Revealing a sustained increase in relative avian forelimb length through the Mesozoic

The evolution of avian flight is characterised by a gradual accumulation of various morphologies through time—yet an explicit temporal trend in wing length remains elusive. This is despite an expectation of increasing forelimbs leading to birds as seen in the fossil record and a clear theoretical expectation for an increase in wing length through time as a consequence of selection for improved flight performance from the earliest birds to the effective fliers we see today. Using Bayesian phylogenetic comparative methods across multiple phylogenies, we test for temporal trends in relative total forelimb length and individual forelimb bones through theropod evolution, whilst accounting for allometric relationships and assessing heterogeneity in the rates of evolution. We provide the first explicit evidence of a long-term trend towards increasing forelimb length in Mesozoic birds occurring independently of allometry-associated elongation and explain prior findings of heterogeneous rates in the evolution of the avialan forelimb bones. The use of methods which simultaneously account for relationships between traits and multiple hypotheses of their shared histories allows us to better understand the tempo and mode of transitions in locomotion—this is vital in understanding how flight evolved in early birds and how the elongation of the forelimbs played a part in their ultimate domination of the skies.