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Low Beta Diversity of Maastrichtian Dinosaurs of North America

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Low Beta Diversity of Maastrichtian Dinosaurs of North America Low beta diversity of Maastrichtian dinosaurs of North America Matthew J. Vavrek1 and Hans C. E. Larsson Redpath Museum, McGill University, Montreal, QC, Canada H3A 2K6 Edited by David Weishampel, Johns Hopkins University, and accepted by the Editorial Board March 19, 2010 (received for review November 30, 2009) Beta diversity is an important component of large-scale patterns of zone; and an interior Triceratops zone (7,8). The boundaries ofthese biodiversity, but its explicit examination is more difficult than that three regions have been slightly modified as more fossils have been of alpha diversity. Only recently have data sets large enough been found, and later studies incorporated these new dinosaur as well as presented to begin assessing global patterns of species turnover, previous pterosaur (Quetzalcoatlus) finds (8). especially in the fossil record. We present here an analysis of beta The hypothesized provincial delineations lead to the prediction diversity of a Maastrichtian (71–65 million years old) assemblage of that beta diversity for this region as a whole should be relatively dinosaurs from the Western Interior of North America, a region that high. However, previous work was based on the occurrence of a covers ≈1.5 × 106 km2, borders an epicontinental sea, and spans few well-known individual genera that have well-defined geo- ≈20° of latitude. Previous qualitative analyses have suggested graphic boundaries within relatively small spatial scales. Such high regional groupings of these dinosaurs and generally concluded that levels of endemism would be unprecedented for any modern large- there were multiple distinct faunal regions. However, these studies bodied terrestrial fauna. did not directly account for sampling bias, which may artificially We use modern approaches to search for statistical support for decrease similarity and increase turnover between regions. Our areas ofhigh endemism. These approaches included rarefaction and analysis used abundance-based data to account for sampling inten- species estimators to calculate beta diversity and minimum spanning sity and was unable to support any hypothesis of multiple distinct trees (MST) and nonmetric multidimensional scaling (NMDS) to faunas; earlier hypothesized faunal delineations were likely a sam- search for readily apparent provinces (see Materials and Methods for pling artifact. Our results indicate a low beta diversity and support a full explanation). If there are strongly delineated provinces present, single dinosaur community within the entire Western Interior we predict that beta diversity for the Maastrichtian Western Interior region of latest Cretaceous North America. Homogeneous environ- dinosaur fauna is high and that the NMDS/MST ordination will be ments are a known driver of low modern beta diversities, and the fragmented into discrete clusters of sites. warm equable climate of the late Cretaceous modulated by the epi- contenental seaway is inferred to be an underlying influence on the Results low beta diversity of this ancient ecosystem. Using observed values of generic richness, with alpha diversity calcu- lated from the average richness of all formations (n ¼ 24; α ¼ 5:46), biogeography | macroecology | paleoecology | Cretaceous beta diversity is relatively high at βH1 = 8.24 (Table 1). This corrob- orates the high endemism found by previous workers but takes no lpha diversity is defined by Whittaker (1, 2) as species richness account of sampling effects. There is an obvious and unsurprising on the local or habitat scale, and beta diversity is defined as the correlation between generic richness and sample size for this dataset A R2 P << difference in the types of species found in different areas of alpha ( = 0.79, 0.001; Fig. S1), illustrating the bias of differential diversity. Alpha and beta diversity together make up species sampling intensity. Only 11 of the 24 formations have more than 10 richness at the landscape scale, called gamma diversity. Because specimens recorded, and thus low alpha diversities are due to insuf- ficient sample sizes rather than actual conditions. This range of sam- beta diversity measures turnover across an area, it is closely related ECOLOGY to the numbers of endemic species within each community, which pling intensity causes beta diversity estimates to be higher than those in turn can be used to assess biotic provinciality. Quantitative obtained from evenly, well-sampled data (5). estimates of modern beta diversity recover surprisingly low beta To reduce this bias when calculating the average alpha diversity, diversity values despite broad taxonomic and geographic sampling, we eliminated all formations with fewer than 100 specimens. This regardless of the motility of the group in question (3–5). Instead, eliminated all but four formations, namely Hell Creek (Montana), beta diversity seems most correlated with climate evenness (4–6). Hell Creek (North Dakota), Horseshoe Canyon (Alberta), and We present beta diversity estimates for an ancient terrestrial Lance (Wyoming). Formations with more than 100 specimens are ecosystem. The Western Interior of North America is perhaps the expected to give the most robust values of actual alpha diversity most intensely sampled dinosaur-bearing region in the world (7). and are therefore more informative than keeping formations with fewer specimens. We believed that using only formations with The terrestrially deposited rock sequence comprises sparsely more than 100 specimens was an appropriate compromise, main- exposed Aptian (121–112 Ma) formations to extensively exposed taining high numbers of specimens while retaining enough local- Maastrichtian (71–65 Ma) formations. These Maastrichian rocks ities for reasonable estimates. Average alpha diversity uncorrected are largely floodplain deposits along the western shores of the epi- for sampling bias (Sobs) for these four localities was 13.5. However, continental Western Interior Seaway (Fig. 1). The rich Maas- for the estimates of gamma diversity, we retained all formations for trichtian deposits have been a focus of research on patterns of dinosaur distributions and biogeography on a subcontinental scale (7–13). We used this dinosaur assemblage for our analysis of beta Author contributions: M.J.V. designed research; M.J.V. performed research; M.J.V. contrib- diversity in the fossil record because it is currently the only data set uted new reagents/analytic tools; M.J.V. and H.C.E.L. analyzed data; and M.J.V. and H.C.E.L. large enough and the only region with extensive previous work on wrote the paper. endemism and provinciality. Previous studies have concentrated on The authors declare no conflict of interest. dinosaur faunal provinciality at local scales on the basis of presence/ This article is a PNAS Direct Submission. D.W. is a guest editor invited by the absence data. Specifically, these analyses have suggested high levels Editorial Board. of endemism at local, formational scales (13). The most widely 1To whom correspondence should be addressed. E-mail: [email protected]. accepted community hypothesis divides the dinosaur fauna into This article contains supporting information online at www.pnas.org/cgi/content/full/ three zones: a northern Leptoceratops zone; a southern Alamosaurus 0913645107/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.0913645107 PNAS | May 4, 2010 | vol. 107 | no. 18 | 8265–8268 Downloaded by guest on September 27, 2021 Table 1. Results for rarefaction and alternate methods of species estimation for the highlighted formations and full region Locality No. of specimens Generic richness (observed) Rarefaction Chao 1 ACE Jackknife 1 Average estimated richness* Hell Creek Montana 211 17 14.38 19.58 15.66 18.2 17.81 Hell Creek North Dakota 268 9 6.68 8.5 10.15 8.58 9.08 Horseshoe Canyon Alberta 120 14 12.85 28.99 14.94 18.73 20.88 Lance Wyoming 111 14 13.88 14.58 14.35 15.39 14.77 Average α† 177.5 13.5 11.95 17.91 13.77 15.22 15.64 ‡ Western Interior 997 45 21.95 40.55 28.02 31.72 33.43 βH1 — 3.33 1.84 2.26 2.03 2.08 2.14 *Average estimated richness is the mean value of Chao 1, ACE, and Jacknife 1. † Values for the average α were calculated using only the four listed formations. ‡Values for the entire Western Interior region were calculated using all 24 localities from the dataset. the calculation. This method would likely overestimate gamma When beta diversity was recalculated with an average alpha diversity, because there were many genera known for this region diversity calculated from only the four formations mentioned that have not been found within these four formations. above, beta diversity dropped greatly to βH1 = 3.33. However, the sampling intensities for each formation and the region in total vary greatly, from n = 111 (Lance Wyoming) to n = 268 (Hell Creek North Dakota). Rarefaction methods were used to com- pensate for these large differences (Fig. 2). After all samples were rarefied, generic richness for each formation dropped com- paratively little (α ¼ 11:95), whereas gamma diversity decreased by nearly half. Consequently, beta diversity also showed a large decrease to βH1 = 1.84. One criticism of rarefaction is that, although accounting for sample size, it does not reflect different rates of increase in species richness due to differences in underlying species evenness (14, 15). Total Hell Creek Montana Hell Creek North Dakota Horseshoe Canyon Alberta Lance Wyoming 10 15 20 25 30 35 05 A B Species Richness (S) Fig. 1. Approximate locations of Maastrichtian-aged dinosaur-bearing for- mations from the Western Interior of North America used in this analysis. (A) Abbreviations of formation names are as follows: A, Aguja; D, Denver; Fe, Ferris; 0 5 10 15 20 25 30 35 C D Fr, Frenchman; HM, Hell Creek Montana; HN, Hell Creek North Dakota; HS, Hell Creek South Dakota; HC, Horsehshoe Canyon; J, Javelina; Kp, Kaiparowits; Kd, 0 204060801000 20 40 60 80 100 Kirtland; LM, Lance Montana;LS, Lance South Dakota; LU, Lance Utah;LW, Lance Number of Individuals (N) Wyoming; LaC, Laramie Colorado; Law, Laramie Wyoming; M, McRae; N, North Horn; P, Pinyon Conglomerate; Sc, Scollard; SA, St.
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