Fossil skeletons have long fascinated researchers as a window into prehistory. But until now, little was known about the details of sexual development in extinct creatures. An international team of researchers from China, Germany and Japan, with major participation from the University of Bonn, reported the discovery of Keichousaurus in the journal Current Biology. It is a small marine viviparous reptile that lived about 240 million years ago in what is now China.
Deer antlers, peacock feathers, and lion’s mane are known as expressions of sexual dimorphism. These are clearly different physical characteristics between the sexes of a species. These visible differences usually only become apparent after the animal reaches adulthood, and they provide important information for understanding growth and reproduction.
In today’s animal world, “sexual dimorphism” is often seen at puberty, which represents the attainment of reproductive capacity or sexual maturity. However, in some species, especially reptiles, it is difficult to determine the onset of sexual maturity. Differences between the sexes are usually seen only in color or body size. Both are difficult to prove from fossils.
The current study focuses on Keichousaurus, a species discovered in Triassic sediments—dating back to about 240 million years ago—in southwestern China. This species is known for the large number of specimens, including specimens with embryos, and the apparent sexual dimorphism. For example, males of this species grow larger than females. Additionally, adult males of Keichousaurus had stronger upper arm bones (humeri), which also looked different from females. In cross-section, the humerus of a male is triangular and that of females is round-oval.
Studies of bone tissue now show differential deposition in males, resulting in a triangular cross-section. In contrast, females retain an oval cross-section similar to juveniles. The deposited bone tissue before, during and after puberty also provides important insights into sexual growth and development in this group.
Analyzes of bone complexity and growth rate confirmed rapid growth through puberty. Differences in growth rate and maximum attained body size are strategies of the sexes to ensure survival to reproductive maturity and success with the opposite sex. For example, Keichousaurus females apparently found larger males more attractive than smaller males. Males that grow faster than others are more likely to breed successfully. The observed increase in bone density after puberty in both boys corresponds to a growth slowdown, suggesting a shift of energy from growth to reproduction.
The change in humeri midshafts in males corresponds to an enlargement of the muscle attachment areas, suggesting stronger forelegs. Keichousaurus males may have been subject to higher demands from territorial disputes. This puts more stress on their front legs. Strong forelegs can also be beneficial in copulatory posture during the mating process. The interplay of endogenous hormonal regulation during puberty and external pressure stimuli probably simultaneously contribute to the morphologic changes in the male humerus during this phase.
More information:
Qiang Li et al, Puberty in a Mesozoic reptile, Current Biology (2023). DOI: 10.1016/j.cub.2023.05.073
Provided by the University of Bonn
Citation: Fascinating insights into the sexual development of an extinct marine reptile (2023, June 30) retrieved on 4 July 2023 from https://phys.org/news/2023-06-insights-sexual- extinct-marine-reptile.html
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