And the reason I bring it up is because there is a new paper on the subject:
Rowe, T., T. H. Rich, P. Vickers-Rich, M. Springer and M. O. Woodburne (2008 January 29). The oldest platypus and its bearing on divergence timing of the platypus and echidna clades. PNAS 105(4):1238–1242. doi:10.1073/pnas.0706385105
It's not in print yet (hence the futuristic date), but it's been posted online. (Free access! Yes!)
The answer turns out to be rather bewildering. Well, first, let me clarify that I am actually asking how long stem-platypuses have been around. Platypuses proper (i.e., the crown group) have presumably not been around that long, but their ancestors go back to some time when the platypus lineage joins the lineage of echidnas (Tachyglossidae, a.k.a. spiny anteaters), an equally bizarre type of living, egg-laying mammal. Their final common ancestor, the ancestral monotreme, turns out to be difficult to pin down.
Rowe et al. review several molecular clock studies which have suggested a wide variety of dates for the origin of Monotremata, the platypus-echidna clade, from 17 to 131 million years ago. They can't even agree on whether one or two pan-monotreme lineages survived the Cretaceous-Paleogene extinction event (about 65 million years ago). Some of the more reliable estimates are older, but Rowe et al. consider the oldest dates to be most likely.
Why? Teinolophos trusleri.
This is a fossil species from 112.5 to 121 million years ago. It's usually regarded as a stem-monotreme, meaning that it would have no bearing on the platypus-echidna split. But Rowe et al. have done a morphology-based cladistic analysis that places it as a stem-platypus, the oldest one known. (They also find Steropodon galmani, a 110-million year old fossil based on an opalized jaw, to be a stem-platypus.) They also consider that it was ecologically platypus-like, i.e., that it was amphibious.
This would mean that things like platypuses were swimming around in the Early Cretaceous, under the snouts of gigantic dinosaurs. It would also mean that the echidna and platypus lineages had diverged well before the Cretaceous-Paleogene extinction, perhaps sometime around the Jurassic-Cretaceous boundary. So any theory as to the cause of the extinction would have to allow for both stem-echidnas and stem-platypuses to survive, instead of a single stem-monotreme lineage.
It also means that many of the molecular analyses would be quite wrong. And even the more correct ones would be just barely right. This means one of two things:
- Monotreme evolution is much slower than that of other mammals, and molecular clock studies are based on presumed rates that are too fast.
- Rowe et al. are wrong, and Teinolophos is a stem-monotreme, not a stem-platypus.
As for #2, Teinolophos is only known from jaw material, and echidnas do have highly modified, toothless jaws. If #2 is true, then echidnas would have evolved from a somewhat platypus-like ancestor, an intriguing possibility. (In fact, that could be the case even if #2 is not true.)
But #1 is perfectly possible as well. As the authors note, it might help explain why there are a thousand times more therian mammals than monotreme mammals alive today. (Theria is the crown group that includes marsupials and placentals, i.e., all living mammals apart from monotremes.) Molecular clock estimates are notoriously sketchy. Only further evidence (e.g., Mesozoic monotreme fossils) will help clarify the situation.
* No, the plural is not "platypi". If you want to be fancy and use the proper classical plural, it's "platypodes" (like "Antipodes"). But who wants to be that fancy?