Rowan C. Martindale

Detecting Photosymbiosis in Fossil Scleractinian Corals

The evolutionary success of reef-building corals is often attributed to photosymbiosis, a mutualistic relationship scleractinian corals developed with zooxanthellae; however, because zooxanthellae are not fossilized, it is difficult (and contentious) to determine whether ancient corals harbored symbionts. In this study, we analyze the δ15N of skeletal organic matrix in a suite of modern and fossil scleractinian corals (zooxanthellate- and azooxanthellate-like) with varying levels of diagenetic alteration. Significantly, we report the first analyses that distinguish shallow-water zooxanthellate and deep-water azooxanthellate fossil corals. Early Miocene (18–20 Ma) corals exhibit the same nitrogen isotopic ratio offset identified in modern corals. These results suggest that the coral organic matrix δ15N proxy can successfully be used to detect photosymbiosis in the fossil record. This proxy will significantly improve our ability to effectively define the evolutionary relationship between photosymbiosis and reef-building through space and time. For example, Late Triassic corals have symbiotic values, which tie photosymbiosis to major coral reef expansion. Furthermore, the early Miocene corals from Indonesia have low δ15N values relative to modern corals, implying that the west Pacific was a nutrient-depleted environment and that oligotrophy may have facilitated the diversification of the reef builders in the Coral Triangle.

Microbialite Fabrics and Diminutive Skeletal Bioconstructors in Lower Norian Summit Point Reefs, Oregon, United States

Abstract Microfacies analysis of five patch reefs from the Martin Bridge Formation (Upper Triassic/Norian, Summit Point, Oregon, United States) reveals that microbialite fabrics dominate reef construction versus corals or other large metazoans. Other reef bioconstructors include characteristic Late Triassic branching corals and diminutive calcifiers (solenoporacean red algae, foraminifera, and sponges). The patch reefs exhibit dense growth fabrics and had elevation above the seafloor, making them ecological reefs or true reefs. Robust binding and encrusting organisms inhabited higher-energy areas, whereas red algae, followed by phaceloid coral colonies, inhabited zones of decreasing water energy, defining a subtle zonation consistent with wave energy in the paleoenvironment. The dominance of microbial fabrics and diminutive bioconstructors make the Summit Point reefs distinct among Upper Triassic reefs from northeastern Panthalassa. Summit Point was originally identified as a Dachstein-type reef (e.g., high diversity, dominated by corals and sponges, massive bedding, large reef cavities). However, the dominance of microbial fabrics over larger metazoans, the (comparatively) modest faunal diversity, and lack of abundant, multigenerational epibionts or cements clearly differentiates the Summit Point reefs from the Dachstein reefs of northern Europe. The Summit Point reefs do, however, bear a striking resemblance to some Middle Triassic (Anisian) reefs from the Tethys, particularly with the abundance of microbialite fabrics and diminutive, binding or encrusting bioconstructors.

A new Early Jurassic (ca. 183 Ma) fossil Lagerstätte from Ya Ha Tinda, Alberta, Canada

Lagerstatten —deposits of exceptionally preserved fossils—offer vital insights into evolutionary history. To date, only three Konservat-Lagerstatten are known from Early Jurassic marine rocks (Osteno, Posidonia Shale, and Strawberry Bank), all located in Europe. We report a new assemblage of exceptionally preserved fossils from Alberta, Canada, the first marine Konservat-Lagerstatte described from the Jurassic of North America. The Ya Ha Tinda assemblage includes articulated vertebrates (fish, ichthyosaurs), crinoids, crustaceans, brachiopods, abundant mollusks (coleoids with soft tissues, ammonites, gastropods, bivalves), wood, and microfossils. Paired bioand chemostratigraphies show that Lagerstatte deposition occurred during the late Pliensbachian through early Toarcian, capturing the carbon isotope excursion associated with the Toarcian Oceanic Anoxic Event. Therefore, the Panthalassan Ya Ha Tinda biota is coeval with Toarcian Lagerstatten from the Tethys Ocean (Posidonia Shale and Strawberry Bank). Comparisons among these deposits permit new insights into the diversity, ecology, and biogeography of Jurassic marine communities during a time of pronounced biological and environmental change (e.g., expanded subsurface anoxia, warming, and extinctions). They also highlight the possibility that Mesozoic Oceanic Anoxic Events are temporal foci of exceptional preservation.

FROM FORE REEF TO LAGOON: EVOLUTION OF THE UPPER TRIASSIC DACHSTEIN CARBONATE PLATFORM ON THE TENNENGEBIRGE (SALZBURG, AUSTRIA)

ABSTRACT Carnian and Norian (Upper Triassic) limestones and dolostones along the Eisriesenwelt trail on the Tennengebirge (Salzburg, Austria) reveal a progression from fore-reef to lagoonal environments, including a small Norian reef with both Carnian and Norian characteristics. Here, we provide high-resolution biostratigraphic age constraints of the Tennengebirge platform carbonates and describe an early Norian patch reef that is built by large Retiophyllia corals and encrusted by “Tubiphytes,” sponges, and microbial fabrics. The tall (up to 4 m), narrow phaceloid Retiophyllia coral colonies exhibit phototropic growth patterns—coral branches that are at the top of the colony grew longer than those on the side of the colony—thus, we suggest that these corals had a symbiotic association with photoautotrophs (most likely zooxanthellae). The well-constrained ammonoid and conodont biostratigraphy presented here establish that the Tennengebirge patch reef was deposited in the early Norian, nevertheless, it contains features typically associated with Carnian reefs (small, encrusting sponges, Carnian-style microbial crusts, and “Tubiphytes”). The Carnian traits of the Norian reef emphasize the importance of accurate biostratigraphic dating; obtaining independent age estimates for reef outcrops is crucial to correctly determine the timing and magnitude of transitions in Triassic reef ecology.

Mixed Siliciclastic/Carbonate Systems

Abstract The ichnology of mixed siliciclastic/carbonate successions, in which sediment admixture has had a clear effect on infaunal populations, is discussed. Fundamental influences that allow for the development of unique ichnological signatures in mixed systems include grain size, grain shape, and early diagenetic alteration. Shell debris within dominantly siliciclastic successions introduces complexities into the infaunal habitat that are clearly reflected in trace-fossil assemblages. Some differences are preservational, with trace fossils inherently more difficult to recognize in coarser bioclastic intervals than in fine-grained siliciclastic intervals. Faunal level differences include those where some taxa are excluded by admixed bioclastic detritus or by reduced ecospace availability. Diagenetic processes in mixed systems also operate at different scales and temporal intervals, leading to fundamental differences in preservation and in substrate consistency. In mixed systems, quintessential carbonate ichnotextures co-occur with prototypical siliciclastic ichnotextures, such as successions with interstratified or coplanar firmground and hardground assemblages.

Crustose coralline algae increased framework and diversity on ancient coral reefs

Crustose coralline algae (CCA) are key producers of carbonate sediment on reefs today. Despite their importance in modern reef ecosystems, the long-term relationship of CCA with reef development has not been quantitatively assessed in the fossil record. This study includes data from 128 Cenozoic coral reefs collected from the Paleobiology Database, the Paleoreefs Database, as well as the original literature and assesses the correlation of CCA abundance with taxonomic diversity (both corals and reef dwellers) and framework of fossil coral reefs. Chi-squared tests show reef type is significantly correlated with CCA abundance and post-hoc tests indicate higher involvement of CCA is associated with stronger reef structure. Additionally, general linear models show coral reefs with higher amounts of CCA had a higher diversity of reef-dwelling organisms. These data have important implications for paleoecology as they demonstrate that CCA increased building capacity, structural integrity, and diversity of ancient coral reefs. The analyses presented here demonstrate that the function of CCA on modern coral reefs is similar to their function on Cenozoic reefs; thus, studies of ancient coral reef collapse are even more meaningful as modern analogues.