Fossil skeletons reveal puberty in extinct reptile

The fascinating world of fossil skeletons has always captivated researchers, offering a glimpse into the past. However, the details of sexual development in extinct creatures have remained largely elusive. Recently, an international team of scientists from China, Germany, and Japan, including researchers from the University of Bonn, shed light on the puberty of Keichousaurus, a small marine reptile that existed approximately 240 million years ago in what is now China. Their findings were published in the journal Current Biology.

Sexual dimorphism, which refers to distinct physical differences between the sexes of a species, is exemplified by features like deer antlers, peacock feathers, and lion’s manes. These characteristics are typically only apparent in adulthood and provide valuable insights into growth and reproduction.

In modern-day animals, sexual dimorphism usually becomes evident during puberty, which signifies the attainment of sexual maturity and reproductive capacity. However, in certain species, particularly reptiles, detecting the onset of sexual maturity can be challenging. Differences between the sexes often manifest as variations in coloration or body size, making it difficult to ascertain from fossil remains.

The study focused on Keichousaurus, a species abundant in Triassic sediments from southwestern China dating back to around 240 million years ago. This species is renowned for its numerous specimens, including those with preserved embryos, and its prominent sexual dimorphism. Notably, male Keichousaurus individuals grew significantly larger than their female counterparts. Moreover, adult males exhibited more robust upper arm bones, known as humeri, which differed noticeably from those of females. Males had triangular humeri in cross-section, while females had round-oval ones.

Analysis of bone tissue revealed differential deposition in males, resulting in a triangular cross-section, while females retained an oval cross-section similar to juveniles. The study of bone tissue deposition before, during, and after puberty offered critical insights into growth and sexual development within this group.

Bone compactness and growth rate analyses confirmed rapid growth until reaching puberty. Differences in growth rate and maximum attained body size represent strategies employed by each sex to ensure survival to reproductive maturity and success in attracting mates. For example, female Keichousaurus individuals appeared to find larger males more attractive than smaller males. Consequently, males that experienced faster growth were more likely to achieve successful reproduction. The observed increase in bone density after the end of puberty in both sexes suggested a shift of energy allocation from growth to reproduction.

The transformation of the midshafts of male humeri corresponded to an enlargement of muscle attachment sites, indicating more robust forelegs. This suggests that Keichousaurus males may have faced greater demands from territorial fights, placing additional stress on their front legs. Strong forelegs could have also been advantageous in copulatory postures during the mating process. The morphological changes in the male humeri during this phase likely resulted from a combination of endogenous hormonal regulation and external pressure stimuli.

Source: University of Bonn

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