AN UNUSUAL RECEPTACULITID MORPHOLOGY FROM THE ORDOVICIAN POGONIP GROUP OF THE GREAT BASIN

Receptaculitids are an enigmatic fossil taxon that has fueled a phylogenetic debate among paleontologists for over 200 years. They have been variously classified as sponges, calcareous green algae, and an extinct clade of organisms unrelated to any other taxa. These globose, calcareous marine fossils have a relatively primitive gross morphology but a mineralogically complex skeleton, characterized by complex helicoid arrangements of hundreds of individual skeletal elements in whorls.

Ordovician receptaculitids are present across the Great Basin in association with the geographically extensive oncolitic shoal system that ran along the western coast of Laurentia. A large number of well-preserved Middle Ordovician receptaculitids have been collected from the Pogonip Group Member F of the Arrow Canyon Range (ACR) in south central Nevada. Although these specimens vary widely in size and shape, they can all be assigned to a single receptaculitid species that exhibits remarkably variable gross morphology. The differences in morphology can be attributed to both environmental influences and ontogenic variability.

Younger, smaller specimens have skeletal structures shaped like inverted, hemispherical domes with an open top, later growing and slumping over into a cornucopia-like shape, and ultimately growing as large as 20 cm in length in a unique but consistently preferred body type. This large morphotype resembles an ancient oil lamp, shaped like a shallow, open bowl with one section of wall tapered to a point. The only other example of this particular “oil lamp” receptaculitid morphology was described by M. Foster of samples he collected in temporally equivalent strata in the Grapevine Mountains of eastern California, approximately 200 km west of our ACR locality (Foster, 1973). Foster also speculated that the highly variable specimens all belonged to a single receptaculitid species. Our field location allowed for a larger sampling of over 200 specimens, which has provided a more clear view of the complex ontogeny of these organisms and supports the assertion that these highly morphologically variable specimens are indeed a single species.