Morphometric analysis of dinosaur eggshells: constraints of size on shape

Studies on fossil eggs and eggshells have been traditionally focused on analysis of microstructure and systematic classification, but few attempts have been made to examine the variability of eggshell characters. This work examines the variability of one diagnostic character, shape of the calcitic units, of some dinosaur eggshells by using outline shape analysis (elliptic Fourier analysis). The studied material belongs to five different oospecies of the parafamily Megaloolithidae Zhao, 1979. To test shape variation within and between oospecies, an analysis on 129 outlines was carried out and their variability was summarised by principal component analysis. We then tested if the shape of megaloolithid units changes according to size (i.e. allometry). A regression analysis between eggshell unit shape and two different size measurements (centroid size and eggshell thickness) showed that the thinnest eggshells (thickness ≤ 1 mm) have different shapes than thicker ones ( ≥ 2.5 mm). Our results stress that the variability in the shape of eggshell units in Megaloolithus oospecies is correlated to size (i.e. eggshell thickness), a phenomenon that should be then considered carefully when determining oospecies, since a single oospecies can show both a wide range of eggshell thickness and combined with different eggshell unit shapes.