theropod teeth from the prince creek formation | BioOne

Journal of Vertebrate Paleontology 20(4):675–682, December 2000 q 2000 by the Society of Vertebrate Paleontology

THEROPOD TEETH FROM THE PRINCE CREEK FORMATION (CRETACEOUS) OF NORTHERN ALASKA, WITH SPECULATIONS ON ARCTIC DINOSAUR PALEOECOLOGY ANTHONY R. FIORILLO1 and ROLAND A. GANGLOFF2 Dallas Museum of Natural History, P.O. Box 150349, Dallas, Texas 75315; 2 University of Alaska Museum, University of Alaska, Fairbanks, Alaska 99775 1

ABSTRACT—Theropod teeth are taxonomically diagnostic components of dinosaur assemblages. Seventy teeth have been recovered from six different localities in the Kogosukruk Tongue of the Prince Creek Formation (Upper Cretaceous) of the North Slope of Alaska. This assemblage of teeth shows slightly less diversity compared to well documented assemblages of teeth from the slightly older Judith River Formation of south-central Montana, the Aguja Formation of west Texas, and the Hell Creek Formation of eastern Montana. In addition, in contrast to the Judith River Formation assemblage of teeth in south-central Montana, the teeth assigned to Troodon dominated the Alaskan assemblage. The dominance of Troodon is attributed to adaptation by this theropod to low light conditions while overwintering at a high paleolatitude.

INTRODUCTION Vertebrate paleontologists have long been aware of the taxonomic utility of isolated dinosaur teeth (e.g., Leidy, 1856). However, it has been clearly shown that this taxonomic utility is variable with respect to dinosaur group (Horner, 1990; Coombs, 1990). In their seminal paper, Currie et al. (1990) showed that within dinosaurs, the teeth of small theropods provide the most reliable criteria for generic and even species identification. Subsequent workers have utilized the work by Currie et al. (1990) as a means to document theropod faunas from a variety of localities around the world (Chure, 1994; Fiorillo and Currie, 1994; Zinke and Rauhut, 1994). Isolated theropod teeth are usually found in dinosaur-bearing deposits dominated by other dinosaurian taxa. And like other dinosaurs, these teeth were continuously replaced during the lifetime of these animals. Given that most dinosaur-bearing deposits contain the remains of several types of theropods, and for the Upper Cretaceous many of these types are recurring through several rock units, one can infer that these are sympatric animals. Furthermore, the frequency of these sympatric taxa in various faunas should provide insight into the variation in guild structure of contemporaneous ancient ecosystems. As shown in below, there are at least four morphologically similar theropod taxa within the Late Cretaceous ecosystems of western North America. This structure is of particular interest to ecological studies because nowhere in modern ecosystems do four morphologically similar carnivorous vertebrates co-exist in one ecosystem. Rather, in modern ecosystems three carnivore systems can occur. In these ecosystems, shape divergence (e.g., Van Valkenburgh and Wayne, 1994) or size divergence (Crabtree and Sheldon, 1999) within these carnivore taxa are the evolutionary processes that accommodate these compressed niche spaces. This paper will document the taxonomic occurrences of theropod dinosaur teeth from the Upper Cretaceous Prince Creek Formation of the North Slope of Alaska. In addition, we will discuss the frequency distribution of these taxa as related to theropod tooth assemblages from the Late Cretaceous of Montana and Alberta. And lastly, we will discuss the paleoecological implications of the Alaskan teeth assemblage in terms of theropod guild structure for high-latitude dinosaurs from above the paleo-arctic circle in North America. Clemens (1994) provided a detailed history of exploration for dinosaur remains on the North Slope of Alaska along the

Colville River. The initial discoveries occurred in 1961 by the late Robert L. Liscomb, a geologist for Shell Oil Company. These discoveries largely languished, due in part presumably to Liscomb’s untimely death, and the general lack of interest in fossil terrestrial vertebrates in Mesozoic rocks of this region. Substantive work on the North Slope dinosaurs did not begin until the mid-1980s. The U.S. Geological Survey located the area where Liscomb did his initial work in 1984, and by 1985 this major bonebed was termed the Liscomb bonebed. The University of California Museum of Paleontology and the University of Alaska Museum mounted several joint expeditions in the late 1980s, while the University of Alaska Museum shouldered the responsibility for work in this region throughout the 1990s. As a result of work along the lower Colville River since the mid-1980s, a series of prolific University of Alaska Museum fossil sites and quarries have now been established (Fig. 1): Liscomb bone bed, Byers Bed, Sling Point site, Old Bone Beach, Kikak-Tegoseak bone bed, and the Niniluk site. The first five localities provided all but one specimen discussed in this report. Of these five localities, the Liscomb is the most prolific in terms of specimens and has been systematically quarried and mapped since 1988. The specimens from all five localities are the basis for the paleoecological interpretations presented here. The Niniluk site is in the underlying Nanushuk Group (see below), rather than the Prince Creek Formation. This site is mentioned in this report because of the stratigraphic significance of the locality. The majority of these localities are in the Kogosukruk Tongue (Campanian to Maastrichtian in age) of the Prince Creek Formation of the Colville Group (Fig. 2; Gangloff, 1994, 1998). The Prince Creek Formation is the non-marine component of the Colville Group and is Coniacian to Maastrichtian in age (Fig. 2: Gangloff, 1998). Unconformably overlying the Prince Creek Formation in most of the area studied is the Pliocene to Holocene Gubik Formation. Underlying the Prince Creek Formation is the Lower Cretaceous Nanushuk Group, which is exposed up river (Mull, 1985). Paleomagnetic studies of the Colville Group suggest this part of Alaska was at 858 to 678 N during the Late Cretaceous (Witte et al., 1987; Besse and Courtillot, 1991). The Colville Group consists of a wedge of sedimentary marine and non-marine rocks that are in excess of 1.5 km thick (Sable and Stricker, 1987). Dinosaur-bearing beds which have been found in organic-rich siltstones have been interpreted as overbank or crevasse splay deposits (Phillips, 1990) while those

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FIGURE 1. Stratigraphic distribution of dinosaurs and succession of rock units for the western half of northern Alaska. Compiled from Huffman (1985), Mull (1985), and Lindberg (1987). Shaded and stippled units are dominantly marine rocks.

found in conglomeratic sandstones have been interpreted as channel lag deposits (Gangloff, 1998). Based on the abundance of organic rich layers, megafloral remains, and wood, and paleogeographic reconstructions that place the rising Brooks Range 100s of kilometers away from the area, we postulate that the Prince Creek Formation was formed in a well-vegetated,

coastal, lowland environment. The age of the Prince Creek Formation in the vicinity of the Liscomb bone bed is Maastrichtian in age based on pollen (Wodehouseia spinata Zone, Fredriksen, 1990). Conventional K-Ar and Ar/Ar dates from glass shards from tephra beds through the local section yield dates that range from 68 to 71 MA, with the weighted mean of 69 Ma (Conrad et al., 1990). A recent reanalysis using 40Ar/39Ar and single sanidine crystals (Obradovich, pers. comm., 2000) would place the Liscomb bone bed very close to the Campanian-Maastrichtian boundary of Obradovich (1993). The fauna recovered from quarry excavations and accumulated beach float include specimens of chondrichthyian and osteichthyan fishes, large and small theropods, a pachycephalosaur, ceratopsian and hadrosaurian dinosaurs, as well as multituberculate, marsupial, and placental mammals (Nelms, 1989; Rich et al., 1997; Fiorillo et al., 1999). Given the isolated and unassociated nature of the theropod teeth, we assume in this study that the vast majority of the teeth are randomly distributed and belong to separate individuals. An exception is a cluster of four Troodon formosus teeth recovered from one quarry (AK383V) in the Liscomb bone bed that suggest these four teeth were from a single individual. SYSTEMATIC PALEONTOLOGY

FIGURE 2. Map of major geographic features of northern Alaska with principal dinosaur collecting sites indicated. Modified from Huffman (1985).

Descriptions of the teeth of Dromaeosaurus albertensis, Saurornitholestes langstoni, Troodon formosus and the tyrannosaurids are brief because detailed descriptions have been provided elsewhere (Currie, 1987; Currie et al., 1990). The distribution frequency of these taxa is provided in Table 1. All num-

FIORILLO AND GANGLOFF—CRETACEOUS THEROPOD TEETH, NORTHERN ALASKA TABLE 1. Summary of occurrence of Alaskan theropod tooth taxa for all sites discussed in this report (N 5 60). Taxon D. alber- S. langs- T. formo- Tyrannotensis toni sus saurid

Locality Liscomb Quarry Kikak-Tegoseak Quarry Old Bone Beach Sling Point Quarry Byers Bed Ninuluk Bluff

8 2 0 0 3 1

0 0 2 0 0 0

42 0 0 2 1 0

5 2 0 0 2 0

bers refer to specimens housed at the University of Alaska Museum in Fairbanks. Order SAURISCHIA Suborder THEROPODA Family DROMAEOSAURIDAE Subfamily DROMAEOSAURINAE DROMAEOSAURUS ALBERTENSIS Matthew and Brown, 1922 Referred Specimens—AK-497-V-001FT, AK232-V-073, AK381-V-052, AK306-V-035, AK335-V-013, AK292-V-03, AK238-V-009, AK390-V-041, AK383-V-180, AK385-V-002, AK490-V-170, UAM-AK83.V-090, AK456-V-024, AK456-V025, AK211-V-001 Discussion—Thirteen teeth are attributed to Dromaeosaurus albertensis based on the morphology of the denticles on the carinae, and the characteristic lingual twist of the anterior carina (Currie et al., 1990). As Currie et al. (1990) could not distinguish between maxillary and dentary teeth in Dromaeosaurus albertensis, we make no further distinction within this set of teeth. One tooth in this group deserves special mention because of the split near the base of the anterior carina, UAM-AK83.V90. Erickson (1995) examined hundreds of teeth assigned to the tyrannosauridae and documented the widespread occurrence of such split carinae. To account for these splits, Erickson (1995) proposed three hypotheses, trauma, aberrant tooth replacement, or genetic factors. Of these three hypotheses, he favored the last. The discovery of a split carina on the tooth assigned to an entirely separate taxon (Dromaeosaurus albertensis) suggests that rather than genetic factors, one of the other hypotheses should be reconsidered. In addition to the split anterior carina, there is a small line of 15 denticles near and separate from the posterior carina, but diverging towards the base of the tooth at an approximate 258 angle. This line of denticles is just above the base of the tooth. Because of this additional abnormality we suggest that all these features are the result of aberrant tooth replacement as defined by Erickson (1995) or aberrant tooth development. A single, poorly preserved, additional tooth is attributed to Dromaeosaurus albertensis Matthew and Brown, 1922 (AK211-V-001). This specimen represents the only dinosaur body fossil recovered form the thick 3,000 meter Nanushuk Group. The tooth was questionably assigned to the tyrannosaurid genus Alectrosaurus (Gangloff, 1998). However, the lack of blood grooves between the denticles and the presence of strong lateral compression better support an assignment to Dromaeosaurus albertensis. Subfamily VELOCIRAPTORINAE Saurornitholestes langstoni Sues, 1978 Referred Specimens—AK289-V-034, AK289-V-035 Discussion—The least commonly recovered theropod teeth

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are the easily recognizable teeth attributed to Saurornitholestes langstoni. These teeth are laterally compressed, and have distinctive individual denticles that are diagnostic for the taxon. Both teeth exhibit much smaller denticles on the anterior carinae than those on the posterior carinae. Currie et al. (1990) point out that for this taxon, the presence or absence of denticles on the anterior carinae are variable even within the same jaw. Family TROODONTIDAE TROODON FORMOSUS Leidy, 1856 Referred Specimens—AK491-V-143, AK491-V-168, AK490-V-086FT, AK490-V-008FL, AK490-V-004, AK299-V134, AK385-V-001, AK385-V-002, AK387-V-000FT, AK387V-017, AK300-V-017, AK300-V-021, AK300-V-042, AK300V-055, AK300-V-129, AK300-V-060, AK383-V-018, AK383V-183, AK383-V-137, AK383-V-140, AK383-V-176, AK382V-015, AK382-V-105, AK233-V-054, AK392-V-007, AK284-V-024, AK285-V-008, AK285-V-013, AK285-V-037a, AK335-V-012FT, AK282-V-001, AK282-V-010, AK282-V052, AK282-V-056, AK283-V-017, AK283-V-115, AK-497-V002, AK-498-V-001, AK-498-V-002, AK-498-V-003, AK83-V095, AK335-V-076, AK459-V-011, AK388-V-002, AK388-V082 Discussion—Teeth attributable to Troodon formosus are easily the most commonly found theropod material recovered in this assemblage. Of these teeth, based on data provided by Currie (1987), only one tooth is recognized as a premaxillary tooth (AK285-V-008), while the remaining teeth are either from the dentaries or the maxillae. Of the teeth that show wear facets (N 5 27), just over three-quarters of them show wear on only the lingual side of the tooth (N 5 21). A few teeth show wear at the apex only (N 5 3) or at the apex and lingual side (N 5 3). Family TYRANNOSAURIDAE Referred Specimens—AK461-V-001, AK383-V-172, AK383-V-175, AK491-V-089, AK300-V-086, AK298-V-031, AK390-V-034, AK390-V-041, AK455-V-001 Discussion—Members of the large theropod group, the tyrannosauridae are represented by nine teeth. Two of these teeth have a basal D-shaped cross-section and lack of anterior and posterior carinae that is characteristic of teeth from the premaxilla. Though the denticles are of similar shape to those of a Dromaeosaurus albertensis tooth, the remaining seven teeth were distinguished from this dromaeosaur taxon by the diagnostic blood groove between denticles that extends obliquely towards the base of the tooth (Currie et al., 1990; Abler, 1992). PALEOECOLOGICAL HYPOTHESES AND SPECULATIONS It has been suggested elsewhere (Paul, 1988; Fiorillo and Gangloff, 1999) that the dinosaurs found in the Prince Creek Formation were likely inhabitants of this paleoarctic region year round rather than seasonal migrants. Therefore, we examine the paleoecological implications of the theropod fauna from the perspective of nonmigrating dinosaurs. There is great similarity in the tooth-based theropod faunas throughout the Campanian and Maastrichtian of North America (Currie et al., 1990; Rowe et al., 1992; Fiorillo and Currie, 1994; Fiorillo, 1998 ) demonstrating the temporal longevity of these taxa at the generic and specific level. Quantitative data are uncommon for tooth-based theropod faunas. If we accept Obradovich’s (1993) time scale mentioned earlier, the quarries that yielded these specimens are at or very near the CampanianMaastrichtian boundary. Therefore, two data sets available for

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TABLE 2. Comparison of theropod taxa from the Prince Creek Formation of Alaska, the Judith River Formation of Montana, and the Aguja Formation of Texas. The data for the Montana and Texas faunas are from Fiorillo and Currie (1994) and Rowe et al. (1992), respectively. No teeth attributed to tyrannosaurids were reported by Fiorillo and Currie (1994) for the theropod fauna from the Judith River Formation of south-central Montana, rather the large theropod was referred to as theropod ‘‘A.’’

Dromaeosaurus albertensis Saurornitholestes langstoni Troodon formosus Richardoestesia gilmorei Tyrannosauridae/theropod ‘‘A’’

Alaska

Montana

Texas

X X X

X X X X X

X X X X X

X

comparison are from the Maastrichtian Horseshoe Canyon Formation of southern Alberta (Ryan et al., in press) and from the Campanian Judith River Formation of south-central Montana (Fiorillo and Currie, 1994). We include the latter data set for two reasons. First the Montana sample contains the same taxa as the Alaska sample, thus it is valid to compare our Maastrichtian fauna from the North Slope to a late Campanian fauna. And secondly, the taxa in the Campanian sample from Montana are represented in similar relative abundances to the southern Alberta sample. The Montana sample then, adds robustness to our frequency comparisons. Table 2 shows the occurrence of recently documented tooth taxa for the Prince Creek Formation of northern Alaska, the Judith River Formation of south-central Montana (Fiorillo and Currie, 1994), the Horseshoe Canyon Formation of southern Alberta (Ryan et al., 1998), and includes a fourth fauna from the Aguja Formation of west Texas (Rowe et al., 1992). Frequency data for the Aguja fauna are unavailable. These last three faunas were chosen because they are detailed accounts of theropod faunas that follow the landmark work initiated by Currie et al. (1990). Also, the wide geographic distribution of the faunas provides baseline data for a latitudinal gradient along most of western North America during this time frame. Table 2 shows great similarity between all four faunas with the decided exception that Richardoestsia gilmorei, a small theropod, is missing from northern Alaska. It is tempting to speculate from these data that additional work will yield the teeth of Richardoestsia gilmorei. However, in the modern world, taxonomic diversity decreases in higher latitudes (e.g., Bee and Hall, 1956; Pisanty-Baruch et al., 1999). This diversity change is likely linked to the temperature gradient between low and high latitudes. Though the climate of the Cretaceous is often referred to as more equable than that of the modern world, climate modeling for the Late Cretaceous still shows the presence of a pronounced temperature gradient from low to high latitudes (DeConto et al., 1999). Therefore, the reduced taxonomic diversity within the Alaska sample may be evidence of reduced high latitude diversity in the Late Cretaceous of North America, the same biogeographic phenomenon as observed in the modern world. Given the absence of one small theropod from the North Slope fauna (Richardoestsia gilmorei) compared with faunas to the south, we ask the question, are there additional differences in the structure of this northern fauna compared with those to the south? In other words, are there significant differences in relative abundance of taxa from the North Slope compared with faunas from lower latitudes? Fiorillo and Currie (1994) and Ryan et al. (1998) provide data for the relative frequencies of theropod taxa from the Judith River Formation of south-central Montana and southern Alberta, respectively. Comparison of these data with the abundances of the taxa from the North Slope shows a striking con-

TABLE 3. Relative abundance data for the North Slope theropod tooth fauna (N 5 60) compared to a similar fauna from south-central Montana (N 5 129; Fiorillo and Currie, 1994). Alaska

Dromaeosaurus albertensis Saurornitholestes langstoni Troodon formosus Richardoestesia gilmorei Tyrannosauridae/theropod ‘‘A’’

Specimen total 13 2 45 0 9

Montana

%

Specimen total

%

19 3 65 0 13

9 72 8 12 28

7 56 6 9 22

trast in the dominant taxa (Table 3). In contrast to the Montana and Alberta faunas that are dominated by Saurornitholestes langstoni, the North Slope fauna is dominated by Troodon formosus. In addition to the dominance of this taxon, there is also an increase in the relative abundance of Dromaeosaurus albertensis teeth in the Alaskan fauna. If this sample is representative, we can infer from these data that this pattern is ecologically significant. In the Alaskan fauna the preserved potential prey items include hadrosaurs, ceratopsians and pachycephalosaurs (Fiorillo, 1989, 1991; Nelms, 1989; Gangloff, 1994, 1998; Fiorillo et al., 1999), taxa also preserved in the Montanan and Albertan dinosaur faunas. Therefore, functionally these theropods were using similar food resources in both areas. To quantify the potential use of these food items, we made simple maximum length to maximum height ratios for the lower jaws of these theropods as an estimator of bite force (Table 4). We assume that length and height are the most important dimensions with respect to jaw loading and that similarities in ratios relate to similarity in feeding behavior. These measurements were obtained from published illustrations or unpublished photographs of these taxa. Basic biomechanical modeling shows that a more gracile jaw would sustain lower bite forces than a more robust one (e.g., Hildebrand, 1988). Because we are interested only in relative bite force, we have not included units of length for this column in our table. Even though tyrannosaurids are several times the size of these smaller theropods, we include a tyrannosaurid in the table to examine the potential competition within small theropod groups from juvenile tyrannosaurids. Both Carr (1999) and Carpenter (1992) studied the ontogenetic changes within tyrannosaurids. They showed that the muzzle shortens with ontogeny, and that there is a strong similarity in skull dimensions between small adult theropods and juvenile tyrannosaurids. Examination of Table 4 suggests that tyrannosaurs had a relatively stronger bite force than did any of the other theropods. Table 4 also suggests that basic jaw shape and presumably bite loads in the remaining small theropods were comparable. Therefore, we conclude that these small theropods were in direct competition for food resources and likely developed some means to partition food resources (see below). TABLE 4. Estimated relative bite force for the theropod taxa found in the Kogosukruk Tongue of the Prince Creek Formation of the North Slope of Alaska. Measurements of length and width were obtained from published illustrations or photographs of the lower jaws of these taxa.

Dromaeosaurus albertensis Saurornitholestes langstoni Troodon formosus Tyrannosauridae

l : w ratio

Approximate adult body length (m)

.19 .20 .19 .24

1.8 1.8 1.8 8.0–11.8

FIORILLO AND GANGLOFF—CRETACEOUS THEROPOD TEETH, NORTHERN ALASKA TABLE 5. Summary list of factors affecting canid sympatry in modern ecosystems that might also have influenced theropod sympatry in Cretaceous ecosystems. Size Dominance Interference Competition Character Displacement Home-Range Interspersion Scavenging Spatial and Temporal Partitioning

With respect to the increased relative abundance of Dromaeosaurus albertensis in the Alaska sample, Currie (1995) has suggested that this taxon was somewhat unspecialized for Late Cretaceous theropods. We suggest that this generalist body plan was better adapted for the high latitude climate of its time. With respect to the difference in abundance between Saurorntholestes langstoni and Troodon formosus, some other factor other than food type must account for this difference. The data from this study, combined with the faunal data elsewhere in western North America highlight an interesting ecological phenomenon, sympatry of several similarly shaped and sized carnivorous animals in one terrestrial ecosystem. For the fauna of the North Slope, this assemblage consists of three taxa, with part-time coexistence of a fourth taxon. More specifically, Dromaeosaurus albertensis, Troodon formosus, and Saurornitholestes langstoni all achieve virtually identical adult body lengths (Table 4). Additionally, the tyrannosaurids had to pass through this same size category during growth. To the south (Alberta and Montana), this situation is further complicated by the inclusion of Richardoestesia gilmorei. Factors affecting sympatry in mammalian carnivores have been examined by others (e.g., Johnson et al., 1996; Crabtree and Sheldon, 1999). With respect to morphologically similar carnivores sharing the same ecosystem (i.e., canids), Crabtree and Sheldon highlighted several factors that can contribute to this sympatry. Table 5 is a summary list of the most prominent of those factors. Given the similarity in jaw shape and approximate overall body length (Table 4), we submit that these small theropods are morphologically similar with respect to food utilization and that the factors in Table 5 are likely relevant to theropod ecology as well. By this assumption, differences in the denticles of these theropods are considered minor to the larger feeding plan strategy. Of the factors listed in Table 5, the similar body sizes of the small theropods from these fossil assemblages preclude size dominance as the factor contributing to the differences in abundance of S. langstoni and T. formosus between assemblages. Interference competition, that is one species driving another species away which may or may not result in one species killing another, is untestable given our current data set and therefore is given no further consideration here. Similarly, home-range interspersion and character displacement are also considered untestable given our current data. The issue of scavenging is difficult to address, and all microwear patterns on the teeth of these small theropods from south-central Montana show similar patterns and presumably, similar food use (Fiorillo, 1997). Therefore, we can examine only the issue of spatial and temporal partitioning in more detail. Russell and Seguin (1982) examined the cranial capacity of the small theropod formerly known as Stenonychosaurus inequalis, but now referred to as Troodon formosus (Currie, 1987). In their study, they demonstrate the increased relative brain size of this taxon, but more important to this discussion, they also highlight the relative increased eye orbit diameter of T. formosus compared to other small theropods. From this observation, they suggest that Troodon formosus was a crepuscular or nocturnal predator, that is a predator that was adapted to low levels of sunlight. With respect to the theropod fauna from Montana and Al-

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berta, the dominance of Saurornitholestes langstoni suggests a competitive advantage for this small theropod. The co-existence of Troodon formosus, with its presumed crepuscular or nocturnal preferences would be explained by a temporal partitioning of food resources. We submit that the dominance of Troodon formosus in the Alaskan fauna is likely an example of spatial partioning. Smiley (1969) and Parrish and Spicer (1988) have shown that a highlatitude flora did exist for the North Slope of Alaska during the Late Cretaceous, a flora dominated by the deciduous conifer Parataxodium wigginsii. Further, Spicer (1987) shows that at the paleolatitude for this region, light is limited ten months of the year. Therefore, for the majority of the year, adaptations for low-light conditions would be advantageous. But what of the two months of moderate to high-light conditions where an animal adapted for low-light conditions would presumably at a disadvantage due to more intense light? Otto-Bliesner and Upchurch (1997) and Upchurch et al. (1999), in their paleoclimate modeling, have suggested that increased high-latitude temperatures for the Cretaceous can be explained by increased forest cover in those regions. If correct, given that the dominant floral component is deciduous, then we suggest that this leaf canopy was sufficient to keep light levels down such that Troodon formosus maintained a competitive advantage, even during summer months, in this coastal lowland environment. Alternatively, Ryan et al. (1998) discuss a similar occurrence of Troodon sp. remains from a site in the Horseshoe Canyon Formation of south-central Alberta. From their data, they suggest that a specialized relationship existed between Troodon sp. and Edmontosaurus sp., where Troodon sp. was specialized to prey on the young of the latter. While we do not doubt such a predator-prey relationship could have occurred at some time in the Cretaceous, we suspect this to have been exceptional rather than the normal behavior for Troodon. The relative abundance of Troodon formosus in the Liscomb bone bed, which is dominated by young juveniles of Edmontosaurus, may suggest that this theropod was adapted to a similar predation strategy at higher latitudes. However, in no mammalian carnivore, where body sizes and jaw dimensions can show great feeding specializations (e.g., Van Valkenburgh, 1991, 1996), does such a specialized relationship between predator and prey exist (e.g., Murie, 1940, 1944; Craighead et al., 1995; Johnson and Crabtree, 1999). Given the similarity of body size and jaw dimensions of the North Slope theropods (Table 4), which we infer illustrate a less specialized feeding strategy for any given small theropod, we prefer a model to explain the data presented here that invokes faunal adaptation to a high latitude environment. The model we prefer is similar to that proposed by Rich and Rich (1989) and Vickers-Rich et al. (1999) which emphasizes the possession of large orbits for low-light conditions. Our model differs from theirs in an important concept. In their model, an adaptation (large orbits) within a uniquely occurring taxon (Leaellynasaura amicagraphica) is attributed to the low light conditions of the high latitude of the Lower Cretaceous of southern Australia. Unfortunately the two skull fragments reported for this taxon are from immature animals (Rich and Rich, 1989). Proportionately large eyes in juvenile animals are an expected condition. We, on the other hand, are describing a faunal adaptation or response. This adaptation is expressed as an increase in the relative abundance of a cosmopolitan taxon (Troodon formosus) typically uncommon in lower latitudes and this increase in relative abundance is in response to the constraints of the physical environment. High latitude vertebrate faunas today show tremendous variation in specialized adaptations for existing in these extreme environments. If we are correct in our conclu-

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TABLE 6. Ratio of total skeletal elements collected to total theropod teeth. These data are used as a means to predict which of the remaining quarries is most likely to provide data to support or refute the model proposed in this report.

Liscomb bone bed Kikak-Tegoseak bone bed Norton Bed Sling Point site Byers Bed Ninuluk

the high latitudes of its time. This adaptation provided a competitive advantage over otherwise morphologically similar contemporaries.

Tnt/Tne/nc

Range

ACKNOWLEDGMENTS

1:3 1:7 1:20 1:29 1:11 NA

1:18–1:192 1 02:1:29 NA NA 1 04:1:133 NA

The authors thank Dr. David Varricchio for comments during the writing of this manuscript, as well as providing photographs of the lower jaw of Saurornitholestes langstoni. This manuscript was greatly improved by the comments provided by Drs. Blaire Van Valkenburgh, Louis Jacobs, Kenneth Carpenter, and an anonymous reviewer on earlier versions of this work. We also thank Michael J. Ryan for access to his manuscript. The National Science Foundation and the Jurassic Foundation provided financial support for fieldwork on the North Slope. We gratefully acknowledge the support of the Dallas Museum of Natural History, American Airlines, Arco Alaska, Inc., Alyeska Pipeline Service Company, Inc., and the Arctic Management Unit of the Bureau of Land Management for additional support in the field. Support for the work on the Judith River Formation of south-central Montana was provided largely by National Science Foundation grants EAR 84-08446 and EAR 87-21432.

Tnt 5 Total number of teeth collected. Tne 5 Total number of skeletal elements collected. nc 5 number of collecting events.

sions, then our study provides some insight into the evolutionary constraints of the individual theropod taxa, in contrast to the apparent plastic nature of a faunal response to the physical environment. Falsifying our model can be done by detailed excavation of the additional dinosaur bone beds along the Colville River to examine the robustness of the frequency data presented here. Further, an analysis of the total number of theropod teeth (Tnt) compared to the total number of skeletal elements (Tne) collected at each quarry or collected randomly in situ or along beaches below known fossiliferous beds was made (Table 6). In addition, this ratio was divided by the number of separate collecting or sampling events (nc). The results are summarized above. Further discussion is deemed necessary to allow a fuller understanding of the collecting methods and their possible impact on the validity of the statistics. The Liscomb bone bed locality is, by far, the most extensively and systematically collected source of bones and teeth in Alaska. The data set used in this analysis was compiled from 1985 to 1999. A total of 26 collecting events were recorded, 17 of these (65%) are part of a series of 1 meter by 1 meter by 0.3 to 1 meter quarries. Each quarry was mapped using Cartesian x, y, z coordinates to a precision of 1 cm. Although the tooth-to-skeletal elements ratio ranges widely (1:18 to 1:192), ratios of 1:48 to 1:79 typical represent approximately 47% of the cases. Collections from the Liscomb bone bed exhibit the greatest range of ratios and are commonly higher than those collected as float at the base of a bone bed, such as at Sling Point and Kikak-Tegoseak. It should be noted that the collections of float were made without the use of screening techniques. The Byer’s Bed collections represent a highly selective process that resulted in low ratios due to collector bias. Liscomb bone bed quarrying represents an attempt to collect and invoice all skeletal elements encountered. It appears that when all factors affecting the sampling ratios under discussion are taken into account, the Kikak-Tegoseak bone bed shows great promise for the recovery of theropod teeth during future quarrying efforts and is an excellent site to further test the model proposed in this study. If, for example, Kikak-Tegoseak yields a large sample of theropod teeth with the high frequency of Troodon formosus reported here then the faunal adaptation model would be supported. If however this site yields a frequency pattern more like that observed in Alberta and Montana, then the model proposed by Ryan et al. (in press) should be favored. In summary, the theropod fauna recovered from the Prince Creek Formation of the North Slope of Alaska contains many of the taxa found further to the south. However, the taxonomic abundances are dramatically different. We suggest that the reason why the North Slope fauna was dominated by a small theropod with exceptionally large eyes (Troodon formosus) was because it was a taxon adapted for the low-light conditions of

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