#ophiomorpha

浅い砂地の海の地層に残った生きものの巣穴の化石　前期更新世（約100万年前）　長岡市

Another ichnofossil, thought to be a crustacean burrow. They're generally smooth inside, and lumpy outside, where the sediment was packed in tight to support the burrow. They date as far back as the early Permian (298mya).

Unlike Zoophycos, Ophiomorpha is usually found in rocks that form from sand off beachs and in tidal sandflats, although some have been found in turbidite layers where those turbidites were well-oxygenated. We can only presume that the Zoophycos organisms are much more tolerant of low oxygen levels than Ophiomorpha. Other ichnofossils from the same near-shore rocks include Skolithos, Arenicolites, and Diplocraterion. In fact that assemblage of four ichnospecies is called the Skolithos ichnofacies, and is used to infer a shallow-water paleoenvironment.