302 Evolution? The Fossils Say YES!
mammals evolved from cynodonts in the Late Triassic at exactly the same time as the early
dinosaurs (which may have outcompeted the last of the larger synapsids that once ruled the
Triassic). But while dinosaurs soon came to dominate the globe for the next 130 million years,
mammals remained small and inconspicuous, seldom getting much larger than the size of
a house cat. Most mammals apparently lived in the nooks and crannies of the world of the
“terrible lizards,” hiding in the vegetation, and probably coming out mainly at night. In fact,
the first two-thirds of mammalian history was the story of these tiny Mesozoic mammals.
Only after the nonavian dinosaurs vanished at the end of the Cretaceous did the world open
up for mammals so that they could dominate the planet.
For over a century, very little was known about Mesozoic mammals. Because they were
so tiny and delicate, the best we could find were tiny pinhead-sized teeth and partial jaws
from animals the size of shrews. None were known from almost any other part of the skel-
eton. When I first began to work on Jurassic mammals for my master’s thesis in 1977, this
state of affairs was still true after a century. I had access to a new collection of jaws and
teeth from the famous Upper Jurassic Morrison Formation dinosaur quarries at Como Bluff,
Wyoming. This allowed me to survey all the early Mesozoic mammals known at the time
and conduct the first cladistic analysis of all these poorly understood species. I did several
different papers on that research and published them. One result was that I was able to
show that the long-abused paraphyletic group “Pantotheria” was a hopeless wastebasket
and needed to be abandoned. Indeed, most paleontologists since then have stopped using
this obsolete concept. Around the campfire in the long field season of 1977, my graduate
advisor Malcolm McKenna, my fellow graduate students, and I daydreamed about what
it would be like if we had skulls or even partial skeletons of these critters, instead of tiny,
frustratingly incomplete teeth and jaws. For a century before us, everyone else who worked
on Mesozoic mammals must have felt the same way. But they did the best they could with
what they had.
I moved on from Mesozoic mammals shortly thereafter because there were no more
new specimens to study at the time. I also preferred working on larger and more easily
studied mammals like camels, horses, and rhinos, which do not require a microscope to see
or photograph. Since then, there has been a veritable explosion of new Mesozoic mammal
fossils. Not only do we have many more new species based on jaws and teeth, but there have
been some extraordinary finds that have decent skulls (fig. 13.9), and in a few cases, even
articulated skeletons for many of the different groups. What these specimens show is that
most Mesozoic mammals were small, insectivorous creatures, living much like the modern
shrew in most of their habits. A few were slightly larger in body size, and one specimen from
the Cretaceous of China, known as Repenomamus, was over a meter long and actually has
a baby psittacosaur dinosaur in its stomach. Generally speaking, however, it appears that
mammals avoided the dinosaurs and were not in any position to compete with them, let
alone eat them.
The most amazing specimens of Mesozoic mammals come from the same Lower Creta-
ceous Liaoning lake beds that yielded the many feathered dinosaurs and early birds discussed
in the previous chapter. These include a complete specimen of the oldest known marsupial,
Sinodelphys szalayi (fig. 13.10), which preserves not only the bones in articulation but even
the impressions of the fur and soft tissues. This fossil shows that marsupials (the pouched
mammals, which today include opossums, kangaroos, and koalas) had already split off from
the main mammalian stem at 120 million years ago, and opossum-like marsupial teeth are