the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Apr 2025
Early Permian longitudinal position of the South China Block from brachiopod paleobiogeography
Abstract. Knowledge of the past location of tectonic plates is essential to understanding the evolution of climate, ocean systems, and mantle flow. Tectonic reconstructions become increasingly uncertain back in geological time. Paleomagnetic data constrain the past latitude of continental blocks, however, their past longitude is unconstrained. For example, the longitude of the South China Block during the Early Permian is unknown. Paleobiogeographic data, which have long been used in tectonic reconstructions, make it possible to evaluate the faunal similarity between continental blocks. In this study, we use the Early Permian global brachiopod distribution from the Paleobiology Database to evaluate the correlation between faunal similarity and physical distance of continental blocks for three distinct tectonic reconstruction models. We use this approach to assess which of the three tectonic scenarios places the South China Block in a location that best accounts for the Early Permian brachiopod distribution data. Based on this analysis, the preferred tectonic reconstruction places the South China Block in a central position within the Paleo-Tethys Ocean instead of on its outskirts. The framework developed in this study is openly available and our approach could be applied to other tectonic blocks, time periods, and faunal data.
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Status: closed
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RC1: 'Comment on egusphere-2025-1018', Anonymous Referee #1, 28 Apr 2025
-
AC1: 'Reply on RC1', Robert Marks, 18 Jun 2025
We sincerely thank the two reviewers for their constructive comments and suggestions. We propose to revise our manuscript considering these comments and suggestions (in bold below).
Reviewer: This study presents a novel and ambitious approach to constrain the Early Permian longitudinal position of the South China Block (SCB) using brachiopod paleobiogeography, offering a creative solution to the longstanding challenge of reconstructing paleolongitude in deep-time tectonic models. By integrating quantitative faunal similarity indices (Jaccard, Simpson, and cME) with global plate reconstructions, the authors provide a framework that bridges paleobiology and geodynamics, marking a significant methodological advance. The conclusion that the SCB occupied a central position within the Paleo-Tethys Ocean (as per Young et al., 2019) challenges previous marginal placements and has implications for paleoceanographic and climatic interpretations. The open accessibility of the analytical framework further enhances its utility for future studies.
Reviewer: First, the reliance on brachiopod distribution assumes that faunal similarity inversely correlates with physical distance, yet environmental heterogeneity, larval dispersal barriers (e.g., landmasses, currents), and sampling biases (e.g., uneven fossil preservation/collection) could decouple this relationship. While the authors acknowledge these issues, the extent to which they influence the indices—particularly given the SCB’s disproportionately large dataset—remains unclear. For instance, the Jaccard index’s poor performance highlights the vulnerability of binary presence-absence metrics to sampling disparities, suggesting that results may overemphasize reconstruction Y19’s plausibility.
Response: We propose to further discuss these limitations and how they impact faunal similarity correlations with the South China Block (SCB), focussing particularly on different dispersal barriers (as a result of differing geographic locations and potentially different ocean current scenarios) and how sample size disparities limit the different indices. It is unclear to us how these know limitations emphasise the plausibility of any of the three considered reconstructions.
Â
Reviewer: Second, the tectonic models themselves inherit uncertainties. The assumption of fixed LLSVPs in Matthews et al. (2016) versus their potential mobility in Young et al. (2019) reflects debated geodynamic hypotheses, yet the study does not fully disentangle how these contrasting assumptions propagate into the faunal-distance correlations.
Response: The assumptions/uncertainties within the tectonic models are part of determining tectonic plate locations. The central idea of the paper is to use faunal similarity-distance correlations to test these locations. We propose to emphasise this in the revised manuscript.
Â
Reviewer: Additionally, the choice of 277 Ma as a representative time slice overlooks temporal dynamics within the ~27 Myr Early Permian, during which climatic shifts (e.g., deglaciation) and biotic turnover could skew biogeographic patterns.
Response: We agree and plan to split the data and perform the analysis for two distinct periods: Artinskian-Kungurian times (warmer climate) and Asselian-Sakmarian times (cooler climate). We have performed an exploratory data analysis to ensure the Permian brachiopod age data is of sufficient resolution to split into these two periods and, highlighting the scalability of the framework to other periods, have reanalysed for the two periods and found promising results. One consideration is that in reconstruction W13 the positions of plates were defined at 277 Ma and 306 Ma, with positions for Asselian-Sakmarian times (295 Ma) inferred from linear interpolation.
Reviewer: A critical but unaddressed issue lies in the taxonomic accuracy of brachiopod genera extracted from the Paleobiology Database. Fossil identifications in large-scale databases are prone to errors due to misclassification, synonymies, or outdated taxonomy. For example, brachiopod genera with overlapping morphological features or poorly preserved specimens may be mis-assigned, directly distorting faunal similarity calculations. Such inaccuracies could artificially inflate or diminish correlations between biogeographic indices and physical distance. To strengthen the robustness of the analysis, future iterations of this framework should involve systematic re-evaluation of the brachiopod taxonomic data by domain experts to resolve ambiguities and validate species assignments. I believe some of the authors are brachiopod experts, not sure if they reviewed the taxonomy of the genera extracted from PBDB.
Response: We propose to clarify in the revision that we checked the taxonomic reliability of the database for the Permian brachiopod records and concluded that the taxonomic data were reliable at the genus level. This minimises the impact of inaccurate fossil identification.
Â
Reviewer: Lastly, the statistical approach—while rigorous—simplifies complex biogeographic processes into linear relationships. Nonlinear effects (e.g., threshold distances for provinciality) or geographic barriers (e.g., continental shelves) may distort correlations, particularly for marine taxa like brachiopods.
Response: The relationship between faunal similarity and physical distance is complex and may not be linear. The considered logarithmic relationships (logarithmic transformations of the similarity coefficients) suggests similar correlations as the linear relationships.
Â
Reviewer: The framework’s scalability to other taxa/periods, though promising, requires validation against independent datasets (e.g., paleomagnetic or stratigraphic constraints).
Response: We agree that the framework is best used to provide evidence supporting plate tectonic configurations using faunal data in conjunction with other, independent datasets.
Citation: https://doi.org/10.5194/egusphere-2025-1018-AC1
-
AC1: 'Reply on RC1', Robert Marks, 18 Jun 2025
-
RC2: 'Comment on egusphere-2025-1018', Anonymous Referee #2, 27 May 2025
This interesting and innovative manuscript studies the palaeolongitude of the South China Block (SCB) during the Early Permian by investigating the faunal affinity of brachiopods between the SCB and other tectonic plates. Based on three different paleogeographic reconstructions, the manuscript employs strict statistical analysis to examine the relationship between brachiopod faunal similarities and physical distances. The study supports that the SCB were positioned in the central part of the Palaeo-Tethys Ocean, rather than at its periphery, challenging the conventional views. However, some weaknesses remain in the research methods. Detailed comments/suggestions follow:
1. Comparing faunal affinities between SCB and other plates across the entire Early Permian (spanning ~17 Ma) is problematic. The SCB remained in the palaeoequatorial region throughout this interval, maintaining consistent Tethyan warm-water brachiopods. In contrast, other tectonic units, particularly the Cimmerian Terranes, underwent significant faunal transitions, evolving from Gondwanan cold-water taxa to cool- or even warm-water elements throughout the early Permian. Thus, the brachiopod faunas of these mobile blocks could shift from being very different to closer to those of the SCB over this timespan. To obtain more reliable results, I strongly recommend dividing the early Permian into two intervals (Asselian-Sakmarian and Artinskian-Kungurian) for separate analyses.
2. As noted by the authors, the North American brachiopod faunas exhibit significant diversity during the Early Permian. However, they were excluded from the analyses due to their far distance (>12,000 km) from the South China Block in all three reconstruction maps. The North America Plate was situated in the palaeoequatorial region, and its faunas likely maintained biogeographic connections with South China via ocean currents. Thus, its inclusion in the analyses would provide a more comprehensive assessment of faunal affinities versus distances.
3. Another issue concerns the inconsistent distance thresholds applied in the faunal similarity analyses. In W13 (Fig. 3), the data appear to have a global scope, with distances extending up to 20,000 km. In contrast, Y19 (Fig. 6) restricts the analysis to plates within an 8,000 km distance limit. In addition, relationships between biogeographical indexes and distance based on M16 are absent. What is the basis for the choice of distance limits in these analyses?
4. The authors consider that the latitudinal positions of the SCB were relatively stable in three configurations, its longitudinal variation significantly affects distance-based analyses. However, the latitudinal uncertainties of the SCB affect the distance of other plates to its north and south. For example, the Australian Plate exhibits substantial discrepancies in distance between the SCB in different reconstructions: its centroid ranges between 4000-6000 km in W13, but 6000-8000 km in Y19 and M16. Considering the high diversity of brachiopods of the Australian Plate, the differences in the distance could have a large impact on the results. In addition, the faunal appearances of Western Australia and eastern Australia are really different, it is unclear whether this study treats the Australian Plate as a single plate or divides it into two geographic units.
5. For Fig. 1, I suggest to add the citations and abbreviations in the blank space of each map, such as Young et al., (2019, Y19), which will make the article more readable.
6. For Fig. 2a, I am wondering if the number of plates includes all plates at that range or only those containing brachiopods. Displaying the number of plates with brachiopod records would be more meaningful, as only those would be used in the analysis.
Citation: https://doi.org/10.5194/egusphere-2025-1018-RC2 -
AC2: 'Reply on RC2', Robert Marks, 18 Jun 2025
Reviewer: This interesting and innovative manuscript studies the palaeolongitude of the South China Block (SCB) during the Early Permian by investigating the faunal affinity of brachiopods between the SCB and other tectonic plates. Based on three different paleogeographic reconstructions, the manuscript employs strict statistical analysis to examine the relationship between brachiopod faunal similarities and physical distances. The study supports that the SCB were positioned in the central part of the Palaeo-Tethys Ocean, rather than at its periphery, challenging the conventional views. However, some weaknesses remain in the research methods.
Reviewer: Comparing faunal affinities between SCB and other plates across the entire Early Permian (spanning ~17 Ma) is problematic. The SCB remained in the palaeoequatorial region throughout this interval, maintaining consistent Tethyan warm-water brachiopods. In contrast, other tectonic units, particularly the Cimmerian Terranes, underwent significant faunal transitions, evolving from Gondwanan cold-water taxa to cool- or even warm-water elements throughout the early Permian. Thus, the brachiopod faunas of these mobile blocks could shift from being very different to closer to those of the SCB over this timespan. To obtain more reliable results, I strongly recommend dividing the early Permian into two intervals (Asselian-Sakmarian and Artinskian-Kungurian) for separate analyses.
Response: We agree and propose to separately analyse Artinskian-Kungurian and Asselian-Sakmarian times. As stated above, we have performed an exploratory data analysis to ensure the Permian brachiopod age data is of sufficient resolution to split into these two periods and, highlighting the scalability of the framework to other periods, have reanalysed for the two periods and found promising results. One consideration is that in reconstruction W13 the positions of plates were defined at 277 Ma and 306 Ma, with positions for Asselian-Sakmarian times (295 Ma) inferred from linear interpolation.
We agree that the Cimmerian Terranes present significant faunal transitions throughout Early Permian times, and propose to discuss the possible contributions of i/ changes in climate and ii/ changes in the latitudinal position of these terranes, which are limited in all three considered reconstructions.
Â
Reviewer: As noted by the authors, the North American brachiopod faunas exhibit significant diversity during the Early Permian. However, they were excluded from the analyses due to their far distance (>12,000 km) from the South China Block in all three reconstruction maps. The North America Plate was situated in the palaeoequatorial region, and its faunas likely maintained biogeographic connections with South China via ocean currents. Thus, its inclusion in the analyses would provide a more comprehensive assessment of faunal affinities versus distances.
Response: For the North American plate, there is a discrepancy between the physical distance to the SCB and the distance of dispersal pathways for marine fauna. North America is across the Tethys Ocean from the SCB. However, the North American brachiopod faunas are on the Panthalassan coastline and could disperse either across the Panthalassa Ocean or north around the Siberian block (Fig. 1 in the manuscript), both of which are much greater distances than the great circle distance measured across the Tethys Ocean.
Â
Reviewer: Another issue concerns the inconsistent distance thresholds applied in the faunal similarity analyses. In W13 (Fig. 3), the data appear to have a global scope, with distances extending up to 20,000 km. In contrast, Y19 (Fig. 6) restricts the analysis to plates within an 8,000 km distance limit. In addition, relationships between biogeographical indexes and distance based on M16 are absent. What is the basis for the choice of distance limits in these analyses?
Response: We propose to clarify the implementation of distance limits in the revised manuscript. Figure 3 illustrates that a global analysis does not necessarily provide a valuable correlation as it is impacted by various issues, such as the abovementioned differences between physical distances and dispersal pathways. Figure 4 illustrates the relationship between biogeographical indexes and distance for all three reconstructions, with distance limits on the X-axis, and correlation coefficient between biogeographic indexes and physical distance on the Y-axis. While we present the correlation for Y19 with the 8,000 km distance limit as the most successful reconstruction within this framework, Figure 4 shows the strength of the correlation for all considered reconstructions and distance limits.
Â
Reviewer: The authors consider that the latitudinal positions of the SCB were relatively stable in three configurations, its longitudinal variation significantly affects distance-based analyses. However, the latitudinal uncertainties of the SCB affect the distance of other plates to its north and south. For example, the Australian Plate exhibits substantial discrepancies in distance between the SCB in different reconstructions: its centroid ranges between 4000-6000 km in W13, but 6000-8000 km in Y19 and M16. Considering the high diversity of brachiopods of the Australian Plate, the differences in the distance could have a large impact on the results.
Response: We agree that the SCB is largely in the Southern Hemisphere in W13, which affects the distance between the SCB and the Australian plate (as well as other plates). We propose to clarify section 5.1 of the discussion to emphasise this point. Reconstructions M16 and Y19 can be compared more directly because they use the same set of tectonic plates and the location of the SCB primarily differs in its longitude. Reconstruction W13 was developed using a different approach and is more significantly different from M16 and Y19. Nevertheless, the presented correlations provide a valuable measure of the overall validity for the SCB placement within a given global tectonic configuration.
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Reviewer: In addition, the faunal appearances of Western Australia and eastern Australia are really different, it is unclear whether this study treats the Australian Plate as a single plate or divides it into two geographic units.
Response: We considered the Australian Plate as a single plate. We acknowledged in the discussion that this is not ideal for large plates that likely contain multiple, distinct faunal provinces. Breaking the analysis down to faunal provinces could be done in future work.
Â
Reviewer: For Fig. 1, I suggest to add the citations and abbreviations in the blank space of each map, such as Young et al., (2019, Y19), which will make the article more readable.
Response: We agree that this change will improve readability.
Reviewer: For Fig. 2a, I am wondering if the number of plates includes all plates at that range or only those containing brachiopods. Displaying the number of plates with brachiopod records would be more meaningful, as only those would be used in the analysis.
Response: We propose to clarify in the revision that only the plates that contain brachiopods are considered in Figure 2a.
Citation: https://doi.org/10.5194/egusphere-2025-1018-AC2
-
AC2: 'Reply on RC2', Robert Marks, 18 Jun 2025
Status: closed
-
RC1: 'Comment on egusphere-2025-1018', Anonymous Referee #1, 28 Apr 2025
This study presents a novel and ambitious approach to constrain the Early Permian longitudinal position of the South China Block (SCB) using brachiopod paleobiogeography, offering a creative solution to the longstanding challenge of reconstructing paleolongitude in deep-time tectonic models. By integrating quantitative faunal similarity indices (Jaccard, Simpson, and cME) with global plate reconstructions, the authors provide a framework that bridges paleobiology and geodynamics, marking a significant methodological advance. The conclusion that the SCB occupied a central position within the Paleo-Tethys Ocean (as per Young et al., 2019) challenges previous marginal placements and has implications for paleoceanographic and climatic interpretations. The open accessibility of the analytical framework further enhances its utility for future studies.
However, several uncertainties and limitations warrant caution. First, the reliance on brachiopod distribution assumes that faunal similarity inversely correlates with physical distance, yet environmental heterogeneity, larval dispersal barriers (e.g., landmasses, currents), and sampling biases (e.g., uneven fossil preservation/collection) could decouple this relationship. While the authors acknowledge these issues, the extent to which they influence the indices—particularly given the SCB’s disproportionately large dataset—remains unclear. For instance, the Jaccard index’s poor performance highlights the vulnerability of binary presence-absence metrics to sampling disparities, suggesting that results may overemphasize reconstruction Y19’s plausibility.
Second, the tectonic models themselves inherit uncertainties. The assumption of fixed LLSVPs in Matthews et al. (2016) versus their potential mobility in Young et al. (2019) reflects debated geodynamic hypotheses, yet the study does not fully disentangle how these contrasting assumptions propagate into the faunal-distance correlations. Additionally, the choice of 277 Ma as a representative time slice overlooks temporal dynamics within the ~27 Myr Early Permian, during which climatic shifts (e.g., deglaciation) and biotic turnover could skew biogeographic patterns.
A critical but unaddressed issue lies in the taxonomic accuracy of brachiopod genera extracted from the Paleobiology Database. Fossil identifications in large-scale databases are prone to errors due to misclassification, synonymies, or outdated taxonomy. For example, brachiopod genera with overlapping morphological features or poorly preserved specimens may be mis-assigned, directly distorting faunal similarity calculations. Such inaccuracies could artificially inflate or diminish correlations between biogeographic indices and physical distance. To strengthen the robustness of the analysis, future iterations of this framework should involve systematic re-evaluation of the brachiopod taxonomic data by domain experts to resolve ambiguities and validate species assignments. I believe some of the authors are brachiopod experts, not sure if they reviewed the taxonomy of the genera extracted from PBDB.
Lastly, the statistical approach—while rigorous—simplifies complex biogeographic processes into linear relationships. Nonlinear effects (e.g., threshold distances for provinciality) or geographic barriers (e.g., continental shelves) may distort correlations, particularly for marine taxa like brachiopods. The framework’s scalability to other taxa/periods, though promising, requires validation against independent datasets (e.g., paleomagnetic or stratigraphic constraints).
In summary, this work innovatively leverages paleobiogeography to address a critical gap in plate reconstructions, but the conclusions should be tempered by the inherent uncertainties in fossil data completeness, model assumptions, and temporal/spatial resolution. Future studies could strengthen the approach by incorporating multivariate biogeographic methods, higher-resolution time slices, cross-validation with geodynamic models that explicitly test LLSVP mobility, and rigorous taxonomic vetting of fossil datasets.
Citation: https://doi.org/10.5194/egusphere-2025-1018-RC1 -
AC1: 'Reply on RC1', Robert Marks, 18 Jun 2025
We sincerely thank the two reviewers for their constructive comments and suggestions. We propose to revise our manuscript considering these comments and suggestions (in bold below).
Reviewer: This study presents a novel and ambitious approach to constrain the Early Permian longitudinal position of the South China Block (SCB) using brachiopod paleobiogeography, offering a creative solution to the longstanding challenge of reconstructing paleolongitude in deep-time tectonic models. By integrating quantitative faunal similarity indices (Jaccard, Simpson, and cME) with global plate reconstructions, the authors provide a framework that bridges paleobiology and geodynamics, marking a significant methodological advance. The conclusion that the SCB occupied a central position within the Paleo-Tethys Ocean (as per Young et al., 2019) challenges previous marginal placements and has implications for paleoceanographic and climatic interpretations. The open accessibility of the analytical framework further enhances its utility for future studies.
Reviewer: First, the reliance on brachiopod distribution assumes that faunal similarity inversely correlates with physical distance, yet environmental heterogeneity, larval dispersal barriers (e.g., landmasses, currents), and sampling biases (e.g., uneven fossil preservation/collection) could decouple this relationship. While the authors acknowledge these issues, the extent to which they influence the indices—particularly given the SCB’s disproportionately large dataset—remains unclear. For instance, the Jaccard index’s poor performance highlights the vulnerability of binary presence-absence metrics to sampling disparities, suggesting that results may overemphasize reconstruction Y19’s plausibility.
Response: We propose to further discuss these limitations and how they impact faunal similarity correlations with the South China Block (SCB), focussing particularly on different dispersal barriers (as a result of differing geographic locations and potentially different ocean current scenarios) and how sample size disparities limit the different indices. It is unclear to us how these know limitations emphasise the plausibility of any of the three considered reconstructions.
Â
Reviewer: Second, the tectonic models themselves inherit uncertainties. The assumption of fixed LLSVPs in Matthews et al. (2016) versus their potential mobility in Young et al. (2019) reflects debated geodynamic hypotheses, yet the study does not fully disentangle how these contrasting assumptions propagate into the faunal-distance correlations.
Response: The assumptions/uncertainties within the tectonic models are part of determining tectonic plate locations. The central idea of the paper is to use faunal similarity-distance correlations to test these locations. We propose to emphasise this in the revised manuscript.
Â
Reviewer: Additionally, the choice of 277 Ma as a representative time slice overlooks temporal dynamics within the ~27 Myr Early Permian, during which climatic shifts (e.g., deglaciation) and biotic turnover could skew biogeographic patterns.
Response: We agree and plan to split the data and perform the analysis for two distinct periods: Artinskian-Kungurian times (warmer climate) and Asselian-Sakmarian times (cooler climate). We have performed an exploratory data analysis to ensure the Permian brachiopod age data is of sufficient resolution to split into these two periods and, highlighting the scalability of the framework to other periods, have reanalysed for the two periods and found promising results. One consideration is that in reconstruction W13 the positions of plates were defined at 277 Ma and 306 Ma, with positions for Asselian-Sakmarian times (295 Ma) inferred from linear interpolation.
Reviewer: A critical but unaddressed issue lies in the taxonomic accuracy of brachiopod genera extracted from the Paleobiology Database. Fossil identifications in large-scale databases are prone to errors due to misclassification, synonymies, or outdated taxonomy. For example, brachiopod genera with overlapping morphological features or poorly preserved specimens may be mis-assigned, directly distorting faunal similarity calculations. Such inaccuracies could artificially inflate or diminish correlations between biogeographic indices and physical distance. To strengthen the robustness of the analysis, future iterations of this framework should involve systematic re-evaluation of the brachiopod taxonomic data by domain experts to resolve ambiguities and validate species assignments. I believe some of the authors are brachiopod experts, not sure if they reviewed the taxonomy of the genera extracted from PBDB.
Response: We propose to clarify in the revision that we checked the taxonomic reliability of the database for the Permian brachiopod records and concluded that the taxonomic data were reliable at the genus level. This minimises the impact of inaccurate fossil identification.
Â
Reviewer: Lastly, the statistical approach—while rigorous—simplifies complex biogeographic processes into linear relationships. Nonlinear effects (e.g., threshold distances for provinciality) or geographic barriers (e.g., continental shelves) may distort correlations, particularly for marine taxa like brachiopods.
Response: The relationship between faunal similarity and physical distance is complex and may not be linear. The considered logarithmic relationships (logarithmic transformations of the similarity coefficients) suggests similar correlations as the linear relationships.
Â
Reviewer: The framework’s scalability to other taxa/periods, though promising, requires validation against independent datasets (e.g., paleomagnetic or stratigraphic constraints).
Response: We agree that the framework is best used to provide evidence supporting plate tectonic configurations using faunal data in conjunction with other, independent datasets.
Citation: https://doi.org/10.5194/egusphere-2025-1018-AC1
-
AC1: 'Reply on RC1', Robert Marks, 18 Jun 2025
-
RC2: 'Comment on egusphere-2025-1018', Anonymous Referee #2, 27 May 2025
This interesting and innovative manuscript studies the palaeolongitude of the South China Block (SCB) during the Early Permian by investigating the faunal affinity of brachiopods between the SCB and other tectonic plates. Based on three different paleogeographic reconstructions, the manuscript employs strict statistical analysis to examine the relationship between brachiopod faunal similarities and physical distances. The study supports that the SCB were positioned in the central part of the Palaeo-Tethys Ocean, rather than at its periphery, challenging the conventional views. However, some weaknesses remain in the research methods. Detailed comments/suggestions follow:
1. Comparing faunal affinities between SCB and other plates across the entire Early Permian (spanning ~17 Ma) is problematic. The SCB remained in the palaeoequatorial region throughout this interval, maintaining consistent Tethyan warm-water brachiopods. In contrast, other tectonic units, particularly the Cimmerian Terranes, underwent significant faunal transitions, evolving from Gondwanan cold-water taxa to cool- or even warm-water elements throughout the early Permian. Thus, the brachiopod faunas of these mobile blocks could shift from being very different to closer to those of the SCB over this timespan. To obtain more reliable results, I strongly recommend dividing the early Permian into two intervals (Asselian-Sakmarian and Artinskian-Kungurian) for separate analyses.
2. As noted by the authors, the North American brachiopod faunas exhibit significant diversity during the Early Permian. However, they were excluded from the analyses due to their far distance (>12,000 km) from the South China Block in all three reconstruction maps. The North America Plate was situated in the palaeoequatorial region, and its faunas likely maintained biogeographic connections with South China via ocean currents. Thus, its inclusion in the analyses would provide a more comprehensive assessment of faunal affinities versus distances.
3. Another issue concerns the inconsistent distance thresholds applied in the faunal similarity analyses. In W13 (Fig. 3), the data appear to have a global scope, with distances extending up to 20,000 km. In contrast, Y19 (Fig. 6) restricts the analysis to plates within an 8,000 km distance limit. In addition, relationships between biogeographical indexes and distance based on M16 are absent. What is the basis for the choice of distance limits in these analyses?
4. The authors consider that the latitudinal positions of the SCB were relatively stable in three configurations, its longitudinal variation significantly affects distance-based analyses. However, the latitudinal uncertainties of the SCB affect the distance of other plates to its north and south. For example, the Australian Plate exhibits substantial discrepancies in distance between the SCB in different reconstructions: its centroid ranges between 4000-6000 km in W13, but 6000-8000 km in Y19 and M16. Considering the high diversity of brachiopods of the Australian Plate, the differences in the distance could have a large impact on the results. In addition, the faunal appearances of Western Australia and eastern Australia are really different, it is unclear whether this study treats the Australian Plate as a single plate or divides it into two geographic units.
5. For Fig. 1, I suggest to add the citations and abbreviations in the blank space of each map, such as Young et al., (2019, Y19), which will make the article more readable.
6. For Fig. 2a, I am wondering if the number of plates includes all plates at that range or only those containing brachiopods. Displaying the number of plates with brachiopod records would be more meaningful, as only those would be used in the analysis.
Citation: https://doi.org/10.5194/egusphere-2025-1018-RC2 -
AC2: 'Reply on RC2', Robert Marks, 18 Jun 2025
Reviewer: This interesting and innovative manuscript studies the palaeolongitude of the South China Block (SCB) during the Early Permian by investigating the faunal affinity of brachiopods between the SCB and other tectonic plates. Based on three different paleogeographic reconstructions, the manuscript employs strict statistical analysis to examine the relationship between brachiopod faunal similarities and physical distances. The study supports that the SCB were positioned in the central part of the Palaeo-Tethys Ocean, rather than at its periphery, challenging the conventional views. However, some weaknesses remain in the research methods.
Reviewer: Comparing faunal affinities between SCB and other plates across the entire Early Permian (spanning ~17 Ma) is problematic. The SCB remained in the palaeoequatorial region throughout this interval, maintaining consistent Tethyan warm-water brachiopods. In contrast, other tectonic units, particularly the Cimmerian Terranes, underwent significant faunal transitions, evolving from Gondwanan cold-water taxa to cool- or even warm-water elements throughout the early Permian. Thus, the brachiopod faunas of these mobile blocks could shift from being very different to closer to those of the SCB over this timespan. To obtain more reliable results, I strongly recommend dividing the early Permian into two intervals (Asselian-Sakmarian and Artinskian-Kungurian) for separate analyses.
Response: We agree and propose to separately analyse Artinskian-Kungurian and Asselian-Sakmarian times. As stated above, we have performed an exploratory data analysis to ensure the Permian brachiopod age data is of sufficient resolution to split into these two periods and, highlighting the scalability of the framework to other periods, have reanalysed for the two periods and found promising results. One consideration is that in reconstruction W13 the positions of plates were defined at 277 Ma and 306 Ma, with positions for Asselian-Sakmarian times (295 Ma) inferred from linear interpolation.
We agree that the Cimmerian Terranes present significant faunal transitions throughout Early Permian times, and propose to discuss the possible contributions of i/ changes in climate and ii/ changes in the latitudinal position of these terranes, which are limited in all three considered reconstructions.
Â
Reviewer: As noted by the authors, the North American brachiopod faunas exhibit significant diversity during the Early Permian. However, they were excluded from the analyses due to their far distance (>12,000 km) from the South China Block in all three reconstruction maps. The North America Plate was situated in the palaeoequatorial region, and its faunas likely maintained biogeographic connections with South China via ocean currents. Thus, its inclusion in the analyses would provide a more comprehensive assessment of faunal affinities versus distances.
Response: For the North American plate, there is a discrepancy between the physical distance to the SCB and the distance of dispersal pathways for marine fauna. North America is across the Tethys Ocean from the SCB. However, the North American brachiopod faunas are on the Panthalassan coastline and could disperse either across the Panthalassa Ocean or north around the Siberian block (Fig. 1 in the manuscript), both of which are much greater distances than the great circle distance measured across the Tethys Ocean.
Â
Reviewer: Another issue concerns the inconsistent distance thresholds applied in the faunal similarity analyses. In W13 (Fig. 3), the data appear to have a global scope, with distances extending up to 20,000 km. In contrast, Y19 (Fig. 6) restricts the analysis to plates within an 8,000 km distance limit. In addition, relationships between biogeographical indexes and distance based on M16 are absent. What is the basis for the choice of distance limits in these analyses?
Response: We propose to clarify the implementation of distance limits in the revised manuscript. Figure 3 illustrates that a global analysis does not necessarily provide a valuable correlation as it is impacted by various issues, such as the abovementioned differences between physical distances and dispersal pathways. Figure 4 illustrates the relationship between biogeographical indexes and distance for all three reconstructions, with distance limits on the X-axis, and correlation coefficient between biogeographic indexes and physical distance on the Y-axis. While we present the correlation for Y19 with the 8,000 km distance limit as the most successful reconstruction within this framework, Figure 4 shows the strength of the correlation for all considered reconstructions and distance limits.
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Reviewer: The authors consider that the latitudinal positions of the SCB were relatively stable in three configurations, its longitudinal variation significantly affects distance-based analyses. However, the latitudinal uncertainties of the SCB affect the distance of other plates to its north and south. For example, the Australian Plate exhibits substantial discrepancies in distance between the SCB in different reconstructions: its centroid ranges between 4000-6000 km in W13, but 6000-8000 km in Y19 and M16. Considering the high diversity of brachiopods of the Australian Plate, the differences in the distance could have a large impact on the results.
Response: We agree that the SCB is largely in the Southern Hemisphere in W13, which affects the distance between the SCB and the Australian plate (as well as other plates). We propose to clarify section 5.1 of the discussion to emphasise this point. Reconstructions M16 and Y19 can be compared more directly because they use the same set of tectonic plates and the location of the SCB primarily differs in its longitude. Reconstruction W13 was developed using a different approach and is more significantly different from M16 and Y19. Nevertheless, the presented correlations provide a valuable measure of the overall validity for the SCB placement within a given global tectonic configuration.
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Reviewer: In addition, the faunal appearances of Western Australia and eastern Australia are really different, it is unclear whether this study treats the Australian Plate as a single plate or divides it into two geographic units.
Response: We considered the Australian Plate as a single plate. We acknowledged in the discussion that this is not ideal for large plates that likely contain multiple, distinct faunal provinces. Breaking the analysis down to faunal provinces could be done in future work.
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Reviewer: For Fig. 1, I suggest to add the citations and abbreviations in the blank space of each map, such as Young et al., (2019, Y19), which will make the article more readable.
Response: We agree that this change will improve readability.
Reviewer: For Fig. 2a, I am wondering if the number of plates includes all plates at that range or only those containing brachiopods. Displaying the number of plates with brachiopod records would be more meaningful, as only those would be used in the analysis.
Response: We propose to clarify in the revision that only the plates that contain brachiopods are considered in Figure 2a.
Citation: https://doi.org/10.5194/egusphere-2025-1018-AC2
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AC2: 'Reply on RC2', Robert Marks, 18 Jun 2025
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This study presents a novel and ambitious approach to constrain the Early Permian longitudinal position of the South China Block (SCB) using brachiopod paleobiogeography, offering a creative solution to the longstanding challenge of reconstructing paleolongitude in deep-time tectonic models. By integrating quantitative faunal similarity indices (Jaccard, Simpson, and cME) with global plate reconstructions, the authors provide a framework that bridges paleobiology and geodynamics, marking a significant methodological advance. The conclusion that the SCB occupied a central position within the Paleo-Tethys Ocean (as per Young et al., 2019) challenges previous marginal placements and has implications for paleoceanographic and climatic interpretations. The open accessibility of the analytical framework further enhances its utility for future studies.
However, several uncertainties and limitations warrant caution. First, the reliance on brachiopod distribution assumes that faunal similarity inversely correlates with physical distance, yet environmental heterogeneity, larval dispersal barriers (e.g., landmasses, currents), and sampling biases (e.g., uneven fossil preservation/collection) could decouple this relationship. While the authors acknowledge these issues, the extent to which they influence the indices—particularly given the SCB’s disproportionately large dataset—remains unclear. For instance, the Jaccard index’s poor performance highlights the vulnerability of binary presence-absence metrics to sampling disparities, suggesting that results may overemphasize reconstruction Y19’s plausibility.
Second, the tectonic models themselves inherit uncertainties. The assumption of fixed LLSVPs in Matthews et al. (2016) versus their potential mobility in Young et al. (2019) reflects debated geodynamic hypotheses, yet the study does not fully disentangle how these contrasting assumptions propagate into the faunal-distance correlations. Additionally, the choice of 277 Ma as a representative time slice overlooks temporal dynamics within the ~27 Myr Early Permian, during which climatic shifts (e.g., deglaciation) and biotic turnover could skew biogeographic patterns.
A critical but unaddressed issue lies in the taxonomic accuracy of brachiopod genera extracted from the Paleobiology Database. Fossil identifications in large-scale databases are prone to errors due to misclassification, synonymies, or outdated taxonomy. For example, brachiopod genera with overlapping morphological features or poorly preserved specimens may be mis-assigned, directly distorting faunal similarity calculations. Such inaccuracies could artificially inflate or diminish correlations between biogeographic indices and physical distance. To strengthen the robustness of the analysis, future iterations of this framework should involve systematic re-evaluation of the brachiopod taxonomic data by domain experts to resolve ambiguities and validate species assignments. I believe some of the authors are brachiopod experts, not sure if they reviewed the taxonomy of the genera extracted from PBDB.
Lastly, the statistical approach—while rigorous—simplifies complex biogeographic processes into linear relationships. Nonlinear effects (e.g., threshold distances for provinciality) or geographic barriers (e.g., continental shelves) may distort correlations, particularly for marine taxa like brachiopods. The framework’s scalability to other taxa/periods, though promising, requires validation against independent datasets (e.g., paleomagnetic or stratigraphic constraints).
In summary, this work innovatively leverages paleobiogeography to address a critical gap in plate reconstructions, but the conclusions should be tempered by the inherent uncertainties in fossil data completeness, model assumptions, and temporal/spatial resolution. Future studies could strengthen the approach by incorporating multivariate biogeographic methods, higher-resolution time slices, cross-validation with geodynamic models that explicitly test LLSVP mobility, and rigorous taxonomic vetting of fossil datasets.