DEPOSITIONAL CONDITIONS OF THE NEOGENE GASTROPOD,
TURRITELLA PLEBIA


        All paleobiologists must be concerned with the changes that occur during fossilization. The inherent problem is that the process of fossilization can produce postmortem changes in fossils that are easily mistaken for events that occurred while the organism was still alive. Unwary researchers can unwittingly construct attractive scenarios that have little basis in fact.

        For example, at the Berlin Ichthyosaur State Park in the Shoshone Mountains of Nevada there is a well known fossil assemblage of over thirty individuals of the giant ichthyosaur, Shonisaurus.


From Marshall's Art

The curious feature of this assemblage is that all of the individuals are oriented in a uniform, parallel alignment, facing in the same direction. The most popular interpretation of this assemblage is that, like modern whales, pods of ichthyosaurs occasionally died in accidental mass beachings (C. L. Camp. 1976. Vorläufige Mitteilung über grosse Ichthyosaurier aus der oberen Trias von Nevada. Sitzungsberichte der Österreichischen Akademie der Wissenschaften, Mathematisch-naturwissenschaftliche Klasse, Abteilung I, 185: 125-134). Such an interpretation is plausible, because it is consistent with an observed behavior seen in ecological similar extant animals. But how probable is this interpretation?

        A mass beaching is characterized by more than simply the orientation of the skeletons. First, the skeletons should be preserved in coarse, well sorted beach sand, as opposed to poorly sorted subtidal sands. Second, if all individuals died at the same time, they should be preserved in a single layer within the sediments, Finally, waves breaking on a beach would quickly dissociate the skeletons, winnowing away smaller, lighter bones and abrading exposed bones so that surface details are obscured.

        Detailed observations on the Shonisaurus assemblage indicates that none of these predictions is true (C. L. Camp. 1980. Large ichthyosaurs from the Upper Triassic of Nevada. Palaeontographica A 170: 139-200). The skeletons are (1) preserved in fine, poorly sorted, deep water sediments, (2) occur in different layers within the sediment, and (3) are largely intact, with little surface abrasion. The most likely explanation is that the assemblage represents an incidental accumulation of skeletons over a few centuries or millennia. The parallel body orientation is probably due to the carcasses becoming aligned by a bottom current.

        The error made in initially interpreting the Shonisaurus assemblage was in inferring a premortem biotic explanation without first disproving alternative postmortem taphonomic explanations. Careful paleobiological studies must first determine the conditions under which an organism was deposited and fossilized before attempting a reconstruction.
 


Paleobiology of Turritella plebia

        Probably the most abundant gastropod in Neogene sediments of the Atlantic coastal plain is the tall-spired shell of Turritella plebia (Fossils of Calvert Cliffs handout). This species is unusual in that it frequently occurs in huge numbers in sediments otherwise devoid of fossils. Further, the shells are not scattered throughout the sediment, but are concentrated into single-species stringers or lenses.


From Miocene Fossils of Maryland

        Turritella is an extant genus within the widely distributed family Turritellidae. Members of this family are unusual for gastropods in several aspects of their life style. First, unlike other gastropods, which are typically either herbivores or carnivores, turritellids use their gills for filter feeding. Second, rather than actively crawling on a large muscular foot, movement in turritellids is slow and intermittent. The small foot is forced into the sediment and then inflated with hemolymph to form an anchor while the shell is pulled forward a short distance. The foot is then repositioned and the process repeated. Finally, turritellids are gregarious, forming huge, relatively dense settlements of individuals.

        Given the known biology of living turritellids, stringers of T. plebia can plausibly be interpreted as entombed settlements of gregarious individuals. But, as was the case with Shonisaurus discussed above, there is a taphonomic explanation that needs to be disproved. Postmortem winnowing of shells is a common phenomenon on soft sediments and also produces monospecific stringers that closely resemble those of T. plebia.

        The purpose of this exercise is for you to determine if T. plebia stringers are the result of gregarious behavior or postmortem winnowing.

        There are both field and lab components to this exercise. During the field trip to Calvert Cliffs several of us (1) took measurements on the orientation of individual shells within a stringer, (2) collected shells for study in the laboratory, and (3) collected sediment samples from the stringer. Back in the laboratory you will (1) analyze the behavior of Turritella shells under different current regimes, and (2) determine the particle size distribution for the collected sediments.
 


Field Trip Data

        Turritella stringers most commonly occur in dense, bluish-gray sediments that are devoid of other macrofossils. The compass orientations (in degrees) for the apices of approximately 50 shells were recorded. These data are recorded in the turri spreadsheet on the laboratory computers. A number of Turritella shells and their associated sediment were also collected for analysis in the laboratory.
 


Laboratory Investigations

        In the laboratory, use a dissecting needle to carefully remove all sediment form the interior of each shell. Also, break a small (= about 1 × 1 mm) opening near the apex of the shell. This opening will facilitate the removal of air bubbles from the shell during your investigations, but will not seriously alter the behavior of the shells during your experiments.

        All of the laborattory data will need to be entered into the turri spreadsheet for analysis. Carefully review each section of the procedures to determine where data should be entered.

        Three aspects of postmortem shell behavior will be investigated: (1) shell orientation in a unidirectional current, (2) shell orientation in an oscillating current, and (3) hydraulic sorting by particle size. For all of these studies you should use shells that have been soaked in water and agitated in a test tube shaker to remove all internal air bubbles.
 

Orientation in a Unidirectional Current
        Randomly place twenty shells in a flow tank and increase the current velocity until the shells rotate to a stable orientation in the water flow, but do not increase it to the point that the shells actually start to slide downstream. Once this has happened, stop the current and, using a compass, determine the orientation of each shell, and record this data in the spreadsheet. Turn off the propeller motor and remove the shells.
 

Orientation in an Oscillating Current
        For this, the flow tank will substitute for a wave tank. As above, randomly place the same twenty shells in the tank. Establish oscillating waves by turning the flow tank on and off regularly. Continue for about thirty seconds. Using a compass determine the bearing of each shell, and record in your spreadsheet.

Hydraulic Sorting
        Since particles (including fossils) can be sorted by water movement, it is important to know whether a given specimen was fossilized where it lived, or was transported from elsewhere. Since water tends to sort particles by size, a comparison of fossil size with the size of adjacent sedimentary particles could be used to determine the presence of hydraulic sorting. If fossil and sedimentary particles are of similar size, sorting is likely to have occurred. If, on the other hand, fossils are consistently larger than adjacent particles, sorting is very unlikely.

        There is, unfortunately, a problem with this simplistic approach; fossils and sedimentary particles have very different shapes and densities. A Turritella shell resting on sand has a much larger surface area to volume ratio and a much lower density than the siliceous, roughly spherical sand grains. The simplest method to account for these differences is to calculate the quartz equivalent (= the size of a spherical sand grain with the same settling velocity) for each fossil. Since any particle can be hydraulically transported by a current moving at more than twelve times its settling velocity, sand grains and fossils having comparable quartz equivalents should sort together. The quartz equivalent diameter (Dqe; in mm) of a fossil can be calculated as:

The volume of a fossil (Vf; in cm3) can be determined by immersion in water.  Fill a 5 mL graduated cylinder approximately half full with tap water. Record the volume to the nearest 0.05 mL (= one-half gradation) and record in your spreadsheet. Take a shell that has been soaking in water and shake out all excess water. Slip the shell, apex first, into the graduated cylinder and vibrate the cylinder for 10-15 seconds on a test tube shaker. This will force air bubbles out of the shell interior. Record the new water volume in your spreadsheet; the difference between the final and initial water volumes is the shell volume. Repeat this procedure for a total of ten to fifteen shells. Record this data in your spreadsheet.

        Use a paint brush to push about several dozen grains of sediment onto a white index card. Use a dissecting microscope and a sediment comparator card to determine particle sizes for the first twenty grains you observe. Do NOT search for the largest grains to measure; simply measure the twenty you observe. Record this data in your spreadsheet. Once all of the data has been obtained it should be graphically analyzed:

1.  Graph the field orientation data. Use the bins-sorting function in Excel to group the data into eight categories (0-45 degrees, 46-90 degrees, etc.). Plot this data as a radar chart (filled subtype).

2.  Analyze and graph the unidirectional and oscillating current data in a similar manner.

3.  Analyze the shell volume data to obtain quartz equivalent sizes. Plot this data with the quartz grain diameters data as frequency distributions, after sorting each data set into appropriate size classes.


Questions

Do T. plebia shells exhibit random or clumped compass orientations in
        (i) the field?
        (ii) a unidirectional current?
        (iii) an oscillating current?

Are the quartz equivalent sizes of the shells and the sediment particle sizes similar to each other? What does this suggest about the deposition of these Turritella?