We do, in fact, find this out in the third book of the series, Life, the Universe and Everything.
The Guardian is a national daily newspaper in the UK with a left-wing editorial slant. Its readers are stereotyped as the anxious, well-meaning, politically-correct middle classes.
Douglas Adams had a great gift for memorably odd and humorous character names. He claimed that he only thought up such bizarre names to torment the secretary of the radio producer who oversaw the first radio series upon which the novel is based, as she had to laboriously type them all out in the scripts!
So Long, and Thanks for all the Fish is the title of the fourth book in the increasingly self-referential series of Hitchhikers books. It was also turned into a spectacular musical number (Spotify link) in the film adaptation of The Hitchhiker's Guide to the Galaxy.
Ivan Pavlov was a Nobel Prize winning Russian scientist famous for being the first to describe "classical conditioning", also known as "Pavlovian conditioning". The best-known of his experiments into conditioning involved training dogs to salivate at the sound of a bell by causing them to associate the bell with food being served to them.
The Cenozoic era is the most recent of the three geological eras in the Earth's history. It runs from 65.5 million years ago to the present day. The dinosaurs had died out by this stage to be replaced by, among other species, the mammoth and the sabre-toothed tiger.
The stegosaurus was a dinosaur of the Late Jurassic period.