Your Turtle rock was produced during during Pre-Camrbian period of the Palaeozoic era, and was used as a sunning rock for various early organisms during the late-the Cretacious period. In fact, your rock, as you described it is actually one of the more rare "mating platforms" used by early arthropods. Amazingly, the odds of your rock being a platform for "nocturnal activities" that would generate what is now the entire lobster family is close to 40%. That's pretty incredible, considering lobsters are pretty dang tasty.
Other notable information about your turtle shaped rock, is that it's not actually a rock. It's a collection of waste from a rare algae and fungal symbiotic relationship in which the algae produced energy derived from the toxic gasses in the atmosphere at the time and an early type of photosynthesis... the by-product was consumed by the fungus, who then extruded a concrete like substance that solidified and hardened into the rock like formation you see today. This lifted the algae further into the atmosphere thus completing the symbiotic relationship. Originally sharped edged, with tall extrusions, the rock spires wore easily in the various environmental conditions, and what remains is the hardened shell beneath. Also clear by your description, is the fact that these colonies of symbiotic organisms were actually mobile in nature and often branched out and broke off into newer colonies. The head and tail shapes of your "rock" are actually a snapshot in this process which was probably interrupted at the start of the late-Cambrian period (which, as you know was marked by strong climate shift, likely interrupting the delicate balance of the symbiotic relationship, thus halting the actually branching of the rock (which would have simply resulted in several large rocks next to each other). Quite a remarkable specimen you have!
The rock type you described has been noted, in later periods of the Mesozoic era, as a highly probable location of a small lookout for rodents of unusual size (Though, I don't think they exist, as fossil record is gapped at this point).
Lastly, your rock might also be located on a lay-line in the earth's living field. While remote in possibility, if you check around the edges you might discover crystals that are forming. Once reaching about 1/4" in total length, these crystals will give your virility and "oomph" if worn around the wrist or neck.
And I'd just like to say, it's not often people come here looking for information like this. It was a real pleasure being able to share this knowledge with you. I can hardly wait to see what else you ask us to interpret for you. woot!
Skippy
