Let me begin with a digression. (Hey, we may as well start as we mean to go on).
Citations in scientific writing are used for two very different reasons, but because the two cases have the same form we often confuse them. We may cite a work as an authority, to lend its weight to our own assertion, as in “Diplodocus carnegii seems to have had fifteen cervical vertebrae (Hatcher 1901)”; or we may cite a work to give it credit for an observation or concept, as in “… using the extant phylogenetic bracket (Witmer 1995)”.
The conflation of these two very different modes of citation causes some difficulty because, while many authors would never cite (say) a blog post as an authority, most would feel some obligation to cite it in order to give credit. You might not want to cite this SV-POW! post as authority for a length of 49 m for Amphicoelias fragillimus; but you would hardly use the sacrum illustration from this post in your own work without crediting the source.
So the fact citations do two rather different jobs causes confusion.
source: svpow (07/08/12)