No matter how outdated a Charles Knight illustration, I still love them. When drawing Amphicoelias Knight borrowed from the descriptions of the time and from Cope's description of Amphicoelias as well as descriptions of other sauropods, notable Diplodocus. The aquatic life of Amphicoelias, if it had been able to live such a life and raise its head like a periscope, there is no telling if the vertebrae of this animal would have allowed for such range of motion because we do not have them to look at, it would have been safe from almost every predator that could have lived in the water. In fresh water its size would have granted it near immunity from the crocodilians (Deinosuchus even probably) and in the marine world, has it ventured there with this once popular lifestyle idea, it most certainly would have been safe from the so called ginsu sharks (Cretoxyrhina) and the largest Mosasaurs (seeing as how they were more of swallow whole predators only parts of Amphicoelias would be on the menu anyhow). However, Knight's depiction is sorely outdated by modern science and so, as much fun as it is to look at, we know that they would not have lived in such an environment. It is still a very interesting illustration.
What about those pesky land predators, the theropods? Yesterday's image compared the two species of Amphicoelias to a human being, this image, however, is comparing Tyrannosaurus rex and Amphicoelias fragilimus to one another. Clearly the largest predator of all time in North America would have had trouble taking down a monstrous behemoth like Amphicoelias. This introduces an interesting quandary: What was the ecological impact of Amphicoelias? Certainly other Jurassic hunters would have had issues with taking down the adults if a T. rex was this much smaller than A. fragilimus. Even a pack of ravenous Allosaurus probably could not muster up the killing ability to take down a healthy adult. The herds of Amphicoelias could not have grown unchecked or the other herbivores of the Jurassic would have found themselves under a great deal of pressure to find vegetation because the sheer size of this animal would have forced it to eat ridiculous amounts of vegetation itself. I think we must have an animal here that roamed alone and, more than likely, in small numbers at best. This is the type of animal that more than likely laid hundreds of eggs so that 2 or maybe 3 individuals could grow to adulthood. Hatching day was probably a banquet for smaller predators and mammals.
From Carpenter 1995
The sheer bulk of Amphicoelias was its best defense. Think of it in terms of elephants, other than a high power rifle handled by some jerk after ivory (sorry, I try to keep my personal opinions to a minimum around here but I find it very hard when mentioning poachers), an elephant has few if any predators to worry about; when an elephant is sick or when it is young it is at its most vulnerable and even then the herd is typically fairly protective. Amphicoelias, with a vertebra this large, probably only had to worry about predators until it was about the size of your average Diplodocus, and even then the only predators that it would worry about would be packs of Allosaurs and the occasional desperate family of Ceratosaurus. Past that size there really is almost nothing to worry about except stubbing a toe, a sickness that makes defense (i.e. standing up and looking ridiculously tall), or maybe stepping on the Amphicoelias equivalent of a splinter; which in this case would probably be an entire log. The fact that a full grown man, someone taller than myself (I'm 5'5" on a good day in boots), could be so much shorter than just a backbone makes this animal almost unfathomable to the imagination. However, I think that paleontologists as a group are probably the most imaginative scientists aside from anyone dealing with space in one form or another. This allows us to let science do the talking, but allows our brains to picture this behemoth of dinosauria to walk down our street (in about two steps probably) and to flesh out the details of its life. Educated guesses based on imagination and science; that is the best we can do for Amphicoelias at this point, but perhaps someday we will get very lucky and find another of these enormous creatures.