Biomineralization and taphonomic responses

The spectrum of arthropod preservation in the fossil record spans from taxa that are non-biomineralizing through ones that are well biomineralized. Klompmaker et al. (Reference Klompmaker, Portell and Frick2017), studying present-day arthropods, discussed differences in preservation potential among taxa, stemming largely from their relative extent of biomineralization. In general, biomineralizers suffer the effects of biodegraders less than non-biomineralizers and have a better or more complete fossil record (Mikulic, Reference Mikulic and Culver1990). Their remains dominate the arthropod faunas of concentration deposits (Konzentrat-Lagerstätten). Notable examples include the trilobites, and certain crustaceans, particularly ostracodes, barnacles, and biomineralizing decapods such as lobsters and brachyuran crabs. Plotnick and McCarroll (Reference Plotnick and McCarroll2023) recognized that different parts within biomineralized exoskeletons may be variably mineralized and therefore can have different taphonomic outcomes: the thickness and extent of biomineralization, as well as the Mg:Ca ratio and phosphorus content in exoskeletons, are variables influencing the preservation of arthropod remains.

At the other end of the preservational spectrum, non-biomineralizing marine arthropod taxa, which stand a relatively increased probability of complete breakdown through the action of biodegraders, tend to have weak fossil records in most sedimentary environments apart from conservation deposits (Konservat-Lagerstätten). Examples include various crustaceans such as amphipods, copepods, and krill, as well as radiodonts, naraoiids, marrellomorphs, lobopodians other than the armored lobopods, emeraldellids, luolishaniids, and tardigrades. Intermediate intervals along the preservational spectrum are occupied by taxa that have weakly biomineralized exoskeletons, such as some decapods (e.g., certain shrimps and prawns) or heavily sclerotized but non-biomineralized skeletons, such as some xiphosurans.

The relationship between biomineralization and preservation in the fossil record is heavily influenced by the role of biodegraders, which recycle organic materials back into the ecosystem, and are important in the cycles of carbon, calcium, phosphate, oxygen, and even sulfur. Biodegraders include predators, scavengers, herbivores, and microbial agents—organisms that in sum represent all domains and kingdoms (animals, plants, fungi, protistans, and archaeans). Among the variety of organisms that are adapted, or even specialized, for the breakdown of chitin (Beier and Bertilsson, Reference Beier and Bertilsson2013) are some bacteria (e.g., Gooday, Reference Gooday1990; Jiang et al., Reference Jiang, Li, Chen, Zhang, Wang, Wang and Sheng2022), fungi (Gooday, Reference Gooday1990), archaeans (e.g., Huber et al., Reference Huber, Stöhr, Hohenhaus, Rachel, Burggraf, Jannasch and Stetter1995; Tanaka et al., Reference Tanaka, Fujiwara, Nishikori, Fukui, Takagi and Imanaka1999; Gao et al., Reference Gao, Bauer, Shockley, Pysz and Kelly2003), algae, including diatoms (Vrba et al., Reference Vrba, Filandr, Nedoma, Simek and Muzzarelli1996, Reference Vrba, Kofroňová-Bobková, Pernthaler, Simek, Macek and Psenner1997; Štrojsová and Dyhrman, Reference Štrojsová and Dyhrman2008), rotifers (Štrojsová and Vrba, Reference Štrojsová and Vrba2005), and higher animals (Beier and Bertilsson, Reference Beier and Bertilsson2013). In the terrestrial realm, even carnivorous plants can break down chitin (e.g., Gooday, Reference Gooday1990).

Thicker, more resistant, biomineralized skeletal elements generally stand a better chance of long-term survival in sediments than less resistant, non-biomineralized bodily elements. This principle underlies the concept of the “preservation paradox,” wherein the most abundant and diverse taxa in a living ecosystem are proportionally underrepresented in the resulting fossil assemblage (Babcock et al., Reference Babcock, Stigall, Leslie, Elliot and Briggs2006) Among the best examples of this principle are some of the crustaceans, such as amphipods, copepods, and krill, which account for a large percentage of the biomass in Holocene marine ecosystems and presumably were similarly abundant in the geologic past, but which have meagre fossil records.

In some instances, conservation deposits such as the Burgess Shale (Cambrian; e.g., Walcott, Reference Walcott1912; Conway Morris, Reference Conway Morris1985, Reference Conway Morris1998; Whittington, Reference Whittington1985; Briggs et al., Reference Briggs, Erwin and Collier1994), Chengjiang (Cambrian; e.g., Zhang and Hou, Reference Zhang and Hou1985; Hou et al., Reference Hou, Aldridge, Bergström, Siveter, Siveter and Feng2004), Sirius Passet (Cambrian; e.g., Budd, Reference Budd, Fortey and Thomas1997; Babcock and Peel, Reference Babcock and Peel2007; Harper et al., Reference Harper, Hammerlund, Topper, Nielsen, Rasmussen, Park and Smith2019), Emu Bay (Cambrian; e.g., Bicknell et al., Reference Bicknell, Holmes, Pates, García-Bellido and Paterson2022a; Gaines et al., Reference Gaines, García-Bellido, Jago, Myrow and Paterson2024), Kaili (Cambrian; e.g., Zhao et al., Reference Zhao, Zhu, Babcock and Peng2011), ‘orsten’ of the Alum Shale (Cambrian; e.g., Müller and Walossek, Reference Müller and Walossek1985, Reference Müller and Walossek1987), the Fezouata Formation (Ordovician; e.g., Van Roy et al., Reference Van Roy, Briggs and Gaines2015), Bertie Group dolostones (Silurian; e.g., Clarke and Ruedemann, Reference Clarke and Ruedemann1912; Vrazo et al., Reference Vrazo, Brett and Ciurca2016, Reference Vrazo, Brett and Ciurca2017), Waukesha (Silurian; Mikulic et al., Reference Mikulic, Briggs and Kluessendorf1985a, Reference Mikulic, Briggs and Kluessendorfb; Wendruff et al., Reference Wendruff, Babcock, Kluessendorf and Mikulic2020) and other Silurian plattenkalk deposits (e.g., von Bitter et al., Reference von Bitter, Purnell, Tetreault and Stott2007), Mazon Creek and similar deposits (Carboniferous; e.g., Nitecki, Reference Nitecki1979; Baird et al., Reference Baird, Shabica, Anderson and Richardson1985a, Reference Baird, Sroka, Shabica and Beardb, Reference Baird, Sroka, Shabica and Kuecher1986; Shabica and Hay, Reference Shabica and Hay1997; Cotroneo et al., Reference Cotroneo, Schiffbauer, Mccoy, Wortmann, Darroch, Peng and Laflamme2016), and the Solnhofen Limestone (Jurassic; e.g., Barthel et al., Reference Barthel, Swinburne and Conway Morris1990) provide a counterbalance to the preservation paradox. In these and comparable sedimentary settings, the actions of biodegraders have been limited by a variety and combination of means that include, but may not be limited to, dysoxia or anoxia, salinity fluctuation, desiccation, rapid burial through event deposition or tidal sedimentation, and microbially mediated sedimentary sealing (e.g., Allison and Briggs, Reference Allison, Briggs and Donovan1991a, Reference Allison, Briggs, Allison and Briggsb, Reference Allison and Briggs1993; Feldman et al., Reference Feldman, Allen, Kvale, Cunning, Maples and West1993; Brett et al., Reference Brett, Baird, Speyer, Brett and Baird1997; Babcock et al., Reference Babcock, Zhang and Leslie2001; Briggs, Reference Briggs, Krumbein, Paterson and Zarvarzin2003a; Schiffbauer and Laflamme, Reference Schiffbauer and Laflamme2012; Vrazo et al., Reference Vrazo, Brett and Ciurca2016; Wendruff et al., Reference Wendruff, Babcock, Kluessendorf and Mikulic2020; Albani et al., Reference Albani, Mazurier, Edgecombe, Azizi, Bekhouch, Berks and Bouougri2024). Conservation deposits may provide a more realistic picture of the original diversity and abundance of arthropods in ancient marine ecosystems than we might infer from “ordinary” deposits. Conservation deposits have been of incalculable benefit for providing details pertaining to the morphology, origins, phylogeny, systematics, paleoecology, and taphonomy of numerous arthropod clades, as well as many non-arthropodan organisms (e.g., Conway Morris, Reference Conway Morris1985, Whittington, Reference Whittington1985; Barthel et al., Reference Barthel, Swinburne and Conway Morris1990; Allison and Briggs, Reference Allison and Briggs1991c; Budd, Reference Budd, Fortey and Thomas1997; Butterfield, Reference Butterfield2003; Schiffbauer and Laflamme, Reference Schiffbauer and Laflamme2012; Lerosey-Aubril et al., Reference Lerosey-Aubril, Hegna, Babcock, Bonino and Kier2014, Reference Lerosey-Aubril, Gaines, Hegna, Ortega-Hernández, Van Roy, Kier and Bonino2018, Reference Lerosey-Aubril, Kimmig, Pates, Skabelund, Weug and Ortega-Hernández2020; Robison et al., Reference Robison, Babcock and Gunther2015; Cotroneo et al., Reference Cotroneo, Schiffbauer, Mccoy, Wortmann, Darroch, Peng and Laflamme2016; Wendruff et al., Reference Wendruff, Babcock, Kluessendorf and Mikulic2020; Pates et al., Reference Pates, Lerosey-Aubril, Daley, Kier, Bonino and Ortega-Hernández2021, and references therein).

Fortuitously, some of the well-studied conservation deposits coincide with critical intervals in the evolutionary history of arthropods, and provide important morphologic information, which in turn provides insight into evolutionary processes and driving factors, phylogenetic relationships, morphologic patterns, and other aspects of the biology of ancient arthropods. Our perception of the phylogenetic history and relationships of and within arthropods would be far less complete were it not for conservation deposits, and especially ones in Paleozoic strata.

Collecting and monographic biases

Few studies on marine Fossil-Lagerstätten consider the full suite of body fossils present at a site (see English and Babcock, Reference English and Babcock2010, for an exception). Often reports emphasize one group of fossils in preference to others, for reasons of monographic priority, perceived abundance or preservational quality at a site, or perhaps because collecting in a truly unbiased fashion would be challenging (Miller, Reference Miller, Brett and Baird1997). Indeed, in particularly rich deposits such as the Cincinnatian “series” (Upper Ordovician of Ohio, Kentucky, and Indiana, USA), collecting 100% of body fossils would present enormous logistical and time-related challenges in the field-collection phase, as well as accessioning and storage challenges in museum repositories, where space for collections is commonly at a premium.

Consider stratigraphic units that are rich in arthropod sclerites, of which there are many in the Phanerozoic (e.g., Clarke and Ruedemann, Reference Clarke and Ruedemann1912; Eldredge, Reference Eldredge1972; Brandt Velbel, Reference Brandt Velbel and Curran1985; Whiteley et al., Reference Whiteley, Kloc and Brett2002; Hunda et al., Reference Hunda, Hughes and Flessa2006; Brett et al., Reference Brett, Zambito, Hunda and Schindler2012; Bonino and Kier, Reference Bonino and Kier2024). Specimens in these deposits are commonly separated, and often broken, sclerites (Figs. 3.2, 4.2). Fully articulated exoskeletons are much less common, but even when present they often show some displacement of sclerites (Fig. 2.3–2.5). These sites provide valuable paleobiological or taphonomic information that often is exploited only in part. In particular, such sites may convey useful information bearing on questions of predation and scavenging, molting, and non-biologically induced physical breakage (e.g., Brandt, Reference Brandt1993; Pratt, Reference Pratt1998; Babcock, Reference Babcock, Kelley, Kowalewski and Hansen2003; McCoy and Brandt, Reference McCoy and Brandt2009; Bicknell et al., Reference Bicknell, Paterson and Hopkins2019a, Reference Bicknell, Holmes, Pates, García-Bellido and Paterson2022a, Reference Bicknell, Smith, Howells and Fosterb). Anecdotally, it is my experience that trilobite remains, especially separated sclerites such as thoracic segments, librigenae, hypostomes, and rostral plates, are collected and/or recognized in numbers well below their true abundance, especially in trilobite-rich Cambrian, Ordovician, and Devonian strata. For example, commercial and other heavily collected trilobite quarries in the Cambrian of the Great Basin (see Robison et al., Reference Robison, Babcock and Gunther2015; Bonino and Kier, Reference Bonino and Kier2024) are commonly littered with separated trilobite sclerites rejected by collectors. In addition, trilobite grainstones or “coquinas” (e.g., Babcock, Reference Babcock1994; Terfelt, Reference Terfelt2003; Babcock et al., Reference Babcock, Robison, Rees, Peng and Saltzman2007, Reference Babcock, Peng, Brett, Zhu, Ahlberg, Bevis and Robison2015; Calner et al., Reference Calner, Ahlberg, Lennart and Erlström2013) are rarely collected intensively except for faunal documentation.

To provide the most complete understanding of arthropod biodiversity through time, and abundance in individual ecosystems, documentation is necessary. Even incomplete materials that cannot be easily assigned to taxa initially, should be documented, because they may provide clues to the identities of other fragments that emerge upon further work. Sometimes rare taxa are documented in preference to the more common taxa, which can lead to a distorted impression of taxic abundance at a site. It is just as important to collect and document the common taxa as the less-common taxa.

Concretions

One of the most common sources of well-preserved marine arthropods is in concretions (e.g., Branisa, Reference Branisa1965; Müller, Reference Müller1979, Reference Müller1983; Feldmann and McPherson, Reference Feldmann and McPherson1980; Feldmann and McKenzie, Reference Feldmann and McKenzie1981; Feldmann and Zinsmeister, Reference Feldmann and Zinsmeister1984; Feldmann, Reference Feldmann1985, Reference Feldmann1988, Reference Feldmann1990, Reference Feldmann1992a, Reference Feldmannb; Müller and Walossek, Reference Müller and Walossek1985, Reference Müller and Walossek1987; Weidner and Feldmann, Reference Weidner and Feldmann1985; Bishop, Reference Bishop1986; Tucker et al., Reference Tucker, Feldmann, Holland and Brinster1987; Feldmann and Wilson, Reference Feldmann, Wilson, Feldmann and Woodburne1988; Tshudy and Feldmann, Reference Tshudy, Feldmann, Feldmann and Woodburne1988; Feldmann et al., Reference Feldmann, Tshudy and Thomson1993, Reference Feldmann, Schweitzer and Marenssi2003; Hannibal et al., Reference Hannibal, Feldmann, Rolfe and Suárez-Soruco1993; Walossek and Müller, Reference Walossek, Müller, Fortey and Thomas1997; Bishop et al., Reference Bishop, Feldmann and Vega1998; Schweitzer and Feldmann, Reference Schweitzer and Feldmann2000a, Reference Schweitzer and Feldmannb; Crawford et al., Reference Crawford, Feldmann, Waugh, Kelley and Allen2006; Schwimmer and Montante, Reference Schwimmer and Montante2007; Feldmann et al., Reference Feldmann, Franƫescu, Franƫescu, Klompmaker, Logan, Robins, Schweitzer and Waugh2012; Cotroneo et al., Reference Cotroneo, Schiffbauer, Mccoy, Wortmann, Darroch, Peng and Laflamme2016; Tashman et al., Reference Tashman, Feldmann and Schweitzer2019; Bicknell et al., Reference Bicknell, Ortega-Hernández, Edgecombe, Gaines and Paterson2021; Fig. 1). Concretions have varied compositions, among them are calcareous concretions (especially calcite, siderite, or ankerite, e.g., Fig. 1.2, 1.3, 1.5, 1.6), siliceous concretions (often quartz, or quartz sand combined with other minerals such as glauconite; Fig. 1.1, 1.4), phosphatic (e.g., francolite) concretions, and iron sulfide (pyrite or marcasite) concretions. Feldmann et al. (Reference Feldmann, Franƫescu, Franƫescu, Klompmaker, Logan, Robins, Schweitzer and Waugh2012) noted, from concretions in the Bearpaw Shale (Cretaceous) of Montana, that concretions often contain more than one mineral, the result of multiple steps in concretion formation. In that specific example, framboidal pyrite, probably resulting from mineralization of a microbial sheath surrounding lobster cuticle, was present, in addition to francolite, which replaced the lobster cuticle. Most of the concretionary mass, however, was composed of calcium carbonate.

Originally, Seilacher at al. (Reference Seilacher, Reif and Westphal1985, p. 19) referred to concretions as “a subset of stagnation (and obrution?) deposits,” a form of conservation deposit, and some may have such an origin. However, a range of biological and biostratinomic circumstances may result in concretion development. Dhami et al. (Reference Dhami, Greenwood, Poropat, Tripp, Elson, Vijay and Bosnan2023) reviewed recent literature pertaining to the compositions of fossil-bearing concretions and factors involved in their formation, including microbial mediation and geochemical pathways. As for the origin of the specimens illustrated here (Fig. 1), obrution and biofilm-mediated early mineralization are likely, but compelling evidence for stagnation is lacking. Some concretions may qualify, in part, as skeletal concentration deposits, owing in part to event deposition followed by early diagenesis (Brett et al., Reference Brett, Baird, Speyer, Brett and Baird1997, Reference Brett, Zambito, Hunda and Schindler2012).

One unifying characteristic of concretions is the tendency for their stratigraphic occurrence to be along horizons that have wide lateral extent (e.g., Babcock and Speyer, Reference Babcock and Speyer1987; Hellstrom and Babcock, Reference Hellstrom and Babcock2000; Whiteley et al., Reference Whiteley, Kloc and Brett2002), indicating an origin with physical events, coupled with biological processes of decay and microorganism-mediated mineralization in sediment where pore-water conditions or chemical microenvironments within biofilm “decay halos” took place (e.g., Allison and Briggs, Reference Allison, Briggs, Allison and Briggs1991b; Borkow and Babcock, Reference Borkow and Babcock2003; Briggs, Reference Briggs, Krumbein, Paterson and Zarvarzin2003a; Babcock et al., Reference Babcock, Peng, Brett, Zhu, Ahlberg, Bevis and Robison2015; Cotroneo et al., Reference Cotroneo, Schiffbauer, Mccoy, Wortmann, Darroch, Peng and Laflamme2016). Obrution, condensation plus obrution, and perhaps anoxia, in addition to biological factors, played central roles in the development of certain concretion beds that are rich in marine arthropod remains. Some carbonate concretions evidently developed in places, or during times, of siliciclastic sediment starvation (e.g., Babcock et al., Reference Babcock, Peng, Brett, Zhu, Ahlberg, Bevis and Robison2015).

Certain concretionary horizons rich in enrolled trilobite remains have been ascribed a relationship to storm events. In an example from the Alden Pyrite Bed in the Ledyard Shale Member of the Ludlowville Formation (Devonian) of New York, Babcock and Speyer (Reference Babcock and Speyer1987) inferred that phacopine trilobites became buried in sediment, enrolled, as advancing storm conditions stirred up sediment. Burial in anoxic muds and failure to emerge from the bottom-smothering sediment following the storm resulted in the trilobites succumbing in their enrolled postures. Concretion growth followed shortly thereafter. This obrution model can be invoked for other instances of beds rich in enrolled trilobite corpses, including ones known from the Cambrian of Missouri (Stitt, Reference Stitt1983), the Ordovician of southwest Ohio and adjacent Kentucky and Indiana (Osgood, Reference Osgood1970; Brandt Velbel, Reference Brandt Velbel and Curran1985; Hunda et al., Reference Hunda, Hughes and Flessa2006; Brett et al., Reference Brett, Zambito, Hunda and Schindler2012), the Devonian of Ohio, Michigan, and Indiana (Stewart, Reference Stewart1927; Kesling and Chilman, Reference Kesling and Chilman1975), and the Permian of Kansas (Whittington, Reference Whittington1992).

Apart from the examples from the Alden Pyrite Bed, these occurrences of enrolled trilobites from the United States have not been commonly considered as concretions, but indeed they seem to be, because the fossils are preserved in rounded masses of, or containing, mineralized material, and their origins appear to be related to microorganism–sediment interactions. There are varied ways enrolled trilobites have been preserved, however. In the Alden Pyrite Bed, the trilobites are fully to partly pyritized except for the calcite-reinforced exoskeleton. In the Silica Shale (Devonian, e.g., Stewart, Reference Stewart1927; Kesling and Chilman, Reference Kesling and Chilman1975) and the Cincinnatian “series” (Ordovician, e.g., Brandt Velbel, Reference Brandt Velbel and Curran1985; Babcock, Reference Babcock, Feldmann and Hackathorn1996; Hunda et al., Reference Hunda, Hughes and Flessa2006), pyrite accompanies mud-size carbonate and occasionally carbonate spar. Pyrite is present is much greater quantity within the “capsules” formed by the Silica Shale trilobites than in the Cincinnatian trilobites, where it preserves appendages and internal soft tissues (Vayda and Babcock, Reference Vayda and Babcock2022). Preservation of non-biomineralized anatomy of trilobites is rare in outstretched trilobites from these two deposits. In both the Missouri (Cambrian) and Kansas (Permian) examples, preservation is almost exclusively in mud-sized sediment, now calcite.

Another important instance of trilobite preservation in concretions is the layers of outstretched trilobites in the Wheeler and Marjum formations (Cambrian) of Utah (Bright, Reference Bright1959; Gaines and Droser, Reference Gaines and Droser2003; Robison and Babcock, Reference Robison and Babcock2011; Robison et al., Reference Robison, Babcock and Gunther2015). Both corpses and molts are preserved in the concretions, with cone-in-cone development extending stratigraphically downward (Gaines and Droser, Reference Gaines and Droser2003; Robison and Babcock, Reference Robison and Babcock2011). Babcock et al. (Reference Babcock, Peng, Brett, Zhu, Ahlberg, Bevis and Robison2015) remarked that cone-in-cone development is commonly associated with anoxic or dysoxic conditions, and Gaines and Droser (Reference Gaines and Droser2003) inferred that the Elrathia-rich beds of the Wheeler Formation represented sedimentation in the exaerobic zone during the Cambrian. Preservation of appendages and internal soft tissues is common in the concretions from the Wheeler and Marjum formations (Fig. 1.6).

In another example of concretionary development, Babcock et al. (Reference Babcock, Peng, Brett, Zhu, Ahlberg, Bevis and Robison2015) noted that large “orsten”-type concretions in the Cambrian of Scandinavia and China may have a relationship to eustatic sea level, coinciding with transgressive systems or maximum flooding, and siliciclastic sediment starvation in basinal areas. “Orsten”-type carbonate concretions have produced exquisitely preserved, small arthropods that sometimes retain appendages and soft, non-biomineralized tissues (e.g., Müller, Reference Müller1979, Reference Müller1983; Müller and Walossek, Reference Müller and Walossek1985, Reference Müller and Walossek1987; Walossek and Müller, Reference Walossek, Müller, Fortey and Thomas1997, Reference Walossek, Müller and Edgecombe1998; Babcock et al., Reference Babcock, Dong and Robison2005).

Arthropods and many other organisms have been documented from sideritic Mazon Creek-type concretions of North America and Europe (e.g., Meek and Worthen, Reference Meek and Worthen1865, Reference Meek and Worthen1866; Lesquereux, Reference Lesquereux1866; Johnson and Richardson, Reference Johnson and Richardson1966; Nitecki, Reference Nitecki1979; Baird et al., Reference Baird, Shabica, Anderson and Richardson1985a, Reference Baird, Sroka, Shabica and Beardb, Reference Baird, Sroka, Shabica and Kuecher1986; Brett et al., Reference Brett, Baird, Speyer, Brett and Baird1997; Cotroneo et al., Reference Cotroneo, Schiffbauer, Mccoy, Wortmann, Darroch, Peng and Laflamme2016; Bicknell et al., Reference Bicknell, Ortega-Hernández, Edgecombe, Gaines and Paterson2021; Fig. 1.2). Most of these concretions formed in marginal-marine to shallow-marine, siliciclastic-dominated deltaic paleoenvironments. The concretions preserve a combination of marine, nonmarine aqueous, and terrestrial biota. Together, these concretions probably result from a variety of biologic factors and incorporate aspects of both concentration and conservation models. Cotroneo et al. (Reference Cotroneo, Schiffbauer, Mccoy, Wortmann, Darroch, Peng and Laflamme2016) introduced a model for the formation of the sideritic Mazon Creek concretions, involving encasement of decaying organic remains by early diagenetic pyrite and siderite, which was mediated by sulfate-reducing bacteria. High-porosity proto-concretions were cemented prior to compaction by siderite resulting from the methanogenic production of bicarbonate. Proto-concretion formation must have occurred on the order of days after settling of organic remains on the sediment surface, because details of the soft anatomy of arthropods and other organisms are commonly preserved (e.g., Nitecki, Reference Nitecki1979; Baird et al., Reference Baird, Sroka, Shabica and Kuecher1986; Bicknell et al., Reference Bicknell, Ortega-Hernández, Edgecombe, Gaines and Paterson2021).

In numerous Mesozoic and Cenozoic examples, arthropods, and especially decapods, are common in concretions (e.g., Feldmann and McPherson, Reference Feldmann and McPherson1980; Feldmann and Zinsmeister, Reference Feldmann and Zinsmeister1984; Feldmann, Reference Feldmann1985; Bishop, Reference Bishop1986; Tucker et al., Reference Tucker, Feldmann, Holland and Brinster1987; Feldmann and Wilson, Reference Feldmann, Wilson, Feldmann and Woodburne1988; Tshudy and Feldmann, Reference Feldmann1988; Feldmann et al., Reference Feldmann, Tshudy and Thomson1993, Reference Feldmann, Schweitzer and Marenssi2003, Reference Feldmann, Franƫescu, Franƫescu, Klompmaker, Logan, Robins, Schweitzer and Waugh2012; Bishop et al., Reference Bishop, Feldmann and Vega1998; Schweitzer and Feldmann, Reference Schweitzer and Feldmann2000a, Reference Schweitzer and Feldmannb; Crawford et al., Reference Crawford, Feldmann, Waugh, Kelley and Allen2006; Babcock et al., Reference Babcock, Feldmann and Grunow2024). Many of these occurrences are inferred molt ensembles or incomplete exoskeletons (Fig. 1.1, 1.4, 1.5), although some corpses are undoubtedly represented. Among many excellent examples, the Cretaceous–Paleogene and Eocene deposits of Seymour Island, Antarctica, are particularly noteworthy (Feldmann and Zinsmeister, Reference Feldmann and Zinsmeister1984; Feldmann, Reference Feldmann1985, Reference Feldmann, Wilson, Feldmann and Woodburne1988; Feldmann and Wilson, Reference Feldmann, Wilson, Feldmann and Woodburne1988; Feldmann et al., Reference Feldmann, Schweitzer and Marenssi2003; Babcock et al., Reference Babcock, Feldmann and Grunow2024). Here, mostly large adult decapods are preserved in dense, siliceous and glauconitic concretions from shallow siliciclastic marine environments. Most remains appear to be molted exoskeletons in various states of completeness, from fully intact to separated sclerites. Some lobsters have been preserved as molt ensembles in “Salterian” position, suggesting that concretionary growth was rapid following the time of molting (Feldmann et al., Reference Feldmann, Tshudy and Thomson1993; Babcock et al., Reference Babcock, Feldmann and Grunow2024; Fig. 1.4).

Growth of concretions, at least initially, has been demonstrated experimentally to be related to the rapid development of a microbial biofilm or microbial sheath (a so-called “decay halo”) surrounding recently dead or shed organic remains (Borkow and Babcock, Reference Borkow and Babcock2003; Briggs, Reference Briggs, Krumbein, Paterson and Zarvarzin2003a, Reference Briggsb). The decay halo, in studied examples, comprises a network of intertwined fungal hyphae that form a three-dimensional, rounded envelope around decaying matter. Bacterial cells associated with this consortium can include inferred autolithifiers (Borkow and Babcock, Reference Borkow and Babcock2003; see also Feldmann et al., Reference Feldmann, Franƫescu, Franƫescu, Klompmaker, Logan, Robins, Schweitzer and Waugh2012) that mediate the early stages of lithification in the decay halo, ultimately resulting in early diagenesis of a concretion. Experiments on the timing of arthropod decay and disarticulation (e.g., Plotnick, Reference Plotnick1986; Briggs and Kear, Reference Briggs and Kear1993; Babcock and Chang, Reference Babcock and Chang1997; Babcock et al., Reference Babcock, Merriam and West2000, Reference Babcock, Dong and Robison2005; Borkow and Babcock, Reference Borkow and Babcock2003; Briggs, Reference Briggs2003b) indicate that, for soft tissues to be preserved, mineral replication (for example, precipitation of a thin layer of mineral over soft tissue) must take place within about 7–10 days of death (Babcock et al., Reference Babcock, Merriam and West2000, Reference Babcock, Dong and Robison2005). Disarticulation along arthrodial membranes is a longer process but normally completes in about 30–40 days. The implication is that arthropods preserved with soft tissues, such as internal organs, muscles, or some connective tissues intact, were, in many instances, corpses that were quickly preserved by taphonomic processes including mineral replication and perhaps early burial, sediment sealing, or even microbial sealing. Articulated exoskeletons lacking these true soft tissues, in many instances, probably represent molts or corpses that remained at or near the sediment–water interface that underwent decay for up to several weeks.

Clusters

Clustered associations of trilobite remains have been well documented (e.g., Laudon, Reference Laudon1939; Esker, Reference Esker1964; Ludvigsen, Reference Ludvigsen1979; Speyer and Brett, Reference Speyer and Brett1985; Speyer, Reference Speyer and Boucot1990; Whittington, Reference Whittington1992; Levi-Setti, Reference Levi-Setti1993; Karim and Westrop, Reference Karim and Westrop2002; Whiteley et al., Reference Whiteley, Kloc and Brett2002; Babcock, Reference Babcock, Kelley, Kowalewski and Hansen2003; Robison and Babcock, Reference Robison and Babcock2011; Brett et al., Reference Brett, Zambito, Hunda and Schindler2012; Robison et al., Reference Robison, Babcock and Gunther2015; Corrales-García et al., Reference Corrales-García, Esteve, Zhao and Yang2020; Secher, Reference Secher2022; Bonino and Kier, Reference Bonino and Kier2024; Randolfe and Gass, Reference Randolfe and Gass2024; Figs. 2.2–2.5, 3.2, 4.2). Most such occurrences are dominated by exoskeletons within narrow size classes, inferred to have been molted (for recognition criteria, see Henningsmoen, Reference Henningsmoen1975; McNamara and Rudkin, Reference McNamara and Rudkin1984; Speyer, Reference Speyer1985; Brandt, Reference Brandt1993; Babcock, Reference Babcock, Kelley, Kowalewski and Hansen2003; Brett et al., Reference Brett, Zambito, Hunda and Schindler2012). In most examples, the trilobites are outstretched and, with rare exception (Corrales-García et al., Reference Corrales-García, Esteve, Zhao and Yang2020), are adult (holaspid) exoskeletons (e.g., Speyer and Brett, Reference Speyer and Brett1985; Speyer, Reference Speyer and Boucot1990; Karim and Westrop, Reference Karim and Westrop2002; Babcock, Reference Babcock, Kelley, Kowalewski and Hansen2003; Brett et al., Reference Brett, Zambito, Hunda and Schindler2012). Clusters may consist of complete exoskeletons or incomplete ones (lacking, for example, the librigenae).

Horizons yielding clusters of trilobites commonly have wide aerial extents, indicating an origin with physical events, coupled with biological processes of decay and microorganism-mediated mineralization (Brett et al., Reference Brett, Zambito, Hunda and Schindler2012). In some classic examples, such as in the Hamilton Group (Devonian) of New York (Speyer and Brett, Reference Speyer and Brett1985; Brett et al., Reference Brett, Zambito, Hunda and Schindler2012), and the “butter beds” of the Arnheim Formation (Ordovician) of Ohio, Kentucky, and Indiana (e.g., Osgood, Reference Osgood1970; Brandt Velbel, Reference Brandt Velbel and Curran1985; Babcock, Reference Babcock, Feldmann and Hackathorn1996; Hunda et al., Reference Hunda, Hughes and Flessa2006; Brett et al., Reference Brett, Zambito, Hunda and Schindler2012), the occurrence of large numbers of outstretched trilobites in single sedimentary beds has been attributed to obrution (sediment smothering) on storm-prone shallow marine shelves (Speyer and Brett, Reference Speyer and Brett1985; Babcock, Reference Babcock, Feldmann and Hackathorn1996; Whiteley et al., Reference Whiteley, Kloc and Brett2002; Brett et al., Reference Brett, Zambito, Hunda and Schindler2012). In some instances, biological activity, such as synchronized mass molting, likely preceded the accumulation of molts, which was then followed by obrution and diagenesis (Speyer and Brett, Reference Speyer and Brett1985; Karim and Westrop, Reference Karim and Westrop2002; Babcock, Reference Babcock, Kelley, Kowalewski and Hansen2003; Gutiérrez-Marco et al., Reference Gutiérrez-Marco, Sá, García-Bellido, Rábano and Valério2009; Robison and Babcock, Reference Robison and Babcock2011; Brett et al., Reference Brett, Zambito, Hunda and Schindler2012; Corrales-García et al., Reference Corrales-García, Esteve, Zhao and Yang2020). Biological mediators and anoxia may have also played roles in the taphonomic history of clusters.

Most cluster associations probably qualify as short-term condensation (concentration) deposits, perhaps on the order of hours to days to weeks. Others, especially where separated and time-averaged suites of sclerites are present (Figs. 3.2, 4.2), likely reflect longer time intervals, perhaps through years in duration. Where non-biomineralized parts are preserved in specimens buried soon after death, such as in pyritized trilobites from the Ordovician of New York (Brett et al., Reference Brett, Zambito, Hunda and Schindler2012), they also may qualify as conservation deposits.

Trilobite cluster associations could have been the result of animal behavior, current activity, sediment smothering, and potentially other factors. Cluster associations were among the earliest taphonomic associations to be discussed for fossil arthropods: clusters of trilobites in discrete, fine-grained limestone layers in the lower Wanakah Shale (Devonian) of western New York came to be known as “Grabau’s trilobite beds” (Whiteley et al., Reference Whiteley, Kloc and Brett2002), following their documentation in the late nineteenth century by Grabau (Reference Grabau1899). Most occurrences of trilobite clusters in “Grabau’s trilobite beds” and elsewhere in the Hamilton Group (Devonian) of New York, are either monospecific or disproportionately dominated by one species (Speyer and Brett, Reference Speyer and Brett1985; Fig. 2.4). Speyer and Brett (Reference Speyer and Brett1985) attributed this pattern to the aggregation of animals at the time of mating, during which time older exoskeletons were cast off and abandoned. Shortly afterward, the abandoned molts were buried under a cloud of sediment introduced by storm activity (obrution). Karim and Westrop (Reference Karim and Westrop2002) reached a similar conclusion for Ordovician trilobites from Oklahoma that are preserved in large clusters. Gutiérrez-Marco et al. (Reference Gutiérrez-Marco, Sá, García-Bellido, Rábano and Valério2009) documented large clusters of Ordovician trilobites from Portugal and suggested this gregarious behavior offered temporary refuge from predators and represented synchronous molting and reproduction. In contrast, Corrales-García et al. (Reference Corrales-García, Esteve, Zhao and Yang2020) documented clusters of trilobites from the Cambrian of South China that exhibited gregarious activity attributed to the need for protection during synchronized molting, which was unrelated to reproductive behavior.

Robison and Babcock (Reference Robison and Babcock2011) documented non-random orientations in trilobite clusters from the Cambrian of Utah that suggest bidirectional or multidirectional current redistribution prior to sediment covering. This tends to support the inference that the remains were molted, although the possibility that some corpses were present cannot be ruled out entirely. These exoskeletal remains may have accumulated at the sediment–water interface over a period of days to a few weeks prior to rapid, event-related sediment covering. Remains could have been initially scattered over a rather broad area and concentrated to some extent by current activity prior to burial.

Some clustered associations of arthropods can be reasonably attributed to events involving mass mortality. Robison et al. (Reference Robison, Babcock and Gunther2015) illustrated a cluster of Cambrian trilobites from Utah with opisthoparian facial sutures and retaining their librigenae in place, indicating a mass mortality rather than an accumulation of molts. Similarly, Crawford et al. (Reference Crawford, Casadío, Feldmann, Griffin, Parras and Schweitzer2008) documented a mass mortality of decapod crustaceans from the Miocene of Argentina, citing Andean volcanism as a factor in the mortality and burial event.

Clusters of non-trilobite arthropods in marine or other aquatic settings have received less attention. Obrution can be invoked as a major factor in their occurrence, but prior concentration of remains through current activity and/or biologic activity is likely in various instances. Layers in Paleozoic carbonates that are unusually rich in monospecific assemblages of ostracodes may be explained as coterie assemblages of crustaceans that gathered in mass aggregations for mating, events that were accompanied by synchronized molting. Alternatively, these could represent mass mortality events. One such layer, from the Greenfield Dolomite (Silurian) of Ohio is illustrated here (Fig. 2.1). The exoskeletons may have been concentrated by currents. Afterward, rapid sedimentary influx, perhaps by storm suspension and smothering, led to sedimentary sealing and, ultimately, preservation.

Similar to mass accumulations of trilobites and ostracodes reviewed above, Vrazo and Braddy (Reference Vrazo and Braddy2011) ascribed some accumulations of eurypterid remains to mass molting and mating events, followed by burial of the molted exoskeletons. Vrazo et al. (Reference Vrazo, Brett and Ciurca2017) placed these associations in a broader ecological–sedimentary context, interpreting the mass fossilization of eurypterids in Silurian and Devonian deposits of the Appalachian Basin to be a result of the interplay of several factors, including habitat preference, burial by storms or microbialite sediment baffling, and sequence-stratigraphic history. Vrazo et al. (Reference Vrazo, Brett and Ciurca2017) interpreted the eurypterids as preferentially inhabiting nearshore marine ecosystems following freshening of water during times of transgression. After burial of molted exoskeletons, long-term preservation of remains was facilitated by regression and cyclical shallowing-upward deposition that promoted hypersalinity and anoxia.

Bicknell et al. (Reference Bicknell, Pates and Botton2019b) documented a cluster of the belinurid xiphosuran Euproops from the Carboniferous of England, inferring that they exhibited mass mating behavior. They speculated that this gregarious behavior may have helped decrease the effect of predation, increased genetic diversity, or both.

Another example of arthropod clusters is illustrated by phyllocarid remains from a cannel-coal-type bed, described as a marine horizon (Bennington, Reference Bennington, Rice, Martino and Slucher1992) in the Breathitt Formation (Carboniferous) of Kentucky. One slab (Fig. 3.3) illustrates outstretched exoskeletons in various states of completeness with a nearly orthogonal arrangement of their long axes. The valves of the carapace are opened in a “butterflied” pattern and internal non-biomineralized parts are not preserved. The mandibles are in place in some specimens. This concentration of exoskeletons, which are probably molts, is inferred to have been influenced by currents. Burial leading to preservation may have been storm related, although their introduction to a dysoxic or anoxic channelform deposit in a transitional marine–nonmarine environmental setting indicates that low oxygen conditions played a significant role in their preservation. Naimark et al. (Reference Naimark, Kalinina, Shokurov, Markov, Zaytseva and Boeva2018) documented the role of clay minerals, especially kaolinite, in facilitating preservation of arthropod remains in sediments. It is likely that clay minerals present in the cannel coal also played an important role in preservation of these phyllocarids.

Event beds

Tempestites, or storm deposits, sediment–gravity flow deposits, and ash beds resulting in rapid burial of remains, and sediment sealing are important sources of arthropod fossils (Taylor, Reference Taylor1976; Speyer and Brett, Reference Speyer and Brett1985; Babcock, Reference Babcock1994; Robison, Reference Robison1994; Brett et al., Reference Brett, Baird, Speyer, Brett and Baird1997, Reference Brett, Zambito, Hunda and Schindler2012; Peng et al., Reference Peng, Babcock and Lin2004a, Reference Peng, Babcock and Linb; Albani et al., Reference Albani, Mazurier, Edgecombe, Azizi, Bekhouch, Berks and Bouougri2024). Event-bed deposition was discussed in part in connection with both concretionary horizons and cluster associations (above). Beds enriched in fossils can accumulate over temporal scales ranging from geologically “instantaneous” to thousands of years (Kidwell and Jablonski, Reference Kidwell, Jablonski, Tevesz and McCall1983; Kidwell, Reference Kidwell, Allison and Briggs1991; Kidwell and Bosence, Reference Kidwell, Bosence, Allison and Briggs1991). Those containing arthropod remains, however, tend to represent shorter time spans. They are primarily event-bed concentrations, but occasionally composite concentrations, in the terminology of Kidwell (Reference Kidwell, Allison and Briggs1991). Brett et al. (Reference Brett, Baird, Speyer, Brett and Baird1997) recognized that short-term events can incorporate live burial of organisms (mass mortality events), or passive burial of separated body parts (skeletal concentrations). Skeletal concentrations can be either parautochthonous or allochthonous.

Current-related transportation, stunning or killing of live animals, and obrution have been invoked for some deposits such as the Burgess Shale (Conway Morris, Reference Conway Morris1985, Reference Conway Morris1998), where animals may have been transported from shallow shelf areas into deeper anoxic water and buried. Such transport by means of turbidites or other forms of sediment-gravity flow, would have resulted in an aggregation of both corpses and molts that had concentrated near the sediment–water interface.

Some of the richest sources of Cambrian trilobites, mostly disarticulated sclerites, are from sediment-gravity deposits on distal to marine carbonate shelves to slopes. The Henson Gletscher Formation of North Greenland (Babcock, Reference Babcock1994; Robison, Reference Robison1994) and the Huaqiao Formation of South China (Peng et al., Reference Peng, Babcock and Lin2004a, Reference Peng, Babcock and Linb) both show concentrations of remains, high in abundance and species richness, in carbonate-hosted, gravity-flow deposits. Separated sclerites that are transported and buried in sediment-gravity flows are sometimes more numerous towards the upper portions of flow beds. In these instances, relatively light sclerites, easily suspended in a water-charged turbid flow of sediment, evidently settled out of suspension as part of the relatively light fraction of particles. This pattern contrasts with winnowed shell-bed-type concentrations, where less grading of particles may have occurred.

On stormy marine shelves, clouds of suspended sediment are inferred to have buried organismal remains present at the sediment surface, sealing them in sediment, and leading to their preservation near the bases of tempestite beds (Speyer and Brett, Reference Speyer and Brett1985). Alternatively, storm-related currents may have picked up, winnowed (washed) or concentrated remains, and redeposited the parautochthonous remains some distance from their prior resting sites. Thin limestone beds rich in disarticulated and often broken trilobite sclerites plus other fossils are a persistent feature of such deposits as the Rochester Shale (Silurian) of New York and Ontario, and the Cincinnatian “series” (Ordovician) of Ohio, Kentucky, and Indiana. These tempestite beds may be rich in arthropod remains but for the most part remain understudied for their taphonomic and paleobiologic potential (see Miller, Reference Miller, Brett and Baird1997). Whereas broken trilobite sclerites in such deposits could be assumed to be related to physical transport, it is at least equally likely that they were broken through the action of predators (see Pratt, Reference Pratt1998; Babcock, Reference Babcock, Kelley, Kowalewski and Hansen2003). If so, remains preserved in tempestites could offer interesting insights into predator–prey relationships.

Albani et al. (Reference Albani, Mazurier, Edgecombe, Azizi, Bekhouch, Berks and Bouougri2024) documented Cambrian trilobites that were entombed in volcanic ash from Morocco. Extraordinarily well-preserved specimens from this deposit have yielded remarkable details about their non-biomineralized anatomy and its relationship to biomineralized morphology.

In the context of event deposition, a noteworthy trace-fossil-rich Lagerstätte pertains in part to the fossil record of arthropods. Tempestite beds, cemented with carbonate (commonly siderite) in the Chagrin Shale Member of the Ohio Shale (Devonian) of Ohio, yield a rich and diverse trace fossil assemblage (Hannibal and Feldmann, Reference Hannibal and Feldmann1983; Stukel, Reference Stukel1987; Hannibal, Reference Hannibal, Feldmann and Hackathorn1996). Some of these traces, Chagrinichnites, record the burial of phyllocarid crustaceans under a rapidly deposited sediment layer, followed by the animals burrowing through the sediment layer, emerging from the surface (Feldmann et al., Reference Feldmann, Osgood, Szmuc and Meinke1978; Hannibal and Feldmann, Reference Hannibal and Feldmann1983). This example of live arthropods extricating themselves from under rapidly deposited sediment serves as a model for other marine arthropods that had the capacity for burrowing in sediment. Many other arthropods, such as trilobites, if they were live animals at the time of burial, were probably able to escape to safety. This was not necessarily true for trilobites enrolled below the sediment surface under inhospitable conditions, however.

Microbial sealing

In certain fine-grained carbonate (limestone or dolostone) deposits, arthropod and other remains have been preserved through a combination of factors that include microbial sealing (Barthel et al., Reference Barthel, Swinburne and Conway Morris1990; Allison and Briggs, Reference Allison, Briggs, Allison and Briggs1991b; Vrazo et al., Reference Vrazo, Brett and Ciurca2016). Such deposits commonly are described as lithographic-type carbonates (Barthel et al., Reference Barthel, Swinburne and Conway Morris1990; Wendruff et al., Reference Wendruff, Babcock, Kluessendorf and Mikulic2020). Seilacher et al. (Reference Seilacher, Reif and Westphal1985) referred to exceptional preservation in the Solnhofen Limestone (Jurassic) of Germany, a true lithographic limestone, as deposition that took place under obrutionary stagnation conditions, citing a depauperate benthos, few sediment-penetrative traces, and features consistent with cyanobacterial mats such as thin laminations, near-lack of erosion, rip-ups, and ruffling of sediment surfaces.

Wendruff et al. (Reference Wendruff, Babcock, Kluessendorf and Mikulic2020) attributed exceptional preservation in the lithographic dolostone of the Waukesha Lagerstätte (Silurian of Wisconsin; Mikulic et al., Reference Mikulic, Briggs and Kluessendorf1985a, Reference Mikulic, Briggs and Kluessendorfb), to “microbial entombment.” As described by Wendruff et al. (Reference Wendruff, Babcock, Kluessendorf and Mikulic2020), microbial entombment is a microorganism-mediated sedimentary and early diagenetic process, incorporating elements of microbial sealing, microbially mediated mineral precipitation (sometimes accompanied by carbonate dissolution), and physical sediment accumulation within a microbial mat. The Waukesha Lagerstätte shows exceptional preservation of arthropod cuticle, characteristic of a conservation deposit, but the most common remains, trilobite exoskeletons in disarticulated or complete but outstretched condition (Wendruff et al., Reference Wendruff, Babcock, Kluessendorf and Mikulic2020; Randolfe and Gass, Reference Randolfe and Gass2024), imply a concentration of these remains at the sediment surface. Dissolution of carbonate from skeletal materials, including trilobite exoskeletons, can be attributed to geochemical conditions within microbial mats (Wendruff et al., Reference Wendruff, Babcock, Kluessendorf and Mikulic2020). Because the trilobites retain moderate but not original convexity, and lack exoskeletal fracturing, their remains must have become stuck on mat surfaces, covered rapidly by cyanobacteria and perhaps other microbes, then endured loss of carbonate.

The celebrated eurypterid-rich layers of the Bertie Group, a Silurian lithographic-type dolostone cropping out in New York and Ontario (Clarke and Ruedemann, Reference Clarke and Ruedemann1912; Kluessendorf, Reference Kluessendorf1994; Tetlie et al., Reference Tetlie, Tollerton and Ciurca2007; Vrazo and Braddy, Reference Vrazo and Braddy2011; Vrazo et al., Reference Vrazo, Brett and Ciurca2016, Reference Vrazo, Brett and Ciurca2017), and the Eramosa Formation (Silurian of Ontario; von Bitter et al., Reference von Bitter, Purnell, Tetreault and Stott2007), another fine-grained dolostone, both show features broadly similar to those reported from the Waukesha Lagerstätte (Wendruff et al., Reference Wendruff, Babcock, Kluessendorf and Mikulic2020) (see also Vrazo et al., Reference Vrazo, Brett and Ciurca2017, and discussion above in the section titled “Clusters”), therefore it can be inferred that similar preservational circumstances (conservation and concentration) prevailed. In these microbial-sealing associations, a combination of sedimentary and microbially mediated processes led to fossilization of accumulated or freshly dead organic remains, molted exoskeletons, and traces.