Not quite alike – Changing skull shapes in archeopterodactyloidean ontogeny could point towards heterochony mediated niche differentiation

Archeopterodactyloidea are a monophyletic group of early branching Pterosaurians that likely originated in the early to middle Triassic. They are often characterized by their elongated skull and neck as well as their short tail. Their feeding ecology is a much debated topic, describing them to be anything from insectivores to piscivores. Here we attempt to understand their feeding ecology better by trying to quantify skull shape as a possible ecological proxy. A major focus here are differences between immature and adult specimens of the same species and possible implications for their diet. We used Elliptic Fourier Analysis to quantify pterosaurian skulls outlines as a quantitative representation of overall skull shape. Our results show a varying degree of heterochrony for all four species of archeopterodactyloidean pterosaurians in the analysis. This heterochronic change is largely cause by the elongation of the skull throughout ontogeny. Germanodactylus cristatus shows clear differences between short immature skulls and more elongated adult skulls. In Pterodactylus antiquus these differences become even more pronounced. In Aurorazhdarcho micronyx the difference lessens, with more elongated immature skulls. In Ctenochasma elegans these differences are even less pronounced, with already strongly elongated immature skulls. This could either imply early branching archeopterodactyloideans had some sort of ecological niche differentiation between stages or that they were more nidicolous. This then likely changed through phylogeny with different late branching representatives of archeopterodactyloidea being either less ecologically differentiated between stages or possibly being more nidifugous.