Evaluating variation in Solnhofen avialans

As the oldest known fossil bird, Archaeopteryx is pivotal to the study of avian origins. Fifteen avialan fossils have been described from the Upper Jurassic Solnhofen Limestones, 14 of which have been referred to Archaeopteryx. Recently, this sample has been hypothesized to include archaeopterygids, non-archaeopterygid avians and anchiornithines, although interpretations concerning newly erected non-archaeopterygid avialans have not been critically examined, and currently, there is no taxonomic consensus. Morphometrics, as a powerful tool for understanding variation within a sample, can recognize non-random patterns that can support taxonomic hypotheses. Here, we analyse linear skeletal measurements from all Solnhofen avialans and compare them to the non-avian avialan Anchiornis. Results strongly suggest all Solnhofen avialans belong to a single taxon conforming to a growth curve. If more than one taxon is represented within the sample, these taxa are proportionately indistinguishable when growth is considered. Results also indicate that, while the forelimb becomes proportionately longer with increasing maturity, the opposite pattern is observed in Anchiornis, supporting interpretations that anchiornithines are non-volant. We examine the validity of the genera Ostromia and Alcmonavis and argue that purported diagnostic features do not provide robust support for their distinctiveness. Our results suggest that all known Solnhofen avialans represent Archaeopteryx.