As Cyborg from the Teen Titans cartoon once exclaimed...
BOOYAAAAHHHHH!!
The Coop - Team Old Scoolers - 2,202,100 - Area 16
Pure dumb luck is how I got the bonus. Those weird and large "V"-like ships that drop off smaller "V"-like ships were coming up from the lower right side of the screen (moving to the left), just as the fossil in question started to show up. I darted over to the right where the hidden bonus was, sank slowly back as I point blanked it which kept all the smaller ships off my ass, and it blew up just before it got completely under my ship where the lasers couldn't hit it anymore. Area 15 was surprisingly short, and the new enemy patterns for the ships from stage 1 were pretty easy to lure around. Then Area 16 came along and reminded me that the game didn't like me getting this far
