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Late Cretaceous Coprolites from Southern Saskatchewan: Comments on Excretion Plasticity and Ichnological Nomenclature: GEOLOGICAL NOTE
Ferruginous coprolites associated with the Whitemud Formation (Late Cretaceous) exposures in south-central Saskatchewan represent both the expelled and unexpelled faeces of a large nonmarine fish, possibly a sturgeon. The physico-chemical controls on the plasticity of the faeces at the time of expulsion and the mode of preservation of the faeces during diagenesis minimizes the utility of binomial ichnological classification for coprolites.
Ferruginous bodies, previously described by Worcester (1950) as ironstone concretions, are irregularly distributed throughout kaolinitic silts, muds and clays of the Whitemud Formation (Late Cretaceous) of south-central Saskatchewan, western Canada. The depositional environment is a broad, shallow channel of an alluvial plain. The abundance of these bodies locally downgrades the economic potential of otherwise valuable ceramic clays. Features, interpreted as being organic in origin, notably gross morphology, surface ridge patterns and scarce, internal folds, indicate that the bodies are coprolites. They represent both the expelled and unexpelled faeces of fishes. These constitute the first recorded evidence of vertebrates from the formation. The argument is presented here that, although coprolite structures are to some extent a reflection of internal anatomy of the animal of origin, they also provide a record of physico-chemical controls of 1) plasticity of the faeces at the time of expulsion, and 2) mode of preservation of the faeces during diagenesis. The role played by these latter factors in coprolite preservation minimizes the utility of "generic" and "specific" designations, normally favoured in ichnological taxonomy (Hantzschel, 1975; Sargeant and Kennedy, 1973).
Coprolites were collected from disused clay pits over a distance of some 220 m along a minor scarp face, where they have been concentrated by slope wash in densities of up to 30 specimens per 1,000 sq cm. Maximum lengths of specimens are for the most part in the range 2 to 10 cm, although values of up to 22 cm have been recorded. The coprolites have been preserved as a result of nearly complete replacement of the original faecal material by goethite with subordinate siderite, lepidocrocite and pyrite, determined from X-ray diffraction, differential thermal and thermogravimetric analyses carried out by Dr. A. E. Foscolos, Department of Energy, Mines and Resources (Calgary, Alberta). The coprolites do not incorporate skeletal material of any kind. Associated plant debris is largely restricted to fragments, adhering to the outer surfaces of some faecal masses.
A limited number of coprolite types was distinguished on the basis of gross morphology: uncoiled (linear, segmented-elongate and spindle-shaped), regularly (spirally and helicoidally) coiled, and irregularly coiled (contorted and piled-lobate) forms are the most common. Variable tightness of coiling is characteristic for spiral and helicoidal
forms among the Whitemud specimens (, ); spiral coiling has been used widely by previous workers to distinguish coprolites from mineral segregations of inorganic origin (Amstutz, 1958; Hantzschel et al., 1968). Coprolites, belonging to the main morphologic types, exhibit excellent preservation of fine surface ridges and striations, interpreted as the impressions of sphincter and intestinal muscles. These ridges and striations are longitudinal in uncoiled coprolites and oblique to the long axes of coiled forms. Sphincter-pinched constrictions and terminal necks are also common. Scarce, subcircular perforations, observed on several specimens, may be the result of gas escape, when the faeces were soft and cohesive. A surface feature, common to all morphologic types among the Whitemud coprolites, is the occurrence of polygonal patterns of cracks, probably of synaeresis origin. All of the features, noted above, are masked to varying degrees by concretionary overgrowths of iron minerals around the original faecal masses.
The presence of a spiral ridge or valve in the small intestine of the animal of origin is a prerequisite for the production of regularly coiled faeces (Zangerl and Richardson, 1963; Williams, 1972). Sturgeons are considered to be the most likely animals of origin for the Whitemud coprolites, because they possess spiral intestinal valves and attain large size (up to 1,000 kg); they have an almost entirely cartilaginous skeleton and skull with no teeth, which might, at least in part, account for the absence of skeletal remains in the coprolite-bearing strata; their diet of invertebrates, sifted from bottom muds, would account for the absence of vertebrate-skeletal debris from the coprolites; and scarce, fossilized forms have been recorded from the Cretaceous strata of Montana (McAlpine, 1947). Since some spiral coprolites among the Whitemud specimens display intricate systems of internal folds, similar to the mucosal folds figured and discussed by Williams
(1972), it is reasonable to conclude that these forms represent the fossilized intestines of sturgeons and that the excrement was unexpelled at the time the fishes died. However, the fairly common occurrence of plant fragments, adhering to the outer surfaces of coprolites, belonging to all morphologic types suggests that most were expelled on to a subaqueous depositional surface, strewn with plant debris.
That different morphologic types among the Whitemud specimens possess similar ridge patterns of probable sphincter- and intestinal-muscle origin not only suggests a faecal origin for all types, but also raises the possibility of a common animal of origin. According to this line of reasoning, plasticity of the original faeces is considered to have been the most important, single factor controlling the final morphology. Different coprolite morphologies, possessing essentially similar, small-scale surface ridge patterns, may represent different degrees of uncoiling of originally spiral expelled faeces (Williams, 1972); variations about an optimum plasticity of faeces, which would favour coiling, while deviation from the optimum plasticity would produce uncoiled or irregularly coiled forms (Zangerl and Richardson, 1963); or droppings from different phases within a single episode of excretion, comparable to the fore, middle and end droppings, distinguished by Kao (1962). One or more of the above conditions, controlled by physico-chemical properties of the faeces with a bearing on plasticity, might have been operative during formation of the Whitemud coprolites. Thus it is possible for different coprolite morphologies to result from defaecation by the same animal; e.g., from different physico-chemical responses to the same phase of behaviour of an animal. This influence of nonbiological controls on coprolite form is compounded by the modification of surface features and gross morphology by propagation of synaeresis cracks and development of concretionary overgrowths. Clearly a binomial system of nomenclature, such as the one for trace-fossils, discussed in draft form by Sargeant and Kennedy (1973), is inappropriate for coprolites, in the light of the importance and even possible dominance of nonbiological factors in determining coprolite morphology.
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