UNIVERSITY OF CINCINNATI
Date:______
I, ______, hereby submit this work as part of the requirements for the degree of: in:
It is entitled:
This work and its defense approved by:
Chair: ______
Taxonomic Diversity, Faunal Analysis and Paleoecology of a microvertebrate site
in the Late Cretaceous Meeteetse Formation, Northern Wyoming
A thesis submitted to the
Division of Research and Advanced Studies
of the University of Cincinnati
in partial fulfillment of the
Requirements of the degree of
MASTER OF SCIENCE
In the Department of Geology
Of the College of Arts and Sciences
2005
by
Ji-Yeon Shin
B.S., Ewha Women’s University, 2002
Committee Chair: Dr. Glenn W. Storrs
ABSTRACT
A microvertebrate study of a fossil locality in the Late Cretaceous Meeteetse
Formation, northern Wyoming, has resulted in the discovery of more than one thousand specimens and at least fifteen vertebrate taxa.
The Meeteetse vertebrate fauna is highly aquatic. Dominance of fresh water fish, such as Myledaphus and Lepisosteus, soft-shelled turtles and crocodiles is indicative. A small theropod tooth belongs to Saurornitholestes. Hadrosaurs and ceratopsians are present. A rare multituberculate molar and an incisor were also recovered and referred to Mesodma.
Analysis of sedimentology and ecology of the vertebrates and invertebrates indicate that the Elk Basin site in the Meeteetse Formation was deposited in a broad flood plain associated with ponds and small lakes in humid climate conditions in the early Maastrichtian.
ii ACKNOWLEDGMENTS
This study was carried out under the guidance of my committee chair, Dr.
Glenn W. Storrs, to whom I am especially grateful for suggesting this study, valuable advice, deep interest and support of the project. I would like to thank my other committee members, Dr. Carlton E. Brett and Dr. David L. Meyer for contributing many critical suggestions and valuable discussion. Dr. Carlton E. Brett especially gave constant encouragement, for which I am very grateful.
I would like to thank Dr. Warren Huff for giving valuable comments on dating and generously helping me with photographing small specimens using his facility. I am also grateful to Dr. David L. Meyer for helping with initial photography. Mason Jane
Milam gave help and advice on photography at Cincinnati Museum Center.
I am especially grateful for Bill Garcia and Dale Gnidovec who spent so many hours to help collect samples and screen in the hot weather. Their help was indispensable. I am also thankful to volunteers at Cincinnati Museum Center who helped me with detailed screening.
I would like to thank Dr. Donald B. Brinkman for providing a detailed illustrated guide to identify microvertebrate specimens, for which I am most grateful.
iii I must thank Ana Londono for helping with screening and sorting at Cincinnati
Museum Center and for constant support in the office. She has been a source of knowledge and advice in geology, and provided constant friendship throughout this study.
Finally, special thanks to my family at home for their endless support, for cheering me up in the low times and encouraging me to pursue my dreams.
Financial support for this research was provided by Cincinnati Museum
Center and the University of Cincinnati Department of Geology.
iv TABLE OF CONTENTS
ABSTRACT…………………………………………………...…………………………….ii
ACKNOWLEDGMENTS ...…………...………...……….…...…...……………….……….. iii
TABLE OF CONTENTS …………………………………………………………………….v
LIST OF FIGURES ...………………………………………………………...………………vii
LIST OF TABLES ...... ix
CHAPTER 1
INTRODUCTION ……………………………………………………………………...……. 1
BACKGROUND …………………………………………………………………….…...…... 7
General stratigraphy and sedimentology…………………………………...……. 7
Depositional history during the Late Cretaceous in Wyoming...…………...…. 13
Stratigraphy and sedimentology of the field site in Elk Basin ...... 15
METHODS ………………………………………………………………………………….. 22
CHAPTER 2
FAUNAL LIST OF VERTEBRATES...... 25
SYSTEMATIC PALEONTOLOGY …………………………………………………...…..... 28
INVERTEBRATE FAUNA ...... 56
ANALYSIS OF MEETEETSE VERTEBRATE FAUNA ……………………………….... 57
PALEOECOLOGICAL ANALYSIS ……………………………………………………...... 60
Paleoenvironment of the Meeteetse Formation based on sedimentology ...... 61
Paleoecology of the Meeteetse Formation based on vertebrates ………….…..61
v CONCLUSIONS …………………………………………………………………….……... 65
REFERENCES …………………………………………………………………….………. 67
vi LIST OF FIGURES
Figure No. Page No.
1. Map of Wyoming showing location of the field site at Elk Basin and
adjacent basins 2
2. Geographic location of field site of the Meeteetse Formation at
Elk Basin, Wyoming 3
3. Overview of field site of the Meeteetse Formation at Elk Basin, Wyoming 4
4. Latest Cretaceous (Maastrichtian) paleogeographic map of western North
America 8
5. Stratigraphic position of exposures in Big Horn Basin and adjacent
regions in Wyoming 10
6. General stratigraphy of the Elk Basin section 11
7. Overview of the field site at Elk Bain 16
8. Closer view of the field site 17
9. Stratigraphic position of the Meeteetse Formation at field site at Elk Basin 18
10. A thirty cm deep trench at field site for describing micro-stratigraphic
section 19
11. Columnar section of the field site of the Meeteetse Formation at Elk Basin 21
12. Buckets used to collect sediments from field 23
13. Wood screens before matrix distribution 23
14. Screens in water before processing 24
15. Wet screening device at Cincinnati Museum Center 24
vii 16. Lonchidion selachos 29
17. Myledaphus bipartitus 31
18. A dentition of extant Raja sp. 32
19. Acipenser sp. 33
20. Kindleia fragosa 36
21. Lepisosteus sp. 38
22. Teleost 40
23. Plesiobaena sp. 43
24. Basilemys sp. 44
25. Adocus sp. 44
26. Aspideretes sp. 45
27. Borealosuchus sp. 47
28. Saurornitholestes langstoni 49
29. Hadrosaur 50
30. Ceratopsian 52
31. Ornithischian ossified tendons 53
32. Mesodma sp. 54
33. Relative abundance of vertebrates based on groups 58
viii LIST OF TABLES
TABLE NO. PAGE NO.
1. Micro-stratigraphic description of the Meeteetse Formation at field site 20
2. Relative abundance of Meeteetse Fauna 57
3. Relative abundance of vertebrates based on groups 58
4. Environmental preferences of selachian taxa in the Meeteetse Formation 62
ix Introduction
Vertebrate microfossil studies provide an important source of paleoecological information. Microvertebrate localities are those where small skeletal remains, such as small bones and teeth of various vertebrates, have become concentrated. Materials can be obtained through surface collecting and wet screening. Earlier microvertebrate studies of the Late Cretaceous showed that paleoenvironmental interpretations can be made based on faunal and sedimentological analyses. These studies have provided an understanding of the changes in paleocommunity structure during the Late Cretaceous
(Estes, 1964; Estes and Berberian, 1970; Sahni, 1972; Dodson, 1983; Lehman, 1987;
Brinkman, 1990). In this study, non-marine vertebrate microfossils from the Late
Cretaceous Meeteetse Formation were analyzed and paleoecology was assessed based on taxonomic diversity and sedimentology.
The Meeteetse Formation was first described by Hewett (1914) from exposures near the town of Meeteetse in the southwestern part of the Big Horn Basin (Fig. 1) in south-central Montana and northern Wyoming. The Meeteetse Formation is a predominantly terrestrial unit, ranging from Campanian to early Maastrichtian in age.
The field site is located near an oil well pad at Elk Basin, northeast of Powell, Wyoming
(Fig. 2 and Fig. 3). Exact field site details are on file at Cincinnati Museum Center
(CMC).
1
Figure 1: Map of Wyoming showing location of the field site of the Meeteetse Formation at Elk Basin and adjacent basins. Modified from Hicks (1993)
2
Figure 2: Geographic location of field site of the Meeteetse Formation at Elk Basin, Wyoming
3
Figure 3: Overview of field site of the Meeteetse Formation at Elk Basin, Wyoming, scale in meter.
4 The Meeteetse Formation has yielded relatively few vertebrate fossils until now.
The discovery of a new microvertebrate site in Elk Basin, Wyoming has given the
Meeteetse Formation new potential. Preliminary finds include isolated dinosaur teeth and bone fragments (Ornithischia and Theropoda), portions of turtle shells, gar scales
(Lepisosteus) and freshwater skate teeth (Myledaphus) (Hicks, 1993). In addition, the
Elk Basin locality appears highly fossiliferous, and holds the potential for further discovery of rare squamates and mammalian taxa, as indicated by the discovery of multituberculate teeth during this study.
This project examined the taxonomic diversity and paleoecology of the Elk
Basin locality. A detailed collection of microvertebrates from the new locality recovered over one thousand of specimens. A complete faunal analysis of bulk samples, including species identification and relative abundances, was conducted. The paleoenvironmental condition of the site was assessed on the basis of the fauna, taphonomy of the samples, sedimentology, and regional stratigraphy.
Estes, (1964) classic work on the Lance Formation (late Maastrichtian) resulted in the recovery of more than thirty thousand microvertebrate specimens, representing over seventy-five different species, from the field site. Estes’ ecological analyses provided important paleoenvironmental information on the Lance Formation. The fauna of the Meeteetse Formation was compared to those of adjacent assemblages, which
5 include the underlying Judith River Formation (late Campanian) and the overlying
Lance Formation (late Maastrichtian). Faunas from these three different formations, the
Judith River, Meeteetse and Lance Formation, showed strong similarities (Estes, 1964;
Sahni, 1972; Dodson, 1983; Lehman, 1987; Brinkman, 1990). However, the Meeteetse vertebrate fauna is less diverse and abundant than those of the Lance and the Judith
River due to limited time of sampling.
6 Background
General Stratigraphy and sedimentology- The Meeteetse Formation was first described by Hewett (1914) from exposures near the town of Meeteetse in the southwestern part of the Big Horn Basin, Wyoming (Fig. 1). Hewett (1926) later extended the term Meeteetse Formation to exposures in the Grass Creek-Golden
Eagle area. Later, Love and others (1951) indicated an extension into the northwestern
Wind River Basin. The Meeteetse Formation was deposited on the western shore of the Western Interior Seaway (Fig. 4) and represents the time equivalent of the Bearpaw
Shale of Montana overlying the Judith River regressive sediments (Keefer, 1965). The maximum thickness of the Meeteetse Formation is 396 meters in the western part of the Big Horn Basin. It thins towards the eastern part of the Big Horn Basin, interfingers with the Lewis Shale, and is absent in the southwestern Powder River Basin. The thickness of the Meeteetse Formation varies from about 120 meters to 210 meters in the southeastern part of the Wind River Basin. A succession of alternating thin beds of sandstone, siltstone, shale, tuffaceous siltstone or mudstone, carbonaceous shale, and lenticular coal beds appears in the Meeteetse Formation (Love, 1956; Windolph et al.
1986). Most of its strata are soft and easily eroded (Keeper and Rich, 1957) so that the formation generally underlies conspicuous strike valleys (Rich, 1958). The Meeteetse
Formation presents a uniquely gray, black,
7
Figure 4: Latest Cretaceous (Maastrichtian) paleogeographic map of western North America Modified from a U.S.G.S. paleogeography map http://energy.usgs.gov/factsheets/cret.coals/maas.map.html
yellow, and brown banded appearance, where good exposures exist. The age determination of the formation is based on paleontologic evidence. Hewitt (1926) collected fossil plants along the western margin of the Big Horn Basin and indicated a probable Judith River or Bearpaw age (late Campanian) for the Meeteetse Formation.
8 A record of the repetition of transgressive and regressive cycles relating to eustatic sea-level change of the epicontinental sea that invaded most of the Western
Interior is preserved in the Meeteetse Formation (Lillegraven and Ostresh 1990). A sequence of bentonites overlies the late Campanian regression recorded in carbonaceous mudstones and fluvial sandstones with a vertebrate fauna of ceratopsian and hadrosaurian dinosaurs. The Bearpaw transgression features tabular sandstone and shale beds in the sequence. Deposition of prodeltaic and fluvial sandstones appears in the final Maastrichtian regression, and presents a record of freshwater environments. Representative environments include widespread swamps, broad flood plains, lagoons, and deltas formed during the brief invasions and final retreat of the
Lewis Sea across central Wyoming (Rich, 1958). These regressive Maastrichtian lake sediments carry a fauna of various mollusks, turtles, crocodiles, gar-pike (Lepisosteus sp.) and freshwater skate (Myledaphus bipartitus). These taxa indicate a fluvio- lacustrine environment near the delta margins. The succession shows a cycling of marine and terrestrial environments across a delta lobe in response to sea-level change (Hicks, 1989). According to Lillegraven (1987), the Meeteetse and Lance formations have been searched for microvertebrate fossils (emphasis on mammals) in the southern half of the Bighorn Basin. The new locality at Elk Basin appears rich and the potential exists for the discovery of squamate and further mammalian remains
9 (Storrs, 1991). Recovered multituberculate teeth are the first known mammal specimens from the Meeteetse Formation.
The Meeteetse Formation and Lewis Shale, equivalent to the Bearpaw Shale, are considered as stratigraphic equivalents throughout central and north-central
Wyoming (Cobban and Reeside, 1952). The name Lewis Shale is applied to marine facies, while the Meeteetse Formation is used for equivalent non-marine strata. These formations are used for the interval between the Mesaverde and Lance formations in
Wyoming (Fig 5).
Powder River Basin Powder River Basin Wind River Basin Green River Basin Bighorn Basin (western) (northeast)
Lance Lance Lance Lance Lance
Formation Formation Formation Formation Formation
Fox Hills Sandstone Meeteetse Meeteetse Lewis Shale Bearpaw Shale Formation Formation
Pierre Upper Cretaceous Upper Cretaceous Mesaverde Almond Mesaverde Parkman Shale
Formation Group Formation Formation Sandstone Mesaverde Mesaverde
Figure 5: Chart showing the stratigraphic position of exposures in the Big Horn Basin and adjacent regions in Wyoming. Modified from Keeper (1965)
The Lewis Shale consists of a series of dark gray marine shales and thinly bedded olive-gray sandstones, about 350 meters thick in the southwestern Powder
10 River Basin (Fig. 1), and interfingers with the Meeteetse Formation in the western
Powder River Basin, thinning out rapidly. In most places, the lower Lewis tongue underlies thin, lenticular, channel-type sandstone. In the Elk Basin (Fig. 6), the lower tongue of the Lewis Shale is equivalent to the Bearpaw Shale of Montana.
Figure 6: General Stratigraphy of the Elk Basin section. Modified from Hicks (1993)
11 Fossil collections from the lower Lewis tongue in the southeastern part of the
Wind River Basin include the Pelecypods: Inoceramus, Pteria, Modiolus and
Glycimeris; the cephalopods: Baculites and Acanthoscaphites; and a gastropod,
Turritella, which are considered to be late Bearpaw age. The fauna of the Lewis Shale from the Wind River Basin indicates shallow-water forms, indicating that the seas covered a very wide shelf area during the deposition of the Lewis (Rich 1958).
The contact between the Meeteetse Formation and the overlying Lance
Formation is generally conformable. Their lithologic similarities and the lenticular character of individual beds make these formations difficult to determine (Rich, 1958).
However, most of the upper part of the Meeteetse Formation includes a greater amount of tuffaceous, bentonitic siltstone and coal beds than the strata of the Lance formation.
The Lance Formation is the uppermost unit of Late Cretaceous age in the area, and is characterized by a series of buff and gray, massive lenticular and moderately porous sandstone beds with minor interbedded carbonaceous shale and thin coal. The basal part of the formation consists of extensive sandstone units. The strata of the
Lance Formation were deposited after the final withdrawal of the Cretaceous sea. They are mostly of freshwater origin except for local brackish-water facies (Rich, 1958).
12 Depositional history during the Late Cretaceous in Wyoming- During the time of the Cody Shale, marine conditions predominated over the area. The Late Cretaceous sea gradually withdrew eastward in central Wyoming in the early Campanian (Keefer &
Rich, 1957). The last succession of marine deposition is represented by the Cody
Shale in the early Campanian (Telegraph Creek, Eagle and Claggett) in central
Wyoming. The lower sandstone unit of the Mesaverde Formation was deposited during the eastward regression of the Cody sea. Thin tongues of marine shale in the lower part of the Mesaverde Formation in the central and eastern parts of the basin indicate minor re-advances of the sea. Most of the strata of the Mesaverde were deposited in broad flood plains, coastal swamps, deltas, and lagoons. These depositional conditions were generally similar throughout the rest of Late Cretaceous time, except for brief marine invasions during deposition of the Lewis Shale (Keeper, 1965).
Major Laramide orogenic movement in west-central Wyoming began in Late
Cretaceous (middle and late Montana) time (Horberg et al., 1949). Probable downwarping of the Wind River Basin floor took place during deposition of the
Meeteetse Formation and Lewis Shale, and became more pronounced in latest
Cretaceous time (Keeper, 1965). Evidence of middle and late Montana folding and faulting in west-central and southwestern Wyoming was also found (Eardley, 1962).
Upwarping of the Granite Mountains and Washakie Range along the south and
13 northwest margins of the Wind River Basin began during deposition of the lower Lance.
Beginning in Late Cretaceous time, uplift west of the present Idaho-Wyoming boundary moved the main sites of deposition eastward (Keeper, 1965).
The central part of Wyoming was characterized by progradation of extensive flood plains over highly carbonaceous strata of the Meeteetse Formation. The surface of the plain was probably gently tilted eastward and broad deltas extended into the sea.
No major tectonic activity was present during deposition of the Lewis and Meeteetse formations except for local minor uplift, which provided fine-grained sediment into the eastward flowing rivers. Volcanic ash from the western part of the Yellowstone region was deposited throughout central Wyoming (Keeper, 1965). Fairly rapid withdrawal of the sea prevented a significant amount of littoral and near-shore sandstone deposition and may have led to the absence of regressive sandstone at the base of the Meeteetse
Formation. After this event, the Cretaceous sea withdrew entirely from the central part of Wyoming and far to the east. However, there were a few marine invasions which are represented in the upper Lewis Shale. Continuous non-marine deposition from the
Meeteetse Formation into the Lance Formation occurred in the western part of the area, while a conformable succession of upper Lewis Shale followed by the Lance Formation was deposited in the southeastern Wind River Basin and southwestern Powder River
Basin (Fig. 1). In the east part of the Powder River Basin, there was massive
14 deposition of littoral sand near the margin of the retreating sea represented by the Fox
Hills Sandstone (Rich, 1958). The Fox Hills Sandstone was succeeded conformably by the overlying Lance Formation (Dobbin & Reeside, 1929).
Stratigraphy and sedimentology of the field site in Elk Basin
The field site of the Meeteetse Formation at Elk Basin corresponds to the uppermost five meters of the formation. Its lateral extant is approximately 34 meters
(Fig. 7 and Fig. 8). The lower boundary of the Meeteetse Formation in this locality consists of a three meters thick, poorly cemented bentonite bed. Radiometric dating of the bentonite layer did not give an accurate age determination for the deposit (Huff, pers. comm., 2004). However, two different features of the Meeteetse Formation in the field site give evidence of its age. The Meeteetse Formation in the field site underlies the Lance Formation, approximately 244 meters thick. And the Meeteetse is the laterally equivalent of the Bearpaw Formation. From this, the Meeteetse Formation in the field site most likely appears to be a terrestrial deposit of early Maastrichtian age
(Fig 9).
15 Lance Formation
Meeteetse Formation
Judith River Formation
Figure 7: Overview of the field site at Elk Basin, scale bar in meter
16 Lance Formation
Meeteetse Formation
N
Figure 8: Closer view of the field site, scale bar in meter
17
Figure 9: Stratigraphic position of the Meeteetse Formation at field site at Elk Basin
The strata in the Meeteetse Formation are mainly non-resistant and poorly cemented, and dominated by a series of massive mudstones and siltstones (Hicks,
1993) with cross-bedded sandstone. The Meeteetse Formation in the field site can be divided into a lower unit of cross-bedded sandstone, siltstone, shale, carbonaceous shale, and coal, and an upper unit consisting largely of lenticular sandstone. The lower
18 unit presents a gray, yellow, and brown banded appearance. A thin coal layer is present in a mudstone. Two relatively thin beds of mudstone and siltstone in the lower unit are fossiliferous. The gradation in sedimentology from the base to the top of the
Meeteetse Formation in the field site indicates an environmental change from a fluvio- lacustrine environment to a relatively arid fluvial environment with climatic seasonal change (Hicks, 1993). A 30 cm deep trench (Fig. 10) was dug across the gently dipping beds to describe a micro-stratigraphic section that was divided into 11 sections. A total of 5 meters was exposed. The detailed micro-stratigraphic description is shown in Table
1 and Fig. 11.
Figure 10: A 30 cm deep trench in field site for describing the micro-stratigraphic section
19
Table 1: Micro-stratigraphic description of the Meeteetse Formation in the field site
Unit (from top) Thickness (cm) Description Large cross-bedded sandstone, dark brown color, 1 88 well cemented and well sorted, fine grained Gray to dark yellow mudstone, 2 151 poorly cemented Cross-bedded sandstone, lenticular, well cemented 3 15 and well sorted, medium grained Gray to yellow mudstone, irregular and 4 33 convoluted bedding, bioturbated Finely cross-bedded sandstone, yellow to gray 5 15 color, well cemented, medium grained, carbonized plant impression at the base Sandstone with silt, yellow into gray color, 6 28 irregularly bedded, bioturbated Mudstone, dark gray to brown color, irregularly 7 53 bedded, poorly cemented, fine grained, coal, bone bearing unit Siltstone, dark gray, irregularly, fine grained, 8 15 bioturbated, bone bearing unit Mudstone, gray into yellow color, irregularly 9 38 bedded, poorly cemented Massive sandstone, lenticular, bioturbated 10 20 (burrowing) Cross-bedded sandstone, yellow color, 11 55 well cemented, fine grained
20
Figure 11: Columnar section of the field site of the Meeteetse Formation at Elk Basin
21 Methods
Fossil sampling of the site was accomplished by collecting 239 19-liter buckets of sediment (Fig. 11). These came mostly within 30 cm depth of the surface due to the large lateral extent of the site. The first three buckets of the total sample collected were processed by on-site dry sieving using brass screens of mesh size three mm to determine the degree of microvertebrate density of the site and future collecting direction. Dry sieving proved inefficient in the recovery of microvertebrate fossils and might possibly damage fossils during processing. Therefore, wet screening was used to recover microvertebrate material from the rest of the collected material. This method dramatically increases the efficiency of collection of useful samples of small-scale microvertebrate fossil specimens from large volumes of weakly consolidated sediment
(McKenna et al, 1994). Ten wood screens of mesh size four mm (Fig. 12) were used daily in the field to produce concentrates, which were processed later with finer screens in the lab. Once the boxes were filled with sediment, they were allowed to soak in a creek until ready for processing (Fig. 13). Processed boxes were dried out in the sun for a couple of hours. Concentrates were brought to Cincinnati Museum Center for further screening and sorting in a wet lab using a specially designed wet screening device designed by Dr. Storrs (Fig. 14). Samples were sorted into morphological categories under magnifiers and microscopes. Fossil identification was accomplished
22 via comparative morphology. The Estes’ (1964) monograph and an illustrated microvertebrates guide produced by Brinkman (2002) were especially helpful in this effort.
Figure 12: Buckets used to collect sediments from field
Figure 13: Wood screens before matrix distribution
23
Figure 14: Screens in water before processing
Figure 15: Wet screening device at Cincinnati Museum Center
24 Faunal list of vertebrates
Class Elasmobranchii
Order Heterodontiformes
Suborder Selachii
Lonchidion selachos
Order Rajiformes
Family Dasyatidae
Myledaphus bipartitus
Class Osteichthyes
Order Acipenseriformes
Family Acipenseridae
Acipenser sp.
Order Amiiformes
Family Amiidae
? Kindleia fragosa
Order Lepisosteiformes
Family Lepistosteidae
Lepisosteus sp.
25 Infraclass Teleostei
Unidentified genus and species
Class Reptilia
Order Testudines
Family Banidae
? Plesiobaena sp.
Family Dermatemydidae
Basilemys sp.
Adocus sp.
Family Trionychidae
Aspideretes sp.
Order Crocodylia
Suborder Eusuchia
Family Crocodylidae
Subfamily Crocodylinae
Borealosuchus sp.
26 Order Saurischia
Suborder Theropoda
Family Dromaeosauridae
Saurornitholestes langstoni
Order Ornithischia
Suborder Ornithopoda
Family Hadrosauridae
Unidentified genus and species
Suborder Ceratopsia
Family Ceratopsidae
Unidentified genus and species
Class Mammalia
Infraclass Allotheria
Order Multituberculata
Family Ptilodontidae
Mesodma sp.
27 Systematic Paleontology
All of the fossils discussed below were collected from the Meeteetse Formation,
Wyoming. All material is catalogued in the Cincinnati Museum Center (CMC),
Cincinnati, Ohio.
Class Elasmobranchii
Order Heterodontiformes
Suboder Selachii
Lonchidion selachos
Material- CMC VP 8256. Complete lateral teeth.
Description- Lonchidion is a small hybodont shark; several lateral teeth were found from the Meeteetse Formation. The lateral teeth (Fig. 16) are 2-5 mm wide, horizontally elongated and low crowned with reduced cusplets. The tooth surface is smooth and the distal parts of the crown are pointed (Cappetta, 1987). A well developed occlusal crest runs along the tooth and shows small cusps: a labially protruded low central cusp with up to two pairs of cusplets, anterior and posterior lateral cusps (Estes, 1964). The root is incomplete due to wear and breakage and the crown-root junction is wider than the lowermost part of the crown. Other morphological characteristics of the tooth of L. selachos are: moderate and monognathic heterodonty, cutting-crushing dentition type, and strong and parallel-sided labial protuberance (Rees
28 and Underwood, 2002).
A B
C
Figure 16: Lonchidion selachos, CMC VP 8256, lateral tooth: A, labial; B, lingual; C, root views, scale bar in mm.
Discussion- Lonchidion is a small hybodont selachian genus from the
Ladinian of the Middle Triassic to the Maastrichtian of the Late Cretaceous of Laurasia and Gondwana (Cappetta, 1987). Most of the systematics of the hybodont sharks is based upon isolated teeth because few species of these sharks are known from well preserved skeletal remains. This has resulted in confusing and inconsistent classification schemes of hybodonts by different researchers (Rees and Underwood,
2002). Lonchidion selachos was first described by Estes (1964) based on isolated teeth, associated fin spines and cephalic spines of L. selachos from the Late Cretaceous
29 Lance Formation, eastern Wyoming.
This genus has been found from both freshwater (including lacustrine units) and marine deposits. This suggests that this genus was able to tolerate a wide range of salinities (Wrobleski, 2004). However, the hybodont sharks were more diverse in freshwater environments in the Late Cretaceous (Rees and Underwood, 2002).
Order Rajiformes
Family Dasyatidae
Myledaphus bipartitus
Material- CMC VP 8257, dermal denticles; CMC VP 8263, numerous isolated
teeth.
Description- The teeth consist of a root and a crown with the crown always larger than the root. The crown has a hexagonal to rhomboidal shape (Fig. 17-B) and its sides are flattened with numerous vertical wrinkles (Fig. 17-A). The root is bifid and normal to the crown and its ends are rounded. Denticles were found along with the teeth (Fig. 17-D and E). Some of the smaller teeth exhibit a lenticular shape, rather than the typical hexagon, while some of them are very irregular. The range of shape differences are shown in Figure 17-C.
30
A B
C
D E
Figure 17: Myledaphus bipartitus, CMC VP 8263: isolated teeth: A, labial; B, occlusal views; C, teeth are showing variation in size and shape, CMC VP 8257, dermal denticles: D, occlusal; E, lateral views. Scale bar in mm.
31 Discussion- Myledaphus teeth are among the most common fossils in the
Meeteetse Formation as well as in other mid-continent Late Cretaceous nearshore and estuarine deposits (Breithaupt, 1982; Case, 1987; Brinkman, 1990). They are also present in Paleocene and early Eocene deposits of the same region. M. bipartitus is found not only in freshwater deposits but marine and marginal-marine deposits, which indicates that M. bipartitus was able to tolerate a wide range of salinities similar to modern dasyatids (Wroblewski, 2004). Modern freshwater skates are distributed in warm shallow waters and fully grown individuals can reach three meters in size. They are powerful swimmers and feed on mollusks, crustaceans and fish. Their dentition consists of numerous plate-like teeth (Fig. 17) and is used in crushing prey.
Figure 18: A dentition of extant Raja sp., exhibit in National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.
32 Class Osteichthyes
Order Acipenseriformes
Family Acipenseridae
Acipenser sp.
Material- CMC VP. 8258. Denticles.
Description-The acipenserids are a group of primitive bony fish, which includes extant sturgeon and paddlefish. The sturgeon is represented by dermal denticles from the Meeteetse Formation. These teeth are less than 5 mm in size with irregular spiky denticles (Fig.19). Most acipenserid fossils are known from large fragmentary dermal scutes and other ossifications and it is hard to distinguish fossil Acipenser species from each other or from most living sturgeons (Findeis, 1997).
Figure 19: Acipenser sp., CMC VP 8258, denticles. Scale bar in mm.
Discussion- Acipenserid fossils were widely distributed in freshwater drainages along the western interior seaway of North America during the Cretaceous and Tertiary
33 periods (Choudhury and Dick, 1998). The living relative of fossil acipenserids, lake sturgeon Acipenser fulvescens, is characterized by a robust, torpedo-shaped body covered by five rows of scutes: 1 dorsal, 2 lateral and 2 ventral (derived from ganoid scales), and a pointed snout. Lake sturgeon has a heterocercal tail in which the vertebral column extends into the upper lobe of the caudal fin making a large upper lobe and a small lower lobe. Acipenserids share this characteristic with sharks and other cartilaginous fishes (Smith, 1979). Lake sturgeon has a long snout with 4 barbels followed by a ventral mouth and spiracles, small respiratory opening behind each eye, and no teeth (Wener, 2004). They can reach up to 2.4 meters in length and weigh up to
136 kilograms. Average individuals are from 0.5 meters to 1.4 meters long and weigh 1 to 27 kilograms (Harkness and Dymond, 1961).
Lake sturgeon Acipenser fulvescens, can be found in large rivers and lakes and generally stays close to the bottom (Werner, 2004). Lake sturgeon prefer nearshore environment with water temperatures from 10 to 18°C and depths of 4.5-9 meters, generally avoiding aquatic vegetation (Hay-Chmielewski, 1997). They inhabit fresh water for their entire lives while other sturgeon species dwell in the ocean and only migrate into freshwater rivers to spawn (Kempinger, 1988). Lake sturgeon is an opportunistic benthivore which forages over gravel using its sensory barbells to detect prey- small invertebrates such as insect larvae, crayfish, snails, clams and leeches
34 (Harkness and Dymond, 1961). Interestingly, lake sturgeons are long-lived fish with females living for 80-150 years and males for 55 years (Houston, 1987).
Order Amiiformes
Family Amiidae
? Kindleia fragosa
Material- CMC VP 8259, incomplete mid-centra.
Description- Incomplete amiid mid-centra were found from the Meeteetse
Formation. The centra are 1.2 cm (centrum # 1) and 0.7 cm (centrum #2) wide, each with slim lateral profiles and solid lateral surfaces. The amiid mid-centra have a pair of pits mid-ventrally, but they are hard to see in these specimens (Fig. 20).
Discussion- Amiidae, bowfins were widely distributed in freshwater deposits in the Late Cretaceous represented by Kindleia and the extant Amia (Gray, 1988).
Schultze & Wiley (1984) noted that Kindleia and Amia are the most advanced amiids and the only ones that dwell in freshwater. These two freshwater amiids had longer lifespans than their marine relatives and they played important roles in Late Cretaceous freshwater ecosystems Kindleia became extinct by the late Eocene (Berra, 1981).
35
A B
C D
Figure 20: Kindleia fragosa, CMC VP 8259, centra: A, centum #1, anterior view; B, lateral views; C, centrum #2, anterior view; D, lateral views. Scale bar in mm.
Extant Amia calva is a highly predaceous fish and prefers weedy, stagnant, sluggish and shallow rivers. It has an abbreviated heterocercal tail in which the vertebral column extends partially into the upper lobe of the caudal fin. A. calva has a very long dorsal fin consisting of 45 soft rays, which runs along most of the back.
The bowfin is a long-lived fish and some individuals live up to 30 years. A fully grown bowfin is 30-60 cm in length and weighs 1-1.4 kilograms (Werner, 2004).
36 Order Lepisosteiformes
Family Lepisosteidae
Lepisosteus sp.
Material- CMC VP 8262, a posterior dorsal centrum; CMC VP 8265, scales;
CMC VP 8272, skull bone fragments; CMC VP 8273, teeth.
Description- The rhomboidal shaped dermal plates of Lepisosteus are some of the more common elements of the Meeteetse fauna. The thick enamel-covered scales of this gar are widespread in most of the deposits of the Cretaceous, which makes them a remarkable indicator for microvertebrate localities (Estes, 1964). Ganoid scales
(Fig. 21-C, D) vary from 2 to 7 mm in size. Lepisosteus centra are unique in possessing ball and socket joints with nearby centra. The centrum found from the Meeteetse formation is an elongated posterior centrum (Fig. 21-A and B). Anterior dorsal centra are shorter. Skull bone fragments (Fig. 21-E) were also found and have unique and prominent enamel tubercles on their external surfaces. The teeth (Fig. 21-F) are conical with a moderate degree of curvature; striations are present at the base and the tips become constricted at the base (Brinkman, 2002). Lepisosteus is first known from the Lower Cretaceous in fresh and probable brackish water deposits (Wiley & Schultze
1984). Wiley and Stewart (1977) reported Lepisosteus from the Niobrara Formation, western Kansas, as the first and only gar from fully marine deposits.
37
A B
C D
E F
Figure 21: Lepisosteus sp., CMC VP 8262, posterior dorsal centrum: A, lateral; B, posterior views, CMC VP 8265, scales: C, diamond shape ganoid scale; D, scale surface, CMC VP 8272, E, skull fragment, CMC VP 8273, tooth: F, lateral view. Scale bar in mm.
Discussion- Lepisosteus remains are very distinctive because of their heavy,
38 diamond-shaped ganoid scales. Gars have a relatively long snout and a jaw full of teeth which makes it a very effective predator. Gars usually live in shallow, weedy lakes and rivers (Gray, 1988). In adult fish, the vertebral column extends into the upper lobe of the caudal fin. Females live as long as 22 years, while males have a shorter life of about 11 years. Gars can grow up to 1.3 m long, but average individuals are about 60 cm long and weigh around 1 kilogram (Werner, 2004).
Only eight species are known worldwide and modern gars are restricted to central and eastern North America and from southern Canada (~50°N Latitude) south to Costa Rica (~10°N Latitude). The closely related extant alligator gar, L. spatula, is restricted to freshwater of the Gulf of Mexico coastal plain from Florida to Vera Cruz,
Mexico. Lepisosteus were more widely distributed in the Cretaceous, which indicates that it was a more important fish in freshwater at that time (Gray, 1988). Gars are restricted to freshwater, but they are able to tolerate brackish and nearshore salinities.
Infraclass Teleostei
Unidentified genus and species
Material- CMC VP 8280, 8281, 8282, 8283, 8284, centra; CMC VP 8285, tooth.
Description- Unidentified teleost centra (Fig. 22 A-J) and a tooth (Fig. 22-K) were collected from the Meeteetse Formation. These are included for completeness.
39
A B
C D
E F
40
G H
I J
K
Figure 22: Teleostei, unidentified genus and species, CMC VP 8280, centrum: A, anterior; B, lateral views, CMC VP 8281, centrum: C, anterior; D, lateral views, CMC VP 8282, centrum: E, anterior; F, lateral views, CMC VP 8283 centrum: G, anterior; H, lateral views, and CMC VP 8284, centrum: I, anterior; J, lateral views, CMC VP 8285, tooth: K, lateral view, scale bar in mm.
41 Class Reptilia
Order Testudines
Numerous turtle shell fragments and a few partial limb bones were found in the
Meeteetse Formation during the course of this study. Disarticulated and fragmentary turtle remains make identification difficult. The following provisional assignments are based on characteristic patterns of shell fragments.
There are four different turtle suborders: Proganochelydia, which inhabited late
Triassic terrestrial and swamp environments; Amphichelydia were mainly Jurassic and
Cretaceous turtles with a probable terrestrial and aquatic-amphibious lifestyle; freshwater snake-necked or side-necked turtles, Pleurodira, from the Late Jurassic to the present; and extinct and extant Cryptodira including marine, freshwater and terrestrial turtles (Gray, 1988).
Suborder Amphichelydia
Family Baenidae
? Plesiobaena sp.
Material- CMC VP 8275, numerous shell fragments.
Description- Various sizes of smooth shell fragments (Fig. 23) of Plesiobaena were found in the Meeteetse Formation during field work. Baenid turtles are primitive amphichelyid turtles confined to North America (Brinkman, 2002).
42
Figure 23: Plesiobaena sp., CMC VP 8275, detailed surface view of the shell fragment. Scale bar in mm.
Discussion- Baenidae turtles were the most abundant and diversified Late
Cretaceous turtles. They had an amphibious life style (Gray, 1988). Baenid turtles are known with diverse freshwater fish and amphibians in a similar vertebrate fauna from floodplain deposits bordering the retreating Late Cretaceous inland sea in North
America (Estes, 1970; Gray, 1988).
Suborder Cryptodira
Family Dermatemydidae
Basilemys sp.
Material- CMC VP 8276, numerous shell fragments.
Description- Numerous shell fragments of Basilemys were collected from the
Meeteetse Formation. Basilemys is a primitive soft shelled turtle or trionychoid.
43 Basilemys has triangular shaped shell (Fig. 24- A, B) ornamentation (Hay, 1908).
A B Figure 24: Basilemys sp., CMC VP 8276, detailed surface of shell fragments: A, dorsal; B, ventral view. Scale bar in mm.
Adocus sp.
Material- CMC VP 8277, numerous shell fragments.
Description- Adocus also has a shell ornamented with shallow triangular shaped pits (Fig. 25), usually set in lines like Basilemys, but Adocus is a smaller turtle with thinner shells and finer pits (Brinkman, 2002).
Figure 25: Adocus sp., CMC VP 8277, detailed surface view of the shell fragment. Scale bar in mm.
44 Family Trionychidae
Aspideretes (Trionyx) sp.
Material- CMC VP 8278, numerous shell fragments.
Description- This trionychoid turtle has heavily pitted shell ornamentation and the pits are deeper (Fig. 26) than those of Adocus and Basilemys. The Side of the shell is straight and the bottom of the pits are flat (Estes, 1964).
A B
Figure 26: Aspideretes sp., CMC VP 8278, detailed surface of shell fragments: A, dorsal; B, ventral view. Sale bar in mm.
Discussion- Soft-shelled turtles, the Trionychidae, were widely distributed freshwater carnivores that feed on vertebrates and mollusks (Gray, 1988). Carpenter
(1979) mentioned that Trionyx is the only valid genus of Trionychidae living today in
North America.
45 Order Crocodylia
Suborder Eusuchia
Family Crocodylidae
Subfamily Crocodylinae
Borealosuchus sp.
Material- CMC VP 8267, isolated teeth; CMC VP 8268, scutes.
Description- Crocodile remains are one of the more abundant vertebrate fossils in the Meeteetse Formation locality. They are represented by various types of teeth (Fig.
27 A-E) and scutes (Fig. 27-F). Crocodile scutes have large, deep pits compared to turtle shells. Scutes with a well-developed mid-line ridge are typical in the dorsal region of the neck. The crocodile found in the Meeteetse Formation is referred to
Borealosuchus sp., which formerly called “Leidyosuchus” (Brochu, 2000).
Borealosuchus is a common crocodilian in Late Cretaceous deposits in North America.
Borealosuchus can be distinguished from another Late Cretaceous crocodile,
Albertochampsa, by the shape of its crushing tooth (molar, Fig. 27-E). According to
Brinkman (2002), the crushing teeth of Albertochampsa are round in crown view, while they are oval-shaped in Borealosuchus. The crushing teeth in the back of the tooth-row are a much better indicator of the size of the animal than the cheek teeth. No other skeletal elements of Borealosuchus were found in the Elk Basin, Meeteetse Formation
46 site.
A B C
D E F
Figure 27: Leidyosuchus sp., CMC VP 8267, isolated teeth, A and B, tall conical teeth; C, low conical tooth from the posterior of the teeth row; D, germ tooth; E, crushing tooth; F, triangular- shaped scute with well-developed mid-line ridge, scale bar in mm.
Discussion- According to Brochu (2001), Borealosuchus is considered as the endemic widespread crocodilian in non-coastal regions in the Late Cretaceous. Modern
47 crocodiles are water dependent and some are heavily aquatic. They feed on almost all kinds of animals that are accessible- fish, crabs, shrimp, molluscs, insects, arachnids amphibians, reptiles, water birds and mammals. Their feeding behavior, however and diet change significantly as they mature (Gray, 1988). Like modern crocodiles,
Borealosuchus probably fed on a variety of animals. Crocodiles shed their teeth throughout their life, so they have different sizes of teeth in various stages of growth.
Crocodiles are sensitive to climatic changes which makes them an important source of paleoenvironmental information. Where suitable climatic conditions are present with available water such as rivers and lakes, these animals flourished throughout the world
(Gray, 1988).
Order Saurischia
Suborder Theropoda
Family Dromaeosauridae
Saurornitholestes langstoni
Material- CMC VP 8260, isolated dentary tooth.
Description- An isolated dentary tooth of a small theropod (Fig. 28) was found from the Meeteetse Formation. Currie and others (1990) made the first comprehensive study of theropod teeth from the Judith River Formation with detailed descriptions and illustrations of teeth. In that paper, they showed that it is possible to identify theropod
48 taxa based on the shape of the denticles and other features. The measurements of the dentary tooth are based on Currie and others (1990) and include: fore aft basal length
(FABL), 7 mm; tooth height, 16.5 mm; cross-sectional thickness, 3 mm, denticles per millimeter, 5; denticles width, 0.2 mm and height, 0.3 mm. Total length of the tooth can not be measured because of breakage at the tip but FABL of the tooth is relative to the total length of the tooth. The tooth is laterally compressed and the denticles become smaller and rounded near the base of the tooth. The tooth is a flattened oval shape in mid cross-section and hour-glass shaped in basal cross-section. The hour-glass shape of the tooth is formed by a slight indentation on the lingual surface (Sankey et al, 2002).
A B
C Figure 28. Saurornitholestes langstoni, CMC VP, 8260, isolated dentary tooth: A, lingual view; B, lingual enlargement of posterior denticles; c, enlargement of denticles. Scale bar in mm.
49 Discussion- The isolated denticle tooth from the Meeteetse Formation belongs to a small theropod Saurornitholestes langstoni based on measurements and morphology (Currie et al., 1990). Saurornitholestes is one of the more abundant small theropods in Late Cretaceous deposits. Saurornitholestes teeth are flatter and more gracile than those of Dromaeosaurus albertensis. The narrow blade-like teeth suggest that they functioned mostly in cutting and slicing (Sankey et al, 2002).
Order Ornithischia
Family Hadrosauridae
Material- CMC VP 8269, an incomplete vertebra; CMC VP 8271, teeth fragments.
Description- Teeth fragments (Fig. 29-D, E) and an incomplete vertebra (Fig. 29-
A, B) of hadrosaurs were obtained from the Meeteetse Formation deposit in Elk Basin.
Not enough is preserved for more precise identification.
Discussion- Hadrosaurs range through the Late Cretaceous and were distributed throughout the world. Hadrosaurian habitat has been debated in terms of how aquatic or terrestrial these dinosaurs may have been. Romer (1966) suggested that hadrosaurs were amphibious based on their anatomy. He interpreted that the webbed forelimb (later found to be a taphonomic artifact) was an adaptation for aquatic life and the hind limbs and tail were effective swimming appendages. On the contrary,
50 Ostrom (1964) concluded that hadrosaurs were more terrestrial than aquatic. The highly specialized dentition of hadrosaurs was suited to hard and fibrous terrestrial plants. Galton (1970) suggested that hadrosaurs were proficient runners holding their body horizontally, as in other bipedal dinosaurs, and the small hooves on their digits were used in slower movement. Hadrosaur remains are found in low-lying coastal plain deposits and the margins of rivers (Carroll, 1988). This suggests that duck-billed dinosaurs lived in fluvial environments.
A B
C D
Figure 29: Hadrosaur elements, CMC VP 8269, incomplete vertebra: A, anterior view; B, lateral view, CMC VP 8271; C and D: teeth fragments, scale bar in mm.
51 ? Family Ceratopsidae
Material- CMC VP 8264, 8288 incomplete phalanges.
Description- Ceratopsian partial phalanges were collected from the Elk Basin
Meeteetse Formation locality. The incomplete phalanges are 2.5 cm and 1.8 cm wide
(Fig. 30-A) and 2.2 cm tall (Fig. 30-B), each. Dinosaur unguals or ‘toe bones’ come in a wide range of shapes and sizes. Many are difficult to assign to the genus or family level, however, tentative identification can be made when found in association with other bones or teeth. Ceratopsian unguals on the first three digits are small, dorsoventrally compressed, and the distal parts are blunt and irregular (Dodson et al, 2004).
A B
Figure 30: Ceratopsian phalanges, A: CMC VP 8264, B: CMC VP 8288. Scale bar in mm.
Discussion- The horned quadrupedal herbivores of the family, Ceratopsidae are mainly known form the Late Cretaceous of North America. Distinctive ceratopsian features are a triangular-shaped skull and a beak-like rostral bone at the tip of the
52 snout. Most advanced ceratopsians are only found in western North America (Carroll,
1988). The later ceratopsian skeleton was well adapted for galloping. Ceratopsians have bundles of ossified tendons over the hips, and neck and trunk vertebrae with high neural spines for the attachment of powerful muscles to support the head (Benton,
1997). Ceratopsians were originally considered as inland animals. However, according to Brinkman and others (1998), they were more common in coastal areas.
Unidentified genus and species
Material- CMC VP 8270, ossified tendons
Description- Several ornithischian ossified tendons (Fig. 31) were obtained from the Meeteetse Formation. This can be associated with either hadrosaurs or ceratopsians found in Elk Basin, Meeteetse Formation locality.
Figure 31: Ornithischian ossified tendons, CMC VP 8270, scale bar in mm
53 Class Mammalia
Order Multituberculata
Family Ptilodontidae
Mesodma sp.
Material- CMC VP 7767, isolated left lower second molar; CMC VP 8255 isolated upper incisor.
Description- The two isolated teeth from the Meeteetse Formation are thought to belong to the genus Mesodma, based on their shapes and sizes (Clemens, 1963). The molar (Fig. 32-A, B) is 1.2 mm wide and 1.8 mm long and the cusp formula is 4:2. The incisor is 4 mm long and 1.5 mm wide (Fig. 32-C).
A B
Figure 32: Mesodma sp., CMC VP 7767, lower left second molar: A, occlusal view, CMC VP 8255, isolated upper incisor: B, labial view.
Discussion- Multituberculates were highly successful and long ranging early mammals. The age range of the multituberculates is 100 million years, which exceeds
54 that of any other mammalian order. The first multituberculates appeared in the Late
Jurassic; they flourished in the Late Cretaceous and survived through the Eocene.
Multituberculates were the first herbivorous mammals with specialized skulls and teeth suitable for eating plants, however, the shearing lower premolars show a pattern that is particularly close to that of modern omnivorous marsupials (Carroll, 1988). The skeleton indicates that multituberculates were terrestrial to arboreal rather than fossorial. Multituberculates had a very specialized dentition; a pair of large incisors is developed above and below, similar to rodents, and there was a well-developed series of cheek teeth (Romer, 1966). The molars are elongated and have two or three parallel rows of cusps which were used for mastication (Carroll, 1988). The great success of the multituberculates may have been related to the radiation of angiosperms in the late
Mesozoic and early Cenozoic, which were a good food source for them (Clemens et al,
1979).
55 Invertebrate Fauna
The invertebrates collected from the Meeteetse Formation, Elk Basin site are poorly preserved and hard to identify. The presence of gastropods indicates a shallow water environment. Following is a tentative Meeteetse invertebrate faunal list:
Faunal list of invertebrates
Class Pelecypoda
Order Unionoida
Family Unionidae
Class Gastopoda
Order Architaenioglossa
Family Viviparidae
? Viviparus sp.
? Campeloma sp.
56 Analysis of Meeteetse vertebrate fauna
Fifteen taxa and more than one thousand fossil elements were recognized in this study. These taxa are present in other Late Cretaceous microvertebrate localities of western North America. The latter provided important information for identification of the material from the Meeteetse Formation (Estes, 1964; Sahni, 1972; Carpenter,
1979; Breithaupt, 1982; Brinkman, 1990). The relative abundance is given in Tables 2 and 3, with dominant and rare fossil elements being indicated in these tables. Figure 32 graphically compares the dominance of each group.
Table 2: Relative abundance of Meeteetse fauna. NIE: number of identifiable elements, RA: relative abundance (% of total NIE), ENE: estimated number of contributing elements possible for each taxon (Estes and Berberian, 1970), MNI: minimum number of individuals.
Taxon NIE RA ENE MNI Lonchidion 10 .79 130 1 Myledaphus 427 33.62 400 2 Acipenser 2 .16 80 1 Kindleia 1 .08 170 1 Lepisosteus 353 27.80 200 2 Teleost 19 1.49 200 1 Plesiobaena 5 .39 190 1 Basilemys 55 4.33 190 1 Adocus 47 3.70 190 1 Aspideretes 63 4.96 190 1 Borealosuchus 222 17.48 200 2 Saurornitholestes 1 .08 Var. 1 Ornithischians 63 4.96 Var. 2 Mesodma 2 .16 84 1 Total 1270 100.00 18
57
Table 3: Relative abundance of vertebrates based on groups.
Groups of animals Relative abundance Elasmobranchs 34.41 Bony fish 29.53 Turtles 14.10 Crocodile 17.48 Dinosaurs 5.04 Multituberculate 0.16 Total percent 100.00
Elasmobranchs Bony fish Turtles Crocodile Dinosaurs Multituberculate
Figure 33: Relative abundance of vertebrates based on groups.
Referring to Tables 2 and 3, the Meeteetse fauna is highly aquatic. The dominance of Myledaphus and Lepisosteus is related to selective preservation of hard objects with regular shapes such as scales and pavement teeth. They represent more
58 than sixty percent of the Meeteetse microvertebrate fauna. Only elements more than half complete were counted as NIE (number of identifiable elements) to eliminate the possibility of counting the same individual elements several times. No squamate remains were obtained from the Elk Basin Meeteetse Formation site. Fragile lizard skeletons are hard to preserve and weather quickly through subaerial erosion (Estes,
1964). Mammalian remains are also rare. One of the reasons for this low occurrence of mammals is that isolated multituberculate teeth without roots are small enough to pass through the washing screen (Clemens, 1963). Numerous coprolites (including shark) were not included in the paleoecological analysis since it was not possible to examine their contents. The main reason for the low occurrences of particular taxa from the
Meeteetse Formation compared to those of Lance and Judith River Formation (Estes,
1964; Eberth and Brinkman, 1997) is due to limited time of sampling. The variation of sedimentary processes and environmental difference among these formations can be considered as well. Therefore, occurrences of certain animals of the Meeteetse
Formation may have been more common than it appears. However, the differences in faunal occurrences and abundances among sedimentologically similar sites may show original paleoecological differences, rather than preservational variation (Eberth, 1990).
Although the Meeteetse fauna is not as diverse as expected based on other
Late Cretaceous microvertebrate sites, the Elk Basin locality appears highly
59 fossiliferous, and holds the potential for discovery of rare squamates and mammalian taxa, as indicated by the discovery of a multituberculate tooth during this study. Due to limited collecting time, this study may not reveal the entire microvertebrate fauna of the
Elk Basin locality. Further study is needed to assess this important Late Cretaceous microvertebrate site.
Paleoecological analysis
Paleoecological interpretation of various fossil vertebrate assemblages provides useful information about community development through geologic time. The analysis must consider taphonomic changes between the original life assemblage and the fossil assemblage. Fossil assemblages can be preserved in or near their natural habitats. However, the original assemblages can be modified by a variety of factors including hydrodynamic sorting, physicochemical, biological and ecological effects
(Estes and Berberian, 1970; Dodson, 1973; Wolff, 1973, Behrensmeyer 1978; Korth,
1979; Fiorillo, 1991; Blob, 1997). Also the size and shape of the microfossils can vary even among localities from the same sedimentary facies (Blob and Fiorillo, 1996). In addition, emphasis on isolated teeth in paleoecological studies can produce a potential problem. Teeth can be reworked several times from old deposits and transported downstream by current. Isolated bones and articulated skeletons can also be affected by flowing water, and reworking from older sediments is possible for isolated bones
60 (Currie, 1990).
Paleoenvironment of the Meeteetse Formation based on sedimentology
The Meeteetse Formation was deposited in broad flood plains associated with ponds, lakes and fluvial environments that lay along the western edge of the Late
Cretaceous sea. Thinly bedded carbonate deposits of the Meeteetse Formation, with primarily aquatic faunas, suggest a lacustrine environment that changed over time. The presence of organic and fine silts suggests that currents were not significant in these lakes (Ash et al., 1978). Fine-grained and cross-bedded sandstones are thought to have been deposited in low-gradient river channels. Fine-grained and bioturbated sandstones indicate weakly developed paleosols and river channels that mark a short floodplain interval. Mudstones in the lower part of the Meeteetse Formation can be explained as a low-energy riverine flood basin, possibly associated with ponds or small ephemeral lakes (Difley & Ekdale, 2002).
Paleoecology of the Meeteetse Formation based on vertebrates
Selachians are the most sensitive paleoenvironmental indicators among fossil vertebrates from the latest Cretaceous and Paleocene. Selachian teeth are typically preserved in sandy estuarine and tidally influenced fluvial channel deposits including
Lonchidion and Myledaphus. Both Myledaphus and Lonchidion have relatively stout teeth that are not easily transported (Wroblewski, 2002). Although most hybodonts
61 appear in marine or brackish water, Lonchidion and Myledaphus, of the Meeteetse
Formation, show freshwater affinities based upon distribution in relation to other vertebrates in Late Cretaceous deposits of the Meeteetse and Lance formations (Table
4). According to Patterson (1966), the hybodont sharks of the Early Cretaceous avoided competition with more advanced marine sharks of similar niches by moving into freshwater where they had top predator status.
Table 4: Environmental preferences of selachian taxa in the Meeteetse Formation. Modified from Wroblewski (2004) Taxon Common name Salinity Climate Environment Dasyatids Stingrays Marine-Fresh Tropical-Temperate Shallow coasts, bays, estuaries, rivers, lakes Lonchidion N/A Marine-Fresh Tropical Estuaries, bays, lakes
Some of the Myledaphus teeth collected from the Meeteetse Formation have relatively shorter roots and more elongated and flatter crowns. Mechanically abraded and worn teeth indicate substantial transportation and reworking before they were buried. Recent dasyatids (stingrays) are primarily inhabitants of shallow coastal and estuarine environments, where they lie covered in sand or mud and feed on crustaceans, mollusks, and small fish.
The environment of the acipenserids is somewhat ambiguous, but the fossil record shows that most are from fresh and brackish water environments of coastal fluvial plains (Choudhury and Dick, 1998). Estes (1964) interpreted the depositional
62 environment of Acipenser eruciferus from the Upper Cretaceous Lance Formation as a
‘broad, lowland coastal plain’. This data and abundance of the fossil sturgeon remains from the Meeteetse Formation indicates the probable existence of a large river draining into a brackish-water bay.
The extant amiid Amia calva is very closely related to the Cretaceous Kindleia and lives in weedy, stagnant, sluggish and shallow freshwater environments. It is a highly predaceous fish and fossil Kindleia probably occupied a similar ecological niche as that of Amia.
Modern gars are able to tolerate brackish and nearshore salinities, although the alligator gar, Lepisosteus spatula, is restricted to freshwater. Gars usually inhabit shallow, weedy lakes and rivers. Wider distribution of fossil Lepisosteus indicates that gar were a more important fish in freshwater at that time (Gray, 1988).
The turtle Basilemys lived in semi-arid fluvial environments rather than a wet environment (Brinkman & Nicholls, 1993). In contrast, other turtles (Adocus and trionychids), crocodiles, fish and mollusks were particularly adapted to a wet environment. Based on these facts, the occurrence of Basilemys with aquatic animals can be interpreted as non significant paleoenvironment indicator (Difley and Ekdale,
2002). The long term accumulation under various energy conditions produced the fragmented shells and bone fragments of these turtles (Martin, 1999).
63 Some of the aquatic fauna such as crocodiles, soft-shell turtles and acipenserids are extant today. Crocodile, Trionyx (trionychids), Amia, Lepisosteus and
Acipenser appear together on the Gulf coastal plain, but there is no other geographic area where these genera occur together. The composition of the fossil aquatic fauna indicates that the best modern analogue of the humid coastal plain environment of the
Meeteetse Formation is that of the Gulf and south Atlantic coastal plains of the United
States (Estes, 1964).
64 Conclusions
1. The early Maastrichtian Meeteetse Formation was deposited in broad flood plains associated with ponds and small lakes in fluvial environments.
2. At least fifteen vertebrate taxa and more than one thousand elements were recovered from the Meeteetse Formation, Elk Basin, northern Wyoming.
3. The vertebrate fauna of the Meeteetse Formation was poorly known prior to this study. Wet screening and surface collecting have resulted in the recovery of a small but varied microvertebrate fauna consisting of fish, turtles, dinosaurs and mammals.
4. The preservational condition of fossil materials from the Meeteetse Formation site at Elk Basin is poor. All Meeteetse vertebrate material is disarticulated.
Fragmentary and heavily eroded skeletal remains make tentative identification difficult.
5. Myledaphus and Lepisosteus are the dominant fish taxa, occupying more than sixty percent of the Meeteetse microvertebrate fauna.
6. Most teleost centra from the Meeteetse Formation require further identification. The fish fauna may have been more diverse than it appears.
7. Amphibians and squamates were not recognized in this study, but they are abundant in other Late Cretaceous microvertebrate assemblages. Their fragile skeletal elements and limited time of sampling of this study may have been major reasons for
65 their absence in the Meeteetse microvertebrate fauna.
8. Four different turtle taxa are found and their presence indicates a freshwater environment, except for Basilemys which preferred a more arid environments.
9. The crocodile Borealosuchus is present, but no remains other than scutes and teeth were found.
10. A small theropod tooth is referred to Saurornitholestes. Hadrosaurs and ceratopsians are present, but the material is fragmentary.
11. A multituberculate molar and incisor were found and referred to Mesodma.
12. Sedimentological and ecological data indicate that the paleoenvironment of the Meeteetse Formation was a low current, shallow freshwater setting with a subtropical climate. Invertebrate fauna also supports this interpretation.
13. Further work is needed to recover the entire microvertebrate fauna from the highly fossiliferous Elk Basin locality.
66 References
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