Three Indeterminate Forms of Scalidophoran Worms from the Cambrian Fortunian of South China

Abstract

A wide variety of scalidophoran worms have been found in the Cambrian Fortunian in recent years, suggesting that scalidophorans diversified during this age. Here, we report new scalidophoran material obtained from the 2nd member of the Kuanchuanpu Formation (Fortunian Stage) in Zhangjiagou Lagerstätte, Xixiang County, Shaanxi Province, South China. Three indeterminate forms of scalidophoran worms are described in detail. The morphology of these worms differs from that of any coeval worms and may represent new taxa. However, because these fossils are fragmented and lack detailed information on the arrangement pattern of introvert scalids, they are named here in open nomenclature. This study enriches the diversity of the earliest scalidophorans to 7 genera and species and 10 indeterminate forms. Based on the new fossils, Fortunian scalidophoran worms can be divided into three categories: (1) worms with microannuli; (2) worms with macroannuli; and (3) worms without annuli.

Growth pattern of Fortunian scalidophoran sclerites

“…Among the previously reported Fortunian armored scalidophorans, most of them have similar morphology and size to the sclerites within the same annulus, such as Zhongpingscolex (Shao et al, 2020a), Shanscolex (Liu et al, 2019), and Dahescolex (Shao et al, 2020b), Forms 1-3 and 5-6 (Liu et al, 2019;Qin et al, 2023).…”

“…A pair of rounded large spinose sclerites are located at the anterior part of the trunk, and a pair of large oval spinose sclerites are located on the eighth and ninth annuli. UMCU 22XX01 is described in Qin et al, 2023; UMCU 16CHD0409-004 is described in Liu et al, 2019. The description of UMCU 22XX02 follows Qin et al, 2023, with only minor revisions to the sclerites.…”

“…UMCU 22XX01 is described in Qin et al, 2023; UMCU 16CHD0409-004 is described in Liu et al, 2019. The description of UMCU 22XX02 follows Qin et al, 2023, with only minor revisions to the sclerites. Two small sclerites appear on its second annulus (Figure 3B in Qin et al, 2023;Figures 2E,F); they seem to be newly formed.…”

“…There are at least five large sclerites (lss, Figure 1; Figure 3) on the trunk, in order from anterior to posterior: the first pair of large sclerites (1-lss), the second pair of large sclerites (2-lss), and the third large single sclerite (3-lss). Comparisons of Form 3 with other worms can be found in Liu et al, 2019 andQin et al, 2023 and will not be repeated here.…”