UP IN ARMS: ENVIRONMENTAL FACTORS CONTRIBUTING TO ARM DISARTICULATION IN AN OPHIUROID MASS MORTALITY BED IN THE TIRABUZÓN FORMATION, BAJA CALIFORNIA SUR

Preservation of fully articulated ophiuroids (ophiuroid Lagerstätten) is rare due to rapid disarticulation of their exoskeletons. Recently a mass mortality bed of exceptionally preserved ophiuroids was found in the Pliocene Tirabuzón Formation, 4 km north of Santa Rosalía, Baja California Sur. The ophiuroid fossil assemblage appears to be monotaxic and was tentatively identified as the genus Ophiocnemis. The exceptional articulation of the ophiuroids in the indurated Tirabuzón samples suggests: (1) rapid burial; (2) no decomposition prior to interment; and (3) lack of post-mortem reworking or bioturbation.

The samples contain an anomalous (152:1) ratio of arms to discs. To determine the cause, live ophiuroids were subjected to simulated wave motion coupled with temperature, salinity, and/or turbidity changes. Individual ophiuroids were placed in a round tank fitted with a motor and agitator and subjected to motion and turbidity for 180 minutes. The following factors were varied: (1) presence of sediment; (2) speed of rotation; (3) temperature; (4) salinity. The ophiuroid was then placed in a holding tank for 24 hours to observe for autotomization and disarticulation.

Agitation in normal salinity/temperature water caused orientation difficulties and varied arm-curling responses. One ophiuroid exhibited autotomization. Initially, 1 cm of one arm was lost from the 2.49 cm total. Within 48 hours all five arms disarticulated into 42 pieces. Changes in suspended sediment (0 - 235 mg/l) had no effect on the ophiuroids. However, 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. Autotomization occurred once as a result of exposure to hyposaline water. After 36 hours only 0.3 cm of each arm remained attached to the disk. Arm flexibility of ophiuroids subjected to hypersaline water was restored post mortem, but ophiuroids exposed to hyposaline water exhibited prolonged arm rigidity. Pressure caused the rigid arms to break easily. High energy storm waves would break these rigid ophiuroid arms. Sorting of the elongate arms from the flat central disks could account for the abnormal arm to disk ratio in the Tirabuzón samples.