Reappraisal of Schaubcylindrichnus: A probable dwelling/feeding structure of a solitary funnel feeder

The ichnogenus Schaubcylindrichnus has been considered as a cluster of multiple J-shaped tubes, all of which show congruency. Based on such a reconstruction, the trace fossil has commonly been regarded as dwelling tubes of gregarious, head-down deposit-feeding animals.

Well-preserved specimens of Schaubcylindrichnus are found in Miocene and Pleistocene strata of Japan. The trace fossil consists of a bundle of closely spaced, thickly lined tubes, each of which shows a gentle bow-like bend curving downward in a vertical to oblique plane. As individual tubes in a single bundle frequently show size-gradation of diameter, branching and/or cross-cutting, it is probable that the bundle was formed through successive burrowing by a solitary producer. In a longitudinal section of well-preserved specimens, a funnel-like, sediment-filled structure, which is attached to an upper end of a limb of the bow-shaped bundle, is observed. At the other end of the limb, there is a mound-like structure of sediment that is piled up from the bedding surface. The trace fossil was probably a burrow system of a solitary funnel-feeding animal, like an enteropneust.

According to comparisons with type specimens of the ichnospecies of Schaubcylindrichnus, namely, topotypes of Schaubcylindrichnus coronus and holotype and paratypes of S. freyi, S. freyi can be regarded as a junior synonym of S. coronus. As the topotype specimens of S. coronus have many common features with the traces in Japan, the latter should be included in the former. S. coronus therefore is concluded as the burrow system of a solitary funnel feeder.     

Foraminifera-walled Schaubcylindrichnus coronus Frey and Howard, 1981, from the Middle Eocene,Kachchh, Western India

Bioclast lined tubes of Schaubcylindrichnus coronus are not much reported around the world. As of now only two reports are available. Interestingly in both these reports and in the current find, tests of foraminifers are used as tube building material. Current report of Schaubcylindrichnus is from the subtidal deposits of the Naredi Formation (Ypresian) of Kachchh Basin, Western India. As the host rock here abounds in the tests of larger benthic foraminifera, concentration of same is noticed within the tubes. The natural selection of lighter and larger calcitic grains of tests over the heavier mineral grains is the main reason for presence of the tests in tube building; moreover, presence of inner smooth wall of tube also indicates organisms’ preference for comfort within their burrow.