Trilobite expansion into estuarine environments during the Ordovician radiation

MÁNGANO, M. GABRIELA; BUATOIS, LUIS A.; WAISFELD, BEATRIZ G.; MUÑOZ, DIEGO F.; VACCARI, N. EMILIO; ASTINI, RICARDO A.

Tallin

Simposio; 14th International Symposium on the Ordovician System; 2023

Trilobites have traditionally been considered fully marine. Through the integration of ichnological,palaeobiological and sedimentological datasets within a sequence-stratigraphic and strati -graphic palaeobiology framework, we challenge this assumption. This analysis is based on thestudy of incised fluvio-estuarine valley deposits from the Furongian Tilcara Member (TM) andthe latest Furongian Pico de Halcón Member (PHM) of the Santa Rosita Formation, the early lateTremadocian Cardonal Formation (CF), and the Dapingian–Darriwilian Alto del Cóndor Formation(ACF), from Cordillera Oriental of northwest Argentina. These valleys were incised into wavedominated shallow-marine strata and filled with transgressive deposits that accumulated intide-dominated estuaries. Whereas the TM lacks any body or trace fossil evidence of thepresence of trilobites in estuarine settings, the other three units reveal that trilobites were ableto inhabit these settings. The PHM and CF are host to trilobite trace fossils in outer estuarine facies,both containing various ichnospecies of Cruziana (e.g., C. omanica and C. semiplicata in the TM)and Rusophycus (e.g., R. latus in both units). In addition, the PHM also contains body fossils ofthe olenid trilobite Neoparabolina frequens argentina in the same deposits in which the tracefossils are preserved, as well as from middle estuarine facies. The ACF displays trilobite tracefossils of the C. rugosa group in inner, middle, and outer estuarine deposits, illustrating furtherlandward incursions. This unit also contains body fossils of the asaphid trilobite Ogyginus sp.Accordingly, our data indicate two attempts of landward exploration via brackish water: phase 1in which the outer to middle portion of estuaries were colonized by olenids (Furongian–early lateTremadocian) and phase 2 involving exploration of the inner, middle, and outer es tuarine zonesby asaphids (Dapingian–Darriwilian). Our study indicates that these trilobites were tolerant tosalinity stress and able to make use of the ecological advantages offered by marginal-marineenvironments migrating up-estuary, following salt wedges either reflecting amphidromy or aseuryhaline marine wanderers. It is suggested that tolerance to salinity stress arose inde -pendently among different trilobite groups as a result of the broad array of behaviors andadaptations of trilobites during the Ordovician radiation. We speculate that the assumption thatall trilobites were stenohaline may have resulted in the misinterpretation of some tide-dominatedestuarine deposits as fully marine.