Perhaps some dinosaurs died of malaria

Back in January I wrote about the book 'What bugged the dinosaurs? Insects, Disease and Death in the Cretaceous', by George and Roberta Poinar (click here to read what I said then). We have now obtained a copy for indexing for CAB Abstracts, and I have had a chance to take a quick look at it.

Much of what I read about the book concentrated on its suggestion that disease-transmitting insects played an important role in the extinction of the dinosaurs. One of the authors was quoted in an entry at Science Blog as saying that insects and diseases, although not the only cause of dinosaur extinction, provide the explanation of why the dinosaurs were already declining at the time of their sudden extinction. It is stated in the National Geographic news item about the book that he was convinced that many deadly diseases were emerging at the end of the Cretaceous period. It was unclear to me why such diseases would have become more severe or widespread then, given that the best evidence¹ of insect-borne disease in prehistoric reptiles dated from millions of years earlier.

In fact the book does not put much emphasis on insects as a possible cause of extinction; much of it is a more general account of their importance in the Cretaceous, based on the insects that the authors and others have found fossilized in amber, and on analogy with their modern-day counterparts. It is argued that insects were important as consumers of plants, as pollinators of early angiosperms, as consumers of dung and corpses, as food for dinosaurs and as vectors of plant and animal diseases. The relevant insect groups (as well as ticks and mites) are all known to have been present at the time, as are the groups to which the pathogens belong.

Evidence that the insects were transmitting diseases is rather harder to come by, and the book makes frequent use of words and phrases like 'may have' and 'no reason why not'; but as far as I'm qualified to judge, such speculation seems quite reasonable. The example¹ already mentioned demonstrates the presence of Leishmania-like protozoa and apparently reptilian blood cells in a sandfly, and the protozoa were found to be common in sandflies from the same amber deposits. Another study² found a variety of insect pathogens, including viruses related to known pathogens of vertebrates, in a biting midge. Modern reptiles are attacked by biting insects, so a scaly skin is clearly no protection.

It is only the last chapter of the book that discusses dinosaur extinction. The authors agree that there is evidence for both sudden and gradual extinction, and that there are differences between groups of organisms and between locations. They argue that at the end of the Cretaceous the dinosaurs would also have been affected by climate change, volcanism and meteor impact, which would have favoured the spread of diseases and made populations more vulnerable to them; they conclude that 'you cannot discount the possibility that diseases, especially those vectored by miniscule insects, played an important role in exterminating the dinosaurs'.

Comparing the book and what I had heard about it shows the importance of reading the original source of information rather than relying on hearsay -- people may focus on something that catches their attention rather than the main point. My opinion about insects and dinosaur extinction is still that the idea is interesting but hard to prove; why not read the book and see what you think?

 

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 Poinar, R.: Fossil evidence of insect pathogens. Journal of Invertebrate Pathology (2005) 89 (3), pp. 243-250. doi:10.1016/j.jip.2005.05.007