Chiba et al.—Systematic re-evaluation of Medusaceratops
However, Longrich (2013) retained the original centrosaurine designation, noting that the number of epiparietals is variable in Chasmosaurinae, and the parietosquamosal contact more closely resembles that of Centrosaurinae. Here, we describe newly collected centrosaurine material
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from the Mansfield bonebed, re-examine the type series of Medusaceratops, reassess the known specimens from the bonebed, and conduct a systematic revision of the taxon. New data allow Medusaceratops to be included in a numerical cla- distic analysis for the first time. We confirm the subfamilial identity of Medusaceratops as an early centrosaurine, which has implications for understanding the early diversification of ceratopsids.
Geological context
The Mansfield bonebed is located in the badlands of Kennedy Coulee, north of Rudyard, Hill County, Montana, USA (Fig. 1.1). Here, the middle Campanian alluvial deposits, com- monly referred to the lower Judith River Formation (Goodwin and Deino, 1989), crop out extensively along the drainage systems flowing toward the Milk River Valley in the north (Fig. 1.2, 1.3). Following recent stratigraphic revision of the Judith River Formation by Rogers et al. (2016), the beds exposed at Kennedy Coulee correlate to the McClelland Ferry Member to the south, but also with the upper Foremost and overlying lower Oldman formations of southern Alberta to the north, including the Taber Coal Zone (TCZ) and the Herronton Sandstone Zone (HSZ; Ogunyomi and Hills, 1977; Eberth and Hamblin, 1993; Cullen et al., 2016). The TCZ, which represents the top of the Foremost Formation in Alberta and correlative
coal deposits exposed to the south, represents a datum for calibrating stratigraphic sections and associated fossil taxa (Eberth and Hamblin, 1993; Brinkman et al., 2004; Ryan, 2007; Evans and Ryan, 2015; Freedman Fowler and Horner, 2015; Cullen et al., 2016; Ryan et al., 2017). The Mansfield bonebed occurs ~10m above the top of the Marker A Coal layer, which is equivalent to the top of the TCZ (sensu Goodwin and Deino, 1989) based on multiple sections measured in the Kennedy Coulee and at the Probrachylophosaurus bergei Freedman, Fowler, and Horner, 2015 locality (MOR locality JR-518), which is ~8km to the north east. There are two ben- tonite layers occurring ~10m below and ~25m above the TCZ, which provides radiometric ages of 79.02 and 78.01 Ma, respectively (Roberts et al., 2013, but see Freedman Fowler and Horner, 2015 and Fowler, 2016 for discussion), that bracket the Mansfield bonebed and constrain its chronostratigraphic age. The bonebed host beds sit on top of a 10m thick interval of interbedded organic-rich mudstones with discontinuous carbonaceous seams, siltstone, and sandstones (Fig. 1.4). Fossils occur at the base of a 3m thick, fining-upward sandy channel deposit that is rich in ironstone nodules; paleocurrent measure- ments suggest predominant flows toward the ESE. The stratigraphic occurrence of Medusaceratops places the
taxon above the Herronton Sandstone Zone in the same strati- graphic interval where Albertaceratops and Wendiceratops were recovered ~25km to the northwest in southern Alberta. Correlation to the top of the TCZ places Albertaceratops slightly lower in the section (8mabove the TCZ) with respect to Medusaceratops (10m above the TCZ) and Wendiceratops (12m above the TCZ), making them virtually indistinguishable stratigraphically. Due to the discontinuous nature of the beds
Figure 1. (1) Locality map of the Mansfield bonebed. (2, 3) Overview photos of the locality area. (4) Stratigraphic column of the locality area. The bonebed is indicated by a black star.
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