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The study investigates early leaf decay in marine and freshwater environments, using an experiment in which Arbutus menziesii (Pacific Madrona) leaves were placed at the sediment-water interface and allowed to decay for 20 days. The leaves show distinct changes at a morphological level from very early stages of decay (i.e., after 5 days). The pattern of degradation in the marine environment is different from that in freshwater. Comprehensive megascopic and microscopic examination using SEM images reveal that decay is essentially due to microbes: bacteria and fungi that form a biofilm. Degradation in water, either marine or freshwater, is more rapid than in sediment, due to oxic conditions and unrestricted mobility of microbes. The lower surface of the leaves shows greater alteration due to: (1) a thinner cuticle; (2) the presence of stomata, which act as an entry point of degradative agents; and (3) cuticular ridges that increase surface area and act as a platform for trapping microbes, detritus, and diatoms. The upper surface remains largely unaffected.
Chemical changes accompanying the morphological alterations have been assessed by molecular analysis of the whole leaves using Pyrolysis-Gas Chromatography-Mass Spectrometry. Various moieties released by the pyrolysis of polysaccharides, lignin, protein, cutin, and cutan (which account for the bulk of the leaf tissue) have been identified and changes in the relative abundances of these traced to the degraded leaves. Relative abundances of toluene and phenol (derived from protein), 2-furaldehyde, 2-hydroxymethylfuran and 2,3-dimethylcyclopenten-1-one (derived from polysaccharides), and 2-methoxyphenol and 2,6-dimethoxyphenol (derived from lignin) decrease after 20 days in both settings. All other pyrolysis products show no distinct change. This shows that, despite morphological changes, the bulk of the tissue, including proteins and carbohydrates (considered labile), survives early decay.
The effects of unidirectional currents on the behavior of empty shells of the articulate brachiopod Terebratalia transversa were studied both in a laboratory flume and in a tidal-channel environment. Shells were tested on a hard, high-friction surface in the flume and on unconsolidated, well-sorted sands both in the flume and in the field. The velocity of the current competent to move terebratuloid shells is controlled by the starting orientation of the shell and is not strongly related to size or shape within the range of shells tested. Most shells assumed a preferred final orientation, both in the laboratory and field, with the anterior-posterior axis parallel to the current, ventral valve up, and pedicle opening facing up-current. Brachiopod shells that achieved hydrodynamically stable positions tended to be transported mainly by tumbling, with the anterior-posterior axis parallel to the current direction. Observations of shells on natural, current-swept sand substrates indicate that scour marks are formed around the up-current side of the brachiopod, with the scoured sediment deposited as a wedge down current from the shell. If scour continues, the brachiopod may slide down into the up-current depression, assuming variable inclinations. If the shell slides sufficiently far down into the depression, the solenoidal vortex that generates the scour may weaken or fail. Sediment carried in the bedload then buries the shell.
The littleneck clam Protothaca staminea in Argyle Creek and Argyle Lagoon on San Juan Island (Washington, USA) provides an ideal opportunity to test the effect of life habits on the taphonomic signature of shells. This bivalve exhibits two different modes of life in adjacent habitats: infaunal in muds and muddy sands (Argyle Lagoon) and free epifaunal on gravels (Argyle Creek). The mode of life significantly affected the taphonomic signature of both live and dead shells. Epifaunal P. staminea exhibit more damage than infaunal shells, suggesting that the infauna has a greater fossilization potential and may be more heavily affected by time-averaging than the epifauna. Both live infauna and epifauna suffered important taphonomic modifications after death, especially on the internal surface of the shell, but infauna did not reach the high level of damage acquired by the epifauna. In Argyle Creek, taphonomic agents were more effective at the sediment-water interface than within the sediment.
Because mode of life has a significant influence on processes of preservation, different taphonomic patterns in fossil bivalves do not necessarily imply different postmortem histories of shells, even when the taphonomic analysis is restricted to a single species. Some external modifications and internal shell damage cannot be regarded as unambiguously postmortem since edge and color modification, external corrasion and encrustation, and internal bioerosion can occur during the lifetime of the animal. Finally, this paper shows that a single bivalve species can exhibit more than one mode of life even within closely proximate environments. The typical mode of life is reflected in shell morphology while the secondary one is not. Thus, functional-morphology studies of extinct species can lead to incomplete interpretations of the range of a bivalve's life habits. An integrated approach combining functional morphology, comparisons with close relatives, and lithofacies analysis can be useful in paleoecological interpretations of extinct bivalve species.
The process of time-averaging can have deleterious effects on the recognition of morphological variability in the fossil record. To explore this issue, a geometric morphometric study was conducted on a life and death assemblage of the terebratulide brachiopod Terebratalia transversa. The results from several geometric morphometric techniques (including Procrustes analysis and thin-plate spline) confirm a high degree of morphological variability with little change in mean shape between the living and sub-fossil assemblages. Additionally, there is no evidence of distinct morphogroups in either assemblage, as postulated for the species in previous studies. These trends persist at all depths and size classes. The similar range of morphological variability at each site suggests a common causal factor such as a similar array of microenvironments available at all depths. One implication of this consistency in morphological variability between the living and sub-fossil assemblages is that the variability of a fossil assemblage of this species could be used to estimate single-generation variability during the time-averaged interval. Furthermore, the potential for recognizing the full range of shape variability in the sub-fossil record of a highly variable species is encouraging for the pursuit of species recognition in the fossil record. Very good fidelity of the sub-fossil assemblage with respect to morphological variability is documented here for the first time in brachiopods, and agrees well with the findings of similar studies of other taxa.
Actualistic comparison of size-frequency distributions (SFDs) of life and death assemblages of the brachiopod Terebratalia transversa from the San Juan Archipelago, Washington State, USA, revealed significant differences in the fidelity of SFDs between hard-bottom and mixed-bottom habitats. In relatively shallow (36–55 m), high-energy, and hard-bottom settings with pebbles and cobbles, the SFD of death assemblages is shaped primarily by taphonomic processes and its compositional fidelity is very low. Juvenile specimens are absent, probably due to winnowing and mechanical destruction. In deeper (64–84 m), low-energy, and mixed-bottom settings with a high proportion of soft substrata, population dynamics have a greater influence on the SFD of death assemblages; compositional fidelity is relatively high and original between-habitat variations in the population structure are preserved. Shell maceration probably is an important destruction process in both settings. In addition to environmental factors, intrinsic factors involving postmortem durability and population dynamics (related to timing and frequency of dead-shell production) are important in influencing preservation potential and fidelity of brachiopod death assemblages. There is differential preservation between small and large specimens. Smaller specimens are always characterized by better preservation and probably are dominated by cohorts that died most recently. Without continual input of juveniles, these soon will disappear from the death assemblage. More durable, larger specimens are characterized by higher taphonomic damage. These data point to size-selective taphonomic processes, leading to disharmonious time-averaging with respect to the relative abundance of size classes. Based on the assumption of short-term variations in population structure and evidence about high rate of destruction of juveniles and differential postmortem durabilities of juveniles and adults, high fidelity of SFDs of death assemblages in deeper, mixed-bottom settings is also the consequence of limited time-averaging. In contrast to previous studies of SFDs of death assemblages, it is emphasized that in addition to extrinsic factors, the interplay of inherent durability and frequency of dead-shell production is important in understanding their fidelity. A simple deterministic model predicts that on a short time scale, subtle differences in rate of destruction and recruitment frequency (related to rate of dead-shell production) lead to quite different probability of preservation of juveniles, even with the same mortality and growth rate. In settings comparable to the San Juan Archipelago, preservation potential of punctate brachiopods will depend on the rate of burial. This fact has a significant implication for distribution pattern of fossil punctate brachiopods, because unless rapidly buried, their distribution patterns can be strongly biased.
Owls are important contributors to the Tertiary small-vertebrate fossil record. They concentrate small-vertebrate remains by producing pellets rich in skeletal material that provide a sample of the small-vertebrate fauna of an area. A common assumption is that different predators inflict unique fragmentation and skeletal element representation signatures, thus providing a method for identifying a field assemblage as pellet derived, and possibly identifying the predator. In addition to the digestive process of pellet formation, the taphonomic history of a pellet includes the post-regurgitation processes of weathering, disintegration, transport, and burial, all of which can introduce biases into an assemblage and confound paleoecological interpretation. Analysis of a modern accumulation of small-vertebrate remains from Great Horned Owl (Bubo virginianus) pellets in a temperate forest environment on San Juan Island, Washington, reveals that fragmentation and skeletal-element representation change with residence time on the forest floor as pellets disintegrate and skeletal elements become dispersed. Matted hair initially protects the skeletal elements. As the pellet breaks down, the bones become dispersed, fragmentation of the bones increases (from 99% intact bones in intact pellets to 75% intact bones in fully dispersed pellets), and small, fragile skeletal elements are lost, resulting in a residual concentration of larger, more robust skeletal elements. The spatial distribution of skeletal elements below the roosting site follows a right-skewed, bimodal pattern. Skeletal elements are preserved in the soil to a depth of three centimeters. Post-regurgitation processes have the potential to distort the original faunal and skeletal composition of pellet-derived assemblages, thus masking any original predator-specific signatures. Actualistic taphonomic studies are necessary in order to understand how well pellet-derived assemblages capture information on local ecological and environmental conditions. This is a critical question that must be addressed to enable correction for such biases before pellet-derived assemblages are used for assessment of small-vertebrate community change and paleoenvironmental reconstruction.
Data from modern subtidal brachiopod-mollusk dominated death assemblages are used to investigate the effect of sieve size on taphonomic signature, comparing bivalves and brachiopods. The data comprise a total of 4439 specimens, mainly fragments (93%), sorted into five size fractions, and scored for four taphonomic variables: fragmentation, surface alteration, bioerosion, and encrustation. The taphonomic signatures of bivalve and brachiopod shells change as a function of sieve size, primarily from loss of encrusters on small, mobile fragments. Bivalves and brachiopods show significant differences (Wilcoxon rank-sum, α = 0.05) in taphonomic signature that become more pronounced in the fine fractions, mainly as a result of different levels of bioerosion. Bivalves seem to become fragmented after weakening from bioerosion, whereas brachiopods apparently are more prone to mechanical breakage. Thus, brachiopod shells may be shorter-lived in the coarse fractions and enter the fine fractions with less accumulated damage. Due to intrinsic taxonomic differences in the response to taphonomic processes, death assemblages with similar levels of taphonomic modification may have accumulated on different temporal scales. Comparative studies seeking taphonomic control must strive to use material from the same size fractions to avoid spurious results based on sampling procedure alone.
The consistency of taphonomic data collected by multiple operators in taphofacies studies is examined in order to determine if the employment of multiple operators is an acceptable way of generating the large amount of data required in taphofacies studies. Through two exercises designed to train operators in scoring taphofacies data, the consistency/repeatability of scoring data in the damage categories commonly used in taphofacies analysis was examined. The results of these tests seem to indicate that using multiple operators within a single study introduces vast amounts of inconsistency into the data. Most troubling is that consistency among operators in their scoring is so low that any attempt at comparing the results between studies by different authors would seem destined to encounter tremendous variation in the scoring of taphonomic data and not be feasible. Closer examination of the causes of the inconsistency among the operators, however, suggests that, with increased experience and a clear understanding of the definitions being employed, these problems should be reduced greatly. Based on this work, it is recommended that any taphonomic study making use of multiple operators first use a series of exercises in order to minimize the tremendous amount of error possible if consistent operators are not identified and used. While this work has focused on taphofacies studies and multiple operators, the results and recommendations presented here likely are relevant to many biological and paleontological studies that employ multiple operators.