Taphonomy of a Pliocene ophiuroid mass mortality lagerstätte in the Tirabuzón Formation, Baja California sur

UNCW Author/Contributor (non-UNCW co-authors, if there are any, appear on document)
René Anne Shroat-Lewis (Creator)
The University of North Carolina Wilmington (UNCW )
Web Site: http://library.uncw.edu/
Tricia Kelley

Abstract: Preservation of fully articulated ophiuroids (ophiuroid Lagerstätte) is rare due to rapid disarticulation of their endoskeletons. Recently, material from a mass mortality bed(s) of well preserved ophiuroids was found in the Pliocene Tirabuzón Formation, 4 kilometers north of Santa Rosalía, Baja California Sur. Unconsolidated sediment samples from the Tirabuzón Formation indicate a high abundance, yet low diversity, benthic foraminiferal assemblage. Three species are most abundant: (1) Virgulina californensis; (2) Bolivina interjuncta; and (3) Uvigerina attenuata. The high frequency of Bolivina interjuncta and Uvigerina attenuata throughout the formation suggests the Tirabuzón Formation was deposited in 200-500m water depth. The ophiuroid fossil assemblage appears to be monotaxic and was identified as the genus Ophiocnemis. Few of the ophiuroids in these samples were found whole. Instead, an anomalous (152:1) ratio of arms to discs exists. The individual arms and discs display a high degree of articulation, i.e. arms with spines still attached and articulated jaw structures. The high degree of articulation found in the arms suggests: (1) rapid burial; (2) no decomposition prior to interment; and (3) lack of post mortem reworking and bioturbation. To determine the possible cause of the abnormal ratio of arms to discs, live ophiuroids were subjected to simulated wave current coupled with turbidity, temperature, and/or salinity changes. Agitation in normal salinity/temperature seawater caused orientation difficulties and varied arm-curling responses. Changes in suspended sediment (0-235 mg/l) had no effect on the ophiuroids over a 180 minute period. Warm water (>30° C) caused arm rigidity and cool water (<18° C) caused substrate attachment difficulties. Both hypersaline (>52 ppt) and hyposaline (<24 ppt) water produced: (1) arm rigidity; (2) attachment difficulties; (3) lethargy; and/or (4) death. Arm flexibility of live ophiuroids subjected to hypersaline water was restored post mortem, but live ophiuroids exposed to hyposaline water exhibited prolonged arm rigidity. Prolonged arm rigidity, high energy storm waves, and sorting of the elongate arms from the flat central discs may account for the abnormally high arm to disc ratio in the Tirabuzón samples.

Additional Information

A Thesis Submitted to the University of North Carolina at Wilmington in Partial Fulfillment of the Requirement for the Degree of Masters of Science
Language: English
Date: 2009
Fossils--California--Baja, Geology--California--Baja, Paleontology--California--Baja
Geology -- California -- Baja
Fossils -- California -- Baja
Paleontology -- California -- Baja

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