The other week I heard about a recently published book ('What bugged the dinosaurs? Insects, Disease and Death in the Cretaceous', by George and Roberta Poinar) which argues that disease-transmitting insects played an important role in the extinction of the dinosaurs. I have ordered a copy of the book so it can be indexed in CAB Abstracts, and in the meantime I have investigated some blogs, book reviews and news items that discuss it, and looked to see what else I can find on the subject in CAB Abstracts.
Most people reading this will probably be aware of the well-known theories that the dinosaurs became extinct as a result of the worldwide effects of an asteroid impact or large-scale volcanic activity. The book apparently argues that this cannot be the whole explanation as they had already been in decline for some time before the final extinction. This is consistent with what I had read before, although this news item from Science Daily refers to research suggesting the opposite, and the National Geographic news item about the book quotes others who say that there was no such decline.
A search of CAB Abstracts for articles by one or both of the authors of the book (whose work helped inspire the book and film Jurassic Park) revealed numerous studies on fossil insects preserved in amber. One1, published in 2004, describes the investigation of an early Cretaceous sandfly; its gut contained both reptilian blood cells and protozoan parasites similar to the modern genus Leishmania, which is transmitted between hosts by sandflies. A number of others describe the finding in biting insects of protozoa similar to modern insect-borne pathogens; and another2, of fossilized dinosaur faeces rather than insects, reports the presence of protozoan and helminth (trematode and nematode) parasites.
The suggestion is that insect-borne diseases emerged in the late Cretaceous period and had a significant effect on populations that had not previously been exposed to them; according to this blog entry at Science Blog, the authors compare the situation to the arrival of avian malaria in Hawaii, which killed off many of the native bird species. It is also suggested that insect pollinators may have promoted the evolution of flowering plants (an idea supported by several articles cited in a review with the intriguing title 'Did dinosaurs invent flowers?'3, which answers that question in the negative), which may have been less suitable for dinosaurs to eat. A further suggestion is that intestinal parasites may have been spread by non-biting insects. All these factors would have made dinosaur populations more vulnerable to asteroid impact, climate change and other likely causes of extinction.
Not everyone is convinced by the idea; see for example this blog entry by Daniel Cressey on nature.com -- although the identification of the parasites does not depend on the DNA evidence criticized there -- as well as some of those mentioned above. From what I have seen so far, it seems to me that it is an interesting idea, but that it would be difficult to prove. It is not clear to me why insect-borne pathogens would have suddenly developed virulence in the hosts in which they evolved (as opposed to being introduced to a previously unexposed host population as in the Hawaiian example). Also, the examples of parasites in fossilized insects are from the early Cretaceous period, tens of millions of years before the dinosaurs became extinct, so I don't know what the evidence is that they became more important in the late Cretaceous. I hope that we will be able to obtain a copy of the book to shed some light on this.
1: Poinar, G., Jr. and Poinar, R.: Evidence of vector-borne disease of early Cretaceous reptiles. Vector-Borne and Zoonotic Diseases (2004) 4 (4), pp. 281-284. doi:10.1089/vbz.2004.4.281.
2: Poinar, G. Jr. and Boucot, A. J.: Evidence of intestinal parasites of dinosaurs. Parasitology (2006) 133 (2), pp. 245-249. doi:10.1017/S0031182006000138
3: Barrett, P. M. and Willis, K. J.: Did dinosaurs invent flowers? Dinosaur-angiosperm coevolution revisited. Biological Reviews (2001) 76 (3), pp. 411-447. doi:10.1017/S1464793101005735.