Fibrolamellar bone in basal Archosauromorpha: Tracing the evolution of high growth rates.

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The origin of endothermy is a major issue in the evolutionary biology of vertebrates. Traditionally, it has been explained by selection pressure for increased agility or improved parental care. However, endothermy may also be linked to the evolution of high growth rates, which in turn require high basal metabolic rates, which lead to endothermy. High growth rates appear to be a precondition for the evolution of multi-tone land animals (such as sauropods and elephants) because low growth rates lead to a dramatically decreased likelihood of survival to maturity. The only way to detect high growth rates in an extinct tetrapod is through bone histology. Specifically, fibrolamellar bone tissue serves as a proxy for high growth rates. While our research in the previous funding period has shown that high growth rates were already present in the earliest sauropod dinosaurs, also suggesting they were a prerequisite for gigantism, it is not clear whether these high growth rates evolved due to the selection pressure for increased body size or in the framework of another selective regime. Thus, we need to comprehensively sample the pre-Jurassic and nondinosaurian part of the archosauromorph phylogeny to detect the patterns of occurrence of fibrolamellar bone. At present, a single origin with repeated loss appears equally as likely as a convergent origin in several archosauromorph groups. Understanding the pattern of fibrolamellar bone evolution in basal archosauromorphs will address the hypothesis of high basal metabolic rates being an adaptation to large body size.