Specs are a waste of time and an excuse for not committing your ideas to code. Each developer should take a shot at solving the problem and integrate their code with the other contributing developers. Demo the code for marketing, the PM's and other stakeholders early and often. Get everybody involved as soon as possible. Going off in a corner and developing a spec is busywork if you are isolated and really don't understand the big picture. Developers don't like to write specs anyway so don't ask them to do stuff they don't want to do.