An official website of the United States government
Scalidophoran worms represent common infaunal components of early and middle Cambrian Burgess Shale-type fossil biotas. Early scalidophorans resemble extant priapulids based on overall morphology, but the genusSelkirkiarepresents the earliest record of tube dwelling for the group. Despite its ubiquitous presence in exceptional marine deposits, whether the exclusively Cambrian occurrence ofSelkirkiareflects its entire evolutionary history or is affected by taphonomic biases remains unresolved. Here, we demonstrate the post-Cambrian survival ofSelkirkiabased on new material from the Lower Ordovician Fezouata Shale biota of Morocco. The discovery ofSelkirkiain the Fezouata Shale extends the biostratigraphic range of the genus by 25 million years and its palaeobiogeographic occurrence to the high latitudes of Gondwana, strengthens the evolutionary links between Cambrian and Ordovician Burgess Shale-type biotas and increases scalidophoran diversity for the Fezouata Shale biota otherwise consisting exclusively of the palaeoscolecidPalaeoscolex? tenensis. The tube ofSelkirkiaunderwent negligible external change for over 40 million years, indicating a high degree of morphological stasis during the Early Palaeozoic. A tubicolous mode of life is rare among extant priapulids and expressed only inMaccabeus, which forms a delicate tube from agglutinated plant debris, unlike the macroscopic secreted cuticular tube ofSelkirkia.
more »
« less
New fossil assemblages from the Early Ordovician Fezouata Biota
Abstract 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 thein-situpreservation 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.
more »
« less
- Award ID(s):
- 2047192
- PAR ID:
- 10385191
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Scientific Reports
- Volume:
- 12
- Issue:
- 1
- ISSN:
- 2045-2322
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract The Fezouata Shale Formation has dramatically impacted our understanding of Early Ordovician marine ecosystems before the great Ordovician biodiversification event (GOBE), thanks to the abundance and quality of exceptionally preserved animals within it. Systematic work has noted that the shelly fossil subassemblages of the Fezouata Shale biota are typical of open-marine deposits from the Lower Ordovician, but no studies have tested the quantitative validity of this statement. We extracted 491 occurrences of recalcitrant fossil genera from the Paleobiology Database to reconstruct 31 subassemblages to explore the paleoecology of the Fezouata Shale and other contemporary, high-latitude (66°S–90°S) deposits from the Lower Ordovician (485.4–470 Ma) and test the interpretation that the Fezouata Shale biota is typical for an Ordovician open-marine environment. Sørensen's dissimilarity metrics and Wilcoxon tests indicate that the subassemblages of the Tremadocian-aged lower Fezouata Shale are approximately 20% more heterogenous than the Floian-aged upper Fezouata Shale. Dissimilarity metrics and visualization suggest that while the lower Fezouata and upper Fezouata share faunal components, the two sections have distinct faunas. We find that the faunal composition of the lower Fezouata Shale is comparable with other Tremadocian-aged subassemblages from high latitudes, suggesting that it is typical for an Early Ordovician open-marine environment. We also find differences in faunal composition between Tremadocian- and Floian-aged deposits. Our results corroborate previous field-based and qualitative systematic studies that concluded that the shelly assemblages of the Fezouata Shale are comparable with those of other Lower Ordovician deposits from high latitudes. This establishes the first quantitative baseline for examining the composition and variability within the assemblages of the Fezouata Shale and will be key to future studies attempting to discern the degree to which it can inform our understanding of marine ecosystems just before the start of the GOBE.more » « less
-
Animals originated in the Neoproterozoic and ‘exploded’ into the fossil record in the Cambrian. The Cambrian also represents a high point in the animal fossil record for the preservation of soft tissues that are normally degraded. Specifically, fossils from Burgess Shale-type (BST) preservational windows give paleontologists an unparalleled view into early animal evolution. Why this time interval hosts such exceptional preservation, and why this preservational window declines in the early Paleozoic, have been long-standing questions. Anoxic conditions have been hypothesized to play a role in BST preservation, but recent geochemical investigations of these deposits have reached contradictory results with respect to the redox state of overlying bottom waters. Here, we report a multi-proxy geochemical study of the Lower Cambrian Mural Formation, Alberta, Canada. At the type section, the Mural Formation preserves rare recalcitrant organic tissues in shales that were deposited near storm wave base (a Tier 3 deposit; the worst level of soft-tissue preservation). The geochemical signature of this section shows little to no evidence of anoxic conditions, in contrast with published multi-proxy studies of more celebrated Tier 1 and 2 deposits. These data help confirm that ‘decay-limited’ BST biotas were deposited in more oxygenated conditions, and support a role for anoxic conditions in BST preservation. Finally, we discuss the role of iron reduction in BST preservation, including the formation of iron-rich clays and inducement of sealing seafloor carbonate cements. As oceans and sediment columns became more oxygenated and more sulfidic through the early Paleozoic, these geochemical changes may have helped close the BST taphonomic window.more » « less
-
Abstract The Ediacara biota are an enigmatic group of Neoproterozoic soft‐bodied fossils that mark the first major radiation of complex eukaryotic and macroscopic life. These fossils are thought to have been preserved via pyritic “death masks” mediated by seafloor microbial mats, though little about the chemical constraints of this preservational pathway is known, in particular surrounding the role of bioavailable iron in death mask formation and preservational fidelity. In this study, we perform decay experiments on both diploblastic and triploblastic animals under a range of simulated sedimentary iron concentrations, in order to characterize the role of iron in the preservation of Ediacaran organisms. After 28 days of decay, we demonstrate the first convincing “death masks” produced under experimental laboratory conditions composed of iron sulfide and probable oxide veneers. Moreover, our results demonstrate that the abundance of iron in experiments is not the sole control on death mask formation, but also tissue histology and the availability of nucleation sites. This illustrates that Ediacaran preservation via microbial death masks need not be a “perfect storm” of paleoenvironmental porewater and sediment chemistry, but instead can occur under a range of conditions.more » « less
-
null (Ed.)The Barstow Formation in the Mojave region of California was deposited in an extensional-basin setting of the Basin and Range province and preserves diverse middle Miocene mammalian assemblages. Six facies associations represent the dominant depositional environments in the basin, which changed through time from alluvial-fan and playa-dominated settings to floodplains and spring-fed wetlands. The majority of fossil localities and specimens occur in later-forming facies associations. We analyzed the taphonomic characteristics of fossil assemblages to test whether basin-scale facies associations or locality-scale facies exert more control on the preservational features of mammalian assemblages through the formation. We documented the facies settings of 47 vertebrate localities in the field in order to interpret depositional setting and the mode of accumulation for fossil assemblages. We evaluated skeletal material in museum collections for taphonomic indicators, including weathering stage, original bone-damage patterns, hydraulic equivalence, and skeletal-element composition. We evaluated four alternative modes of accumulation, including attritional accumulation on the land surface, accumulation by fluvial processes, carnivore or scavenger accumulations, and mass-death events. The majority of localities represent attritional accumulations at sites of long-term mortality in channel-margin, abandoned-channel, poorly drained floodplain, and ephemeral-wetland settings. Skeletal-element composition and taphonomic characteristics varied among facies, indicating an important role for depositional setting and landscape position on fossil-assemblage preservation. We find that locality-scale facies have a greater influence on the taphonomic characteristics of fossil assemblages; the taphonomy of each facies association is influenced by the facies that compose it. The facies composition and distribution within facies associations change through the formation, with a greater variety of depositional settings forming later in the history of the basin. Heterogeneous landscapes present more settings for fossil accumulation, contributing to the increase in fossil occurrence through the depositional history of the formation.more » « less
