Palaeoenvironmental distribution of Ordovician dasycladaleans in the Tarim Basin, Xinjiang, China

Abstract

green algae are common in the Phanerozoic, and have been reported from various lagoons and other environments of post-Ordovician periods. There is a paucity of studies exploring palaeoecology of Ordovician dasycladaleans. Based on examination of 1,089 large thin sections from the Upper Ordovician Lianglitag Formation in 12 drilling wells in the Tarim Basin, Xinjiang, China, dasycladaleans mainly occur in platform margins, less abundant in open platforms, and absent from restricted platform tidal-flats and lagoons. Lagoons used here are in a geological sense, referring to only water bodies enclosed by reefs and banks and featured by abnormal salinity and monotonous eurytopic animals. According to lithology and biota, the lagoons in previous studies are actually open platforms. Thus, this study shows that dasycladaleans are not adapted to lagoons, and this nature of them did not change with time. Statistical analysis shows that occurrence frequency of the Ordovician dasycladaleans is 52% in platform margin environments, 22% in open platform environments, and 0% in restricted platform environments.

The latest encrinurid trilobites from the Lower Devonian of Xinjiang, Northwest China

“…However, the exact locations of encrinurids and conodonts are uncertain, so it is controversial whether encrinurids described by Duan and An (2004) appeared in the Early Devonian. The age of encrinurids from the lower part of the Wutubulake Formation in the Mangkelu section of western Junggar, Xinjiang was also considered to be the Early Devonian based on benthic animals such as other trilobites and corals (Zhang, 1983;Wang, 1991), but later studies of graptolites from this formation changed its age to Pridoli (Cai et al 1993;Ni et al 1998).…”

“…Most scholars believe that encrinurids became extinct to the end of Silurian (Ormiston, 1977;Boucot, 1985;Chlupác, 1994), so they were used to determine the Silurian-Devonian boundary (SDB). Nevertheless, some reports suggest that encrinurids can survive into the Devonian (Maksimova et al 1973;Fletcher, 1975;Biske et al 1977;Bourque & Lesperance, 1977;Nikiforova, 1977;Maksimova, 1980;Strusz, 1980;Zhang, 1983;Owens et al 2010). But these so-called 'earliest Devonian encrinurids' were not age constrained by index graptolites or conodonts, and do not have an undoubted earliest Devonian age.…”