Fast post-mortem encrustation of razor shells: examples from the Irish Sea and palaeontological implications

Instructive taphonomic principles are demonstrated by the skeletons of dead invertebrates. Bivalves have resistant skeletons and are common fossils. The thin-valved razor shells, Ensis spp., have a good fossil record despite being fragile. This may be due, in part, to rapid post-mortem encrustation of valves by mineralized invertebrates. Two Recent specimens of Ensis siliqua (Linnaeus), encrusted post-mortem, are described from the Irish Sea coast of Southport, Merseyside. An articulated shell with an intact ligament is encrusted posteriorly on all surfaces by the balanid Balanus crenatus Brugiére. Barnacles inside the shell are smaller than those externally, yet may represent the same spatfall; those inside were constrained by growing in an enclosed space. To a palaeontologist, a mollusc valve encrusted inside and out by cementing organisms would be interpreted as having had a long residence time on the seafloor. This specimen demonstrates the disjunction between loss of soft tissues (days?) and loss of the ligament (weeks, perhaps months), between which encrusters may settle inside the shell, early in its post-mortem history. Similar patterns of encrustation by balanids are now known two species of Ensis and the cockle Cerastoderma edule (Linnaeus).A fragment of a single valve is encrusted only on the internal surface by serpulids and bryozoans. This is balanulith-like, but is only encrusted on the inner surface and not by balanids. Ensis valves can be reinforced by a range of calcareous encrusting organisms; an Ensis fragment encrusted both inside and out by serpulids would be worthy of being named a serpulith.

Taphonomy of a Panopea Ménard de la Groye, 1807 shell bed from the Pisco Formation (Miocene, Peru)

Invertebrate taphonomy can provide significant information about the post-mortem processes that affected the fossil record. In the East Pisco Basin of southern Peru, a Panopea Ménard de la Groye, 1807 shell bed was found in the upper Miocene strata of the Pisco Formation, hinting at a peculiar biostratinomic and diagenetic history. This bed contains abundant invertebrate fossil molds cemented by dolomite. The specimens of the deep infaunal bivalve, Panopea sp., occur together with bivalves representative of shallow infaunal species (Trachycardium sp. and Dosinia ponderosa [Gray, 1838]) and balanid barnacles, which are sessile encrusters. The Panopea specimens host compound molds evidencing an abundant encrusting fauna, including serpulids, ?foraminifera, bryozoans, and barnacles that colonized the inner surfaces of the valves before their final burial. We hypothesize that short-term, storm-related processes exhumed the living bivalves, resulting in a sedimentological concentration of relatively well-preserved shells. After the death of the exhumed bivalves, the inner surfaces of the articulated Panopea shells, representing hard-substratal, sheltered environments on an otherwise unstable sandy seafloor (i.e., “benthic islands”), were colonized by different encrusting organisms. Following the final burial, dolomite precipitated, cementing the sediment infill of the valves. Lastly, a decrease of pH occurred at the sulfate reduction-methanogenesis boundary, inducing the dissolution of the shell carbonate.

Spatio-temporal variation in rate of carbonate deposition by encrusting organisms in different reef microhabitats from Eastern Pacific coral reefs

Reef encrusting calcifiers (non-scleractinian species) constitute assemblages that participate in the carbon cycle at coral reefs. Despite their apparent secondary role in building the reef framework, they contribute to the reef consolidation binding sediments and inducing larval recruitment from other epilithic invertebrates. The contribution of encrusting calcifiers on reef accretion was examined by the assessment of their rate of carbonate deposition on four different simulated reef microhabitats using calcification accretion units (CAUs) during 12 months at Playa Las Gatas and Islote Zacatoso, two coral communities from the coast of the Mexican Pacific. Encrusting calcifiers from Playa Las Gatas, the most impacted site, showed a rate of carbonate deposition (mean ± SD) four times higher than at Islote Zacatoso (10.02 ± 3.22 g CaCO3 m−2 d−1vs 2.48 ± 1.01 g CaCO3 m−2 d−1). Overall, the rate of carbonate deposition on surfaces protected from sedimentation and light was up to 1.8 times higher than on exposed ones (11.40 ± 4.35 g CaCO3 m−2 d−1vs 6.18 ± 3.13 g CaCO3 m−2 d−1). Carbonate deposition by calcareous algae was higher on the well-lit exposed surfaces while filter-feeding invertebrates showed the major contribution on the shaded cryptic surfaces. Although rate of carbonate deposition by encrusting calcifiers seems to be lower than hermatypic corals, it seems to be relevant on coral reefs affected by anthropogenic impacts where coral calcification is low. Under global demise of coral reefs by environmental degradation and climate change, encrusting calcifiers may become relevant for the process of carbonate deposition.

‘Cenoceras islands’ in the Blue Lias Formation (Lower Jurassic) of West Somerset, UK: nautilid dominance and influence on benthic faunas

Substantial numbers of the nautilid Cenoceras occur in a stratigraphically limited horizon within the upper part of the Lower Jurassic (Sinemurian Stage) Blue Lias Formation at Watchet on the West Somerset Coast (United Kingdom). Individual nautilid conchs are associated with clusters of encrusting organisms (sclerobionts) forming ‘islands’ that may have been raised slightly above the surrounding substrate. Despite the relatively large numbers of nautilid conchs involved, detailed investigation of their preservation suggests that their accumulation reflects a reduction in sedimentation rates rather than an influx of empty conches or moribund animals. Throughout those horizons in which nautilids are present in relative abundance, the remains of ammonites are subordinate or rare. The reason for this unclear, and preferential dissolution of ammonite conchs during their burial does seem to provide a satisfactory solution to the problem.

FACIES ASSOCIATIONS AND MICROFOSSILS FROM THE MIDDLE-UPPER TRIASSIC LIMESTONES OF THE TRANSILVANIAN NAPPES (PERȘANI MOUNTAINS, EASTERN CARPATHIANS, ROMANIA)

Triassic limestones that belong to the Transilvanian Nappe system, form extensive outcrops in the central part of the Persani Mountains, near Racos locality, on the southern and northern sides of the Olt Gorges. Triassic carbonate olistoliths were sampled in four distinct locations belonging to the Transilvanian Nappe: the Old Racos Quarry, Olt Gorges, Tipia Racosului Hill and Tipia Ormenisului Hill. The identified facies types include brecciated limestones with encrusting organisms and cyanobacteria, packstone-grainstone with sponges and encrusting organisms, boundstone with grainstone internal sediment, peloidal wackestone-packstone, peloidal packstone-grainstone, intraclastic grainstone-rudstone. Such microfacies characterize reefal bioconstructions or fore-reef domains located in the vicinity of shelf crest areas. They share common features with similar Wetterstein facies carbonates from Italy, Austria or Hungary. The micropaleontological assemblage indicates the Anisian-Ladinian to Carnian ages for the studied limestones, the present study bringing new data concerning the age of these carbonates. In addition, the presence of some foraminifera and dasycladalean algae are mentioned for the first time in this area.

Domination of mesophotic ecosystems in the Wakatobi Marine National Park (Indonesia) by sponges, soft corals and other non-hard coral species

Mesophotic ecosystems have been relatively poorly studied in the Indo-Pacific and in particular within the Coral Triangle region. Here we used a mini-ROV to explore the changes in major benthic groups at two sites (~200 m apart) in the Wakatobi Marine National Park, SE Sulawesi, Indonesia spanning shallow water coral reefs (5 m) to deeper water mesophotic ecosystems (80 m). We found very similar patterns at both sites where coral cover peaked at 15 m, declined rapidly by 30 m, and was virtually absent at 50 m. As coral declined there was a marked increase in sponges, soft corals and other encrusting organisms (including ascidians, bryozoans, tubeworms, gorgonians and molluscs). Importantly, our results differ from most previous studies in other geographic locations where hard corals extend much deeper. It is unclear what drives this difference but it may be related to higher levels of turbidity and therefore reduced light penetration in the Wakatobi compared with other areas, which limits the vertical extent of coral development.

Carbonate production rates of encruster communities on a lagoonal patch reef: Vabbinfaru reef platform, Maldives

Coral reefs are formed by the growth and calcification of primary coral framework and secondary encrusting organisms. Future scenarios of reef health predict global declines in coral cover and an increase in the relative importance of encrusting organisms to gross reef calcification. Numerous coral growth studies are available; however, there are few quantitative estimates of secondary carbonate production on reefs. The present study used vertically orientated PVC pipe to generate rates of carbonate production (g cm–2 year–1) by encruster communities on Vabbinfaru reef platform, Maldives (4°18′35″N, 73°25′26″E). Maximum carbonate production by encrusters was 0.112 g cm–2 year–1 (mean ± s.d.: 0.047 ± 0.019 g cm–2 year–1). Encruster community composition was dominated by non-geniculate coralline algae (mean ± s.d.: 76 ± 14.2%), with other encrusting taxa being quantitatively unimportant to total substrate cover (mean ± s.d.: 9 ± 16.7%). Rates of encruster calcification at Vabbinfaru fell within the range of values reported for other reef-building provinces. There is a particular need for more quantitative field-based measurements of reef-organism calcification rates because such values strengthen regional and global estimates of gross carbonate production and have direct implications for net reef accretion and the development of reef sedimentary environments.