What were mammals like when dinosaurs roamed the earth? Discussing Mesozoic critters with Dr. Brian Davis, a paleontologist at the University of Louisville, we learn what these rodent-sized beings looked like, how they lived and how we’re able to learn about them millions of years later.
Science Moab: When was the Mesozoic Era?
Davis: The Mesozoic Era began 250 million years ago, and includes the geologic periods Triassic, Jurassic—which was the heyday of the dinosaurs—and Cretaceous. The Cretaceous Period was punctuated by a big extinction event about 66 million years ago. Fossils of the first dinosaurs and the first mammals appear at about the same time, which surprises a lot of people. Many think of the Mesozoic as the age of dinosaurs, but other major groups of vertebrates around the world coexisted with the dinosaurs. Mammals appear at about the same time as dinosaurs, and although they stayed small, they were just as diverse and interesting as they are later in their history.
Science Moab: Can you describe what these early mammals looked like?
Davis: They were mostly small and probably would have looked like a little shrew or mouse. They were likely insectivorous and somewhat nocturnal. In the Jurassic period, this ancient group of mammals appears that left no descendants. They’re not related to modern mammals. Some have fossilized skin flaps along the flanks of their bodies and may have been gliders like the modern flying squirrel. Some of them have wide tail bones and evidence of thick pelts and may have been semi-aquatic like otters or beavers.
There are some cases where mammals did get big, to maybe the size of a raccoon or a badger. There’s a mammal from the Early Cretaceous of China, about 125 million years ago, with a big, heavy skull, sharp teeth, and the remains of some baby dinosaurs in its belly. Most mammals ended up as lunch for dinosaurs, so it’s cool that there was some turnabout.
Science Moab: What conditions allowed these fossils to exist until our time?
Davis: Where something dies has everything to do with its chances of becoming a fossil. It’s necessary for the body to become buried immediately. The longer something is left out in the open, the more exposed it is to scavengers or to natural breakdown. Much of the fossil record is found in ancient aquatic environments. Animals that are strictly terrestrial, like most mammals and small dinosaurs, are usually underrepresented — not because they may have been rare, but because they were less likely to end up in the conditions you need for fossilization.
In addition, not every part of an animal has an equal chance of being fossilized, even if it ends up in the perfect conditions. What we do find are the parts of the animals that are especially hard. For mammals that means enamel from teeth. So most of our record comes from teeth and the heavy jawbones that hold teeth together. Then, to actually find a fossil, the rocks they are in need to be exposed. Geologists have mapped the surface of the earth, so we know the age and type of environment of almost any outcropping of rock. If you’re interested in early mammals, for example, then you find a Jurassic exposure from a terrestrial environment, and you go to where those rocks are. So, becoming a fossil is certainly a non-random event in that many of the right conditions have to be met.
Science Moab: What kind of information can you obtain from an animal’s teeth and jaws?
Davis: There’s actually quite a bit you can learn about an animal from its teeth. A key feature of mammals’ teeth is that they are intricately interlocking and used to chew food. They also have a really specific shape and don’t tend to change during an animal’s life or vary within a species. So, you can pretty readily figure out what kind of animal you’re looking at from its teeth, and understand its diet based on the size of the animal and the way the teeth work together. By measuring the isotopes that are in the teeth, it’s possible to measure mineral content and get an idea about the environment the food came from and understand different aspects of the animal’s life history.
To learn more and listen to the rest of Brian Davis’ interview, visit www.soundcloud.com/user-495802209/mammals-during-the-time-of-the-dinosaurs. This interview has been edited for clarity.
Science Moab investigates early shrew-sized mammals during the Mesozoic