Abstract
Three new chasmosaurines from the Kirtland Formation (~75.0–73.4 Ma), New Mexico, form morphological and stratigraphic intermediates between Pentaceratops (~74.7–75 Ma, Fruitland Formation, New Mexico) and Anchiceratops (~72–71 Ma, Horseshoe Canyon Formation, Alberta). The new specimens exhibit gradual enclosure of the parietal embayment that characterizes Pentaceratops, providing support for the phylogenetic hypothesis that Pentaceratops and Anchiceratops are closely related. This stepwise change of morphologic characters observed in chasmosaurine taxa that do not overlap stratigraphically is supportive of evolution by anagenesis. Recently published hypotheses that place Pentaceratops and Anchiceratops into separate clades are not supported. This phylogenetic relationship demonstrates unrestricted movement of large-bodied taxa between hitherto purported northern and southern provinces in the late Campanian, weakening support for the hypothesis of extreme faunal provincialism in the Late Cretaceous Western Interior.
Highlights
Intermediate or “transitional” fossils are an expected product of evolution, and are especially celebrated when they occur within major evolutionary transitions (Anderson and Sues, 2007; Wellnhofer, 2010; Daeschler et al, 2006)
Based on the morphology of the posterior end of the parietal, here we show that MNA Pl.1747 and UKVP 16100 should be referred to a different taxon from AMNH 1624 and 1625
As the holotype specimens are probable subadults or adults, Taxon A and Taxon B can be appropriately compared with other taxa which are based on putative adults
Summary
Intermediate or “transitional” fossils are an expected product of evolution, and are especially celebrated when they occur within major evolutionary transitions (Anderson and Sues, 2007; Wellnhofer, 2010; Daeschler et al, 2006). In the Upper Cretaceous rocks of North America a combination of increasingly intensive sampling and newly refined stratigraphy is beginning to fill in gaps in the dinosaur record This is revealing hitherto unknown morphotaxa that link previously disparate or misunderstood morphologies, and/or define new ‘end-members’ that extend or emphasize stratigraphic morphological trends, challenging previously held assumptions about the mode and tempo of dinosaur evolution (Horner et al, 1992; Sampson, 1995; Holmes et al, 2001; Ryan and Russell, 2005; Wu et al, 2007; Currie et al, 2008; Sullivan and Lucas, 2010; Evans et al, 2011; Scannella and Fowler, 2014; Scannella et al, 2014)
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