CONCRETIONS

Certain carbonate concretions are recognized for their fish fossils, particularly from the Triassic Karoo Formation of Madagascar, the Cretaceous Santana Formation of Brazil, and Holocene Green’s Creek of Ottawa, Canada. While several species of fish are known from Canadian concretions (McAllister et al. 1981), the capelin, Mallotus villosus, still alive today, is most common. As with most preservation in concretions, it is presumed that microbial activity starts in on the organic matter, providing carbonate, and precipitation occurs rapidly, while decay diminishes as encasement occurs (Mojarro et al. 2022). Microbial films are sometimes fossilized in fish concretions, accentuating the important role they play in forming concretions and preserving the fossils (Martill 1988).

Many different ammonites, gastropods (section header above), cephalopods, and other marine organisms have been found associated with concretions at this location.

Siderite (iron carbonate) or ironstone concretions that sometimes preserve fossil organisms are well known from the Carboniferous Mazon Creek beds of Illinois’ Francis Creek Shale Member (Foster 1988). They can also be found in other parts of the world, including from near Carterville, Illinois, Terre Haute, Indiana, Windsor, Missouri, Coseley, England, Montceau-les-Mines, France, and Sosnowiec, Poland (Pacyna and Zdebska 2012). Mazon Creek concretions are particularly prized due to the exquisite preservation of even soft-bodied fauna, soft tissues, pigments, and biomolecules (Clements et al. 2019; McCoy et al. 2020; Tripp et al. 2022). Lin et al. (2020) showed that siderite precipitation can be rapid with sufficient iron and alkalinity. The dissolution of calcium carbonate minerals provides alkalinity as well as seeds for nucleation.

Around 465 faunal and 450 faunal species (at least) have been recorded from the Mazon Creek lagerstätte (lagerstätte being a fossil site with exceptional preservation), and it is believed to have represented a marine bay where closely associated terrestrial and marine organisms were buried together (Foster 1988; Clements et al. 2019). These concretions suggest rapid burial, precipitation of ‘proto-concretions’ around organic remains that protected against bacterial activity, and due to local environmental conditions, precipitation of siderite after methane fermentation supplied carbonate (Clements et al. 2019). Different locations within Mazon Creek proffer different proportions of species, suggesting different environments within the region rather than homogenous mixing (Foster 1988).

This is the holotype for a Carboniferous shark, juvenile form, housed at the Field Museum of Natural History, Chicago. This elasmobranch may have been related to the modern sawfish.

Photograph by James St. John (CC-BY 2.0).

Mazon Creek fossil collectors first encountered the Tully Monster in 1955, which after examination of hundreds of specimens first appeared to be a “bilaterally symmetrical, soft-bodied marine invertebrate animal with no known ancestors or descendants” (Carman 1989). The strange Tullimonstrum has been difficult to characterize, but researchers now point to molecular biomarkers that suggest it was a vertebrate. McCoy et al. (2020) noted: “The ancient disparity of soft-bodied jawless vertebrates (e.g., lampreys, hagfish) may have been much greater than presently known.” That fits well within a creationist orchard framework.

Several locations around the world are known for their crab-enclosed concretions (Oregon, New Zealand, Mississippi, etc.). In some cases the concretions have to be prepared top-down or the crab will be damaged, while in others, the concretions are just cracked open and then prepared and repaired.

(Another interesting crab concretion are the travertine-encased crabs of Turkey, from a Pleistocene deposit.)

The Whitby coastline in Yorkshire, United Kingdom, is well-recognized for its Jurassic fossils. Ammonites in particular are sometimes preserved inside concretions that can be cracked open for preparation. In matrix, these concretions often have a pyrite rind, but this is sometimes lost if the concretion weathers out (as above). When polished, a pyrite-rind concretion is termed a 'golden cannonball,' and is prized by collectors if it contains an interesting fossil.

Split fossil-bearing concretions will sometimes show the positive and negative impression of a fossil.