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169 11 A New Chasmosaurine (Ceratopsidae, Dinosauria) from the Upper Cretaceous Ojo Alamo Formation (Naashoibito Member), San Juan Basin, New Mexico ROBERT M. SULLIVAN AND SPENCER G. LUCAS Ojoceratops fowleri, a new genus and species of chasmosaurine ceratopsid from the lower Maastrichtian Naashoibito Member (Ojo Alamo Formation), San Juan Basin, New Mexico, is diagnosed by an apomorphic squamosal. Ojoceratops fowleri is presently the only diagnosable ceratopsid from the Ojo Alamo Formation and antedates Eotriceratops xerinsularis, Triceratops horridus, Diceratops hatcheri, and Torosaurus latus, and is a temporal equivalent of ‘‘Torosaurus’’ utahensis. Introduction and Background Nearly 100 years ago, Barnum Brown (1910) briefly described the first ceratopsid remains from the Upper Cretaceous strata of the San Juan Basin, New Mexico (Fig. 11.1). The material he noted included uncollected thin squamosal fragments with ‘‘deep vascular grooves’’ and a small, smooth ‘‘supraorbital horncore’’ (AMNH 5798). While the material came from an area known to contain rocks of both the upper part of the Kirtland Formation (‘‘upper shale’’ of traditional usage) and the Ojo Alamo Formation (formerly Ojo Alamo Sandstone ), the exact stratigraphic horizon remains uncertain. The light brown-tan preservation of the ‘‘supraorbital horncore’’ (AMNH 5798) suggests that this specimen came from the lower Kirtland or upper Fruitland Formation and not from the stratigraphically higher De-na-zin Member (upper Kirtland Formation) or the overlying Ojo Alamo Formation. Gilmore (1916) subsequently reported ceratopsid remains from the same region. These included four specimens from the Ojo Alamo Formation; one specimen consisting of frill fragments, a second specimen consisting of frill fragments with ‘‘deep radiating vascular impressions,’’ a third consisting of ‘‘similar fragments,’’ and the fourth consisting of isolated teeth. Lull (1933) later cited these (and two others) as uncataloged material in the USNM collections, but none of this uncataloged material is presently housed at the Smithsonian (M. Brett-Surman pers. com. 2006), and their disposition is unknown. All of these specimens reportedly came from strata above the ‘‘lower conglomerate [which is at the base of the Ojo Alamo Formation],’’ with the possible exception of the third, whose stratigraphic position is less certain. In a subsequent paper, Gilmore (1919) noted the fragmentary remains (‘‘posterior median or dermosupraoccipital’’) of a ceratopsid from the Ojo Alamo Formation. Comparing it to Triceratops, Ceratops , or Monoclonius, he interpreted it as ‘‘a fragmentary frill in which the openings are apparently smaller than in either of the genera mentioned above. In all probability it represents an undescribed form’’ (Gilmore 1919: 65). Unfortunately, the whereabouts of this specimen is also unknown. For over 50 years, no additional ceratopsid material was collected from the Ojo Alamo Formation. In 1977, the Bureau of Land Management conducted a paleontological survey in advance of potential coal mining activity in the San Juan Basin, New Mexico. A few fragmentary 170 sullivan & lucas FIGURE 11.1. Regional map of the San Juan Basin, New Mexico, showing outcrops of the Ojo Alamo Formation and principal localities for Ojoceratops fowleri, gen. nov., sp. nov. (1) Head of Hunter Wash (SMP localities: 313b, 319b, 364b); (2) head of Willow Wash/Alamo Wash (NMMNH localities 4726 and 5841; SMP localities 410, 411, 415); (3) head of De-na-zin Wash (SMP locality 403); (4) head of Coal Creek (SMP loc. 375); and (5) north flank of Betonnie Tsosie Wash (SMP loc. 374). ceratopsid teeth were collected that year, and mandibular and rostral fragments were later recovered in 1983 and 1984, respectively . Lehman (1981) reviewed the history of collecting vertebrate fossils in the Upper Cretaceous strata of the San Juan Basin and added to the list of material from the Naashoibito Member, a unit previously considered part of the Kirtland Formation but recently reassigned to the Ojo Alamo Formation (see Sullivan and Lucas 2003, 2006; Sullivan et al. 2005b). It wasn’t until 1997 that a major effort to resample the dinosaur faunas from the Ojo Alamo and underlying Kirtland formations commenced, separately initiated by both the New Mexico Museum of Natural History and Science and the State Museum of Pennsylvania. Consequently, numerous ceratopsid and other specimens have been recovered from these formations during the last 13 years. In addition to acquiring new specimens, the stratigraphy and biostratigraphy of the terrestrial Upper Cretaceous rocks of the San Juan Basin continues to be refined. All specimens herein come from the Naashoibito Member of the Ojo Alamo Formation. Exposures of this member stretch along the northern border of the Bisti/De-na-zin Wilderness (head of Hunter Wash across to Willow and Alamo washes...

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