There's a funny little piece of trivia attached to Iguanodon on the Internet, that once upon a time someone suggested that the famous thumb spikes were not merely wielded as pointed instruments of close-quarters defense, but may have also delivered venom. This tidbit is most readily found in the Iguanodon entry on Wikipedia, where up until recently the idea was described as coming from Tweedie (1977) and being refuted in Naish and Martill (2001) and a Dinosaur Mailing List post by Darren Naish based on the absence of any anatomical evidence (e.g., hollow spike, grooves, open tip). All in all, the whole thing just comes across like an example of an out-there Dinosaur Renaissance concept.
There is a catch, though. "Love in the Time of Chasmosaurs" recently covered Tweedie's book in two posts, but the venomous Iguanodon did not make an appearance, as documented in the comments. I vaguely remembered reading the same thing about Iguanodon back in the day, though, so I thought I'd have a look, and I already had another source in mind: "Dinosaur Mysteries", a pop-sci-type dinosaur book from 1987 (Elting and Goodman 1987). (Coincidentally enough, LITC covered this book almost nine years ago to the day in their old digs.)
Problem: while I used to have a copy of this book (in fact, I think I had two at one point, there being relatively few dinosaur books available for many relatives looking for birthday and Christmas presents in the late 1980s), I don't now. Solution: it just so happened that Internet Archive includes a copy in their surprisingly thorough dinosaur collection. I simply signed in and there I was. Right there in the Table of Contents was "The Case of the Poisoned Spike". On page 46 was this passage:
"But could the spikes have contained poison—the way a snake's fang's hold venom? One scientist thinks that poisoned thumbs might have been a good form of protection. That is a mystery still to be solved."
And there we have it: a reference to venomous Iguanodon. It has about as much substance as your typical daydream ("one scientist" would have Wiki editors reaching for their templates), but it exists. But what of Tweedie?
It turns out Tweedie does have something to say. On page 69, we read this:
"It [the thumb spike] is usually regarded as a defensive weapon but no one has explained how it could have been effective against the great claws and rending teeth of a large theropod. There are many things concerning dinosaurs that the fossil record can never tell us about. If these living animals were known to us only as fossils, who would be bold enough to suggest that the spur on the hind leg of the platypus or the spine on a sting-ray's tail were weapons charged with venom? Dinosaurs must have been very diversely adapted animals, and it is reasonable to suppose that most of the more obvious defensive devices seen among modern animals were evolved by them."
This is a fascinating piece of work. Nowhere in it does the author explicitly propose that Iguanodon had venomous thumb spikes. Read as a whole, though, the effect is that the reader is drawn to that conclusion. To me, someone involved in the writing of this book really, really wanted to include a venomous Iguanodon but either couldn't pull the trigger on stating it plainly or was argued (or edited) out of it. Did Elting and/or Goodman read this passage and come to the unstated conclusion, making Tweedie "one scientist"? Fittingly, Naish and Martill (2001) write that Tweedie "implied" this conclusion.
Elting, M., and A. Goodman. 1987. Dinosaur mysteries. Platt & Munk, New York, New York.
Naish, D., and D. M. Martill. 2001. Ornithopod dinosaurs. Pages 60–132 in D. M. Martill and D. Naish, editors. Dinosaurs of the Isle of Wight. The Palaeontological Association, London, United Kingdom. Field Guide to Fossils 10.
Tweedie, M. W. F. 1977. The world of the dinosaurs. Weidenfeld & Nicolson, London, United Kingdom.