Permian lobed Zoophycos as the product of the terrestrialization process: Behavioral innovation in the Tahkandit Limestone (Yukon River, Alaska, USA)

Abstract

Paleontological survey in the remote Yukon-Charley Rivers National Preserve in Alaska led to the discovery of lobed Zoophycos from the lower Tahkandit Limestone (informally named Sandstone unit), an interval characterized by grayish-green glauconitic sandstone and conglomerate of coastal origin. The studied Zoophycos consists of a lobate skirt-like spreite bounded by a marginal tube. Smaller tongue-shaped lobes branch off from larger parent lobes that share the same tongue-like shape. Sedimentological features, together with body fossils and associated trace fossils ( Planolites, Chondrites ), indicate a shoreface habitat for the Zoophycos producer. This shallow-marine environmental setting is in contrast with the deeper bathymetries in which lobed Zoophycos are recovered in post-Palaeozoic times. The producer of the lobed Zoophycos of the Yukon River is interpreted as a deposit-feeder that used sensory-driven, directed search for locating heterogeneously distributed trophic resources. The Zoophycos producer filled its burrow with Coprolus- like fecal pellets, possibly complementing deposit feeding with microbial gardening and/or food caching. Data presented here provide useful insight into the morphological evolution and bathymetric distribution of Zoophycos , suggesting two ‘Golden Ages’ for lobed Zoophycos: (1) Carboniferous–Permian and (2) Cretaceous–Neogene. This stratigraphic distribution supports the important ecological role of major terrestrialization events, that are, the Palaeozoic expansion of land plants and the Mesozoic expansion of angiosperms. The consequent increased input of nutrients to coastal areas was an important contributor to declining trends in porewater oxygen concentrations. This phenomenon favored adaptive traits to exploit nutrient-rich but oxygen-poor niches, among which the U-shaped marginal tube of lobed Zoophycos was an efficient adaptation to bring oxygenated water into low-oxygen substrates. • New ichnological information is added to a poorly understood Permian rock unit along the Yukon River in eastern Alaska. • This report illustrates the important role of terrestralization events in driving the Zoophycos morphology. • In contrast to previous work, this study shows larger spirals of Zoophycos preserved in more inshore environments. • This is one of the oldest known trace morphologies of Zoophycos with such a complex vertical structure.