Fossils provide first evidence of puberty onset in ancient marine reptile

Bone sections of about 18 different individuals of this species were examined for this study. 

The scientists were able to distinguish various traits of Keichousaurus male and female based only on the differential fossilized bone structures. 

The analysis revealed that males of this species, for example, grew larger than females.

Furthermore, adult male Keichousaurus possessed stronger upper arm bones (humeri) than females. The humerus bone of a male appears triangular in cross-section, while that of a female looked round-oval.

“Studies of bone tissue now indicate differential deposition in males, resulting in a triangular cross-section. In contrast, females retained an oval cross-section similar to that of juveniles. The deposited bone tissue before, during, and after puberty also provides important insights into growth and sexual development in this group,” mentioned the press release by the University of Bonn.

Bone changes linked to puberty

The researchers discovered that the shape change of male bones in Keichousaurus presumably begins with the onset of puberty. In addition, sex hormones likely led to puberty in Keichousaurus.

Using growth curves, the scientists hypothesize that such bone shifts happened when the Keichousaurus was one year old.

According to the official release, disparities in male and female development rates, as well as their maximum body size, were probable survival tactics to achieve reproductive maturity. 

Female Keichousaurus, for example, reportedly considered bigger males more appealing than smaller males. Males who matured quicker than others were thus more likely to successfully breed. 

The study concluded that the observed increase in bone density after adolescence ends in both sexes correlates to a slowdown of development, implying a shift of energy from growth to reproduction.

The results have been published in the journal Current Biology. 

Study abstract:

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. 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 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.