The issue is contentious because the fossil record suggests that placental mammals did not expand, in terms of the number of different species, until after 65 million years ago. A plausible reason is that all the dinosaurs had been killed off, except the line that evolved into birds, and the placental mammals speciated into the ecological niches that had been left vacant.

But biologists reconstructing the mammalian tree of evolution from the DNA of living species have come to a different conclusion. Their molecular clock data suggest a much earlier speciation, perhaps prompted by the breakup of the ancient supercontinent of Gondwana around 120 million years ago. In this view, placental mammals evolved into distinct species before the asteroid hit of 65 million years ago, and many would have survived the mass die-out that polished off the dinosaurs.

Michael Novacek, an expert in mammalian paleontology at the American Museum of Natural History in New York, said the louse tree was very interesting and showed that lice were diversifying during the Cretaceous. But the fossil record of placental mammals is reliable and does not record a speciation until later. “The fossil record continues to show that the origin of modern placental mammals postdates or is at the Cretaceous-Paleogene boundary,” he said. “So the contradiction between the fossils and molecular clocks remains.”

In his view, the hosts on which lice were speciating during the Cretaceous could have been a different branch of the mammalian family tree, all of whose species are extinct.

Dr. Smith said Dr. Novacek was correct in saying that there were nonplacental mammals around in the Cretaceous on which lice would doubtless have fed. But these nonplacental mammals, which included several lineages of marsupials, all became extinct, taking their parasites with them. These lice would not show up in his tree, Dr. Smith said, unless they had been able to transfer to the placental mammalian species, and most lice do not regularly switch hosts.

Dr. Smith believes that lice infested birds before mammals, in part because lice are so common on birds. Every bird family but one has lice, and there is a species of bird, the great tinamou, that harbors 18 different species of lice, perhaps because it has many different kinds of feathers, each offering a special niche. Given the ancient root of the louse family tree, it is likely that the first louse infested the feathered dinosaurs that were the birds’ ancestors, he concludes.