Sometimes it is really hard to find exciting illustrations and paintings of some of the animals which we discuss here for Saturday mornings. Today, talking about a giant, plodding, deep in thought turtle of ancient Earth history, that is quite an accurate description of the challenge. Archelon was a giant sea turtle. As a giant sea turtle it was, as stated above, a slow moving animal- slower than Elasmosaurus certainly- and plodded through the oceans at a nice steady pace, its size being its largest defense. The size, pictured here in the type specimen, was an astounding find for a turtle, or any other entire fossil at that time. The lack of only one flipper (in terms of major appendages and parts of the skeleton anyhow) is pretty fantastic really, considering how many fossil animals are "discovered" and named using only small parts of the skeleton such as teeth or claws alone. The "shell" structure is easily seen here as well. We can see the struts over which the leathery or bony carapace would have been stretched; the shells of modern turtles are solid carapaces with their strength in the shell itself, not as trusses and struts. We can actually think of the shell of Archelon in terms of support beams in a house; strong beams in key points to best defend, in this case, the inner "living area."
The fleshed out Archelon is, as is every fossil animal, always open to some sort of interpretation. The shell is the most open part of the skeleton to interpretation, I think, for artists. Flippers are open to some interpretation, however, they are fairly solid and non-changing and their structure delineates what they would have looked like fleshed out nearly perfectly given our understanding of extant turtles, and other animals with flippers. The flesh of the neck and the fore and hind limbs are also open to a bit of interpretation, but it is definitely the shell that we are most interested in. The shell in this illustration is not entirely tightened over the shell ribs that are seen in the fossil, but some depressions of the area between the ribs are clearly evident.
Another version of the shell, illustrated here, shows a much more bony carapace than a leathery carapace stretched over the shell ribs. The bony carapace strengthens the ribs and emphasizes the struts. With the bony carapace stretched over the ribs the depressions between the ribs are not as pronounced; this could have the ability to additionally protect the turtle much better than the leathery carapace which, when stretched, still "sagged" between the ribs. The argument could also be made, though, that the more taunt and therefore less giving carapace in a bony format like this would be more easily punctured as it would have no or very little flexibility in the areas not protected by the dorsal ribs.