Large Phymatoderma in pliocene slope deposits, northwestern Ecuador: Associated ichnofauna, fabrication, and behavioral ecology

Abstract

Phytnatoderma is a complex, branching biogenic structure that is seldom mentioned in the ichnologic literature, although it may have been misidentified recently as “Zonarites” and large pellet‐filled “Chondrites”. The ichnogenus is usually interpreted as the product of subsurface deposit‐feeding animals. Well‐preserved examples from Pliocene slope mudstone in Ecuador are assignable to P. granulatum (Schlotheim, 1822). These structures are typically 30–40 cm in overall diameter; consist of outward‐branching, horizontal to subhorizontal, overlapping tunnels grouped in bundles; and have tunnel fills consisting of pelleted volcanic ash. The Ecuadorian Phymatoderma occur with a moderately rich ichnoassemblage that includes Alcyonidiopsis ichnosp., Chondrites intricatus, several kinds of Phycodes‐like structures, Phycosiphon incertum, Planolites ichnosp., Rorschachichnus amoeba, Scolicia ichnosp., Taenidium barretti, and Zoophycos ichnosp. Benthic foraminiferids and geologic context point to deposition on the upper trench slope, ca. 3–4 Ma. Ecuadorian Phymatoderma are large, structurally intricate burrow systems that not only represent surface deposit feeding and below‐surface pellet stowage/food caching, but also behavior that apparently included revisiting runnels to utilize the pellets for food, in response to a variable trophic regime. Together with other morphologically intricate, ethologically complex marine ichnogenera (e.g., Zoophycos, Chondrites, Paleodictyon), Phymatoderma deserves special attention as a complex animal artefact, rather than being considered merely a snapshot of a single or dominant behavior. Such complicated structures appear to record deliberate activity to restructure habitats, modulate natural disturbances, and to control trophic resources.