Introduction

The terrestrial Early Cretaceous (ca. 121-98.9 mya) of North America is represented by several formations that crop out between the Gulf Coast and the Great Basin. These formations record a wide temporal and geographic range of environments and document major faunal changes between the Late Jurassic and the Late Cretaceous. However, despite new discoveries which have improved our sample of taxa from these faunas, little is known about the paleoecology of these ancient communities.

Vertebrate microfossil assemblages provide a promising means of addressing paleoecology by providing large, diverse samples of parautochthonous taxa. Ongoing work has revealed rich vertebrate faunas in the Cedar Mountain Formation (Cifelli 1993) and the Trinity Group (Winkler et al. 1990), but much less is known about the intervening Antlers (Cifelli et al. 1997) and Cloverly Formations. The last major assessment of the Cloverly fauna was conducted by Ostrom (1970). Lacking microvertebrate sites, Ostrom focused entirely on large vertebrates, including the recently discovered Deinonychus (Ostrom 1969).

In this study, newly discovered microvertebrate samples are used to determine how the Cloverly ecosystem was constructed, whether Cloverly environments were uniform or heterogeneous, and how the Cloverly ecosystem compares with those of penecontemporaneous formations. It is hypothesized that the Cloverly records a heterogeneous paleoenvironment similar to those of the other Early Cretaceous formations.

Methods

Potentially rich vertebrate microfossil sites were identified in the Cloverly Formation outcrops of the Bighorn Basin, Wyoming, during the 2004-2005 Smithsonian dinosaur expeditions. Bulk samples were collected from four sites, and wet-sieved to < 1 mm sieve size. Fossil material was removed from this concentrate in the Vertebrate Paleontology Laboratory of the National Museum of Natural History. The vertebrate microfossils were then sorted into taxonomic and anatomical categories. Specimens were identified to the lowest possible taxonomic level (see images below). Diversity and abundance tallies were calculated for each site, and Simpson's Dominance Index and Pielou's J diversity metrics were calculated. General anatomical and phylogenetic information were used to infer diet and habitat for each organism in the sample, allowing a broad examination of the paleoecology of each site. In addition, the Cloverly results were compared to published and museum data for the coeval Antlers, Twin Mountains, Paluxy, and Cedar Mountain Formations.

A.) Teleost vertebra, B.) Albanerpeton salamander jaw, C.) turtle shell, D.) gar scale, E.) Deinonychus tooth, F.) crocodile tooth, G.) ornithopod dinosaur tooth

Results

The Cloverly Formation exhibited relatively high diversity and evenness as evidenced by Simpson's Dominance Index and Pielou's J values, which were consistent across all sites.

USNM 42181 and USNM 42175 show a predominantly terrestrial signal, in which dinosaurs comprise the greatest abundance and diversity. Conversely, sites USNM 42157 and USNM 42168 show an aquatic signal with osteichthyan fishes more abundant and diverse. Dinosaurs and mammals were more diverse than abundant, while crocodylians were more abundant than diverse. Using the inferred ecologies of Cloverly taxa, approximately 50% of the taxa were terrestrial, while 50% were either aquatic or amphibious. Approximately 70% of Cloverly vertebrate taxa were carnivorous. Herbivorous invertebrates were not included in the microsite analysis.

Cloverly diversity appears most similar to that of the Antlers and Cedar Mountain Formations, especially in the dominance of terrestrial taxa as shown in above graphs. The Twin Mountain and Paluxy Formations show a more aquatic signal, but all five formations show generally similar taxonomic composition, with many widely distributed forms.

Discussion

Identification of microvertebrates from the four Cloverly sites greatly increased the known diversity of the Cloverly Formation and allowed preliminary paleoenvironmental and paleoecological reconstructions. The appearance of both terrestrially and aquatically dominated sites confirmed that the Cloverly Formation does record a heterogeneous environment, one with similarities to other Early Cretaceous formations. An abundance of crocodylians, chondrichthyans, and osteichthyans in the fauna conform to expectations about the microvertebrate assemblages, however taphonomic and taxonomic biases might be reflected in the assemblages as evidenced by a lack of herbivores. Similarities between the faunal compositions of the Cloverly Formation and other Early Cretaceous formations suggest that the Cloverly records a representative North American Early Cretaceous fauna.

Acknowledgements

• Fred Grady, Peter Kroehler, Steve Jabo, Jorge Velez-Juarbe, Jennifer Young, Robert Hill, and Alison Massagli provided help with collection, screenwashing, and sorting
  
• Jason Head, Don Brinkman, and Bob Purdy assisted with identifications
  Shannon Peters provided the biodiversity calculators, and Dale Winkler allowed us to study unpublished Trinity Group data.
• Funding for this project was provided by the NMNH Office of the Director and the Smithsonian Walcott Fund.
• Additional support was provided by the RTP organizers, specifically Mary Sangrey.

Cloverly Formation outcrop, Bighorn Basin, Wyoming