Were all trilobites fully marine? Trilobite expansion into brackish water during the early Palaeozoic.

Affiliation

Mángano MG(1), Buatois LA(1), Waisfeld BG(2)(3), Muñoz DF(2)(3), Vaccari NE(2)(3)(4), Astini RA(2)(3).
Author information:
(1)Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan S7N 5E2, Canada.
(2)Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba, Vélez Sarsfield 1611, Ciudad Universitaria, Córdoba X5016CGA, Argentina.
(3)Consejo Nacional de Investigaciones Científicas y Técnicas
(CONICET), Centro de Investigaciones en Ciencias de la Tierra
(CICTERRA), Edificio CICTERRA, Av. Vélez Sársfield 1611, Ciudad Universitaria, Córdoba X5016CGA, Argentina.
(4)Universidad Nacional de La Rioja, Av. Luis M. de la Fuente s/n, F5300 La Rioja, Argentina.

Abstract

Trilobites, key components of early Palaeozoic communities, are considered to have been invariably fully marine. Through the integration of ichnological, palaeobiological, and sedimentological datasets within a sequence-stratigraphical framework, we challenge this assumption. Here, we report uncontroversial trace and body fossil evidence of their presence in brackish-water settings. Our approach allows tracking of some trilobite groups foraying into tide-dominated estuaries. These trilobites were tolerant to salinity stress and able to make use of the ecological advantages offered by marginal-marine environments migrating up-estuary, following salt wedges either reflecting amphidromy or as euryhaline marine wanderers. Our data indicate two attempts of landward exploration via brackish water: phase 1 in which the outer portion of estuaries were colonized by olenids (Furongian-early late Tremadocian) and phase 2 involving exploration of the inner to middle estuarine zones by asaphids (Dapingian-Darriwilian). This study indicates that tolerance to salinity stress arose independently among different trilobite groups.