Los Angeles - The lowly louse may have a more impressive pedigree than once thought: Dinosaurs may have hosted the parasitic bugs, a study says.

The findings, published Tuesday in the journal Biology Letters, also show - through comparison of lice - that mammals and birds may have begun to flourish before the dinosaurs went extinct 65 million years ago. That's counter to a long-held idea that they ascended and diversified only once the dinosaurs were gone.

Lice spend their entire lives on one species, so they evolve along with their hosts. That means their similarities can be used as a way to study evolutionary relationships between the creatures they live on.

"Some people have described lice as heirlooms - they're just inherited - which is why they tend to track the evolution of their hosts pretty well," said study coauthor Kevin Johnson, an ornithologist at the University of Illinois, Champaign.

The international team of researchers looked at DNA differences between 69 modern lineages of lice that feed on various birds and mammals. Based on these DNA differences, the scientists were able to infer how much time had passed since various lice shared common ancestors.

The scientists also used fossils in building their louse family tree: a 44-million-year-old bird louse fossil, and a 100-million-year-old book louse fossil (a nonparasitic louse closely related to parasitic lice). Based on the fossils' physical characteristics, the scientists were able to determine where they fit on the family tree of the lice they had analyzed genetically. Those anchoring time-points allowed them to fix dates at which other lice in the tree branched to become separate species.

From this, the researchers concluded that lice began diversifying around 125 million years ago, so their hosts - birds and mammals - must have begun diversifying well before the dinosaurs die-off.

Dinosaurs may even have been the first animals to have lice, Johnson said. Reptiles have no hair or feathers onto which a louse could latch, but lice might have lived on the first feathered dinosaurs, then continued to do so as those dinosaurs evolved into birds.