The Mesozoic is commonly called "the Age of Reptiles" because the largest animals at the time (and the largest land animals of all time) were all reptiles, from non-avian dinosaurs on land, pterosaurs in the sky, and mosasaurs, plesiosaurs, and ichthyosaurs in the water. Around the same time as the first dinosaurs in the late Triassic, the first true mammals evolved, but due to reptiles occupying all the large niches, they rarely reached a metre long.
Since Most Writers Are Human and Reptiles Are Abhorrent, it's easy to sympathise with our distant ancestors. In fiction, non-fiction, and Speculative Documentaries alike, they're usually portrayed as underdogs whose potential is yet to be unleashed as they cower beneath their dinosaur overlords. This trope may have risen to popularity in The '80s, when Hollywood Prehistory became a Dead Horse Trope coupled with the Dinosaur Renaissance influencing popular culture, establishing dinosaurs as complex and prosperous even today.
In Real Life, this trope wasn't the case, as they were quite a diverse and successful group back then, with swimmers, gliders, and burrowers among their ranks. Some of them were so big and fierce that they even ate dinosaurs.
Primate Versus Reptile is a related concept, where a mammal and a dinosaur-like reptile are seen as adversaries with the mammal usually being the more "heroic" of the two.
- Were-mammals in the world of Beast Fables revere a Moses-like figure called the Torch Bearer, a Didelphodon who led their ancestors to safety away from the Tyrant King, implying that other mammals at the time were meek.
- Age of Reptiles: "Tribal Warfare" features a primitive primate-like mammal as a Chekhov's Gunman in the Tyrannosaurus nest. It's easily chased off even by a T. rex baby, but it gets the last laugh in the end. When the last surviving Tyrannosaurus returns to the nest after the Final Battle, he discovers the mammal eating the last egg.
- Dinosaur: The lemur tribe (which live on a tiny isolated island) is extremely fearful of dinosaurs before adopting Aladar, and evidently dinosaurs do not think much of them either. When Baylene and Eema first notice the lemurs riding on Aladar's back, the two equate them to skin parasites.
- Doraemon: Nobita's New Dinosaur: As the gang trek through a Cretaceous forest, they come across a colony of Deltatheridium scampering across their path. However, Doraemon flees up a tree in terror at the sight of the small mammals, due to them resembling mice. Later, Doraemon encounters a Repenomamus, a larger mammal that preyed on small dinosaurs, that eats his piece of the Tomodachi Chocolate meant for Cole the Tarbosaurus, and it promptly flees when Cole comes their way.
- The Land Before Time:
- In the first film, a recently-hatched Ducky briefly chases after a small, shrew-looking mammal, who runs away and hides under a snapping turtle.
- The Land Before Time IV: Journey Through the Mists plays with this with Tickles, a ratlike mammal. While she is prone to fleeing away from larger animals as the rest of her kind are, she has a moment of bravery when she saves Ducky from being eaten by Ichy and Dil.
- You Are Umasou: During his mad search for Umasou, Heart terrifies a squirrel-looking mammal into wetting itself.
- Super Mario Bros. (1993): The animated prologue shows Brooklyn 65 million years ago. Two grazing dinosaurs (a sauropod and a Triceratops) are depicted, and a small mammal scurrying beneath them (represented by a skunk of all things). The skunk takes one look at the Triceratops, gets so scared its stripes fly off, and runs away in terror.
- The page spread about dinosaurs in Guinness World Records 2011 mentions that 115 million years ago, the number of mammal species was increasing, but their size was restricted due to the dominance of dinosaurs.
- In Homchen by Kurd La▀witz, the protagonist (a monkey-like marsupial in the Late Cretaceous) attacks and kills a reptile at the beginning through deception, and it is treated as some great crime and rebellion against the natural order both by the reptiles and the other mammals (even his own parents).
- The New Dinosaurs: An Alternative Evolution: It's made clear that the continued dominance of the dinosaurs has prevented any possible diversification of mammals, and they have all remained small and rodent-like. Only one mammal gets any focus, the desman-like zwim, although unnamed mammals appear as generic prey animals in a few other entries.
- Raptor Red: One chapter focuses on a tiny, shrew-like mammal that lives in the same area as Red and her family, and his struggles to avoid being eaten or stepped on by the massive dinosaurs. Though he does escape one dinosaur's attempt to eat him by biting it on the snout.
- Silverwing: The distant prequel Darkwing is set shortly after the K/Pg extinction event. The small mammals have noticed that the mighty dinosaurs and pterosaurs (called "saurians") have been decimated by some incomprehensible cataclysm and many of the survivors are dying of a rotting plague. The mammals form a pact to work together and destroy as many saurian eggs as possible to prevent them from ever dominating again (because adult dinosaurs, or even juveniles, are far too powerful for the mammals to destroy).
- Dinosaurs: Small talking mammals make up the dinosaurs' main diet (followed closely by other dinosaurs). They are shown kept alive inside their refrigerators, and will often argue with their predator, even in the middle of being eaten.
- Mammals vs. Dinos: The entire documentary shows how tiny mammals evolved and developed in the shadow of the dinosaurs, either as prey or utterly beneath the notice of the reptilian giants. Even Repenomamus, a relatively large Mesozoic mammal and shown as a predator of dinosaur hatchlings, is depicted being quickly killed and eaten by a pack of Dilong.
- Prehistoric Planet: In the episode "Ice Worlds", an Alaskan troodontid is shown using fire to smoke out a number of small burrowing mammals (identified by Word of God as Cimolodon), and it easily captures and kills one of them. This is the only time in the entire series mammals appear.
- The Opening Narration for Walking with Beasts describes mammals as living in the shadows of dinosaurs for over 160 million years, over footage of two mammals hiding while a theropod and an ankylosaur fight, but that their time would come when the asteroid hit.
- Walking with Dinosaurs:
- The cynodonts from "New Blood" are stem-mammals, and while they put up more of a fight than their descendants in the series, they ultimately have to abandon their burrows to escape the predations of the Coelophysis (note this is a world where dinosaurs aren't yet the rulers of the land).
- The unnamed mammal in "Spirits of the Ice Forest" (identified as Steropodon in the book and live-acted by a coati) is wimpy enough that a dinosaur the size of a small child scares it off by throwing debris at it.
- Didelphodon is the representative of the mammals in "Death of a Dynasty", and is invariably shown as either a nest raider or a scavenger, with several individuals getting eaten by a female Tyrannosaurus after trying to raid her nest. Here, though, the mammals are actually doing fairly well, as the age of the dinosaurs is about to close out.
- Discussed in Animal Crossing: New Horizons. Blathers describes Juramaia (one of the collectable fossils, a mammal from the Jurassic) as being small and (possibly) nocturnal so it could hide from dinosaurs. "They needed every advantage to live among those behemoths."
- Averted in Zniw Adventure, which takes place in a prehistoric world where dinosaurs are anthropomorphic and civilized. Small mammals appear, but they are not anthropomorphic. Instead, they are animalistic creatures that the dinosaurs treat as pests. The player is clearly meant to sympathize more with the dinosaurs than the mammals. Some dinos are tasked with catching mammals to keep them out of their food supplies. It's never outright stated if any dinos eat mammals, although there is one segment where Zniw helps a dino catch some mammals, and each time Zniw gives a mammal to the dino, he turns away from the camera and does... something with it. Considering that it's heavily implied that some dinosaurs still eat each other despite being anthropomorphic, it's definitely possible that some dinos eat mammals.
- While all the vertebrates in Hamster's Paradise are mammals, the original draft features an era called the Mouseozoic that resembles the Mesozoic. The largest animals at the time resemble dinosaurs, but there's a group of small hamster-descendants that have changed little from their ancestors called neomice, that are mostly noctural and low on the food chain. They're among the few lineages to survive the ice age that ends the Mouseozoic.
- Discussed in Moth Light Media's video "The Mammals that Lived Alongside the Dinosaurs". He says that framing Mesozoic mammals as hiding in trees or burrows from their "saurian oppressors" is unfair because they were far more diverse than most people think.
- 64,000,000 Years Ago: The documentary short centres around a primitive mammal surviving at the end of the Late Cretaceous (which, in reality, was about 66 million years ago, not 64). Even the smallest dinosaur shown, the Ornithomimus is a threat to it, and the narrator states how the dominance of the dinosaurs prevents mammals from growing larger or diversifying beyond tiny insect-eaters cowering in their shadows.
- Dinosaur Train:
- "Tiny's Tiny Friend" introduces Cindy Cimolestes, a possum-like mammal who's terrified of the local dinosaurs (and Pteranodons), including the herbivores. Tiny convinces Cindy that she and her siblings are no threat, and they eventually become friends.
- Downplayed with Vlad Volaticotherium; while he's much more confident than Cindy, he explains to the 'saurs in his debut episode ("Haunted Roundhouse") that most mammals are nocturnal so they can hunt for insects without being disturbed by the "big scary dinosaurs".
- Averted with the other mammals, who aren't scared of dinosaurs at all.
- Futurama: Discussed in "A Clockwork Origin". The crew gets attacked by robotic versions of dinosaurs, other Mesozoic reptiles, and a Dimetrodon, which are then wiped out by a solar flare bringing out an electric surge that short-circuited them. The Professor proclaims that "only puny mammal-like robots cowering in caves could survive such a catastrophe", just as Bender comes out of a cave annoucing that he had taught himself to knit during the time.
- The Land Before Time TV series: In the episode "Stranger From the Mysterious Above", Spike encounters a colony of Ceratogaulus or horned gophers (which are actually from the Cenozoic, not the Mesozoic) that mistake him for "The Big Wise One" from their legends. They later ask him if he could deal with the "Great Hideous Beast" that's been terrorizing them... which turns out to be a Microceratus not much bigger than them or Ducky.
- The Simpsons: One Couch Gag depicts the evolution of Homer, starting from a single-celled microorganism and ending with his modern self. His counterpart from the Mesozoic is a small rat-like mammal who is hunted by a T. rex that resembles Bart. The chase ends with Homer hiding in a hole, enabling him to not only escape the predator but also to survive the meteor that wipes out all dinosaurs.