than Confuciusornis. Both features indicate slow growth, which, according to Chinsamy and colleagues, suggests low metabolic rates. Full endothermy, therefore, would have evolved late on the evolutionary line leading to modern birds. This view was contested by subsequent studies, which pointed out that slow growing bone is not necessarily an indicator for low metabolic rates, and in the case of Mesozoic birds was rather a result of the decrease in body size that characterized the early evolution of birds. A more comprehensive study based on thin sectioning of bones was published by Armand de Ricqlès and colleagues in 2003. Based on 80 thin sections taken from an adult Confuciusornis exemplar, this study confirmed the high growth rates proposed by Zhang and colleagues. The fast-growing fibrolamellar bone tissue was similar to that seen in non-avian theropods, and the sampled individual probably reached adult size in much less than 20 weeks. Small body size was not primarily achieved by slowing growth but by shortening the period of rapid growth. The growth rate estimated for Confuciusornis is still lower than the extremely fast growth characteristic for modern birds (6–8 weeks), suggesting that that growth was secondarily accelerated later in avian evolution. In 2008 Chiappe and colleagues conducted a statistical analysis based on 106 specimens to explore the relationship between body size and the possession of long tail feathers. The population showed a clear bimodal distribution of the size of the animals with two distinct weight classes. However, there was no correlation between size and the possession of the long tail feathers. From this it was concluded that either the sexes did not differ in size or both sexes had the long feathers. The first case was deemed most likely which left the size distribution to be explained. It was hypothesized that the smaller animals consisted of very young individuals, that the large animals were adults and that the rarity of individuals with an intermediate size was caused by Confuciusornis experiencing a growth spurt just prior to reaching adulthood, the shortness of which would have prevented many becoming fossilized during this phase. This initially slow growth followed by a growth spurt would have resulted in a S-shaped growth curve, similar to that inferred for non-avian dinosaurs. Such an extended dinosaurian mode of growth conflicts with the earlier histological findings of de Ricqlès that suggest a much shorter, avian-style growth. Alternatively, the observed size distribution