Live birth in an archosauromorph reptile.

Area of Science:

• Paleontology
• Evolutionary Biology
• Reproductive Biology

Background:

• Live birth has evolved independently multiple times in vertebrates, including mammals and various reptiles.
• The major clade Archosauromorpha, encompassing modern birds and crocodilians, has not previously shown evidence of live birth.

Purpose of the Study:

• To report the discovery of a pregnant fossil reptile, Dinocephalosaurus, providing the first evidence of live birth within Archosauromorpha.
• To investigate the reproductive biology and evolutionary history of Archosauromorpha.

Main Methods:

• Discovery and analysis of a fossilized pregnant specimen of Dinocephalosaurus from the Middle Triassic of China.
• Phylogenetic modeling to infer reproductive strategies and sex determination mechanisms.

Main Results:

• The fossil represents a pregnant Dinocephalosaurus, confirming live birth in Archosauromorpha approximately 245 million years ago.
• Phylogenetic models suggest Dinocephalosaurus utilized genotypic sex determination (chromosomes) rather than environmental factors.

Conclusions:

• This finding pushes back the evidence of reproductive strategies in Archosauromorpha by approximately 50 million years.
• Genotypic sex determination in Dinocephalosaurus provides crucial insights into amniote adaptation to aquatic environments.