Abstract
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.
1 Introduction
The origin and evolution of medusa are the focus of both paleontologists and modern biologists (; Wang D et al., 2020). The most distinctive characteristics of medusozoans are their radial symmetry and their ability to swim (). Paleontologists have found numerous medusozoans in the early Cambrian (531.8–536.4 Ma) (; ; ; ; ; ; Wang et al., 2017; ; ; ; Wang X et al., 2020). In the Zhangjiagou Section, South China, various Fortunian radial animals were found such as Olivooides, Quadrapyrgites, and Hexaconularia (; ; ; ; Van Iten et al., 2014; ). These taxa were compared to cnidarians as they show a radial body plan with an apical–abapical differentiation (; ; ; ; Van Iten et al., 2014; ). However, most olivooids structures are lacking from living cnidarians. Olivooid tubes had prominent annulations and fine longitudinal striations with the oral aperture being the only opening of these tubes (; ). Therefore, olivooids were considered as coronate scyphozoans, and their elongate tubes were considered as polyp stages that develop after hatching (; ; ; ; ; ; ; ; Wang et al., 2017).
The microscopic animal, Qinscyphus, was first reported from the early Cambrian Kuanchuanpu Formation at the Zhangjiagou section in South China (). 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 (; ). Post-embryonic tissue was tube-shaped with densely spaced and slightly raised annuli (). Several circlets of triangular thickenings formed five longitudinal rows, pentaradially arranged on post-embryonic tissues, each straddling several annuli (). Moreover, Qinscyphus might be a direct developer due to the comparable morphology of the oral part in both embryos and adults (; ). 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) (). This paper reports five new three-dimensionally 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.
2 Materials and methods
The studied specimens are deposited at the University Museum of Chang’an University (UMCU), Xi’an, China. They were extracted from the phosphatic limestone of the lower part of the second member of the Kuanchuanpu Formation, Zhangjiagou section, Xixiang County, southern Shaanxi Province, South China (Figure 1). The lithostratigraphy and biostratigraphy of the Zhangjiagou section were studied by and , . The same locality and horizon have previously yielded abundant and beautifully preserved microfossils, such as specimens of Olivooides (; ; ; ; ), Pseudooides, the oldest known priapulid–like cycloneuralians as well as other scalidophorans (; ; ; ; Wang et al., 2019; Wang D et al., 2020) and possibe Saccorhytus (; ). The fossil-bearing horizon correlates with the Anabarites trisulcatus–Protohertzina anabarica assemblage zone (; ), which has an estimated age of 531.8–536.4 Ma and falls within the Fortunian Stage ().
FIGURE 1
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.
3 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 (Figures 2A,H and Figures 3A,F).
FIGURE 2
FIGURE 3
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).
FIGURE 4
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 indicate that the new crest with five distinct triangular thickenings formed at the oral part (Figures 2F,I). Around the oral aperture, perradial belts and interradial belting are demarcated by marginal furrows, with a median furrow bisecting each interradial belt (Figures 2F,G,I,J). The morphology and number of triangular thickenings at the oral part are comparable to Olivooides multisulcatus (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.
4 Discussion
The Cambrian fossil embryos are represented by taxa of Markuelia, Pseudooides, Olivooides, Quadrapyrgites and Qinscyphus (; ; ; ; ; ). However, the affinity of many Fortunian radial animals remains debatable. For example, Pseudooides was previously regarded as the embryonic stage of some co-occurring arthropod-like fossil (). But it was challenged because the features were unusual for any arthropod or even bilaterian (). Olivooides was proposed as a coronate scyphozoan (; Yue and Bengtson, 1999; ; ) or an echinoderm (), a cubozoan (; ), a stem-group cycloneuralian () or a diploblastic stem-group eumetazoan (Yasui et al., 2013). Quadrapyrgites might be the sister group of Olivooides (), and has been proposed as a stem-group cycloneuralian (), a coronate scyphozoan (), or a cubozoan (). Qinscyphus might also be a coronate scyphozoan (; ). The coronate scyphozoan hypothesis for Olivooides, Quadrapyrgites and Qinscyphus is favoured here.
The possible polypoid cnidarians from the Fortunian stage correspond to Anabaritidae (; ), Hexangulaconulariidae (; ; ), Carinachitidae (; ; Yue and Bengtson, 1999; ; ; ), Olivooidae (; ; ; ) and Family uncertain (; ; ). They had comparable radial symmetry, tube morphology, and developmental cycles. Furthermore, Olivooidae might include several genera with similar morphologies and developmental modes: Olivooides (; Yasui et al., 2013; ), Quadrapyrgites (; ; ; ; ; ), Qinscyphus (; ), Octapyrgites ().
Olivooides multisulcatus (Figure 4) represents the closest analogue to Q. necopinus. Both are cup-shaped with an apical part and an abapical part (Figure 5). They had a similar body of pentaradial symmetry, five longitudinal rows of triangular thickenings and were found in coeval strata from South China. However, Q. necopinus and O. multisulcatus still had significant differences (Table 1). The apical part of Q. necopinus lacked tiny stellate protuberances on the external surface (Figures 4D,E). The newly described material shows multiple distinct transverse folds (Figure 2A) rather than an apparent longitudinal edge. Moreover, the triangular thickennings of O. multisulcatus occurred continuously and independently on each annulus (Figures 4A–C), whereas for Q. necopinus they appeared every four to seven annuli and straddled the next four to five annuli (Figures 3A,F and Figures 5C,D). Qinscyphus necopinus annuli were slightly raised rather than enormously extended ridged ones (Figures 3C,D). Those annuli might have contributed to a better stretching, contraction, and buffering in the living environment. Qinscyphus necopinus and O. multisulcatus also differed from each other by the absence/presence of stellate structures, and the shape of the perradial lobes and interradial belts in their embryonic stages. The perradial lobes of Q. necopinus had an elliptical end. The interradial belts had median and marginal furrows (Figure 5B). However, the interradial lobes of O. multisulcatus were strongly squeezed by the perradial lobes (Figure 5A) (see ).
FIGURE 5
TABLE 1
| Characteristics\taxa | Olivooides multisulcatus | Qinscyphus necopinus | |
|---|---|---|---|
| Embryo | oral part | 5 sharply V-shaped perradial lobes and 5 interradial lobes | 5 perradial belts with elliptical end and 5 interradial belts |
| lower part | stellate structures | smooth | |
| Hatched stages | tube shape | cup-shaped | cup-shaped |
| oral lobes | 5 | 5 | |
| apical/abapical differentiation | yes | yes | |
| apical part | stellate structures | smooth | |
| triangular thickenings | 5 longitudinal rows (continuously and independently on each annulus) | 5 longitudinal rows (every four to seven annuli and straddled the next four to five annuli) | |
| Annuli | enormously extended outwards to be ridges | slightly raised | |
| radial symmetry | 5 | 5 | |
| growth pattern | direct development | direct development | |
Morphological comparisons of Olivooides multisulcatus and Qinscyphus necopinus.
Previous work suggested that the apical part of Q. necopinus is an inverted pentagonal pyramid with five prominent longitudinal edges (; ; ). 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 (), 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 Quadrapygites (; ). These soft transverse folds were absent in previous reconstructions of Olivooides and Quadrapygites (), 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 (), 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 (; ).
FIGURE 6
5 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.
Statements
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.
Funding
The National Natural Science Foundation of China (grant numbers 41872014, 41572007 and 41572009), the Strategic Priority Research Program of Chinese Academy of Sciences (grant number XDB26000000), the State Key Laboratory of Paleobiology and Stratigraphy, Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences (grant number 193123), the Basic Research Plan of Natural Science of Shaanxi Province (grant number 2018JM4002).
Acknowledgments
We thank Xiaoke Sun and Mingjin Liu, as they helped revise some of the manuscript’s images and language issues.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Publisher’s note
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
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Summary
Keywords
medusozoan, olivooids, apical complement, Zhangjiagou section, developmental sequence
Citation
Zhang Y, Liu Y, Shao T and Qin J (2023) New Qinscyphus material from the Fortunian of South China. Front. Earth Sci. 11:1038686. doi: 10.3389/feart.2023.1038686
Received
07 September 2022
Accepted
28 February 2023
Published
17 March 2023
Volume
11 - 2023
Edited by
Farid Saleh, Yunnan University, China
Reviewed by
Jean Vannier, Université Claude Bernard Lyon 1, France
Peiyun Cong, Yunnan University, China
Jonathan Antcliffe, Université de Lausanne, Switzerland
Updates
Copyright
© 2023 Zhang, Liu, Shao and Qin.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Yunhuan Liu, yunhuanl@chd.edu.cn
This article was submitted to Paleontology, a section of the journal Frontiers in Earth Science
Disclaimer
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.