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Stable isotope analyses (δ18O, δ13C) of Spirula spirula, a mesopelagic cephalopod with a loosely coiled internal calcareous (aragonitic) shell, suggest that δ18O is precipitated in equilibrium with the surrounding water, recording the temperature of the seawater inhabited by the animal through its life. The δ18O trends are interpreted to reveal a life that begins in deep waters characterized by cool temperatures before rising to warm surface waters to feed during its juvenile stage. Following this brief period in warmer waters, the isotopes suggest that the remainder of the organism's life is spent in progressively cooler (deeper) waters. The incorporation of isotopically light metabolic carbon, however, significantly affects the stable carbon isotope signal recorded in S. spirula, effectively obscuring the record of δ13C of seawater dissolved inorganic carbon archived in the shell carbonate. This may relate to the internal position of the shell, in which the growing margin is anchored in soft tissue and separated from the ambient seawater within the mantle cavity. By analogy, δ13C of extinct cephalopod shells may, thus, prove useful as a guide to the amount of soft tissue surrounding the growing margin of the shell. Changes in δ13C of the shell may also indicate a change of diet concurrently with the inferred rise to warm surface waters. The results of this study have important implications for understanding ancestors of S. spirula, such as belemnites, in terms of the constraints on equilibrium precipitation of shell carbonate, sought in terms in paleoenvironmental studies.
Although limpets are common in rocky intertidal shores, little is known about drilling predation on them. Drilling intensity and preferences by Nucella (Muricidae) on three Lottiidae species (Lottia pelta, L. digitalis, and Tectura scutum) were explored in a modern limpet death assemblage from False Bay (San Juan Island, Washington, USA). Of the 1,531 shells, only 61 (4%) were drilled, with drilling frequencies of 5.9% (L. digitalis), 2.4% (L. pelta) and 0.5% (T. scutum). The higher drilling frequency observed for L. digitalis may reflect spatial differences in prey distribution within the intertidal zone. Hole diameter correlated positively with limpet size, suggesting that larger predators drill larger prey. No differences in drilling frequency were observed due to prey ornamentation or size; however, drill holes were never observed on the largest and thickest L. pelta shells, suggesting a possible size refugium. The majority of holes occurred near the apex, indicating stereotypic attack behavior. Uniform frequency distributions across taphonomic grades and similar central tendencies between drilled and undrilled shells suggest that holes were not affected by taphonomic bias. The preservation of drilled and undrilled shells differed significantly, however; thus, drill holes may have negatively affected the preservation potential of shells, possibly by weakening the shell. Poor shell preservation indicates that biostratinomic effects may play a larger role in preservational biases and underestimation of predation frequencies than previously thought. Studies using drilling frequencies demand careful identification of predatory traces when shells are poorly preserved. In addition, careful evaluation of predation frequency is needed when predatory strategies that may not leave visible traces are possible.
Biological and physical factors govern the distribution of fossils, but it is not always clear which is more important. The preservation of late Eocene vertebrates at the UNESCO World Heritage site of Wadi Al-Hitan, Western Desert of Egypt, is controlled primarily by the physical processes responsible for sequence stratigraphic architecture on a siliciclastic shelf. Three types of stratigraphic surface, each characterized by a taxonomically and taphonomically distinct fossil assemblage, yield most of the known vertebrate fossils. Complete, partially articulated whale skeletons, primarily Basilosaurus isis, are abundant in offshore marine flooding surfaces (MFS) in the late transgressive systems tract (TST) of the first Priabonian sequence (TA4.1), where low net sedimentation rates and environmental averaging in offshore environments promoted the accumulation of carcasses on traceable stratigraphic surfaces. Complete, well-articulated whales, primarily Dorudon atrox, are more widely scattered on minor erosion surfaces in rapidly accumulating shoreface sediments of the overlying falling stage systems tract. Fragmented and abraded vertebrate remains are abundant and diverse in a discontinuous conglomerate that marks the first sequence boundary above the base of the Priabonian (Pr-2), which has not been previously recognized in Egypt, but which formed incised valleys with at least 45 m of total relief. Fossils in this variably thick lag conglomerate include skeletal elements reworked by rivers from underlying marine deposits and bones of terrestrial animals living in the fluvial environment. Marginal marine vertebrates, primarily dugongs, occur on shelly marine ravinement surfaces above Pr-2, in the early TST of the second Priabonian sequence. Most vertebrate remains in Wadi Al-Hitan occur in condensed stratigraphic intervals and taxonomic composition changes with sequence position, both important considerations in interpretation of paleobiological patterns.
Stratigraphic architecture exerts a powerful influence on the vertical distribution of fossils and should be considered before biostratigraphic and paleobiologic interpretations to avoid interpretations of patterns that are actually stratigraphic artifacts. Previous numerical models of the stratigraphic distribution of fossils were developed for siliciclastic settings, which also work well for mixed carbonate-siliciclastic settings. These models are evaluated here using the Upper Ordovician Bighorn Dolomite, which was deposited on a tropical carbonate platform. The Bighorn Dolomite contains three third-order, unconformity-bounded depositional sequences, which correspond closely to the Steamboat Point, Leigh, and Horseshoe Mountain Members and are correlative with the C2, C3, and C5 sequences of the eastern United States. The Steamboat Point and Leigh Members record greenhouse conditions and are characterized by meter-scale parasequences with weakly defined flooding surfaces and minor internal facies changes. Absent in these units are well-defined parasequence sets with strong upward-deepening or upward-shallowing trends. In contrast, the Horseshoe Mountain Member reflects a transitional climate and consists of 10-m-thick, well-developed parasequences that clearly stack into upward-deepening (transgressive) and upward-shallowing (highstand) systems tracts. Fossil associations within the Bighorn Dolomite fall into five biofacies that reflect depositional environment and age. The brachiopod biofacies is found in deep subtidal facies, with the gastropod facies and crinoid biofacies in shallow subtidal facies. The dasyclad biofacies and coral biofacies are limited to the shallow subtidal facies of the Steamboat Point and Horseshoe Mountain sequences, respectively. In the greenhouse sequences, biofacies transitions are gradational, whereas they are abrupt and correspond to major flooding surfaces in the transitional climate sequences. Similarly, first and last occurrences are not clustered in the greenhouse-climate sequences, although they are elevated near the maximum flooding zone. First and last occurrences are clustered in the transitional-climate sequence where open marine facies border the sequence boundary, at major flooding surfaces, and where open marine facies are last expressed.
The benthic foraminiferal turnover and extinction event (BEE) associated with the negative carbon isotope excursion (CIE) across the Paleocene–Eocene Thermal Maximum (PETM) is analyzed in the Zumaia section (Spain), one of the most complete and expanded deep-water sequences known worldwide. New biostratigraphic, paleoecologic, and paleoenvironmental data on benthic foraminifera are correlated to information on planktic foraminiferal and calcareous nannofossil turnover in order to evaluate possible causes and consequences of the PETM. Gradual but rapid extinction of 18% of the benthic foraminiferal species starts at the onset of the CIE, after the initial ocean warming (as inferred from calcareous nannofossils) recorded in the last 46 kyr of the Paleocene. This gradual extinction event culminated ∼10.5 kyr after the onset of the CIE and led to the main BEE, affecting 37% of the species. Therefore, extinctions across the PETM affected a total of 55% of the benthic foraminiferal species at Zumaia. The gradual extinction occurred under inferred oxic conditions without evidence for carbonate dissolution, indicating that carbonate corrosivity and oxygenation of the ocean bottom waters were not the main cause of the event. An interval characterized by dissolution occurs above the main BEE, suggesting that bottom waters became corrosive after the main extinction. Carbonate is progressively better preserved through the overlying deposits, and carbon isotope values gradually return to background levels. These data are consistent with a slow deepening of the carbonate compensation depth after its initial rise owing to abrupt acidification of the oceans. Microfossil data support a rapid onset of the PETM, followed by long-term effects on calcareous plankton and benthic foraminifera.
Trophic interactions between sharks and other marine vertebrates are represented by both indirect and direct evidence from the fossil record. Indirect evidence includes such traces as shark tooth marks and gouges on the bones of prey, such as fish, reptiles, whales, dolphins, and seals. Direct evidence is represented by the presence of shark teeth in definite association with prey species. In this paper, we report direct evidence for trophic interactions between a white shark (Carcharodon sp.) and a mysticete whale from the lower Pliocene (∼4–5 Ma) Pisco Formation of Peru: a partial mandible of an unidentified mysticete whale with a partial tooth of a white shark embedded within the cortical bone. Modern white sharks are known predators of many marine mammal species, and both active hunting and scavenging have been well documented. In this instance, we interpret this specimen to represent a scavenging event. This fossil is unusual because it represents a seldom-reported event that preserves direct evidence of trophic interactions.
The first fossil balanuliths are reported from a Pliocene (3.5 Ma) coquina on Mejillones Peninsula, northern Chile. While balanuliths were recently described for the first time, fossil balanuliths are not that uncommon, but they apparently have been unreported. Recently described living balanuliths form around broken parts of bivalve shells that were encrusted while still attached to the complete shell. Here balanuliths are reported on nuclei of trochid and, to a lesser extent, muricid gastropods.