The Practicality of Vertical Cephalopod Shells As Paleobathymetric Markers
Total Page：16
File Type：pdf, Size：1020Kb
The practicality of vertical cephalopod shells as paleobathymetric markers REX E. CRICK Department of Geology, University of Texas at Arlington, Arlington, Texas 76019 ABSTRACT the shell, total volume of the shell, density motion to describe the sinking behavior of of sea water, and shell geometry, the the shells of Nautilus by applying the phys- The depth to which an intact coiled observed sinking behavior of Nautilus can ics of the sinking of hollow, rigid bodies. cephalopod shell will sink with its plane of be duplicated experimentally using equa- Experiments using shells of Nautilus yielded symmetry vertical is related to shell geome- tions developed by Weaver and Chamber- sinking velocities in agreement with their try. Equations that relate shell geometry to lain (1976) and Chamberlain and Weaver calculated values and also showed that the physical constants of pressure, sea-water (1978). Given that the earliest coiled cepha- orientation of a sinking Nautilus shell varies density, and gravity yield the maximum lopods had shell construction similar to that as the phragmocone fills with water. With a depth of stable verticality of a shell. Water of Nautilus (Flower, 1964; Mutvei, 1972) small negative buoyancy, the shell sinks depths calculated from the geometries of and assuming that constants such as atmos- with its plane of symmetry upright (verti- vertical shells of Cymatoceras hilli (Shat- pheric pressure, acceleration of gravity, and cal), but as the phragmocone fills, the shell tuck), Drakeoceras drakei Young, and density of sea water have not changed sig- begins to rock from side to side until the Mortoniceras wintoni (Adkins) from the nificantly during the Phanerozoic, the phragmocone is about 55% full, at which Fort Worth Formation (Upper Cretaceous, necessary parameters (total volume of the point the shell leans over and sinks with its Albian) of north-central Texas are used to shell and shell geometry) can be measured plane of symmetry horizontal. Weaver and reconstruct a portion of the paleobathyme- or estimated with sufficient precision to Chamberlain were also able to refute the try of the East Texas Embayment during allow for the description of the sinking common argument that vertical shells may late Fort Worth time. The maximum water behavior of the coiled shells of fossil result from embedding upon impact with depth to which shells with these geometries cephalopods. In their experimental work, the bottom. The observed and calculated would have sunk with the plane of symme- Weaver and Chamberlain (1976) and sinking velocity of a Nautilus shell was try vertical range from 1.6 m for the least Chamberlain and Weaver (1978) placed shown to be approximately 30 cm/sec, a stable shell geometry (£). drakei) to 2.6 m emphasis on the behavior of the shell during velocity too slow to cause embedding upon for the most stable shell geometry (C. hilli). actual sinking because it is during this por- impact. They were able to show empirically Water depths were deeper than 2.6 m at tion of the post-mortem history that shell that the maximum depth to which Nautilus localities where no vertical shells were ob- geometry determines the attitude of a shell sinks in a vertical position ranges from less served and shells with the plane of symmetry at the time of burial. This statement than 7 m for rapidly filling shells to as much horizontal showed no evidence of being assumes that other physical constants were as 600 m for slowly filling shells. In the lat- reworked from the vertical orientation. invariable or nearly so. Thus, a coiled shell ter case, however, the shell will quickly of a given geometry, mass, and volume will, assume a horizontal orientation because it if the shell maintains its integrity, pass will continue to fill after reaching bottom INTRODUCTION through a series of events that can be de- and the stability of the vertical shell will be scribed using equations of motion in sink- removed within a few hours or a few days, For most shell-bearing animals, death is ing. If, during its descent through the water depending on depth. Chamberlain and followed by a fairly mundane set of events, column, the shell reaches bottom in a verti- Weaver (1978) modified the equations in- but post-mortem history of the chambered cal position and remains in this position troduced in their 1976 paper to allow for cephalopod Nautilus is far more spectacular long enough to be preserved, reasonable variations in shell geometry, the mass of the than that of other important marine inver- estimates of maximum water depth can be shell, and the total volume of the shell (shell tebrates. After death, gases released during calculated. material plus camerae). By incorporating decay of the visceral mass (including the the effects of hydrodynamic stability and siphuncular cord) often increase the buoy- Weaver and Chamberlain (1976) tested loss of buoyancy during sinking, they were ancy of the shells (if not already of positive ideas put forth by Reyment (1958, 1970) able to calculate sinking velocity and pres- buoyancy) so that they rise to the surface of and Raup (1973) concerning the presinking sure across the shell wall as a function of the water, where they float until stranded and postsinking history of coiled shells and depth for shells of any size and shape. The near a shoreline or become waterlogged and did so by using shells of Nautilus to describe results of this analysis indicated that the sink. Due to the relationships between the sinking behavior of chambered shells. largest single source of variation in the acceleration of gravity, buoyancy, mass of They were able to derive equations for Geological Society of America Bulletin, v. 94, p. 1109-1116, 7 figs., 1 table, September 1983. 1109 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/9/1109/3444777/i0016-7606-94-9-1109.pdf by guest on 26 September 2021 1110 R. E. CRICK observed and calculated depth limits of tions derived from equations used by naval Filling of the Shell shells is the geometry of the shell. Although architects and others to describe motions in the mass and total volume of the shell may the sinking of hollow, rigid bodies. Only the If the integrity of a shell has not been lost vary from shell to shell and from taxon to equations necessary for the calculation of as it sinks (no breakage of the phragmo- taxon, these differences are so slight that depth estimates are discussed below. The cone), filling will occur in one of two ways: they do not noticeably affect the depth reader is directed to Weaver and Chamber- rapid or slow. During rapid filling, the gas limit. Thus, as Raup (1973) suggested, shell lain (1976) and Chamberlain and Weaver contained in the shell will be trapped and geometry as it describes the stability of the (1978) for the derivation of these equations. the influx of fluid into the camerae will be shell is the key to estimating water depth In their experiments, Chamberlain and proportional to the pressure difference from a vertical shell. Weaver (1978) considered an incompres- between the external fluid (sea water) and Aside from occasional published reports sible shell sinking from the surface of a the internal gas. If the siphuncular tube has prior to 1958 of coiled cephalopod shells stationary fluid (sea water) of constant den- deteriorated, leaving a conduit for the found preserved in a vertical position, sity. This was done for convenience, with entrance of sea water through the open sep- Reyment (1958) was the first to report on full knowledge that sea water is not motion- tal necks, the pressure difference will be the phenomenon as it relates to the distribu- less and that its density varies slightly as a small and the shell can be said to fill rapidly. tion and buoyancy of fossil cephalopods. function of salinity and temperature. The Sinking by rapid filling was probably a rare Although Reyment made no prediction of variation in these factors, however, did not event because well-preserved shells com- water depth, he did recognize that all occur- prove large enough to introduce a signifi- monly show that the siphuncle was intact at rences were in facies he believed to be cant error in their calculations. It was also the time of fossilization. Slow filling, shallow-water and which quite possibly assumed that the chambers of the shell con- whereby sea water moves slowly through represented strandline deposits. Reyment tained an ideal gas and an amount of exter- the wall of the siphuncular tube by diffu- (1970) later examined a number of cephalo- nal fluid (sea water) that had entered by sion, was probably the common mode of pod-bearing localities unrelated in space diffusion through the wall of the siphuncu- filling for most shells (Collins and Minton, and time for the presence of vertical shells. lar tube. 1967). Owing to the slow addition of weight At each locality, he was able to document as the shell sank, stability would have been that at least a few coiled ammonites or nau- prolonged to a much greater depth. How- ever, if a shell reached the bottom under tiloids were in a vertical orientation as a q«0 result of the shell coming to rest on the bot- conditions of slow filling in water deeper & than the lower limit of its vertical stability, tom under conditions whereby verticality was sustained either by rapid burial or pro- I filling would have continued and the verti- longed buoyancy plus a rate of sedimenta- cal stability of the shell would have been tion sufficient to anchor the shell in a 6 lost within hours or days (Chamberlain and vertical position.