Abstract
Abstract. Body size changes have been reported across crisis intervals. Belemnites – now considered extinct stem-decabrachians – have rarely been investigated for this purpose, and the few studies have resulted in ambiguous outcomes. Here we investigate two Toarcian belemnite accumulations in southern Germany from a morphometric point of view with the support of computed tomography data. The aim of this study is to test whether a difference in size can be observed between the rostra of the two studied samples, from individual lineage to community, and which proxy is more reliable. A significant decrease in median size from the Early Toarcian (Dactylioceras tenuicostatum Zone) to the Middle Toarcian (Haugia variabilis Zone) is recognized. This is observed at the community level of organization, considering the whole assemblage, but also within Passaloteuthis–Acrocoelites lineage, at the genus level. It is also demonstrated that diameter-based measurements or maximum preserved length are not reliable proxies for size, and therefore apical length or three-dimensional approximations, such as the geometric mean or the post-phragmocone volume, are more advisable. This is especially important when comparing specimens with markedly different rostrum shapes. Further studies are, however, still necessary to disentangle the mechanisms behind the reduction in rostrum size within the Toarcian and their putative environmental causes.
Highlights
The Early Toarcian coincides with a multi-phased crisis (Harries and Little, 1999; Caruthers et al, 2013), which has been mainly attributed to warming and/or anoxia (Little and Benton, 1995; Harries and Little, 1999; Hesselbo et al, 2000; Pálfy and Smith, 2000; Wignall et al, 2005; Danise et al, 2013)
When considering the whole belemnite assemblage, the two battlefields have significantly different distributions, according to the Kolmogorov–Smirnov test (Fig. 6 and Table 1). This can be observed when taking into account either the individual morphometric parameters (L, l, Dv, Dl and Dmax) or the three-dimensional parameters (GM, total volume (TV) and postphragmocone volume (PPV)), with at least a 90 % confidence level (Table 1)
By comparing the rostrum size of two different belemnite battlefields, we demonstrate that, for the entire assemblage, the size changes indicated by empirical volumetric data are equivalent to the results given by the geometric mean, which takes into account post-phragmocone measurements (Fig. 7)
Summary
The Early Toarcian coincides with a multi-phased crisis (Harries and Little, 1999; Caruthers et al, 2013), which has been mainly attributed to warming and/or anoxia (Little and Benton, 1995; Harries and Little, 1999; Hesselbo et al, 2000; Pálfy and Smith, 2000; Wignall et al, 2005; Danise et al, 2013) This crisis is reflected in biotic communities through the occurrence of two major extinction events and through changes in morphological disparity (Dera et al, 2010, 2016). Some authors even consider that, in some cases, the body size reduction might reflect a preservation or collection artefact (McGowan et al, 2009; Brayard et al, 2010)
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