Egg shape changes at the theropod bird transition, and amorphometric study of amniote eggs

Abstract

The eggs of amniotes exhibit a remarkable variety of shapes, from spherical to elongate and from symmetrical to asymmetrical. We examine eggshell geometry in a diverse sample of fossil and living amniotes using geometric morphometrics and linear measurements. Our goal is to quantify patterns of morphospace occupation and shape variation in the eggs of recent through to Mesozoic birds (neornithe plus non-neornithe avialans), as well as in eggs attributed to non-avialan theropods. In most amniotes, eggs show significant deviation from sphericity, but departure from symmetry around the equatorial axis is mostly confined to theropods and birds.