While my favorite lifeform from the age of the dinosaurs (and before) is the trilobite, I also have a soft spot for the hard-shelled ammonite. Leonid An has built a scientifically accurate, albeit fictional, ammonite that he’s dubbed Ammonoidea fictum. The Bionicle pieces make an excellent shell, and I love the big yellow eye glaring from behind a mouthful of tentacles.
Sadly, ammonites died out around the same time as the dinosaurs, survived by the similar (but only distantly related) nautilus. If you like this LEGO ammonite, check out the white nautilus we featured here back in 2009.