Abstract
Dinosaur embryos are among the rarest of fossils, yet they provide a unique window into the palaeobiology of these animals. Estimating the developmental stage of dinosaur embryos is hindered by the lack of a quantitative method for age determination, by the scarcity of material, and by the difficulty in visualizing that material. Here we present the results of a broad inquiry, using 3D reconstructions from X-ray computed tomography data, into cranial ossification sequences in extant saurian taxa and in well-preserved embryos of the early branching sauropodomorph dinosaur Massospondylus carinatus. Our findings support deep-time conservation of cranial ossification sequences in saurians including dinosaurs, allowing us to develop a new method for estimating the relative developmental percentage of embryos from that clade. We also observe null-generation teeth in the Massospondylus carinatus embryos which get resorbed or shed before hatching, similar to those of geckos. These lines of evidence allow us to confidently estimate that the Massospondylus carinatus embryos are only approximately 60% through their incubation period, much younger than previously hypothesized. The overall consistency of our results with those of living saurians indicates that they can be generalized to other extinct members of that lineage, and therefore our method provides an independent means of assessing the developmental stage of extinct, in-ovo saurians.
Highlights
Dinosaur embryos are among the rarest of fossils, yet they provide a unique window into the palaeobiology of these animals
We develop a numerical method for coding the ossification stage of each bone in each specimen, and use a dissimilarity matrix to assess the relative developmental percentage of the Massospondylus carinatus embryos
The best exemplars of each bone in the Massospondylus carinatus embryos were extracted as surface mesh (.stl) files and combined in order to reconstruct an articulated skull for visualization purposes (Fig. 1)
Summary
Dinosaur embryos are among the rarest of fossils, yet they provide a unique window into the palaeobiology of these animals. Our findings support deep-time conservation of cranial ossification sequences in saurians including dinosaurs, allowing us to develop a new method for estimating the relative developmental percentage of embryos from that clade. Recent research has shown that the shape of the bird skull can be explained by paedomorphic retention of juvenile, non-avian dinosaur features[4] Despite these findings, little is known about the cranial ossification sequence of dinosaurian embryos, and how this sequence compares to other saurians[8,9]. In non-avian reptiles, there is very slight interspecific and intraspecific variability in both the pattern and timing of cranial ossification[13,14,18,19,20,21,26] Understanding these ossification sequences as well as the phylogenetic relationships between the taxa is important for the clarification of heterochronic processes in macroevolution. The degree of sutural closure is difficult to assess in very immature specimens such as embryos
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