Dawn of segmented bilaterians unveiled by soft-bodied fossils in microbial pseudomorphs from the Basal Cambrian
Curation statements for this article:-
Curated by eLife
eLife Assessment
This important manuscript reports exceptionally preserved, millimeter-scale segmented bilaterian microfossils from the early Cambrian Fortunian Kuanchuanpu Formation in South China. The authors propose that these non-biomineralized animals were preserved through microbial pseudomorphs rather than direct phosphatization, filling a critical taphonomic and evolutionary gap in early animal evolution. The physical preservation and high-resolution imaging of the fossils are convincing, but the theoretical grounds for establishing "true segmentation" versus superficial annulation/homonomy remain underdeveloped and need revision and further context. The new fossils and their taphonomy will be of broad interest to the paleontology community and those interested in early animal evolution.
This article has been Reviewed by the following groups
Listed in
- Evaluated articles (eLife)
Abstract
It is estimated that the evolution of segmented bilaterians occurred during the Ediacaran, whereas most of their unambiguous body fossils did not appear until Cambrian Stage 3. This fossil gap hampers our understanding of the early history of body segmentation, which is a crucial evolutionary innovation for bilaterians. Trace fossils from the late Ediacaran and basal Cambrian suggest this gap is likely to represent a taphonomic bias, implying that the progenitors of segmented bilaterians that existed within this temporal span were hardly preserved. Here, we report a variety of segmented bilaterians from the lowermost Cambrian of South China. These fossils are preserved as microbial pseudomorphs, rather than as ordinary phosphatization. These findings demonstrate that microbial pseudomorphs represent a major pathway for the preservation of segmented bilaterians during this period, as well as an effective mechanism for overcoming the taphonomic bias that affects micro-animals with delicate, non-biomineralized bodies. Moreover, the newly described animals, among the earliest segmented bilaterians, reveal a high diversity of segmented bilaterians during Ediacaran-Cambrian transition, shedding new light on the evolution of body segmentation.
Article activity feed
-
eLife Assessment
This important manuscript reports exceptionally preserved, millimeter-scale segmented bilaterian microfossils from the early Cambrian Fortunian Kuanchuanpu Formation in South China. The authors propose that these non-biomineralized animals were preserved through microbial pseudomorphs rather than direct phosphatization, filling a critical taphonomic and evolutionary gap in early animal evolution. The physical preservation and high-resolution imaging of the fossils are convincing, but the theoretical grounds for establishing "true segmentation" versus superficial annulation/homonomy remain underdeveloped and need revision and further context. The new fossils and their taphonomy will be of broad interest to the paleontology community and those interested in early animal evolution.
-
Reviewer #1 (Public review):
Summary:
This manuscript reports exceptionally preserved, millimeter-scale segmented bilaterian microfossils from the Fortunian Kuanchuanpu Formation in South China. The authors propose that these non-biomineralized animals were preserved through microbial pseudomorphs rather than direct phosphatization, filling a critical taphonomic and evolutionary gap in early animal segmentation.
Strengths:
This paper filled the critical taphonomic and evolutionary gap in early animal segmentation.
Weaknesses:
Extremely similar work has already been published by Xian et al., 2026, PNAS, with specimens from absolutely the same formation, and they also proposed that these animals are segmented. So in this context, this work's significance is eclipsed.
-
Reviewer #2 (Public review):
Summary:
The manuscript by Yang and authors presents an interesting set of new microfossils from the early Cambrian of South China. These are interpreted as segmented bilaterians, and so may be the oldest such examples in the fossil record, with implications for our understanding of early animal evolution. Further, the preservation of these fossils as microbial pseudomorphs is a unique pathway to fossilize such important organisms.
Strengths:
The fossils are impressive and arguments for their affinities as segmented bilaterians are convincing. Discussion of somewhat novel preservation is detailed and helpful to understand this taphonomic mode.
Weaknesses:
It needs to be clear what the authors' definition of a "segment" is and how that matches (or not) other interpretations. Arguments by Hannibal and Patel …
Reviewer #2 (Public review):
Summary:
The manuscript by Yang and authors presents an interesting set of new microfossils from the early Cambrian of South China. These are interpreted as segmented bilaterians, and so may be the oldest such examples in the fossil record, with implications for our understanding of early animal evolution. Further, the preservation of these fossils as microbial pseudomorphs is a unique pathway to fossilize such important organisms.
Strengths:
The fossils are impressive and arguments for their affinities as segmented bilaterians are convincing. Discussion of somewhat novel preservation is detailed and helpful to understand this taphonomic mode.
Weaknesses:
It needs to be clear what the authors' definition of a "segment" is and how that matches (or not) other interpretations. Arguments by Hannibal and Patel (2013, EvoDevo) convincingly define all segmented organisms based on polarity within repeated units of bilaterally symmetrical animals, but others may disagree. The manuscript would benefit from discussion of this in the introduction, laying out a hypothesis for what we might expect to find as evidence of the earliest segmented animals and then describing these new fossils within that context.
As it currently reads, the definition of a segment is not presented until the discussion and is somewhat ambiguous, seemingly referring to any organism with bilateral symmetry and repeated units. However, modern (e.g. tardigrades) non-segmented animals exhibit these features but are not considered truly segmented. This is critical as various Precambrian fossils (Arborea (Dunn et al., 2019, Palaeontology); Cloudina (Schiffbauer et al., 2020, Nature Communications) Dickinsonia (Gold et al., 2015, Evolution and Development); Parvancorina (Darroch et al., 2017, Biology Letters); Spriggina (Evans et al., 2026, Scientific Reports); Yilingia (Chen et al., 2019, Nature) etc.), have been argued to possess bilateral symmetry ad have bodies composed of repeated units. These older examples should be acknowledged and discussed in detail. Stating that "the identity of these [Precambrian] organisms is controversial" (line 51) is not justification for the main claims of this paper, and it should be clear what distinguishes the new fossils from previously proposed evidence of earlier segmented bilaterians. I believe there is a case to be made in support of the authors' claims, but this context is vital to make that argument convincingly. It is also worth discussing evidence of segmented bilaterians in the early Cambrian trace fossil record, such as Treptichnus pedum (Kesidis et al., 2019, Proceedings B) and Rusophycus (Daley et al., 2018 PNAS), that may predate or be roughly coeval with these fossil remains.
-
Author response:
Reviewer #1 (Public review):
Summary:
This manuscript reports exceptionally preserved, millimeter-scale segmented bilaterian microfossils from the Fortunian Kuanchuanpu Formation in South China. The authors propose that these non-biomineralized animals were preserved through microbial pseudomorphs rather than direct phosphatization, filling a critical taphonomic and evolutionary gap in early animal segmentation.
Strengths:
This paper filled the critical taphonomic and evolutionary gap in early animal segmentation.
Weaknesses:
Extremely similar work has already been published by Xian et al., 2026, PNAS, with specimens from absolutely the same formation, and they also proposed that these animals are segmented. So in this context, this work's significance is eclipsed.
Thanks the reviewer for the constructive …
Author response:
Reviewer #1 (Public review):
Summary:
This manuscript reports exceptionally preserved, millimeter-scale segmented bilaterian microfossils from the Fortunian Kuanchuanpu Formation in South China. The authors propose that these non-biomineralized animals were preserved through microbial pseudomorphs rather than direct phosphatization, filling a critical taphonomic and evolutionary gap in early animal segmentation.
Strengths:
This paper filled the critical taphonomic and evolutionary gap in early animal segmentation.
Weaknesses:
Extremely similar work has already been published by Xian et al., 2026, PNAS, with specimens from absolutely the same formation, and they also proposed that these animals are segmented. So in this context, this work's significance is eclipsed.
Thanks the reviewer for the constructive comments. In Xian et al., 2026, PNAS, the taphonomic mechanism has not been explained in details and in our manuscript we propose the mode that how microbes pseudomorph the entire bodies of micro-animals. In addition, the specimens we presented in this manuscript demonstrate the abundance and diversity of segmented bilaterians in the basal Cambrian. In the revision, we will majorly discuss the evolution of body segment based on our new materials and add a new figure to complement the comparison of microbial pseudomorphs vs Ediacaran-type and Burges-Shale type.
Reviewer #2 (Public review):
Summary:
The manuscript by Yang and authors presents an interesting set of new microfossils from the early Cambrian of South China. These are interpreted as segmented bilaterians, and so may be the oldest such examples in the fossil record, with implications for our understanding of early animal evolution. Further, the preservation of these fossils as microbial pseudomorphs is a unique pathway to fossilize such important organisms.
Strengths:
The fossils are impressive and arguments for their affinities as segmented bilaterians are convincing. Discussion of somewhat novel preservation is detailed and helpful to understand this taphonomic mode.
Weaknesses:
It needs to be clear what the authors' definition of a "segment" is and how that matches (or not) other interpretations. Arguments by Hannibal and Patel (2013, EvoDevo) convincingly define all segmented organisms based on polarity within repeated units of bilaterally symmetrical animals, but others may disagree. The manuscript would benefit from discussion of this in the introduction, laying out a hypothesis for what we might expect to find as evidence of the earliest segmented animals and then describing these new fossils within that context.
As it currently reads, the definition of a segment is not presented until the discussion and is somewhat ambiguous, seemingly referring to any organism with bilateral symmetry and repeated units. However, modern (e.g. tardigrades) non-segmented animals exhibit these features but are not considered truly segmented. This is critical as various Precambrian fossils (Arborea (Dunn et al., 2019, Palaeontology); Cloudina (Schiffbauer et al., 2020, Nature Communications) Dickinsonia (Gold et al., 2015, Evolution and Development); Parvancorina (Darroch et al., 2017, Biology Letters); Spriggina (Evans et al., 2026, Scientific Reports); Yilingia (Chen et al., 2019, Nature) etc.), have been argued to possess bilateral symmetry ad have bodies composed of repeated units. These older examples should be acknowledged and discussed in detail. Stating that "the identity of these [Precambrian] organisms is controversial" (line 51) is not justification for the main claims of this paper, and it should be clear what distinguishes the new fossils from previously proposed evidence of earlier segmented bilaterians. I believe there is a case to be made in support of the authors' claims, but this context is vital to make that argument convincingly. It is also worth discussing evidence of segmented bilaterians in the early Cambrian trace fossil record, such as Treptichnus pedum (Kesidis et al., 2019, Proceedings B) and Rusophycus (Daley et al., 2018 PNAS), that may predate or be roughly coeval with these fossil remains.
Thanks for the valuable suggestions and references from the reviewer. In the revision, we will introduce the definition of segment and former researches in the introduction part as suggested. And we will discuss the comparisons of our specimens with the Precambrian fossils mentioned above in more detail. The major difference between our materials and Spriggina and Yilingia is the undisputed bilateral symmetry. While the paired appendages distinguished the Cambrian fossils from Parvancorina, Cloudina, and Dickinsonia. Since the pseudomorphing mechanism caused the lost of internal structures of the original, the well-developed paired appendages and outer segment could be the major evidence of their identity. In addition, we will make a more detailed introduction and discussion on the trace fossils of during the Ediacaran-Cambrian transition.
-
-
-
Yang et al. (2026-06 preprint, bioRxiv) 「カンブリア紀最初期から産出した微生物による仮晶に保存された軟体化石が明らかにする，体節をもつ左右相称動物の黎明」 Dawn of segmented bilaterians unveiled by soft-bodied fossils in microbial pseudomorphs from the Basal Cambrian doi.org/10.64898/202...
-
Dawn of segmented bilaterians unveiled by soft-bodied fossils in microbial pseudomorphs from the Basal Cambrian https://www.biorxiv.org/content/10.64898/2026.06.18.733126v1
-
Dawn of segmented bilaterians unveiled by soft-bodied fossils in microbial pseudomorphs from the Basal Cambrian https://www.biorxiv.org/content/10.64898/2026.06.18.733126v1
-
