The New York Times has a story up, After the Dinosaurs’ Demise, Many Mammals Seized the Day. It’s a write-up of a new paper that is open access, Temporal niche expansion in mammals from a nocturnal ancestor after dinosaur extinction.
This research illustrates how computational power has changed evolutionary biology. There has long been an intuitive verbal model that mammals were ancestrally night-adapted creatures based on aspects of their biology, as well as the evolutionary reality that for most of the lineages’ existence they were overshadowed by dinosaurs (remember, more than half of our evolutionary history predates the Cenozoic).
But today we do more than posit models which match and predict the fossil (or genetic) data. Computationally intensive phylogenetic frameworks are tested using extant lineages to generate probabilities of given scenarios generating the data we see given particular models. Something like the Reversible-jump Markov chain Monte Carlo (which is used in this paper) could actually be done manually…if a phylogeneticist had thousands of slaves to do all the computations. Obviously, the emergence of powerful computers accessible to all really changed the game in terms of analytic power.
And yet I wonder about the sense of precision that people gain from these methods. Verbal models are necessarily vague. When you give a probability of a given hypothesis being 0.71, that gives understanding a solidity. But is it warranted? Though researchers understand all the individual moving parts of the phylogenetic framework, only a computer can really bring it all together.
It’s something to consider. This is to a great extent the future of evolutionary biology. Positing models, and put it into a calculating machine like Leibniz dreamed of.
Citation: Temporal Niche Expansion In Mammals From A Nocturnal Ancestor After Dinosaur Extinction
Roi Maor, Tamar Dayan, Henry Ferguson-Gow, Kate Jones
Addendum: This is stupid of me, but only after reading the above paper did I reflect that most amniotes are diurnal and that mammals are the exception. Think about it, birds. And reptiles are probably more sluggish at night.
So, mammals evolved thermal regulation, i.e, evolved as endotherms, in order to function after dark and avoid dinosaurs? The same reasoning would not apply to birds since they descended from (some) dinosaurs and did not have to compete with them. Why would dinosaurs have evolved thermal regulation, then? Just because it made them more fit (seems kind of circular)?
The problem with the temporal partitioning hypothesis is touched upon lightly in the discussion portion of the paper: Dinosaurs do not appear to have been strictly diurnal. A study in 2011 which measured fossilized scleral rings found that most plant-eating dinosaurs were probably active during both the day and night, and that some predatory dinosaurs (including smaller ones which could hunt mammals) were nocturnal. Therefore, while it appears to be true that mammals stayed nocturnal and small(ish) throughout the Mesozoic, the reason for it wasn’t simply that mammals were competed out of the daytime niches and had to stay in the dark where it was safe.
Honestly, the evidence that dinosaurs and mammals directly competed during the Mesozoic in general is fairly limited. Even considering small non-avian dinosaurs, with only a few exceptions, they tended to weigh more than a pound, making them at least an order of magnitude larger than the average Mesozoic mammal. It may well be that mammals were pushed into their evolutionary redoubt mostly because dinosaurian physiology simply didn’t work well at the scale of a few tens of grams, yet mammals remained competitive with other small undergrowth animals like lizards and frogs.
The Freaks Come Out At Night, then?
From a lay reading endothermy / proto endothermy developed in both lines leading to mammals and dinosaurs (birds) before the Mesozoic.
Indeed, as you may know, the mammals were actually predominant in the age preceding the dinosaurs. They show trends toward improved gait, endothermy, encephalization. Nearing the end of the Permian, evidence of developing endothermy also begins to show in dinosaur ancestors (the archosaurs).
What might have happened? Well, we know that the Permian Triassic extinction event is actually the bigger, more impactful extinction: the biggest of all time. It is thought that the world of the Triassic began as a very arid, very low oxygen world, no doubt related to tectonic activity and the formation of Pangaea. Large parts of the interior of that supercontinent would be a desert hellscape.
What do we know of birds, and of reptiles generally? They do not urinate. They excrete. Nor do they sweat. Birds have superior respiratory systems to mammals.
Among the very earliest dinosaurs we see the development of a proto pseudo raptor form. So imagine, if you will, a relatively agile, relatively warm blooded omnivore, that doesn’t lose water to sweat or urine, that has the beginnings of a superior oxygen extractive system, running the sand dunes that are now the norm of a dusty, barren world