| Summary: | Pachyrhinosaurus perotorum, a paleo-Arctic centrosaurine ceratopsid from the Kikak-Tegoseak Quarry (North Slope, Alaska) represents a unique opportunity to add to the understanding of ceratopsian bone histology, which is poorly understood due to the minimal preservation of growth markers (e.g. lines of arrested growth) and limited histological sampling across the ceratopsian lineage. Histological analyses of eight rib fragments from P. perotorum were conducted to add to the understanding of ceratopsian growth dynamics. Cyclical growth is preserved within ribs from P. perotorum allowing for the assignment of relative ontogenetic ages. One juvenile (DMNH 23891), 4 sub-adults (DMNH 21574, DMNH 24384, DMNH 24228, and DMNH 23888), and one adult (DMNH 24237) were identified. Radial and reticular fibrolamellar bone is prevalent in juvenile and sub-adult individuals indicating P. perotorum grew rapidly during ontogeny. Dense secondary bone is widespread in adult and three sub-adult individuals, which obscures most primary bone tissue and lines of arrested growth (LAGs). The degree of remodeling is higher than that previously reported in dinosaur rib histology, and may be attributable to differences in element-specific growth rate, environmental or biomechanical stresses. However, more histological studies of P. perotorum comparing growth between different postcranial long bones are needed to constrain the controls of secondary bone within this paleo-Arctic species. Although previous studies have interpreted taphonomy of the Kikak-Tegoseak Quarry (KTQ) using sedimentological and paleontological data, less is known about the geochemical taphonomy of this assemblage. P. perotorum bone has been altered from carbonate-hydroxyapatite to carbonate fluorapatite. XRD full width half maximum (FWHM) values display narrower peak widths (0.29-0.35°) than modern bone indicating a more crystalline apatite lattice structure. ATR-FTIR infrared splitting factor (IR-SF) values in P. perotorum specimens are greater (3.3-3.6) than in modern bone indicating that apatite crystallite sizes are larger than what is typically found in non-fossil bone. Higher crystallinity is a common result in fossil bone due the growth of authigenic apatite. ATR-FTIR spectra reveal elevated carbonate due to the addition of B-type carbonate into the apatite lattice. Relative amounts of carbonate correlates with ontogeny which could reflect a biological signal, although the effect of diagenetic alteration cannot be ruled out. Based on the elevated carbonate within bones of P. perotorum, there is potentially significant diagenetic alteration of the δ18Ocarbonate signal, therefore future stable isotope studies from the KTQ P. perotorum specimens should be cautious. Determining the potential chemical alteration of the δ18Ophosphate is more difficult since crystallinity data cannot differentiate between biogenic and secondary phosphate in bone. However, depleted carbonate: phosphate ratios can indicate the addition of more phosphate and thus, potential diagenetic alteration of the δ18Ophosphate. REE spider patterns yield different patterns between P. perotorum bones, which indicates taphonomic reworking (spatial and/or temporal) due to the preservation of different redox conditions and diffusion periods between bones. This observation is surprising due to the lack of significant/variable bone weathering or abrasion. Geology
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