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 – echidnas and the platypus – is that they give birth to live young. This “viviparity” contrasts with the “oviparity” 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 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. Extraordinary ichthyosaur fossil shows its last meal was a pterosaur “Whether mammalian ancestors were oviparous or viviparous has been considered a mystery,” says Leandro Gaetano at Argentina’s National Scientific and Technical Research Council. 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’s leg bones. At that point, the class noticed a curious growth line inside the bone. Gaetano and his colleagues investigated further and eventually concluded that this line, analogous to a tree’s growth ring, recorded the moment that the ancient mammal relative went through a growth spurt after its birth.
Fossil fruits show flowering plants flourished in time of dinosaurs 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. 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. Millions of fossil whale bones found in deep-ocean ‘necropolis’ “Further, as they argue, the apparent absence or rarity of actual eggs of any kind in the Permian and Triassic has always been puzzling – the so-called amniote egg gap,” he says. Oliver Griffith 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.
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