Biogenic Iron Preserves Structures during Fossilization: A Hypothesis: Iron from Decaying Tissues May Stabilize Their Morphology in the Fossil Record

The effects of clays on bacterial community composition during arthropod decay

Fossilization, or the transition of an organism from the biosphere to the geosphere, is a complex mechanism involving numerous biological and geological variables. Bacteria are one of the most significant biotic players to decompose organic matter in natural environments, early on during fossilization. However, bacterial processes are difficult to characterize as many different abiotic conditions can influence bacterial efficiency in degrading tissues. One potentially important variable is the composition and nature of the sediment on which a carcass is deposited after death. We experimentally examined this by decaying the marine shrimp Palaemon varians underwater on three different clay sediments. Samples were then analyzed using 16S ribosomal RNA sequencing to identify the bacterial communities associated with each clay system. Results show that samples decaying on the surface of kaolinite have a lower bacterial diversity than those decaying on the surface of bentonite and montmorillonite, which could explain the limited decay of carcasses deposited on this clay. However, this is not the only role played by kaolinite, as a greater proportion of gram-negative over gram-positive bacteria is observed in this system. Gram-positive bacteria are generally thought to be more efficient at recycling complex polysaccharides such as those forming the body walls of arthropods. This is the first experimental evidence of sediments shaping an entire bacterial community. Such interaction between sediments and bacteria might have contributed to arthropods' exquisite preservation and prevalence in kaolinite-rich Lagerstätten of the Cambrian Explosion.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13358-024-00324-7.

Comment on "The lower Cambrian lobopodian Cardiodictyon resolves the origin of euarthropod brains"

Strausfeld et al. (Report, 24 Nov 2022, p. 905) claim that Cambrian fossilized nervous tissue supports the interpretation that the ancestral panarthropod brain was tripartite and unsegmented. We argue that this conclusion is unsupported, and developmental data from living onychophorans contradict it.

Response to Comment on "The lower Cambrian lobopodian Cardiodictyon resolves the origin of euarthropod brains"

Budd et al. challenge the identity of neural traces reported for the Cambrian lobopodian Cardiodictyon catenulum. Their argumentation is unsupported, as are objections with reference to living Onychophora that misinterpret established genomic, genetic, developmental, and neuroanatomical evidence. Instead, phylogenetic data corroborate the finding that the ancestral panarthropod head and brain is unsegmented, as in C. catenulum.

New fossil assemblages from the Early Ordovician Fezouata Biota

The Fezouata Biota (Morocco) is a unique Early Ordovician fossil assemblage. The discovery of this biota revolutionized our understanding of Earth's early animal diversifications-the Cambrian Explosion and the Ordovician Radiation-by suggesting an evolutionary continuum between both events. Herein, we describe Taichoute, a new fossil locality from the Fezouata Shale. This locality extends the temporal distribution of fossil preservation from this formation into the upper Floian, while also expanding the range of depositional environments to more distal parts of the shelf. In Taichoute, most animals were transported by density flows, unlike the in-situ preservation of animals recovered in previously investigated Fezouata sites. Taichoute is dominated by three-dimensionally preserved, and heavily sclerotized fragments of large euarthropods-possibly representing nektobenthic/nektic bivalved taxa and/or hurdiid radiodonts. Resolving whether this dominance reflects a legitimate aspect of the original ecosystem or a preservational bias requires an in-depth assessment of the environmental conditions at this site. Nevertheless, Taichoute provides novel preservational and palaeontological insights during a key evolutionary transition in the history of life on Earth.

The lower Cambrian lobopodian Cardiodictyon resolves the origin of euarthropod brains

For more than a century, the origin and evolution of the arthropod head and brain have eluded a unifying rationale reconciling divergent morphologies and phylogenetic relationships. Here, clarification is provided by the fossilized nervous system of the lower Cambrian lobopodian Cardiodictyon catenulum, which reveals an unsegmented head and brain comprising three cephalic domains, distinct from the metameric ventral nervous system serving its appendicular trunk. Each domain aligns with one of three components of the foregut and with a pair of head appendages. Morphological correspondences with stem group arthropods and alignments of homologous gene expression patterns with those of extant panarthropods demonstrate that cephalic domains of C. catenulum predate the evolution of the euarthropod head yet correspond to neuromeres defining brains of living chelicerates and mandibulates.

Probability-based preservational variations within the early Cambrian Chengjiang biota (China)

The Chengjiang biota (Yunnan Province, China) is a treasure trove of soft-bodied animal fossils from the earliest stages of the Cambrian explosion. The mechanisms contributing to its unique preservation, known as the Burgess Shale-type preservation, are well understood. However, little is known about the preservation differences between various animal groups within this biota. This study compares tissue-occurrence data of 11 major animal groups in the Chengjiang biota using a probabilistic methodology. The fossil-based data from this study is compared to previous decay experiments. This shows that all groups are not equally preserved with some higher taxa more likely to preserve soft tissues than others. These differences in fossil preservation between taxa can be explained by the interaction of biological and environmental characteristics. A bias also results from differential taxonomic recognition, as some taxa are easily recognized from even poorly preserved fragments while other specimens are difficult to assign to higher taxa even with exquisite preservation.

The Chengjiang Biota inhabited a deltaic environment

The Chengjiang Biota is the earliest Phanerozoic soft-bodied fossil assemblage offering the most complete snapshot of Earth’s initial diversification, the Cambrian Explosion. Although palaeobiologic aspects of this biota are well understood, the precise sedimentary environment inhabited by this biota remains debated. Herein, we examine a non-weathered core from the Yu’anshan Formation including the interval preserving the Chengjiang Biota. Our data indicate that the succession was deposited as part of a delta influenced by storm floods (i.e., produced by upstream river floods resulting from ocean storms). Most Chengjiang animals lived in an oxygen and nutrient-rich delta front environment in which unstable salinity and high sedimentation rates were the main stressors. This unexpected finding allows for sophisticated ecological comparisons with other Burgess Shale-type deposits and emphasizes that the long-held view of Burgess Shale-type faunas as snapshots of stable distal shelf and slope communities needs to be revised based on recent sedimentologic advances.

The Chengjiang Biota is the earliest most diverse animal community from the Cambrian Explosion (~518 million years ago). This biota is shown to have colonized a delta, highlighting the importance of this shallow environment in recording early snapshots of life on Earth.

The complex role of microbial metabolic activity in fossilization

Bacteria play an important role in the fossilization of soft tissues; their metabolic activities drive the destruction of the tissues and also strongly influence mineralization. Some environmental conditions, such as anoxia, cold temperatures, and high salinity, are considered widely to promote fossilization by modulating bacterial activity. However, bacteria are extremely diverse, and have developed metabolic adaptations to a wide range of stressful conditions. Therefore, the influence of the environment on bacterial activity, and of their metabolic activity on fossilization, is complex. A number of examples illustrate that simple, general assumptions about the role of bacteria in soft tissue fossilization cannot explain all preservational pathways: (i) experimental results show that soft tissues of cnidaria decay less in oxic than anoxic conditions, and in the fossil record are found more commonly in fossil sites deposited under oxic conditions rather than anoxic environments; (ii) siderite concretions, which often entomb soft tissue fossils, precipitate due to a complex mixture of sulfate- and iron reduction by some bacterial species, running counter to original theories that iron reduction is the primary driver of siderite concretion growth; (iii) arthropod brains, now widely accepted to be preserved in many Cambrian fossil sites, are one of the first structures to decay in taphonomic experiments, indicating that their fossilization processes are complex and influenced by bacterial activity. In order to expand our understanding of the complex process of bacterially driven soft tissue fossilization, more research needs to be done, on fossils themselves and in taphonomic experiments, to determine how the complex variation in microbial metabolic activity influences decay and mineralization.

Leanchoiliidae reveals the ancestral organization of the stem euarthropod brain

Fossils provide insights into how organs may have diversified over geological time.¹ However, diversification already accomplished early in evolution can obscure ancestral events leading to it. For example, already by the mid-Cambrian period, euarthropods had condensed brains typifying modern mandibulate lineages.² However, the demonstration that extant euarthropods and chordates share orthologous developmental control genes defining the segmental fore-, mid-, and hindbrain suggests that those character states were present even before the onset of the Cambrian.³ Fossilized nervous systems of stem Euarthropoda might, therefore, be expected to reveal ancestral segmental organization, from which divergent arrangements emerged. Here, we demonstrate unsurpassed preservation of cerebral tissue in Kaili leanchoiliids revealing near-identical arrangements of bilaterally symmetric ganglia identified as the proto-, deuto-, and tritocerebra disposed behind an asegmental frontal domain, the prosocerebrum, from which paired nerves extend to labral ganglia flanking the stomodeum. This organization corresponds to labral connections hallmarking extant euarthropod clades⁴ and to predicted transformations of presegmental ganglia serving raptorial preocular appendages of Radiodonta.⁵ Trace nervous system in the gilled lobopodian Kerygmachela kierkegaardi⁶ suggests an even deeper prosocerebral ancestry. An asegmental prosocerebrum resolves its location relative to the midline asegmental sclerite of the radiodontan head, which persists in stem Euarthropoda.⁷ Here, data from two Kaili Leanchoilia, with additional reference to Alalcomenaeus,^8,9 demonstrate that Cambrian stem Euarthropoda confirm genomic and developmental studies^10-15 claiming that the most frontal domain of the euarthropod brain is a unique evolutionary module distinct from, and ancestral to, the fore-, mid-, and hindbrain.