What fossils can tell us about the evolution of viviparity and placentation.

The research paper discusses a fossil discovery that sheds light on the evolution of viviparity (live birth) and placentation in vertebrates, revealing this process could have begun as far back as 380 million years ago.

Background

• The study is centered around a significant fossil discovery – a preserved fetal skeleton and cord within an ancient jawed fish dating back to the Devonian period.
• This fossil marks the earliest known evidence of vertebrate placentation, significantly extending the ancestry of this characteristic back to 380 million years ago.

Fossil Evidence for Viviparity

• The research offers a review of fossil evidence related to viviparity, which represents the condition where young are born alive rather than being hatched from eggs.
• Specifically, it highlights fossil proof of viviparity present in marine reptiles and early mammals, marking a significant evolutionary advance.

Evolutionary implications of Horse Fossils

• The fossils of horses provide essential insight into early viviparity. It was shown that horses, from an early stage, had adopted a reproductive strategy that favored a single, well-developed newborn, also known as being precocial.
• This indicates a significant early shift from a strategy of producing numerous underdeveloped offspring (which is common among reptiles and fishes) towards producing fewer, more mature newborns – a strategy predominantly seen in mammals today.

Early Placentation in Horses

• In one particular instance of a preserved horse fossil, the preserved soft tissues hint towards the existence of a diffuse placenta.
• This supports predictions made by previous phylogenetic analyses of placentation, which suggested that diffuse placentas were present in earliest horses.

Conclusion

• The research concludes by asserting that these fossils increase our understanding of the evolutionary trajectory of viviparity and placentation. In essence, they show the reproductive strategies have evolved over an even longer period than previously thought.