Enigmatic vertebrate swimming trace fossils from the Wapiti Formation, Alberta, Canada, and their implications for paleoenvironmental reconstruction

Abstract

Vertebrate ichnofossil research, when compared with invertebrate ichnological analyses, tends to put greater emphasis on identifying the trace makers rather than associating the trace fossils with animal behaviors to interpret local paleoenvironmental conditions. In 2011, several sandstone blocks bearing unusual deformational structures were collected from the Wapiti Formation exposed near Red Willow Falls, Alberta, Canada. The sandstone blocks were analyzed for their sedimentary characteristics and the morphology of the enigmatic structures in order to identify their origins and their significance for paleoenvironmental interpretation. The deformational features are interpreted to be the swim tracks of small vertebrates attempting to escape a flash flood event. The sedimentology of the sandstone suggests a rapid increase in water flow in an overbank setting, and the orientation of the traces indicate that the trace maker was initially moving against, and then moving perpendicular to, the current. This is likely the first case of a trace fossil exhibiting clear association between vertebrates and flash flood events. As such, these traces provide an excellent example of the utility of vertebrate trace fossils for assessing organism response to changing environmental conditions, thus facilitating interpretation of local geological events.