Scientists Report Discoveries that Suggest Live Birth as Early as 280 Million Years Ago
Newly discovered fossils from Uruguay and Brazil may hold the key to resolving one of the great scientific debates associated with the early conquest of the land by vertebrates. The fossils are embryos (unhatched young) of semi-aquatic reptiles known as Mesosaurs and a specimen of a pregnant female. They may be the oldest examples of live birth in Tetrapods, an important step towards adapting to a much more terrestrial based life.
Tetrapods include amphibians, reptiles, birds and mammals but there is an important distinction between amphibians and the other types of vertebrates that make up this group. Amphibians breed in water. Animals such as frogs, salamanders and newts lay unprotected eggs which are externally fertilised. In contrast, reptiles, birds and mammals use internal fertilisation. In reptiles and birds, the embryo develops in fluid surrounded by a protective, calcium based shell – an adaptation to a more terrestrial lifestyle, without the need to be close to a source of water for breeding. In nearly all extant mammals (monotremes are the exception – echidna, platypus etc.), the developing young is surrounded by fluid and a protective membrane, but it is retained inside the mother’s body for some time before birth. The presence of a protective membrane, known as the amnion, around the embryo allows the further classification of Tetrapods into two distinct groups, amphibians in one group with the rest of the vertebrates in another – the Amniotes.
A Breakthrough in the Conquest of the Land
Amniotes are more independent of water than amphibians, but fossil evidence of this important evolutionary step has been very difficult to find – until now that is. Early Tetrapod fossils do not preserve evidence of reproductive habits, but in a paper published by palaeontologists studying at the University of the Republic (Uruguay), they report on the discovery of fossils that show that Mesosaurs may have been capable of live birth, thus marking an important advance in Tetrapod evolution. things to do in anaheim this weekend
The research team have studied two beautifully preserved fossils dating from the Cisuralian epoch of the Permian (280 million-years-ago). The fossils represent amniotic embryos and are the earliest found to date. The embryos are young Mesosaurs, semi-aquatic, primitive reptiles that are descended from terrestrial animals but returned to a marine environment.
The fossils are very small, the largest no bigger than a man’s thumb nail, they were unearthed in Brazil and Uruguay. Excavated from a gypsum laden matrix it suggests that these reptiles lived in salty, anaerobic water, which helped to preserve the embryos. The Mesosaurs lived alongside a wide range of invertebrates such as burrowing worms and crustaceans as fossils of these creatures have been found too.
It is remarkable that such delicate objects could have been preserved, yet alone survive in the fossil record for more than 250 million years.
Is this Evidence of Marine Reptile Embryos?
Intriguingly, the embryos lacked recognisable eggshells. Moreover, one well-developed embryo was found within an adult presumed to be a pregnant female. This suggests that these reptiles had evolved the ability to give birth to live young a strategy adopted by other later marine reptiles such as Ichthyosaurs and seen in some extant members of the Order Squamata today like vipers. The fossil indicates that Mesosaurs may have been viviparous.
One of the well-developed Mesosaur embryos was discovered on its own, not inside an adult. This might indicate that the Mesosaurs laid eggs after embryos reached advanced stages of development. Alternatively, this specimen could represent a miscarried embryo.