Abstract
The problematic calcareous microfossil Halysis is abundant in the Middle Ordovician Darriwilian Stage of the western edge of the Ordos Basin, North China. The rich and well-preserved specimens of Halysis in this area facilitate detailed studies for its skeletal construction and tube microstructure. Halysis differs from calcified cyanobacteria and calcareous red and green algae in morphology, skeletal construction and microstructure, as well as reproduction mode. Halysis typically consists of multiple juxtaposed parallel tubes arranged in sheets (‘multiple-tube’ type) or is just composed of one tube (‘single-tube’ type). In ‘multiple-tube’ Halysis, tube fission by bifurcation results from the insertion of a microcrystalline wall at the center of a mother tube. This study demonstrates for the first time that the tube walls of Halysis have a laminofibrous (fibronormal) microstructure, composed of fibrous calcite perpendicular to wall surface, and recognizes the ‘single-tube’ type Halysis composed of one tube; in addition, for the first time, this study finds out that ‘multiple-tube’ Halysis develops buddings from the conjunction of two tubes and ‘single-tube’ Halysis shows wide-angle Y-shaped branchings. Based on these findings, this study further compares Halysis with tabulate corals. Halysis appears stratigraphically earlier than Catenipora and Aulopora, and has a smaller tube size. ‘Multiple-tube’ Halysis resembles Catenipora and ‘single-tube’ Halysis resembles Aulopora in skeletal construction and microstructure, and in their tube walls of laminofibrous microstructure composed of fibrous calcite perpendicular to the tube wall surface. Catenipora and Halysis are both characterized by the absence of septal spines. The similarities suggest that Halysis may be the ancestor of Catenipora-like and Aulopora-like tabulate corals.
Highlights
Halysis Høeg 1932 is a problematic calcareous microfossil occurring commonly in Early- to Mid-Paleozoic carbonate rocks (Shen and Neuweiler 2015)
Halysis has been studied for nearly one hundred years, its systematic position remains in dispute
Based on an examination of two hundred thin sections during this study, we found that (1) Halysis specimens from the Ordos Basin have a wall microstructure similar to those of tabulate corals, but different from those of cyanobacteria, coralline red algae and green algae; (2) many specimens of Halysis are composed of a single tube
Summary
Halysis Høeg 1932 is a problematic calcareous microfossil occurring commonly in Early- to Mid-Paleozoic carbonate rocks (Shen and Neuweiler 2015). It has been reported from shallow marine limestones of the Lower Ordovician to Devonian Systems (Guilbault et al 1976; Poncet 1986; Mamet and Shalaby 1995; Munnecke et al 2001; Riding and Braga 2005; Feng et al 2010). Halysis was originally described as a chain of spherical cells by Høeg (1932). Others considered it to be composed of a. Halysis has been studied for nearly one hundred years, its systematic position remains in dispute
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