Abstract
Non-avian dinosaurs such as oviraptorosaurs and troodontids share several important reproductive characters with modern birds, including eggshell microstructure and iterative egg production. Nevertheless, debate exists concerning their incubation strategies. Here we estimate incubation period for the troodontid, Troodon formosus, by examining a near-term embryonic tooth. Synchrotron scanning and histologic thin sections allowed counting of daily (von Ebner) growth lines. The tooth preserves 31 intact lines with an average spacing of 3.3 ± 0.96 μm. Adding 8 more for the missing crown tip gives a total age of 39 days. Modern crocodilians begin to establish their functional dentition at approximately 47% through incubation. Thus, this tooth age suggests a Troodon incubation period of 74 days, falling midway between avian (44.4 days) and reptilian (107.3 days) values predicted by the Troodon egg mass (314 g). An accelerated incubation relative to modern reptiles supports brooding and concurs with a suite of features in oviraptorosaurs and troodontids (sequential laying, large complex clutches, and precocial young) that appear dependent upon both adult body and incubation temperatures elevated over ambient conditions. However, the largely buried condition of Troodon clutches may have prohibited efficient brooding, necessitating longer incubation than that of modern birds with fully exposed eggs.
Highlights
Much debate exists concerning incubation strategies in these non-avian dinosaurs and whether it is homologous to that of modern birds
Based on daily incremental lines, they aged embryonic teeth in two dinosaur taxa and calculated their incubation periods based on estimates of the establishment of hatchling functional dentitions
The predicted values are more similar, 111% and 83%, respectively, to incubation periods estimated by a modern reptilian model[31]
Summary
Much debate exists concerning incubation strategies in these non-avian dinosaurs and whether it is homologous to that of modern birds. Based on daily incremental lines (von Ebner lines), they aged embryonic teeth in two dinosaur taxa and calculated their incubation periods based on estimates of the establishment of hatchling functional dentitions Both species examined, Hypacrosaurus and Protoceratops, exhibited long, reptile-grade incubation periods[30] with the estimated values 209% and 208%, respectively, of the incubation period predicted for an avian egg of similar size using the regression equation of Deeming et al.[31]. The predicted values are more similar, 111% and 83%, respectively, to incubation periods estimated by a modern reptilian model[31] We apply these embryonic aging methods[28,29,30] to test the incubation strategy of the troodontid, Troodon formosus, a dinosaur, like oviraptorosaurs, hypothesized to have a reproductive mode (Fig. 1D) intermediate between reptiles with fully buried clutches produced en masse and modern birds with iteratively produced eggs incubated free of sediment[12,34]. Using a Troodon egg mass of 314 g20 and equations of Deeming et al.[31], Troodon would be expected to have an incubation period of 107.3 days with reptilian soil burial but only 44.4 days for avian sediment-free brooding
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