Complex sentence: Soon the term "Testudo" was only being used for tortoises as opposed to all chelonians, with tortoises defined by completely terrestrial behaviors, heavy shells, and elephant-like limbs with nails but no visible toes.
Split into simpler sentences: Soon the term "Testudo" was only being used for tortoises as opposed to all chelonians. The tortoises were defined by completely terrestrial behaviors, heavy shells, and elephant-like limbs with nails but no visible toes.