From dinosaurs to birds: paleomacroecological analysis of the body size inversion in the maniraptor lineage

Abstract

Large taxonomic groups, distributed over vast geological periods, may exhibit wide patterns of morphological variation, such as variation in body size and complexity of living beings. Some studies seek to determine whether these trends of increasing or decreasing size and complexity result from persistent directional forces (e.g., natural selection) or from non-directional (random) trends produced by constraint limits. These mechanisms have been characterized as "active or directed" and "passive" trend processes, respectively. Understanding these variation patterns allows assessing hypotheses within a paleomacroecological context, such as Cope's Rule, which suggests that lineages with longer evolutionary time spans tend to increase in size. This pattern has been demonstrated in various groups of fossil vertebrates, such as dinosaurs. However, the Maniraptora lineage (ancestral to groups that gave rise to birds) seems to exhibit an inverted pattern, with a reduction in body size in recent lineages. Tests indicate that this asymmetry in size evolution can be explained by passive or active trends, i.e., random size variation or directional selection. In this case, an "Inverted Cope's Rule" could indicate directional selection for size reduction in the Maniraptora lineage towards the derivation of birds. Body size and time distribution vectors will be derived for each lineage, evaluating the general trend of size variation. If asymmetry in size evolution is observed, a Subclade Test will be conducted to assess if a specific subclade repeats the asymmetric distribution pattern. The goal of this project is to assess active and/or passive trends related to body size variation in Maniraptora dinosaurs, using fossil data from the Paleobiology Database and the Natural History Museum Database.