Puberty in a Mesozoic reptile.

The histology of bone can be preserved virtually unaltered for hundreds of millions of years in fossils from all environments and all vertebrate taxa, giving rise to the flourishing field of paleohistology. 1 The shafts of long bones are formed by the apposition of periosteal bone tissue, similar to the growth of wood, and preserve, an often cyclical, record of the growth of the individual and events in its life history. One such event is sexual maturation or puberty, during which hormonal changes transform the juvenile into a sexually mature adult. Puberty has been well studied in humans and some other living vertebrates. Here, we describe puberty in Keichousaurus , a small sexually dimorphic and live-bearing marine reptile from Middle Triassic rocks of SW China, about 240 million years old. Using a combination of bone histology and morphology, we detected puberty 2 as one of the four life stages (the others being fetus, juvenile, and adult). Adult Keichousaurus males have a more robust humerus than females, with pronounced muscle attachment sites and a triangular shaft cross section. Midshaft sections of the humeri of the males show the transition from the rounded juvenile cross section to the triangular adult cross section, as reflected in the contour of the growth marks. This shape change is produced by differential bone apposition of the periosteum, presumably triggered by sex hormones, as in humans, 3 and influenced by changes in loading regime during puberty. This is the first report of puberty in a fossil amniote. [Display omitted] • Puberty is recognized in a Triassic reptile called Keichousaurus by bone histology • Shape change of male humeri in Keichousaurus starts from the onset of puberty • Sex hormones may trigger puberty in Keichousaurus • Loading regime during puberty can result in differential bone apposition in humeri Li et al. report puberty for the first time in a fossil amniote. The bone thin sections show the onset of pubertal growth and reveal the development of sexually dimorphic features in a live-bearing marine reptile called Keichousaurus from the Triassic of South China. [ABSTRACT FROM AUTHOR]