The cataclysmic event that occurred 66 million years ago, when an asteroid struck the Earth, is infamous for leading to the demise of the dinosaurs. But what is perhaps less known is that this mass extinction event also marked the rise of a new class of mammals, our own distant relatives.
The last mass extinction set the stage for the dominance of mammals on Earth. However, the commonly held beliefs about which types of mammals survived these challenging times are being rewritten.
For many years, the widely accepted theory among scientists was that mammals and their kin, which persevered through hard times like the mass extinctions, were generalists. In layman’s terms, these creatures could eat a variety of foods and adapt to diverse living conditions.
This theory, known as the “survival of the unspecialized”, has been prevalent since the 1800s. However, a new study delving into the family tree of mammals through several mass extinctions suggests that this might not be the complete picture.
Ken Angielczyk, the MacArthur Curator of Paleomammalogy at the Field Museum and the senior author of this revolutionary study, challenges the old paradigm.
He explains, “The conventional wisdom is that generalized animals are the least likely to go extinct. But we found that the ones that survived more often only seemed generalized in hindsight, when compared with their later descendants.”
These survivors, he points out, were fairly sophisticated for their era, possessing traits that not only aided their survival but also gave them an evolutionary edge.
This shattering of conventional wisdom comes as a result of rigorous research. Spencer Hellert, an Assistant Professor at Columbia College Chicago and a co-lead author of the study, paints a clearer picture of the preconceived notions.
“What’s been thought previously is that every time a new group of mammals evolves, you start out with a small generalist animal,” he explains. He contrasts this with the case of a panda bear, a specialist which primarily feeds on bamboo, suggesting that such specialists were not the typical survivors of mass extinctions.
The research undertaking was monumental. Hellert curated one of the most comprehensive family trees of synapsids – a group which encompasses all mammals and their extinct kin. This meticulous family tree, which encompasses numerous fossil records, was designed to summarize and amalgamate previous family trees in a more systematic manner. As Angielczyk pointed out, a colossal family tree was integral for testing their hypotheses.
David Grossnickle, another co-lead author and an Assistant Professor at the Oregon Institute of Technology, earlier highlighted that small, insect-eating mammals were the ones often surviving challenging epochs. This current study was an attempt to check if such trends persisted among ancestral mammals.
However, the discoveries were astounding. The team, which included co-authors Graeme Lloyd and Christian Kammerer, found that synapsid evolution was not merely a story of “survival of the small and unspecialized”. At various junctures, it was the larger synapsids that prevailed, and they weren’t confined to insect-eating.
Grossnickle elucidates on the surprise: “We were pretty surprised— it’s pretty well-established that those mammal radiations go from these small insectivores into the bigger taxa repeatedly.” This pattern, however, began to vanish as the researchers delved deeper into synapsid history.
A closer examination of the survivors showcased that these animals were more specialized than previously believed. A pertinent example lies in the dental adaptations of many mammals from the dinosaur era.
While most had teeth suited for tearing into prey, a select few possessed a more evolved tooth structure reminiscent of a mortar and pestle. This nuanced tooth design could have been crucial during times of scarce food, allowing these mammals to consume a diverse array of foods.
The study’s findings bring to light that the survivors of mass extinctions may not always be generic or unspecialized creatures. Grossnickle concludes, “You often need an extinction event like the one that killed the dinosaurs to knock out some of those older groups, and then it allows those fancier animals to persist and diversify.”
The broader implications of this study shed light on our understanding of evolution. As Angielczyk aptly summarizes, “The fact that we see this complexity in the diversifications of mammals and their ancient relatives means we need to examine other groups to see if the situation in mammals is an exception, or business as usual.”
In essence, this study not only challenges long-held beliefs but also paves the way for fresh inquiries into the annals of evolution. And as with all groundbreaking research, it leaves us with more questions than answers, pushing the boundaries of our knowledge and understanding.
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