Ask the average person (or high schooler) on the street, and he’ll guess that mammals didn't appear on the scene until after the dinosaurs went extinct--and that the last of the dinosaurs evolved into the first of the mammals. The true story, though, is very different: in fact, the first mammals evolved from therapsids ("mammal-like reptiles") at the end of the Triassic period, and coexisted with dinosaurs all through the Mesozoic Era. But part of the popular folk tale has a grain of truth: it was only after dinosaurs went kaput 65 million years ago that mammals were able to evolve beyond their tiny, mouselike forms into the widely specialized species that populate the world today.
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Popular misconceptions about the mammals of the Mesozoic Era are easy to explain: scientifically speaking, dinosaurs tended to be very, very big and early mammals tended to be very, very small. With a couple of exceptions, the first mammals were truly tiny creatures, rarely exceeding a few inches in length and a few ounces in weight. With their low profiles, these hard-to-see critters could feed on insects and small reptiles (which bigger dinosaurs tended to ignore), and they could also scurry up trees to avoid getting stomped on (or eaten) by larger ornithopods and theropods.
The Evolution of Mammals
Before discussing how mammals evolved, it's helpful to define what distinguishes mammals from other animals (and especially reptiles). Female mammals have milk-producing mammary glands with which they suckle their young; most mammals have hair or fur, and all have warm-blooded metabolisms. In terms of the fossil record, paleontologists distinguish mammals from reptiles by the shape of their head and neck bones, as well as the presence (in mammals) of two small bones in the inner ear (in reptiles, these bones constitute part of the jaw).
As mentioned above, mammals evolved toward the end of the Triassic period from therapsids, the line of "mammal-like reptiles" that arose in the early Permian period and produced such uncannily mammal-like beasts as Thrinaxodon and Cynognathus. By the time they went extinct in the mid-Jurassic, therapsids had evolved various mammalian traits (fur, cold noses, warm-blooded metabolisms, and possibly live births) that were further elaborated upon by their descendants, the true mammals.
As you can guess, paleontologists have a hard time distinguishing between highly evolved therapsids and newly evolved mammals. Late Triassic creatures like Eozostrodon, Megazostrodon and Sinoconodon appear to have been intermediate forms between therapsids and mammals, and even in the early Jurassic, Oligokyphus sported reptilian ear and jaw bones at the same time as it showed every other sign (rat-like teeth, the habit of suckling its young) of being a mammal. (If this seems confusing, bear in mind that the modern-day platypus is classified as a mammal, even though it lays distinctly reptilian, soft-shelled eggs rather than giving birth to live young.)
Lifestyles of the Early Mammals
The most distinctive characteristic of the mammals of the Mesozoic Era is how small they were. Although their therapsid ancestors had attained respectable sizes (for example, the late Permian Biarmosuchus was about the size of a large dog), very few early mammals were larger than mice, for a simple reason: dinosaurs had already become the dominant land animals. The only ecological niche open to mammals entailed a) feeding on plants, insects and small lizards, b) hunting at night (when predatory dinosaurs were less active), and c) living high up in trees. Eomaia, from the early Cretaceous, and Cimolestes, from the late Cretaceous, were fairly typical in this regard.
This isn't to say that all early mammals had identical lifestyles. For example, the North American Fruitafossor had a pointed snout and mole-like claws, which it clearly used to dig for insects (and probably to hide underground when predators were afoot), and the late Jurassic Castorocauda was built for an aquatic lifestyle, with a long, beaverlike tail and hydrodynamic arms and legs. Perhaps the most spectacular deviation from the mammalian body plan was Repenomamus, a three-foot-long, 25-pound carnivore that is know to have fed on dinosaurs (a specimen of Repenomamus has been found with the remains of a Psittacosaurus in its stomach).
Next Page: The Age of Mammals, and Profiles of Mesozoic Mammals
