In the Triassic Epoch, a snapshot of Earth’s past reveals the enigmatic Thrinaxodon, an ancient relative of mammals, akin in size and form to today’s mink. Notably, this creature hints at the size of the common ancestor shared by a group of mammalian kin, known as cynodonts, and hints at their penchant for animal-based sustenance. Image Credit: April Neander
Contrary to prior assumptions, the mammals that endured the tumultuous aftermath of mass extinctions were not mere generalists; many harbored unique traits that proved pivotal in ensuring their survival. This revelation prompts a fundamental reevaluation of our understanding of evolutionary dynamics.
Some 66 million years ago, a catastrophic asteroid impact wreaked havoc upon Earth, ushering in a cataclysmic mass extinction event. In its wake, dinosaurs (with the exception of a few avian survivors) perished, along with a substantial portion of the mammalian population. Nevertheless, a resilient cadre of diminutive mammals managed to endure, sowing the seeds for the diverse array of mammals we observe today.
For decades, scientists had held the belief that mammals and their related lineages surviving such trying times, like those marked by mass extinctions, did so because of their adaptability and generalist tendencies, enabling them to subsist on a variety of resources. A groundbreaking study, delving into the annals of the mammalian family tree across numerous mass extinctions, unveils a startling revelation: the surviving species were far from being generic, and their success lay in possessing novel and distinctive characteristics.
Ken Angielczyk, the MacArthur Curator of Paleomammalogy at the Field Museum and senior author of the study, asserts, “The concept of ‘survival of the unspecialized’ harks back to the 1800s, with the conventional wisdom positing that generalized creatures are least susceptible to extinction. Yet, our findings challenge this notion. The survivors often exhibited traits that, in retrospect, may appear generalized but, in reality, represented advanced adaptations for their time, bestowing upon them the evolutionary flexibility needed for survival.”
Examination of ancient synapsids, distant mammalian relatives, presents a thought-provoking picture of inferred ancestor sizes during significant synapsid evolutionary diversifications. Varanosaurus from the Permian Period, akin in size to the forebear of all synapsids, and Morganucodon from the Jurassic Period, roughly mirroring the progenitors of most mammals from the Age of Dinosaurs and contemporary mammals, provide insights. Both of these creatures subsisted on other animals. Image Credit: Photo by Ken Angielczyk
The common notion had been that with each emergence of a new mammalian lineage, a small and versatile creature served as its progenitor, as these were thought to be the ones most likely to persevere during catastrophes—able to conceal themselves effectively and feed opportunistically on available resources. In essence, the survivors of mass extinctions were not specialists, akin to a panda subsisting solely on bamboo.
David Grossnickle, an Assistant Professor at the Oregon Institute of Technology and co-lead author, previously published research in 2019 highlighting the proclivity of small insectivorous mammals to withstand challenging epochs, including the extinction event that spelled doom for the dinosaurs, while also foreshadowing major diversifications. The question arose: did this trend apply to earlier mammals and their predecessors?
Spencer Hellert, an Assistant Professor at Columbia College Chicago, a research associate at the Field Museum, and co-lead author, meticulously constructed a sprawling family tree encompassing synapsids, the animal group from which mammals ultimately descend. This family tree represents one of the most extensive fossil trees ever compiled, integrating previous scientific lineage diagrams into a more formal, rigorous, and reproducible synthesis of information.
Angielczyk elaborates, “A comprehensive family tree of this magnitude was essential for testing this idea. We needed to include pertinent information about the diets and body sizes of these creatures. Subsequently, we scrutinized the developments that occurred throughout the five major synapsid evolutionary radiations. In these episodes, a handful of species diversified significantly. However, when confronted with a new catastrophe, the majority of these species fell victim to extinction, marking the cycle’s repetition.”
Intriguingly, the research team, which also included co-authors Graeme Lloyd and Christian Kammerer, discovered that the narrative of synapsid evolution defied the expected “survival of the small and unspecialized” paradigm. At various points, larger synapsids emerged as survivors, and the victors were not exclusively generalist insect-eaters.
Grossnickle expresses his surprise, noting, “It’s well-established that mammalian radiations typically commence with small insectivores that subsequently evolve into larger taxa, a pattern I expected to observe when tracing back through synapsid history. Yet, as we ventured further back in time, this pattern began to fade.”
Novel Traits: The Evolutionary Edge
While some of the survivors of mass extinctions initially seemed unspecialized, closer scrutiny unveiled the presence of innovative traits. For example, many mammals from the dinosaur era exhibited teeth adept at cutting through prey. A select few possessed dental structures resembling mortar and pestle, capable of grinding food in addition to cutting. This more sophisticated dental adaptation might have conferred an advantage during lean times, enhancing their ability to consume a broader array of sustenance.
These findings do not imply that highly specialized animals like pandas, which solely subsist on bamboo, are less susceptible to extinction compared to their more generalist counterparts, such as raccoons with diverse dietary preferences. Rather, the study underscores that the mammalian relatives that weathered mass extinctions were far from generic as previously surmised.
Grossnickle concludes, “Animals endowed with novel traits, such as unique tooth features or improved jaws for breaking down a variety of food items, do not typically dominate ecologically until older, incumbent lineages succumb to extinction. It often takes an extinction event, akin to the one that eliminated the dinosaurs, to clear the way for these more specialized creatures to persist and diversify.”
This research holds broader implications for our comprehension of evolutionary processes. Angielczyk contends, “We are left questioning whether there exists a consistent set of attributes commonly possessed by the ancestors of evolutionary diversifications. The complexity we discern in mammalian and ancient relatives’ diversifications suggests the need to investigate other groups to determine whether the scenario in mammals represents an anomaly or a recurring phenomenon.”
Reference: “Derived faunivores are the forerunners of major synapsid radiations” by Spencer M. Hellert, David M. Grossnickle, Graeme T. Lloyd, Christian F. Kammerer, and Kenneth D. Angielczyk, 5 October 2023, Nature Ecology & Evolution.
Frequently Asked Questions (FAQs) about Mammal Survival Strategies
What is the main finding of the study mentioned in the text?
The main finding of the study highlighted in the text is that mammals and their ancient relatives that survived mass extinctions were not necessarily generic generalists as previously believed. Instead, they often possessed unique and novel traits that aided in their survival. This challenges the traditional notion of “survival of the unspecialized” and suggests that advanced adaptations played a crucial role in their ability to endure catastrophic events.
How did the study challenge conventional wisdom regarding survival during mass extinctions?
The study challenged conventional wisdom by demonstrating that survivors of mass extinctions were not always small and versatile generalists. While it was commonly believed that such generalist creatures were more likely to endure catastrophic events, the research revealed that some survivors exhibited advanced traits, including specialized dental structures and other unique features. These traits provided them with an evolutionary edge, enabling them to adapt and thrive in challenging times.
What role did dental features play in the survival of some mammalian relatives?
Dental features played a significant role in the survival of certain mammalian relatives. Many of these creatures possessed teeth that were not only adept at cutting into prey but also had more specialized structures resembling mortar and pestle, allowing them to grind food. These “fancier” dental adaptations may have been advantageous during periods of food scarcity, as they enabled a broader range of dietary options. This suggests that unique dental traits were one of the factors contributing to their survival.
Does the study suggest that highly specialized animals are less vulnerable to extinction?
No, the study does not suggest that highly specialized animals, such as pandas that exclusively eat bamboo, are less vulnerable to extinction compared to more generalist species. Instead, it emphasizes that survivors of mass extinctions were not as generic as previously assumed and often possessed novel traits. The research underscores the importance of adaptability and the acquisition of innovative characteristics in the face of changing environments.
What broader implications does this study have for our understanding of evolution?
This study has broader implications for our understanding of evolution. It raises questions about whether there is a consistent set of characteristics typically possessed by the ancestors of evolutionary diversifications. The complexity observed in the diversification of mammals and their ancient relatives challenges preconceived notions and suggests the need to explore other groups to determine if similar patterns are evident in different evolutionary contexts.