Dichotomy in formation and growth of bones of Yanornis martini (Pygostylia, Ornithuromorpha): study of thermal regime in an extinct bird

Abstract

ABSTRACT Thermophysiology is a factor that influences metabolic and growth rates in organisms, for which bone microstructure is used as a proxy to determine them in extinct amniotes. We investigate if volumetric data can be used to determined variations in bone growth, and if quantitative correlations can be established between osteohistological features and thermal regimes in the case of this extinct bird. Furthermore, we investigate neurovascular volumetric data to determine growth differences between the tibia and pubis of the fossil bird Yanornis martini. Moreover, we also compare 2D osteohistological features (e.g. lumina of osteonal canals) from the tibia of Yanornis martini with those of various extant endothermic and ectothermic amniotes whose thermal regime is known. The 3D analysis shows a dichotomy regarding growth rates, with the pubis growing faster than the tibia. The comparative 2D analyses indicate that Yanornis martini exhibited a thermal regime similar to an extant endothermic marsupial. We conclude that 2D analytical approach can be used to collect preliminary quantitative indicators for thermal regimes in stem birds, particularly when only physical thin sections are available, yet needs to be carefully considered. Therefore, further in-depth stereometric analyses on 3D-rendered models are required to refine our observations.