| 28 Aug 2025
An unpredictable body size response to the Permo–Triassic climate crisis
Abstract. A predictive ecological response to both the present and past climate crises is that marine ectotherm species will become smaller before going extinct or fluctuate in abundance and size with environmental conditions. The problem with studying past climate events with high rates and magnitude of warming, which may serve as analogues for projected climate change, is that very few species, or even genera, survived such events. Here, we utilized one of the few records of marine bivalves that spans the Permian–Triassic climate crisis with specimen-level data and at a high resolution. These measurements come from the Bellerophon and Werfen formations of the Dolomites in Italy, representing relatively shallow marine environments. At the species-level, there is almost a complete turnover, and the newly evolved species are typically significantly smaller, but not unusually small, whereas the three surviving species do not show a significant body size change. Our results clarify that the observed temporary size reduction at the genus-level is primarily driven by the preferential evolution of smaller species after the extinction, rather than, as often assumed, by a size decrease within existing species; this challenges the universal validity of the 'Lilliput effect' in the sense of direct intra-species dwarfing, but confirms it as a consequence of faunal turnover. Subsequently, there are two pulses of genus-level body size recovery determined by different mechanisms. The first phase (late Griesbachian) is driven by the size-increase of the existing species, whereas the second phase (early Spathian) is also due to the evolution of larger species. The effects of abiotic and biotic factors in controlling these body size dynamics are superimposed during the Early Triassic. These results suggest a mechanism to explain size reductions during climate crises, but does not find a species-level body size reduction to be a forecastable response to extreme climate warming.
Dear colleagues,
I greatly appreciate the detailed and high-resolution analyses of bivalve size responses observed within the dolomites of Italy across the Permian-Triassic extinction and its aftermath. I can only imagine how much effort and work it took also judging from preparation for our own studies. In our meta-analysis, we observed greater size responses related to hyperthermal crises - at least with the currently available published data . As you rightly point out, there is need for more data to better understand the phenomenon. I agree that various factors may contribute to these patterns and as you rightly point out (changes in species versus genus level) and the difficulty of finding locations where species and genera can be studied across such crises particularly during mass extinction events like the Permian-Triassic erasing a large part of diversity as well as associated changes in sedimentation, sea level and environmental factors complete this. I however feel 3 additional factors can be important to understand mechanisms of size changes and their discrepancies between localities/regions, groups and time intervals. In this aspect i remain cautious in generalising the efforts from a single region or study but find all new data to these questions meaningful and important to publish and study. Unfortunately, not a lot of studies are available offering the kind of resolution like yours to allow for it (only a small fraction in literature allow for investigating these mechanisms).
It may however be interesting to these others potential aspects discussed in the discussion. In this respect, to what degree we can generalise mechanisms across groups or studies beyond species/genus. Before me proceeding, i want to say i greatly appreciate the collection and study effort, rigorous analyses and do not want to criticise it in any way as you did the greatest effort ever done on this topic in this group and region. I also acknowledge that this points may not always be easy to study from own experiences related to availability and preservation of specimens. We have been trying to test this in some groups but requires considerable effort and data and not all aspects are possible within the same datasets. I therefore greatly appreciate you tackling at least some of them here. I will draw here from own experiences/studies as i know them best but does not mean other studies do not exist or less interesting or appropriate in any way.
Points i would like a brief discussion on (at least as far as your data allows for it):
1) The use of species versus genus: i agree fully that there is a need to understand and compare patterns at the species level versus genus or higher. However, i am unsure how well resolved this can be across an extinction event or any intervals for that matter (particularly when including gaps in record or different to poor preservational issues) as it would require a precise taxonomy and understanding of intraspecific and intrageneric including understanding of ontogenetic changes and phenotypic plasticity across different environments to fully understand these differences. How can we for example be sure our assignments to particular fossil species or genera is correct if taxa show marked changes through ontogeny and in addition marked different distribution in size and phenotypic variation across time? Can we be sure authors would not assign them to new species or inverse to similar genera if they have marked different or similar morphologies (in relationship to size). We tried to more fully understand this for example in our studies of belemnites (Rita et al. 2019: https://doi.org/10.1098/rsos.190494; Nätscher et al. 2021: https://doi.org/10.1038/s41598-021-93850-0; De Baets et al. 2021: https://doi.org/10.1186/s13358-021-00242-y) or ostracods (Nätscher et al. 2023: https://doi.org/10.1111/pala.12667). How can we be sure that smaller or larger specimens at particular times (particularly when looking differently) are new species without large understanding about ontogeny, growth rates and intraspecific variation which are challenging to obtain? This may the most difficult to tackle as i am not sure how well this is/can be understood in taxa and with data/preservation you have available but if not in the dolomites i would not know where else.
2) Mechanisms within species versus mechanisms at higher levels: it remains the question what are the main drivers of changes in size we observe across time and even we can even capture them particularly in species differing in reproductive strategies (if we have no knowledge on all ontogenetic stages). A single locality/region may not necessarily be sufficient to fully grasp variation in species size and ontogenetic representation (if different ontogenies live in different places). In the case of belemnites across the TOAE (where we also have gaps but have material across a latitudinal gradient), it seems response can differ at different localities (Rita et al. 2019: https://doi.org/10.1098/rsos.190494; Nätscher et al. 2021: https://doi.org/10.1038/s41598-021-93850-0; De Baets et al. 2021: https://doi.org/10.1186/s13358-021-00242-y). For example in Peniche we could document a clear decrease in size across the Pliensbachian-Toarcian boundary when all ontogenetic stages are represented and adults changes consistently but differed between localities) while we consistently see an increase in (adult) size in the TOAE driving by new species arriving (similar to your study). The issue across the TOAE in part seems to be that the new species seems mostly represented by later (ontogenetic) stages (do no live their entire life in same localities). These factors therefore may complicate the interpretations - particularly to understand if patterns with species or even at higher level are driven by shifts in ontogenetic representation (e.g., higher degree of juveniles) or adult size? Given at least some of your are experts on Triassic bivalves i feel you may be best able to tackle this question.
3) The last factor is changes in environmental and depositional conditions across time or space. Ideally we would have representation of species across all habitats to fully control for this but this may not be always possible (e.g., sometimes there are no full representation or comparable environments/conditions before or after an event = from my understanding a large event like Permian-Triassic extinction could qualify as such an event). Sometimes we may only have high resolution data available for a single section which does not make it less interesting to publish and discuss. We tried to understand this be looking at a dataset on size distributions of ammonoid species belonging to a single family across space not time (De Baets et al. 2022: https://doi.org/10.2110/palo.2021.063). Our working hypothesis was initially that specimens / size distribution getter bigger on average at higher latitudes. Interestingly, we found that lithology (before genus assignment or collection effort) has the largest effect on our dataset. If we correct for it, we even see that specimens/ size distributions get larger towards lower latitudes (exactly the opposite from our hypothesis). How could changes in environment or depositional conditions have potentially have affected changes size distribution/representation? You at least seem to correct for abundance and environmental factors and multiple localities so i assume potential depositional environments/facies changes could also be tackled or ruled out.
Unfortunately, datasets able to study such aspects like yours are still to far and few between (i learned this the hard way).
I therefore look forward to see your interesting work published and i hope i convinced you that at least some discussion of these factors may be meaningful as far as possible or at least considering them as aspects worth studying in the future.