Revisiting an age-old question: How to determine developmental age in fossil vertebrates?

How do we determine the ontogenetic (developmental) age of a fossil vertebrate, like a dinosaur? It ain’t easy. One of the best methods is thin-sectioning long bones and counting the growth rings inside, akin to determining the age a tree. However, this isn’t always possible for a variety of reasons, including the unavailability of appropriate fossil material or the prohibition of destructive analysis (museums tend not to like their fossils destroyed). Therefore, we often must turn to other proxies to estimate the relative maturity of our fossils. These proxies might include things like size, bone surface texture, development of sociosexual signals (like horns or crests), or the fusion of adjacent bones in the skull and spine (Hone et al., 2016).

         The last criterion is commonly used by paleontologists (e.g., Bakker et al., 1988; Sereno et al., 2009; Mallon et al., 2015). As the thinking goes, as individual vertebrate animals age, they tend to fuse up the bones in their skull and spine, so that the sutures between adjacent bones become obliterated by adulthood. We’re familiar with this phenomenon in human babies: the “soft spot” on babies’ heads represents the unfused junction between the frontal and parietal bones of the skull, which closes around one year of age. Until recently, the same principle has been widely applied to the skulls of dinosaurs and other fossil vertebrates, but recent findings by Bailleul et al. (2016) must give us pause for thought.

Skull bone fusion in the emu (Dromaius novaehollandiae). Note that as skull length increases in progressively older individuals, the average degree of fusion between the individual skull bones increases, too. (Image from Bailleul et al., 2016).
         In a recently published paper, the authors carefully document the pattern of skull bone fusion in both the emu and American alligator. The authors found that, although emu skulls follow the expected pattern of sutures closing with age, those of alligators do not. Instead, alligators show the opposite trend, whereby the skulls are highly integrated very early on, only to have the sutures between adjacent bones re-open as growth continues. Why this disaggregation of the alligator skull happens is unknown, but Bailleul et al. (2016) suggest that it might facilitate the distribution of stresses throughout the skull as older animals achieve increasingly more forceful bites.

Skull bone fusion in the American alligator (Alligator mississippiensis). Note that as skull length increases in progressively older individuals, the average degree of fusion between the individual skull bones actually decreases, unlike in the emu. This may have some serious implications for dinosaur growth models. (Image from Bailleul et al., 2016).

         What implication does this have for the relative aging dinosaurs? Next to birds, alligators and other crocodylians are the closest relatives of dinosaurs. Therefore, the fact that alligators do not fuse their skulls with age implies that maybe some dinosaurs didn’t, either. If not, there remains a distinct possibility that some of our growth models are wrong. Those growth models relying solely on the skull fusion criterion are most suspect, in which case, it’s worth going back to test them again using the other maturity proxies given above. So goes science.

Posted by: Jordan Mallon, Canadian Museum of Nature
Literature cited

Bailleul, A.M., Scannella, J.B., Horner, J.R., and Evans, D.C., 2016. Fusion patterns in the
skulls of modern archosaurs reveal that sutures are ambiguous maturity indicators for the Dinosauria. PLOS ONE, 11(2), p.eIm0147687.

Bakker, R.T. Williams, M., and Currie, P.J., 1988. Nanotyrannus, a new genus of pygmy
tyrannosaur, from the latest Cretaceous of Montana. Hunteria, 1(5), p.1-30.

Hone, D.W., Farke, A.A., and Wedel, M.J., 2016. Ontogeny and the fossil record: what, if
anything, is an adult dinosaur? Biology Letters, 12(2), p.20150947.

Mallon, J.C., Ryan, M.J., and Campbell, J.A., 2015. Skull ontogeny in Arrhinoceratops
brachyops (Ornithischia: Ceratopsidae) and other horned dinosaurs. Zoological Journal of the Linnean Society, 175(4), pp.910-929.

Sereno, P.C., Tan, L., Brusatte, S.L., Kriegstein, H.J., Zhao, X., and Cloward, K., 2009.
Tyrannosaurid skeletal design first evolved at small body size. Science, 326(5951), pp.418-422.
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