Septal complexity in ammonoid cephalopods increased mechanical risk and limited depth

Abstract

The evolution of septal complexity in fossil ammonoids has been widely regarded as an adaptive response to mechanical stresses imposed on the shell by hydrostatic pressure. Thus, septal (and hence sutural) complexity has been used as a proxy for depth: for a given amount of septal material greater complexity permitted greater habitat depth. We show that the ultimate septum is the weakest part of the chambered shell. Additionally, finite element stress analyses of a variety of septal geometries exposed to pressure stresses show that any departure from a hemispherical shape actually yields higher, not lower, stresses in the septal surface. Further analyses show, however, that an increase in complexity is consistent with selective pressures of predation and buoyancy control. Regardless of the mechanisms that drove the evolution of septal complexity, our results clearly reject the assertion that complexly sutured ammonoids were able to inhabit deeper water than did ammonoids with simpler septa. We suggest that while more complexly sutured ammonoids were limited to shallower habitats, the accompanying more complex septal topograhies enhanced buoyancy regulation (chamber emptying and refilling), through increased surface tension effects.

Non-invasive imaging methods applied to neo- and paleo-ontological cephalopod research

“…One postulates a mechanical strengthening of the shell against hydrostatic pressure, while the other favors physiological reasons (pumping out and refilling of chamber liquid). During the last two decades two attempts were made to solve this problem with the help of mathematical algorithms and computers (Daniel et al, 1997;Hassan et al, 2002), resulting in opposite results. Our applications show a potential in 3-D models obtained from original fossil material to contribute greatly to this discussion.…”

The Evolution of Larval Morphology and Swimming Performance in Ascidians

“…This model may be used to generate predictions of performance for each position in the morphospace and thereby generate a performance landscape (also known as a performance surface: Arnold 2003; a fitness landscape: Gilchrist and Kingsolver 2003; or a functional morphospace: Moore and Ellers 1993). Performance landscapes have been used to test adaptive hypotheses that ammonite shell geometry generates high locomotor stability (Raup 1967) and strength against hydrostatic pressure (Daniel et al 1997), to determine whether morphological disparity among labrid fishes facilitates disparity in function (Hulsey and Wainwright 2002), to identify developmental constraints in the body shape of sea urchins (Ellers 1993), and to investigate the effects of environmental change on the macroevolution of vascular plants (Niklas 1997). The models used to generate performance landscapes may come in the form of simple algebraic equations (e.g., Moore and Ellers 1993) or elaborate computational simulations (e.g., Daniel et al 1997), depending on the complexity of the functional system.…”

“…A model's predictions may vary tremendously depending on the assumptions used by the investigator. For example, both Daniel et al (1997) and Hassan et al (2002) created sophisticated finite-element models of the shells of extinct ammonites to test how septal complexity affects shell strength. Daniel from related extant species, it is difficult to evaluate which investigators more accurately replicated the biomechanics of ammonoid shells.…”

Diversity dynamics of ammonoids during the latest Bajocian and Bathonian (Middle Jurassic) in the epicratonic Polish Basin

“…The presence of prosopid crabs associated with small oyster buildups or brachiopod lumachelles in the uppermost Bajocian ore-bearing clays point to a shallow-water environment (Krobicki and Zatoń 2008). Although water-depth may have been one of the factors limiting the development of these oceanic forms (but see Daniel et al 1997), lower temperatures in northern areas as compared to those in Tethyan regions may have been a more likely ecological barrier for their further development (Fernández-López and Gómez 2004). The temperatures estimated on the basis of stable isotopes derived from belemnite rostra were rather low, ranging from 6 to 10°C during the latest Bajocian Bomfordi Chron in the Polish Jura sector of the Polish Basin (see Wierzbowski and Joachimski 2007: fig.…”