Ornithischian incubation longer and relatively longer than bird incubation

A new paper by Erickson et al. 2017 reports:
“Birds stand out from other egg-laying amniotes by producing relatively small numbers of large eggs with very short incubation periods (average 11–85 d). Here, nonavian dinosaurian incubation periods in both small and large ornithischian taxa (Protoceratops and Hypacrosaurus) are empirically determined through growth-line counts in embryonic teeth. Our results show unexpectedly slow incubation (2.8 and 5.8 mo) like those of outgroup reptiles.”

Now let’s do the math:
2.8 mo @ 30 days/month = 84 d. Hey! That’s one less than the upper limit in brds! 5.8 mo = 178 days (a few 31 day months added). Actual figures are 83 d for Protoceratops. 172 d for the much larger Hypacrosaurus. At this point, let’s remind ourselves that larger mammals have larger gestation/incubation times, too. And it’s also important to note that no theropod eggs were tested. Oviraptor embryos have no teeth.

Now let’s see some details
Comparison of Protoceratops incubation period relative to that typical for birds with same-sized eggs shows greater than twofold slower values (83.16 vs. 39.72 d). Relative to that typical for reptiles Protoceratops was modestly faster values (∼17%, 83.16 vs. 100.40 d) than predicted for typical reptiles.

Comparison of Hypacrosaurus incubation period relative to that typical for birds with same-sized eggs shows greater than twofold slower values (171.47 vs. 81.54 d). Relative to that typical for reptiles Hypacrosaurus was modestly faster values (∼12%, 171.47 vs. 153.72 d) than predicted for typical reptiles.

Phylogenetically 
all phytodinosaurs, including Ornithischia, are about as distant from birds as are the crocs, which are also proximal outgroups to the Theropoda in the LRT, contra many other studies that nest crocs much more distantly.

Figure 1 Full chart from Erickson et al. 2017. See figure 2 for details.

Figure 1 Full chart from Erickson et al. 2017. See figure 2 for details.

If you’re curious
The ostrich (Struthio) egg is not listed in the Erickson chart. Ostrich eggs are the largest of all birds, but the smallest bird eggs in relation to the adult bird’s size. Their incubation range is well within the Ercikson bird cloud.

Figure 2. Closeup of figure 1. In both cases Struthio was added.

Figure 2. Closeup of figure 1. In both cases Struthio was added. Two ornithischian dinos are shown. No theropods are shown. Cd – crocs.

The slowest incubation period among birds,
is among the Procellariformes, a clade of seabirds including the albatrosses and petrels. Not the chart above it’s blue and labeled Pr. See how closely it comes to the Protoceratops icon?

Oddly, turtles
have a relatively faster incubation time than do lizards and crocs.

Apparently no data yet on theropod dinosaur embryo teeth.
I’m sure that’s where it gets even more interesting (i.e. closer to birds).

References
Erickson GM, Zelenitsky DK, Kay DI, and Norell MA 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  PDF

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