Biogenic Iron Preserves Structures during Fossilization: A Hypothesis

Abstract

It is hypothesized that iron from biological tissues, liberated during decay, may have played a role in inhibiting loss of anatomical information during fossilization of extinct organisms. Most tissues in the animal kingdom contain iron in different forms. A widely distributed iron‐bearing molecule is ferritin, a globular protein that contains iron crystallites in the form of ferrihydrite minerals. Iron concentrations in ferritin are high and ferrihydrites are extremely reactive. When ancient animals are decaying on the sea floor under anoxic environmental conditions, ferrihydrites may initialize the selective replication of some tissues in pyrite FeS2. This model explains why some labile tissues are preserved, while other more resistant structures decay and are absent in many fossils. A major implication of this hypothesis is that structures described as brains in Cambrian arthropods are not fossilization artifacts, but are instead a source of information on anatomical evolution at the dawn of complex animal life.

Insights into soft-part preservation from the Early Ordovician Fezouata Biota

“…Maghemite is found associated with pyrite in some analyzed samples under Raman Spectroscopy (Fig. 6) (Saleh et al, 2020c and Thistlethwaite, 2002). Ferrihydrite is a widely distributed biological mineral (Aldred et al, 2009;Hoda et al, 2013;Dunaief et al, 2014).…”

“…quantity of decayed organic matter is not enough to reduce SO 4 2-) (Gabbott et al, 2004). However, when comparing internal labile tissues to each other, the model based on H 2 S limitation cannot explain why some tissues are pyritized, while others decayed and disappeared (meaning they reduced SO 4 2-) without pyritizing (Saleh et al, 2020c).…”

“…In these sediments, C is also associated with pyrite, possibly suggesting that the original mode of preservation in the Fezouata Shale was comparable to that of the Burgess Shale and the Chengjiang Biota comprising both organic material and authigenic minerals (Gaines et al, 2008;Saleh et al, 2020a,c). As no fossil shows complete pyritization and pyrite precipitation remains rare and tissue-selective, other parameters were also likely controlling pyrite precipitation in the Fezouata Shale (Saleh et al, 2020c). For instance, cuticles of many arthropod taxa are preserved without any pyrite crystals.…”

Central nervous system of a 310-m.y.-old horseshoe crab: Expanding the taphonomic window for nervous system preservation

“…Such a membrane in E. danae may have slowed the decay of the synganglion (relative to other internal structures) and facilitated the rapid molding of these delicate tissues. Given the ferruginous pore-water conditions that led to the precipitation of siderite, it is also possible that dissolved Fe 2+ may have played a role in delaying decay of neural tissue (Schweitzer et al, 2014;Saleh et al, 2020). It is also possible that reduction of porosity around the CNS via the rapid growth of concretion matrix further delayed its decay until after the neural tissue was robustly molded (McCoy et al, 2015a(McCoy et al, , 2015b).…”

Intrinsic Iron May Have Promoted Ancient Nervous Tissue Fossilization

“…propose a novel solution to this fossilization conundrum—a way to explain why nervous tissues are preserved in some fossils, not despite their propensity to decay but in part because of it. [ 3 ] Inspiration for their hypothesis comes from the observation that some fossilized nervous tissues are mineralized in pyrite (FeS 2 ), replicating them in a long‐lived mineral. Pyritization necessitates sources of iron, sulfate, and organic carbon—a lack of any can limit its formation.…”