Just a quick snapshot of this crucial issue in palaeontology, whose protagonists on both sides are, unusually, both favourites of mine.
It’s about whether the mammals just scraped through the K-T boundary, or not. It’s about whether the big third group of mammals, i.e. us, who neither lay eggs nor have pouches, diversified while the dinosaurs were still alive, or just afterwards.
It’s all the more fascinating because exactly the same argument is going on over birds: did the big third group of birds, (i.e. the ones that aren’t one of the big flightless land birds like the ostrich, kiwi etc, or the tinamous, their still-flying relatives, nor are they in the duck and chicken group), did the group of all the rest – Neoaves – diversify alongside the dinosaurs or suddenly just after their demise?
Saying the placental mammals diversified in the Cretaceous, is Springer and his crew, who endeared themselves to me not just by stressing strenuously that parallelisms caused by similar environmental constraints on creatures from different lineages, could invalidate cladograms, but also used the term “ecological vicars” in expressing the view. Will all readers please send in their sketches or more sophisticated depictions of these ecological vicars lurking in trees, ready to pounce on the unwary! I envisage long roughly-knitted green cardigans or pullovers, and sandals, with or without socks, and maybe all sitting huddled next to each other on a branch like… say bee-eaters or howler monkeys, or perhaps distributed randomly around the trees like Robin Hood and his Merry Men. The rest I leave to your imagination. Also, Springer stresses the advantage of making trees from DNA data rather than from the morphology of bones, with which I also agree.
O’Leary et al. say… well I’m not sure of the details of what they say because it’s frightfully complicated, and I’m not going to spare the couple of days needed to understand it all in detail. But basically they’ve combined DNA and morphologically based trees to produce their one big mammal tree.
Oddly, I find myself agreeing with much of what the Springer side says, but not their conclusion. Also, I am less predisposed to agree with what O’Leary et al. are saying, but I suspect they may well be right, and especially in their conclusion.
Basically, Springer is saying that if the big diversification happened after the Cretaceous, the rates of mutation would need to be astonishingly high – as fast as viruses.
O’Leary says it doesn’t have to be that fast, but anyway, there may well be an increase in the speed of the genetic clock, ie. the rate of mutations, or at least the speed at which mutations get fixed into the genetic pool(s), in the explosive recovery after a mass extinction.
This is a vital issue. I don’t think any theoretical work has been done on how genetic clocks may accelerate in an explosion. It would seem particularly odd to assume they don’t… especially since, in both the bird and mammal cases, when we construct the trees, they both show a massive explosion, particularly in the dominant third group. (Why they should both have exactly three groups I’m not sure!) And in both cases, it’s extremely hard to identify the initial rapid branching processes, and put them into order. But why would both groups show these extraordinary explosions, and at approximately the same time… but then, if these were before the end of the Cretaceous, not show evidence of anything like a similar explosion after the end of the Cretaceous?!
That would require a massive result of an invisible environmental event, and then an invisible result of a definite massive environmental event – twice over, i.e. in both birds and mammals.
There was a bird near-equivalent of the O’Leary big-tree paper: Hackett et al. 2008, but they took the Springer view, of, in their case, Neoaves exploding well back in the Cretaceaous.
Another thing I like about O’Leary et al., is that they’re not scared of ancestors. Cladists, which O’Leary et al. don’t seem to be, despite their making cladograms, are too scared ever to mention the concept of ancestors, even though their precious cladograms specify character sets at the nodes, each of which is an ancestor – and a valid one, if the cladogram is sound. O’Leary peeked into their big cladogram, fished out the character set it contained at its root, and reproduced the animal, at least as a picture.
(It’s genetic reconstruction still lies in the future!) It looked like a tree shrew. (Actually, come to think of it, I don’t know how they decided exactly where the root was.)
I had been thinking that O’Leary’s view, where only three mammal species survived the end of the Cretaceous but also left modern descendants, explains better why there were no marsupials outside Australia and South America until recently, and why there were no placentals inside South America and Australia apart from bats (until a few rodents somehow crept across fairly recently).
Actually though, I suppose they each might have evolved, in the north and south respectively, but before the end of the K, even if Springer were right. I think Africa separated from he southern continents about the start of the Cretaceous or just before. But the Springer view – actually I’m going to separate the protagonists from the theory – the pre-K-T view, would not also include a mass extinction that accounted for why so many other mammal groups disappeared.
But then again, not all the other mammal groups did disappear.
Anyway, my favoured scenario is this:
Shortly after the end of the Cretacous, a primitive duck survived somewhere, probably fairly near Antarctica. Ditto, a tinamou-like thing that gave rise to the ratites; also, there was the Great Third Ancestor that gave rise to all the other modern birds. What this looked like, and why, I guess at in my book. There probably weren’t any other bird survivors because, since birds are both small and mobile, they would probably have avoided extinction for long enough for us to have noticed them. But nonetheless there may have been.
At the same time, just after the dinosaurs died out, there was an ancestral mammalian egg-layer, which probably looked more like the platypus than an echidna, since people suspect the echidna’s ancestry was aquatic. That was probably living in Australia or Antarctica, or just maybe South America (Australia was still joined to Antarctica until surprisingly recently). There was also an ancestral marsupial, quite possibly just one of them, that might have been in Australia, Antarctica or South America. There was something small and primitive in New Zealand, or perhaps a number of different such lineages there. There may have been a few other lineages elsewhere in the southern continents (not including Africa) that have died out.
There were multitubercualates at least in the northern hemisphere but not in the south, and there was probably just the ancestral placental in the north, but not the south. There may have been a very few others in the north where conditions after the meteorite were more harsh than in the south, because it was heading north-west when it hit. Multituberculates survived because they were hugely over-represented, and they included many versions that were not only burrow-dwellers but could eat both insects, which would have been abundant just after the apocalypse, and seeds which they might have stored. The ancestral placental would probably have managed on insects, which were probably its normal diet anyway.
So no birds survived in the northern hemisphere; perhaps just two mammalian species survived in the north, and perhaps just three in the south. That explains the north-south mammalian segregation. However, if the mammals diversified during the Cretaceous, why would there be so few mammal lineages today?… unless there was mass extermination? And if there was mass extermination, why does no second explosion appear in the mammal trees? I suspect the Springer account can’t really explain the mammalian geographical distribution after all.