30 ene 2012
Non-avian feathered dinosaurs might have spent much of their time nibbling off insects and scratching themselves, suggests a new study that proposes certain dinosaurs were the first animal hosts of lice.
“Our analysis suggests that both bird and mammal lice began to diversify before the mass extinction of dinosaurs," principal investigator Kevin Johnson was quoted as saying in a University of Illinois at Urbana-Champaign press release. “And given how widespread lice are on birds, in particular, and also to some extent on mammals, they probably existed on a wide variety of hosts in the past, possibly including dinosaurs."
That view is strengthened in the journal paper itself. The researchers write: "These data give an early- to mid-Cretaceous origin (115 - 130 million years ago) for lice, which suggests that these parasites probably infested feathered theropod dinosaurs."
Johnson, an ornithologist with the State Natural History Survey at the university, and his colleagues came to the dino-lice conclusion after constructing a partial family tree for lice. They did this by comparing the DNA sequences of genes from 69 present-day louse lineages. Changes in gene sequence are believed to be a reliable measure of relatedness among different species in the same basic animal group. Because these changes accumulate over time, they can also be used to create a rough timeline of the evolution of related groups of organisms.
The lice family tree indicates the wingless insects were around long before dinosaurs bit the ultimate dust 65 million years ago.
While anyone who has dealt with lice, such as head lice on kids, knows how tenacious and hearty these insects can be, it is still surprising to learn that lice have been around for so long. The oldest fossils found so far that resemble modern bird and mammal groups are less than 65 million years old, Johnson explained. That's what led to the prior belief that major bird and mammal lineages -- as well as their body-dwelling insects -- appeared only after the dinosaurs went extinct.
But the lice DNA findings indicate that modern birds and mammals were already around when the non-avian dinosaurs died out. The researchers can see that because each louse species evolves specialized traits to suit a particular host animal. Wing lice, for example, have long bodies that allow them to insert themselves between the barbs in a feather and thus evade preening. Gopher lice have grooves in the tops of their heads that clasp onto a single shaft of hair.
These specialized traits make it hard for lice to shift to other hosts. As a result, their evolutionary history coincides very closely with that of their hosts. This history provides evidence of the existence of mammals and birds before 65 million years ago.
“Lice are like living fossils," explained co-principal investigator Vincent Smith, who was a former postdoctoral researcher in Johnson’s lab and is now at the Natural History Museum in London. “The record of our past is written in these parasites, and by reconstructing their evolutionary history, we can use lice as markers to investigate the evolutionary history of their hosts.”
If dinosaurs did indeed have lice, as the researchers strongly suspect, Johnson said, "maybe birds just inherited their lice from dinosaurs."
