Note: I sent this email to my college devs mailing list on November 4, 2012. I just bumped into it and thought of publishing it (with some minor grammar edits).
Few weeks ago, someone asked me on this mailing list:
Why do many Operating Systems' books have dinosaurs on their front covers?
I didn't knew back then but I know the answer to this question now.
Quoting Tanenbaum  from his standard text for Operating Systems :
"...There was no way that IBM (or anybody else) could write a piece of software to meet all those conflicting requirements. The result was an enormous and extraordinarily complex operating system [i.e. OS/360].... One of the designers of OS/360, Fred Brooks, subsequently wrote a witty and incisive book describing his experiences with OS/360 (Brooks, 1995). While it would be impossible to summarize the book here, suffice it to say that the cover shows a herd of prehistoric beasts stuck in a tar pit. The cover of Silberschatz et al. (2004) makes a similar point about operating systems being dinosaurs."