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Ichnofacies stand today as one of the more elegant but widely misunderstood concepts in ichnology, especially where paleobathymetry is concerned. Marine ichnofacies are not intended to be paleobathometers, as some workers continue to imply (e.g., Lockley et al., 1987; Ekdale, 1988); rather, they are archetypical facies models based upon recurring ichnocoenoses (Frey and Pemberton, 1984, 1985, 1987). If a particular ichnocoenose tends to occur repeatedly within a given bathymetric setting, so much the better; but water depth per se is rarely, if ever, a governing factor. Ichnocoenoses and ichnofacies, therefore, are best viewed in the context of actual depositional conditions or environmental gradients, wherever they occur (Figure 1).
The ichnogenus Didymaulichnus has previously been reported in rocks of upper Precambrian-Carboniferous age (Häntzschel, 1975; Pickerill et al., 1984). Didymaulichnus has been postulated by different authors to be an epistratal grazing trace of trace makers ranging from molluscs and other soft-bodied animals to arthropods (Young, 1972; Häntzschel, 1975; Bradshaw, 1981; Hakes, 1985). If this is the case, it is reasonable to assume Didymaulichnus occurs in sediments and sedimentary rocks that are as young as the modern. In an examination of the Cardium Formation (Upper Cretaceous, Turonian) this ichnogenus occurred with relative abundance.
The traces of macroboring organisms are known throughout the Phanerozoic, with diversification and exploitation of the macroboring niche paralleling variations in the development of skeletal metazoa. The oldest macroboring biota is an abundant yet low diversity fauna in hardgrounds and reefs of Lower Cambrian age. Following the extinction of archaeocyathids at the end of the Lower Cambrian (and thus the demise of skeletal reefs until the Middle Ordovician), boring organisms appear to be restricted to submarine hardgrounds. With the development of skeletal reefs in the Middle Ordovician the macroboring fauna shows a rapid speciation and a dramatic increase in diversity. This same pattern occurs again in the Devonian. This record appears to represent refuge of the fauna in low stress, hardground environments when skeletal reefs were not present and radiation in the high stress environment of the reef when large skeletal metazoa were abundant and diverse.
Stromatoporoids from the Late Devonian (early Frasnian) Waterways Formation near Fort McMurray, Alberta, contain well preserved Trypanites Mägdefrau. The stromatoporoid heads are formed of an initial growth of Clathrocoilona inconstans Stearn that is encased by a second stage growth of Trupetostroma papulosum Stearn. These two stages were separated by a period of no growth and erosion. The first two generations of boring penetrated the skeleton of C. inconstans while the third generation borings penetrated both C. inconstans and T. papulosum. The borings in the stromatoporoids are filled with light colored micrite, dark colored micrite, skeletal fragments, dolomite, non-ferroan calcite, and ferroan calcite. Analysis of the borings, the growth stages of the stromatoporoids, the boring fill, and the orientation of the geopetal fabrics indicates that the stromatoporoids were subjected to repeated cycles of growth-boring-filling and reorientation. This complex interaction of biologic and physical reworking had a profound influence on the diagenetic transformation of the stromatoporoid heads. This example clearly illustrates the role that biogenic agents can play in the production of diagenetic fabrics of hard carbonate substrates.
Three ichnospecies of Bergaueria—B. perata, B. hemispherica, and B. radiata—are described from the Lower Cambrian Gog Group near Lake Louise, Alberta. Usually populations of Bergaueria are monospecific and this occurrence is unique. Since all three ichnospecies are found on a single slab, the criteria used for ichnospecies differentiation appear valid.
Schaubcylindrichnus, a distinctive sheaf of arcuate, well lined, congruent dwelling tubes, although poorly known in general, is a good indicator of shoreface depositional settings for many Cretaceous rocks of the United States. Until recently, in fact, it was known only from those rocks; but it has now been recognized in Tertiary rocks of Taiwan, possibly in slightly more distal depositional settings. In addition to its distinctive morphological and environmental significance, the ichnogenus also may have substantial biostratigraphical significance. Further reconnaissance is needed to confirm these seemingly unique distributions, that is, to help explain why the trace fossil so far seems to be restricted to certain Cretaceous rocks in North America and certain Tertiary rocks in Asia.
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