Fossils and the Flood

Last edited: 07/09/2002

Sessile benthic marine organisms, such as sponges and most corals, are organisms that live fixed in place on or in the sediment. The fossils of calcified sessile benthic marine organisms are abundant in the Phanerozoic geologic record. Particular fossils and fossil assemblages of sessile benthic organisms may be interpreted either as in-place, buried where they grew, or transported from their place of growth and buried elsewhere. Flood geology is the hypotheses that all or most of the fossiliferous sedimentary record was deposited by a months-long global flood. A logical retrodiction of this hypothesis is that all of the sessile benthic marine fossils within so-called "flood deposits" were transported from their preflood growth location prior to burial, and are therefore allochthonous. Obviously the duration of the flood was too short to accomodate the syn-depositional growth of most benthic organisms.

Contrary to this retrodiction, sedimentary strata attributed to each of the Phanerozoic systems contain fossils and fossil assemblages of sessile benthic organisms which, judged by various sedimentary and taphonomic criteria, have not been transported from their site of growth, and are therefore autochthonous. Such assemblages show that, numerous times during the deposition of the sedimentary record, sedimentation ceased or slowed long enough to allow the development of in-place communities of sessile benthic organisms. Hence, it is false that most of the fossiliferous sedimentary record was deposited by a single, months-long depositional event.

Whether a given fossil or fossil assemblage has been buried in-place or was transported prior to burial can be inferred on the basis of criteria such as preservational state, growth orientation of individuals, growth interactions amongst adjacent individuals, and size-frequency distributions comparable to ecological communities. The significance of each of these criteria to specific cases of benthic marine fossils and fossil assemblages are discussed below.

Preservational state refers to features such as abrasion, breakage and disarticulation, or lack thereof. Some calcified benthic organisms form durable hard parts that can sustain transport with minimal alteration, but other organisms are extremely delicate and would be virtually obliterated by transport. If one considers that there are sedimentary basins filled with >10km of Phanerozoic sediment, and that these basins would have to be filled in a matter of months, then it is clear that most benthic organisms would have been subjected to fantastic hydrodynamic and mechanical forces prior to burial.

Growth orientation means the fossil is preserved in the same orientation it would have had when alive on the sea floor. Usually the orientation of individuals (or, in assemblages, the pattern of orientation) expected from in-place preservation is very different from that expected from transport. In particular those benthic taxa with erect growth forms and high vertical height/basal diameter (v/b) ratios (such as many coral, sponge, stromatoporoid, erect bryozoans, and calcerous algae taxa) would be expected to be oriented with their long axis parallel to bedding if they settled from suspension, whereas in-place preservation may result in many individuals with their long axes oriented perpendicular to bedding. On the other hand, disorientation of such fossils does not in itself indicate transport, since organisms can easily be toppled or disoriented and yet remain at or very near their site of growth (e.g. Kobluk et al., 1977).

Growth interactions amongst adjacent individuals, for instance growth deformation between adjacent brachiopods on a bedding plane, or 'competitive overgrowth' interactions between adjacent bryozoans cemented to a hardground surface, or intergrowth between adjacent corals in reef framework, are only explicable if the individuals are preserved with the same spatial relationship they possessed during life. Such interactions may result in fusion, in overgrowth, where once individual overgrows the other, or in a competitive 'standoff,' where both individuals cease growth at a common boundary and do not overgrow each other. Such interactions are common amongst modern and ancient benthic organisms (e.g. Stebbing, 1973; Fagerstom et al., 2000).

Size-frequency relationships are usually quite different for individuals in an ecological community than for an assemblage of transported individuals. For instance, whereas currents will tend to sort an assemblage of individuals by size, a buried ecological community may contain individuals of all sizes, and may have more than one size-frequency 'peak.' Also, in an ecological community, different species in an assemblage may display very different size-frequency and/or spatial distributions. Cummins (1986) states that "the degree of covariance between individuals of several species of of similar hydrodynamic propensity is dependent on the amount and intensity of post-mortem movement. The more species that covary, and the larger the size classes that covary, the more likely that transportation played an important role in the species' distribution pattern. Conversely, the absence of covariance suggests that, for at least some species, biological factors determined the species' distribution pattern" (p. 1). If one considers the long distances that many benthic organisms must have been transported and the strength of the currents required to transport them, then hydrodynamic sorting should be a major factor governing the vertical and lateral distribution of fossils.

Brachiopods

Brachiopods may be recognized as in-place based on preservational state, orientation, growth interactions, and possibly size-frequency relationships. For instance, Ziegler et al. (1966) describe several horizons of in-place Pentamerus sp. brachiopods from the Silurian of Alabama and Scotland. These brachiopods are preserved in life orientation, and "many of the specimens show growth irregularities caused by interference with their neighbors" (p. 1032). Such growth interference "are of several kinds and can be distinguished from deformations caused by compaction of sediments after the animals were dead" (p. 1032). Similar assemblages exhibiting growth interference have been reported from the Silurian of Michigan and Wales (p. 1033), and the Lower Permian Speiser Shale of Kansas (Cuffey, 1994).

Hallam (1961) describes brachiopod clusters from the Lower Jurassic of Leicestershire which are interpreted as in-place based on preservational state and size-frequency distribution. Hallam notes that there are few signs of physical disturbance, such as abrasion or disarticulation of shells, the shells remained mostly empty of sediments and are filled with calcite cement. The clusters contain a wide range of sizes and thus do not appear to be current-sorted, and it is "inherently improbable" that currents would deposits brachiopods in isolated clusters in the first place. This feature is highlighted by the fact that the size-frequency distribution of two species within the same clusters are quite different. Hallam notes (p. 656) that "there is no reason to expect similarity if the fossils represent original living associations, whereas if the shells had been sorted to any extent . . . they would tend to be sorted by size regardless of biological affinities."

Groups of the brachiopod Rostricellula preserved in growth position within the Middle Ordovician Witten Formation of Tennessee are figured in Walker and Alberstadt (1975). These groups can be inferred to be in growth position based upon their preservation in a gravitationally unstable (life) orientation, their 'clumpy' distribution, the size-frequency distribution of individuals in the groups, and the preservational state.

Some taxa of brachiopods possessed thin, hollow, hair-like spines projecting out from the shell. In most instances, these spines do not remain intact, having broken off as a result of preburial transport, or post-burial breakage. In instances where the such shells are preserved in life orientation with spines intact and unbroken, it can be inferred that the brachiopods are in-place or very nearly so, since transport would quickly result in breakage of these spines. Examples of such preservation in productids have been reported from Late Mississippian thrombolite mounds from Arkansas (Webb, 1987, p. 693), and from Permian deposits of west Pakistan (Grant, 1966, 1968) and west Texas (Cooper and Grant, 1975).

Sponges

Stromatoporoids are calcified sponges that formed rigid, accretionary skeletons referred to as coenostea. They are prominent components of many Ordovician-Devonian reefs and buildups (e.g. Manten, 1972; Wood, 2000). They possess an accretionary growth style, resulting in a layered-appearance in section view. They range in size from ~100mm to several m in diameter, and from >100mm to ~1.5m tall. Stromatoporoid coenostea display a wide range of growth forms, most of which can be described by the terms laminar, tabular, domical, columnar, digitate or expanding conical (Kershaw, 1998). Laminar forms are defined as those with a vertical height/basal diameter ratio (V/B) less than 0.1. Those with V/B ratios from 0.1 to 0.5 and from 0.5. to 1 are referred to as low domical and high domical respectively. Forms with V/B ratios exceeding 1 are columnar. Stromatoporoid morphologies are further subdivided on the basis of surface ornamentation such as surface protuburences known as mamelons, grooves known astrorhizae and internal arrangment of growth lines. Stromatoporoid coenostea often contain borings, and are often encrusted by bryozoans and other hardground fauna (e.g. Kershaw, 1980; Meyer, 1981). Corals sometimes are intergrown with stromatoporoids (Kershaw, 1987). Because many fossil (and mondern) corals display annual density banding, growth rates can be estimated for some fossil stromatoporoids. Such data indicate that stromatoroids may grow rapidly in the horizontal dimension, up to 23mm/yr, but vertical growth is very slow, ~4mm/yr or less (Meyer, 1981). Some of the most famous stromatoporoid buildups are those from the Silurian of Gotland, Sweden (Manten, 1971), and New York, the from the Devonian of west Australia (Playford, 1981; Wood, 2000), Alberta, Canada.

That many stromatoporoids and stromatoporoid assemblages are in-place can be inferred on the basis of several criteria. For instance, expected orientation would be very different for transported assemblages than for in-place assemblages (Kershaw, 1979). Laminar forms, for instance, would be about as stable upside down as upright, and therefore a transported assemblage would be expcted to contain roughly as many overturned as upright individuals. Low domical forms on the other hand may be easily uprighted. As the V/B ratio increases and exceeds 1 (columnar, expanding conical), uprighting of transported forms becomes increasingly unlikely, and an assemblage of transported assemblage would be expected to contain few if any individuals in growth orientation. This contrasts strongly with the orientation of stromatoroids seen in the geologic record. Many buildups contain a large majority of individuals in growth orientation, approaching 100% in some cases. This includes many examples with high V/B ratios. As one example, Kapp, 1974, fig. 1 shows a mushroom-shaped stromatoporoid in upright, growth orientation. The specimen is about 80cm tall, has a 'cap' about 75cm wide, and a 'stem' about 35cm wide. Thus V/B is >2, and the stromatoporoid is exceptionally top-heavy.

The degree of physical damage to the stromatoporoid is another criterion for distinguishing between transported and in-place stromatoporoids. For instance, many forms of stromatoporoid possess lateral extensions extending outward from the main body of the coenostea, interdigitating with adjacent sediment. These may thin to only a few mm thick at their distal edges. Transport would likely result in breakage of these thin lateral projections, as the result of repeated impacts with the substrate and other transported objects. Such breakage is indeed seen in stromatoproids which are known to have been toppled and moved (rotated stromatoroids will show an abrupt reorientation of the growth axis). Such projections may be well-preserved in the case of in-place burial. The presevational state of most Paleozoic stromatoporoids indicates preservation in or near growth location (e.g. Kershaw and Brunton 1999, p. 318, 323). This includes some giant stromatoroids, up to 5m in diameter, that are inferred to have lived ~500 years (Wood, 2000).

In-place preservation can also be inferred in some cases based upon the nature of the relationship between the basal layer of the stromatoroid and the underlying substrate. Although most stromatoporoids have more or less flat bases, some stromatoporoids have irregularly-shaped, concave bases, and can be seen to encrust the substrate in such a way that the basal layer conforms to irregularities of the substrate, which may consist of skeletal debris such as crinoid fragements, for example, or even igneous rock exposed at an unconformity (Johnson et al., 2001). In such cases it is clear that the stromatoporoid has been preserved at the site of growth. Some examples of this type of relationship are reported from the Ordovician of Vermont (Kapp, 1974, 1975), the Silurian of Gotland, Sweden (Kershaw, 1980), Mongolia (Johnson et al., 2001), and Iowa (Philcox, 1971), and the Devonian of western Australia (Wood, 2000).

Chaetetids are a type of calcerous sponge known from the Ordovician onward. They developed a variety of growth forms comparable to stromatoporoids and some corals (Kershaw and West, 1991), including laminar, domical, columnar, and branching forms. Some examples of Chaetetid sponges preserved in growth orientation have been noted at several stratigraphic levels within Pennsylvanian limestones of Kansas. Suchy and West (2002) describe two superposed horizons with an abundant fauna of in-place chaetetid sponges and associated benthic organisms from the Middle Pennsylvanian Higginsville Limestone. Many of the chaetetids have narrow bases and wide tops, and therefore a transported assemblage of chaetetids would be expected to have few if any individuals in a growth orientation. Yet 95% of the chaetetids are in growth orientation, as are 95% of the tabulate corals, most of the rugose corals and brachiopods, and "nearly all" of the encrusting of the encrusting bryozoans (p. 436). Nearly all of the fossils are well-preserved and unabraded. Also present are buildups up to 2.3m thick formed by intergrown chaetetids and corals. Based on growth rates of modern analogues, Suchy and West (p. 441) estimate that the larest buildups would have taken a minimum of 10,000 years to form.

Crinoids

Crinoids are a class of benthic marine organisms known from the Ordovician to the recent, and abundant from the Ordovician to the Jurassic. Most forms consist of an upright stalk composed of a series of stacked columnar plates (there are also stalkless forms where the calyx attaches directly to the substrate). At the top of the stalk is a calyx or 'cup.' Brachia or 'arms' radiate outward from the cup, and smaller extensions known as pinnules radiate outward from the brachia. In crinoids which live attached to the seafloor, the stalks are attached by a variety of structures, such as disc-shaped holdfasts (common on hard substrates) or structures resembling roots penetrating the substrate (soft substrate) (see Brett (1981) for a review of crinoid attachment structures).

Preservational state can also provide evidence for in-place preservation. Like all other echinoderms, the crinoid skeleton is composed of numerous individual calcite plates bound together by soft tissue. These plates rapidly disarticulate after the death of the crinoid. The vast majority of crinoid material in the geologic record is found as individual plates or groups of plates which cannot be identified to the species level. Crinoids will also disarticulate as a result of transport. Complete crinoids are therefore likely to have undergone little if any transport.

Although rare, several examples of complete, well-preserved crinoids and crinoid assemblages interpreted as in-place have been described. Some of the examples described in Hess et al. (1999) are reported from the Ordovician of Tennessee (Lebanon Limestone), Iowa and Minnesota (Dunleith Formation), New York (Trenton Group), and Ontario (Bobcaygeon and Verulam Formations), from the Silurian of Staffordshire, UK (Much Wenlock Limestone), New York and Ontario (Rochester Shale), the Devonian of Germany (Hunsruck Slate), the Mississippian of Indiana (Edwardsville Formation), the Triassic of south China (Falang Formation), the Jurassic of Bradford, UK (Bradford Clay), and the Cretaceous of Mississippi (Prairie Bluff Chalk). Two of these occurences, the Ordovician Bobcaygeon and the Jurassic Bradford Clay, are part of classic hardground faunas (see Brett and Liddell, 1978; Palmer and Fursich, 1974), which preserve numerous in-place fossil belonging to several benthic taxa. Another probable example of in-place preservation are the dense thickets (up to 100 per 930cm2) of erect, in-place crinoids with preserved root structures within the Late Mississippian Pitkin Limestone of Arkansas (Webb, 1987, p. 692).

In many cases, the crinoid has broken off at the base of the stalk, leaving the attachment structure behind. The relationship of the attachment structure to the substrate can sometimes be demonstrative of in place preservation. For instance, hold fasts preserved on sloping surfaces may show an asymmetrical growth form, such that the base of the stalk attachment surface is horizontal (e.g. Hybocystites holdfasts from Orodovician hardgrounds of Ontario; Brett and Liddell, 1978). This is analogous the asymmetrical growth of tree stumps which grow on a hillside, and proves the in situ growth of crinoids even though most of the crinoid itself is not preserved.

Bryozoans

Bryozoans are filter- feeding, colonial metazoans known from the Ordovician to the recent. Individual members of bryozoan colonies are known as zooids. The founding zooid of a colony is referred to as the ancestrula, from which bud descendent zooids. Most zooids form a calcified exoskeleton known as the zooecium, while the exoskeleton of the colony as a whole is referred to as the zoarium. Most bryozoans occur on hard substrates, such as rocks or shells. In the case of hardgrounds, where the seafloor itself has been hardened by syndepositional marine cementation, bryozoans will encrust the seafloor itself. Like many colonial metazoans, bryozoans exhibit a wide variety of growth forms, including flat, encrusting sheets, erect, low mound, and branching forms. Colonies may range in size from >1mm to ~1m.

Most bryozoans are quite delicate, particularly thin, single-layer, encrusting forms. They would be fragmented and obliterated by extensive transport. Therefore, well-prerved, complete bryozoans are unlikely to have undergone much transport. Bryozoan fossils may show growth interactions with adjacent fossils or with the substrate, which demonstrates that they are preserved in place. Palmer and Palmer (1977) describe several types of growth interaction between bryozoans and Trypanites from a Middle Ordovician hardground in Iowa (Trypanites refers to a type of small, tube-shaped boring ubiquitous on hardground surfaces, and which may be formed by several different soft-bodied borers). For instance, holdfasts of the erect bryozoans Escharopora are bored by Trypanites, which extend through the holdfast and into the substrate. Palmer and Palmer (1977) state that "there is no doubt that the two organisms lived and grew contemporaneously, for the fibrous texture of the bryozoan is distorted around the aperature, the strands parting around the hole and meeting again to run parallel on the distal side of the Trypanites" (p. 194). Two responses are noted where the encrusting bryozoan Ceramoporid encounter Trypanites. The Bryozoan may arch up and over the hole, in which case the bryozoan appears to be repelled by the Trypanites, or the bryozoan may grow down into the boring, in which case the boring appears to have been occupied (p. 195).

Whole, unfragmented bryozoans in growth orientation are also common in mounds, reefs, and cavities. For instance, thin, encrusting bryozoans, sometimes forming stacked accumulations, are documented within Late Missippian bioherms of the Pitkin Formation of Atrkansas (Webb, 1987, p. 690, fig. 8a). Well-preserved erect bryozoans are reported from Ordovician mounds within the Cystoid Limestone of northeast Spain (Vennin et al., 1998, p. 126). Examples of in-place bryozoans preserved in cryptic habitats, where they may encrust the walls, floors, and ceilings of the crypts, are described in Palmer and Fursich (1974), Brett and Liddell (1984), and Wilson (1998), among others. Brett and Liddell (1984) for instance discuss examples of bryozoans encrusting overhangs at the margins of cavities excavated below hardgrounds.

Cuffey (1997) documents numerous horizons of in-place, hemispherical Prasopora bryozoans within the Middle Ordovician Coburn Limestone of Pennsylvania. The Coburn consists of cyclically-bedded packages of micrite-shale-calcerenite, 10-30cm thick. Each cycle displays an erosional base overlain by a fossil hash containing randomly-oriented bryozoans which clearly have been disturbed, probably by storms, and are not in growth position (although the lack of abrasion and physical damage suggests they have not been transported far). The calcerous shale units, on the other hand, contain Prasopora colonies which are "uniformly upright, in growth position" (p. 122). The Coburn contains approximately 300-1000 of these Prasopora horizons.

Corals

There are many examples of in-place corals preserved within the geologic record, both as components of bioconstructions (reefs, biostomes) and in non-reef sediments as well. As an example, Adams (1984) describes pinnacle-shaped reefs from the Lower Carboniferous of Furness, northwest England. These reefs are up to 5m thick and 15m tall (and thus V/B ratios up to 3 or more), and are composed of primarily of the tabulate corals Syringopora and Michelinia, solitary rugoes corals, bryozoans, and calcerous algae (p. 236), most of which are preserved in growth orientation. Spiny brachiopods with attached spines have been noted within framework cavities, as have delicate frondose bryozoans (p. 241). There is no evidence that these reefs themselves have been transported, and the upright orientation of the tall, slender reefs is makes such transport exceedingly unlikely.

Scoffin (1971) describes patch reefs from the Middle Silurian Wenlock Limestone of Shropshire, England. The reefs are up to 100m wide and 20m thick, and are composed primarily of tabulate and rugose corals, stromatoporoids and bryozoans, most of which are well-preserved and remain in growth orientation. Many corals and stromatoporoids remain in growth orientation within the adjacent off-reef sediments as well. That these reefs are preserved at their site of growth is indicated by their relationship to the surrounding sediment. First, 'fingers' of reef limestone extend laterally outward from the reefs at several levels. These fingers would break off unless they were underlain by sediment. Second, there are 'indentations' in the margins of the reefs that correspond to bentonite beds in the adjacent sediments. This indicates that the reefs formed on-site and were affected by contemporaneous volcanism. From exposures in Coates Quarry, for instance, "the average of the widths of all the reefs 1m below a bentonite band, at the level of the bentonite band, and 1m above the band are 11.7m, 9.1m, and 12.0m, respectively" (p. 190). Also, some of the reefs have tall, slender profiles and would not be expected to be preserved upright if they had been transported.

Bivalves

Bivalves are a class of mostly marine, filter-feeding, benthic molluscs. They have been a common component of the shallow marine sedimentary record since the Triassic onwards, before which their particular ecological niche was dominated by brachiopods. Bivalve engage in a wide variety of life habits. Some groups are burrowers and live within the sediment. Others cement themselves (their left valves) to solid substrates, such as hardgrounds or the shells of other organisms. Some possess the ability to produce borings in solid rock, again including hardgrounds, 'rockgrounds,' and the shells and hardparts of other organisms (corals, sponges, etc.).

For many bivalve assemblages it is difficult to establish autochthony. For instance, they may remain articulated even though they have been transported, and they may possess hydrodynamically-stable orientations that are the same as the same as growth orientation. Nevertheless, in some instances a strong case can be made for in-place preservation. For instance, certain varieties of cylindrical to conical rudist bivalves ('elevator' morphotype of Skelton & Gili, 1991) with erect growth habits formed buildups worldwide during the Cretaceous, examples being described from France, Israel, Austria, the Caribbean, the USA, and elsewhere. Several photographs of rudist assemblages and bibliography can be found at the Rudist Bivalve Paleontological Database, and a large (214k) image illustrating some rudist specimens can be found here. Rudist buildups may contain sparsely- to -densely packed individuals, usually with nearly all preserved in upright, growth orientation (e.g. Gili et al., 1995, p. 253, 261). In densely-packed assemblages, individual rudists may show attachments and growth disturbances amongst adjacent individuals. Rudists have often been described as reef builders, yet it has been shown that such buildups only rarely possessed significant relief above the sea-floor. In most cases, the rudists appear to have grown embedded in sediment, with only the top of the rudists protruding (constratal growth in the sense of Gili). Hence, most rudist buildups were not reefs in the strict sense, which implies significant topographic relief above the sea-floor.

Bivalve borings (most of them anyway) are referred to the trace-fossil genus Gastrochaenolites. Unlike fossil organisms themselves, there can be no question that the borings are preserved in-place. And there is also no question that they are borings, because they can be seen to cross-cut and truncate shells and intraclasts in the substrate. Such borings are common in modern reefs, rocky shores, and other hard substrates. Observation of modern boring bivalves indicates that such borings take months to form (Kleeman, 1996). See this page for an example of recent Gastrochaenolites, and this page for an example of bivalve borings from a Cretaceous hardground from Glen Rose, Texas. Examples of such bivalve borings have been reported from hardgrounds and hardground cavities from Early Pennsylvanian Hale Formation of Arkansas, US (Wilson and Palmer, 1998), from two hardground horizons at the Carboniferous-Jurassic unconformity at Somerset, England (Bromley, 1975, p. 408), from the Middle Jurassic of Wiltshire, England (Palmer and Fursich, 1974), from Middle Jurassic hardgrounds in the Carmel Formation of Utah (Wilson and Palmer, 1994) and Sundance Formation of Wyoming (Wilkinson et al., 1985), from hardgrounds from the Cretaceous of Israel (Lewy, 1985), from hardgrounds and hardground clasts from the Late Cretaceous of Qahlah Formation of Oman (Wilson and Taylor, 2001), from hardgrounds and hardground cavities developed on Late Cretaceous chalk at Vise, Belgium (Bromley, p. 421), from hardgrounds at the Cretaceous-Paleocene contact at Tunorqo, Nuggsuag peninsula, west Greenland (several generations; Bromley, 1975, p. 419), amongst others. Where bivalve borings are found penetrating a fossiliferous stratum or fossiliferous cobbles derived from an underlying stratum, this proves that the underlying fossiliferous sediments were deposited, then lithified, then bored (and/or encrusted), and then buried.

Worm Tubes

A variety of Polychaete worms (e.g. serpulids) construct calcerous tubes, in which they live. This picture, this picture, and this picture show modern serpulid worm tubes. These tubes may be recumbent or erect, and typically have a diameter 0.5-5mm, and many be up to tens of cm long. These tubes usually attach to a hard substrate such as a shell. Once a colony is established, successive generations can attach to existing tubes. In some cases, aggregations of these tubes may build small framestone reefs and mounds, usually less than ~2m thick, as have been described from modern and recent environments of the Western Mediterranean (Fornos et al., 1997), Eire, Ireland (Bosence, 1973), and Baffin Bay, Texas (Andrews, 1964). Buildups containing such tubes in growth position have also been described in the geologic record. Examples have been described from the Ordovician of Ottawa, Canada (Steele-Petrovitch and Bolton, 1998), the Devonian Martin Formation of Arizona (Beus, 1980), and the Carboniferous Lower Borden Group of Cumberland and Roxburghshire, UK (Leeder, 1973). Of these, the largest example is that described by Leeder, 1973, which takes the form of a bioherm about 13m wide and 2m high, composed of closely-packed, erect tubes, some of which are cemented to each other. The lack of broken tubes in these mounds and off-mound sediments, as well as the erect (life) orientation, supports in-place preservation.

Calcerous Macroalgae

Some algae form fragile calcified skeletons or thalli. A widespread, extant genus of calerous algae is Halimeda. A geologically important family of calcerous algae are the Dasycladales. This family is characterized by sack or stick-like growth forms, and erect growth orientation (Chuvasov and Riding, 1989). Such algae are easily toppled and disarticulated by currents. The continual growth and breakdown of such algae can be a major source of fine carbonate in some tropical carbonate depositional environments. The preservation of such algae as whole, unbroken, individuals clearly requires a quiet depositional environment and little if any preburial transport. Samankassou (1999) describes algal mounds up to ~22m thick and 60m long from the Lower Pseudoschwagerina Limestone (uppermost Carboniferous) of Austria. These mounds possess densely intergrown frameworks of algal thalli (Anthacoporella), unbroken and in erect orientation, and a matrix of pelloidal micrite. The micrite has a peloidal textures similar to modern microbial carbonates crusts.

Stromatolites and Calcimicrobial Mounds

Stromatolites are "attached, laminated, lithified sedimentary growth structures, accretionary away from a point or limited surface of initiation" (Semikhatov et al., 1979). Observations of modern stromatolites, such as those in the Bahamas, indicate that they form as the result of the grain-trapping, carbonate-precipitating, and grain-welding activities of cyanobacteria (e.g. Reid et al., 2000). Stromatolites assume a variety of growth forms, depending upon the environmental regime under which they form. The most common forms are domical/hemispherical, columnar, and club-shaped. These organo-sedimentary structures have a long and abundant geologic record, extending from the Archean to the recent.

As discussed above with respect to stromatoporoids, the expected orientation of a transported assemblage of stromatolites would be very different from the orientation expected from in-place preservation. For instance, columnar and club-shaped stromatolites, which have high V/B ratios, would rarely if ever be expected to be found upright in a transported assemblage, whereas an in-place assemblage may contain many such stromatolites in an upright, growth orientation. Therefore, stromatolite assemblages containing many columnar stromatolites, all in growth orientation, are very unlikely to have undergone transport from growth location.

As an example, Eagen and Liddell (1997) describe a laterally extensive horizon of stromatolites from the Middle Cambrian Ute Formation of Utah and Idaho. This horizon, which can be correlated over an area of 2400km2 (p. 298), contains many closely-packed columnar and club-shaped stromatolites, with diameters of 0.15-0.3m, and heights of 1-2m, and thus V/B ratios ~6. All of the stromatolites illustrated are in growth orientation, and no toppled or disorientated individuals are noted. Thus, the orientation of these stromatolites is strong evidence for in-place preservation. Furthermore, there are very thin layers of silt that can be seen to pass through the stromatolites and into the adjacent sediment, which demonstrates that "the stromatolites were forming while the sediment was being deposited around them" (p. 302).

Lehrmann (1999) document numerous calcimicrobial buildups from the earliest Triassic of south China. Unlike the laminated fabric characteristic of stromatolites, these buildups consist largely of clusters of Renalcis, which are interpreted as calcified coccoid bacteria (Pratt, 1984), and/or bacterial biofilms (Stephens and Sumner, 2002). The base of the section described in Lehrmann (1999) displays a more or less flat, biostromal calcimicrobial layer up to 15m thick. Above this biostrome occur ~ 150m of cyclically-bedded limestones, with many beds containing mound and inverted-conical calcimicrobial buildups. At the Dajiang section, 24 mound-bearing beds are noted, with a cumulative mound thickness of ~30m (p. 360).

Conclusion

Very little attention has been paid in creationist writings regarding the recognition of in-place fossils and fossil assemblages within the geologic record. There are a few notable exceptions, however. For instance, flood geologists Lance Hodges and Ariel Roth (Hodges and Roth, 1986) studied the orientation of corals and stromatoporoids in some Paleozoic and Pleistocene reef cores. They concluded that the Devonian reef in their study was a 'true reef,' and was either in-place, or had been transported without rotation. Many European flood geologists now actually regard the entire Mesozoic and Cenozoic as 'post-flood' deposits, based in part upon such evidence (e.g. Scheven 1988, Robinson 1996, Garner 1996). Robinson (1996) for instance states that in-place organisms are "just the sort of direct evidence one would look for in order to test whether certain rocks are Flood rocks," and that "in situ organisms and structures are common enough in the Mesozoic to constitute a refutation of the post-Cretaceous model by themselves" (p. 59). As we've seen, however, probable in situ benthic fossils and fossil assemblages are hardly restricted to the Mesozoic. Instead, they occur in sediments attributed to all of the Phanerozoic systems.

References

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