Commensal worm traces and possible juvenile thalassinidean burrows associated with Ophiomorpha nodosa, Pleistocene, southern Brazil

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Abstract

The Pleistocene Chuí Formation at Osório (Rio Grande do Sul, Brazil) consists of coastal marine and eolian sands, the former containing abundant and well-preserved Ophiomorpha nodosa burrow systems. Detailed ichnological study has revealed interesting features associated with them. Small-sized Ophiomorpha, here assigned to a new ichnospecies, O. puerilis, are interpreted as possible burrows of juvenile thalassinidean crustaceans probably belonging to the same species as the producers of larger O. nodosa. Additionally, helicoidal burrows with thick, concentrically laminated linings are associated with the walls of O. nodosa. They are assigned to the new ichnospecies Cylindrichnus helix, and they are interpreted as dwellings of commensal annelid worms. The association of these three ichnospecies constitutes a fossil example of the role of thalassinideans as ecosystem engineers able to modify their environment and to create new space and resources usable by other organisms.

Introduction

Ophiomorpha is one of the best-known trace fossils for paleontologists and sedimentary geologists due to its abundance in Mesozoic and Cenozoic shallow and marginal marine deposits. The ichnogenus designates multiple-branching gallery systems of variable complexity characterized by having a thick pelletal lining. Early works revealed the striking similarity between Ophiomorpha and modern burrows of thalassinidean crustaceans (Weimer and Hoyt, 1964, Suguio and Martin, 1976, Frey et al., 1978). Ophiomorpha is a substrate-controlled ichnogenus that occurs almost exclusively in fine- to medium-grained sandy deposits (Ekdale, 1992). Although very abundant in shallow marine environments, some ichnospecies of Ophiomorpha are known from deeper water settings (e.g., Tchoumachenko and Uchman, 2001). The extensive literature regarding Ophiomorpha includes papers focused on paleoenvironmental interpretation and ichnofabrics (e.g., Pollard et al., 1993, Anderson and Droser, 1998), paleobiological and neoichnological aspects (e.g., Frey et al., 1978, Miller and Curran, 2001) and evolutionary paleoecology (e.g., Bottjer et al., 1988).

Despite being a very well known ichnogenus, Ophiomorpha often occurs only as cross-sections that do not allow in depth analysis of important attributes, such as architecture or relation with other traces. Some outcrops, however, offer the possibility of such detailed studies, which reveal interesting features of the paleobiology of the tracemaker. The Pleistocene Chuí Formation in Brazil includes some outcrops with those characteristics. Ophiomorpha occurs here in well-sorted loose sands that allow careful cleaning of the burrow systems. The study of Ophiomorpha nodosa from the Chuí Formation, besides providing the opportunity to analyze certain aspects of its 3-dimensional architecture, has revealed interesting new details about the paleoecology of this trace fossil. Small-sized Ophiomorpha (O. puerilis nov. isp.) and helicoidal Cylindrichnus (C. helix nov. isp.) have been found in direct association with Ophiomorpha nodosa. They are here interpreted as burrows of juvenile thalassinidean crustaceans and commensal worms, respectively. The objective of this paper is to analyze these features in the light of biological data on modern thalassinideans and their burrows.

Section snippets

Geological setting

The Chuí Formation crops out in the Coastal Plain of Rio Grande do Sul (Planicie Costeira do Rio Grande do Sul, PCRS) in southern Brazil. This plain extends for about 33,000 km2 along the eastern part of the state of Rio Grande do Sul, parallel to the present shoreline (Fig. 1). The PCRS was formed during the Quaternary by the progradation of sediments deriving from the western highlands. The proximal part of the plain consists of alluvial fans fed by the Precambrian Sul-riograndense Shield and

The Osório outcrops

The outcrops studied for this project are located in two adjacent quarries, Jazida Gomes and Transareia, situated in the surroundings of Osório in the northeastern part of Rio Grande do Sul (Fig. 1). The sedimentology of these outcrops was previously studied by Tomazelli et al. (1982).

Several sections were made to establish the stratigraphy of the Chuí Formation at Osório. A representative section (Gomes Quarry) is shown in Fig. 3. Two main units are differentiated: a lower sandy marine unit

Ophiomorpha nodosa burrow systems

Ophiomorpha burrow systems in the Chuí Formation constitute complex labyrinths with horizontal to vertical elements and multiple branching. Qualitative observations show that horizontal and subhorizontal elements are important as constituent of mazes connected to less-branched vertical shafts (Fig. 6B). Vertical elements are rectilinear, while the rest are rectilinear to gently curved. The branching style of the mazes is mostly Y-shaped but also T-shaped (Fig. 6A, C).

The burrows are circular in

Ophiomorpha nodosa

Weimer and Hoyt (1964) demonstrated the striking similarity between the burrows of the callianassid thalassinidean Callichirus major (formerly known as Callianassa major) and the ichnospecies Ophiomorpha nodosa. This fact led many authors to inadequately establish an identity between the two. Quaternary Ophiomorpha from the south Atlantic coast have been referred as ‘fossil tubes of Callianassa’ (Suguio and Martin, 1976), ‘fossil tubes of Callichirus’ (Suguio et al., 1985), or ‘callianassid

Conclusions: thalassinideans as ecosystem engineers

The association of Ophiomorpha nodosa, O. puerilis and Cylindrichnus helix in the Pleistocene Chuí Formation constitutes a fossil example of how thalassinidean crustaceans modify their environment favoring the occupation of new ecological niches. Modern thalassinideans and other crustaceans play a very important ecological role, particularly in shallow marine environments, as a result of their intense burrowing activity and high-density populations. They can be considered as true ‘physical

Acknowledgements

The authors wish to thank Marcelo Zagonel de Oliveira for his assistance in the field. Tony Ekdale and Richard Bromley contributed with their suggestions to improve the manuscript. This paper is a contribution to the projects 31.00.006/01-0 and 31.00.002/04-4 of UNISINOS, 524415/1996-0 and 474345/03-3 of CNPq (Brazilian Scientific and Technological Development Council), BTE 200-0584 of the Spanish Government and GRC 2001/SGR/00077 of the Generalitat de Catalunya.

One of the authors, Marcelo E.

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