Rostrum size differences between Toarcian belemnite battlefields

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.

Fossilized leftover falls as sources of palaeoecological data: a ‘pabulite’ comprising a crustacean, a belemnite and a vertebrate from the Early Jurassic Posidonia Shale

“…We suggest that the association of the complete belemnite arm crown with a complete rostrum and some soft parts represent the remains of the meal of a vertebrate predator, which had learned enough about belemnite anatomy to spare the rostrum. This lends further credibility to the hypothesis that belemnite predation might contribute to belemnite accumulations (so-called battlefields) under certain circumstances (Doyle & Macdonald, 1993), although in deposits lacking soft-tissue preservation likely current concentration and/or condensation are important factors (Rita et al, 2018(Rita et al, , 2019Urlichs, 1971).…”

Mechanisms and drivers of belemnite body-size dynamics across the Pliensbachian–Toarcian crisis

“…The more complete belemnites are suspected to be a less time-averaged sample [68], because a better preservation is usually an indicator of quick burial and little transport of the fossil. Conveniently, complete specimens (with at least part of the alveolar region preserved) are easier to assign taxonomically and ontogenetically [69]. The fragmented specimens were only used to calculate the belemnite abundance per bed.…”

“…However, in all known belemnites with soft tissue preservation, the fins attach to the rostrum and the soft parts closely track the outline of their internal skeleton, as in their extant relatives [73,74]. For these reasons, the rostrum can be considered a reasonable proxy for body-size in the absence of the preserved soft tissue [69]. This is also supported by a comparison between hard parts and soft parts in well-known taxa showing that larger rostra correspond to a larger mantles [75].…”

“…Previous research has demonstrated that the geometric mean (GM), a unidimensional metric that combines the apical height, width and length (normalGM=normalDv×normalDl×l 3) is the most robust proxy to compare the body-size between morphologically different forms belonging to different species or ontogenetic stages within a species [69]. In addition, the geometric mean is more comparable to the proxies used in extant coleoids [76].…”

Jaws of a large belemnite and an ammonite from the Aalenian (Middle Jurassic) of Switzerland

“…With the ten-armed coleoids, the decabrachians, they share an internal skeleton largely surrounded by a muscular mantle, a large brain compared to other invertebrates, ten arms, chitinous jaws, large lateral eyes, and a predatory mode of life (e.g., Naef 1922;Reitner and Urlichs 1983;Doguzhaeva et al 2002Doguzhaeva et al , 2003Weis and Delsate 2006;Klug and Fuchs 2010;Klug et al 2010aKlug et al , b, 2016Keupp and Mitta 2015;Clements et al 2016;Donovan & Fuchs 2016; Klug and Tajika 2018;Wani et al 2018;Jenny et al 2019;. In contrast to other hard parts, the low magnesium calcite rostra of belemnites represent abundant fossils in the Jurassic and Cretaceous, sometimes occurring in rock-forming numbers (Doyle and Macdonald 1993;Rita et al 2018). All other body parts are much less commonly preserved and true belemnite soft parts were not described before 1983 (Reitner and Urlichs 1983), after a series of fakes had been erroneously published as first records of belemnite soft parts (Huxley 1864;Wiesenauer 1976;Rietschel 1977;Seilacher and Wiesenauer 1978).…”