Carpenter and Tidwell (2005) accepted Hatcher's argument that there is only one species of sauropod dinosaur known from the Arundel Formation and that "Astrodon johnstoni" is it. Interestingly, the majority of the bones of "Astrodon" are of juveniles, and Carpenter and Tidwell considered the two species named by Marsh, "P. nanus" and "P. altus", as different growth stages of "Astrodon johnstoni".