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Late Ordovician bryozoan carbonate mounds are present in the upper part of the Deschambault Formation (Trenton Group) at the Montmorency Falls locality, northeast of Quebec City. These mounds are local features within a stratigraphic interval otherwise characterized by a bedded sequence of bryozoan-rich deposits. From the core of the mound to its margins and into the well-bedded off-mound sediments, there is a taphonomic gradient in terms of preservation, automicrite formation, fragmentation, transport, growth versus shelter porosity, and marine cementation. By ruling out both local seepage of nonmarine fluids (nonspecialist fauna, normal-marine carbon and oxygen stable isotopes, near PAAS rare earth element distribution patterns) and differential growth rate (bryozoan zooecium size), mound formation is explained by a positive taphonomic feedback mechanism. Centimeter-size patches of automicrite in mound cores are considered crucial in explaining subtle variations of microtexture, microfacies, and subsequent mound formation. Automicrite (M1) is dark gray, contains microbioclasts, has sharply defined, scalloped outer margins, is commonly gravity defying, and occasionally joins up to bridge multiple bryozoan skeletons. Clusters of microtubules are present within M1 and a first generation of infiltrated microcrystalline carbonate sediment (M2). The size, geometry, and arrangement of these microtubules suggest an origin either from the attachment of nonspicular, keratosan sponges, or from the assimilatory action of marine fungi (so-called Wedl tunnels). In conclusion, a subtle and small-scale secondary reinforcement of bryozoan skeletons by cryptobiontic ephemeral substrates is critical for the development of mounds out of an essentially level-bottom bryozoan community.
This study focuses on quantitative analysis of the species composition and distribution of living and dead mollusks to assess fidelity of death assemblages in a protected tidal flat and a shallow subtidal lagoon behind a fringing reef from the Indo-Pacific province. We evaluate how well the death assemblage (DA) reflects the original living community and whether there was a recent ecological shift in species composition. Quantitative analysis of 18 samples from unvegetated sandy substrata shows that dead individuals dominate in the shallow subtidal lagoon, but living and dead individuals were roughly equally abundant on the tidal flat. This difference points to rapid degradation or export of dead shells from tidal flats, leading to smaller potential for time averaging. Rarefied species richness and diversity of DAs is higher than that of the living assemblages (LAs) at the scale of habitats and at the scale of the study area, and this difference in richness is stronger and live-dead (LD) agreement in composition is smaller in the subtidal than in the intertidal habitats. Distinct assemblages characterized intertidal and subtidal habitats both in LAs and DAs, and the rank-abundance distributions of DAs generally corresponded to that of LAs. We suggest that anthropogenic impact in the area did not result in a major environmental change in the subtidal environment during the past few decades because existing small differences between LAs and DAs can be explained by some degree of time averaging and small-scale redistribution of shells. On the tidal flat, however, the lower time-averaging of the DA and the generalistic life habits of prominent members in the LA do not exclude a major shift.
We describe plant–arthropod associations from the Middle Pennsylvanian (late Bolsovian–early Asturian) Pennant Sandstone Formation of southern Britain. Our material comprises calcified cordaitaleans and tree-fern axes, preserved in braided channel deposits, and interpreted as remains of subhumid riparian vegetation distinct from that of coeval coal swamps. The first plant–arthropod association, attributed to herbivorous insects, comprises cambial damage to cordaitalean leafy branches, resulting in traumatic wound response. The second and most widespread association, attributable to detritivorous oribatid mites, includes tunnels and galleries containing widely scattered, clustered, or densely packed microcoprolites within the inner root mantle of marattialean tree ferns and cordaitalean trunks and branches. Diameter data for tunnels and microcoprolites are multimodal, recording four or five instars of oribatid mites that parallel instar-based fecal pellet and body lengths in modern taxa. The third association attributed, possibly, to an arthropleurid, comprises a single, very large (19 × 14 mm) coprolite. Included plant fragments support a previous conjecture that arborescent lycopsids formed part of this iconic arthropod's diet. Mucus-lined burrows within the macrocoprolite imply that fecal material was processed by annelids. The high diversity and frequency of plant–arthropod associations are unusual for Mid-Pennsylvanian time, and may reflect previously undetected interactions in those ecosystems that lay outside “coal forest swamps.”
Dinosaur reproductive biology is often inferred from the biology of extant taxa; however, taphonomic studies of modern nest sites have focused exclusively on avian, rather than reptilian species. We documented eight Agassiz's desert tortoise (Gopherus agassizii) nests and ten loggerhead sea turtle (Caretta caretta) nests. Gopherus agassizii excavated burrows up to 70 cm long and laid rigid-shelled eggs 10–12 cm below the burrow floor. The 19 cm × 12 cm depressions consisted of hard consolidated sand surrounded by a 3–4-cm-high rim and contained 2–5 hatched eggs in a single layer. These hatched egg bottoms represent ∼ 25% of the original egg, and five of 27 contained fully developed dead neonates. Desiccated membrane separated from the egg interior forming pockets that filled with eggshell and sand. Of 106 and 79 eggshell fragments in the hatched egg and surrounding sand, 48% and 23% occurred concave up, respectively. However, the combined numbers of eggshell fragments inside the eggs and in the immediately surrounding sand approximates the 60∶40 ratios at in situ avian nests. Therefore, this ratio may provide reliable evidence for hatching sites regardless of the incubation strategy employed by the adult. Caretta caretta nests differed from those of tortoises in their greater depth (∼ 50 cm) and occurrence in moist, cohesive sand. Clutches contained over 100 pliable-shelled eggs that tore and collapsed upon hatching, without brittle fracture. Failed eggs in two clutches showed five development stages, indicating that the deaths occurred over an extended time period. With the exception of predation, the G. agassizii and C. caretta nests showed no significant eggshell or hatched eggs above the egg chamber.
Age spectra of Mulinia lateralis shells from the top 0–10 cm of the sediment column in Copano Bay, Texas, show three distinct populations: a young population with a highly skewed distribution ranging from 0 to 1 years, a middle-aged population that is more symmetrical ranging from 1 to 10 years with a peak ∼ 4 years, and a small, very old population ranging from 100 to > 10000 years. The young population is interpreted to record the rapid loss of shells from the taphonomically active zone at or near the sediment surface. The middle-aged population is interpreted to record a sequestered population of shells that has had time to accumulate below the taphonomically active zone. Although surface age spectra differ among depositional environments and at different locations, both of these populations are present in all sampled facies at multiple locations within the bay, indicating that the overarching controls on surface age spectra affect the entire bay. The very old surface population is present only in bay-margin sites and is interpreted to represent shells exhumed from eroding Holocene deposits. An 86-cm-long core taken at a bay-margin site near the mouth of the Aransas River contains shells spanning < 1 to 14,000 years but not preserved in stratigraphic order. The lack of stratigraphic order and the presence of distinct breaks in the postmortem age distribution suggest that 67% of shells in the core were recycled from older deposits, resulting in a 2–3 order of magnitude increase in the amount of time averaging in the sediment column relative to surface samples.
Neoproterozoic carbonate successions provide a new taphonomic window into the diversification of eukaryotes. We report recently discovered macroscopic organic warty sheets (MOWS) in macerates of limestone from the ca. 662–635 Ma Taishir Formation (Tsagaan Olom Group, Mongolia). Sheets are applanate. One surface contains raised ridges and conspicuous, ∼ 100-µm-tall warty protuberances with depressed tops that enclose internal cavities containing cellular structures. The Taishir MOWS may be the remains of unusual bacterial, protistan, or fungal biofilms, or a previously undocumented, extinct taxon. However, multiple lines of evidence including the morphology of warty protuberances and the presence of cellular architecture within protuberances support the interpretation of MOWS as marine algae, perhaps a member of the Rhodophyta. Regardless of their specific taxonomic affiliation, MOWS increase the diversity of biota reported from the Cryogenian glacial interlude and indicate the presence of macroscopic and morphologically complex multicellular organisms in the Cryogenian.