Abstract
Osteoarthritis is extremely rare in wild mammal populations (less than 1%) and varies in frequency according to species (0–25%) in recent birds, where it is inversely related to size. Large population samples of Early Cretaceous birds in China permit us to analyze its frequency in one of the earliest avian radiations. In these samples, the larger bird (Caudipteryx) shows a high frequency (30%). The earliest previous documentation of primary osteoarthritis in any animal is in a family of Early Cretaceous dinosaurs (Iguanodontidae). We document its occurrence in a basal bird and in one of the forms considered by some to be a feathered dinosaur. These occurrences are 20 million years older than the next oldest occurrence of osteoarthritis.
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