February 7, 2006
Our Venomous Forefathers?
11:47 pm
When asked to think of a venomous animal, most people will picture spiders or snakes. But a recent paper in Acta Palaeontologica Polonica suggests that venom might be a hallmark of mammals, or at least it may have been early on in their history.
The platypus is one of the few venomous mammals alive today.* Unlike snakes and spiders, it delivers its venom using sharp spurs on the inside of its ankles; kicking at enemies or rivals like a duck-billed cowboy. The platypus’ closest living relatives, several species of spiny, ant-eating echidna, also have these extratarsal spurs, although these are non-venomous.
In their new paper, Jørn H. Hurum, Zhe-Xi Luo, and Zofia Kielan-Jaworowska review what is known of the ankles in several groups of Mesozoic mammals, and find that the ankle-spur has a much wider distribution at the base of the mammal family tree. Monotremes (the platypus and echidnas) had it, but it is also present in mammals that are more closely related to us, like eutriconodonts, multituberculates, and “symmetrodonts.”
Newly described “symmetrodont” Akidolestes is mentioned—the holotype preserves traces of the keratin that covered its spur in life. (See if you can find the spurs in this diagram.)
Plugging these groups into a (very simplified) mammal family tree shows an interesting pattern:
Distribution of ankle-spurs in early mammals. From info in Hurum, Luo, & Kielan-Jaworowska 2006.
Some sort of ankle-spur is present in most groups of early mammals. It is unknown whether any of these Mesozoic mammals armed their spurs with venom, but the paper suggests it is an intriguing possibility. Perhaps our ancestors would have found a little poison useful when fending off attacks from predatory dinosaurs.
The groups that happen to be the most successful today—marsupials and placentals—have lost the ankle-spur. But the fossils suggest that this is the exception, not the rule.
It may be that kicking venom is as fundamentally mammalian as fur or milk.
Hurum, J.H., Luo, Z−X., and Kielan−Jaworowska, Z. 2006. Were mammals originally venomous? Acta Palaeontologica Polonica 51 (1): 1–11. PDF
Tip of the toupee to Trevor Dykes for the heads-up on this one. Trevor’s Mesozoic Eucynodonts site is a good place to spend a few days if you’re interested in finding out more about the mammal groups mentioned here.
*Actually, only the males produce venom. So in all fairness to the non-poisonous females, the platypus really is only a half-venomous mammal.
—Matt Celeskey.
File under: Mammals, Mesozoic, Recent Discoveries, Synapsids.
One Response to “Our Venomous Forefathers?”
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afarensis, on February 8th, 2006 at 12:38 pm Said:
Wow. Cool! Didn’t know they were poisonus. A early ancestor of T-rex has been discovered…
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