Oxford University and Macquarie University researchers have concluded that mid-Jurassic mammals had an explosive evolution compared to later eras. As scientists collect evidence and data, more questions pop up about what fueled the growth.
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Jurassic mammals faced the most explosive evolution during the mid-period, according to researchers.Scientists from Oxford University and Macquarie University in Australia researched why mammal evolution up to ten times faster in the period, unlike the beginning and end.
The answer? New adaptations.
During the Mesozoic era, roughly 252-66 million years ago, mammals were considered to be nocturnal and insect-eaters, but the past decade has shown that adaptations helped bring about new changes. For example, combined improvements in eating and locomotion offered more possibilities. When new food’s available through gliding, digging, and swimming, mammals are able to find more diverse and nutritious options. Most of this information is widely available thanks to discoveries in China and South America.
Oxford researches did a large-scale analysis of skeletal and dental changes in era mammals. Evolutionary rates indicate the burst occurred around 200-145 million years ago.
Working in the university’s Department of Earth Sciences, Dr. Roger Close, notes that the changes show “mammal ‘experimentation’ with different body-plans and tooth types peaked in the mid-Jurassic.”
What does that mean, exactly? “This period of radical change produced characteristic body shapes that remained recognizable for tens of millions of years.” To record the changes, the team documented all changes that happened every million years and found a frequency of up to 8 changes per million years. That’s almost ten times the rate at the end of the era.
Therian mammals, the forebears of placental mammals and marsupials, surpassed even though numbers. At a rate of 13 times faster than average for other mammals, the slow down leaves questions on why the numbers feel so suddenly in the late Jurassic period.
“We don’t know what instigated this evolutionary burst.” Close’s team has a theory, however. “It could be due to environmental change, or perhaps mammals had acquired a ‘critical mass’ of ‘key innovations’ – such as live birth, hot bloodedness, and fur – that enabled them to thrive in different habitats and diversify ecologically.”
When ecological diversity evolves, the “pace of innovation slowed” down significantly. In other words, when the basic changes become the body base, things tend to taper off. Take the multituberculates for instance. Despite showing drastic changes to teeth and skeletons, the rodent-like body form and distinctive teeth remained until their extinction sometime around 130 million years later in the early Oligocene.
Multituberculates demise has been credited with the rise of modern rodents in some circles. Controversy surrounds the idea since rodents lived alongside the predators for quite some time. What can be determined is that as the planet’s vegetation and climate changed, predatory pressure also changed. Essentially, Gregg E. Ostrander's 1984 theory concludes that it wasn’t only competitive inferiority, but all the elements combined to wipe out the large mammals. As long extinctions happened, the adaptable animals were the most likely to survive.
But the rise in the mid-Jurassic is more “characteristic of other ‘adaptive radiation’ events of this kind, such as the famous ‘Cambrian explosion,” says Dr. Close. “In the Jurassic we see a profusion of weird and wonderful bodies suddenly appear and these are then ‘winnowed down’ so that only the most successful survive.”
The old saying “only the strong survive” seems rather appropriate. But even more impressive is the fact the researchers “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.’”
As information becomes more available through investigation and discovery, who knows what else will be added to the explosions of life on Earth before humankind walked the planet.
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The research report is available in the journal Current Biology.