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

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