this type of hypothesis testing. Unfortunately, my proposal has its own flaws. Even after making the thin sections, there's no guarantee the histological features won't have been obliterated by other processes (bacterial invasion, permineralization). And as it turns out, bacterial invasion is particularly common in cranial bones (Horner, personal comm). You may also remember my comment about how sauropod skull material is "rare," and how making thin sections involves "cutting." Not surprisingly, those repositories lucky enough to house sauropod cranial material are reluctant enough to allow access to it, let alone hand it out to be sliced up like deli meat, with the chance that there will be no appreciable data gained in the end.
"Hey, thanks for letting us borrow your horse! Unfortunately, all of our tests were inconclusive..."
REFS--
Chinsamy-Turan, A. 2005. The microstructure of dinosaur bone: deciphering biology with fine-scale techniques, Johns Hopkins University Press, Baltimore: 216 p.
Horner, J.R., Weishampel, D.B., and Forster, C.A. 2004. Hadrosauridae, in Weishampel, D.B., Dodson, P., and Osmólska, H., eds, The Dinosauria, 2nd edition, University of California Press, Berkeley: pp. 438-463.
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