Body size evolution in Mesozoic birds

Area of Science:

• Paleontology
• Evolutionary Biology
• Comparative Anatomy

Background:

• Cope's Rule describes a general trend of increasing body size in evolving lineages.
• Avian evolution presents a unique case due to flight constraints, suggesting potential limitations on body size increase.
• Previous interpretations of early avian evolution have not consistently supported size increase.

Purpose of the Study:

• To investigate body size trends in Jurassic and Cretaceous flying birds.
• To determine if avian evolution followed Cope's Rule despite flight constraints.
• To analyze body size changes within specific avian clades.

Main Methods:

• Analysis of body size data from fossil flying birds across the Jurassic and Cretaceous periods.
• Statistical examination of body size trends over a 70-million-year timespan.
• Comparative analysis of body size changes within major avian clades (Pygostylia, Ornithothoraces, Ornithuromorpha).

Main Results:

• Demonstrated an overall increase in body size across flying birds from the Early Jurassic to the end of the Cretaceous.
• Identified a nonrandom, though not strictly directional, increase in avian body size over 70 million years.
• Observed body size increase within Pygostylia and Ornithothoraces, but a decrease in the crownward lineage Ornithuromorpha (ancestral to extant birds).

Conclusions:

• Avian evolution, contrary to some expectations, exhibited an overall increase in body size during the Mesozoic Era.
• Diversification after the origin of flight likely facilitated this size increase, potentially through increased variance.
• The decrease in body size in Ornithuromorpha may have contributed to their survival across the Cretaceous-Palaeogene boundary.