Stefano Dominici

Taphonomy and Paleoecology of Shallow Marine Macrofossil Assemblages in a Collisional Setting (Late Pliocene–Early Pleistocene, Western Emilia, Italy)

Abstract The late Pliocene to early Pleistocene siliciclastic succession of western Emilia (northern Italy) forms part of the infill of the Po Plain-Adriatic foredeep, a collisional basin with active thrusting during deposition, and is characterized by 5-to-20-m-thick fining-upward sedimentary cycles. Late Pliocene cycles were deposited in an inner-to-middle shelf setting at the end of a long-term uplift-denudation cycle controlled by tectonics. Cycles are characterized by shell beds of biogenic origin, particularly abundant in the middle of each cycle. The stacking pattern of paleocommunities, with particular reference to their paleodepths, reflects that each cycle is the product of sea-level variation. In particular, early transgressive deposits are marked by a Nuculana community, indicating a shelly-sand bottom of the inner shelf; late transgressive deposits and the condensed section are marked by a shelly bottom Timoclea community developed at times of lowered clastic input; highstand deposits are characterized by a muddy soft bottom Pinna community. The situation changed near the boundary of the early Pleistocene, when a new uplift-denudation cycle started, renewing sediment availability and increasing the slope of the nearby mountain front. Local climatic conditions periodically triggered the sudden delivery on the shallow shelf of large amounts of sediment through hyperpycnal flows and related turbidity currents generated by catastrophic river floods. Sandy shelfal lobes form the lower reaches of the delta front. These are characterized by densely packed shell beds and the occurrence of echinoderm obrution deposits. Recurring sedimentological assemblages from sandstones, characterized by the sandy bottom Spisula and Ditrupa communities, and in situ faunas from intervening mudstones, characterized by the muddy-bottom, hypoxic Arctica (and Corbula?) community, suggest that the lower part of a cycle formed during deglaciation following glacial maxima, at times of high input of freshwater and river-borne nutrients and high summer pelagic productivity. Mudstones from the upper part of the cycle, with dispersed remains of the muddy-bottom Venus community, indicate return to full interglacial conditions and to humid, low seasonality climates. Thus, early Pleistocene cycles could be correlative to sapropel-non sapropel cycles typical of the eastern Mediterranean deep water successions. Moreover, the western Emilia fossil and sedimentary records suggest that cyclic deposition was controlled by factors other than eustasy, providing a case study for future sequence stratigraphic modeling in active tectonic settings.

Mediterranean fossil whale falls and the adaptation of mollusks to extreme habitats

The hypothesis that sunken carcasses of Mesozoic marine reptiles and Cenozoic whales acted as evolutionary stepping stones to deep-sea reducing habitats is underlain by the question of whether vent-like, chemosymbiotic specialization fi rst evolved at shelf depths. Fossil skeletons of large whales have long been known from ancient shallow-water strata, but they have never been considered as a source of information on ecosystem development. We present a study on a 3 Ma old fossil whale fall and a survey of other Pliocene fossil skeletons to show that the associated biota is dominated by heterotrophs, with subsidiary chemoautotrophs. The taphonomy of the Mediterranean shelf whale falls shows some differences with respect to deep-water studies. Quantitative analyses of abundance data within a large data set on fossil and modern mollusk families confi rm that deep- and shallow-water communities at reducing habitats are composed of a different set of taxa, i.e., specialists occurring only below the shelf break. Mediterranean carcasses sunken in coastal settings do not seem to be favorable for the evolution of whale-fall specialists among the mollusks. The situation reverses as the shelf break is approached.

Evidence of Osedax worm borings in Pliocene (∼3 Ma) whale bone from the Mediterranean

Osedax worms subsist entirely on vertebrate skeletons on the seafloor, using root-like tissues to bore into and degrade the bones. Paleontologists have only recently begun to appreciate the possible destructive effect that these worms may have had on the marine vertebrate fossil record and little is known of their evolutionary history. Using microcomputed tomography, we document Osedax-like borings in a fossil whale bone from the Pliocene of Italy and present new data on the borings of extant Osedax worms. The fossil borings are distinguished from those of other known borers and identified as traces of Osedax activity based on diagnostic features. Our results suggest that it is necessary to isolate individual borings for the confident identification of Osedax traces. This is only the second paleogeographic occurrence of Osedax in the fossil record and indicates that by the Pliocene these worms had colonised a large portion of the worlds oceans. This is the first evidence for Osedax in the Mediterranean, past or present, and suggests that more species await discovery in this region.

DEPOSITIONAL DYNAMICS AND THE RECORD OF ECOSYSTEM STABILITY: EARLY EOCENE FAUNAL GRADIENTS IN THE PYRENEAN FORELAND, SPAIN

Abstract We studied mollusk-dominated multispecies assemblies from the south Pyrenean foreland in Spain by using relative abundance data in a framework of high-frequency depositional sequences for an interval spanning 2 myr across the early Eocene climatic optimum. The sequences are part of the Figols (middle Ypresian) and Castigaleu (upper Ypresian) allogroups and together document environmental changes influenced by tectonics at the lower frequency and driven by sea level and climate at the high-frequency temporal scale. We applied ordination through multidimensional scaling and other techniques to explore the structure of the data set. Six Figols and eight Castigaleu communities, linked along onshore-offshore gradients, were interpreted on uniformitarian grounds. Paleoenvironments ranged from mangrove forest to tidal flat, tidal creek, estuary, delta front, shoreface, carbonate ramp, and inner shelf. Some habitats were represented throughout the interval, allowing examination of the effect on coastal ecosystems of environmental disturbances tied to sea-level lowstands, as deduced from the sequence stratigraphic analysis. These disturbances presumably amounted to extirpation by river floods, burial, increased turbidity, high river-derived nutrient input, and decreased ecospace availability. The major effect on the mangrove-estuary-delta gradient was coincident with the unconformity separating the two allogroups. Mangrove communities suffered a change in rank of dominating species and a turnover of rare species. Subtidal estuarine and delta communities were affected in their relative abundances, but we observed no change in rank or turnover. We suggest that the different niche breadth of organisms leads to different responses to perturbation of several scales. Many intertidal cerithioidean gastropods went extinct, whereas subtidal turritelline gastropods were unchanged, during the major environmental variation. Mangrove gastropods experienced increased originations in the upper Ypresian. The carbonate ramp heterotrophs did not change across the Figols-Castigaleu perturbation, although throughout the early Eocene the phototroph guild within the same ecosystem was undergoing frequent turnover events. All studied associations showed significant changes in the relative abundances of constituent species across unconformities of minor entity, proving that soft-bottom marine communities conform to an open-membership model of ecosystem recruitment, as suggested by studies of past open marine ecosystems. Persistence to a degree is suggested by mangrove communities, indicating a slightly more limited membership in low-nutrient estuarine habitats, a response more similar to that of coral reef tropical ecosystems. This study confirms the idea that different species and communities may experience opposite effects from the same events and shows that faunal distributions in estuarine and deltaic systems reflect more than just bathymetric change. High onshore origination also conforms to the theory of onshore-offshore faunal change.

Mollusk species at a Pliocene shelf whale fall (Orciano Pisano, Tuscany)

Abstract The recovery of an intact, 10 m long fossil baleen whale from the Pliocene of Tuscany (Italy) offers the first opportunity to study the paleoecology of a fully developed, natural whale-fall community at outer shelf depth. Quantitative data on mollusk species from the whale fall have been compared with data from the sediments below and around the bones, representing the fauna living in the muddy bottom before and during the sinking of the carcass, but at a distance from it. Although the bulk of the fauna associated with the fossil bones is dominated by the same heterotrophs as found in the surrounding community, whale-fall samples are distinguishable primarily by the presence of chemosymbiotic bivalves and a greater species richness of carnivores and parasites. Large lucinid clams (Megaxinus incrassatus) and very rare small mussels (Idas sp.) testify to the occurrence of a sulphophilic stage, but specialized, chemosymbiotic vesicomyid clams common at deep-sea whale falls are absent. The Orciano whale-fall community is at the threshold between the nutrient-poor deep sea and the shallow-water shelf, where communities are shaped around photosynthetic trophic pathways and chemosymbiotic specialists are excluded by competition.

Onshore-offshore gradient in metacommunity turnover emerges only over macroevolutionary time-scales

Invertebrate lineages tend to originate and become extinct at a higher rate in onshore than in offshore habitats over long temporal durations (more than 10 Myr), but it remains unclear whether this pattern scales down to durations of stages (less than 5 Myr) or even sequences (less than 0.5 Myr). We assess whether onshore–offshore gradients in long-term turnover between the tropical Eocene and the warm-temperate Plio-Pleistocene can be extrapolated from gradients in short-term turnover, using abundances of molluscan species from bulk samples in the northeast Atlantic Province. We find that temporal turnover of metacommunities does not significantly decline with depth over short durations (less than 5 Myr), but significantly declines with depth between the Eocene and Plio-Pleistocene (approx. 50 Myr). This decline is determined by a higher onshore extinction of Eocene genera and families, by a higher onshore variability in abundances of genera and families, and by an onshore expansion of genera and families that were frequent offshore in the Eocene. Onshore–offshore decline in turnover thus emerges only over long temporal durations. We suggest that this emergence is triggered by abrupt and spatially extensive climatic or oceanographic perturbations that occurred between the Eocene and Plio-Pleistocene. Plio-Pleistocene metacommunities show a high proportion of bathymetric generalists, in contrast to Eocene metacommunities. Accordingly, the net cooling and weaker thermal gradients may have allowed offshore specialists to expand into onshore habitats and maintain their presence in offshore habitats.

ICHNOLOGICAL RECORD OF ENVIRONMENTAL CHANGES IN EARLY QUATERNARY (GELASIAN–CALABRIAN) MARINE DEPOSITS OF THE STIRONE SECTION, NORTHERN ITALY

ABSTRACT Nineteen ichnotaxa occur in a 94-m-thick section of Gelasian–Calabrian siliciclastic deposits along the Stirone River. The most common trace fossils are grouped in 17 clusters based on a K-Means Cluster Analysis. This grouping is related to distal, archetypal and proximal Cruziana ichnofacies. Successions of ichnotaxa and clusters as represented in Detrended Correspondence Analyses determine environmental trends that helped establish a sequence stratigraphic scheme, which is not always obvious in the sedimentary record. The depositional sequences are separated by three sequence boundaries (A–C), which are well expressed in the ichnological record. Primary fabrics are less frequent in the archetypal and proximal Cruziana ichnofacies than in their idealized models. This is probably due to less intense storms in the small and protected paleo-Adriatic Sea and to intense bioturbation. Small-scale intervals with low ichnodiversity and ichnofabrics commonly dominated by one trace fossil are present. This suggests stressed conditions and opportunistic colonization related to small, but probably frequent, seafloor disturbances. The record of these disturbances— possibly caused by storm and bottom currents, deposition, or possibly erosion—has been obliterated by subsequent bioturbation. The disappearance of Schaubcylindrichnus, the smaller size of Scolicia and Ophiomorpha, and a slight decrease in trace fossil diversity in the Calabrian part of the section is interpreted to record climate cooling. This is also supported by the general decrease in ichnodiversity in Pleistocene versus Pliocene shallow-marine to slope siliciclastic facies.

Coralgal buildups associated with the Bolca Fossil-Lagerstätten: new evidence from the Ypresian of Monte Postale (NE Italy)

The Monte Postale and Pesciara di Bolca sites are well-known Eocene Fossil-Lagerstätten. The origin of these deposits has been previously related to a lagoon environment, protected from the open sea by a barrier formed by some kind of reef. However, no detailed description of a bioconstructed system has ever been reported in the area. This study provides the first detailed characterization of the upper Ypresian Monte Postale reef, made of in situ corals, calcareous algae, and encrusting foraminifera. Three main facies associations have been recognized: (1) a coralgal rim, roughly E/W oriented, (2) lagoon deposits in the south and (3) fore-reef sediments towards the north. The coralgal rim is represented by a wave-resistant framework made of heavily calcified organisms associated to Alveolina accumulations; the adaptation of most of these organisms to the euphotic zone promoted their growth in proximity to the sea level, allowing this structure to act as an effective barrier. In particular, during relative sea-level lowstands or highstands, the coralgal rim could have limited the water exchange between the lagoon and the open sea, reducing the oxygenation in the lagoon and allowing the formation of the Fossil-Lagerstätten. During relative sea-level transgressions, the water could have risen well above the margin, establishing normal oxygenation and hampering the fossils’ preservation. The Monte Postale succession represents one of the few examples of lower Eocene bioconstructions. The description of this biotic assemblage provides new data for the characterization of reef communities during a crucial climatic phase, near the end of the early Eocene climatic optimum global warming.

Quantitative Taphonomy in Sandstones from an Ancient Fan Delta System (Lower Pleistocene,Western Emilia, Italy)

Abstract Many ancient settings differ from Holocene shallow seas, and studies on quantitative taphonomy carried out in modern shell beds do not necessarily answer all questions that the fossil record may raise. In order to help characterize past environments, sedimentological shell beds from a Lower Pleistocene fan-delta system are analyzed. The studied sandstone intervals, cyclically alternated with a mudstone facies, were deposited in a tectonically active setting during phases of advance of fan deltas. High-density flows triggered by river floods mixed skeletal remains from different shallow-water communities. Standardized quantitative taphonomic procedures tested in modern settings are applied for the first time to fossil bivalves. The study shows that intrinsic factors did not overprint the pattern dictated by extrinsic agents. Damage levels are inversely related to sedimentation rate and directly related to the volume of high-density flows, although the influence of other agents cannot be eliminated. The most revealing taphonomic variables are fine-scale alteration, edge modification, fragmentation, and discoloration, while bioerosion and encrustation were insignificant, due to generally high rates of sedimentation. The study suggests different sources of skeletal remains. Fine-scale alteration, edge modification, and fragmentation are higher in shells from aerobic muddy sediments, and it is suggested that these remains came from erosion of older, tectonically exposed sediments of the mudstone biofacies. The majority of shells are fresh and came from nearshore sandy and muddy biofacies. These are interpreted as contemporary, or nearly so, to the depositional events. The overall results underline the importance of active tectonics in causing repeated burial/exhumation cycles and episodic burial of articulated, or otherwise pristine, skeletal remains, causing the frequent mixing of taphonomic signatures through lateral transport. This situation may be characteristic of other fan-delta systems from ancient structurally confined basins.

Infidelities of fossil assemblages

It is widely accepted that palaeontologists and earth scientists in general should be cautious in extrapolating modern processes and rates to the geologic past (Gould 1965). A chance of error is nested in the cyclic behavior of the Earth system. At the scale of the Phanerozoic, the Holocene forms part of an icehouse interval and the present world’s climatic belts are to be compared only to those occurring in similar glacial-interglacial Earths, Ordovician or upper Palaeozoic, for instance. On the other hand, when looking at the Cambrian, mid-Palaeozoic or Paleogene, we are offered with visions of a greenhouse planet, a ‘strange old world’ indeed (Kauffman 1987; Pfefferkorn 1995; Clyde 1999; Soreghan 2004). At finer temporal scales a hierarchy of sea level and climate variations produces cycles with frequencies spanning orders of 10–10 my (Vail et al. 1991). When examining the geologic record under the perspective of high-frequency cycles, the Holocene becomes a thin skin. We live and act in a time of transgressed shelves, while the thickest parts of the sedimentary record were built at times of progradation, during sea level lowstands and highstands (Clifton 2004). Sea-level lowering during glacial times dramatically impacted the physiography of the continental margins, leading to strong erosion, sediment bypass of coast and shelf areas and to exceptional sediment supply to the deep basins. Although everyone should be aware of the need to reform the uniformitarian approach, inferences are still directly drawn from the Modern to the past in an understatement of the risks involved in forgetting historical pathways. By placing the focus on the issue of spatial fidelity of fossil assemblages, the non-uniformitarian approach is here re-emphasized and an invitation is made to consider its influences in taphonomy. Actualistic taphonomic studies suggest that out-of-habitat transport is not an overwhelming problem in ordinary levelbottom taphonomic settings (Kidwell & Flessa 1996; Behrensmeyer et al. 2000). Allochthony is considered to be frequent only in the presence of a steep depositional gradient or in settings with episodically very high pulse-type energy (Kidwell & Bosence 1991, p. 158). Donovan (2002) has already made the point that steep depositional gradients may be much more common than generally assumed, and actualistic studies support his view (Hubbard 1992; Hohenegger & Yordanova 2001). Here, we raise the point that also on gently sloping shelves considerable shell transport may be more common than indicated by actualistic studies. Transport most likely will result from historically rare, but geologically frequent, catastrophic events: a single major storm or turbidity current over decades or centuries may be sufficient to transport skeletal remains into deeper water environments, cover them with other transported clastics and increase their likelihood of being preserved (e.g. Bries et al. 2004). Under the ever-ruling second law of thermodynamics – methodological uniformitarianism works – we expect shells to move down-slope and not to come back (but see Flessa 1998). It may therefore be premature to deduce lack of out-ofhabitat transport for fossil assemblages from such environments, because none of the actualistic study sites has yet finished its taphonomic history. It is also suggested that the importance of shell transport could be underestimated in many Recent and fossil assemblages, because thin event beds are frequently reworked by bioturbation into the normal sedimentary record (e.g. Sepkoski et al. 1991; Zuschin & Stanton 2002; Keen et al. 2004). To make things worse, tempestitic and turbiditic deposits can only be seldom related to their source areas, which are rarely preserved in the geological record. This discrepancy reflects a bias against intertidal and shallow subtidal settings, typically used for actuopalaeontological studies, without considering that, due to erosion, they are probably bound to be lost, especially in the older geological record. The fossil record is full of shelly event beds. A survey of the literature will convince even the most skeptical that most shell beds are produced by geologically rare and catastrophic events. Tempestitic and turbiditic shell beds are among the most common skeletal concentrations throughout the Phanerozoic (Kidwell & Brenchley 1994; Li & Droser 1997; Kowalewski & Bambach 2003). Based on actuopalaeontology, however, the view is that spatial fidelity is generally high. Should we adhere to this knowledge when studying fossil assemblages? Interestingly, the view that out-ofhabitat displacement of skeletal material can be a common feature of fossil shell beds, once held by palaeoecologists (Johnson 1960; Park 1968), was kept alive in the work of sedimentary geologists (Goldring & Bridges 1973; Kazmierczak & Goldring 1978; Mutti et al. 1996), against a mounting wave of actuopalaeontological studies. Why was this so? Perhaps because researchers familiar with the fact that gravels, mud clasts, and sands do get transported over long distances, expect bioclasts to be transported too. In fact, hyperpicnal flows and related turbiditic currents, among the most important factors of delta progradation (Mutti et al. 1996, 2003; Plink-Bjorklund & Steel 2004), can carry shells and other organic remains up to hundreds of miles from their source area (van Straaten 1960; Sarnthein & Bartolini 1973; Pilkey & Curran 1986; Mutti et al. 1996, 2003; Squires 1998; Donovan 2002; BeavingtonPenney 2004). Accordingly, many direct analytical studies of fossil shell beds reveal palaeoecological and taphonomic evidence that bioclasts were frequently carried out of their life site (e.g. Plaziat 1984; Fursich & Flessa 1987; Dominici 2004; Tomasovych 2004; Zuschin et al. 2005; Beavington-Penney et al. 2005). Such evidence suggests exercising caution when interpreting taphonomic facies and approaching the study of palaeocommunity composition and diversity patterns from shell beds. Further attention should be paid to the structural setting from which we derive our models. Many of the above-mentioned palaeontological studies are from tectonically mobile basins in collisional belts, where, due to mountain building, high quantities of sediment are discharged in high-subsidence basins. A variety of fluidal sediment gravity flows (Mutti et al. 1996, 2003; Benvenuti & Martini 2002; Plink-Bjorklund & Steel 2004) occurred in flooddominated fluvio-deltaic systems, delivering huge volumes of sediment to rapidly subsiding basins and displacing bioclastic material. In Holocene transgressed and slow-subsiding coastal and shallow marine areas, allochthonous shell beds are expected to be very rare, and the few high-magnitude events that punctuate an overall condition of ‘normal’ processes determine very limited displacement. Spatial fidelity of modern assemblages is expected to be generally high, whereby low sedimentation rates lead to mixing of shells produced at significantly different times (Kowalewski & Bambach 2003). In contrast, allowing for the opposite condition at times of maximum progradation of fluvial and coastal systems, we would expect low spatial-fidelity of fossil assemblages, at the same time predicting time-averaging to be less significant than suggested by modern shell beds.