The Lake Valley Formation has long been known for the crinoids that are abundant in the formation. The crinoids were noted in the first descriptions of the formations in the late 1800’s, but there has never been a comprehensive study done on the crinoids and as a result there is no complete list of crinoids that existed in south-central New Mexico during the Early Mississippian. Although all subclasses of crinoids are found in the Nunn Member, it is the camerate crinoids that dominate the fauna and will be the focus of this study. Most of the specimens studied came from the Macurda collection from the University of Michigan. This collection was supplemented with specimens from the Laudon collection from the University of New Mexico and additional samples collected in the field. These collections provided more than 7000 specimens to be examined, with approximately 4500 of those being identifiable camerate crinoids.
The first chapter of this paper is a systematic review of the camerates. Sixty-one species are recorded from the Nunn Member, five of which are new species: Blairocrinus macurdai, Iotacrinus novamexicana , Agaricocrinus alamogordoensis, Uperocrinus kuesi, and Collicrinus laudoni. An updated list of camerates found in the Lake Valley allows for better understanding and easier comparison of crinoid faunas across North America during this time. Environmental preferences were also examined to see if there was a difference between the faunas found in the shallow water shelf and those found associated with Waulsortian mounds in deeper water.
The majority of the camerates found in the Lake Valley Formation are from the Family Actinocrinitidae. This family has been a source of much taxonomic confusion due to generic concepts not equally applied between Europe in North America. The second chapter of this paper objectively defines the actinocrinitids by discrete characters and reevaluates the generic assignments of North American species. A phylogenetic hypothesis is presented for the relationships of the Actinocrinitidae genera based on a parsimony-based analysis and plotted against stratigraphic ranges. Although groupings were revealed in this analysis, the Actinocrinitidae cannot be readily divided into subfamilies. Twenty-one genera of Actinocrinitidae are described, 17 of which are found in North America. A total of 125 species were evaluated of which 34 species and one open-nomenclature taxon are reassigned to different genera, and four species are designated as nomina dubia.
The third chapter evaluates the similarities of camerate genera in three coeval North American formations with the Lake Valley Formation. These formations are the lower Burlington Limestone of the Mississippi Valley, the Redwall Limestone of Arizona, and the Anchor Limestone of Nevada. These similarities are based on presence-absence data with the Jaccard Coefficient and with rarefaction curves. The lower Burlington and Lake Valley faunas are nearly equal in generic richness (35 and 31 respectively) and have a high similarity (0.61). The Redwall and Anchor limestones have approximately half the number of genera that the lower Burlington and Lake Valley formations have and are very dissimilar to each other (0.18).
The disparity in number of genera in the Redwall and Anchor limestones as compared to the lower Burlington and Lake Valley are likely taphonomic. The camerates of the Redwall are poorly preserved, likely due to early diagenesis of chert and dolomite. In the Anchor, the camerates were likely moved in debris flows from shallow water settings, diminishing their preservation potential. Because neither the Redwall nor the Anchor have significant numbers of unique genera and both have genera that are a subset of the lower Burlington fauna, the seas of North America were likely well connected during the Early Mississippian, allowing for the camerates to widely disperse. Rarefaction curves indicate that the Redwall and Anchor limestones have been incompletely sampled. Thus, poor preservation and limited sampling in these formations have produced an apparently endemic pattern for faunas that were originally cosmopolitan.
|School:||West Virginia University|
|School Location:||United States -- West Virginia|
|Source:||DAI-B 74/02(E), Dissertation Abstracts International|
|Keywords:||Biogeography, Crinoids, Mississippian, New Mexico|
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