Reevaluating the Miocene Mollusk Systematics, Little Cove Point Member, St. Marys Formation, and Examining Their Paleoecology

Home , Acteocina, Boreotrophon, Bulloidea, Busycotypus, Cardioidea, Carditida

REEVALUATING THE MIOCENE MOLLUSK SYSTEMATICS, LITTLE COVE POINT

MEMBER, ST. MARYS FORMATION, AND EXAMINING THEIR PALEOECOLOGY,

PALEOBIODIVERSITY.

A dissertation submitted

to Kent State University in partial fulfillment

of the requirements for the

degree of Doctor of Philosophy

Heedar Bahman

August 2021

Except for previously published materials

Dissertation written by

Heedar Bahman

B.S., Kuwait University, 2011

M.S., Kent State University, 2018

Ph.D., Kent State University, 2021

Approved by

Rodney M. Feldmann , Co-chair, Doctoral Dissertation Committee

Carrie E. Schweitzer , Co-chair, Doctoral Dissertation Committee

Joseph D. Ortiz , Members, Doctoral Dissertation Committee

Oscar J. Rocha

Accepted by

Daniel K. Holm , Chair, Department of Geology

Mandy Munro-Stasiuk , Interim Dean, College of Arts and Sciences

TABLE OF CONTENTS

TABLE OF CONTENTS ...... iii

LIST OF FIGURES ...... iv

ACKNOWLEDGMENTS ...... viii

SUMMARY ...... 1

INTRODUCTION ...... 2

GEOLOGICAL SETTING ...... 5

METHODS ...... 6

SYSTEMATIC PALEONTOLOGY ...... 9

RESULTS ...... 43

CONCLUSION ...... 50

APPENDIX ...... 51

FIGURES ...... 56

REFERENCES ...... 64

iii

LIST OF FIGURES

Figure

1 Collection locality of samples in Little Cove Point along the Chesapeake Bay in the Little Cove

Point Member of St. Marys Formation, Maryland, USA. Map adapted from Kidwell et al. (2015).

...... 56

2. A. Crassatella turgidula, internal view of right valve, CMM-I-4822. B. Crassatella

turgidula, external view of right valve, CMM-I-4822. C. Dallarca idonea, internal view of

right valve, CMM-I-4820. D. Dallarca idonea, external view of right valve, CMM-I-4820.

E. Mercenaria tetrica, internal view of right valve, CMM-I-4806. F. Mercenaria tetrica,

external view of right valve, CMM-I-4806. G. Dosinia thori, internal view of right valve,

CMM-I-4802. H. Dosinia thori, external view of right valve, CMM-I-4802. I. Spisula

subcuneata, internal view of right valve, CMM-I-4808. J. Spisula subcuneata, external view

of right valve CMM-I-4808. K. Corbula inaequalis, internal view of right valve, CMM-I-

4818. L. Corbula inaequalis, external view of right valve, CMM-I-4818. Scale bar = 1 cm ...... 57

3. A. Lirophora parkeria, internal view of left valve, CMM-I-4816. B. Lirophora

parkeria, external view of left valve, CMM-I-4816. C. Chesacardium laqueatum

laqueatum, internal view of left valve, CMM-I-4814. D. Chesacardium laqueatum

laqueatum, external view of left valve, CMM-I-4814. E. Callocardia subnasuta,

internal view of right valve, CMM-I-4800. F. Callocardia subnasuta, external view

of right valve, CMM-I-4800. G. Mercenaria cuneata, internal view of right valve,

CMM-I-4804. H. Mercenaria cuneata, external view of right valve, CMM-I-4804. I.

Chesapecten covepointensis, internal view of right valve, CMM-I-4812. J.

Chesapecten covepointensis, external view of right valve, CMM-I-4812. K.

Chesapecten santamaria, internal view of right valve, CMM-I-4810. L. Chesapecten

santamaria, external view of right valve, CMM-I-4810. Scale bar = 1 cm ...... 58

4. A. Ilyanassa marylandica, adapertural view, CMM-I-4827. B. Ilyanassa

marylandica, apertural view, CMM-I-4827. C. Turritella chancellorensis, apertural

view, CMM-I-4873. D. Turritella chancellorensis, adapertural view, CMM-I-4873.

E. Acteon ovoides, adapertural view, CMM-I-4879. F. Acteon ovoides, apertural

view, CMM-I-4879. G. Turrifulgur fusiformis, adapertural view, CMM-I-4830. H.

Turrifulgur fusiformis, apertural view, CMM-I-4830. I. Crucibulum constrictum,

apical view, CMM-I-4859. J. Crucibulum constrictum, apertural view, CMM-I-

4859. K. Crucibulum multilineatum, apical view, CMM-I-4861. L. Crucibulum

multilineatum, apertural view, CMM-I-4861. Scale bar = 1 cm ...... 59

5. A. Crucibulum pileolum, apical view, CMM-I-4857. B Crucibulum pileolum,

adapertural view, CMM-I-4857. C. Laevihastula sublirata, adapertural view, CMM-

I-4849. D. Laevihastula sublirata, apertural view, CMM-I-4849. E. Sediliopsis

angulata, adapertural view, CMM-I-4845. F. Sediliopsis angulata, apertural view,

CMM-I-4845. G. Urosalpinx subrusticus, adapertural view, CMM-I-4839. H.

Urosalpinx subrusticus, apertural view, CMM-I-4839. I. Sinum fragilis, apical view,

CMM-I-4867. J. Sinum fragilis, apertural view, CMM-I-4867. K. Cymatosyrinx

pyramidalis, apertural view, CMM-I-4841. L. Cymatosyrinx pyramidalis,

adapertural view, CMM-I-4841. Scale bar = 1 cm ...... 60

6. A. Propebela parva, apertural view, CMM-I-4851. B Propebela parva, adapertural

view, CMM-I-4851. C. Laevihastula simplex, apertural view, CMM-I-4847. D.

Laevihastula simplex, adapertural view, CMM-I-4847. E. Bulliopsis marylandica,

adapertural view, CMM-I-4823. F. Bulliopsis marylandica, apertural view, CMM-I-

4823. G. Hemipleurotoma communis, adapertural view, CMM-I-4843. H.

Hemipleurotoma communis, apertural view, CMM-I-4843. I. Ecphora gardnerae

germonae, apertural view, CMM-I-4835. J. Ecphora gardnerae germonae,

adapertural view, CMM-I-4835. K. Turritella subvariabilis, adapertural view, CMM-

I-4877. L. Turritella subvariabilis, apertural view, CMM-I-4877. Scale bar = 1 cm ...... 61

7. A. Epitonium sayanum, adapertural view, CMM-I-4869. B Epitonium sayanum,

apertural view, CMM-I-4869. C. Retusa marylandica, apertural view, CMM-I-4877.

D. Retusa marylandica, adapertural view, CMM-I-4877. E. Acteon shilohensis,

adapertural view, CMM-I-4880. F. Acteon shilohensis, apertural view, CMM-I-4880.

G. Boreotrophon laevis, adapertural view, CMM-I-4833. H. Boreotrophon laevis,

apertural view, CMM-I-4833. I. Typhis acuticosta, adapertural view, CMM-I-4839.

J. Typhis acuticosta, apertural view, CMM-I-4839. K. Astyris communis, apertural

view, CMM-I-4828. L. Astyris communis, adapertural view, CMM-I-4828. Scale bar

= 1 cm ...... 62

8. A. cancellaria alternata, adapertural view, CMM-I-4853. B cancellaria alternata,

apertural view, CMM-I-4853. C. Crepidula fornicata, adapertural view, CMM-I-

4855. D. Crepidula fornicate, apertural view, CMM-I-4855. E. Neverita discula,

apical view, CMM-I-4865. F. Neverita discula, apertural view, CMM-I-4865. G.

Euspira heros, apical view, CMM-I-4863. H. Euspira heros, apertural view, CMM-

I-4863. I. Mariacolpus covepointensis, adapertural view, CMM-I-4871. J.

Mariacolpus covepointensis, apertural view, CMM-I-4871. K. Ilyanassa peralta,

adapertural view, CMM-I-4825. L. Ilyanassa peralta, apertural view, CMM-I-4825.

Scale bar = 1 cm ...... 63

9. A. Busycotypus rugosum, adapertural view, CMM-I-4831. B Busycotypus rugosum,

apertural view, CMM-I-4831. C. Dentalium caduloide, side view, CMM-I-4882. Scale

bar = 1 cm ...... 64

10. Trophic levels of Molluscan community (bivalves, gastropods, and scaphopods) of extant

Chesapeake Bay fauna extant and Little Cove Point Member fossil fauna ...... 65

vii

ACKNOWLEDGEMENTS

I would like to thank all those who helped, supported and guided me through my Ph.D. and research. First and foremost I would like to thank my co-advisor Rodney Feldmann for guiding me through my dissertation research, helping me through the difficult times we live in, and supporting me emotionally. He provided me with knowledge and means for success. Also, I would like to thank co-advisor Dr. Carrie Schweitzer, for being there for me through the pandemic.

I would like to thank John Nance from Calvert Marine Museum, Solomons, Maryland, for providing the fossils. Special thanks goes to my committee members Dr. Joseph Ortiz and

Dr. Oscar Rocha for taking the time to teach me. I would also like to thank the administrative secretary of the Geology Department, Kelly Thomasson. My utmost gratitude goes to my mother, wife and three daughters for supporting me and being part of my journey in the last five years.

viii

SUMMARY

The Little Cove Point Member belongs to the St. Marys Formation in Maryland, which is part of the Miocene Chesapeake Group. The objective of the research is to reevaluate the

Miocene mollusk systematics, paleoecology, and paleobiodiversity of the Little Cove Point

Member. Two sets of mollusk fossils were provided by the Calvert Marine Museum that represent bulk, samples of studied materiel taken from one l cubic meter area, and grab samples, samples of studied material taken from one or more small area. One set of data was obtained from the Chesapeake Bay Benthic Monitoring Program that include Holocene Mollusca fauna of the bay. The bulk sample includes 35 genera, the grab sample include 20 genera while the modern sample have 21 described genera. Four species were recorded for the first time in the

Little Cove Point Member in this collection, Chesapecten santamaria, Chesacardium laqueatum laqueatum, Lirophora parkeria, and Spisula subcuneata. The paleobiodiversity was measured using the Shannon index (H). Then richness, the number of different species represented in an ecological community and evenness (E), the measure of the relative abundances of species within a community, of the species was calculated. The similarity between the community of mollusks was tested using Sorensen’s coefficient. The bulk sample has higher biodiversity index than the extant fauna, while the grab sample has a lower biodiversity than the bulk sample. Bulk sampling provides a better representation of biodiversity measurements than a grab sample.

INTRODUCTION

The St. Marys Formation is exposed on the eastern coast of the United States. It is a well- known Miocene rock unit containing more than 600 species of marine organisms—both vertebrate and invertebrate fossils, including diatoms, dinoﬂagellates, foraminiferans, sponges, corals, polychaete worms, abundant and diverse mollusks, ostracods, decapod crustaceans, barnacles, brachiopods, echinoderms, sharks, rays, bony ﬁshes, turtles, crocodiles, pelagic birds, seals, sea cows, odontocetes, and mysticetes as well as isolated and fragmentary remains of large terrestrial mammals, palynomorphs, and land plants (Clark et al., 1904; Gottfried et al.,1994;

Eshelman et al., 2007; Godfrey and Barnes, 2008; Ward and Andrews, 2008). Mollusca are a large diverse group of invertebrates with over 50,000 living species and 35,000 fossil species

(Levin, 1999). They made their first appearance in the marine environment of the Early

Cambrian period. Since then they have evolved into a variety of sizes and shapes and successfully occupied a wide range of environments from marine to freshwater and terrestrial habitats. Molluscan fossils have been used to interpret paleoenvironments as well as biostratigraphy. Case et al. (1904) studied the Miocene fossils of Maryland and described the vertebrate fauna, including mammals, birds, reptiles, and fish. They also examined various phyla of invertebrates, including arthropods, brachiopods, echinoderms, protozoans, and mollusks.

Clark et al. (1904) studied the Miocene deposits of Maryland and described each formation and indicated the abundant fossils within them. Shattuck (1904) presented a detailed

description of the St. Marys Formation and divided the formation into members. He also examined the distribution of various fossils, including mollusks. Marvin L. Wass (1972) compiled a checklist of the biota of the lower Chesapeake Bay. Ward (1992) used Miocene mollusks of the East Coast for biostratigraphic correlation. He selected 145 species, of which 35 were determined to be new species or subspecies. Seven belonged to new genera. Petuch and

Drolshgen (2009) studied the Molluscan assemblages of the Chesapeake Group, which includes the St. Marys Formation. They also presented a detailed description of the geological sitting of the Chesapeake Group.

Biodiversity is the study of the distribution of species in ecology. It can be measured by assessing alpha biodiversity, which is the average species diversity in a specific habitat. When the biodiversity measurement is done for species between two habitats, then it is referred to as beta biodiversity. Those concepts can be applied to the fossil record to assess the paleobiodiversity. Sheehan (1991) used the Shannon index to measure the biodiversity of dinosaur fossils from the Late Cretaceous. He concluded that there was no gradual drop in biodiversity in the Hell Creek Formation to indicate an abrupt extinction event. Powell and

Kovalewski (2002) had a new approach to study the biological diversity of the Phanerozoic.

They incorporated abundance data to evaluate Phanerozoic trends in species evenness. Holland

(2010) introduced a new method for calculating diversity patterns for richness. This method allows the contribution of each species to alpha and beta diversity. He also discussed the use of different methods to measure biodiversity. Fengiang Li et al. (2014) used the Shannon index to calculate the biodiversity of terrestrial mollusks spanning from the late Miocene to Pliocene (7.1-

3.5 Ma). This allowed them to track changes in the fossil assemblages to decipher late Miocene-

Pliocene paleoclimate changes. Selly et. al. (2018) used the Shannon index to measure the

diversity of Mollusca from Pigeon Creek in San Salvador, Bahamas, to look at the influence of environmental gradients—gradual changes in physical, chemical, and biological variables—on mollusks’ diversity, body size, and predator-prey interactions to assess the role of gradients at a single horizon. Gardiner (2001) used the Shannon index to measure the biodiversity of

Pleistocene reef mollusks and recent reef mollusks to assess the stability of Cenozoic reefs. Thus, the Shannon index is an ideal measure of paleodiversity of mollusks from the Little Cove Point

Member.

The primary work in this research will be focused on two samples sent by Mr. John

Nance, Calvert Marine Museum. It is a concentrate from one member of the St. Marys Formation that provides an opportunity to do statistical work. The study will also involve collecting grab samples and comparing the results of biodiversity between the grab samples and the screened samples. The objective of the research is to reevaluate the Miocene mollusk systematics, paleoecology, and paleobiodiversity of the fossils from the Little Cove Point Member of the St.

Marys Formation. The work on paleobiodiversity will allow comparison with recent biodiversity.

Comparing the Miocene and Holocene faunas is important in several aspects. For the purposes of conservation paleobiology, species present in the Miocene and extinct in the Holocene will be identified along with possible causes for their extinctions. The change in communities will be examined to determine why the changes occurred.

GEOLOGICAL SETTING

The St. Marys Formation in Maryland is part of the Miocene Chesapeake Group which is located in the central part of the Atlantic coast (Figure 1). It overlies the Choptank Formation and underlies the Eastover Formation. It is divided into three members, Conoy, Little Cove Point and Windmill Point. The Little Cove Point Member was deposited in a transgression period in a shallow inner bay setting (Ward 1992). This member consists of three beds according to

Shattuck (1904). Bed 21 consists of sand and clay interbeds. Bed 22 is sandy clay with abundant fossils. Bed 23 is composed of sandy clay with scattered fossils. Ward (1992) noted several mollusk shell beds. This suggests that the area was influenced by short marine pulses which changed the environment to an open shelf condition. For detailed geological settings refer to

Bahman et al. (2020).

METHODS

This study is based upon two collections of mollusk fossils that were provided by John

Nance from the Calvert Marine Museum in Maryland. The collections include bivalve and gastropod fossils that were preserved in the Miocene St. Marys Formation, Little Cove Point

Member (Figure 1). The first collection represents a bulk sample. It was very carefully washed at

Kent State University to concentrate the mollusks and other megafauna from a cubic meter of sediment. The second collection represents a grab sample. It was carefully washed to concentrate the mollusks from the sediment. This research is divided into two parts. The first part includes describing and identifying the fossils and updating the systematics of the described specimens.

The second part includes measuring the paleobiodiversity and comparing the diversity measured from grab samples and a bulk sample. The hypothesis is that the bulk sample will more completely reflect the molluscan diversity of the Little Cove Point Member.

Specimens were sorted for examination and description. Measurements of the fossils were taken and recorded using a vernier caliper. All measurements are given in millimeters. The fossils were photographed using a Nikon D3100 digital camera with AF-S Micro-Nikkor 60 mm lens. Small specimens were examined under the microscope to describe the shell morphology.

Examine of the literature and comparison of descriptions of already identified species confirmed identification of the fossils.

The alpha and beta paleobiodiversity was measured using diversity indexes. Alpha diversity was determined using the Shannon index (H) using the following formulas:

푠 H = − ∑푖 = 1(푝푖 퐼푛푝푖)

Pi is the proportion of the n/N. The n is the number of individuals in one particular species, and

N is the total number of individuals in all species. S is the number of species in the sample. The distribution and evenness (E) of the species was tested using the Shannon index, where

Hmax = ln(S)

Evenness = H / Hmax

The null hypothesis was tested with the two Shannon diversity indices, that from the bulk sample and that from the recent sample. A modified t-test formula was used.

2 2 2 2 s = ( ni (log ni) – [ ni log ni] )/ N + (S-1)/2N

2 2 sd = (s 1 + s 2)

t = H1 – H2 / sd

Also, the degrees of freedom in the t-test are calculated with the following formula:

2 2 2 (s 1 + s 2) DF = ______2 2 (s 1) / N1 + (s 2) / N2

Data from previously published work on St. Marys Formation was analyzed, followed by a comparison of community composition between the Little Cove Point Member and the extant molluscan fauna. The similarity between the community of mollusks was tested. Sorensen’s coefficient to measure community similarity was determined. It was also used to determine at the similarity between the bulk sample and grab sample.

SC = 2C / S1 + S2

C is the number of species the two communities have in common. S1 is the total number of species found in community one (the bulk sample). S2 is the total number of species found in community two (the grab sample). Sorensen’s coefficient gives a value from zero to one. The

closer the value is to one, the more the communities have in common. This comparison will be useful in many aspects. For the purposes of conservation paleobiology, the change in species composition of the mollusks in the Miocene and Holocene will be identified. Those changes will be examined to determine the probable cause of a shift in species composition.

SYSTEMATIC PALEONTOLOGY

Abbreviations: CMM-I, Invertebrate collection of the Calvert Marine Museum, Solomons,

Maryland, USA.

Class MOLLUSCA Linnaeus, 1758

Order VENERIDA Gray, 1854

Superfamily VENEROIDEA Rafinesque, 1815

Family VENERIDAE Rafinesque, 1815

Genus Callocardia A.Adams, 1864

Callocardia subnasuta Conrad, 1841

(Figs. 3E, 3F)

Description: "Trigonal, thin, ventricose; anterior side narrowed, slightly produced and subangulated at the extremity; surface with rather prominent concentric wrinkles; posterior margin obliquely arched; beaks distant from anterior extremity, and not nearly central; length

11/8 inch. Allied to C. sayana, but is proportionally longer, less ventricose, narrowed, and more produced anteriorly" (Conrad, 1841: 28).

Emendation to description: Beak positioned anteriorly; growth lines more prominent further from beak. Length equals height. Lunule, escutcheon not very well defined. Pallial sinus moderately deep, U-shaped. Two prominent cardinal teeth. One lateral anterior tooth. Inner shell margin smooth.

Material examined: CMM-I-4800, CMM-I-4801.

Remarks: The species is found in the St. Marys Formation, Calvert Formation, and Choptank

Formation. Only four individuals are identified. All specimens have the entire shell preserved.

The shells are small, and the height is approximately ranging from 7.5-19 mm.

Genus Lirophora Conrad, 1863

Lirophora parkeria Glenn, 1904

(Figs. 3A, 3B)

Description: “Shell triangular, depressed, posteriorly somewhat cuneiform, anteriorly rounded; beaks projecting, acute, approximate; lunule distinct, cordate; base posteriorly emarginate; dorsal surface with about five to eight concentric ribs so perfectly flattened and closely appressed to the valve and each other as to become almost obsolete and be marked only by faint undulations and fine concentric impressed or laminated lines; ribs crossed from beak to base by numerous distinct, regular, radiating lines; cardinal teeth three in each valve; laterals none; muscle impressions deep; pallial sinus a slight notch; margin minutely crenulated” (Glenn, 1904: 310).

Emendation to description: Beak positioned anteriorly. Growth lines visible, uniform throughout shell. Lunule wide, distinct. Escutcheon obsolete. Pallial line visible, not very distinct. Pallial sinus small, not well defined. Muscle scars small, well defined. Three cardinal teeth. Anterior tooth smallest. Median tooth, thick, medium in size. Posterior tooth longest. Inner margin with fine serration. Length slightly greater than height.

Material examined: CMM-I-4816, CMM-I-4817.

Remarks: This species was previously only found in the Calvert Formation. This is the first recorded occurrence of this species in the Little Cove Point Member, St. Marys Formation. Six individuals are identified. The shells are small, and the height is approximately ranging from

13.9-20.6 mm. They are partially preserved and fragile.

Genus Dosinia Scopoli, 1777

Dosinia thori Ward, 1992

(Figs. 2G, 2H)

Diagnosis: “Shell moderately large, discoidal, moderately inflated, with numerous small concentric grooves, at first very regular, becoming somewhat irregular (Ward, 1992: 101)

Description: “Shell moderately large, somewhat friable, nearly discoidal in outline, prosocline, moderately inflated, and thin. Brown coloration marks remnants of periostracum in many specimens. Exterior covered by numerous, finely impressed, concentric grooves that are very regular in the young stages and become more distant and irregular in the adult. Medially the grooves are less well defined and tend to fuse in the adults. Posterior slope very weakly rounded, remainder of margin nearly evenly rounded. Lunule small, thin, impressed, escutcheon absent, beak sharp, umbones subcentral. Interiorly, ligamental area large, elongate, sickle-shaped, striated. Resilial pit small, moderately deep, oval in shape. Hinge plate broad, somewhat straight and moderately heavy. Teeth in the right valve (holotype, USNM 405274) consist of a very thin, small anterior cardinal, a strong medial cardinal, proximal to the anterior cardinal, and a stronger, obliquely angled, somewhat bifid, elongate posterior cardinal. Teeth in the left valve are divergent and widely separated. Very small anterior lateral tooth, only a small protuberance in the left valve and a shallow socket in the right valve. Muscle scars distinct, pallial line less so.

Anterior adductor muscle scar large, roundly lanceolate. Posterior scar larger, ovate. Pallial line weakly impressed and somewhat distant from the ventral margin. Pallial sinus narrow, sharply

V-shaped. Interior surface smooth except for low linear undulations within the area delineated by the pallial line” (Ward, 1992: 101).

Emendation to description: Beak positioned posteriorly. Inner margin of shell smooth. Length equals height.

Material examined: CMM-I-4802, CMM-I-4803.

Remarks: The species is found in the Little Cove Point and Windmill Point members of the St

Marys Formation. Thirty-two individuals are identified. Shells are partially preserved. They are large, and the height is approximately 54.7 mm.

Genus Mercenaria Schumacher, 1817

Mercenaria cuneata Conrad, 1869

(Figs. 3G, 3H)

Description: "Subtriangular, ventricose medially, slightly flattened or contracted above the umbo; outline of the disk nearly straight below the middle; surface with coarse concentric lines; posterior side cuneiform, lower half of posterior margin nearly rectilinear, extremity subacute; inner margin minutely crenulated.

" This species may be distinguished from M. mercenaria in being less oblique, proportionally shorter and more acute at the posterior extremity, and in having a more elongated anterior cardinal tooth" (Conrad, 1868: 139).

Emendation to description: Shell large. Beak positioned anteriorly. Growth lines more prominent distally from beak. Length slightly greater than height. Lunula large, impressed. Escutcheon obsolete. Pallial sinus shallow, V-shaped, small. Inner margin dentate. Three cardinal teeth.

Posterior tooth largest, elongated. Median tooth large. Anterior cardinal tooth reduced in size.

Anterior lateral tooth not well defined.

Material examined: CMM-I-4804, CMM-I-4805.

Remarks: The species was only found in the Calvert Formation and Choptank Formation. Ward

(1992) suggested that fossils found in the Little Cove Point Member of the St. Marys Formation are a subspecies of Mercenaria cuneata based on their size. The characters Ward used is not sufficient enough to support his statement. Eleven individuals are identified. Shells are either partially preserved or entirely preserved. They range in size from large to small, and the height is approximately ranging from 11.1-81.6 mm. The shells exhibit borehole marks from predation.

This species differs from Mercenaria tetrica by having a large lunule and reduced anterior cardinal tooth.

Mercenaria tetrica Conrad, 1838

(Figs. 2E, 2F)

Description: "Shell triangular, cordate, ventricose, moderately thick, with crowded concentric very prominent laminae; posterior side subcuneiform, extremity angulated; summits very prominent; lunule defined by a deeply impressed line.

" This shell has nearly the outline of V. mercenaria, but may be distinguished by its very prominent laminse of nearly equal elevation on every portion of the disk" (Conrad, 1838: 7).

Emendation to description: Beak positioned anteriorly. Lunule small. Escutcheon obsolete.

Ventral margin dentate. Pallial line impressed, pallial sinus short, V-shaped. Three cardinal teeth.

Posterior and median teeth largest. Anterior tooth long, thin. Length slightly greater than height.

Material examined: CMM-I-4806, CMM-I-4807.

Remarks: This species is only found in the Windmill Point Member. Ward (1992) assigned fossils from the Little Cove Point Member to subspecies of Mercenaria tetrica based on shell thickness. The characters Ward used is not sufficient enough to support his statement. Only 9 individuals were identified. Shells are well preserved. They range in size from large to small, and

the height is approximately ranging from 6.4-51.1 mm. This species differs from Mercenaria cuneata by having a small lunule, and the anterior cardinal tooth is long.

Superfamily MACTROIDEA Lamarck, 1809

Family MACTRIDAE Lamarck, 1809

Genus Spisula Gray, 1837

Spisula subcuneata Conrad, 1838

(Figs. 2I, 2J)

Description: "Shell subtriangular, thin, inequilateral, obscurely and transversely striate; beaks somewhat pointed; lateral teeth crossed by equidistant minute striae; excavation of the pallial impression small and rounded; anterior and posterior cicatrices scarcely visible; cavity of the shell somewhat deep; cavity of the beaks rather deep" (Lea, 1833: 212).

Emendation to description: Beak positioned anteriorly. Growth lines impressed. Lunule and escutcheon obsolete. Hinge with two cardinal teeth. Anterior tooth longest, thin. Posterior tooth short, thin. Two lateral teeth serrated. Anterior lateral tooth short. Posterior lateral tooth long.

Inner margin smooth. Length slightly greater than height.

Material examined: CMM-I-4808, CMM-I-4809.

Remarks: This species was previously only found in the Little Cove Point Member. About 1522 individuals are identified. The fossils are very fragile and not well preserved. Shells are small to medium, and the height is approximately ranging from 5.2-15.7 mm.

Order PECTINIDA Gray, 1854

Superfamily PECTINOIDEA Rafinesque, 1815

Family PECTINIDAE Wilkes, 1810

Genus Chesapecten Ward and Blackwelder, 1975

Chesapecten santamaria Tucker, 1934

(Figs. 3K, 3L)

Description: “Shell equilateral, suborbicular, inequivalved. Radial sculpture of 12 to 16, most commonly 12 to 14, broad, well elevated ribs which are approximately 4 mm. narrower, at the ventral margin, than the interspaces. Right valves commonly have about 12 ribs which are about

2 mm. narrower than the interspaces. The right valves are flat, or nearly so, and resemble C. madisonius. Say very closely in outline; left valve convex in the umbonal region; auricles subequal, radial sculpture of fine, scaly threads, the anterior the more coarsely threaded; fasciole broad. Auricles of the left valve subequal, uniformly sculptured with fine scaly threads. Summits and sides of the ribs threaded with numerous fine, scaly threads. Interspaces similarly ornamented; one or two of the threads are usually more strongly developed, especially on the right valve. Submargins have a fine uniform sculpture of closely spaced, scaly threads. Interior ribbed to the umbones. Provinculum present. Ctenolium retained throughout the life of the individual” (Tucker 1934: 615).

Emendation to description: Beak positioned medially. Pallial sinus shallow, not well defined.

One muscle scar, not well defined. Five teeth like serration on anterior dorsal margin. Length and height equal in size.

Material examined: CMM-I-4810, CMM-I-4811.

Remarks: This species was previously only known from the Windmill Point Member. This is the first known occurrence in the Little Cove Point Member. Twenty individuals are identified.

Shells are partially preserved, and some are entirely preserved. The shells range in size from small to large, and the height is approximately ranging from 9.1-73.1 mm. This species differs from Chesapecten covepointensis by having fewer of ribs, 12-16.

Chesapecten covepointensis Ward, 1992

(Figs. 3I, 3J)

Diagnosis: “Valves medium in size, sub-orbicular with small auricles, a deep byssal notch, and

17 very scaly ribs radiating from a low beak.”

Description: “Valves medium-sized for the genus. nearly orbicular in outline. Auricles small for the size of the disk, reflecting the shorter hinge. Ribs averaging 17, with numerous raised scales both on the ribs and in the interspaces. Three rows of scales on each rib and their interspaces in adult forms. Left valve only slightly convex, right valve even less convex. Byssal fasciole on the right valve moderately deep with two small teeth on the active area of the ctenolium on the holotype (USNM 405199). Right valve with scales better developed and low umbonal area. Beak prominent. Valves very thin with ribs clearly reflected on interior of shell except in umbonal region above the muscle scar. Muscle scar weakly impressed and somewhat posterior to the midline” (Ward, 1992: 70).

Emendation to description: Beak positioned medially. Pallial sinus shallow, not well defined.

Muscle scar not well outlined. Three to five toothlike serrations on anterior dorsal margin.

Height slightly greater than length.

Material examined: CMM-I-4812, CMM-I-4813.

Remarks: This species is only found in the Little Cove Point Member. Only 15 individuals are identified. The shells are well preserved and small to medium in size, and the height is approximately ranging from 10-20.8 mm. This species differs from Chesapecten santamaria by having 17 scaly ribs.

Order CARDIIDA Ferussac, 1822

Superfamily CARDIOIDEA Lamarck, 1809

Family CARDIIDAE Lamarck, 1809

Genus Chesacardium Ward, 1992

Chesacardium laqueatum Conrad, 1830

Subspecies Chesacardium laqueatum laqueatum Ward, 1992

(Figs. 3C, 3D)

Description: “Shell subovate, inequilateral elongated posteriorly where it is obliquely truncated, the beak is placed anteriorly. Two cardinal teeth, the anterior one very thick, and the other, located posteriorly, thin. The deep socket between the cardinal teeth triangular in shape; the anterior lateral tooth larger than the posterior. Ligamental groove located posteriorly to the beak, short. Impressions of the anterior and posterior muscle scars shallow. Because the entire shell is very thin, the radial ribs and the furrows which form the exterior sculpture of the shell are marked on the interior surface, especially along the External sculpture of the shell formed wide and flat radial ribs, 43 to 46 in number, separated by very narrow interradial furrows. Ten of the ribs located on the posterior obliquely truncated side of the shell and the other 33 to 36 ribs on the convex surface of the shell. In addition to the radial ribs and the furrows, the surface of the shell is also ornamented by fine, rugose, concentric growth lines” (Oleksyshyn, 1959: 31).

Emendation to description: Lunule and escutcheon obsolete. Pallial line and pallial sinus not well defined. Muscle scar impression small. Anterior cardinal tooth larger than posterior cardinal tooth. Inner marginal surface marked by grooves that represent outer ribs. Length slightly greater than height.

Material examined: CMM-I-4814, CMM-I-4815.

Remarks: This species was previously only known from the Windmill Point Member. This is the first known occurrence in the Little Cove Point Member. Only 62 individuals are identified.

Shells are well preserved. The shells are small to large in size, and the height is approximately ranging from 5.8-45.4 mm.

Order MYIDA Satoliczka, 1870

Superfamily MYOIDEA Lamarck, 1809

Family CORBULIDAE Lamarck, 1818

Genus Corbula Bruguière, 1797

Corbula inaequalis Say, 1824

(Figs. 2K, 2L)

Description: "Shell convex, transversely ovate-trigonal, rough, with unequal coarse wrinkles: anterior margin with a very acute but short rostrum at its inferior termination, separated from the disk by an acute line: base rounded and a little contracted near the anterior angle: umbones not prominent" (Say, 1824: 153).

Emendation to description: Beak positioned medially. Growth lines more distinct toward margin.

Lunule and escutcheon obsolete. One cardinal tooth, large. Pallial line and pallial sinus not well defined. Muscle scars small. Inner margin smooth. Length slightly greater than height.

Material examined: CMM-I-4818, CMM-I-4819.

Remarks: The species is known in Miocene formations along the east coast south to Florida.

Only 11 individuals are identified. Shells are small, and the height is approximately ranging from

4.5-6.7 mm. They are well preserved.

Order ARCIDA Stoliczka, 1871

Superfamily ARCOIDEA Lamarck, 1809

Family ARCIDAE Lamarck, 1809

Genus Dallarca Ward, 1992

Dallarca idonea Conrad, 1832a

(Figs. 2C, 2D)

Description: Cordate, inequivalve, ventricose, and slightly sinuous; ribs about 25, narrow and crenulated; the crenulations most distinct on the larger valve; beaks very prominent and distant; area with undulated grooves; hinge with the series of teeth contracted in the center, and a little decurved at the ends" (Conrad, 1832: 16).

Emendation to description: Beak directed anteriorly. Growth lines not visible. Lunule and escutcheon obsolete. Hinge with row of small teeth, increases in size toward anterior and posterior margin. Pallial line shallow. Muscle scar small, distinct. Shell margin dentate. Length greater than height.

Material examined: CMM-I-4820, CMM-I-4821.

Remarks: This species only occurs in the Little Cove Point Member. Only 23 individuals are identified. Shells are well preserved. They range in size from small to large, and the height is approximately ranging from 6.2-42.2 mm.

Order CARDITIDA Dall, 1889

SUPERFAMILY CRASSATELLOIDEA Férussac, 1822

Family CRASSATELLIDAE Férussac, 1822

Genus Crassatella Ward and Blackwelder, 1987

Crassatella turgidula Conrad, 1843

(Figs. 2A,2B)

Description: "Oblong-ovate, slightly ventricose; surface with coarse lines of growth, and concentric undulations obsolete except on the umbones, where they are strongly marked and wide; beaks submedial; umbones flattened; anterior dorsal margin straight; posterior extremity truncated and nearly direct, more oblique in young shells; basal margin swelling a little anteriorly, posteriorly straight to the extremity which is obliquely angulated" (Conrad, 1843:

307).

Emendation to description: Lunule wide, well defined. Inner ventral margin smooth. Pallial line visible, shallow. Muscle scars distinct, small. HingE with two cardinal teeth, both located anteriorly. First tooth thick, large. Second tooth smaller. Two lateral teeth. Anterior lateral tooth small. Posterior lateral tooth long, wide. Length greater than height.

Material examined: CMM-I-4822.

Remarks: This is the first known occurrence of this species in the St. Marys Formation, Little

Cove Point Member. Only 1 individual is identified. The shell is completely preserved. The shells are large, and the height is approximately 30.5 mm.

Class GASTROPODA Cuvier, 1797

Order NEOGASTROPODA Wenz, 1938

Superfamily BUCCINOIDEA Rafinesque, 1815

Family NASSARIIDAE Iredale, 1916

Genus Bulliopsis Conrad, 1862

Bulliopsis marylandica Conrad, 1862

(Figs. 6E, 6F)

Description: “Oblong-ovate, entire; whorls 6, slightly convex or sub-truncated laterally; suture impressed; aperture about half the length of the shell; columella profoundly callous above, the callus extending beyond the lip” (Conrad, 1862: 287).

Emendation to description: Whorls 5-6, smooth. Apical angle 66o, outer lip straight. Curved near base. Inner lip flat, impressed.

Material examined: CMM-I-4823, CMM-I-4824.

Remarks: This species is only found in Miocene formations. Only 60 individuals are identified.

Some of the fossils are partially preserved while others are completely preserved. The shells are medium in size, and the axial length is approximately ranging from 19.2-26.4 mm.

Genus Ilyanassa Stimpson, 1865

Ilyanassa peralta Conrad, 1868

(Figs. 8K, 8L)

Description: “Elongate turrited, whorls 8, longitudinally ribbed and with revolving impressed lines, about 5 in number on the penultimate volution; above near the suture on all the whorls there is a broader impressed line, which divides the ribs and forms a tuberculous ridge around the summits of the whorls; ribs narrow, numerous; spire acuminate" (Conrad, 1868: 264).

Emendation to description: Suture line distinct. Aperture oval. Siphonal notch short. Inner lip smooth. Apical angle 42o.

Material examined: CMM-I-4825, CMM-I-4826.

Remarks: The species is found throughout the St. Marys Formation. It is the second most commonly identified fossil species, 2309, in the collection. Shells are well preserved and small in size, and the axial length is approximately ranging from 6.5-15.1 mm. The shells exhibit borehole marks. This species differs from I. marylandica in having more ribs that are closely spaced.

Ilyanassa marylandica Martin, 1904

(Figs. 4A, 4B)

Description: “Shell small, solid, elongate (except as noted below ), eight-whorled ; body whorl with 9 to 15 distant raised longitudinal ribs which extend to the suture without any constriction or subsutural tubercles, and with 25 to 30 faint revolving lines of uniform strength which never override the ribs ; whorls of the spire with about 10 revolving lines” (Martin, 1904: 194).

Emendation to description: Aperture round. Suture lines impressed. Siphonal canal short, curved.

Inner lip smooth. Apical angle 29o.

Material examined: CMM-I-4827.

Remarks: This species is most abundant in the Little Cove Point and Windmill Point members of the St. Marys Formation. Only 4 individuals are identified. The shells are small in size, and the axial length is approximately ranging from 8.7-10.3 mm. They are well preserved. This species differs from Ilyanassa peralta in having fewer ribs.

Family COLUMBELLIDAE Swainson, 1840

Genus Astyris H.Adams & A.Adams, 1853

Astyris communis Conrad, 1862

(Figs. 7K, 7L)

Description: “Small, whorls six or seven, smooth and polished; spire rather elevated; body whorl abruptly rounded in the middle, or subangular; submargin of labrum minutely dentate” (Conrad,

1862: 287).

Emendation to description: Apical angle 36 o, revolving lines around siphonal notch. Outer lip dentate. Inner lip small, dentate.

Material examined: CMM-I-4828, CMM-I-4829.

Remarks: The species ranges from the Miocene into the Pliocene. Only 212 individuals are identified. Shells are small in size, and the axial length is approximately ranging from 11.4-12.4 mm. They are well preserved. Some individuals lack the outer lip dentation.

Family BUCCINIDAE Rafinesque, 1815

Genus Turrifulgur Petuch, 1988

Turrifulgur fusiformis Conrad, 1840

(Figs. 4G, 4H)

Description: “Shell fusiform, with spiral striae, obsolete, except on the inferior half of the body whorl, where they are prominent, wrinkled, and alternated in size; spire elevated, whorls with obtuse little prominent tubercles at the angle, which is situated near the suture, and is obtuse”

(Conrad, 1840: 80).

Emendation to description: Five whorls. Faint revolving lines on whorls. Suture lines impressed.

Aperture wide, ovate. Siphonal canal wide. Inner lip distinct. Apical angle 70 o.

Material examined: CMM-I-4830.

Remarks: The species is known from the Choptank Formation and St. Marys Formation. It is not found in higher stratigraphic units. Only 3 individuals are identified. Shells are complete and range in size from small to medium, and the axial length is approximately ranging from 15.7-

52.9 mm.

Busycotypus Wenz, 1943

Busycotypus rugosum Conrad, 1843

(Figs. 9A, 9B)

Description: “Pyriform, with rather coarse rugose revolving lines, disposed to alternate in size, and very distinct numerous lines of growth; whorls scalariform, with a tuberculated carina, the

margin of which presents a waved outline, the tubercles being obtuse; spire prominent, profoundly channelled at the suture, the margin of the channel carinated in young shells. Length,

3 inches. (Conrad, 1861: 82).

Emendation of description: Five whorls, revolving lines on whorls. Aperture wide. Siphonal canal narrow. Apical angle 100 o.

Material examined: CMM-I-4831, CMM-I-4832.

Remarks: The species is known from the Calvert Formation and St. Marys Formation. It is not found in higher stratigraphic units. Only 3 individuals are identified. The shell sizes range from small to large, and the axial length is approximately ranging from 11.9-56.2 mm. They are well preserved.

Superfamily MURICOIDEA Rafinesque, 1815

Family MURICIDAE Rafinesque, 1815

Genus Boreotrophon Fischer, 1884

Boreotrophon laevis Martin, 1904

(Figs. 7G, 7H)

Description: “Shell six -whorled; body whorl with 9 to 13 varices; lower part of the body whorl almost or quite smooth, one of the four re volving ribs of T. tetricus sometimes faintly showing”

(Martin, 1904: 203).

Emendation to description: Shell with 5-6 whorls, ornamented with revolving varices. Apical angle 50 o. Aperture round. Siphonal canal long, narrow.

Material examined: CMM-I-4833, CMM-I-4834.

Remarks: The species is known from the St. Marys Formation. There are no recorded occurrences of this species in other units. Only 8 individuals are identified. Shells are small, and the axial length is approximately ranging from 11.1-12.2 mm. They are well preserved.

Genus Ecphora Conrad, 1843

Ecphora gardnerae Wilson, 1987

Ecphora gardnerae germonae Ward & Gilinsky, 1988

(Figs. 6I, 6J)

Diagnosis: “Shell moderately large, thick, umbilicate pyriform, with four, thick, moderately high, somewhat striate spiral ribs, that are moderately T-shaped in cross section.”

Description: “Shell moderately large in size, pyriform, with a relatively small umbilicus and

Ovate aperture. Ornamentation consists of four thick, moderately high, spiral ribs. Ribs thin and pronounced in the young stages, becoming thicker and more T-shaped in the adult stages. Ribs are somewhat sinuous due to irregularities in the incremental growth of the shell. Ribs separated by wide, flat interspaces devoid of spiral lines but exhibiting very fine incremental growth lines in the early stages that become increasingly pronounced in later stages. Apex of the ribs wide, very slightly rounded, and exhibiting up to 4 incised spiral striae on the summit. Upper rib is the raised shoulder of a nearly flat shelf, which is fused to the previous whorl at the level of the second lowest rib. Suture at this juncture very slightly impressed. Columella below the lowest rib greatly constricted and then again flaring at the moderately small umbilicus. A number of widely spaced growth pauses of the siphonal notch give the siphonal fasciole a regularly scaly appearance. Moderately Wide, triangular gap apparent between the columella and the parietal wall of the aperture below the lowest rib. Aperture siphonostomatous, oval in outline, with a moderately long, thin, deeply impressed siphonal notch. Adapical channel in the aperture very

weakly expressed just below the level of the second lowest rib and near the suture. Aperture strongly notched at the termination of the four ribs. Aperture heavily glazed by white aragonitic shell material which obscures the internal manifestation of the ribs deep in the aperture but does not completely infll the ribs at the aperture” (Ward & Gilinsky 1988: 6).

Material examined: CMM-I-4835, CMM-I-4836.

Remarks: This subspecies is only found in the Little Cove Point Member. Only 10 individuals are identified. The shells are well preserved. They are small in size, and the axial length is approximately ranging from 11.9-20.3 mm.

Genus Typhis Montfort, 1810

Typhis acuticosta Conrad, 1830

(Figs. 7I, 7J)

Description: "Shell with four or five acute foliated varices ending above in a pointed, compressed spire, alternating with four shorter rounded varices ending above in a tube; aperture oval and entire; margin reflected; beak closed, and slightly recurved” (Conrad, 1830: 211).

Emendation to description: Shell moderate in size, conical. Five whorls. Apical angle 60 o.

Suture lines impressed. Columella extends beyond aperture. Siphonal canal absent.

Material examined: CMM-I-4837, CMM-I-4838.

Remarks: This species is only found in the Miocene of the Chesapeake Group. Only 16 individuals are identified. Shells are small in size, and the axial length is approximately ranging from 11.2-15.7 mm. They are well preserved.

Genus Urosalpinx Stimpson, 1865

Urosalpinx subrusticus d’Orbigny, 1852

(Figs. 5G, 5H)

Description: “Shell subfusiform, transversely striated, with short longitudinal ribs or undulations on the large whorl; spire conical, costated; upper part of the whorls concave and plain; right lip toothed within, and plicated on the margin; beak recurved. The striae in general are alternately larger and smaller” (Conrad, 1830: 211).

Emendation to description: Whorl five with 11 to 13 rounded varices. Body whorl with 8 to 10 longitudinal ribs. Aperture oval. Siphonal canal cylindrical. Apical angle 45 o. Suture line depressed.

Material examined: CMM-I-4839, CMM-I-4840.

Remarks: The species is only found in the St. Marys Formation. Only 2 individuals are identified. The shells are small in size, and the axial length is approximately ranging from 9.1-

9.5 mm. They are well preserved.

Superfamily CONOIDEA Fleming, 1822

Family DRILLIIDAE Olsson, 1964

Genus Cymatosyrinx Dall, 1889

Cymatosyrinx pyramidalis Martin, 1904

(Figs. 5K, 5L)

Description: “Spire pyramidal, variable in acuteness; shoulder of body whorl angular; beak short, slightly curved at extremity; columella straight” (Martin, 1904: 160).

Emendation to description: Shell subfusiform, medium size. Aperture oval, small. Six whorls ornamented with short longitudinal ribs. Body whorls with 9-10 ribs. Sutures well defined.

Siphonal canal short. Apical angle 27 o.

Material examined: CMM-I-4841, CMM-I-4842.

Remarks: The species is only found in Miocene rocks. Only 8 individuals are identified. Shells are well preserved. The shells are small, and the axial length is approximately ranging from 9.4-

15.9 mm.

Family TURRIDAE H. Adams & A. Adams, 1853

Genus Hemipleurotoma Cossmann, 1889

Hemipleurotoma communis Conrad, 1830

(Figs. 6G, 6H)

Description: “Shell subfusiform, smooth, with one obtuse carina revolving in the middle of each whorl, except the last, which has three; the lowest one obsolete; beak attenuated and slightly recurved” (Conrad, 1830: 224).

Emendation to description: Shell medium in size. Whorls 7-8. Aperture, narrow, small. Apical angle 28 o. Suture line not well defined, siphonal canal narrow, short, slightly curved.

Material examined: CMM-I-4843, CMM-I-4844.

Remarks: The only known occurrence of this species is in Miocene rocks. Only 163 individuals are identified. The shells are entirely preserved. The size of the shells is small, and the axial length is approximately ranging from 11.6-12.8 mm.

Family PSEUDOMELATOMIDAE Morrison, 1966

Genus Sediliopsis Petuch, 1988

Sediliopsis angulate Martin, 1904

(Figs. 5E, 5F)

Description: “Shell subfusiform; spire and beak attenuated; whorls with two revolving rows of tubercles on each, divided by a striated sulcus; whorls strongly striated at the base; suture undulated; large whorl with strong distant revolving and intervening finer striae. A variety occurs

with only one row of tubercles on each whorl, and an impressed line beneath” (Conrad,

1830:225).

Emended Description: “Spire pyramidal and sharply pointed; body whorl short, angular on the shoulder; beak short, curved; sculptured like that of var. distans” (Martin, 1904: 156).

Emendation to description: Whorls 6-7. Aperture wide, lenticular. Whorls with revolving tubercles. Apical angle 28o. Suture line well defined. Siphonal canal short, small.

Material examined: CMM-I-4845, CMM-I-4846.

Remarks: This species only occurs in the Miocene St. Marys Formation. Only 16 individuals are identified. The shells are well preserved and small in size, and the axial length is approximately ranging from 9.2 12.4 mm.

Family TEREBRIDAE Mörch, 1852

Genus Laevihastula Petuch, 1988

Laevihastula simplex Conrad, 1830

(Figs. 6C, 6D)

Description: “Shell elongate conical, smooth, with plain undivided whorls; sides straight; the lines of growth are very distinct, and the large whorl slopes abruptly towards the base; the aperture is rather large” (Conrad, 1830: 226).

Emendation to description: Whorls 9-10 with faint longitudinal curved lines extending from suture to suture. Apical angle 26 o. Suture lines distinct, slightly depressed. Siphonal canal short, slightly curved.

Material examined: CMM-I-4847, CMM-I-4848.

Remarks: The species is only found in the St. Marys Formation. Only 289 individuals are identified. Shells are well preserved. They range in size from small to medium shells, and the

axial length is approximately ranging from 12.5- 29.8 mm. It differs from Laevihastula sublirata in having less pronounced lirations.

Laevihastula sublirata Conrad, 1863

(Figs. 4C, 4D)

Description: “General form like simplex; suture more distinct; whorls somewhat turreted; faint longitudinal lirations extend from suture to suture; an impressed subsutural line sometimes appears on the body whorl at about one- fourth the distance to the base” (Martin, 1904: 144).

Emendation to description: Shell elongated, smooth. Sides straight. Nine whorls. Lirations pronounced. Apical angle 25 o. Suture line very distinct, slightly depressed. Aperture large.

Siphonal canal short, slightly curved.

Material examined: CMM-I-4849, CMM-I-4850.

Remarks: This species is only found in the St. Marys Formation. Only 11 individuals are identified. The shells are well preserved. They are small to medium in size, and the axial length is approximately ranging from 12.1-34.4 mm. This species differs from Laevihastula simplex in having more pronounced longitudinal curved lines on the whorls.

Family MANGELIIDAE Fischer, 1883

Genus Propebela Iredale, 1918

Propebela parva Conrad, 1830

(Figs. 6A, 6B)

Description: “Shell subfusiform, transversely striated, with oblique longitudinal ribs; upper part of the whorls concave and plain” (Conrad, 1830: 225).

Emended description: “Shell small, seven- to nine-whorled; spire attenuate; beaks short, slightly curved; entire surface marked with fine, regular revolving lines; each whorl with about ten

longitudinal ribs which terminate on the shoulder in obtuse nodes; above the shoulder is a concave constriction which extends to the suture and contains the siphonal notch” (Martin, 1904:

153).

Emendation to description: Apical angle 33 o. Suture lines well defined. Aperture narrow, small.

Siphonal canal short, slightly curved.

Material examined: CMM-I-4851, CMM-I-4852.

Remarks: The species is only found in the St. Marys Formation and Choptank Formation. Only

59 individuals are identified. The shells are well preserved and small in size, and the axial length is approximately ranging from 7.6-8.4 mm.

Superfamily VOLUTOIDEA Rafinesque, 1815

Family CANCELLARIIDAE Forbes & Hanley, 1851

Genus Cancellaria Lamarck, 1799

Cancellaria alternata Conrad, 1834

(Figs. 8A, 8B)

Original Description: “Shell short subfusiform; whorls six, with nine or ten thick, longitudinal, oblique costae, with prominent spiral and finer intermediate striae; spire subconical; aperture less than half the length of the shell; labium with three plaits, decreasing in size inferiorly, as in

Mitra; aperture semilunar. Length, half an inch” (Conrad, 1834: 155).

Emended description: “Whorls 6, rounded, with nine or ten prominent ribs, and prominent revolving distant striae, and an intermediate fine line; spire conical; aperture less than half the length of the shell, sub -ovate; columella 3- plaited, plaits decreasing in size towards the base; umbilicus small; summits of volutions flattened; 5 of the larger revolving lines on the penultimate whorl” (Conrad, 1866): 67.

Emended description: “Very many specimens show six or seven prominent spiral striae, while others have only the five mentioned in the description. Most of them show from four to six fine raised lines on the summit of the whorl, a feature not mentioned in either description, and all have several other lines below the prominent ones mentioned. The form of the aperture of course varies with the proportional length of the shell.” (Whitfield, 1894: 112).

Emendation to description: Aperture large, ovate. Apical angle 50 o. Suture line well defined.

Outer lip with teeth. Siphonal canal short.

Material examined: CMM-I-4853, CMM-I-4854.

Remarks: The species is distributed in the St. Marys, Choptank, and Calvert formations. Only 5 individuals are identified. The shells are well preserved and medium in size, and the axial length is approximately ranging from 11.2-19.9 mm.

Order LITTORINIMORPHA Golikov and Starobogtov, 1975

Superfamily CALYPTRAEOIDEA Lamarck, 1809

Family CALYPTRAEIDAE Lamarck, 1809

Genus Crepidula Lamarck, 1799

Crepidula fornicata Linnaeus, 1758

(Figs. 8C, 8D)

Description: “P. testa integra ovali postice oblique 32 ecurve, labio pos-tico concavo” (Linnaeus,

1758: 781).

Emendation to description: sShell inflated, convex, medium in size, smooth. Growth lines faint but visible. Apex prominent, curved downward. Diaphragm half the shell length.

Material examined: CMM-I-4855, CMM-I-4856.

Remarks: The species is found in Miocene rocks of New Jersey south to Florida. It is not extinct.

Only 19 individuals are identified. The shells are fragile. The shells size ranges from small to medium, and the axial length is approximately ranging from 6.1-23.1 mm.

Genus Crucibulum Schumacher, 1817

Crucibulum pileolum Lea, 1843

(Figs. 5A, 5B)

Description: “Shell irregularly conical, thick, sulcate; sulci radiating, large, irregular; concentric striae minute, small; apex smooth, twisted into two whorls; aperture sub-rotund; cyathus large, wide, angular” (Lea, 1845: 248).

Emended description: “Somewhat conical, with profound irregular ribs, and very coarse concentric wrinkles; apex not central, prominent, obliquely inclined, margin profoundly scalloped; diaphragm ovate, profound, the margins free" (Conrad, 1845: 79).

Emendation to description: Shell with faint concentric horizontal lines. Apex curved downward.

Diaphragm small, ovate.

Material examined: CMM-I-4857, CMM-I-4858.

Remarks: The species ranges from the Miocene to the Pliocene. It can be found on the east coast of USA south to Florida. Only 10 individuals are identified. Shells are complete but they are also fragile. They are small in size, and the axial length is approximately ranging from 3.9-6.5 mm.

The species differs from Crucibulum constrictum and Crucibulum multilineatum in having faint concentric horizontal lines and a straight apex.

Crucibulum constrictum Conrad, 1842

(Figs. 4I, 4J)

Description: “Shell irregular, elevated; laterally compressed, marked with simple lines of growth; apex prominent, with one or two minute volutions; diaphragm very profound” (Conrad,

1842: 194).

“Very irregular, elevated, laterally compressed; transversely rugose; apex submedial, very prominent, obliquely inclined, and with 1 or 2 minute volutions; diaphragm extremely profound, adhering by nearly half the circumference of the margin.” (Conrad, 1845: 80).

Emendation to description: Aperture rounded. Margin smooth. Apex curved.

Material examined: CMM-I-4859, CMM-I-4860.

Remarks: The species is found from Florida to Maryland. It ranges from the Miocene into the

Pliocene. Only 17 individuals are identified. The shells are well preserved and small in size, and the axial length is approximately ranging from 2.9-8.2 mm. The species differ from Crucibulum pileolum in having a curved apex and differs from Crucibulum multilineatum in having a more elevated apex.

Crucibulum multilineatum Conrad, 1842

(Figs. 4K, 4L)

Description: “Subovate, depressed; apex prominent; one side with squamose lines, the opposite with finer ramose lines destitute of scales; diaphragm contracted” (Conrad, 1842: 346).

Emendation to description: Shell smooth. Aperture rounded. Shell margin smooth. Apex slightly curved.

Material examined: CMM-I-4861, CMM-I-4862.

Remarks: The species can be found along the east coast from Maryland to Florida. It ranges from the Miocene until the Pliocene. Only 8 individuals are identified. Shells are well preserved. The species has a small shell, and the axial length is approximately ranging from 3.1-4.5 mm. It

differs from Crucibulum pileolum in having a curved apex and differs from Crucibulum constrictum in being more depressed and having a rounded aperture.

Superfamily NATICOIDEA Guilding, 1834

Family NATICIDAE Guilding, 1834

Genus: Euspira Agassiz ,1837

Euspira heros Say, 1822

(Figs. 8G, 8H)

Description: “Shell suboval, thick, rufo-cinereous; within whitish; columella incrassated; callous not continued over the upper part of the umbilicus, hardly extending beyond a line drawn from the base of the columella to the superior angle of the labrum; umbilicus free, simple" (Say, 1822:

248).

Emendation to description: Umbilicus deep. Not covered. Aperture wide. Shell smooth. Suture lines prominent.

Material examined: CMM-I-4863, CMM-I-4864.

Remarks: The species ranges from the Miocene until the Pleistocene. Only 247 individuals are identified. The shells are well preserved. The shells size rages from small to medium, and the axial length is approximately ranging from 4.1-21.9 mm. The shells exhibit predation marks in the form of boreholes.

Genus Neverita Risso, 1826

Neverita discula Petuch, 1988

(Figs. 8E, 8F)

Description: “Ovate, flattened, discoidal, smooth and shiny; spire flattened, melding with last body whorl; umbilical region completely sealed over by large, bulbous callus; columellar region heavily callused” (Petuch, 1988: 17).

Emendation to description: Suture lines fused with whorls. Aperture large, wide.

Material examined: CMM-I-4865, CMM-I-4866.

Remarks: The species is only found in Miocene rocks. Only 48 individuals are identified. Shells are well preserved. The shells range in size from small to medium, and the axial length is approximately ranging from 4.4-17.1 mm. The shells exhibit boreholes.

Genus Sinum Röding, 1798

Sinum fragilis Conrad, 1830

(Figs. 5I, 5J)

Description: “Shell ovate, thin, fragile, smooth, with fine revolving impressed striae; spire very small; apex acute; aperture extending about four-fifths of the length of the shell; columella much narrowed and arcuated, exhibiting the internal volutions" (Conrad, 1830: 222).

Emendation to description: Umbilicus obsolete.

Material examined: CMM-I-4867, CMM-I-4868.

Remarks: This species is found in the Choptank, Calvert, and St. Marys formations. Only 2 individuals are identified. Shells are well preserved. The shells have a medium size, and the axial length is approximately ranging from 10.6-14.4 mm.

Order CAENOGASTROPODA Cox, 1960

Superfamily EPITONIOIDEA Berry, 1910

Family EPITONIIDAE Berry, 1910

Genus Epitonium Röding, 1798

Epitonium sayanum Dall, 1898

(Figs. 7A, 7B)

Description: “A white shell with nine well-marked varices continuous to the apex, which has a smooth, translucent, pale brown nucleus of about three whorls. . . . The interstices are polished, smooth, with occasional faint microscopic spiral striae” (Dall, 1889: 122).

Emendation to description: Shell subfusiform. Whorls globular. Aperture round, small. Apical angle 23o. Suture line depressed. Siphonal canal obsolete.

Material examined: CMM-I-4869, CMM-I-4870.

Remarks: The species ranges from the Miocene to Pleistocene. Only 5 individuals are identified.

The shells are complete and well preserved. They are small in size, and the axial length is approximately ranging from 6.3-10.1 mm.

Superfamily CERITHIOIDEA J. Fleming, 1822

Family TURRITELLIDAE Lovén, 1847

Genus Mariacolpus Petuch, 1988

Mariacolpus covepointensis Petuch, 2004

(Figs. 8I, 8J)

Description: “Shell of average size for genus, elongated, thin and fragile; suture deeply indented, producing overhanging whorls; spire and body whorls ornamented with 5 thin main spiral cords; between main cords are 1 or 2 extremely fine spiral threads; spiral cord at whorl periphery largest, producing overhanging carina; subsutural area with 2-3 extremely thin spiral threads; body whorl often partially disconnected from previous whorls” (Petuch, 2004: 270).

Emendation to description: Whorls 10. Aperture round, small. Apical angle 21o.

Material examined: CMM-I-4871, CMM-I-4872.

Remarks: The species is only found in the Little Cove Point Member. It is highly fossilized with

3472 individuals identified. Shells well preserved. They are medium to small in size,and the axial length is approximately ranging from 7.7-36.2 mm. Some shells exhibit bore holes, an indication for predation. The spiral threads in small sized shells are well distinguished.

Genus Turritella Lamarck, 1799

Turritella chancellorensis Oleksyshyn, 1959

(Figs. 4C, 4D)

Description: “Shell elongated, tapering gradually, with 11 whorls. Two spiral ridges, equally spaced, with the space between the two spiral ridges twice the width of the distance from the ridge to sutures. The first nuclear whorl turned to the side. Shell covered with very delicate spiral threads intersected with delicate growth lines” (Oleksyshyn, 1959: 31).

Emendation to description: Aperture rounded, small. Apical angle 26o.

Material examined: CMM-I-4873, CMM-I-4874.

Remarks: The species is distributed in Miocene rocks along the east coast. Only 3 individuals are identified. Shells are small, and the axial length is approximately ranging from 6.3-12.3 mm.

They are well preserved. The species differs from Turritella subvariabilis by having fine spirals in between the two main ribs.

Turritella subvariabilis d'Orbigny, 1852

(Figs. 6K, 6L)

Description: “Shell subulate, turreted, tapering to an acute apex; whorls flattened in the middle, with from two to five smooth ribs on each, and transversely striated; suture impressed.

“The ribs are generally three in number, but a variety occurs with two only, or the intermediate one becomes obsolete. The largest specimens, which much exceed the figure in size, sometimes have five ribs on each whorl” (Conrad, 1830: 221).

Emendation to description: Whorls more than 10, sides flat, ornamented with two ribs in middle.

Aperture round, small. Apical angle 23o.

Material examined: CMM-I-4875, CMM-I-4876.

Remarks: The species is found in Miocene stratigraphic units along the east coast. Only 6 individuals are identified. Shells are well preserved and medium in size, and the axial length is approximately ranging from 17.5-26.7 mm. The species differs from Turritella chancellorensis by lacking fine spirals between the ribs.

Order CEPHALASPIDEA Fischer 1883

Superfamily BULLOIDEA Gray, 1827

Family RETUSIDAE Thiele, 1925

Genus Retusa Brown, 1827

Retusa marylandica Martin, 1904

(Figs. 7C, 7D)

Description: “Shell small, nearly cylindrical, slightly smaller at the apex and with a suggestion of a constriction in the middle; surface covered with fine crowded longitudinal lines. This species resembles Retusa sulcata but is not as slender as that species” (Martin, 1904: 135).

Emendation to description: Aperture elongate, narrowest at apex. Apical angle 50o.

Material examined: CMM-I-4877, CMM-I-4878.

Remarks: The species is only found in Miocene rocks of the east coast of USA. Only 4 individuals are identified. Shells are small in size, and the axial length is approximately ranging from 3.1-4.9 mm. They are fragile.

Subclass HETEROBRANCHIA Burmeister, 1837

Superfamily ACTEONOIDEA d'Orbigny, 1842

Family ACTEONIDAE d'Orbigny, 1842

Genus Acteon Montfort, 1810

Acteon ovoides Conrad, 1830

(Figs. 4E, 4F)

Description: “Shell ovate, smooth, polished, transversely striated; spire short and conical; aperture more than half the length of the shell; suture deeply impressed. The striae are about twenty in number on the large whorl, and are impressed; the aperture is long and moderately wide, and the fold large” (Conrad, 1830: 227).

Emendation to description: Body whorl more than half shell in size. Outer lip sharp. Columella fold distinct. Whorls 7. Apical angle 56o.

Material examined: CMM-I-4879.

Remarks: The species is found in the St. Marys, Choptank, and Calvert formations. Only 5 individuals are identified. Shells are small, and the axial length is approximately ranging from

5.6-9.1 mm. The entire shell is preserved but it is very fragile. The species differs from Acteon shilohensis in having striae covering the entire body whorl.

Acteon shilohensis Whitfield, 1894

(Figs. 7E, 7F)

Description: “Shell of about medium size, subglobular or broadly ovate, the transverse diameter being to the height about as three to five; spire short, the apical angle about 70 degrees.

Volutions six, short in the spire, abruptly rounded on the top, giving an almost impressed suture line, and presenting a step-like appearance to the spire, rounded and full below; aperture moderately large, somewhat effuse below, the outer lip sharp; columella short and the fold very distinct and defined. Surface polished, with nearly equidistant impressed lines, except on the upper third of the height or on the exposed portion in the spire, where they are obsolete; lines generally clean, or free from punctae or dots. Some of the interspaces on the lower part of the volution marked by an intermediate finer line.

“This species differs from several forms known in the Eocene formation in being more globular, and in having a shorter spire” (Whitfield, 1894: 137).

Material examined: CMM-I-4880, CMM-I-4881.

Remarks: The species is found in the St. Marys and Calvert formations. Only 2 individuals are identified. Shells are small, and the axial length is approximately ranging from 5.5-6.5 mm. The shells are well preserved, but they are fragile. The species differs from Acteon ovoides in having more spaced striae at the top part of the body whorl.

Class SCAPHOPODA Bronn 1862

Order DENTALIIDA Starobogatov, 1974

Family DENTALIIDAE Children, 1834

Genus Dentalium Linnaeus 1758

Dentalium caduloide Dall 1892

(Fig. 9C)

Description: “Shell small, thin, slightly curved, smooth but not polished, marked only with incremental lines which cross the tube some what obliquely; shell cylindrical, posterior orifice small, circular, the margin without notch or sulcus, rarely even perceptibly deviating from a circle except when worn or chipped” (Dall, 1892: 442).

Material examined: CMM-I-4882.

Remarks: the species is only found in Miocene rocks of the east coast. Only 1 individual is identified. The shell is small, and the axial length is approximately 6.7 mm. It is well preserved.

RESULTS

Three sets of data were analyzed (Table 1). One represents a bulk sample of fossils from the St. Marys Formation, the second data set represents extant mollusk populations from the

Chesapeake Bay, and the third represents grab samples collected from the St. Marys Formation.

The second data set was obtained from the Chesapeake Bay Benthic Monitoring Program, which was established in 1984. Data shows 23 species were identified in the modern sampling, while the bulk sample of fossils had 43 species described and the grab sample included 33 described species. The fossil samples show different ranges and distribution. Only 11% of the species were confined to the Little Cove Point Member. Fossils confined to the St. Marys Formation are 22% of the species. Four species were recorded for the first time in the Little Cove Point Member in this collection, Chesapecten santamaria, Chesacardium laqueatum laqueatum, Lirophora parkeria, and Spisula subcuneata. The majority of the fossils, 46%, are distributed among only

Miocene rock units. Only 16% of the fossilized species extend beyond the Miocene rock units but only one species has survived to the Holocene, Crepidula fornicata.

The modern fauna shows the dominance of one species: Macoma mitchelli, with 1,034 individuals. The bulk sample had three dominant species: Ilyanassa peralta, with 1,788 individuals; Mariacolpus covepointensis, with 1,630 individuals; and Spisula subcuneata, with

958 individuals. The grab sample also had three dominant species: Ilyanassa peralta, with 517 individuals; Mariacolpus covepointensis, with 1,842 individuals; and Spisula subcuneata, with

564 individuals.

The Shannon index was calculated for each sample. It is 0.647 for the extant fauna, 0.478 for the bulk fossil sample, and 0.598 for the grab sample. Typical values for the Shannon index are between 0 and 1 where lower values indicate more diversity while higher values indicate less diversity. Evenness for all samples was calculated (Table 2). The extant fauna shows a higher evenness value, 0.647, than the bulk sample at 0.479 and the grab sample at 0.386. Sorensen’s coefficient to compare similarity between the bulk sample and the grab sample is 0.8421.

Calculations for the t-test to determine significant differences between the bulk and recent

Shannon diversity values is significant at alpha = 0.01. The degree of freedom in the t-test is 9,

(Table 3).

DISCUSSION

Paleoecology

St. Marys Formation was deposited in a shallow marine environment based on the invertebrate macrofossil fauna preserved in the formation (Gernant, 1971). Unit 21 was deposited in a shallow, subtidal area based on the presence of the bivalves Lyropecten Conrad,

1862, and Ensis Schumacher, 1817. The paleoenvironment of unit 22 was shallow, marginal marine based on the presence of a diverse assemblage of macrofossils that includes the gastropods Turritella Lamarck, 1799, Nassarius Duméril, 1805, Mangelia Risso, 1826, and the bivalves Spisula Gray, 1837, and Mercenaria Schumacher, 1817. The presence of large numbers of Nassarius and Mangelia indicate the depth was most likely less than 30 m. Unit 23 had a similar paleoenvironmental setting as unit 22 based on the presence of a similar fossil assemblage. Whitmore (1971) estimated the average paleotemperature was 19.4 °C, with the highest temperature at 23.8 °C within the temperate climate zone. His conclusions were based on studying the vertebrate fauna in the Chesapeake Group, such as cetaceans, sirenians, crocodiles, turtles, fishes, birds, and plants. Ward (1992) suggested that the St. Marys Formation was deposited in a shallow lagoonal environment based on studying the molluscan assemblages. He suggested that the Little Cove Point Member reflected an inner-bay setting.

During the deposition of the Little Cove Point Member, the Salisbury sea was flooded due to the warm climate. This event created a unique environment where ten distinct mollusk communities thrived (Petuch, 2004). The presence of an Ecphora community indicates a

shallow, muddy sand lagoonal environment. The Ecphora prey on bivalves such as Mercenaria and Chesacardium, which indicates it probably coinhabit the environment with bivalves.

Mariacolpus occupies the shallow part of the lagoon—around 5–10 m deep. They mainly feed upon Turricula, Sycopsis, and Euspira. It is safe to assume they coexist in the same environment.

Chesapecten live in the deeper part of the lagoon, around 10–100 m deep, in the sandy bottoms.

The gastropods Turrifulgur and Turricula are the main predators of Chesapecten; hence, they occupy the same environment. Cancellaria occupies the muddy sand of shallow waters, around

5–10 m deep, where they prey on smaller bivalves. The Mercenaria, Dosinia, and Dollorca occupy the shallow water, 0–5 m deep, of the intertidal sand flats. Those bivalves are the main food source for gastropods such as Ecphora and Urosalpinx—the former specializes in large bivalves. Also, Urosalpinx feed on barnacles. The shared Mariacolpus and Turritella habitat extends to deeper tidal channels. The Turritella are the main prey for naticid gastropods such as

Neverita and Euspira. Epitonium live among the Turritella and feed on zoantharian coral.

Ilyanassa and Bulliopsis prefer to scavenge in organic rich mud flats (Petuch, 2004).

Depositional environment.

Shideler (1994) studied the paleo-environment of St. Marys Formation based on the sediments and structures. His observations for the stratigraphic bed are as follows. Unit 20 lacks any structure, which indicates the deposition process occurred by suspension. It consists of argillaceous deposits with low carbonate content. This suggests unit 20 was deposited in a prodelta. Unit 21 has a higher sand ratio than unit 20, but both had a similar setting. This suggests unit 21 was closer to the source area with higher energy levels, most likely a sandy delta front. Unit 22 consists of sandy units with large scale cross stratification. This indicates a higher energy level environment than the previous units. Shideler (1994) suggested that unit 22

represents a beach environment. Unit 23 was deposited in lower energy levels like a lower deltaic plain. The lithological sequence indicates a regression event that led to a shallowing upward sequence in St. Marys Formation

Paleobiodiversity

Recent vs bulk sample

The calculations of the Shannon index for the bulk sample and the extant fauna indicate a high paleobiodiversity for the mollusks during the Miocene whereas the extant fauna have a lower biodiversity. A t-value test indicated with 99% certainty that there was a significant difference between the two calculations of the Shannon index. This might be because the

Miocene had a warmer climate than present day and mollusks thrive better in warmer climate.

Also, during the Miocene the marine basin was more connected to the ocean while in present day the Chesapeake Bay is only connected to the Atlantic Ocean at the mouth of the bay. At the same time there is a lot of freshwater input by rivers discharging in the bay. This led to a decrease in salinity in the present day. The Miocene had a similar setting except for the poor connectivity to the ocean which resulted in higher salinity than present day. The low salinity at present would have restricted the number of species that can thrive there.

The two communities have no similarities indicated by Sorenson’s coefficient, which means there are no common species between the two communities. The complexity of biological communities is a constant balancing act. This balance is maintained by the interactions among organisms and the interactions between organisms and their environment. The change in the mollusk species from the Miocene to modern times suggests the balance was not maintained.

Several factors might have affected this. During the Miocene, the climate was warmer than present day and the position of the continents was slightly different as it is today. This led to a

change in global circulation patterns. This might have affected species range. Also, the change in the geography of the bay would affect environmental settings, which in turn poses pressure on the living organisms. In the early to middle Pleistocene period the Delmarva Peninsula, located in the eastern part of the Chesapeake bay formed a major barrier spit which isolated the bay from the Atlantic ocean except at the southern bay mouth. Other organisms from different phyla would also be a reason behind the changes in species composition between the Miocene and the modern fauna due to competition over resources or changes in prey-predator interaction. A change in the chemistry of the marine basin would also put pressure on the ecosystem. Although the mollusk community has changed in composition, the modern community has maintained its functional role in the ecology (Figure 10). The newly introduced species in the community functioned in a manner that is more balanced than the extinct species. Therefore, the overall community was more stable with that change.

The Miocene fauna had a higher richness than the recent fauna, but the evenness seemed to be lower in the Miocene. The overrepresentation of the Miocene fauna by three species—

Ilyanassa peralta, Mariacolpus covepointensis, and Spisula subcuneata—is the reason behind the decrease in species evenness of the bulk sample. This over representation seems to occur in low level carnivores or suspension feeders that prefer shallow water, 0-10 m, yet some suspension feeders that prefer shallow water are represented by few individuals. This might be due to adaptational advantage of some species over others which favored them to be more widespread. High level carnivores and suspension feeders that prefer deeper part of the water are represented by few individuals compared to low level carnivores or suspension feeders that prefer shallow water.

Bulk sample vs. grab sample

Calculations for the Shannon index suggestes that the bulk sample had higher biodiversity than the grab sample. Also, species richness is higher in the bulk sample than the grab sample, which means that when someone is collecting fossils, a bulk sample would give a better representation for the paleo community. Sorensen’s coefficient between the bulk sample and grab sample indicated the two samples are highly similar, but that is expected since both samples are from the same ecological community. Interestingly, there is a slight difference between the two communities, even though they represent the same ecology. This supports the previous statement that bulk samples give a better representation for the extinct mollusk communities. The evenness for the grab sample is less in comparison with the bulk sample. The bulk sample had adequate representation of each species, while the grab sample had more representation for larger-sized species than for smaller-sized species. This lowered the evenness of the grab sample. Based on those results, we could hypothesize that a bulk sample is a more accurate representation of the fossil fauna. This is due to several factors. The grab sample is subjected to biased collection; the collectors might not accurately collect all the fauna due to the size of the fossils and limited accessibility to certain locations. A grab sample represents a small number of fossilized organisms from different locations, while a bulk sample represents all fossilized organisms from a large block of sediments

CONCLUSION

The Little Cove Point Member of the St. Marys Formation was deposited in a shallow marine environment. The average paleotemperature was between 19.4º C – 23.8º C. The

Miocene mollusks lived in the shallower part of the bay where depth did not exceed 100 m. The modern mollusks differ entirely from the Miocene mollusks. The modern sample has a lower biodiversity index than the bulk sample. Several factors may have played a role in changing species composition from Miocene to recent such as tectonic setting, climate change, or competition between species. The bulk sample gives a better representation of biodiversity measurements than the grab sample.

Appendix

Table 1. Biodiversity measurements for grab sample, bulk sample, and modern sample. The modern sample data was obtained from Chesapeake Bay Benthic Monitoring Program.

Grab Species Ni Pi Bulk Species ni Pi Mod Species ni Pi sam sample ern ple samp le 1 Acteon ovoides 4 0.000758 1 Acteon 1 0.00018 1 Acteocina 42 0.01308 581 ovoides 9645 canaliculata 4112 2 Astyris 122 0.023136 2 Acteon 2 0.00037 2 Campeloma 1 0.00031 communis 734 shilohensis 9291 decisum 1526 3 Boreotrophon 1 0.000189 3 Astyris 90 0.01706 3 Corbicula 219 0.06822 laevis 645 communis 8083 fluminea 4299 4 Bulliopsis 18 0.003413 4 Boreotrophon 7 0.00132 4 Doridella 12 0.00373 marylandica 617 laevis 7518 obscura 8318 5 busycotypus 3 0.000568 5 Bulliopsis 42 0.00796 5 Ferrissia spp. 41 0.01277 coronatum 936 marylandica 5105 2586 6 turrifulgur 2 0.000379 6 turrifulgur 1 0.00018 6 Gastropoda 1 0.00031 fusiformis 291 fusiformis 9645 1526 7 Crepidula 11 0.002086 7 Crepidula 8 0.00151 7 Gemma 173 0.05389 fornicata 099 fornicata 7163 gemma 4081 8 Crucibulum 7 0.001327 8 Crucibulum 10 0.00189 8 Geukensia 1 0.00031 constrictum 518 constrictum 6454 demissa 1526

9 Crucibulum 5 0.000948 9 Crucibulum 3 0.00056 9 Haminoea 4 0.00124 multilineatum 227 multilineatum 8936 solitaria 6106 10 Crucibulum 2 0.000379 10 Crucibulum 8 0.00151 10 Littoridinops 345 0.10747 pileolum 291 pileolum 7163 tenuipes 6636 11 Cymatosyrinx 2 0.000379 11 Cymatosyrinx 6 0.00113 11 Lyonsia 1 0.00031 pyramidalis 291 pyramidalis 7872 hyalina 1526 12 Ecphora 2 0.000379 12 Ecphora 8 0.00151 12 Macoma 665 0.20716 gardnerae 291 gardnerae 7163 balthica 5109 germonae germonae 13 Euspira heros 109 0.020671 13 Epitonium 5 0.00094 13 Macoma 1034 0.32211 345 sayanum 8227 mitchelli 838 14 Hemipleuroto 86 0.016309 14 Euspira heros 138 0.02617 14 Menetus 2 0.00062 ma communis 501 106 dilatatus 3053 15 Ilyanassa 517 0.098046 15 Hemipleuroto 77 0.01460 15 Mulinia 105 0.03271 peralta 653 ma communis 2693 lateralis 028 16 Sediliopsis 2 0.000379 16 Ilyanassa 1788 0.33908 16 Mya arenaria 10 0.00311 angulata 291 peralta 5909 5265 17 Laevihastula 79 0.014981 17 Sediliopsis 14 0.00265 17 Mytilopsis 238 0.07414 simplex 984 angulata 5035 leucophaeata 3302 18 Laevihastula 8 0.001517 18 Laevihastula 210 0.03982 18 Odostomia 1 0.00031 sublirata 163 simplex 5526 engonia 1526 19 Mariacolpus 1842 0.349326 19 Laevihastula 3 0.00056 19 Parvilucina 38 0.01183 covepointensis 759 sublirata 8936 crenella 8006 20 cancellaria 1 0.000189 20 Mariacolpus 1630 0.30912 20 Pisidium 1 0.00031 alternata 645 covepointensi 1942 compressum 1526 s 21 ilyanassa 3 0.000568 21 cancellaria 4 0.00075 21 Pisidium spp. 9 0.00280 marylandica 936 alternata 8581 3738 22 Neverita 28 0.005310 22 ilyanassa 1 0.00018 22 Rangia 260 0.08099 discula 07 marylandica 9645 cuneata 6885 23 Propebela 9 0.001706 23 Neverita 48 0.00910 23 Sayella 1 0.00031 parva 808 discula 2977 chesapeakea 1526

24 Turritella 2 0.000379 24 Propebela 50 0.00948 subvariabilis 291 parva 2268 25 Typhis 3 0.000568 25 Retusa 4 0.00075 acuticosta 936 marylandica 8581 26 Chesapecten 6 0.001137 26 Turritella 3 0.00056 santamaria 872 chancellorens 8936 is 27 Chesacardium 16 0.003034 27 Turritella 6 0.00113 laqueatum 326 subvariabilis 7872 laqueatum 28 Corbula 5 0.000948 28 Typhis 13 0.00246 inaequalis 227 acuticosta 539 29 Dallarca 5 0.000948 29 Urosalpinx 2 0.00037 idonea 227 subrusticus 9291 30 Dosinia thori 21 0.003982 30 Sinum fragilis 2 0.00037 553 9291 31 Spisula 564 0.106959 31 Dentalium 1 0.00018 subcuneata 985 caduloide 9645 32 Mercenaria 9 0.001706 32 Callocardia 4 0.00075 cuneata 808 subnasuta 8581 33 Mercenaria 11 0.002086 33 Chesapecten 15 0.00284 tetrica 099 covepointensi 468 s 34 Chesapecten 12 0.00227 santamaria 5744 35 Chesacardium 46 0.00872 laqueatum 3687 laqueatum 36 Lirophora 6 0.00113 parkeria 7872 37 Corbula 6 0.00113 inaequalis 7872

38 Dallarca 18 0.00341 idonea 3617 0.00037 9291 40 Spisula 958 0.18168 subcuneata 0258 41 crassatella 1 0.00018 turgidula 9645 42 Mercenaria 2 0.00037 cuneata 9291 43 Mercenaria 18 0.00341 tetrica 3617 Sum 3505 5273 3204

Table 2. Evenness measurement for all samples.

Total number of HMAX Evenness species Grab sample 33 3.4965075 0.386508496

Extant sample 23 3.13459421 0.647426717

Bulk sample 43 3.761200116 0.478812221

Table 3. The null hypothesis testing for modern and bulk samples

Site: Modern Bulk Total (N) 3204 5273 Richness 23 43 H 0.647 0.478 2 S H 0.000313 0.000336 t-value 9.55012737 Df 8108 p-value 1.6802E-21

FIGURES

Figure 1. Collection locality of samples in Little Cove Point along the Chesapeake Bay in the Little Cove Point Member of St. Marys Formation. Maryland, USA. Map adapted from Kidwell et al. (2015)

Figure 2. A. Crassatella turgidula, internal view of right valve, CMM-I-4822. B. Crassatella turgidula, external view of right valve, CMM-I-4822. C. Dallarca idonea, internal view of right valve, CMM-I- 4820. D. Dallarca idonea, external view of right valve, CMM-I-4820. E. Mercenaria tetrica, internal view of right valve, CMM-I-4806. F. Mercenaria tetrica, external view of right valve, CMM-I-4806. G. Dosinia thori, internal view of right valve, CMM-I-4802. H. Dosinia thori, external view of right valve, CMM-I-4802. I. Spisula subcuneata, internal view of right valve, CMM-I-4808. J. Spisula subcuneata, external view of right valve CMM-I-4808. K. Corbula inaequalis, internal view of right valve, CMM-I- 4818. L. Corbula inaequalis, external view of right valve, CMM-I-4818. Scale bar = 1 cm

Figure 3. A. Lirophora parkeria, internal view of left valve, CMM-I-4816. B. Lirophora parkeria, external view of left valve, CMM-I-4816. C. Chesacardium laqueatum laqueatum, internal view of left valve, CMM-I-4814. D. Chesacardium laqueatum laqueatum, external view of left valve, CMM-I-4814. E. Callocardia subnasuta, internal view of right valve, CMM-I-4800. F. Callocardia subnasuta, external view of right valve, CMM-I-4800. G. Mercenaria cuneata, internal view of right valve, CMM-I-4804. H. Mercenaria cuneata, external view of right valve, CMM-I-4804. I. Chesapecten covepointensis, internal view of right valve, CMM-I-4812. J. Chesapecten covepointensis, external view of right valve, CMM-I-4812. K. Chesapecten santamaria, internal view of right valve, CMM-I-4810. L. Chesapecten santamaria, external view of right valve, CMM-I-4810. Scale bar = 1 cm

Figure 4. A. Ilyanassa marylandica, adapertural view, CMM-I-4827. B. Ilyanassa marylandica, apertural view, CMM-I-4827. C. Turritella chancellorensis, apertural view, CMM-I-4873. D. Turritella chancellorensis, adapertural view, CMM-I-4873. E. Acteon ovoides, adapertural view, CMM-I-4879. F. Acteon ovoides, apertural view, CMM-I-4879. G. Turrifulgur fusiformis, adapertural view, CMM-I-4830. H. Turrifulgur fusiformis, apertural view, CMM-I-4830. I. Crucibulum constrictum, apical view, CMM-I-4859. J. Crucibulum constrictum, apertural view, CMM-I-4859. K. Crucibulum multilineatum, apical view, CMM-I-4861. L. Crucibulum multilineatum, apertural view, CMM-I-4861. Scale bar = 1 cm

Figure 5. A. Crucibulum pileolum, apical view, CMM-I-4857. B Crucibulum pileolum, adapertural view, CMM-I-4857. C. Laevihastula sublirata, adapertural view, CMM-I- 4849. D. Laevihastula sublirata, apertural view, CMM-I-4849. E. Sediliopsis angulata, adapertural view, CMM-I-4845. F. Sediliopsis angulata, apertural view, CMM-I-4845. G. Urosalpinx subrusticus, adapertural view, CMM-I-4839. H. Urosalpinx subrusticus, apertural view, CMM-I-4839. I. Sinum fragilis, apical view, CMM-I-4867. J. Sinum fragilis, apertural view, CMM-I-4867. K. Cymatosyrinx pyramidalis, apertural view, CMM-I-4841. L. Cymatosyrinx pyramidalis, adapertural view, CMM-I-4841. Scale bar = 1 cm

Figure 6. A. Propebela parva, apertural view, CMM-I-4851. B Propebela parva, adapertural view, CMM-I-4851. C. Laevihastula simplex, apertural view, CMM-I-4847. D. Laevihastula simplex, adapertural view, CMM-I-4847. E. Bulliopsis marylandica, adapertural view, CMM-I-4823. F. Bulliopsis marylandica, apertural view, CMM-I-4823. G. Hemipleurotoma communis, adapertural view, CMM-I-4843. H. Hemipleurotoma communis, apertural view, CMM-I-4843. I. Ecphora gardnerae germonae, apertural view, CMM-I-4835. J. Ecphora gardnerae germonae, adapertural view, CMM-I-4835. K. Turritella subvariabilis, adapertural view, CMM-I-4877. L. Turritella subvariabilis, apertural view, CMM-I-4877. Scale bar = 1 cm

Figure 7. A. Epitonium sayanum, adapertural view, CMM-I-4869. B Epitonium sayanum, apertural view, CMM-I-4869. C. Retusa marylandica, apertural view, CMM-I-4877. D. Retusa marylandica, adapertural view, CMM-I-4877. E. Acteon shilohensis, adapertural view, CMM-I-4880. F. Acteon shilohensis, apertural view, CMM-I-4880. G. Boreotrophon laevis, adapertural view, CMM-I-4833. H. Boreotrophon laevis, apertural view, CMM-I- 4833. I. Typhis acuticosta, adapertural view, CMM-I-4839. J. Typhis acuticosta, apertural view, CMM-I-4839. K. Astyris communis, apertural view, CMM-I-4828. L. Astyris communis, adapertural view, CMM-I-4828. Scale bar = 1 cm

Figure 8. A. cancellaria alternata, adapertural view, CMM-I-4853. B cancellaria alternata, apertural view, CMM-I-4853. C. Crepidula fornicata, adapertural view, CMM-I-4855. D. Crepidula fornicate, apertural view, CMM-I-4855. E. Neverita discula, apical view, CMM-I-4865. F. Neverita discula, apertural view, CMM-I-4865. G. Euspira heros, apical view, CMM-I-4863. H. Euspira heros, apertural view, CMM-I-4863. I. Mariacolpus covepointensis, adapertural view, CMM-I-4871. J. Mariacolpus covepointensis, apertural view, CMM-I-4871. K. Ilyanassa peralta, adapertural view, CMM-I-4825. L. Ilyanassa peralta, apertural view, CMM-I-4825. Scale bar = 1 cm

Figure 9. A. Busycotypus rugosum, adapertural view, CMM-I-4831. B Busycotypus rugosum, apertural view, CMM-I-4831. C. Dentalium caduloide, side view, CMM-I-4882. Scale bar = 1 cm

Figure 10. Trophic levels of Molluscan community (bivalves, gastropods, and scaphopods) of extant Chesapeake Bay fauna and Little Cove Point Member fossil fauna.

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