Who is Carl Untermond?

Terry Pratchett's Strata opens with the following quotation, attributed to Dr Carl Untermond's The Overcrowded Eden:

I met a mine foreman who has a piece of coal with a 1909 gold sovereign embedded in it. I saw an ammonite, apparently squashed in the fossil footprint of a sandal.

There is a room in the basement of the Natural History Museum which they keep locked. Among other oddities in there are the tyrannosaurus with a wristwatch and the Neanderthal skull with three gold fillings.

What are you going to do about it?

It's a lovely quotation, and wonderfully phrased. But -- ironically enough, given its implications -- I wonder about its provenance. The only extant references to Untermond or his book appear to be Pratchett and people quoting Pratchett. Does anyone have independent confirmation of Untermond's existence or better bibliographic details?