GRAIN-SIZE CONTROLS ON THE SILURO-DEVONIAN COLONIZATION OF NON-MARINE SUBSTRATES BY INFAUNAL INVERTEBRATES


 Throughout the history of life on Earth, sedimentary environments have placed controls on the trajectory of evolutionary innovations. To survive and thrive in newly colonized sedimentary environments, organisms have needed to develop novel behaviors: often evidenced in the rock record as architectural innovation and diversification in trace fossil morphology. This study focuses on ichnological diversification as a response to challenges presented by different sediment grain sizes during the late Silurian to Early Devonian colonization of the continents by invertebrate life. The ichnodiversity and ichnodisparity from this interval reveal details of the biological response to newly adopted sedimentary and environmental conditions.
 Characteristics of ichnofaunas from terrestrial and emergent settings are compared across the Silurian-Devonian boundary, within both sand and mud dominated successions, to identify differences associated with different substrate compositions. Two trends are revealed: 1) Successions dominated by mudrock contain a lower ichnodiversity than sandstone-dominated successions of similar age, potentially due to the different challenges associated with burrowing in cohesive versus non-cohesive media. Alternatively, this could be due to preference of the tracemakers for the broader environmental conditions that lead to sand or mud deposition. 2) The maximum size of trace fossils within a given formation is larger in sandstone dominated strata than in mudrock dominated strata. Together, these suggest that the availability of substrates with different grain sizes was one factor determining the constitution of early animal communities and behavioral styles during the colonization of the continents.