Rostrum Size Differences Between Toarcian Belemnite Battlefields

Foss. Rec., 21, 171–182, 2018 https://doi.org/10.5194/fr-21-171-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. Rostrum size differences between Toarcian belemnite battlefields Patrícia Rita, Kenneth De Baets, and Martina Schlott Geozentrum Nordbayern, Friedrich-Alexander Universität Erlangen-Nürnberg, 91054 Erlangen, Germany Correspondence: Patrícia Rita ([email protected]) Received: 20 February 2018 – Revised: 9 May 2018 – Accepted: 14 May 2018 – Published: 7 June 2018 Abstract. Body size changes have been reported across cri- changes in morphological disparity (Dera et al., 2010, 2016). sis intervals. Belemnites – now considered extinct stem- Among these, body size reductions have been reported in decabrachians – have rarely been investigated for this pur- various lineages (Morten and Twitchett, 2009; Martindale pose, and the few studies have resulted in ambiguous out- and Aberhan, 2017; Rodríguez-Tovar et al., 2017). This so- comes. Here we investigate two Toarcian belemnite accumu- called Lilliput effect (Harries and Knorr, 2009) has not only lations in southern Germany from a morphometric point of been reported for the Toarcian Oceanic Anoxic Event but view with the support of computed tomography data. The also for other extinction events (e.g. Twitchett, 2007; Harries aim of this study is to test whether a difference in size can be and Knorr, 2009; Borths and Ausich, 2010; Rego et al., 2012; observed between the rostra of the two studied samples, from Sogot et al., 2014). However, the evolutionary and ecological individual lineage to community, and which proxy is more importance of the Lilliput effect remains poorly understood. reliable. A significant decrease in median size from the Early Furthermore, because of physiological differences, not all or- Toarcian (Dactylioceras tenuicostatum Zone) to the Mid- ganisms necessarily exhibit a body size reduction in the face dle Toarcian (Haugia variabilis Zone) is recognized. This is of warming or other environmental stresses (Gardner et al., observed at the community level of organization, consider- 2011; Ohlberger, 2013). Some authors even consider that, in ing the whole assemblage, but also within Passaloteuthis– some cases, the body size reduction might reflect a preserva- Acrocoelites lineage, at the genus level. It is also demon- tion or collection artefact (McGowan et al., 2009; Brayard et strated that diameter-based measurements or maximum pre- al., 2010). served length are not reliable proxies for size, and therefore The Early Toarcian crisis also coincides with a major apical length or three-dimensional approximations, such as turnover in belemnite faunas. A drop in diversity, mani- the geometric mean or the post-phragmocone volume, are fested by morphological bottlenecks, is followed by episodes more advisable. This is especially important when compar- of belemnite diversification during the recovery interval ing specimens with markedly different rostrum shapes. Fur- in the Middle–Late Toarcian (Riegraf, 1980; Doyle, 1994; ther studies are, however, still necessary to disentangle the Caswell and Coe, 2014; Ullmann et al., 2014; Dera et mechanisms behind the reduction in rostrum size within the al., 2016). A preliminary analysis of the data compiled by Toarcian and their putative environmental causes. Schlegelmilch (1998) on belemnites from southern Germany suggests a decrease in median rostrum size (as a proxy for body size) from the Early to the Middle Toarcian (Fig. 1). Most of the previous studies on belemnite rostrum size 1 Introduction have focused on particular taxa or lineages (Christensen, 2000; Morten and Twitchett, 2009), precluding a proper anal- The Early Toarcian coincides with a multi-phased crisis ysis of the potential differences observed when consider- (Harries and Little, 1999; Caruthers et al., 2013), which has ing rostrum size changes at different levels of organization been mainly attributed to warming and/or anoxia (Little and (i.e. populations, communities, individuals). In addition, the Benton, 1995; Harries and Little, 1999; Hesselbo et al., 2000; methods applied usually focus on the analysis of a single Pálfy and Smith, 2000; Wignall et al., 2005; Danise et al., morphometric parameter, such as maximum diameter. De- 2013). This crisis is reflected in biotic communities through spite the fact that the rostrum diameter is often exposed and the occurrence of two major extinction events and through Published by Copernicus Publications on behalf of the Museum für Naturkunde Berlin. 172 P. Rita et al.: Rostrum size differences between Toarcian belemnite battlefields 50 40 Maximum 30 Upper quartile 20 Median Geometric mean (mm) 10 Lower quartile Minimum 0 U. jamesoni T. ibex P. davoei A. margar. P. spinatum D. tenuic.* H. falcifer. H. bifrons H. variabilis* G. thouars. H. insig.-C.aal. L. opalinum Early Late Early Middle Late Early Pliensbachian Toarcian Aalenian Figure 1. Belemnite body size fluctuations across the Pliensbachian (Uptonia jamesoni Zone)–Aalenian (Leioceras opalinum Zone) interval, according to Schlegelmilch (1998). A body size reduction is observed from the Early to the Middle Toarcian. Timescale according to Dera et al. (2016). The width of the boxes is proportional to the number of observations. Asterisks indicate the sampled levels. Teufelsgraben Buttenheim ChronostratigraphyBiostratigraphyLithostratigraphy section section ChronostratigraphyBiostratigraphyLithostratigraphy * ? ? H. variabilis H. falciferum Lower Toarcian Lower ? Posidonia shale Fm. D.ten. * H. bifrons Toarcian Posidonia shale Fm. H. falcifer. P. spinatum P. Pliensbachian Amaltheenton Fm. D. tenuic. 5 m Berlin Marl Limestone Limestone concretions Bavaria spinatum P. Pliensbachian Clay rich in iron oxides Amaltheenton Fm. 0.5 m Ammonites BT TG Bivalves Crinoids Belemnites Nuremberg Fish remains Munich Vertebrates * Sampled level 200 km Figure 2. Location of the studied sections in Bavaria, southern Germany, and sampled stratigraphic levels: Buttenheim (BT) (49◦47041.8300 N, 11◦2042.4700 E) and Teufelsgraben (TG) (49◦35041.4000 N, 11◦16020.9000 E). Foss. Rec., 21, 171–182, 2018 www.foss-rec.net/21/171/2018/ P. Rita et al.: Rostrum size differences between Toarcian belemnite battlefields 173 for a rostrum size analysis, although various mechanisms 100 % can contribute to the formation of belemnite accumulations 80 % (Urlichs, 1971; Doyle and Macdonald, 1993). In many cases, a certain degree of condensation might be involved, resulting 60 % in temporal and/or spatial averaging. However, this is proba- bly equivalent to the result of merging several layers of less 40 % GM (max) rich accumulations in order to reach an appropriate sample size. Therefore, further analysis is necessary to investigate 20 % whether battlefields can be considered a useful resource for 0 % rostrum size analysis. BT TG Here we compare data from two distinct belemnite battle- Passaloteuthis bisulcata Acrocoelites fields in order to examine whether a rostrum size difference Passaloteuthis sp. Dactyloteuthis digitalis can be recognized between them, as is indicated by the analy- Parapassaloteuthis zieteni Dactyloteuthis incurvata sis of the latest comprehensive literature survey from the considered interval (Schlegelmilch, 1998). The main aim is to Passaloteuthis milleri Dactyloteuthis similis assess the performance of individual traditional morphome- Dactyloteuthis hebetata tric parameters or a combination of these as volume metrics or as proxies for rostrum size, considering either the entire Figure 3. Proportion of the main taxa identified in Buttenheim (BT) assemblage of belemnites (community level of organization) and Teufelsgraben (TG) sections, organized vertically according to or particular lineages (genus level of organization). relative rostrum size (maximum geometric mean). Acrocoelites in- cludes A. quenstedi, A. pyramidalis and A. subbrevis and A. sp., which were merged in order to reach a comparable sample size, re- garding the remaining taxa. 2 Material and methods 2.1 Material easily measurable, it can be misinterpreted in partially em- We focused our study on two Toarcian belemnite battle- bedded or broken specimens. In addition, it has been demon- fields (Doyle and Macdonald, 1993) from Franconia, south- strated that the error on the diameter-based measurements is ern Germany, in the Buttenheim and Teufelsgraben local- comparatively larger than the error associated with length- ities (Fig. 2). We sampled all belemnite rostra from pre- based measurements. This is related to preservation issues as determined well-accessible areas within the two consid- well as the smaller dimension of the diameter in comparison ered belemnite battlefields. The well-preserved specimens with length (De Baets et al., 2013). The rostrum length would (i.e. at least with the rostrum solidum preserved) were de- also be more closely correlated with the mantle length used termined to the species level and measured (Table S2 in in extant coleoid size studies (Roper and Voss, 1983), but the the Supplement; Fig. 3). No epirostrum-bearing specimens alveolar region is often flattened and/or broken, hampering were recorded in the studied sites. Due to the absence of precise measurements. Unidimensional or linear shell mea- common species in the datasets, an individual lineage, the surements, particularly diameter, are potentially imprecise Passaloteuthis–Acrocoelites line (e.g. Schlegelmilch, 1998), when comparing morphologically dissimilar taxa. Therefore, was

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