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Taphonomy, shell microstructure and functional morphology in a new ostreid (Crassostrea sp. nov. ) from the Pleistocene of Ecuador
No full textUntil Panama do us part: New finds from the Pliocene of Ecuador provide insights into the origin and palaeobiogeographic history of the extant requiem sharks Carcharhinus acronotus and Nasolamia velox
No full textThe extant blacknose shark Carcharhinus acronotus is a small-sized, tropical to warmtemperate carcharhinid shark occurring along the western Atlantic coasts from North Carolina (USA) through the Gulf and Caribbean regions to Uruguay. Here, we report on two carcharhinid teeth from lower Pliocene (4.07–3.76 Ma) strata of the Upper Onzole Formation exposed in the vicinities of Camarones (northwestern Ecuador). These specimens are assigned to C. acronotus, of which they apparently represent the first occurrence in the Pacific Ocean. The blacknose shark is regarded as the sister group of the whitenose shark Nasolamia velox, an idiosyncratic carcharhinid that currently inhabits the eastern Pacific coasts from Baja California (Mexico) to Peru; furthermore, the divergence between C. acronotus and N. velox has been recently estimated at about 3.7 Ma, which matches well the final phases of formation of the Isthmus of Panama. In light of these data, our Ecuadorian specimens might document an early Pliocene phase in which the newly originated C. acronotus occurred West of the then-fading Panamanian Seaway, possibly as a consequence of occasional dispersal through the latter. Alternatively, they might represent the teeth of an as yet unnamed C. acronotus-like carcharhine, from which the the morphologically conservative C. acronotus and the highly autapomorphic N. velox later arose by vicariance as the Isthmus of Panama rose. A survey of the fossil record of these two taxa does not falsify either hypothesis. Further research on the fossil chondrichthyans from the Cenozoic marine successions of Ecuador will hopefully shed new light on this issue and, more generally, on the role played by the closure of the Panamanian Seaway as a macroevolutionary trigger in the late Cenozoic marine realm
Shell concentrations as tools in characterizing sedimentary dynamics at sequence-bounding unconformities: examples from the lower unit of the Canoa Formation (late Pliocene, Ecuador)
No full textThe Canoa Basin in Manabì, Ecuador, contains a mainly marine, clastic sedimentary succession of Late Pliocene and Pleistocene Age (Canoa Formation and Tablazo Formation). A stratigraphic and sedimentologic study of the entire sedimentary succession indicated that sedimentary facies recur in consistent deepening–shallowing transgressive–regressive patterns and that on this basis it can be divided into three different informal units (Clow, Cupp, Tb). The lowest of these units (Clow) is composed of at least four depositional sequences, each bounded by a ravinement surface and containing a basal environmentally time-averaged shell bed accumulated during rapid sea-level rise
under conditions of reduced terrigenous sediment supply (transgressive systems tract). Owing to intense heavy bioturbation, the internal architecture of these shell beds is not recognizable. Each of these shell beds is mantled by poorly fossiliferous sandy and silty shales accumulated during subsequent progradation (highstand and regressive systems tract). Fossil assemblages, accounting for inner/middle shelf
settings, are dominated by infaunal suspension-feeding bivalves (Linga cancellaris, Chione mariae, etc.), most of which require sandy substrate or are ubiquitous. In terms of trophic/life-habit groups, the total succession shows an increase of infaunal suspension-feeder species’ richness from the lowest (C1) to the upper (C4) shell bed, whereas epifaunal taxa decrease in richness. Shell packing density also
decreases from C1 to C4. The reconstruction of the short-term sedimentary dynamics based on taphonomic and paleoecologic observations indicates a slightly progradational staking pattern of depositional sequences, representing high-frequency sixth-order eustatic sea-level fluctuations within a third-order tectonically induced cycle
Palaeoecology and taphonomy of an extraordinary whale barnacle accumulation from the Plio-Pleistocene of Ecuador
No full textThe exceptional assemblage of cirripedes from the Late Pliocene-Pleistocene sediments of Canoa and Tablazo Formations includes more than 80 specimens of the whale barnacle Coronula diadema, a species that today lives mainly in the skin of humpback whales (Megaptera novaeangliae). In light of the exclusive ectoparasitism of Coronula on whales and mainly on humpback whales, we infer that during the Late Pliocene-Pleistocene the Canoa Basin was on the seasonal migration route of these cetaceans and that whales remained in the Canoa Basin for sufficient time (breeding?) for large accumulations of Coronula to form. Support for this hypothesis comes from current use of this part of the Ecuadorian coast as a breeding ground for Megaptera and from a rib fragment of ?Megaptera from the Coronula-rich Tablazo Formation. Indirect palaeoclimatic implications may be derived from this as humpback whales require at least 25 °C for breeding. Thus, in the Late Pliocene and the Pleistocene, the Humboldt Current apparently had a northern extension similar to the present, with a 25 °C summer isotherm in the same latitudinal position along the Ecuador coast as today. On the basis of preservation, it is concluded that the Coronula-bearing molluscan shell beds in the Canoa and Tablazo Formations are of two basic types: biogenic and sedimentologic. Coronula shells deposited in the biogenic shell beds are little affected by physical processes (fragmentation, peeling, edge preservation), while modified by biological processes like bioerosion and encrustation. The exterior and interior surfaces of the shells display a variety of bioerosional features and encrusting taxa including barnacles, serpulid polychaetes, small oysters and bryozoans indicating that they were deposited during periods of sedimentary quiescence. In comparison, Coronula that accumulated within sedimentologic concentrations are typically heavily abraded and highly fragmented with little or no encrustatio
Backset lamination produced by supercritical backwash flows at the beachface-shoreface transition of a storm-dominated gravelly beach (middle Pleistocene, central Italy)
Full text linkBedforms and resulting sedimentary structures of interpreted upper-flow-regime origin are fairly common in various ancient and modern depositional settings, yet outcrop examples of shallow-water strata dominated by sedimentary structures arising from such flow conditions are rarely documented. This outcrop-based study presents the first detailed analysis of mid-Pleistocene beachface-shoreface strata containing sedimentary structures interpreted to have been formed by storm-generated backwash flows during the transition in flow regime from supercritical to subcritical. Here we propose that moving down the steep beachface, backwash flows rapidly accelerated and became thinner, experienced abrupt deceleration and flow thickening passing through an erosional hydraulic jump in the trough at the toe of the beachface, and then waned to thicker and subcritical conditions just downflow of the hydraulic jump. In this frame, landward-dipping backset laminae deposited on the downflow side of the trough evolved into thinner, upper-stage plane-parallel laminae further downstream. The transition from the proximal to the distal upper shoreface records a significant increase of preserved wave ripples and burrowed intervals, in concert with a progressive decrease in thickness and grain-size of individual sets of plane-parallel laminae and increase of gravel-filled gutter casts
Long-Term Tectonic Influence on Sequence Architecture and Stacking Pattern Revealed by Hiatal Shell Bed Features: Pleistocene Succession of the Canoa Basin (Central Ecuador)
No full text
Basin physiography and tectonic influence on sequence architecture and stacking pattern: Pleistocene succession of the Canoa Basin (central Ecuador)
No full textcies, shell bed features, and sequence stratigraphic framework for the shallow-marine Pleistocene upper Canoa and Tablazo Formations are presented, based on outcrop data from the southern coast of Cabo San Lorenzo, Ecuador. Sediments of this succession exhibit a distinct cyclic pattern, consisting of a stack of eight depositional sequences (cyclothems) likely developed under the main control of orbitally induced sea-level changes. As a rule, within the studied interval an idealized cyclothem is composed of a transgressive systems tract (TST) and a highstand systems tract (HST), whereas deposits attributable to the lowstand and falling-stage systems tracts are not present. Transgressive lithosomes may be defined by estuarine deposits interposed between the sequence boundary and the ravinement surface (back-barrier wedge) and by upward fining shoreface to inner-shelf facies successions above the ravinement (backstepping shelf wedge). Separated by an expanded siliciclastic core, hiatal shell concentrations occur at the base (onlap shell beds) and the top (backlap shell beds) of the transgressive shelf wedges, and some occur at the base of highstand systems tracts (downlap shell beds). On the basis of sedimentary facies, geometry, taphonomy, and paleoecology of shell beds, and the nature of the transition between siliciclastic and mollusk-bearing sediments, cyclothems were classified into two main types that show dependence upon paleoshoreline morphological configuration: sheltered (in the upper Canoa Formation) and exposed (in the Tablazo Formation). Notwithstanding the different synsedimentary tectonic and climatic regimes, the Ecuadorian cyclothems share basic patterns of condensation and facies assemblages with other roughly coeval cyclothemic successions around the world. This suggests that (1) hiatal shell bed development is not just a temperate-latitude phenomenon; (2) a global process, such as glacio-eustatic sea-level change, is the primary mechanism of control for the general architecture of sequences; and (3) specific paleogeographic settings play an important role by determining the taphonomic and paleoecologic characteristics of key shell beds, the nature of their contacts with the encasing sediments, and the type of the component set of facies. At a multicycle time scale, tectonics influenced the long-term trend of the relative sealevel changes and consequently the large-scale stratigraphic organization. Owing to the continued tectonic uplift of the area, successive high-frequency depositional sequences are nested to form a longer-order falling-stage sequence se
First record of a warm-water Pliocene shell bed from northern Chile.
No full textWe recognized a few Pliocene D. ponderosa shell beds in different localities of Mejillones Peninsula
[Antofagasta (Chile), 23°S]. To the best of our knowledge, this is the first case study of Pliocene fossil concentrations
formed by a warm-water species so well south (more than 2000 km) of its extant southern biogeographic
limit. In this paper we provide taphonomic features of an impressive shell bed largely dominated
by this infaunal bivalve that outcrops at Cuenca del Tiburon locality. Moreover, we discuss its mode
of formation in the light of particular paleogeographic and environmental conditions allowing the immigration,
survival, growth and reproduction of large populations of this tropical species in northern Chile
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