Perhaps “strong arms” made people sexy even in ancient times: paleontologists report on the development of sexual characteristics in small marine dinosaurs that lived around 240 million years ago in what is now China. Examination of the fossil remains of these so-called keichousaurs shows how the males developed strong upper arm structures during puberty, which were probably intended to please the females or to keep rivals at bay.
The technical term is sexual dimorphism: In many animals, apart from the sexual organs, there are sometimes clear physical differences between males and females of a species: deer antlers, peacock wheels or lion’s mane are particularly prominent examples of this. In humans, too, the concept is reflected in the so-called secondary sex characteristics. In today’s animal world and also in humans, sexual dimorphism usually develops as part of the development of reproductive ability – the development phase known as puberty. This process has been well studied in humans and many other modern creatures, but not in primeval animals from the reptile group. One problem is that sexual dimorphism in reptiles is often only characterized by differences in size and sex-specific coloring. Both aspects are difficult to prove from fossils.
Puberty reflected in bone structures
However, as an international team of researchers reports, they have now been able to clearly document puberty in a primeval representative of the so-called amniotes for the first time. These are marine animals that hunted small fish around 240 years ago in what is now China. Although the keichousaurs lived in the water, they are still classified as land vertebrates. Because their ancestors lived on land and then secondarily developed an aquatic way of life. The physique of the keichousaurs is reminiscent of that of the large plesiosaurs of the Jurassic and Cretaceous ages – however, they only reached comparatively “cute” dimensions of less than 20 centimeters. The small reptiles have already become known through fossils “with content”: remains of embryos showed that they did not lay eggs, but were viviparous.
The research team led by senior author Martin Sander from the University of Bonn has now used the extensive fossil record of Keichousaurus to explore whether the development of sexual dimorphism during puberty can be proven in these creatures. There was already evidence that the larger specimens represented males and the smaller specimens represented females. Differently developed upper arm bones (humeri) were also found in different fossils. The paleontologists have now followed this trace with more detailed investigations. To do this, they made thin sections of the bones of various fossils and examined the characteristics.
As the researchers report, growth processes in the course of the development of the animals became apparent in the tree ring-like structures of the bones. First of all, the analysis results reflected an extremely rapid growth to the adult state. Later, however, bone density increased again, indicating a slowdown in growth. Eventually, the small reptiles apparently shifted their energy investment from growth to reproduction. In addition to these fundamental development processes, gender-specific peculiarities are now also becoming apparent, the researchers report: The findings clearly show that Keichousaurus males not only became larger during puberty, but also developed significantly more robust upper arms than the females.
Males with strong upper arms
In cross-section, the humerus of a male appears strikingly triangular and that of the females round-oval, the scientists report. According to the detailed analyses, this was due to a different deposition of bone tissue through the action of hormones after the onset of puberty. The deposition pattern in the females remained similar to that in the juveniles, but differed in the males. As the researchers explain, the transformation of the humeral midshafts in males also reflects an enlargement of the muscle attachment sites, suggesting more robust forelimbs.
According to them, the findings also provide indications of possible preferences or behavior in the small reptiles. It is possible that the Keichousaurus females found larger males with powerful “arms” particularly attractive. Perhaps the strong front legs were also advantageous in the copulation posture during the mating process. They may also have played a role in territorial fights between males. A training effect may also have had an impact on the expression of the characteristic. “The interaction of endogenous hormonal regulation during puberty and external stimuli may have contributed together to the morphological change in the male humerus,” concludes the statement from the University of Bonn.
Source: University of BonnCurrent Biology, doi: 10.1016/j.cub.2023.05.073