Mammal Evolution Exploded During The Jurassic Era

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When thinking about the rise of mammals, the general pervasive notion is that during the Mesozoic era, while early mammals were coexisting with dinosaurs, most of the mammals resembled small, rat-like insectivores. Discoveries over the past decade are beginning to prove this is not necessarily true—early mammals like Yanoconodon were adapted for digging, the beaver-like Castorocauda for swimming, and the original “flying squirrel” Volaticotherium for gliding. Some species discovered during the Jurassic actually weighed up to 1kg, which is large for an early mammal.

The swimming Jurassic mammal, Castorocauda. Image Credit: Nobu Tamura via Wikimedia Commons CC-BY 3.0

How and why did mammals become so diverse in their early history? The apparent morphological diversification of mammals during the Jurassic had been previously recognized but never quantified. In a paper published last week in Current Biology, co-authors Roger Close, Matt Friedman, Graeme Lloyd and Roger Benson from Oxford University and Macquarie University sought to quantify and assess this change in morphological diversity—if in fact mammals did undergo an adaptive radiation, there should be a detectable and significant increase in early rates of morphological evolution leading to increased morphological disparity. Morphological disparity is a paleontologist’s way of saying “diversity of physical forms between organisms.”
Using robust datasets of mammalian teeth and skeletal characteristics, evolutionary rates and morphological disparity were quantified statistically in order to investigate this  Jurassic diversification. The results show that in fact there is a distinct, significant peak of morphological evolution during the Early and Middle Jurassic (about 200 to 163 million years ago), slowing down through the Late Jurassic. There are other small peaks of diversification seen throughout the Mesozoic but none of these are statistically significant when compared to the background noise of the analysis.

A typical Jurassic ecosystem with docodonts, a group of now-extinct early mammals. Image Credit: April Neander.

Looking at these data, it is clear there was a distinct adaptive radiation for crown-group mammals (lineages related to modern living groups). The statistical analyses support the rapid appearance of distinct lineages and key features—such as certain molar shapes—during the Jurassic as not being merely an artifact of the fossil record. Rates of diversification were much higher in the Jurassic than in the Cretaceous, and would not increase again until after the non-avian dinosaurs went extinct.

What was special about the Jurassic that lead to this increased rate of morphological diversification? There is no sure way to answer that question, but the breakup of Pangaea during the Middle Jurassic may be a reason. Perhaps this lead to new ecological niches opening that allowed for these new key innovations to arise and lead to the emergence of the mammalian groups that would continue to dominate the rest of the Mesozoic era.