After the extinction of the dinosaurs, into newly opened niches: from small, chipmunk-like creatures to large mammals
A new era after the dinosaurs
Before 66 million years, K–Pg mass extinction wiped out non-avian dinosaurs, as well as marine reptiles such as mosasaurs, and many other groups. Although the extinction of large land vertebrates was a catastrophe for Mesozoic ecosystems, it liberated ecological "area" that has been pushed aside until then mammals—long after the dinosaurs—could quickly take over. During the later Paleocene, Eocene, and later, these small, isolated animals evolved into a multitude of forms: from giant herbivores (ungulates) to apex predators (credonians, later true predators), to marine whales and flying bats. Modern mammals are the descendants of these exceptional post-Cretaceous dispersal a legacy that illustrates success based on adaptation and innovation.
2. The roots of Mesozoic mammals
2.1 Early mammals: small and often nocturnal
Mammals arose at the same time or slightly earlier than dinosaurs Late Triassic (~225+ million years ago). Their ancestors were synapsids (sometimes called "mammal reptiles"), while early true mammals were small, with advanced jaw and ear bone structure, fur for warmth, and lactation. For example:
- Morganucodon (~205–210 million years ago): Basal mammaliaform, small insectivore.
- Multituberculates: A very successful group during the Mesozoic Era, often compared to rodents in terms of ecomorphic niches.
Such animals coexisted with dinosaurs for more than 100 million years, mostly occupying lesser nocturnal or insectivorous niches, presumably to avoid direct competition with large diurnal reptiles.
2.2 Mesozoic constraints
Larger body sizes and wider daily activities were severely limited by the dominance of dinosaurs. Many mammals remained small (from the size of a chipmunk to a cat). This is confirmed by fossil finds, which rarely show larger Mesozoic mammals. There are exceptions (Repenomamus – a Cretaceous mammal that preyed on baby dinosaurs), but they are rare.
3. K–Pg extinction: a new possibility emerges
3.1 Catastrophic events
Before 66 million years, Chicxulub asteroid impact and perhaps intensified Dean's Stairs eruptions in India caused planetary upheavals: "impact winter", global fires, acid rain, etc. Non-avian dinosaurs, pterosaurs, large marine reptiles and many invertebrate groups became extinct. Smaller, flexible organisms - birds, small reptiles, amphibians and mammals – had a better chance of survival after the catastrophe. Initially, the world was resource-poor, so adaptation became essential.
3.2 Extant mammals
Surviving mammals probably had:
- Small body: Lower absolute food requirements.
- Flexible diet: Insectivores or omnivores could have used temporary resources.
- Safe hiding habits: The creation of caves or nests provided protection from environmental extremes.
After the most severe climatic stress subsided, a world without major competitors opened up for these surviving lines – ideally quickly. radiation.
4. Early Paleocene: Mammalian Radiation
4.1 The Paleocene "explosion"
Paleocene (66–56 million years ago)) was going on brightly mammals a leap in size, diversity and abundance:
- Multituberculates continued to thrive as rodent-like herbivores/omnivores.
- New placental and marsupials Mammalian lineages expanded to include specialized frugivores, predators, and insectivores.
- Condylarths (archaic ungulates) arose, later evolving into modern ungulates.
- Cimolesti whether "Paleocene predators" occupied the niches of small predators.
Apart from dinosaurs, mammals have established themselves in the vacant niches: medium-large herbivores, predators, sticklebacks or flying forms. Fossil sites such as Bighorn Basin in North America, shows an abundance of early Paleocene mammalian remains, reflecting transitional ecosystems recovering from extinction [1], [2].
4.2 Climate and vegetation
The warm weather of the Paleocene, with regenerating forests in place of the destroyed Mesozoic vegetation, provided abundant food sources. Angiosperms (flowering plants), which had spread since the Late Cretaceous, provided fruits and seeds for the new mammalian diets. Meanwhile, insects also recovered, supporting the flourishing of insectivores. Thus, increasingly complex mammalian communities formed.
5. Eocene and further diversity
5.1 The "second" phase of mammalian evolution
Eocene (~56–34 million years ago) mammals became even more specialized:
- Ungulates (ungulates): divided into artiodactyls (even-toed ungulates) and perissodactyls (odd-toed ungulates).
- Primates continued to evolve with tree-adapted features (adapiformes, omomycidae).
- Early predatory mammals (miacids) and other predator lineages began to replace or overshadow older Paleocene predatory groups (e.g., creodonts).
Body size increased in many lineages. Some whale ancestors (pakicetids) transitioned from land to water in the early Eocene, eventually evolving into fully marine cetaceans. Ecological systems became increasingly complex, resembling many modern mammalian orders.
5.2 PETM (Paleocene–Eocene Thermal Maximum)
PETM (~56 million years ago) is a brief period of rapid warming that likely triggered migrations and evolutionary changes in mammals. Many lineages appear in the Northern Hemisphere fossil record as having arrived from further south. Mammalian plasticity—enhanced by endothermy—allowed them to better adapt to climate extremes that other groups might not have been able to withstand.
6. Adaptive innovations and vacated niches
6.1 Body size jump
One of the most striking features of post-Cretaceous mammals was rapid change in body sizeBy the middle Eocene, certain herbivores (such as brontotherians or large perissodactyls) were as large as smaller dinosaurs.Coop rules"Cope's Rule" - the phenomenon that evolution tends to increase average size - partly explains how the extinction of dinosaurs created vacant ecological niches for large mammals.
6.2 Complex social/behavioral strategies
Mammals evolved advanced parental care, possible social groups, and diverse dietary specializations. Endothermy allowed them to function at night or in cooler environments. Some lineages (e.g., rodents) were highly adaptable, with rapid reproduction and flexible diets that replaced the niches of small dinosaurs or large reptiles.
6.3 Stepping into the air and water
Bats (order Chiroptera) evolved into true flight, a function previously performed by pterosaurs.Meanwhile, new lineages of marine mammals (whales, sirenians) moved from land to sea, replacing the Mesozoic niches of marine reptiles as large ocean predators/feeders. In each domain—air, land, sea—mammals took firm positions, no longer overshadowed by dinosaurs or marine reptiles.
7. The most important lines after K–Pg
7.1 Placental units
The present-day orders of placental mammals (primates, carnivores, ungulates, rodents, etc.) derive from Paleocene–Eocene expansions. Phylogenomic studies suggest that the main branches diverged at or after the K–Pg boundary, although the exact date is debated. Some lineages may have begun to diverge as early as the Late Cretaceous, but only really dispersed after extinction [3], [4].
7.2 Vertebrates
Vertebrates flourished in the early Cenozoic, especially in South America and Australia, on these isolated continents. Their distribution in North America was historically limited until later migrations. The K–Pg event may have been a "disease" equalizing factor, temporarily allowing vertebrates to expand before placentals gained the upper hand in many areas.
7.3 Sunset of multituberculates
Multituberculates, the successful "rodent" mammals of the Mesozoic, survived into the Paleocene, but gradually declined, overshadowed by true rodents (which appeared in the Eocene) and other advanced placentals. This suggests that some of the Mesozoic surviving groups eventually gave way to new clades that emerged after winning the competition.
8. Fossil data and sources
8.1 Important Paleocene sites
Regions such as Williston Pool, San Juan Basin and Paris swimming pool has many Paleocene mammal fossils. Each trace reveals the recovery of ecosystems after the K–Pg crisis, with intermediate forms connecting Mesozoic remains and more modern orders. Fine features of the skulls and teeth show how diets quickly divided - some adapted to hard vegetation, others to carnivory or omnivory.
8.2 Eocene Lagerstätten
Messel (Germany), Green River (Wyoming, USA) and Fayum (Egypt) are sites from the Eocene period that have preserved exceptionally well-preserved mammal fossils (sometimes including fur and stomach contents). They provide evidence of transitional forms of early horses, primates, bats, and whales, as well as their lush ecosystems.
8.3 Molecular phylogenetics
Besides fossils, molecular clocks, based on the DNA of modern mammals, help determine the dates of the divergence. Although the time scales of fossil and molecular studies sometimes do not match, both indicate that the great wave of mammalian diversity occurred after the K–Pg boundary, when these lineages were "liberated" by the end of the constraints of the Cretaceous world.
9. Why did mammals emerge?
9.1 Ecological and biological factors
- Small, omnivorous or insectivorous existence: survived the K–Pg cataclysm better than large specialized ones.
- Endothermy and fur: Allowed to regulate heat even in a "nuclear winter."
- Reproduction strategies: Longer parental care, lactation, possibly faster generational change, favorable for adaptation.
These traits gave mammals advantages after the K–Pg, allowing them to quickly occupy vacant niches as the world stabilized.
9.2 Morphological plasticity
Mammals are characterized by flexible body shapes: an upright posture, a dental system composed of various types of teeth (molars, canines, incisors), and adapted limb types.Without dinosaurs as large herbivores/predators, they could have expanded unhindered into new morphological boundaries, from large herbivores to apex predators, climbing fliers, or aquatic specialists.
10. Significance for the history of life on Earth
10.1 Foundation for present-day faunas
The rapid rise of mammals in the Paleogene laid the foundation for modern terrestrial ecosystems – Primates eventually created apes and humans, Predators – cats and dogs, Artiodactyls – cattle and deer, etc. Marine mammals replaced the niches of Mesozoic marine reptiles, whales, seals, etc. evolved. Essentially, the end of the dinosaurs led to the mammal-dominated world we know today.
10.2 Post-extinction patterns
By observing how mammals evolved after the K–Pg, we understand a general pattern of life recovering from mass extinctions. The surviving opportunists evolve into various morphological “experiments.” Millions of years later, these lineages form into new, stable ecosystems. If not for that cosmic collision, large dinosaurs might still reign supreme, forever limiting the evolution of mammals.
10.3 Lessons for contemporary biodiversity
As Earth’s climate and ecosystems change in the face of human activity, the K–Pg extinction highlights the importance of sudden shocks, climate stresses, and the adaptive capacity of certain groups. Mammals only adapted to new environments when extinctions eliminated major competitors. The current ecological crisis may also provide an opportunity for unexpected winners (invasive or omnivorous species) to emerge as specialized species disappear. By studying the post-Cretaceous recovery, we understand how quickly biodiversity can reorganize—and what unexpected consequences it can have.
Conclusion
The rise of mammals The K–Pg extinction event is one of the most important transformations in Earth's history. Mammals, long in the shadow of the dinosaurs, took the opportunity to disperse into the vacant niches and in a relatively short time developed forms spanning the size scale from the size of a chipmunk to rhinoceros-sized giants. In subsequent periods, they further differentiated into primates, raptors, ungulates, bats, marine cetaceans, etc., creating the mammalian world of today.
While dinosaurs remain iconic symbols of prehistory, their demise paved the way for our own success as mammals, highlighting the paradox that a tragic extinction can spur a new wave of innovation. Through the fossil record, morphological changes, and molecular data, paleontologists are telling a dynamic story of how tiny, often nocturnal Mesozoic mammals became the creators of the new Cenozoic world—showing how big shocks can reshape the evolutionary landscape, opening the door to unexpected triumphs.
References and further reading
- Alroy, J. (1999). "The fossil record of North American mammals: evidence for a Paleocene evolutionary radiation." Systematic Biology, 48, 107–118.
- Rose, K. D. (2006). The Beginning of the Age of Mammals. Johns Hopkins University Press.
- O’Leary, M.A., et al. (2013). "The Placental Mammal Ancestor and the Post–K–Pg Radiation of Placentals." Science, 339, 662–667.
- Beck, R. M. D., & Lee, M. S. Y. (2014). "Ancient dates or accelerated rates? Morphological clocks and the antiquity of placental mammals." Proceedings of the Royal Society B, 281, 20141278.