Salamon MA et al. (2021), Shared patterns in body size declines among cri...

ECB-ART-48913

Sci Rep 2021 Oct 13;111:20351. doi: 10.1038/s41598-021-99789-6.

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Shared patterns in body size declines among crinoids during the Palaeozoic extinction events.

Salamon MA , Brachaniec T , Kołbuk D , Saha A , Gorzelak P .

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Crinoids were among the most abundant marine benthic animals throughout the Palaeozoic, but their body size evolution has received little attention. Here, we compiled a comprehensive database on crinoid calyx biovolumes throughout the Palaeozoic. A model comparison approach revealed contrasting and complex patterns in body size dynamics between the two major crinoid clades (Camerata and Pentacrinoidea). Interestingly, two major drops in mean body size at around two mass extinction events (during the late Ordovician and the late Devonian respectively) are observed, which is reminiscent of current patterns of shrinking body size of a wide range of organisms as a result of climate change. The context of some trends (marked declines during extinctions) suggests the cardinal role of abiotic factors (dramatic climate change associated with extinctions) on crinoid body size evolution; however, other patterns (two intervals with either relative stability or steady size increase in periods between mass extinctions) are more consistent with biotic drivers.

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	Figure 1. Trend in mean calyx size of crinoids as a whole in the Palaeozoic. Vertical lines represent error bars.
	Figure 2. Trends in mean calyx size of major crinoid subclades in the Palaeozoic. (a) Camerata, (b) Pentacrinoidea, (c) Disparida, (d) Cladida. Vertical lines represent error bars.
	Figure 3. Plots explaining the calculation of the two major component values (biased extinction, biased origination). Interpretation of these plots works as follows, using the late Katian extinction as an example (a) and the late Givetian extinction as an example (b). (a) In the Katian, 55 genera were extant; their mean biovolume size was 0.0874 log mm3. Of these 55 genera, 43 went extinct in this stage (with the mean size 0.282 log mm3), and 12 survived into the Hirnantian (the mean size of these −0.611 log mm3). Therefore, on average the survivors were smaller than the extinction victims. Thus, extinction was size-biased (larger genera were more likely to go extinct). The change in mean size due to this size-biased extinction was −0.611 log mm3—0.0874 log mm3 = − 0.6984 log mm3. In the Hirnantian, only 4 new genera originated with mean size −0.042 log mm3. The mean size in the Hirnantian of all 16 genera (12 survivors from the Katian and 4 originators) is −0.468 log mm3. Thus the 4 new originators changed the mean size of genera extant in the Hirnantian from −0.611 log mm3 to −0.468 log mm3 (a difference of 0.143 log mm3). In summary, the mean size of Katian taxa is 0.0874 log mm3, and the mean size of Hirnantian taxa is −0.468 log mm3, a change of −0.555 log mm3. This change of -0.555 log mm3 is partitioned into a size-biased extinction component of −0.6984 log mm3. Origination of new genera during the Hirnantian with mean size 0.143 log mm3 slightly mitigated the size decrease imposed by extinction component. Note that −0.6984 – (−0.143) = − 0.555. (b) In the Givetian, 75 genera were extant with the mean biovolume 0.254 log mm3. Of these 75 genera, 55 went extinct in this stage (with the mean size 0.4 log mm3), and 20 survived into the Frasnian (with the mean size −0.145 log mm3). Thus, extinction was size-biased (larger genera preferentially went extinct). The change in mean size due to this size-biased extinction was −0.145 log mm3 −0.254 log mm3 = 0.399 log mm3. In the Frasnian, 46 new genera originated with mean size -0.213 log mm3. The mean size in the Frasian of all 66 genera (20 survivors from the Givetian and 46 originators) is −0.192 log mm3. Thus the 46 new originators changed the mean size of genera extant in the Frasnian from −0.145 log mm3 to −0.192 log mm3 (a difference of 0.047 log mm3). In summary, the mean size of Givetian taxa is 0.254 log mm3, and the mean size of Frasian taxa is -0.192 log mm3, a change of −0.446 log mm3. This change of −0.446 log mm3 is partitioned into a size-biased extinction component of −0.399 log mm3 and a size-biased origination component of −0.047 log mm3. Note that −0.399−0.047 = −0.446. Periods sharing the same superscript indicate that size distributions are not significantly different from each other (p > 0.05).
	Figure 4. Mean calyx size of crinoids as a whole in the Palaeozoic plotted with temperature and δ18O curves inferred from three different sources.

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