Importance of being organized: the effects of changing tooth arrangement on durophagous predation

Abstract

The literature on paleontology and functional morphology both include many studies on the feeding mechanics of organisms. In extinct organisms, physiological data must be inferred from bone structure, or muscles scars, even in the most exceptionally preserved specimens (Benton, 2010). However even reconstruction and analysis of muscle scars in fossils rely heavily on assumptions (Rieppel, 2002). Given the resilient nature of enamel, teeth are often the most commonly found remains of extinct organisms, and in some cases they are only known remains (Adnet et al., 2009; Pol, 2012). Enamel is highly calcified and so, unlike other softer portions of an organism’s anatomy, is readily preserved (Lucas et al., 2008). Fortunately, there is a close relationship between tooth form and function, which allows researchers to infer the ecology of the extinct organisms. Slender, pointed teeth, are ideal for puncturing, and bladed teeth are best suited for tearing and cutting prey, whereas more rounded and blunt teeth are thought to function better in crushing prey (Massare, 1987), a feeding strategy termed durophagy. Durophagous predators typically have robust jaw bones in addition to their distinctive and molariform teeth on the premaxilla, maxilla, dentary or the pharyngeal (Norton, 1988, Wilga and Motta, 2000). It is thought that this molariform tooth morphology, as well as tooth arrangement, may serve to increase tooth surface area, thus reducing the stress applied to teeth (Ramsay and Wilga, 2007) when crushing. Studies on the functional ability and structure of durophagous teeth have focused on a range of both extinct and extant organisms, including borophagines canids, early hominoids, and elasmobranchs (Lee et al., 2011; Lucas et al., 2008; Tseng ZJ and Wang X, 2010). For these groups, tooth form dictates diet and, by extension, habitat selection and population ecology. From there it is possible to study large-scale evolutionary patterns, co-evolutions, and trophic interactions (Lauten 2013). To understand the relationship between form and function in the teeth of extinct organisms, physical models can be constructed to test the limitations of these structures either through fossil re-creation or inference from testing on extant relatives. Single cusped mammalian teeth (Lee et al., 2011), flat surfaces of the labially compressed bamboo shark teeth (Shimada et al., 2009), and teeth with varying concavities representative of eels (Crofts and Summers, 2014), have all been created and tested on an individual basis. These studies found that a single concave tooth required more force to fracture prey than a convex tooth, and Lee et al. (2011) found that a domed tooth surface works well to strengthen the overall tooth structure. However, Ramseys and Wilga (2007) posit that many teeth contacting prey simultaneously could be vital for durophagy, since multiple contact points will serve to spread bite force over a larger surface area. Most modelling studies, to date, focus on the functional morphology of a single tooth, not the effects of multiple teeth on a prey item. While there are a number of durophagous lineages, all specialized on the same hard-prey consuming life-style, the arrangement of teeth varies between species. For example, ancient durophagous elasmobranchs such as Ptychodus occidentalis exhibit cusped teeth of the dentary premaxilla and maxilla in rows similar to modern carnivorous sharks (Shimada et al., 2009). In contrast, Megapiranah paranensis, an extinct Serrasalmid, has a staggered, zig-zag dentition on the oral jaws (Grubich et al., 2012). Some durophagous teleost fishes have completely skipped the oral jaws and use flattened pharyngeal tooth plates for crushing (Hernandez and Motta, 1997). Similarly the extinct marine Sauropterygians, the Placodonts, also evolved tooth plates, but on completely different bones. Those goals of this study are: (i) to determine how tooth arrangement affects the ability to crush a simplified prey item, (ii) to test if and how the effects of tooth arrangement vary with tooth occlusal morphology, and (iii) to determine if crushing idealized prey items is a feasible proxy for natural prey items.