Peter K. Dearden.
One of the fun things about my field of research is that it is broad; I find myself reading papers on paleontology as often as genetics and development. Paleontology often discovers remarkable things, as seen in a recent article in Nature (Huang, D., Engel, M. S., Cai, C., Wu, H. & Nel, A. Nature (2012)).
The paper describes some fantastic fossil insects from two Chinese sites, the Jiulongshan Formation in Inner Mongolia, and the Yixian Formation, Liaoning province. Those of you with an interest in Dinosaurs might know of these sites as places where remarkable fossils of feathered dinosaurs have been found. These are dull compared to the insects!
Insect fossils are quite rare as it takes a very specific environment to fossilize insects well. This creates some problems with our understanding of the evolution of insects, as, especially the more derived, holometabolous, insects diverged very rapidly a long time ago. This makes it hard to reconstruct the relationships between groups of insects by looking at DNA sequence alone. Fossil insects help us understand the transitional forms between these groups, helping us to refine our knowledge. We should care because the holometabolous insects include the bees, of key importance to food production, and the beetles, the most species rich and numerous animals on earth. Also amongst the holometabolous insects are the fleas.
Fleas are highly derived insects, specialized for feeding on mammalian blood. Their ancestors probably fed on nectar or fruit juices, gained from piercing plants. The body shape and behavior of modern fleas are adapted to allow them to cling onto mammalian hair, and pierce the skin. It has been believed that fleas evolved with their mammalian hosts. Some fleas now live on birds, but these have been mammalian feeders that have secondary switched to a feathered host1
The fossils found in China, however, suggest that fleas might not have started out in mammals. The fossils are huge (for fleas) with some of the specimen in the paper stretching to 17 mm long. They are also diverse in form, have very robust feeding mouthparts, and have the key characteristics that show they were adapted to clinging to a feathered or furred host. So what were the hosts? Certainly at that time, and in those deposits, the fossils of ancient mammals have been found, but, according to the authors, these are small. The size of the fleas, and their mouthparts are kind of excessive to be feeding on the wee mammals, so is it possible that they were feeding on the feathered dinosaurs that make these fossil sites so well know?
It’s not clear. We don’t yet have a fossil dinosaur with a flea fossilized on it, and it may be that these ancient fleas were promiscuous as to their hosts. It is a distinct possibility, however, that fleas fed on dinosaurs, and switched to mammals, evolving into the mammalian specific fleas we have today. I like this idea, I like the idea that the dinosaurs have not only left us with the legacy of modern birds, but have perhaps even left us their fleas.
If you lie down with Dinosaurs, you get up with giant Jurassic fleas!