Acanosthega was one of the first animals to crawl up from the sea and onto prehistoric Earth, hundreds of millions of years ago. They mostly kept to the water though.
Here's a larger image showing the full creature.
Whoops, I completely missed that part, I edited it and linked the full image in the description.
Nice picture, but... Its canvas size goes way over the limit of 100 pixels in width for the weekly challenge though...