Lidya G. Tarhan

Exceptional preservation of soft-bodied Ediacara Biota promoted by silica-rich oceans: REPLY

The Ediacara Biota, Earth’s earliest fossilized ecosystem of complex, macroscopic, multicellular organisms, occurs in terminal Ediacaran strata worldwide, yet how the fossils are preserved remains controversial. Ediacara assemblages consist of exceptionally preserved soft-bodied forms of enigmatic morphology and phylogenetic affinity. Many of these fossil assemblages are anactualistically preserved as casts and molds in sandstones (“Ediacara-style” preservation). Here we present evidence from the Ediacara Member of South Australia that Ediacara-style preservation was due to rapid, early-stage precipitation of silica cements, facilitated by the high silica saturation state of the oceans prior to the appearance of prolific silica biomineralizers. An early silicification model provides a coherent, mechanistic and empirically supported explanation for the widespread preservation of soft-bodied organisms of Ediacaran–early Paleozoic age as sandstone casts and molds. The prevalence of early silicification confirms that Ediacara-style fossil assemblages can provide an accurate window into life on the Ediacaran seafloor that can be used to reconstruct critical steps in the development and diversification of early animal ecosystems.

MICROBIAL MAT SANDWICHES AND OTHER ANACTUALISTIC SEDIMENTARY FEATURES OF THE EDIACARA MEMBER (RAWNSLEY QUARTZITE, SOUTH AUSTRALIA): IMPLICATIONS FOR INTERPRETATION OF THE EDIACARAN SEDIMENTARY RECORD

Abstract: The Ediacara Member of the Rawnsley Quartzite of South Australia hosts some of the most ecologically and taxonomically diverse fossil assemblages of the eponymous Ediacara Biota—Earths earliest fossil record of communities comprised of macroscopic, complex, multicellular organisms. At the National Heritage Site, Nilpena, fifteen years of systematic excavation and reassembly of bedding planes has resulted in reconstruction of over 400 square meters of Ediacaran seafloor, permitting detailed and sequential sedimentary, paleoecological and taphonomic assessment of Ediacara fossilized communities and the shallow marine settings in which these ecosystems lived. Sedimentological investigation reveals that the Ediacara Member consists of successions of sandstone event beds and a paucity of other lithologies. Moreover, these Ediacara sandstones are characterized by a suite of sedimentary features and style of stratigraphic packaging uncharacteristic of Phanerozoic sandstone successions considered to have been deposited in analogous shallow marine, storm-dominated environments, including: (1) extremely thin (sub-mm- to mm-scale) bed thickness; (2) lateral discontinuity; (3) textural uniformity, including lack of disparity in grain size, between adjacent beds; (4) lack of amalgamation; (5) lack of erosional bed junctions; (6) doubly rippled bedforms defined by rippled bed tops and bases which crisply cast the tops of underlying rippled beds; (7) ubiquity of textured organic surfaces (TOS); (8) positive correlation between body fossil size and abundance and bed thickness; and (9) texturally immature assemblages of sandstone rip-up clasts along bed tops. We interpret these features to reflect the presence of widespread matgrounds, which facilitated seafloor colonization by and ecological development of Ediacara macroorganisms in high-energy environments. Further, we argue that pervasive matgrounds directly mediated the formation and preservation of non-uniformitarian sedimentary features and stratigraphic packaging in the Ediacara Member and were responsible for the anactualistically complete nature of the Ediacara stratigraphic record.

The ‘Tully monster’ is a vertebrate

Problematic fossils, extinct taxa of enigmatic morphology that cannot be assigned to a known major group, were once a major issue in palaeontology. A long-favoured solution to the ‘problem of the problematica’, particularly the ‘weird wonders’ of the Cambrian Burgess Shale, was to consider them representatives of extinct phyla. A combination of new evidence and modern approaches to phylogenetic analysis has now resolved the affinities of most of these forms. Perhaps the most notable exception is Tullimonstrum gregarium, popularly known as the Tully monster, a large soft-bodied organism from the late Carboniferous Mazon Creek biota (approximately 309–307 million years ago) of Illinois, USA, which was designated the official state fossil of Illinois in 1989. Its phylogenetic position has remained uncertain and it has been compared with nemerteans, polychaetes, gastropods, conodonts, and the stem arthropod Opabinia. Here we review the morphology of Tullimonstrum based on an analysis of more than 1,200 specimens. We find that the anterior proboscis ends in a buccal apparatus containing teeth, the eyes project laterally on a long rigid bar, and the elongate segmented body bears a caudal fin with dorsal and ventral lobes. We describe new evidence for a notochord, cartilaginous arcualia, gill pouches, articulations within the proboscis, and multiple tooth rows adjacent to the mouth. This combination of characters, supported by phylogenetic analysis, identifies Tullimonstrum as a vertebrate, and places it on the stem lineage to lampreys (Petromyzontida). In addition to increasing the known morphological disparity of extinct lampreys, a chordate affinity for T. gregarium resolves the nature of a soft-bodied fossil which has been debated for more than 50 years.

DECAY OF THE SEA ANEMONE METRIDIUM (ACTINIARIA): IMPLICATIONS FOR THE PRESERVATION OF CNIDARIAN POLYPS AND OTHER SOFT-BODIED DIPLOBLAST-GRADE ANIMALS

Abstract: Fossils preserving the anatomy of soft tissues provide rare but essential evidence for the reconstruction of metazoan evolutionary history. Decay is inherent to the fossilization process and features may be distorted, displaced, or missing even in exceptionally preserved fossils, and non-anatomical artifacts may be introduced. Here we describe the results of experimental decay of the epibenthic actinian (sea anemone) Metridium senile and document serial changes in its morphology. Decay proceeded rapidly and followed a consistent, reproducible trajectory, which we divide into six stages; in the final stage, no anatomically illuminating information remained. The column, one of the most salient anatomical features of actinians, contracted near the time of death and changed shape dramatically thereafter. The tentacles decayed from their distal ends. Fibrous bundles representing the interior musculature were among the morphological features most resistant to degradation, and taphonomically induced transverse bands were also long lasting. These experimental results provide a semi-quantitative and predictive framework which can be applied to the interpretation of putative polypoid cnidarian organisms. Furthermore, this decay series shows that diploblast- and triploblast-grade fossils are unlikely to be confused and provides the means to evaluate the taxonomic validity of the long-standing assumption that many enigmatic soft-bodied Ediacaran and lower Cambrian taxa are of actinian grade.

Earth's oldest jellyfish strandings: a unique taphonomic window or just another day at the beach?

Discoidal macrofossils reported herein from the lower Cambrian Zabriskie Quartzite (Great Basin, western United States) record the oldest Phanerozoic medusozoan body fossils, as well as the oldest medusozoan stranding event on record. Moreover, these fossils provide evidence of a significant shift in the taphonomic mode characteristic of preservation of nonmineralized taxa in coarse-grained siliciclastic successions near the onset of the Phanerozoic. Taphonomic and sedimentological evidence recorded by these and younger examples of stranded Cambrian medusae is consistent in suggesting that several of the requirements for preservation of these fossils were holdovers from the Ediacaran Period, including the presence of microbial mats and a lack of carcass disturbance by scavenging and/or bioturbating taxa. To shed further light upon the taphonomic factors necessary for the preservation of Cambrian medusae, we compared the biostratinomy and sedimentology of Cambrian medusa strandings to those of Ediacara Biota assemblages from lithologically similar successions. We find key secular disparities in the taphonomic histories of these two types of fossil assemblage. Inconsistencies between the preservational styles characteristic of fossil assemblages preserved in sandstone lithofacies on each side of the Precambrian–Cambrian boundary are explained by a considerable change in the preferred depositional setting in which these macrofossil assemblages are preserved. Thus, rather than documenting a single taphonomic continuum through the Precambrian–Cambrian transition, the Zabriskie and younger medusozoan body fossil assemblages record the advent of an entirely new, yet still very rarely exploited, taphonomic window exclusive to the Cambrian Period.

MACROSCOPIC STRUCTURES IN THE 1.1 Ga CONTINENTAL COPPER HARBOR FORMATION: CONCRETIONS OR FOSSILS?

Abstract Continental siltstones of the Mesoproterozoic Copper Harbor Formation, Michigan contain macroscopic structures of a size and morphological complexity commonly associated with fossils of eukaryotic macroorganisms. A biogenic origin for these structures would significantly augment the Proterozoic continental fossil record, which is currently poor, and also add to a growing body of sedimentological and geochemical data that, albeit indirectly, indicates the presence of life in continental settings early in Earths history. These three-dimensional structures occur abundantly within a single cm-scale siltstone bed. Along this bedding plane, these structures are generally circular-to-ovoid, range up to several centimeters in diameter, and most specimens possess a transecting lenticular element. Structures exhibit sharp, well-rounded external margins and, in contrast to the surrounding aluminosilicate-rich matrix, are calcitic in composition. Surrounding sedimentary laminae are deflected by and cross cut the structures. A fossiliferous origin is considered but rejected and an authigenic concretionary origin is favored based on these characters. However, a concretionary origin does not exclude the possibility of a biogenic precursor that served as a locus for early diagenetic calcite precipitation. This study highlights the need for careful analysis of morphological, mineralogical, distributional, and sedimentological characteristics when considering the origin of enigmatic structures; morphological complexity alone is an insufficient criterion for assignment of biogenicity. The unusual morphology of these concretions augments known concretion morphologies generally comparable to unusual fossil forms, and draws into question the biogenicity of similarly cryptic Proterozoic structures including, in particular, those of the 2.0 Ga Francevillian B Formation of Gabon.

Meiofauna mute the Cambrian Explosion

Trace fossils from the Ediacaran–Cambrian transition of Brazil point to the existence of bioengineering meiofaunal animals prior to the ‘Cambrian Explosion’.

TAPHONOMIC DISPARITY IN FORAMINIFERA AS A PALEO-INDICATOR FOR SEAGRASS

Abstract Seagrass meadows are a key component of marine ecosystems that play a variety of prominent geobiological roles in modern coastal environments. However, seagrass itself has low preservation potential, and consequently seagrass meadows are hard to identify in the rock record. In this study we combine observational taphonomic data from a modern sparse seagrass meadow with actualistic taphonomic experiments, in order to test whether taphonomic disparity (i.e., evenness in the distribution of taphonomic grades among multiple individuals) in the larger benthic foraminiferan Archaias angulatus has potential as a paleo-indicator for seagrass dominated communities. Our observational study demonstrates that sparse seagrass meadows possess a higher proportion of both pristine and highly altered tests than non-seagrass settings. Our taphonomic experiments, conducted over a six-month period, demonstrate a greater magnitude of bioerosion and diversity of bioerosion types in foraminifera deployed within sparse seagrass patches, than those deployed in patches without any seagrass cover. Although our experimental results in particular have high variability, these combined approaches provide a link between pattern (high taphonomic disparity) and process (higher rates of bioerosion) in developing the taphonomic signature of seagrass meadows. On the basis of these results we suggest several taphonomic criteria that could be used to identify seagrass meadows in the rock record. These criteria are potentially species-independent, and so may have greater utility as seagrass proxies than invertebrate indicator species that frequently have limited temporal or spatial distributions.

Ecological Expansion and Extinction in the Late Ediacaran: Weighing the Evidence for Environmental and Biotic Drivers

The Ediacara Biota, Earths earliest communities of complex, macroscopic, multicellular organisms, appeared during the late Ediacaran Period, just prior to the Cambrian Explosion. Ediacara fossil assemblages consist of exceptionally preserved soft-bodied forms of enigmatic morphology and affinity which nonetheless represent a critical stepping-stone in the evolution of complex animal ecosystems. The Ediacara Biota has historically been divided into three successive Assemblages-the Avalon, the White Sea, and the Nama. Although the oldest (Avalon) Assemblage documents the initial appearance of several groups of Ediacara taxa, the two younger (White Sea and Nama) Assemblages record a particularly striking suite of ecological innovations, including the appearance of diverse Ediacara body plans-in tandem with the rise of bilaterian animals-as well as the emergence of novel ecological strategies such as movement, sexual reproduction, biomineralization, and the development of dense, heterogeneous benthic communities. Many of these ecological innovations appear to be linked to adaptations to heterogeneous substrates and shallow and energetic marine settings. In spite of these innovations, the majority of Ediacara taxa disappear by the end of the Ediacaran, with interpretations for this disappearance historically ranging from the closing of preservational windows to environmentally or biotically mediated extinction. However, in spite of the unresolved affinity and eventual extinction of individual Ediacara taxa, these distinctive ecological strategies persist across the Ediacaran-Cambrian boundary and are characteristic of younger animal-dominated communities of the Phanerozoic. The late Ediacaran emergence of these strategies may, therefore, have facilitated subsequent radiations of the Cambrian. In this light, the Ediacaran and Cambrian Periods, although traditionally envisioned as separate worlds, are likely to have been part of an ecological and evolutionary continuum.