Producing living young, and not external eggs, is a form of
birth that could date back to 280 million years ago or even earlier, a new
study suggests.
Called viviparity, this form of birth is used by humans, but clearly we were far from being the first to
evolve it.
The study, published in the December issue of Historical
Biology: An International Journal of Paleobiology, focuses on mesosaurs,
which were among the world’s first aquatic reptiles. They lived in what are now
South America and South Africa at a time when these two landmasses were united
and part of the giant supercontinent Pangaea.
Mesosaurs, and even their earlier ancestors, possibly “were not able to
produce hard shelled eggs, at least for the first several million years of
their evolution,” lead author Graciela Piñeiro, a paleontologist at Uruguay’s Facultad de
Ciencias, told Discovery News. “After the recent discovery of mesosaur embryos, we can state
with a high degree of confidence that embryo retention developed early in
amniote evolution, given that mesosaurs are among the basal-most reptiles and
that they date from the Early Permian around 280 million years ago.”
Piñeiro and colleagues Jorge Ferigolo, Melitta Meneghel and Michel Laurin
recently discovered the exceptionally well-preserved mesosaur embryos at sites
in Uruguay and Brazil. The environmental conditions at the locations allowed
for the preservation of soft tissues, nerves and blood vessels, she said.
Giving birth in this manner and laying
eggs each come with advantages and disadvantages. Eggs with hard, mineralized
shells, such as those associated with today’s chicken eggs or those of
dinosaurs, are believed to help reproduction on dry land. But many terrestrial
animals, including humans, do not lay eggs, so there must be other benefits to
viviparity.
“We think that the retention of the
eggs may have appeared in amniotes as a useful strategy to avoid predation and
increase survivorship chances for the embryos,” Piñeiro
said.
Parental care often then follows. There is even some
evidence that mesosaurs provided such care, because adults and juveniles have
been associated together in the fossil record.
At least some mesosaurs even had the added challenge of giving
birth and raising young in extremely salty water.
“In Uruguay, mesosaurs may have first colonized the shallow
water environment of the Mangrullo Formation, which under the establishment of
arid climatic conditions that increased evaporation became like a salty marsh
where just a few opportunistic organisms could tolerate the anoxic bottom
conditions generated by the accumulation of high amounts of organic matter,” Piñeiro
explained.
When infant mesosaurs entered the world, they possibly even
had a salt gland and other anatomical adaptations already in place, allowing
them to survive the otherwise challenging conditions.
There is also compelling evidence that giant, carnivorous,
four-flippered reptiles known as plesiosaurs gave birth to live young as well.
Robin O’Keefe of Marshall University and team discovered a big embryonic marine
reptile contained in the fossil of its 15.4-foot-long mother, which lived 78
million years ago.
“The embryo is very
large in comparison to the mother,” O’Keefe said, “much larger than one would expect in
comparison with other reptiles. Many of the animals alive today that give birth
to large, single young are social and have maternal care. We speculate that
plesiosaurs may have exhibited similar behaviors, making their social lives
more similar to those of modern dolphins than other reptiles.”
There is one disturbing side-note to
such prehistoric pothering: cannibalism.
“Intriguingly, cannibalism is more
frequently found in reptiles that are viviparous and develop parental care and
social behavior,” Piñeiro said.
She and her colleagues are continuing to study viviparity in
the fossil record. A paper touching on the connection between mesosaurs and terrestrial
animals is forthcoming.
Image credit: Graciela Piñeiro
