May 12, 2006 — Small prehistoric mammals may have looked like easy pickings for their fearsome contemporaries, but a new study suggests many matched might with venom.

The theory could explain how the fox-sized mammals of the Mesozoic Era 225 to 65 million years ago defended themselves against much larger predators. It also may explain why male platypuses have a fang-like venomous spur on their inside hind legs.

Echidnas — the spiny relatives of the platypus also native to Australia — emerged later but also possess what appears to be a remnant of this spur. But the venom canal found in the platypus appendage is missing in the echidna.

"The echidnas and platypus are relicts of very old stem mammals," said lead author Jørn Hurum, a paleontologist at the University of Oslo, Norway. "All other mammals this primitive are extinct."

The common ancestors of most modern mammals, he explained, "lost the trait when they acquired upright stance on the hind limbs. The more primitive mammals still had the legs more or less sprawling out from the body... . It is a really bad idea to have venomous spurs pointing inwards when your ankles almost meet during walking."

Hurum's team also examined fossils from several prehistoric mammals, including Zhangheotherium, a toothy platypus-like critter, and Sinobaatar, a small herbivore. Fossils from these and other Mesozoic mammals show variations on what could be a venomous spur.

The team's findings are published in the journal Acta Palaeontologica Polonica.

As supporting evidence, Hurum points to a Sinosauropteryx primadinosaur dinosaur fossil dating to 123 million years ago. In its gut are fossils of Zhangheotherium  and Sinobaatar. Hurum and others believe the dinosaur was poisoned to death.

Hurum speculated on what may have happened when the dino encountered its final meal:

"Sinosauropteryx catches the mammal between its hands, but the [mammal] has an unexpected weapon....The poison glands positioned in the muscles on the inside of the femurs pump its venom through a canal in the hollow spur into the dinosaur flesh... . The dinosaur falls on its side and dies."

Thomas Martin, a professor in the Mammal Section of the Senckenberg Forschungsinstitut natural history museum in Frankfurt, Germany, told Animal Planet News that the new study sounds "very convincing to me."

Martin explained, "A defensive device like a venomous tarsal spur makes sense in small animals like Mesozoic mammals. We can only speculate why the small-bodied marsupials and placentals lost this structure, but the reason may be that they evolved other strategies for survival, such as spines and the ability to roll up as in hedgehogs, glands that produce bad-smelling secretions (as in skunks), or scales (armadillos)."

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