Ammonoid Septal Formation and Suture Asymmetry Explored with a Geographic Information Systems Approach Margaret M. Yacobucci and Lori L. Manship
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Palaeontologia Electronica http://palaeo-electronica.org Ammonoid septal formation and suture asymmetry explored with a geographic information systems approach Margaret M. Yacobucci and Lori L. Manship ABSTRACT Given the centrality of suture patterns to ammonoid systematics, it is remarkable how little we understand about the formation of septa. Various models for septal forma- tion have been proposed, but given the spatial complexity of suture patterns, the means to test and constrain them have been lacking. Here we use Geographic Infor- mation Systems (GIS) to test models for septal formation by analyzing spatial con- straints and asymmetries in several ammonoid species, including Cretaceous ammonites (the Middle Turonian acanthoceratacean Coilopoceras and the Early Cenomanian hoplitacean Neogastroplites) and Carboniferous goniatites (the dimor- phoceratacean Metadimorphoceras and the goniatacean Somoholites). These analy- ses show that suture lines are not more strongly constrained at lobe tips than at saddles, nor are lobe tips more strongly inflected than saddles. Rather, the entire suture line is similarly constrained and shaped at both lobes and saddles, suggesting that the process controlling septal folding acts along the entire margin of the septal membrane rather than at a few specified tie points. Right and left opposing sutures from the same specimens do not match precisely. More generally, right and left suture patterns show asymmetries both in lengths and degrees of variability. These asymme- tries are consistent across individuals sampled from different localities, implying that they are species-level traits, rather than pathologies or taphonomic deformations. The directed asymmetry of suture patterns may reflect soft part asymmetry in ammonoids. These results can be used to place constraints on alternative models for septal growth and function; the GIS-based method will permit further testing of such models. Margaret M. Yacobucci. Department of Geology, Bowling Green State University, 190 Overman Hall, Bowling Green, Ohio, 43403-0218, USA. [email protected] Lori L. Manship. Department of Physical Science, University of Texas of the Permian Basin, 4901 East University, Odessa, Texas, 79762, USA. [email protected] KEY WORDS: cephalopods; sutures; GIS; morphometrics; growth; constraint PE Article Number: 14.1.3A Copyright: Paleontological Society March 2011 Submission: 6 August 2007. Acceptance: 25 October 2010 Yacobucci, Margaret M. and Manship, Lori L., 2011. Ammonoid septal formation and suture asymmetry explored with a geographic information systems approach. Palaeontologia Electronica Vol. 14, Issue1; 3A:17p; http://palaeo-electronica.org/2011_1/136/index.html YACOBUCCI & MANSHIP: GIS ANALYSIS OF SUTURES INTRODUCTION from the new chamber and replaced with gas (Ward 1987). Ammonoid cephalopods are notable for their Because the septa in Nautilus are relatively chambered shells, with the septal partitions often smooth, this living analog does not offer much showing complex folding along their margins. Pale- insight into how the pronounced folding of ontologists have long used the suture lines formed ammonoid septa occurs. The margins of by the intersection of the crinkled edges of the ammonoid septa, especially those of the Jurassic- septa and the outer shell to classify ammonoids, Cretaceous suborder Ammonitina, are extensively and for more than a century have conducted vari- folded, producing complex suture patterns where ous studies of suture and septal form and morpho- the septum intersects the outer shell wall (Figure genesis (Buckman 1892; Pfaff 1911; Spath 1919a; 1). The proposed models for ammonoid septal for- Westermann 1966; Mutvei 1967; Seilacher 1973; mation all (quite reasonably) assume that this fold- Ward and Westermann 1976; Bayer 1977a, 1977b, ing occurs within the soft, organic septal 1978a, 1978b; Hewitt 1985; Zaborksi 1986; Sei- membrane, before it is mineralized (Bayer 1978a). lacher 1988; García-Ruiz et al. 1990; Hewitt et al. They diverge, however, in how the folds are pro- 1991; García-Ruiz and Checa 1993; Lutz and Boy- duced and how the same pattern of folding can be ajian 1995; Checa 1996; Checa and García-Ruiz produced repeatedly in each new septum as the 1996; Hammer 1999; Olóriz et al. 1999; Gildner animal grows. 2003; Allen 2006) as well as their supposed func- Perhaps the most commonly-cited idea for tion (Westermann 1971; Seilacher 1975; Wester- septal formation is the tie-point model, best articu- mann 1975; Henderson 1984; Hewitt and lated by Seilacher (1973, 1975, 1988) and Wester- Westermann 1986, 1987; Jacobs 1990; Seilacher mann (1975). This model argues that the new, pre- and LaBarbera 1995; Saunders 1995; Daniel et al. calcified, septal membrane attaches to the outer 1997; Hewitt and Westermann 1997; Hassan et al. shell wall at genetically defined tie-points, located 2002; Lewy 2002; Pérez-Claros 2005; Hammer at the tips of suture lobes (that is, the adapical- and Bucher 2006; Pérez-Claros et al. 2007; De most flexure points; Figure 1). The membrane Blasio 2008). Given all the attention that has been between the tie-points then is blown out toward the paid to these features, it is surprising that we are aperture by positive hydrostatic pressure, rather still unsure of just how septa form or how they like a sheet pinned to a clothesline will billow out on relate to other aspects of shell and soft part anat- a windy day, to form the saddles of the suture. The omy. How did the ammonoid create such a com- more tie-points present, the more complicated and plexly folded, mineralized structure? Why are the crinkly the septal margin will be. The soft mem- suture lines (i.e., the folded margins of the septa) brane then calcifies, preserving the crinkles and so consistently patterned that we can use sutures folds. Of some debate within the tie-point concept for fine-scale taxonomy? What process governs has been whether the posterior mantle of the the addition of elements and increasing fold com- ammonoid was involved in determining the shape plexity of sutures through the ontogeny of an indi- of the septum (e.g., Hewitt et al. 1991), whether it vidual? Does the degree of sutural complexity was smooth or permanently fluted (e.g., Seilacher relate to functional, constructional, developmental, 1975 versus Westermann 1975 and Seilacher or metabolic constraints on the septum? 1988), or even capable of changing form at will Models for Ammonoid Septal Formation (Zaborski 1986; Lewy 2002, 2003; Hewitt and Westermann 2003). Several models for how septa and their folded The viscous fingering model offers an alterna- edges form have been advanced within the paleon- tive to the tie-point concept (García-Ruiz et al. tological literature. Living Nautilus provides a 1990; García-Ruiz and Checa 1993; Checa and potentially observable analog, although the details García-Ruiz 1996; Checa 2003). Here, the com- of how Nautilus produces its septa are still a chal- plex septal folding is due not to genetic constraints lenge to study. When constructing a new septal or the shape of the ammonoid posterior, but to the plate, Nautilus first shifts forward in the body cham- hydrostatic properties of the materials involved. ber while growing new shell around the aperture, Septa represent the interface between two fluids of leaving a fluid-filled gap between its soft parts and differing viscosities, the mantle and the last, fluid- the last septum (Ward 1987). The animal secretes filled chamber. Such a situation will produce fractal a new septal membrane just behind its soft parts “viscous fingering” of one fluid mass into the other; and then begins to mineralize it. Once the new wall effects will produce greater complexity of inter- septum is partially mineralized, the fluid is removed 2 PALAEO-ELECTRONICA.ORG fingering near the shell margin and the siphuncle (García-Ruiz et al. 1990). 1.1 Hammer (1999) challenged the viscous fin- U gering model on the grounds that a purely hydro- static model could not produce such consistent LEFTLEFT RIGHTRIGHT septal forms (but see response by Checa and García Ruiz 2000). He proposed a different way to L think about the problem, using a reaction-diffusion E model that explored how growth of the septal mem- brane could be guided by interactions between a slowly diffusing activator morphogen and a more rapidly diffusing inhibitor morphogen (Hammer to 1999; see also Hammer and Bucher 1999 and aperture Guex et al. 2003 for other applications of this con- saddle L cept). The reactions of these molecules would be E U capable of producing complex (and reproducible) 1.2 structures within the septal membrane without requiring direct genetic control or a complex devel- opmental regulatory system. Hammer (1999) noted venter that his model was consistent with the tie-point lobe concept, as it was concerned with understanding why specific points on the septal margin would be consistently shaped in the same way. foliole Careful study of ammonoid suture patterns can test some of the predictions of these models for septal formation and provide some constraints 1.3 on their parameters. For example, the tie-point model implies that suture lobes should be more constrained in their position than suture saddles, and also suggests that saddle folioles should be lobule more rounded while lobules are more incised. In FIGURE 1. Illustration of a typical ammonite suture, addition, at least as presented by their authors, all with anatomical terms and orientations. 1.1 View of three models make the assumption that septa are ammonoid shell, facing the aperture. Sutures are symmetrical across the shell’s dorsal-ventral mid- divided into external (E), lateral (L), umbilical (U), and line, so that right and left suture patterns match. internal (not shown) elements, moving from a ventral to To evaluate aspects like constraint and asym- lateral to dorsal position. Diagram of shell is redrawn metry in septal form requires a method for compar- and modified from Monks and Palmer (2002, p. 44). 1.2 ing and quantifying subtle differences in these Ammonite suture pattern with key elements labeled.