Palaeontologia Electronica palaeo-electronica.org The hydrostatics of Paleozoic ectocochleate cephalopods (Nautiloidea and Endoceratoidea) with implications for modes of life and early colonization of the pelagic zone David J. Peterman, Christopher C. Barton, and Margaret M. Yacobucci ABSTRACT Understanding the hydrostatic properties of ectocochleate cephalopods is essen- tial to study the functional morphology of their shells. The conditions for neutral buoy- ancy, static orientation of the living animal, and their hydrostatic stability are key factors to determine the physical relationships and constraints on cephalopod life habits and paleoecology. The hydrostatics of early orthoconic and cyrtoconic Paleozoic cephalo- pods were investigated by digitally reconstructing morphotypes within Nautiloidea and Endoceratoidea. Morphotypes were chosen from several traditional orders and include endogastric cyrtocones (Ellesmerocerida and Discosorida), exogastric cyrtocones (Oncocerida), and longi-orthocones (Orthocerida and Endocerida). The extant nautilid, Nautilus pompilius, was also modeled as a reference for the hydrostatics of the Paleo- zoic cephalopods. All cyrtocone models have similar hydrostatic stability to the Nautilus and only the ellesmerocerid was found to be negatively buoyant. The hydrostatics of the neutrally buoyant cyrtocones suggests that they behaved as relatively poor-swim- ming demersal or nekto-benthic predators. This mode of life aligns with the largely ner- itic distribution of individuals belonging to these clades in the Paleozoic. In contrast, the orthocones studied were all capable of neutral buoyancy, much more stable than the Nautilus model, and all had vertical syn vivo orientations in a static setting. The hydro- static properties of these morphotypes and their largely pelagic distribution support their role as vertical migrants. However, adapically distributed cameral deposits and endosiphuncular deposits would reduce stability, easing deviation from a vertical orien- tation. These adaptations, along with dorsal color patterns on some species, suggest that active locomotion in orthocones may have been more important than previously thought. David J. Peterman. Department of Earth and Environmental Sciences, Wright State University, Dayton, Ohio, United States of America. [email protected] Peterman, David J., Barton, Christopher C., and Yacobucci, Margaret M. 2019. The hydrostatics of Paleozoic ectocochleate cephalopods (Nautiloidea and Endoceratoidea) with implications for modes of life and early colonization of the pelagic zone. Palaeontologia Electronica 22.2.24A 1-29. https://doi.org/10.26879/884 palaeo-electronica.org/content/2019/2521-cephalopod-hydrostatics Copyright: May 2019 Paleontological Society. This is an open access article distributed under the terms of Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0), which permits users to copy and redistribute the material in any medium or format, provided it is not used for commercial purposes and the original author and source are credited, with indications if any changes are made. creativecommons.org/licenses/by-nc-sa/4.0/ PETERMAN, BARTON, & YACOBUCCI: CEPHALOPOD HYDROSTATICS Christopher C. Barton. Department of Earth and Environmental Sciences, Wright State University, Dayton, Ohio, United States of America. [email protected] Margaret M. Yacobucci. Department of Geology, Bowling Green State University, Bowling Green, Ohio, United States of America. [email protected] Keywords: cephalopod; Ordovician; buoyancy; orientation; paleoecology; digital models Submission: 3 May 2018. Acceptance: 11 April 2019. INTRODUCTION Okamoto, 1996). This belief is largely due to the difficulty of computing accurate volumes and the Paleozoic ectocochleate cephalopods include total mass distribution. The stability indices derived diverse and paleoecologically important higher by previous authors are proportionate to the results taxa, two of which are the focus of this study: Nau- of the current study in the sense that larger values tiloidea and Endoceratoidea. Understanding the represent larger restoring moments to the equilib- hydrostatic properties of their conches is essential rium condition. Stability values quantify the resis- to better understand their modes of life, especially tance to wave action or ease/difficulty in modifying during their initial liberation from the benthos and syn vivo orientation via active locomotion. A stabil- colonization of the pelagic zone during the Early ity index of zero represents a scenario where the Ordovician. The goals of the present work were to centers of buoyancy and gravity coincide, and 1) create highly accurate, three-dimensional digital there is no preferred orientation (similar to an models of a range of early Paleozoic cephalopod object in zero gravity). Larger stability values will morphotypes; 2) use these models to calculate key result in larger restoring moments acting to return hydrostatic properties, including the conditions the object to its static equilibrium condition. How- necessary to achieve neutral buoyancy, the static ever, no studies have tested the magnitude of orientation and stability of the shell during life; and hydrodynamic restoration for cephalopods with dif- 3) interpret these hydrostatic properties in the con- ferent stability indices. Therefore, stability indices text of these cephalopods’ paleoecology. in the current study are relative to each other and Ectocochleate cephalopod conches consist of the extant ectocochleate analogue, Nautilus a body chamber inhabited by the living animal and pompilius. a phragmocone divided by septa into a series of While there is much variability in shell mor- progressively larger chambers (camerae) through- phology throughout the evolutionary history of out ontogeny. The phragmocone functions as an these cephalopods, only early Paleozoic cyrto- apparatus to regulate weight and buoyancy. This cones and orthocones (curved and straight structure becomes a passive gas float if cameral conches, respectively; Figure 1) were investigated liquid is removed and replaced with gas. This here. The hydrostatics of the extant nautilid, Nauti- decrease in mass allows the phragmocone to be lus pompilius, was used as a reference for these positively buoyant and compensates for the nega- Paleozoic cephalopods. Specific taxa were chosen tive buoyancy of the soft body and mineralized that best represent the putative nautiloid clades shell, which are denser than seawater. Neutral Ellesmerocerida, Orthocerida, Oncocerida, and buoyancy is achieved when the total mass of the Discosorida and the endoceratoid clade, organism is equal to the mass of the displaced Endocerida. Although there are varying degrees of seawater (following Archimedes’ principle). The disparity, common morphotypes that have wide- static orientation of an object occurs when the cen- reaching implications for other taxa were selected ters of buoyancy and gravity are aligned vertically, for study. while the object's stability is dependent upon their degree of separation (Okamoto, 1996; Wester- Ordovician Biodiversification mann, 1998). Hydrostatic stability is quantified by The Great Ordovician Biodiversification Event the stability index (the distance between the cen- marked a drastic morphological and taxonomic ters of buoyancy and gravity normalized by the diversification of many marine organisms (Frey et cube root of the organismal volume). Earlier stud- al., 2004). Consequently, the nautiloids rapidly ies have assumed the center of gravity to coincide diversified and expanded their habitat from shal- with the center of the body chamber (Trueman, low, low latitude paleoenvironments to nearly all 1941; Raup, 1967; Raup and Chamberlain, 1967; 2 PALAEO-ELECTRONICA.ORG FIGURE 1. Depiction of general terms used within this study. Orthocones refer to straight shelled cephalopods, whereas cyrtocones are slightly curved. Long and short varieties of these shells are referred to by the prefixes longi- and brevi-, respectively. Exogastric and endogastric shells have a convex and concave venter, respectively (ventral direction towards the right for both cyrtocones). Each figure is modified from Teichert et al. (1964). marine paleoenvironments around the globe Endocerida consist of mostly straight orthoconic (Kröger and Zhang, 2009). In the earliest Ordovi- conchs that can be much larger than the elles- cian (beginning of the Tremadocian stage) only a merocerids. In the case of the endocerids, it is single nautiloid clade, the Ellesmerocerida, common for species to reach several meters in existed. Their conches can be characterized as length, making them the largest swimming organ- generally small (<10 cm), slightly curved, endogas- isms in the Ordovician (Teichert and Kummel, tric (that is, coiling toward the venter) cyrtocones 1960). One striking difference between the ortho- with close septal spacing, and fewer small longi- cerids and endocerids, aside from size, is the much conic forms (Furnish and Glenister, 1964). The larger siphuncle with common mineralized endosi- appearance of Ellesmerocerida in the Late Cam- phuncular deposits within the cones of Endocerida brian marked the first cephalopods with siphuncu- (Teichert, 1964a). Cameral deposits are commonly lar characteristics that would have increased the found within Orthocerida (and many other nauti- transport capacity of cameral liquid, allowing loids), and their hydrostatic attributes were investi- replacement with gases