First clear evidence that Archaeozostera is not an ancestor of Zosteracean sea-grass but a trace fossil

Many structures that were once interpreted as fossils of plants, sea-grasses, or seaweeds are now treated as ichnofossils produced by ancient marine benthic animals. However, the origin of Archaeozostera has still debated owing to a lack of clear and persuasive evidence. At last, we found special specimens that Archaeozostera is not a fossil of the Zosteracean sea-grass but a trace fossil produced by infaunal benthic animal from the late Cretaceous submarine fan deposits in Japan. Detailed field observations of occurrence, morphological analysis, and X-ray diffraction analysis revealed that previous discussions of Archaeozostera were based not only on incomplete specimens but also on biased outcrop observations. On the basis of morphological analysis of new specimens, we demonstrate the first complete overall morphology of Archaeozostera and clear evidence that Archaeozostera is a trace fossil produced by a sedentary endobenthic detritus feeder, which systematically excreted fecal matter deep in sediments. A representative specimen of Archaeozostera, which has recently found from Eocene deep-sea sediments in Spanish Pyrenees, strongly suggests that it is no longer an endemic trace fossil of the Upper Cretaceous of Japan. •Clear and persuasive evidence that Archaeozostera is a trace fossil was found at last.•Ontogeny of the producing animal is successively recorded in Archaeozostera.•Archaeozostera was produced by an endobenthic detritus feeder adopting sedentary lifestyle in a same burrow.•Complete overall morphology of Archaeozostera was first demonstrated in this paper.•A rhizome of Archaeozostera is a horizontal tunnel of trace fossil Ophiomorpha rudis.