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(Late Maastrichtian) Small Theropods and Birds: Teeth from The Sankey, J.T. and S. Baszio, eds. 2008. Vertebrate Microfossil Assemblages: Their Role in Paleoecology and Paleobiogeography. Indiana University Press. Diversity of Latest Cretaceous (Late Maastrichtian) Small Theropods and Birds: Teeth from the Lance and Hell Creek Formations Julia T. Sankey Department of Physics and Geology, California State University, Stanislaus, Turlock, California 95382. [email protected] 1. Abstract Late Maastrichtian (latest Cretaceous) small theropod dinosaur and bird diversity has been difficult to determine because of scarcity or skeletal remains. Numerous teeth of small theropods and birds from the Hell Creek (Montana) and Lance (Wyoming) Formations were analyzed. Relative abundances are: dromaeosaurids (23%), troodontids (14%), cf. Richardoestesia isosceles (35%), cf. Paronychodon (20%), and bird (8%). This Maastrichtian theropod assemblage is different from the late Campanian one from Alberta in three ways: 1) the abundance (55%) of two unusual small theropods, cf. Richardoestesia isosceles and Paronychodon; 2) the abundance of teeth, from all taxa, with a flat side and longitudinal ridges; 3) the lower abundance and diversity of dromaeosaurids. The late Maastrichtian had a lower diversity of small theropods compared to the late Campanian possibly due to competition for 1 prey between dromaeosaurids and juvenile tyrannosaurids and climatic and/or climatic and environmental changes during the latest Cretaceous. Key Words Theropod, dinosaur, Maastrichtian, Late Cretaceous, Teeth, Diversity 2. Introduction 2.1. Previous work The first detailed descriptions of small theropod teeth was done by CURRIE et al. (1990) based on collections from the late Campanian Judith River Group of Alberta. Identification and reference of isolated teeth to known taxa was possible due to the existence of jaws with teeth from some of the small theropods. This work has been used to identify isolated theropod teeth from Late Cretaceous sites ranging from Texas to Alaska (e.g. ROWE et al. 1992; FIORILLO & CURRIE 1994; FIORILLO & GANGLOFF 2000; SANKEY 2001). After CURRIE et al.’s 1990 paper, the numbers of theropod teeth from the Judith River Group increased, especially because of screen washing by Donald Brinkman and crews from the Royal Tyrrell Museum of Paleontology. BASZIO (1997a-b) studied this larger sample from the Judith River Group and also samples from Maastrichtian sites in Alberta. He documented changes in the relative abundance of small theropods under different paleoclimatic and paleoenvironmental conditions from the mid-Campanian through the late Maastricthian. SANKEY et al. (2002) studied an even larger sample (1700+ specimens) of small theropod and bird teeth from the Judith River Group, described and quantified new morphological groups, and documented a higher diversity of small theropods and birds than previously recognized. 2 2.2 Importance of Microsites This research project is an excellent example of why microsite studies are important, especially for particular groups of fossils with sparse fossil records. Prior to this work, late Maastrichtian (latest Cretaceous) small theropod dinosaur and bird diversity had been difficult to determine because of the scarcity of skeletal remains. However, due to the extensive collection of microvertebrate sites, especially through screen-washing, from the Lance Formation of Wyoming and the Hell Creek Formation of Montana by William Clemens and his students from the University of California at Berkeley Museum of Paleontology (UCMP), a large collection of late Maastrichtian small theropod and bird teeth was produced. This paper describes these teeth, both qualitatively and quantitatively, determines the relative abundance of the major groups of taxa, compares this to the late Campanian, and estimates the small theropod and bird diversity in the latest Cretaceous of North America, just prior to the K/T extinctions. 3. Materials and Methods 3.1. Fossil sites Fossils were collected by the University of California at Berkeley Museum of Paleontology (UCMP), and resulted from extensive surface collection and screen washing by William Clemens and his students from microsites in the Lance Formation of Wyoming and the Hell Creek Formation of Montana. Locality information is on file at the UCMP, and is available to qualified researchers. Photographed specimens were coated with vaporized amonium chloride to highlight surface detail. Tooth measurements are illustrated and described in Figure 1. The dimensions 3 measured on each tooth and analyzed are: fore aft basal length (FABL), curvature (Curv), tooth height (Ht), cross-sectional thickness (CST), denticles per millimeter (dent/mm), and denticle width (Wd) and height (Ht) (Table 1). The following abbreviations are used: RTMP (Royal Tyrrell Museum of Paleontology) and UCMP (University of California at Berkeley Museum of Paleontology). (((FIGURE 1 NEAR HERE))) 4. Selected Systematic Paleontology Order Saurischia SEELY 1888 4.1 Family Dromaeosauridae MATTHEW & BROWN 1922 4.1.1 cf. Dromaeosaurus MATTHEW & BROWN 1922 Figures 2.14-2.17 Table 1 Description. Teeth are long and narrow, slightly recurved, oval in cross section, and have denticles on both carinae, with larger posterior denticles. Denticles are short and wide, unworn tips are slightly rounded, and interdenticle spaces are present. Discussion. Teeth resemble Dromaeosaurus albertensis from the Judith River Group (SANKEY et al. 2002) in tooth shape, size, and denticle morphology (i.e. denticles larger on posterior carinae, denticles usually 0.2 mm wide and 0.3 mm high, and unworn tips are slightly rounded to straight). However, because no UCMP teeth possess a twisted anterior carina, which is a characteristic of Dromaeosaurus albertensis teeth, these are referred to cf. Dromaeosaurus. 4 CURRIE et al. (1990) described the variation of teeth along the tooth row of the type specimen of Dromaeosaurus albertensis. They noted that premaxillary teeth are smaller and less recurved than maxillary teeth. UCMP 187037 (Fig. 2.14-2.17) resembles premaxillary teeth from D. albertensis (SANKEY et al. 2002, fig. 4.5-4.7). 4.1.2 cf. ?Dromaeosaurus – Morphotype A SANKEY et al. 2002 Table 1 Description. Teeth are long and wide, straight (i.e. not recurved), lingual side is flat, weakly developed longitudinal ridges on both sides, but especially the convex side, and cross section shape is oval. Anterior side of tooth tip is worn. Denticles present on posterior carina are minute or absent on anterior. Denticles short and wide, square-shaped, with straight to rounded tips. Discussion. Teeth closely resembles those referred to as ?Dromaeosaurus Morphotype A in SANKEY et al. (2002; fig. 4.9-13) in shape and size, and in particular in possessing the distinctive characteristics of one flattened side, longitudinal ridges, anterior tip worn, and in denticle size and shape. CURRIE et al. (1990) considered teeth with one flat side to be growth abnormalities and referred them to known taxa based on their denticle morphology. For example, they referred flat-sided teeth with Dromaeosaurus-like denticles to Paronychodon-like Dromaeosaurus. However, enough differences were documented in flat-sided teeth that SANKEY et al. (2002) referred them to ?Dromaeosaurus Morphotype A. These teeth were noted to have the following distinctive characteristics: one flat tooth surface, longitudinal ridges, distinctive shape (posterior tooth edge is straight), large size, and long fore-aft basal lengths relative to heights. Support for 5 the validity of this grouping came from the following evidence: 1) No teeth of this flat morphology are present in the type specimen of D. albertensis. 2) Flat morphology teeth have a stratigraphic distribution through the late Maastrichtian (this paper and BASZIO 1997b, Plate VI, 88), and D. albertensis is restricted to the mid-Campanian. 3) The pattern of wear on these teeth is different from that of D. albertensis, suggesting that they functioned differently (SANKEY et al. 2002). (((FIGURE 2 NEAR HERE))) 4.1.3 cf. Saurornitholestes SUES 1978 Figures 2.1-2.13 Table 1 Description. Tooth shape varies from strongly recurved (Fig. 2.5) to only slightly recurved (Fig. 2.3). Teeth are flattened (labial-lingually) and are oval in cross section. Denticles are large on the posterior carinae, and are minute or absent on the anterior. Posterior denticles are long and slender, and vary in size along the carina (Fig. 2.12). Denticle tips are clearly pointed in unworn teeth or rounded in outline (Fig. 2.2). Weakly developed longitudinal ridges are present on both sides of many teeth (Fig. 2.3; 2.10), but not all (Fig. 2.12). Discussion. Teeth resemble Saurornitholestes cf. S. langstoni from the Judith River Group in shape, size, and denticle morphology (SANKEY et al. 2002). However, the UCMP teeth are distinct in having longitudinal ridges, which are not present in Saurornitholestes from the Judith River Group. Many of the UCMP teeth are more strongly recurved (e.g. Fig. 2.5) and wider (i.e. FABL) than those from the Judith River Group. Additionally, in many of the UCMP teeth, 6 denticles are wider and less distinctly pointed than in typical Saurornitholestes langstoni teeth from the Judith River Group (SANKEY et al. 2002). Because of these differences, the UCMP teeth do not resemble any currently described Saurornitholestes taxa from either the Campanian or Maastricthian. (((FIGURE 3 NEAR HERE))) (((TABLE 1 NEAR HERE))) 4.2 Troodontidae 4.2.1 Genus Troodon LEIDY 1856 Troodon sp. Figures 3.9-3.12;
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