With time, the idea that the differing skulls might be representative of individual variation within one (or two) species gained popularity. In 1986, John Ostrom and Peter Wellnhofer published a paper in which they proposed that there was only one species, "Triceratops horridus". Part of their rationale was that generally there are only one or two species of any large animal in a region. To their findings, Lehman added the old Lull-Sternberg lineages combined with maturity and sexual dimorphism, suggesting that the "T. horridus"-"T. prorsus"-"T. brevicornus" lineage was composed of females, the "T. calicornis"-"T. elatus" lineage was made up of males, and the "T. obtusus"-"T. hatcheri" lineage was of pathologic old males. His reasoning was that males had taller, more erect horns and larger skulls, and females had smaller skulls with shorter, forward-facing horns.