What Sparked the Mammal Explosion?

If you think tiny shrew-like mammals scurried afoot below dinosaurs right before they went extinct, you’ve got the wrong picture.

A paper in Current Biology has a curious headline: “Mammalian evolution: A Jurassic spark.” Despite the pun, the title implies something dramatic happened when the first mammals appear in the fossil record—a “spark” of rapid diversification reminiscent of the Cambrian explosion.

Mammals first appear in the fossil record at about the same time as the earliest dinosaurs (∼220 million years ago), and so the first two-thirds of mammalian evolutionary history thus occurred during the Mesozoic ‘Age of Dinosaurs’. Mesozoic mammals were long portrayed as tiny, shrew-like creatures, unable to diversify due to severe competition and predation from dinosaurs and other reptiles. However, discoveries in the past two decades have greatly expanded the known diversity of Mesozoic mammals, revealing the existence of specialised gliders, climbers and burrowers, semi-aquatic forms and even badger-sized carnivores that ate small dinosaurs. Evidence of extensive ecological differences has been found even between closely-related species, and quantitative analyses of the skulls and skeletons of Mesozoic mammals suggest a diverse range of diets and locomotor modes. Although the ecological and functional diversity of Mesozoic mammals has received increasing attention, the tempo of their adaptive radiation has seldom been quantified. In a new paper in Current Biology, Close and colleagues now show that, during the Mesozoic, mammals evolved very rapidly during the early and middle Jurassic (∼201–164 million years ago), with the average rate of change during this period being twice as fast compared to the remainder of the Mesozoic.

This period of rapid evolution also broadly coincides with peaks in morphological disparity (as measured by the average morphological difference between contemporaneous species) and lineage diversity (as measured by the number of contemporaneous branches on the evolutionary tree). Together with previous studies which have highlighted the ecomorphological diversity of Jurassic mammals, these results demonstrate that mammals underwent a sustained and extensive adaptive radiation during the Jurassic, when dinosaurs also underwent a major increase in diversity and disparity.

Why is this not portrayed in museum dioramas of dinosaur habitats? Lee and Beck describe how the old (incorrect) picture of Jurassic mammals proceeded from biased sampling of morphological characters and phylogenetic techniques that incorrectly portrayed temporal modes of evolution. In short: mammals appeared abruptly and rapidly inhabited all kinds of habitats. True, the largest weighed about a kilogram in the Jurassic (up to 10 kilos in the Cretaceous), but the variety of adaptations calls for excessive amounts of mutation and selection for the time available. And you know evolutionists are in trouble when they pull out their magic wand, convergent evolution —

The Jurassic radiation of small mammals also underscores the prevalence of convergent evolution. Phylogenetic analyses of modern mammals have highlighted how similar ecomorphs (e.g. ant-eating forms, gliders, specialised burrowers and carnivores) evolved multiple times during the Cenozoic. Ongoing studies of their fossil relatives are revealing that many of these ecomorphs also evolved repeatedly, and relatively rapidly, during the Age of Dinosaurs. Early mammals, despite living in the shadows of the dinosaurs, were diverse and successful.

Close et al. describe the mammal radiation as an “intense burst” of evolution. They use the word “rapid” three times.

Contrary to the traditional view that Mesozoic mammals were exclusively small, generalized insectivores, discoveries in the last two decades, especially from China, have demonstrated that they were adapted for diverse feeding and locomotor ecologies. These finds extend the early mammal repertoire to include digging, climbing, gliding, and swimming and show that some non-therian lineages achieved surprisingly large body sizes (up to approximately 1 kg

Given these revelations, what else about the fossil record are they not telling us?

Let’s play word association games like the ones pollsters do about politicians. What word comes to mind when you hear “evolutionist”? Answer: liar, untrustworthy, dishonest, …