The first dinosaur egg remains a mystery

A recent study by Norell et al. (2020) described new egg specimens for two dinosaur species, identified as the first soft-shelled dinosaur eggs. The authors used phylogenetic comparative methods to reconstruct eggshell type in a sample of reptiles, and identified the eggs of dinosaurs and archosaurs as ancestrally soft-shelled, with three independent acquisitions of a hard eggshell among dinosaurs. This result contradicts previous hypotheses of hard-shelled eggs as ancestral to archosaurs and dinosaurs. Here we estimate the ancestral condition for dinosaur and archosaur eggs by reanalyzing the original data from Norell et al. and that from a recent study on reptile eggshells (Legendre et al., 2020) with the addition of these new dinosaur specimens. We show that the recovery of dinosaur eggs as ancestrally soft-shelled is conditioned by the discretization of a continuous character (eggshell thickness), the exclusion of turtle outgroups from the original sample, and a lack of branch length information. When using a larger sample, calibrated trees, and a definition of hard-shelled eggs referencing their unique prismatic structure, we recover dinosaur and archosaur eggs as either hard-shelled or uncertain (i.e. equal probability for hard- and soft-shelled). This remaining ambiguity is due to uncertainty in the assessment of eggshell type in two dinosaur species, i.e. ∼1% of the total sample. We conclude that more reptile egg specimens and a strict comparative framework are necessary to decipher the evolution of dinosaur eggs in a phylogenetic context.