Forest J. Gahn and Dr. Scott Ritter, Geology
Purpose and Structure:
The purpose of this paper is to document a newly discovered assemblage of Early Mississippian (Kinderhookian) fossil crinoids from southeast Iowa. I will provide an introduction and background of the project, followed by the results of the research. Cited references are included at the end of the report.
Introduction and Background:
Crinoids are echinoderms or “spiny-skinned animals.” They are most closely related to common marine invertebrates such as asteroids (starfish) and echinoids (sea urchins). Crinoids are often referred to as sea lilies or feather stars for their physical similarity to flowers. These animals live attached to the bottom of the sea floor by a stem or stalk. The stem elevates the crown, consisting of a calyx and arms, into nutrient-rich ocean currents. The calyx houses the mouth and internal organs. The arms are used primarily for the capture of food particles.
Crinoids have represented a diverse and abundant component of marine faunas for at least 450 million years. Approximately 6,000 species of fossil crinoids have been described and about 650 species live in today’s oceans. They reached their acme during the Mississippian Period which began nearly 350 million years ago. Crinoids were so prolific during the Mississippian that it is often referred to as, “The Age of Crinoids.”
The Wassonville Formation was deposited during the Early Mississippian, or Kinderhookian. During the Kinderhooklan, southeast Iowa was part of a tropical, shallow sea situated approximately 20 degrees south of the equator.1 Crinoids flourished in the Wassonville. Nevertheless, their legacy is recorded primarily by disaggregated skeletal elements. Crinoids are composed of hundreds to thousands of individual plates that readily disarticulate. Well-articulated crinoids are rare, and most often absent from crinoidal limestones.
Despite the odds against them, well-preserved crinoids are represented in the fossil record. The proper recovery, description, and study of such material is essential for understanding the history of life on Earth. Fossils record invaluable clues to evolutionary processes, pathways, and relationships among both extant and extinct organisms. They can also be used to correlate rocks of the same age around the world, and inform us of the position of the continents millions of years ago.
Only one species of fossil crinoid has ever been documented from the Wassonville. 2 In order to augment the database of Kinderhookian crinoids for evolutionary studies, I began searching for new material in this formation. Over a period of several years, I found a few complete crinoids in the Wassonville. Nevertheless, I could not determine the exact horizon from which they were coming. This frustration ended in May, 1996, when I excavated a layer representing a storm deposit containing a diverse assemblage of well-preserved echinoderms.
Results:
This grant supported three weeks of continued field work in the Wassonville of southeast and central Iowa. I was able to excavate several small pockets of echinoderms in storm-generated strata of the Wassonville. The greatest surprise of the study was the discovery of another abundant crinoid producing horizon approximately fifteen feet below the first one that contained an entirely distinct fauna.
The lower crinoid horizon was deposited in a bioclastic grainstone. It represents a high-energy marine environment that is relatively more shallow than the upper crinoid zone. This fauna is dominated by the camerate crinoid genera Cusacrinus and Platycrinites. Poteriocrine cladids, taxocrinids, and the blastoid genus Orophocrinus are also abundant. I collected fifteen species of crinoids from the lower horizon. Only two crinoid species from the lower horizon are present in the upper horizon.
The upper crinoid horizon was deposited in a laminated mudstone. It represents the deepest water environment in the Wassonville Formation. The well-preserved echinoderms from this zone were smothered by periodic storms that disrupted the otherwise low-energy ecosystem. This fauna is dominated by the cladid crinoid genera Paracosmetocrinus, Nactocrinus and Gilmocrinus, the camerate Cribanocrinus, the questionable disparid Belemnocrinus, and two species of small echinoids. Twenty-four species of crinoids are known from the upper crinoid horizon. Although dozens of crinolds were collected from this horizon during this field season, only two represented new species.
The echinoderms of the Wassonville formation are most closely related to the crinoids of the Maynes Creek Formation of Le Grand, Iowa.3 The Maynes Creek and Wassonville are very similar both lithologically and paleontologically and may warrant nomenclatural synonymy in the future. The fauna is also similar to the Choteau Formation of Missouri, the Henderson Canyon and Lodgepole Formations of the northwestern United States, the Banff Formation of Alberta, Canada, and the Hook Head Formation of Wexford County, Republic of Ireland.
Not only does the Wassonville share many species of global distribution, it also contains many forms that are unique to the formation. Well over 25% of the fauna represents new and undescribed species of fossil crinoids. Several of these species and genera represent primitive forms that are found representing more advanced characters in stratigraphically younger faunas. Moreover, the species in the Wassonville contain features that will be very useful in cladistic analysis, or the mapping of rates and changes in evolutionary lineages through time.
Currently, I am continuing my research on the echinoderm fauna of the Wassonville as a graduate student at the University of Cincinnati. The research that I completed as an undergraduate will constitute a considerable portion of my thesis and future work as a crinoid paleontologist.
References:
- B.J. Witzke et. a]., University of Iowa Guidebook Series 10, (I 990) 105.
- Strimple, H.L. 1967. Oklahoma Geology Notes 27(4), (1967) 81-85.