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| The Earliest Mammals |  |
The first mammals appeared on Earth together with the earliest dinosaurs about 220 million years ago, during the Mesozoic era. These early mammals were not abundant, but they were quite diverse, and a number of distinct lineages appear early in their history. The animals from one of these lineages, the triconodonts, are distinguished by the three main projections on their back teeth. The triconodonts have a long 
and diverse fossil record that can be traced back 220 million years but ends abruptly in the late Cretaceous.
Despite their significant position in early mammalian evolution, most fossils of triconodonts found to date are merely isolated teeth and fragmentary bones. Although paleontologists have determined that these earliest mammals were insect eaters by their teeth, many other aspects of their biology and behavior have not been well understood. The discovery of Jeholodens by Ji Qiang of the Chinese Academy of Geological Sciences and Luo's comparative studies have offered new insight into the lives of these early mammals.
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