How to get rare dinosaurs Like shell Don?

You collect their "bones" in the same way as regular dinosaurs except they are only in the "cave" areas after you have opened the first six dinosaur sections. It's the area where you can get any of the 6 regular dinosaurs and ShellDon through a random mix.
I found that investing in the free timed exploration (3hours) nets something quite often. Also the other 2 explorations often give excellent rewards (up to 10 diamonds sometimes) to pay for the few more diamonds spent running them. 9 and 23 hours respectively.