Tatankaceratops
Tatankaceratops (meaning "Bison horn face") is a controversial genus of herbivorous ceratopsian dinosaur. It is a small chasmosaurine ceratopsian which lived during the Late Cretaceous period (latest Maastrichtian stage) in what is now South Dakota. It is known from a single partial skull which was collected from the Hell Creek Formation, dating to 66 million years ago. "Tatankaceratops" was described by Christopher J. Ott and Peter L. Larson in 2010 and the type species is "Tatankaceratops sacrisonorum". "Tatankaceratops" is known from one specimen housed at the Black Hills Institute, BHI 6226. In 2011, Nick Longrich published a paper containing  a brief re-evaluation of "Tatankaceratops". Longrich suggested that "Tatankaceratops" appeared to possess a bizarre mix of characteristics from adult and juvenile "Triceratops" specimens; the animal's small size and short, slender brow horns are consistent with the animal being a juvenile, but the gnarled bone and fusion of skull elements to one another are typical of old adult ceratopsians. The elongate nose horn meanwhile was characteristic of "Triceratops", and specifically, the highly advanced "Triceratops prorsus". Longrich noted that this animal could potentially represent a dwarf species of "Triceratops" or simply a "Triceratops" specimen with a developmental disorder which caused  it to stop growing prematurely. Other paleontologists, including Thomas R. Holtz, Jr., have written that they "strongly suspect" "Tatankaceratops" is merely a juvenile specimen of "Triceratops". Authors of subsequent studies involving triceratopsins have not considered "Tatankaceratops" a valid genus. Systematics. The cladogram below follows Ott and Larson (2010).