Neoichnology at lake margins: Implications for paleo-lake systems

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Abstract

Neoichnological studies provide significant information that can aid in the interpretation of ancient ichnological assemblages. Here we examine, for the first time, the pattern in both vertebrate and invertebrate trace distribution and abundance that can be seen in two geographically separate, but broadly analogous, contemporary semi-arid climate lake margin settings in an effort to describe taphonomic processes relevant to paleo-lake systems. Lake margin to supralittoral zone transects were made at Ruby Reservoir, Montana (USA) and La Sotonera, Spain and vegetation cover, trace cover %, trace diversity, and substrate characteristics were recorded. Similar patterns in trace distribution were seen through the eulittoral to supralittoral zones, with the majority of traces identified within a single transect present at both sites. However, a few traces attributed to larger organisms were found to be unique to each and in some instances, could to be assigned to a specific trace maker. The eulittoral zone was considered to have characteristics attributable to the Scoyenia ichnofacies, however, analysis of the supralittoral zone shows some characteristics of both the Coprinisphaera and the Mermia ichnofacies. In addition, a transitional zone between a Scoyenia-like ichnofacies and a fully terrestrial assemblage was found, which may enable continental ichnofacies to be further refined for application to paleo-lake systems. Study of the distribution of traces with distance along the transects also suggests taphonomic biases against the preservation of certain types of traces (e.g., avian) due to seasonal fluctuations of the lake level and the subsequent reduced preservation potential for lake-proximal traces.

Introduction

The study of ichnofossils in ancient strata has been widely applied, because they are a reflection of organism behavior in response to varying paleoecological, paleoclimatic, and paleoenvironmental conditions. Many interpretations of ancient landscapes are based on comparisons to the behavioral patterns of extant organisms and their environment (e.g., Ratcliffe and Fagerstrom, 1980, Hasiotis and Dubiel, 1993, Doyle et al., 2000, Hasiotis et al., 2007). Consequently, by examining the distribution of contemporary traces and the ecosystems in which they arise, comparisons can be drawn to ancient environmental settings and the ichnofossils contained within them. The study of modern assemblages (i.e., neoichnology) allows for more detailed interpretations than purely ichnological studies both because the exact trace maker can be identified, and also because other taphonomic variables, such as firmness of the substrate, may be better constrained (e.g., Radies et al., 2005, Melchor et al., 2006, Minter et al., 2007, Hasiotis et al., 2007).

However, despite the obvious significance of studying modern trace assemblages, relatively little research has been undertaken in this potentially useful area for either marine (e.g., Ekdale, 1980, Wetzel, 1991, Fu and Werner, 2000, Gingras et al., 2004) or non-marine settings (e.g., Pryor, 1967, Metz, 1987, Cohen et al., 1991, Cohen et al., 1993, Lawfield and Pickerill, 2006, Scott et al., 2007). Studies of contemporary continental traces have tended to focus primarily on vertebrate traces (e.g., Cohen et al., 1991, Cohen et al., 1993) or on invertebrate (e.g., Metz, 1987, Muñiz et al., 2006) traces only. Other studies are restricted to a single trace maker either under controlled laboratory conditions or to field-actualistic studies (e.g., Hasiotis, 1990, Hasiotis, 2002, Tschinkel, 2003, Hembree and Hasiotis, 2006).

Herein, we will combine both quantitative and qualitative observations of modern traces made by both vertebrates and invertebrates on two mudflats surrounding La Sotonera lake in northeastern Spain and Ruby Reservoir, south west Montana (Fig. 1, Fig. 2) in order to determine if similar assemblages of traces occur at each locality, and to examine whether the ichnofacies concept is applicable to modern settings, and by extension, to ancient terrestrial landscapes (e.g., Genise et al., 2000, Hasiotis, 2004, Bromley et al., 2007, Hasiotis, 2008). The sedimentological characteristics of lakes and their margins are well known, but biological and ichnological characteristics are less so (e.g., Talbot and Allen, 1996). Each geographically separate locality was chosen to be reasonably analogous in terms of climatic and environmental setting. Comparisons can then be drawn between the assemblages identified at La Sotonera and Ruby Reservoir and compared to those interpreted to have formed in an equivalent paleoenvironmental setting (e.g., Hasiotis, 2004, Lucas et al., 2004, Kim et al., 2005, Radies et al., 2005) in order to examine trace fossil taphonomy.

Section snippets

The study areas

Both of the study sites key out as Bsk in the Koppen-Geiger climate system (Peel et al., 2007), and thus represent directly comparable environmental settings (Fig. 1). The Ruby Reservoir is in the Ruby Valley in Madison County of southwestern Montana (Fig. 2). The lake lies within the Ruby Syncline, an intermontane basin at an elevation of 1900 m and bounded by the ∼ 2500 m Snowcrest and Gravelly mountain ranges (Kron, 2006). The Ruby Reservoir area has a strongly-seasonal, semi-arid, climatic

Methods

La Sotonera and Ruby Reservoir were studied during the summer low water level; mid-August for Ruby Reservoir and early September at La Sotonera. After careful examination of the ecology, landscape, traces and trackways of the lake margins at both localities, transects were chosen so as to be representative of the eulittoral to supralittoral zones at each lake. The eulittoral zone is defined as the area that is only rarely (e.g., seasonally) inundated and the supralittoral zone as the zone that

Results

A total of 16 morphologically different tracks, trails, nests, and burrows were identified along the transects at La Sotonera and Ruby Reservoir, including six which form distinct trackways. The assemblage at Ruby Reservoir contained 12 trace types while La Sotonera had 13 trace types; nine trace types were shared by both assemblages, and the distribution of trace types between different types of organisms was similar in both assemblages (Table 1).

Each transect at Ruby Reservoir and La Sotonera

Discussion

Contemporary traces at La Sotonera and Ruby Reservoir form part of a eulittoral and supralittoral ichnocoenose, which is composed of a combined total of sixteen identified and unidentified traces that are morphologically similar to ichnotaxonomic groups. There were numerous sedimentological and ichnotaxonomic characteristics that are common to both localities (summarized schematically in Fig. 8). Characteristic sedimentological observations have long been recorded at lake margins and

Conclusions

The study of contemporary traces and tracks can provide important information that can aid in the interpretation of ancient ichnological assemblages. From this study several key conclusions can be drawn:

  • 1)

    Significant differences in the assemblage of both vertebrate and invertebrate traces were present at both lakes along the transects from the lake edge to the supralittoral zone.

  • 2)

    Sixteen morphologically different traces and trackways, some with identifiable track makers, were identified reflecting

Acknowledgements

JMMH would like to acknowledge support from a NERC studentship and NDS would like to acknowledge funding from NERC (NE/D000521/1). An anonymous reviewer and Editor-in-Chief Matthew Bennett provided useful suggestions that improved this manuscript significantly.

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