CO?JTRIBI!TIONS FROM THE MUSEUM OF PALEOK.1-OLOCiY

-The nature and extent of enamel ultrastructural variation in mammals has not been thoroughly investigated. In this study we attempt to identify and evaluate the sources of variability in enamel ultrastructural pa t t e rns a t a number of hierarchic levels within t h e extinct order Multituberculata. These levels include: 1) different positions on a single tooth; 2) different depths and orientations of a prepared enamel surface; 3) different teeth from a single individual; 4) isolated teeth assigned to a single species; 5) between congeneric species; 6) between genera; and 7) within suprageneric taxa. Nearly all of the specimens examined can be unambiguously characterized by one of two major ultrastructural types: large (X diameter = 8.2 pm, N = 32, sd = 1.36), arcade-shaped prisms or small (X diameter = 3.6 pm, N = 28, sd = 0.77), circular prisms. Consistent variation in these two types appears only at the level of intergeneric comparisons and above; variation in prism size and shape below this level exists, but is negligible relative to the higher order variation. An analysis of prism spacing and density reveals that small, circular prisms are relatively numerous and closely-spaced per unit area while large, arcade-shaped prisms are relatively few and far between per unit area. In general, there is more interprismatic material between circular prisms, despite their small size. We also compared various modes of preparation used to reveal ultrastructural patterns, and the ways in which patterns have been previously characterized and compared. Our review of Late Cretaceous and early Tertiary multituberculates reveals remarkable consistency in ultrastructural type at the subordinal level. All of the 13 recognized ptilodontoid genera were examined; all but two (Cimolodon and Boffius) possess small, circular prisms that are numerous and closely-spaced. Twelve of 20 recognized taeniolabidoid genera were examined; all but three (Neoliotomus, Xyronomys, and Microcosmodon) possess large, arcade-shaped prisms that are few in number and widely-spaced. Specimens of the taeniolabidoid genus Microcosmodon are unique among multituberculates in having small prisms that are either circular or arcade-shaped. All but one ( Viridomys) of the seven genera currently classified as Suborder incertae sedis possess large, arcade-shaped prisms. ' Department of Geological Sciences and Museum of Paleontology. '1 he l!niversity of M~chigan. Ann Arbor . Department of Anatomical Sciences. Health Sciences Center, State University of New York. Stony Brook.