Syrmosaurus viminicaudus Maleev, 1952

HILL, ROBERT V., WITMER, LAWRENCE M. & NORELL, MARK A., 2003, A New Specimen of Pinacosaurus grangeri (Dinosauria: Ornithischia) from the Late Cretaceous of Mongolia: Ontogeny and Phylogeny of Ankylosaurs, American Museum Novitates 3395, pp. 1-30 : 4

Syrmosaurus viminicaudus Maleev, 1952 )

HOLOTYPE: AMNH 6523 , dorsoventrally crushed skull and mandible with several associated osteoderms, atlas and axis.

TYPE LOCALITY: Djadokhta Formation, Shabarakh Usu, Mongolia (e.g., ‘‘The Flaming Cliffs’’).

REFERRED SPECIMENS: Uncataloged IVPP

specimens; IGM 100/1014; PIN 614, uncataloged PIN specimens; ZPAL MgD­II/1, ZPAL MgD­II/2, ZPAL MgD­II/7, ZPAL MgD­II/9, ZPAL MgD­II/27, ZPAL MgD­II/ 31, ZPAL MgD­II/32.

DIAGNOSIS: As for genus; Pinacosaurus with squamosal dermal ossifications present as weakly developed pyramids, not long spines as in P. mephistocephalus .

DISCUSSION: Abundant material collected by the Sino­Swedish, Polish­Mongolian, Sino­Canadian, and Russian­Mongolian expeditions has been referred to P. grangeri or Pinacosaurus sp. Most of the cranial material collected by these teams has represented juvenile or subadult individuals. All specimens of Pinacosaurus feature the distinctive set of characters found in the holotype, including a highly specialized narial region with at least three pairs of openings, prominent hornlike protuberances above the orbits, and lack of secondary dermal ossifications on the external rostroventral portion of the premaxillary beak. The holotype skull, however, also differs considerably from the numerous subadult crania that have been referred to the genus. The skull is longer than wide, with a narrow premaxillary beak and palate. Maryańska (1977) has attributed this condition to taphonomic processes, but the relatively mild distortion of the pterygoids and premaxillae suggests that the postdepositional reworking of this specimen was limited to dorsoventral crushing. Nevertheless, skulls subsequently referred to P. grangeri consistently exhibit the typical ankylosaurid condition of a skull and premaxillary palate that is wider than long.

A narrow skull can be found in more basal thyreophorans such as Emausaurus ernsti , Scelidosaurus harrisonii , and stegosaurs. The primitive ankylosaurids Tsagantegia longicranialis and Shamosaurus scutatus also exhibit this condition. The presence of a narrow skull in adult Pinacosaurus may be just one indicator of a phylogenetic affinity with these more primitive genera that has previously been obscured, as many of the characters considered diagnostic for the genus (and indeed, for suprageneric ankylosaurian taxa) have been scored from juvenile skulls that do not exhibit all features of the adult. In particular, the exceptionally com­ plete skull of P. grangeri described by Maryańska (1971; 1977), ZPAL MgD­II/1, is frequently cited as displaying typical ankylosaurid morphology, despite the fact that it represents a juvenile individual (e.g., Sereno, 1986; Coombs and Maryańska, 1990).