Ichnology of Fossil Oysters (Bivalvia, Ostreidae) from the Southern Brazilian Coast
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7(2):94-103, jul/dez 2011 © Copyright 2011 by Unisinos - doi: 10.4013/gaea.2011.72.02 Ichnology of fossil oysters (bivalvia, ostreidae) from the southern Brazilian coast Renato P. Lopes Universidade Federal do Rio Grande (FURG), Instituto de Oceanografia, Laboratório de Oceanografia Geológica, Setor de Paleontologia. Av. Itália, km 8, 96201-900, Rio Grande, RS, Brasil. [email protected] ABSTRACT Ichnological features over fossil oysters from southern Brazilian coast are described herein. In the deposits Ostrea puelchana d’Orbigny, 1841 is the dominant oyster, followed by O. equestris Say, 1834 and Crassostrea virginica Gmelin, 1791. Although these taxa are found along the entire coast, they exhibit some geographical variations regarding the presence of borings. The most conspicuous ichnogenera are Caulostrepsis and Entobia, with Gastrochaenolites being common in fossils from the central sector of the coast, and scarce in the southern one. Maeandropolydora and incrustations (made by fouling organisms, e.g. cirripeds and bryozoans) are scarce. Most of the borings are found on the external side of the valves which suggest a dominant bioerosion activity during the life span of the organisms. Key words: Quaternary, Ostreidae, bioerosion, Entobia, Caulostrepsis, Gastrochaenolites. RESUMO ICNOLOGIA DOS OSTREÍDEOS FÓSSEIS (BIVALVIA OSTREOIDA) DO SUL DA COSTA BRASILEIRA. Feições de bioerosão identificadas em ostreídeos fósseis de acumulações da costa sul brasileira são descritas. Os depósitos são caracterizados pela presença dominante de Ostrea puelchana d’Orbigny, 1841, seguida por O. equestris Say, 1834 e Crassostrea virginica Gmelin, 1791. Embora esses táxons sejam encontrados ao longo de toda a costa, exibem variações geográficas com relação à presença de bioerosão. Os icnogêneros mais comuns são Caulostrepsis e Entobia e Gastrochaenolites caracteriza a atividade mais comum entre aqueles do setor central da costa, sendo mais raro no setor sul. Maeandropolydora e incrustrações (p. ex. cirripédios e briozoários) são pouco numerosas. A maioria das perfurações é encontrada na superfície externa das valvas, sugerindo que grande parte dos processos bioerosivos tenha ocorrido enquanto os organismos ainda estavam vivos. Palavras-chave: Quaternário, Ostreidae, bioerosão, Entobia, Caulostrepsis, Gastrochaenolites. INTRODUCTION Bioerosion is the result of alterations organisms, not only after death, but on hard substrates produced by organ- also while the individuals are still alive. Trace fossils are widespread features isms, either in the form of structures such In soft-bottom areas, shells can provide in the geological record and can be con- as tunnels and borings or in the form of the only hard substrate available for sidered as ‘fossilized behaviours’, as they scrapings, rasping and gnawing (Warme, colonization by epi- and endoskeleto- record the activities of the tracemakers 1975). These alterations may be caused zoans. Oyster larvae settle preferentially (Gibert et al., 2004). The assemblages by predators (such as naticid gastropods), near or on other oysters, thus forming containing specific trace fossils are fouling (cirripeds, oysters) and boring large bioherms known as ‘oyster reefs’; characteristic of a given environment, (sponges, bryozoans, polychaetes) organ- because of this habit, plus its usually because the organisms that produce isms. Tunnels, chambers and other struc- large and thick shell, oysters are useful such traces are adapted to specific envi- tures built for protection and sheltering as substrate for the settlement of several ronmental conditions (Frey, 1975). Not are produced by endoskeletozoans (sensu different types of communities of bor- only sediments and rocks, but also dead Taylor and Wilson, 2002) and are assigned ing and encrusting organisms (Parras and remains of organisms consisting of hard to the Domichnia ethological class. Casadío, 2006). parts such as shells and bones, can be Shells of marine molluscs are sub- The Coastal Plain of Rio Grande do altered by such biogenic activities. ject of alteration by a wide array of Sul State (CPRS), in southern Brazil, is Renato P. Lopes known by the presence of several accu- The Complex Multiple Barrier System with recent and fossil shell remains ac- mulations of fossil remains, both marine is subdivided into four major barrier- cumulated on the beach by storm waves. and terrestrial, on the continental shelf lagoon depositional systems and as- 150 specimens were collected in the cen- (Figueiredo Jr., 1975; Lopes and Buch- sociated features (Figure 1B). Each of tral sector of the coast, near Mostardas, mann, 2010). This mixture of marine these systems was formed in response while in the southern sector 80 remains and terrestrial fossils on the continen- to a major sea-level transgression dur- were collected in the Verga lighthouse tal shelf is a result of the Quaternary ing Quaternary interglacial episodes. area and 119 in the Concheiros (Figure sea-level oscillations that affected the The exact ages of such deposits are still 1B). At each collecting site, a 1 km-long Brazilian coast (Tomazelli et al., 2000; unknown, but are correlated to the inter- sector of the beach was surveyed for Angulo et al., 2006). The fossils from glacial maxima at 400 ky (Barrier-Lagoon fossil remains accumulated by storm these accumulations are transported to System I), 325 ky, (Barrier-Lagoon Sys- waves between the surf zone and the the beach by storm events during winter; tem II), 123 ky (Barrier-Lagoon System foredunes. In Mostardas, fossil oysters in the southern portion of the CPRS III) and 6 ky (Barrier-Lagoon System constitute some 80% of the bivalve these events create on the beach large IV). The sediments that constitute remains found on the shoreline; in the concentrations (konzentratt-lagerstätte) these systems are essentially terrigenous Verga lighthouse area, oysters constitute known as “Concheiros”. In a study about sicliciclastic, well-selected and mature, some 20% of the bivalve remains, while the taphonomy of the fossil molluscan with small fractions of organic matter, in Concheiros they constitute less than from this place, Lopes and Buchmann biogenic carbonate and diagenetic clays, 1% of the remains. These differences (2008) included an analysis of the ich- with some important concentrations of nological aspects of the shells. However, heavy minerals (Villwock and Toma- probably reflect the fact that in Conchei- due to the low proportion of fossil zelli, 1995). During the Holocene marine ros the amount of fossil shells accumu- oysters among the analyzed samples, transgression, around 6-7 ky, variations lated on the shoreline is larger than on this group was not properly addressed. in the rates of sea-level rise resulted in the other areas, and the dominant fossil In order to evaluate in detail the ich- the reworking and concentration of taxa in this area are Amiantis purpuratus nological aspects of fossil oysters from terrigenous clastic sediments of the CPRS, several specimens that have been shelf, and erosive terraces were formed collected in the last two years, plus fossil in response to episodes of sea-level sta- specimens from the paleontological col- bilization (Kowsmann and Costa, 1974; lection of Universidade Federal do Rio Martins et al., 1996). Grande (FURG) were analyzed. The re- In the southern portion of the coast, sults of this survey are presented herein. large concentrations of Pleistocene marine bioclasts composed mostly of Location AND GEOLOGICAL rounded shell fragments are found (Bu- SETTING chmann et al., 2009). Figueiredo Jr. (1975) obtained 14C ages between 16 ka and The CPRS is a 618 km-long geomor- more than 38 ka for fossil marine shells phological unit formed by sediments collected from the continental shelf. accumulated in the southern Brazil- These bioclasts can be classified as pa- ian coast (Figure 1A). After the split limpsests, i.e., relict sediments (deposited between South America and Africa in during and/or right after the last glacial the Late Cretaceous, sediments eroded maximum) that are being reworked by from Paleozoic and Mesozoic rocks dynamic processes of today and consti- accumulated in the continental margin, tute some 70% of the sediments from forming the Pelotas Basin, which has a continental shelves around the world maximum thickness of 10 km and had (Emery, 1968; Pilkey and Frankenberg, its uppermost portion influenced by 1964; Swift et al., 1971). eustatic oscillations correlated to the glacial-interglacial cycles between the MATERIALS AND METHODS Neogene and Quaternary. As a result of Figure 1. A. Location of the Coastal Plain of Rio Grande do Sul State (CPRS) these oscillations, two major depositional The fossils described herein were in Southern Brazil; B. geomorphological systems were formed parallel to the all collected on the shoreline, in three subdivisions of the CPRS; the places coastline: the Alluvial Fans System and main areas of the coast. Fossil oysters were the samples were collected are the Complex Multiple Barrier system are found all along the coast of Rio indicated by stars: a. Mostardas; b. Verga; c. Concheiros. (Villwock and Tomazelli, 1995). Grande do Sul State, usually associated Gaea - Journal of Geoscience, vol. 7, n. 2, jul/dez 2011, p. 94-103. 95 Ichnology of fossil oysters (bivalvia, ostreidae) from the southern Brazilian coast Lamarck, 1818, Pitar rostratus Koch, than recent specimens, although most RESULTS 1844 and Glycymeris longior Sowerby, are broken and abraded (Lopes