::Nope, it is just the definition of modern arthropods. It is entirely possible that the ancestor of the trilobite had a much softer exoskeleton, limiting its ecological success. Whether or not it was an arthropod is literally a matter of semantics, not a matter of biology.[[User:FCapra|FCapra]] 18:19, 22 September 2011 (EDT) | ::Nope, it is just the definition of modern arthropods. It is entirely possible that the ancestor of the trilobite had a much softer exoskeleton, limiting its ecological success. Whether or not it was an arthropod is literally a matter of semantics, not a matter of biology.[[User:FCapra|FCapra]] 18:19, 22 September 2011 (EDT) |