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, the earliest evidence of viviparity, an important step towards adapting to a much more terrestrial based life.

Earliest Evidence of Viviparity

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.

Amniotes

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.

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.

The picture above is a composite with a partial skeleton of an adult mesosaur on the right compared to the embryo fossil on the left (scale bar 20 cm).

Researcher Graciela Piñeiro, a palaeontologist at the University of the Republic (Uruguay) commented:

“Despite their age and their delicate nature, they remained in the rocks all that long time almost perfectly preserved.

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.

A Replica of a Mosasaurus

“Ron” the PNSO Mosasaurus replica.

To view the range of PNSO prehistoric animal models and replicas: PNSO Dinosaur Replicas and Prehistoric Animal Models.

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.

Graciela went onto add:

“With the discovery of the mesosaur embryos, we may now have direct evidence that embryo retention or viviparity were strategies developed by early amniotes.”

This is not the first earliest fossil evidence indicating viviparity in the fossil record, an amazing fossil of a placoderm (armoured fish) was discovered a couple of years ago that suggested that at least some of the placoderms may have evolved this reproductive strategy.

To read an article on the placoderm fossil find: Placoderm Parents.

However, this is the earliest case known for a tetrapod.  In 2011, a fossil of a plesiosaur from the genus Polycotylus (marine reptile) that had been found many years earlier in Kansas was analysed and it revealed that these types of marine reptile may also have been viviparous.

To read more about this research: Insight into Plesiosaur Breeding.

This study of mesosaurs pushes back the known records of live birth and amniotic embryos by sixty and ninety million years respectively.  The paper on this mesosaur research has been published in the scientific journal “Historical Biology”.