Picture a world where massive predators stalked the land and pterosaurs ruled the skies. You might think this was a hostile place where small creatures had no chance of thriving. Yet contrary to popular belief, an entire menagerie of mammals flourished , including some that even ate baby dinosaurs for lunch.
The story of Mesozoic mammals is far richer than the traditional narrative suggests. These weren’t just tiny, mouse-like creatures cowering in the shadows of giants. Many of the diverse forms that arose during the Jurassic and Cretaceous resemble species alive today, such as badgers, flying squirrels and even anteaters. Let’s dive into this fascinating world where our ancient ancestors carved out their own remarkable evolutionary paths.
When Giants Ruled and Mammals Emerged
During this period of high evolutionary activity the first dinosaurs appeared slightly earlier than 230 Ma, closely followed by the first mammals only a few million years later. The first mammals also appeared during the Mesozoic, but would remain small – less than 15 kg (33 lb) – until the Cenozoic.
However, size limitations didn’t prevent these early mammals from developing extraordinary adaptations. Those early mammals were small in size and were presumably nocturnal insectivores. They were not able to compete for the main ecological niches with the dinosaurs and other reptile groups, which became the dominant vertebrates at land, the sea, and in the air over the course of the following 175 million years.
Yet this apparent disadvantage would become their secret weapon for survival. The mammaliaforms appeared during this period; their superior sense of smell, backed up by a large brain, facilitated entry into nocturnal niches with less exposure to archosaur predation.
The Mighty Multituberculates
Among all the Mesozoic mammals, none were more successful than the multituberculates. Multituberculata (commonly known as multituberculates, named for the multiple tubercles of their teeth) is an extinct order of rodent-like mammals with a fossil record spanning approximately 120 million years. They are the most diverse order of Mesozoic mammals with more than 200 species known, ranging from mouse-sized to beaver-sized.
These remarkable creatures dominated mammalian ecosystems throughout the age of dinosaurs. They lived and thrived alongside the dinosaurs for more than 100 million years, and outlived them by another 30 million years. Their incredible success came from their specialized dental arrangement, which gave them their name.
The multituberculates had a cranial and dental anatomy superficially similar to rodents such as mice and rats, with cheek-teeth separated from the chisel-like front teeth by a wide tooth-less gap (the diasteme). Each cheek-tooth displayed several rows of small cusps (or tubercles, hence the name) that operated against similar rows in the teeth of the jaw.
Ferocious Predators That Dined on Dinosaurs
Not all Mesozoic mammals were content with insects and plant matter. Repenomamus, a eutriconodont from the early Cretaceous 130 million years ago, was a stocky, badger-like predator that sometimes preyed on young dinosaurs. Two species have been recognized, one more than 1 m (3 ft 3 in) long and weighing about 12–14 kg (26–31 lb), the other less than 0.5 m (1 ft 8 in) long and weighing 4–6 kg (8.8–13.2 lb).
Features of the teeth and jaw suggest that Repenomamus were carnivorous and a specimen of R. robustus discovered with the fragmentary skeleton of a juvenile Psittacosaurus preserved in its stomach represents the second direct evidence that at least some Mesozoic mammals were carnivorous and fed on other vertebrates, including dinosaurs.
Even more dramatically, recent discoveries have revealed actual predation attempts frozen in stone. More evidence suggesting Repenomamus was suited to a predatory lifestyle was later revealed when a specimen of R. robustus was uncovered alongside an adult Psittacosaurus. The intertwined nature of the fossil, similar to the Fighting Dinosaurs fossil of Mongolia, was likely a byproduct of an altercation between the two animals in which the mammal was most likely the instigator of an ongoing predation attempt.
Swimming With the Dinosaurs
Perhaps one of the most surprising discoveries comes from ancient lake beds in China. Uncovered from the Middle Jurassic Jiulongshan Formation of the Inner Mongolia Region, dated approximately 164 million years ago, Castorocauda is the earliest-known mammal that had specialized skeletal and soft-tissue features for swimming and teeth for eating fish.
This beaver-like creature defied all expectations of what Mesozoic mammals should look like. Castorocauda is at least 42.5 cm in body length and more than 6 cm in skull length. Scientists estimate that it weighed about 500 to 800 grams. Castorocauda developed molars specialized for feeding on small fish and aquatic invertebrates, similar to modern seals or the river otters.
The preservation of this fossil was exceptional, revealing details that would normally be lost to time. Castorocauda is preserved with a pelt (guard hairs and under furs), making it the most primitive-known mammal to be preserved with hairs. Carbonized in the fossil, the short and dense under-furs were to keep water from the skin; the longer guard hairs are preserved as impressions on the fossil slab.
Flying Squirrels of the Jurassic
The skies weren’t exclusively dominated by pterosaurs. It had a membrane that acted like a gliding wing which shows it was one of the first flying mammals. It probably glided to access food high off of the ground and to avoid being prey. These early gliders represented another remarkable adaptation that challenged our understanding of mammalian capabilities during the dinosaur age.
Mammaliaforms that arose during the Jurassic radiation included the semi-aquatic, beaver-like Castorocauda; Maiopatagium, which likely resembled today’s flying squirrels; and the tree-climbing Henkelotherium. These diverse forms show that mammals weren’t simply waiting for dinosaurs to disappear before exploring different lifestyles.
The ability to glide provided these ancient mammals with access to resources unavailable to ground-dwelling competitors. It grew up to 9 inches in length and had long fingers. These small primitive flying mammals are not related to extant bats, representing an independent evolution of powered flight in mammals.
Social Networks in the Cretaceous
Recent discoveries have revealed that some ancient mammals lived remarkably complex social lives. Taphonomic and geologic evidence indicates that F. primaevus engaged in multigenerational, group-nesting and burrowing behaviour, representing the first example of social behaviour in a Mesozoic mammal.
Excavated from Egg Mountain in Montana – a well-known dinosaur nesting site – the fossils include skulls or skeletons of at least 22 individuals of Filikomys primaevus, a newly named genus of multituberculate. The small mammal, which was omnivorous or herbivorous, was as abundant in the Mesozoic Era as rodents are today.
This discovery fundamentally changed our understanding of when social behavior evolved in mammals. The trait was thought to have evolved following the Cretaceous-Paleogene mass extinction that killed off most dinosaur species about 66 million years ago, but the study shows that sociality occurred earlier than that, the researchers concluded.
Ecological Pioneers and Niche Masters
Contrary to the old stereotype, Mesozoic mammals explored virtually every available ecological niche. In each of the radiations discussed in the review, mammaliaforms diversified from insect-chomping, rodent-like ancestors and adapted to a variety of ecological niches. New species arose that, for example, could climb, glide or burrow – and ate more specialized diets of meat, leaves or shellfish.
The tally of Mesozoic mammals also includes beasts with beaver-like tails that chased fish in ancient lakes, Jurassic flying squirrel-like creatures, plus mammals that used their powerful forelimbs to tear apart termite nests, and even a badger-like species – called Repenomamus – that’s been found with baby dinosaur bones in its gut contents. Through swimming, gliding, climbing and burrowing, mammals often expanded into ecological roles that dinosaurs did not, undoubtedly assisted by how wonderfully variable mammal teeth can be.
The adaptability of mammalian teeth proved to be a crucial advantage. If you look at the complexity of teeth, it will tell you information about the diet. Multituberculates seem to be developing more cusps on their back teeth, and the bladelike tooth at the front is becoming less important as they develop these bumps to break down plant material.
Masters of the Night Shift
As more time passed, early mammals evolved to become almost entirely nocturnal, and some would eventually master the arboreal lifestyle of living in the trees. Additionally, most dinosaurs were diurnal, but the ones that did eat at night were typically herbivores; it was the carnivores that usually hunted during the day.
This temporal partitioning proved to be a brilliant evolutionary strategy. The nocturnal lifestyle may have contributed greatly to the development of mammalian traits such as endothermy and hair. By avoiding direct competition with large dinosaur predators, mammals could focus on developing the sophisticated sensory systems that would later serve them so well.
The early mammal Morganucodon exemplified this approach. Despite its familiar appearance, Morganucodon had not yet acquired the mammalian physiological advances in regulating body temperature, what we call endothermic or warm bloodedness. Instead, Morganucodon had a slower and almost more reptilian physiology that allowed them to live as many as 14 years – seven times longer than a mouse of similar size today.
Survivors of Multiple Mass Extinctions
The remarkable resilience of Mesozoic mammals becomes even more impressive when you consider the environmental challenges they faced. Adaptive radiation of Mesozoic-era multituberculate mammals began at least 20 million years before the extinction of non-avian dinosaurs and continued across the Cretaceous–Paleogene boundary – probably as a result of dietary expansion towards herbivory during the ecological rise of angiosperms.
But at least one mammal group, rodent-like creatures called multituberculates, actually flourished during the last 20 million years of the dinosaurs reign and survived their extinction 66 million years ago. New research shows that at least one group of small mammals, the multituberculates, actually flourished in the last 20 million years of dinosaurs reign and survived their extinction.
Their success didn’t end with the dinosaur extinction. Following the dinosaur extinction, multituberculates continued to flourish until other mammals – mostly primates, ungulates and rodents – gained a competitive advantage. That ultimately led to multituberculate extinction about 34 million years ago.
Living Fossils of the Mesozoic
Some modern animals provide direct windows into the Mesozoic past. The first mammals were monotremes, or mammals that reproduce by laying eggs. Mammals are common today, but only three monotreme species still exist. These are the duck-billed platypus and a couple of spiny anteaters, or echidnas.
These living fossils represent evolutionary strategies that proved successful throughout the Mesozoic and continue today. Crocodilians have been on the planet for about 240 million years. There are 23 crocodilian species today, including alligators, crocodiles and caimans, demonstrating that survival during the age of dinosaurs required specific adaptations rather than mere luck.
The connection between ancient and modern becomes even clearer when we consider our own lineage. Our own early primate ancestors lived while dinosaurs were still at their apex. A tiny mammal, called Purgatorius by paleontologists, is the earliest known primate – identified from a smattering of bones and teeth found in rock layers both before and after the asteroid impact 66 million years ago. This little omnivore would have scrambled through the trees over the heads of Tyrannosaurus and survived where the non-avian dinosaurs did not.
Conclusion: A Hidden World of Mammalian Success
The ancient mammals of the Mesozoic Era weren’t the cowering, primitive creatures we once imagined. They were sophisticated survivors who developed remarkable adaptations for swimming, flying, burrowing, and even hunting dinosaurs. This is a very old idea, which makes it very hard to defeat regarding the traditional view of mammals during this era.
In fact, without dinosaurs roaming everywhere, mammal evolution probably would not have spun off small, bug-hunting mammals that ran through the trees – or any of the many species that we often don’t notice. The average mass of a modern mammal is under a kilogram, meaning mammals are mostly doing today what they were still doing in the Mesozoic.
These discoveries remind us that evolutionary success isn’t always about size or dominance. Sometimes it’s about finding your niche, adapting brilliantly, and persevering through the most challenging times in Earth’s history. What do you think about these remarkable survivors? Tell us in the comments.
