Megaherbivorous dinosaur turnover in the Dinosaur Park Formation (upper Campanian) of Alberta, Canada

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Abstract

Ongoing research into the biostratigraphy of the upper Campanian Dinosaur Park Formation (DPF) of Alberta has demonstrated that megaherbivorous dinosaur taxa (ankylosaurs, ceratopsids, hadrosaurids) are not homogeneously distributed throughout the unit. This has compelled proposals of different, informal assemblage zone schemes, and the hypothesis that faunal turnover was driven by environmental change associated with marine transgression. The current study tests previous zonation schemes in addition to the hypothesis of turnover pulses in the DPF. Clustering and ordination methods are used to demonstrate the existence of two broad assemblage zones within the DPF, each of which lasted ~ 600 Ka: a lower zone characterized by the presence of the ceratopsid Centrosaurus apertus and the hadrosaurids Corythosaurus, and an upper zone characterized by the presence of the ceratopsid Styracosaurus albertensis and the hadrosaurid Prosaurolophus maximus. These zones can be further sub-divided, based on the distributions of rarer or shorter-lived ankylosaur, ceratopsid, and hadrosaurid species, into ~ 300 Ka sub-zones. Canonical correspondence analysis is used to explore the association between the turnover of the megaherbivorous dinosaurs and various palaeoevironmental proxies. Megaherbivorous dinosaur turnover most closely corresponds to that of fossil palynomorphs. However, none of the palaeoenvironmental proxies explains dinosaur distribution better than a simple time gradient, suggesting that dinosaur turnover was not inextricably linked to environmental change as predicted by the turnover pulse hypothesis.

Highlights

► The Dinosaur Park Formation is composed of two broad megaherbivore assemblage zones. ► These zones span ~ 600 Ka. ► Each zone is divisible into two ~ 300 Ka sub-zones. ► Megaherbivorous dinosaur turnover does not track known palaeoenvironmental proxies.

Introduction

Distinct but essentially contemporaneous northern and southern dinosaur faunal provinces in the Campanian and Maastrichtian (Late Cretaceous) of western North America are believed to reflect adaptation to latitudinal climatic gradients and associated differences in vegetation (Lehman, 1987, Lehman, 1997, Lehman, 2001, Gates et al., 2010, Sampson et al., 2010b). Purported differences in faunal composition between inland and more coastal palaeoenvironments in the northern province imply that proximity to coastline influenced the distribution of dinosaurs and other taxa (Brinkman, 1990, Brinkman et al., 1998, Lehman, 2001). This suggests that, unlike mammals of comparable size, megaherbivorous dinosaurs (> 1 t) had relatively small geographic ranges that were governed by strict habitat preferences, particularly during the late Campanian when the Western Interior Seaway was at its transgressive maximum (Lehman, 2001), and that populations of these animals were sensitive to palaeoenvironmental and palaeoclimatic change.

The palaeontologically well-documented Belly River Group (ca. 79.1–74.8 Ma ago) of Alberta is of particular importance for understanding these patterns of Late Cretaceous dinosaur palaeoecology and biogeography, and testing the sensitivity of megaherbivores to environmental change. The uppermost unit of this group, the upper Campanian Dinosaur Park Formation (DPF), is particularly rich in megaherbivorous dinosaur fossils that represent Ankylosauria, Ceratopsidae, and Hadrosauridae. Recent high-resolution biostratigraphic and detailed taxonomic work has confirmed a non-random distribution of taxa within the DPF. Species are restricted in range to different parts of the formation (Sternberg, 1950, Godfrey and Holmes, 1995, Holmes et al., 2001, Currie and Russell, 2005, Eberth and Getty, 2005, Evans and Reisz, 2007, Evans et al., 2009), prompting the recognition of discrete assemblage (faunal) zones within the DPF where it is best exposed in the area of what is now Dinosaur Provincial Park (DPP) near Brooks (Béland and Russell, 1978, Ryan and Evans, 2005). The DPF was deposited during a major transgressive phase of the Western Interior Seaway (Eberth and Hamblin, 1993), and geological, vertebrate microfossil, and invertebrate fossil data indicate that the local environment became increasingly more coastally influenced up-section at DPP until its final inundation by the sea (Eberth and Hamblin, 1993, Brinkman et al., 1998, Eberth, 2005, Johnston and Hendy, 2005). Species turnover within the formation is often linked to palaeoenvironmental change associated with this marine transgression (Ryan and Evans, 2005, Evans, 2007, Evans and Reisz, 2007), and the possibility of turnover pulses has been suggested (Sampson, 2009, Sampson and Loewen, 2010), but these hypotheses have not been rigorously tested. In this paper, previous zonation schemes of the DPF megaherbivorous dinosaurs, and their potential association with palaeoenvironmental factors, are quantitatively tested using multivariate clustering and ordination methods for the first time. The suggestion that the pattern of megaherbivorous dinosaur distribution is related to turnover pulses is also tested.

The DPF, previously subsumed under the Oldman and Judith River formations, is the uppermost unit of the Belly River Group, which is a transgressive sedimentary succession found throughout much of southern Alberta and southeastern Saskatchewan (Eberth, 2005). The DPF is best exposed in the area of DPP, where it is ~ 70 m thick and disconformably overlies the Oldman Formation. The sediments of the DPF were deposited during the late Campanian, ca. 76.5–74.8 Ma ago, and reflect the third order transgressive event of the Western Interior Seaway (Eberth, 2005). The sandy lower exposures of the DPF comprise alluvial palaeochannel deposits that have been interpreted as having been at least partly influenced by rhythmic tidal backwater conditions, due to the presence of inclined heterolithic strata in the palaeochannel deposits (Koster et al., 1987, Wood et al., 1988, Wood, 1989). However, Thomas et al. (1987) provided evidence that these strata can develop in alluvial (non-coastal) settings as well, and the palaeoenvironment of the lower DPF is currently interpreted as an alluvial plain dominated by meandering river systems (Eberth, 2005). The muddier upper exposures of the DPF comprise overbank facies that culminate in the Lethbridge Coal Zone (LCZ). These exposures are interpreted as poorly drained, organic-rich (swampy) palaeoenvironments that formed in coastal settings. Marine flooding surfaces are common in the LCZ, recognized on the basis of marine microfossils and trace fossils, and lateral continuity with mud-filled incised valleys (Eberth, 1996). The LCZ is estuarine in origin, comprising a coaly to carbonaceous-dominated mixed freshwater, brackish, and marine succession, and was likely deposited along a wave-dominated shoreline (Eberth, 1996). The LCZ interfingers with the overlying marine Bearpaw Formation, which is composed of structureless and laminated shales.

The DPF is best known for its abundant dinosaur fossils, which often occur as articulated skeletons or in bonebeds, although isolated bones and vertebrate microfossil sites are also common (Dodson, 1971, Brinkman, 1990, Brinkman et al., 2005, Eberth and Currie, 2005, Eberth and Getty, 2005). The locations of many of these fossils within the bounds of present day DPP were initially mapped by Sternberg, 1936, Sternberg, 1950, who also used an aneroid barometer to provide hyposmetric elevation data (metres above sea level = masl) for many of the quarries collected until then. With these data, Sternberg (1950) demonstrated that the stratigraphic distributions of the megaherbivorous dinosaur genera of the DPF are not homogeneous. For example, he observed that, among ceratopsids, Centrosaurus occurs low in section, followed successively by Chasmosaurus and Styracosaurus. The hadrosaurids were said to exhibit a similar pattern, whereby the hadrosaurines Gryposaurus and Prosaurolophus appear low and high in section, respectively, and, among the lambeosaurines, Corythosaurus occurs low in the formation, and Lambeosaurus occurs nearer the top. Sternberg (1950) further resolved the stratigraphic distribution of the lambeosaurine genera at the species level. Within Corythosaurus, Co. casuarius appears low in section and Co. intermedius appears higher up. Within Lambeosaurus, L. clavinitialis and L. lambei occur relatively low in section, and L. magnicristatus occurs at the top of the DPF.

Béland and Russell (1978) also commented on the biostratigraphy of the DPF. They arbitrarily divided the DPF into three vertical “levels” and observed the distribution of dinosaur genera within each, using the data of Sternberg (1950) and subsequently collected specimens. They recognized the existence of two assemblage zones within the DPF: a lower one characterized by the presence of the ankylosaur Euoplocephalus, the hadrosaurid Kritosaurus (= Gryposaurus) and the large theropod Albertosaurus (= Gorgosaurus), and an upper one characterized by the presence of the ankylosaur Panoplosaurus, and the hadrosaurids Lambeosaurus and Prosaurolophus.

Unfortunately, these early attempts at describing the biostratigraphy of the DPF were hampered by their reliance on raw altitudes. The beds of the entire Belly River Group, including the DPF, exhibit a gradual dip < 1° to the west (Eberth and Hamblin, 1993, Eberth, 2005), thereby rendering the elevation data of Sternberg (1950) and Béland and Russell (1978) of limited use. More recent work on the biostratigraphy of the DPF has relied on the use of the more sophisticated survey grade Ground Positioning System (MacDonald et al., 2005) and the Oldman/DPF contact as an isochronous datum to more accurately reflect the stratigraphic positions of the fossils within the area of DPP. The contact is not isochronous over a wider geographic area because the DPF thins in an eastward direction so that it is just 31 m thick where it is exposed near Manyberries, Alberta (Eberth and Hamblin, 1993). The most recent studies to comment on the biostratigraphy of the DPF have used the Oldman/DPF contact isochron to refine biostratigraphic patterns within DPP with great success. Different patterns have been noted for the distribution of various fossil palynomorphs (Braman and Koppelhus, 2005), mollusks (Johnston and Hendy, 2005), fish, frogs, turtles (Brinkman, 1990), mammals (Sankey et al., 2005), and dinosaurs (Brinkman, 1990, Brinkman et al., 1998, Currie and Russell, 2005, Eberth and Getty, 2005, Ryan and Evans, 2005). Currie and Russell (2005) noted that, among centrosaurine ceratopsids, Ce. apertus occurs low in section, followed sequentially by S. albertensis and an as-yet-unnamed ‘pachyrhinosaur’ (Ryan et al., 2010a). The chasmosaurine ceratopsids are represented by Ch. russelli low in section, followed sequentially by Ch. belli and “Ch.” (= Vagaceratops) irvinensis. Among hadrosaurine hadrosaurids, G. notabilis occurs low in section and Pr. maximus occurs higher up. Among lambeosaurine hadrosaurids, Corythosaurus occurs low in section, and is replaced by Lambeosaurus in the upper half of the DPF, with the only specimen of L. magnicristatus occurring near the base of the LCZ (Ryan and Evans, 2005, Evans and Reisz, 2007). Evans (2007) further demonstrated that the hypothesized sexual dimorphs of Co. casuarius and L. lambei (sensu Dodson, 1975) are separated stratigraphically, and therefore likely represent either distinct species or perhaps phyletic chronospecies (Evans et al., 2006). Parasaurolophus sp. is restricted to the lower half of the DPF (Evans et al., 2009).

These distributional patterns (Fig. 1) led Ryan and Evans (2005) to propose three informal assemblage zones within the DPF: a lower zone (0–30 m) comprising Centrosaurus, Ch. russelli and Corythosaurus, a middle zone (30–50 m) comprising Styracosaurus, Ch. belli and Lambeosaurus, and a possible third upper zone (~ 50–55 m) comprising rare occurrences of poorly-known taxa including “Ch.” (= Vagaceratops) irvinensis, a ‘pachyrhinosaur’, and L. magnicristatus.

ACM, Beneski Museum of Natural History, Amherst College, Amherst, Massachusetts; AMNH, American Museum of Natural History, New York; CM, Carnegie Museum of Natural History, Pittsburgh; CMN, Canadian Museum of Nature, Ottawa; FMNH, Field Museum of Natural History, Chicago; MCSNM, Museo Civico di Storia Naturale di Milano, Milan; MLP, Museo de la Plata, La Plata, Argentina; NHMUK, Natural History Museum, London; ROM, Royal Ontario Museum, Toronto; SDNHM, San Diego Natural History Museum, San Diego; TMM, Texas Memorial Museum, Austin; TMP, Royal Tyrrell Museum of Palaeontology, Drumheller, Alberta; USNM, Unites States National Museum, Washington; UALVP, University of Alberta Laboratory of Vertebrate Palaeontology, Edmonton; YPM, Yale Peabody Museum, New Haven.

Section snippets

Data collection

Megaherbivorous dinosaur distributions have proved useful in the establishment of assemblage zones within the DPF for the following reasons: (1) ankylosaurs, ceratopsids, and hadrosaurids are collectively among the most abundant vertebrate fossils in the DPF (Dodson, 1983, Brinkman, 1990); (2) they are found throughout the DPF (Sternberg, 1950, Béland and Russell, 1978, Currie and Russell, 2005, Ryan and Evans, 2005); (3) individual species exhibit limited stratigraphic ranges within the DPF (

UPGMA

The first series of Q-mode cluster analyses was performed on the raw data matrix using the UPGMA algorithm. Clustering was generally poor, regardless of the similarity fossil record and insufficient sampling. In order to improve clustering, a range-through assumption was applied to the data matrix. This involves coding all intervals between the first and last appearance data for a taxon as ‘1’, effectively assuming that taxon to be continuously present throughout its entire range in the

Megaherbivore assemblage zones of the DPF

The establishment of assemblage zones in biostratigraphy is useful because it not only facilitates the characterization of the structure underlying the data, but it also allows the study of evolutionary palaeoecology to be framed so that the confounding effects of time-averaging are limited in a biologically meaningful way (as opposed to binning stratigraphic intervals using some arbitrary criterion). The generally symmetric biostratigraphic clustering schemes of the DPF presented above are

Conclusions

The DPF is among the most fossiliferous dinosaur-bearing deposits in the world and records significant palaeoenvironmental changes associated with marine transgression and tectonism (Eberth and Hamblin, 1993). As such, it provides an excellent dataset to test previous hypotheses of megaherbivorous dinosaur distribution, niche segregation, and turnover pulses. This statistical study supports the existence of two broad assemblage zones in the DPF, each lasting ~ 600 Ka: a lower one characterized by

Acknowledgments

Thanks to P. Currie, E. Koppelhus, M. MacDonald, W. Spencer, D. Tanke, and all others who collected and made available the data used here. D. Braman, D. Brinkman, D. Eberth, and D. Tanke offered valuable help and insight. Ø. Hammer provided technical assistance. This manuscript was greatly improved by the editorial efforts of F. Surlyk and two anonymous reviewers. This work was funded by scholarships from the Natural Sciences and Engineering Research Council, Alberta Innovates Technology Futures

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