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Geology.Gsapubs.Org on 30 September 2009 Downloaded from geology.gsapubs.org on 30 September 2009 Geology A surfeit of theropods in the Moroccan Late Cretaceous? Comparing diversity estimates from field data and fossil shops Alistair J. McGowan and Gareth J. Dyke Geology 2009;37;843-846 doi: 10.1130/G30188A.1 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geology Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and science. This file may not be posted to any Web site, but authors may post the abstracts only of their articles on their own or their organization's Web site providing the posting includes a reference to the article's full citation. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. Notes © 2009 Geological Society of America Downloaded from geology.gsapubs.org on 30 September 2009 A surfeit of theropods in the Moroccan Late Cretaceous? Comparing diversity estimates from fi eld data and fossil shops Alistair J. McGowan1* and Gareth J. Dyke2 1Museum für Naturkunde, Leibniz Institute for Research on Biodiversity and Evolution at the Humboldt University Berlin, Invalidenstrasse 43, Berlin D-10115, Germany 2School of Biology and Environmental Science, University College Dublin, Belfi eld 4, Dublin, Ireland ABSTRACT COLLECTORSHIP BIASES An unusually high proportion of large-bodied carnivorous Similar collectorship biases have already been identifi ed in museum theropod dinosaurs has been reported from the Moroccan Late data: Davis and Pyenson (2007) performed a comparative analysis of Cretaceous Kem Kem Formation, a well-known package of North rarefaction curves of North American Paleocene mammal faunas from Africa vertebrate fossil–bearing sediments. We investigate whether museums and fi eld collections and reported a tendency for collector recorded proportions of predator and prey taxa in Kem Kem sedi- curves based on museum data to have steeper initial trajectories because ments are real, or an artifact generated by collecting biases, by com- of preferential sampling of rare taxa. Museums tend not to accumulate paring fi eld data to counts of fossil vertebrates from Moroccan fos- large numbers of common taxa, while rare taxa are biased in collections sil shops. The application of common techniques for standardizing toward “trophy specimens” (Davis and Pyenson, 2007). Guralnick and ecological survey data confi rms that previous workers have been Van Cleve (2005) demonstrated the importance of museum collections misled by the acquisition by museums of specimens from commer- for capturing records of rare taxa, increasing species-richness estimates cial collectors rather than from detailed fi eld surveying. Claims that for extant birds in southern Colorado. an unusual number of theropod dinosaurs were present in North Here we examine Stromer’s Riddle by comparing fi eld-counted ver- Africa Late Cretaceous ecosystems are likely the result of biases due tebrate abundance data from a single stratigraphically well correlated site to both commercial activity and collectorship biases. within the Kem Kem Formation to counts of fossils from Moroccan fossil shops. We suspect that data from fossil shops are analogous to museum INTRODUCTION collections in that they are unlikely to accurately sample true faunal pro- Vertebrate fossils have been known from the Kem Kem Forma- portions at a particular place and time, simply providing data about the tion in the Moroccan Sahara of North Africa for more than 50 years presence of taxa. Biases identifi ed by Guralnick and Van Cleve (2005) and (Lavocat, 1954). This intensively collected sequence (i.e., the Kem Kem Davis and Pyenson (2007) would be magnifi ed in shop data because the beds of Sereno et al., 1996) comprises Late Cretaceous (Albian–Ceno- rarity, or aesthetic appeal, of a fossil will augment its economic value, irre- manian) fl uvial and shallow-marine sediments exposed in southeast- spective of its scientifi c worth. Thus we address two questions. (1) How ern Morocco (Province d’Errachidia) around Erfoud (e.g., Hamada du comparable are fi eld and fossil shop-based surveys? (2) Were there really Guir and Tafi lalt) that are part of the “continental intercalaire” (Lavo- too many theropods in the Kem Kem Formation, or are collecting biases cat, 1954) that extended across much of North Africa during the late the source of reports of an unusual predator:prey ratio? Mesozoic. In terms of described taxa, taking fossil records at face value, large theropod dinosaurs were disproportionately abundant in Kem Kem MATERIALS AND METHODS Formation sediments (e.g., Russell, 1996; Sereno et al., 1996; Amiot et Field data come from the Kem Kem Formation locality of Gara Sba al., 2004; Weishampel et al., 2004), even though such a preponderance (e.g., Lavocat, 1954; Sereno et al., 1996) (030°30.879N, 004°50.224W) of carnivores defi es what we know about modern ecosystems (Farlow, in southeastern Morocco (Table 1). Field data were compared with aver- 1993). How could ecosystems like those that composed the Kem Kem aged counts of vertebrate fossils recorded in six Moroccan fossil dealer- have supported so many predators at the apex of the food chain, while ships (Table 1). Using rarefaction and resampling techniques, we tested relatively fewer potential prey taxa (e.g., herbivorous dinosaurs, smaller vertebrates) are known from the same sequence? We dub this paradox Stromer’s Riddle, after Ernst Stromer, who worked on fossil vertebrates TABLE 1. COUNTS OF INDIVIDUAL KEM KEM FORMATION FOSSILS from North Africa during the 1930s, describing some of the key taxa of AND ASSIGNMENTS TO BROAD VERTEBRATE TAXA large theropods (Stromer, 1936; Russell, 1996; Nothdurft, 2002). Under- Shops* Shops Field Field standing Stromer’s Riddle has wide implications for vertebrate paleo- (counts) (percentages) (counts) (percentages) ecology: is it valid to use vertebrate fossil records, either published or Crocodilians 256 35.3 140 36.8 housed in museum collections, to reconstruct ancient ecosystems? Pterosaurs† 24 3.3 12 3.2 Paleoecological studies based on Moroccan material are further com- Nonavian 178 24.5 57 15.0 plicated because of the widespread commercial fossil trade: many verte- theropods brates described from the Kem Kem Formation were obtained from deal- Sauropods 58 8.0 2 0.5 ers rather than fi eld collected (e.g., Novas et al., 2005). Such collections Ornithischians 14 1.9 5 1.3 Chelonians 158 21.8 194 51.1 have a positive effect on our knowledge of taxonomic richness, as they Birds§ 2 0.3 6 1.6 sample rare taxa in the right tail of the specimens:taxa curve discussed Note: Fossils comprise bones, teeth, and carapace fragments. by Koch (1998), yet biased sampling of rare taxa can produce a distorted *Some fossils counted in the fi eld were not retained; no fossils were picture of ecosystems (Goldwasser and Roughgarden, 1997). We hypoth- purchased as part of this study. esize that this collectorship effect is strongly biasing understanding of the †Fragmentary pterosaur bones and teeth from the Kem Kem Formation are referable to Ornithocheiridae, Pteranodontidae, Tapejaridae, and Azdarchidae relative proportions of taxa in Kem Kem Formation vertebrate faunas. (Barrett et al., 2009). §Fossil birds remain undescribed. An isolated vertebra reported by Riff et al. (2004) from the Kem Kem Formation exhibits no avian synapomorphies. *E-mail: [email protected]. © 2009 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, September September 2009; 2009 v. 37; no. 9; p. 843–846; doi: 10.1130/G30188A.1; 2 fi gures; 1 table. 843 Downloaded from geology.gsapubs.org on 30 September 2009 whether the observed relative proportions of higher taxa in shop and emphasizes the increased proportions of theropod and sauropod dinosaur fi eld data sets are drawn from the same distribution. material in the shop sample, while our fi eld sample contains a higher pro- Rarefaction was used to generate collectorship curves for the two portion of turtles and birds. faunas (“fi eld” versus “shop”) using Analytic Rarefaction 1.4 (Holland, 2003). To test whether the two collections sample from the same pool of Rarefaction and Resampling Results taxa, a resampling program was written by one of us (McGowan; available Rarefaction curves for the shop and fi eld samples have markedly dif- upon request). Resampling with replacement was used, with sampling ferent equations and shapes (Fig. 2A). It is surprising that our fi eld sample probabilities derived from the combined pool of specimens from shop and captures high diversity with fewer specimens. Note that in the central sec- fi eld (Table 1). We generated two distributions, based on 1000 replications tion of these curves, the shop samples provide higher diversity estimates, in each case: one of n = 416 (number of individuals in fi eld sample) and but at higher sample sizes the fi eld collection performs better (Fig. 2A). the other with n = 690 (number of individuals in shop samples). Results Rarefaction results indicate that fi eld data are more consistent at recover- were expressed as proportions of the total sample for comparison with ing the full suite of higher taxa present when samples with more than 100 observed proportions.
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