During the Middle Jurassic about 200 million to 145 million years ago, mammals were evolving 10 times faster than they were by the time the Jurassic was over, according to a new study published in Current Biology. Some of these key innovations include live birth, warm bloodedness, and fur.
The Mesozoic Era – which includes the Triassic, Jurassic, and Cretaceous – is often considered a dino-dominated era with a scattering of tiny, nocturnal, insect-eating mammals. But we’re increasingly finding that that’s just not the case. Earlier this year, the fossils of two surprisingly modern Jurassic mammals were discovered in China: Agile Agilodocodon lived up in the trees, while expert digger Docofossor lived below ground. They’re pictured above along with the swimming, fish-eating Castorocauda, which was discovered last year.
To find out how rapidly new body shapes emerged, Oxford’s Roger Close and colleagues analyzed skeletal and dental changes in 110 groups of mammals. By calculating the rates of morphological evolution across the entire era, they found that mammals underwent an intense burst of innovation and change during the Early to Middle Jurassic. “What our study suggests is that mammal 'experimentation' with different body-plans and tooth types peaked in the mid-Jurassic,” Close says in a statement. “This period of radical change produced characteristic body shapes that remained recognizable for tens of millions of years.”
The frequency of these sorts of significant changes increased up to eight changes per million years per mammal lineage. That’s nearly 10 times higher than that observed for the end of the Jurassic.
The rate of morphological evolution in the ancestors of placental mammals and marsupials were 13 times faster than average in the mid-Jurassic – but by the Late Jurassic, these rates became much lower than average. What’s also interesting is that this slowed-down pace of innovation occurred despite the increase in mammal species. For example, extinct rodent-like mammals called multituberculates experienced major changes in the mid-Jurassic. When they eventually evolved their characteristic rodent-like skeletons and distinctive teeth, they kept this form for the next 100 million or so years, even though they diversified into hundreds of different species.
“We don't know what instigated this evolutionary burst. It could be due to environmental change, or perhaps mammals had acquired a 'critical mass' of 'key innovations'... that enabled them to thrive in different habitats and diversify ecologically,” Close adds. “Once high ecological diversity had evolved, the pace of innovation slowed.”
Perhaps the most famous instance of these sorts of adaptive radiation events is the Cambrian explosion 530 million years ago. That was when marine critters evolved most of the basic body forms we see today. “In the Jurassic we see a profusion of weird and wonderful bodies suddenly appear and these are then 'winnowed down,’” Close says. “What we may have identified in this study is mammals' own 'Cambrian explosion' moment, when evolutionary experimentation ran wild and the future shape of mammals was up for grabs.”