New Qinscyphus material from the Fortunian of South China

Microscopic medusozoans from the Cambrian Fortunian stage of South China are well known for their exceptionally preserved embryos and elongated tubes. However, additional details of their morphology remain unclear. This paper describes new medusozoan fossils showing the whole apical complement and covering the morphological integrity of Qinscyphus. The apical part of Qinscyphus is considered to be soft during early ontogeny, and the inverted pentagonal pyramid may gradually form with growth and development. This discovery of the apical complement of Qinscyphus is novel and essential to complement the external morphology of early medusozoan fossils. More importantly, the new specimens have different annuli with triangular thickenings, providing a more comprehensive view on the developmental sequence of Qinscyphus. Therefore, this work allows a better understanding of early medusozoans ecology and evolution.

The microscopic animal, Qinscyphus, was first reported from the early Cambrian Kuanchuanpu Formation at the Zhangjiagou section in South China (Liu et al., 2017). Qinscyphus necopinus is a cup-shaped animal with an embryonic tissue (apical part) and a post-embryonic tissue (abapical part). The embryonic tissue formed an inverted pentagonal pyramid with a smooth surface (Liu et al., 2017;Shao et al., 2018a). Post-embryonic tissue was tube-shaped with densely spaced and slightly raised annuli (Liu et al., 2017). Several circlets of triangular thickenings formed five longitudinal rows, pentaradially arranged on post-embryonic tissues, each straddling several annuli (Shao et al., 2018a). Moreover, Qinscyphus might be a direct developer due to the comparable morphology of the oral part in both embryos and adults (Shao et al., 2018a;Qin et al., 2020). Qinscyphus was interpreted as a possible coronate scyphozoan for its pentagonal symmetry, its tube (comparable to a periderm), and its oral part (its only opening) (Liu et al., 2017). This paper reports five new threedimensionally phosphatized specimens of Q. necopinus from South China; from which four are complete, unlike the fragmentary nature of previously described material, including partially the holotype.
Nearly 300 kg of rock samples from the key horizon at the Zhangjiagou section ( Figure 1) were cracked into football-sized pieces. Each sample was put in a plastic pail or basin with a capacity of 20 L macerated using diluted acetic acid (~8%). The sieving and changing acid cycle requires three to 7 days, depending on the temperature (ambient temperature around 40°C in summer and 20°C in winter). Undissolvable residues dried naturally and were then handpicked under a binocular microscope. The microfossils were picked out and mounted on aluminium stubs for observation under a Hitachi S4800 environmental scanning electron microscope at Chang'an University, Xi'an. The figures were processed using Adobe Photoshop CS5.

Results
Qinscyphus is rarely found in the Kuanchuanpu Formation, and most specimens in the collection are fragmented. The newly discovered specimens are three-dimensionally preserved and only four intact tubes with a preserved apical complement have been found (  Only one specimen ( Figure 2A) reveals five longitudinal edges by slight folds corresponding to five groups of triangular thickenings ( Figure 2D). These edges merge at the apical end ( Figure 2A), with both length and width of 950 μm. The apical end is similar to the shape of the hatchlings of Olivooides (Figure 4), but has a smooth surface ( Figure 3B). The apical end represents a small percentage of the entire organism ( Figure 2D). However, for the remaining specimens, the apical part shows multiple distinct transverse folds, with no apparent longitudinal edges (Figures 3A,B,G,H).
The post-embryonic tissue (abapical part) is tube-shaped with densely spaced and slightly raised annuli (Figures 3A,F). Five groups of triangular thickenings are developed radially on the abapical part ( Figure 2I). In each group, the triangular thickening occurs repetitively every seven annuli, and straddles the next four to five annuli ( Figures 3C,D). These annuli are only slightly raised, rather than strongly extended outwards to form ridges (Fig. 2C, 3D). The largest complete specimen with six circlets of triangular thickenings is 1.4 mm long and 1.1 mm wide ( Figure 3F). Two oral views  Figure 4F), while the new crest with triangular thickenings did not occur continuously but spaced several annuli to the next crest ( Figure 3C). The new specimens are well preserved and have different circlets of triangular thickenings: one (Figure 2A), two ( Figure 2E), three ( Figure 2H, Figure 3A), and six circlets ( Figure 3F). The surface of these triangular thickenings shows peristaltic deformation and distortion ( Figure 3E), suggesting that the triangular tickenings were soft.
Previous work suggested that the apical part of Q. necopinus is an inverted pentagonal pyramid with five prominent longitudinal edges (Liu et al., 2017;Shao et al., 2018a;Qin et al., 2020). However, the new specimens did not show this feature (Figure 2; Figure 3). The apical part of the new specimens had multiple distinct transverse folds ( Figures 3A,F), but no apparent longitudinal edges. Thus, there are two possible forms of the apical part of Qinscyphus: an inverted pentagonal cone or a circular one. This contrast might be resulting from intraspecific variations. However, considering that the new fossils show a circular cone in early ontogenetic stages (Liu et al., 2017), we suggest that the shape of the apical part might gradually become an inverted pentagonal pyramid with growth and development. We consider that the embryonic tissue (apical part) of Qinscyphus necopinus was soft during early ontogeny (fossils with fewer circlets of triangular thickenings) ( Figure 2A). As such, the transverse folds on the embryonic tissue might be taphonomic artifacts due to the soft nature of the apical part. And it is probable that this part was smooth during life. Even in adult individuals, the embryonic tissues could still show transverse folds in taxa such as Qinscyphus, Olivooides, and

FIGURE 6
Reconstruction of the developmental cycle of Qinscyphus necopinus.
Frontiers in Earth Science frontiersin.org Quadrapygites (Steiner et al., 2014;Dong et al., 2016). These soft transverse folds were absent in previous reconstructions of Olivooides and Quadrapygites (Steiner et al., 2014), with transverse folds only occurring on the upper part of the embryonic tissue. Therefore, we modified the reconstruction of Q. necopinus based on the newly described material, by adding transverse folds on the entire embryonic tissue (Figures 5A,D). Previously reported Qinscyphus were mostly fragmented. The new specimens preserved intact tubes with apical and abapical parts. They also show one (Figure 2A), two ( Figure 2E), three ( Figure 2H and Figure 3A), and six ( Figure 3F) circlets with triangular thickenings respectively. Combined with previous studies, the developmental sequence of Qinscyphus is reconstructed ( Figure 6). The oral part of the embryo had a morphology that is comparable to that of the hatched stages (Shao et al., 2018a), demonstrating that Qinscyphus underwent direct development. After hatching, the tube with transverse annuli and pentaradially arranged triangular thickenings began to develop. New annuli and triangular thickenings might have regenerated from within the abapical end. The circlet of triangular thickenings at the abapical end would have evaginated to form a new circlet of triangular thickenings. Qinscyphus would have elongated its length through the addition of annuli and triangular thickenings at the abapical end (Liu et al., 2017;Shao et al., 2018a).

Conclusion
The newly discovered fossils with the whole apical part complete the morphology of Qinscyphus. The apical portion of Q. necopinus is assumed soft during early ontogenetic stages. And, it does not always manifest as an inverted pentagonal pyramid. More importantly, the new specimens have different annuli with triangular thickenings, completing the developmental sequence and expanding the morphological disparity of Q. necopinus.

Data availability statement
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

Author contributions
YZ coordinated the field work, preparation and curation of fossils, wrote the manuscript, and processed the images. YL edited the manuscript. TS and JQ assisted in collecting the rock samples in the field. All authors contributed to the article and approved the submitted version.