Fig. 6

Wall microstructure of Halysis from the Middle Ordovician Zhuozishan and Kelimoli Formations of the Laoshidan section in Northwest Ordos Basin, North China. a Cross section of a ‘single-tube’ Halysis shows the tube wall composed of fibrous calcite perpendicular to the tube surface, exhibiting laminofibrous microstructure, and the tube interior filled by blocky calcite; b Cross section of a ‘single-tube’ Halysis internally filled with micritic sediments, showing laminofibrous microstructure; c Cross section of a ‘multiple-tube’ Halysis composed of twelve tubes; d Partially enlarged view of the red box in Fig. 6c. Note the laminofibrous microstructure of the tube wall; e Detailed view of wall microstructure of a Halysis tube, composed of radially arranged lath-like calcite crystals perpendicular to the tube surface; f Partial enlarged view of the red box in Fig. 6e. Note thin lath-like calcite crystals arranged in a shingled pattern; g Cross section of a tube of ‘multiple-tube’ Halysis internally filled with blocky calcite; h Detailed view of the calcite in the red box in Fig. 6g; i Longitudinal section of a Halysis tube, with possible tabulae (pointed by yellow arrows); j Detailed view of the red box in Fig. 6i. Note the tube wall displaying laminofibrous microstructure; k Longitudinal section of a Halysis tube. Note that the internal filling is blocky calcite and the wall is composed of calcite flakes perpendicular to the tube surface; l Detailed view of the red box in Fig. 6k. a-d and i-j are thin-section photomicrographs. e-h and k-l are scanning electron microscope images