Diversity, biogeography, and taphonomy of Late Cretaceous chondrichthyans from Montana

The chondrichthyan fauna of the Late Cretaceous of Montana was last comprehensively reviewed by Case in the late 1970s. Collections from vertebrate lag deposits (2002 and 2006) from the Woodhawk Bonebed (WH) and Power Plant Ferry Bonebed (PPF) respectively of the Judith River Formation (JRF) add six additional species to the previous known diversity from the JRF, namely Squalicorax pristodontus, Cretolamna appendiculata, Protolamna sokolovi, Ischyrhiza avonicola, Ptychotrygon hooveri, and Ptychotrygon triangularis. These collections also include previously known species from the JRF, Hybodus montanensis, Cretorectolobus olsoni, Squalicorax kaupi, Squalicorax sp., cf. S. kaupi, Hypotodus grandis, Hypotodus spp., Archaeolamna kopingensis, Archaeotriakis rochelleae, Protoplatyrhina renae, Ischyrhiza mira, Myledaphus bipartitus and a chimaerid, Ischyodus.;The combined faunas from the WH and PPF and Case's studies (1978a and 1979) indicate that the JRF was a coastal, warm shallow marginal marine environment supporting a moderately diverse fauna of mostly lamniforms and rajiforms. Biogeography analyses of elasmobranchs from the Western Interior Seaway (WIS) show that 17 of the JRF taxa are endemic either to the JRF or to the end of the Cretaceous of the WIS. Also present are five cosmopolitan species. The species compositions of Case's study and this study differs, such that, the combined faunas produce a more comprehensive faunal list that is on par with contemporaneous faunas from similar environments to that of the JRF, and that the JRF fauna along with the faunas of the Dinosaur Park Formation (Alberta), "Mcsaverde Formation" (Wyoming), and the Hell Creek Formation (Montana) form the Judith River Province at the generic level in a Parsimony Analysis of Endemicity (PAL).;Most of the species collected are autochthonous; those found in situ are likely to be less abraded than material brought in, as evidenced by taphonomic experiments. Experiments also show that the two locations, despite close proximity to each other, show localized areas of variable energy that affected the faunal composition slightly, the abundance of material, and the quality of preservation. The WH is a higher energy environment where additional material from farther offshore is transported in and mixed with local material, resulting in a higher degree of abrasion of material compared to that preserved at PPF. This has created a mixed marine and estuarine assemblage, which in turn has produced mixed interpretations of the geology and paleontology of the JRF. The experimental approach applied here permits: distinguishing autochthonous from allochthonous fossil vertebrate hard parts, quantification of the amount of transport and wear, and clarification of the potential for postmortem effects such as the loss or distortion of diagnostic skeletal features. In the case of shark teeth, the root lobes and apex of the cusp wear down first, followed by other projections such as cusplets. Loss of these features suggests the effects of taphonomic processes. However, taphonomic insight is achieved at the cost of information on the taxon and the fauna overall. A preliminary product from these observations and experiments is a taphonomic scale that can be applied to moderately worn shark teeth to assess amount of wear and relative environment.