Creatures that bridged the gap between reptiles and mammals may have been bearing live young 90 million years earlier than we thought.
One of the hallmarks of all mammals apart from monotremes 鈥� echidnas and the platypus 鈥� is that they give birth to live young. This 鈥渧iviparity鈥� contrasts with the 鈥渙viparity鈥� seen most other vertebrates, in which females lay eggs that hatch outside their bodies.
One long-standing question has been whether creatures known as cynodonts, which emerged around 260 million years ago during the Permian geological period聽and are directly ancestral to all mammals, laid eggs or gave birth to live young, as no evidence of their reproduction has been discovered either way.
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鈥淲hether mammalian ancestors were oviparous or viviparous has been considered a mystery,鈥� says聽 at Argentina鈥檚 National Scientific and Technical Research Council. 鈥淲e had no clues on how or when one of the most distinctive traits of the majority of present-day mammals, viviparity, evolved in the mammalian lineage.鈥�
This has now changed, thanks to a chance observation made during a postgraduate聽class聽at the University of Buenos Aires in Argentina. The students were examining a fossil of Chiniquodon theotonicus, a cynodont that lived 236 million years ago and was unearthed in the Talampaya National Park in north-western Argentina in 2011. Part of the analysis involved using a microscope to examine thin slices taken from one of the ancient animal鈥檚 leg bones. At that point, the class noticed a curious growth line聽inside聽the bone.
Gaetano and his colleagues investigated further and eventually concluded that this line, analogous to a tree鈥檚 growth ring, recorded the moment that the ancient mammal relative went through a growth spurt after its birth.聽
The researchers reconstructed how large the leg bone and the animal itself would have been when the growth line formed. They calculated that the animal would have weighed 1.68 kilograms at that point. When the animal died as a full-grown adult, it was just shy of 12 kilograms.
This gave the researchers a ratio of newborn to adult size in the ancient animal. They then compared this figure to similar ratios obtained from a wide variety of modern animals: 1869 mammals, 2619 non-avian reptiles and 782 birds.
鈥淲e were amazed to find that聽颁丑颈苍颈辩耻辞诲辞苍听grouped with extant placental mammals,鈥� says Gaetano.
Egg-laying animal groups produce young that are far smaller than adults when they hatch, he says.
For example, some species of large birds have newly hatched young that are just 0.002 per cent of adult body mass, whereas placental mammals often give birth to young that are over 20 per cent of the size of their parents.
鈥淥nly placental mammals produce relatively large newborns, whereas reptile and bird hatchlings are comparatively very small,鈥� says Gaetano. 鈥淗ence, we propose that [the]聽Chiniquodon theotonicus聽reproductive strategy was most similar to that of placental mammals.鈥�
聽at the University of Bristol in the UK says the findings are convincing.
鈥淔urther, as they argue, the apparent absence or rarity of actual eggs of any kind in the Permian and Triassic has always been puzzling 鈥� the so-called amniote egg gap,鈥� he says.
聽at Macquarie University in Sydney says his gut reaction is to be sceptical of the claim of live birth, as we have only indirect evidence. But he is impressed that, in the absence of any direct fossil evidence, the team is still trying to solve the puzzle.
鈥淲e may never get any of the kind of direct evidence that solves the
mystery, for example a fossilised mammalian egg or a fossil mother during
pregnancy,鈥� he says. 鈥淪o it is kind of cool that they鈥檙e taking a different approach to trying to find an answer to that question.鈥�
Frontiers in Mammal Science
