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Re: [dinosaur] 6 month incubation?

From: John Bois <mjohn.bois@gmail.com>

To: dinosaur-l@usc.edu
Sent: Thursday, January 5, 2017 6:29 AM
Subject: [dinosaur] 6 month incubation?

Gregory M. Erickson, Darla K. Zelenitsky, David Ian Kay, and Mark A. Norell (2017)
Dinosaur incubation periods directly determined from growth-line counts in embryonic teeth show reptilian-grade development.
Proceedings of the National Academy of Sciences (advance online publication)
doi: 10.1073/pnas.1613716114

I have several questions regarding assumptions made in this paper...but just one for now: von Ebner lines are assumed to be deposited daily. While circadian rhythms are observed in many animals, this is not universal. In a paper called: Ontogeny of dentinogenesis in the rat incisor...early deposition is ultradian (more than an hour/less than a day) and then after the second to third week after birth, circadian rhythms predominate. This makes sense to me: during gestation there is likely very little periodicity in light reaching the pup. And then, in a dinosaur embryo buried under substrate, light might also be an unreliable cue. And so, since circadian periodicity is apparently not universal in dentin deposition, I'm not sure the estimation of incubation time in this paper is reliable. Any thoughts?


It's certainly interesting that rats have this pattern. The question, though, is whether that pattern is an exception to the rule or not. Erickson ground truthed his assumptions by previously identifying lines of von Ebner deposition in pre and post-hatchling alligators. Those patterns did reflect a daily cycle, as do the patterns of dentine formation in embryonic humans. A brief literature search seems to indicate that ultraradian dentine formation is rare to find compared to circadian-based dentine formation. Though to be fair, there doesn't seem to be that wide a selection of literature out there on embryonic dentine formation.

I would like to see a study or two that looked at pre-hatchling squamate dentine formation, so as to better round all of this out.

Another thing to consider is the shorter days of the Mesozoic (~23 hrs), which produced slightly longer years (~380 days). I don't think Erickson et al. accounted for this (their month estimates suggest they did not), but it would slightly reduce the incubation time (sort of) to closer to 5 months rather than 6 for Hypacrosaurus. Note that there is lots of slop here. The length of a day in the Cretaceous would have been longer than a day in the Triassic, etc.

That said, the authors did make it clear that all of their assumptions were as conservative as possible and that incubation times were likely longer than they have proposed. It will be interesting to see this technique applied to more dinosaur taxa (including some toothed bird taxa), and see just how variable these estimated incubation times were.

http://reptilis.net "I am impressed by the fact that we know less about many modern [reptile] types than we do of many fossil groups." - Alfred S. Romer