I just returned from a two-month trip to Australia, where – for all you twitchers out there – I had a ‘clean sweep’ of Australia’s living monotremes: the platypus and the short-beaked echidna. These are truly bizarre mammals: in addition to obvious modifications of their trophic morphology (duck-bills and “beaks”), they lay eggs and possess a cloaca (on that last subject, we can all be thankful that it was the therian lineage that gave rise to modern humans). Given the phylogenetic and phenotypic distinctiveness of these strange beasts, it is hard to avoid viewing monotremes as “living fossils”, characterized by a long history of low but stable diversity and minimal morphological change. But in this week’s early edition of PNAS, Phillips et al. make the curious claim that terrestrial echidnas are recently derived from aquatic or semi-aquatic ancestors.
The evidence in Phillips et al. hinges on a reinterpretation of the fossil record in conjunction with molecular clock analyses of the echidna-platypus split. Knowing nothing about the cranio-mandibular morphology of monotremes, I am in no position to evaluate much of this work. Still, I found this to be a plausible conclusion and one that seems to make sense: under the new date for the platypus-echidna split (~32 Ma), rates of molecular evolution on both the stem monotreme and subsequent branches are well within the distribution of rates seen in other vertebrates. If, however, the split occurred at least 112.5 Ma, as was proposed previously by Rowe et al. , rates on the monotreme stem would be among the highest reported for vertebrates, and rates after the split would be among the slowest! If Phillips et al. are correct, this is a fascinating bit of natural and evolutionary history. Despite the grumbling of a small but vociferous group of aquatic ape enthusiasts , there are very few well-supported cases of secondarily derived terrestriality from aquatic/semi-aquatic ancestors. So finding that a proto-platypus ecomorph gave rise to Australia’s famous ant-loving “porcupine” is an interesting one indeed.
I think this work challenges some of our assumptions about old, species-poor lineages. Often, such groups are described as “living fossils.” Groups like coelacanths, lungfishes, tuataras, bowfins, and more have persisted for hundreds of millions of years with what appear to be very slow rates of diversification and morphological evolution. Paleontologists have even used the word “inert” to describe the nature of evolution in groups like these. But here we have what appears to be a major adaptive transition between fundamentally different morphologies in one of these living fossil lineages. Why has this group undergone such a radical ecological shift? And conversely, why do so many other groups seem to be inert?