VOL. 32 - N° 1, 2013 Revue De Paléobiologie, Genève (Juin 2013) 32 (1): 249-265 ISSN 0253-6730
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VOL. 32 - N° 1, 2013 Revue de Paléobiologie, Genève (juin 2013) 32 (1): 249-265 ISSN 0253-6730 Hydrostatics, propulsion and life-habits of the Cretaceous ammonoid Baculites Gerd WESTERMANN1 Abstract Experiments with test models support the computer models of orthoconic nautiloids of Westermann (1977). (1) Thin-walled shells with long body-chambers and empty phragmocones (bipartite hydrostatic system) were neutrally buoyant with stable vertical orientation; variants with different wall thickness and body-chamber shape changed the proportions. The test models resemble the real preadult and most adult Baculitidae and support the common assumption that baculites were planktic vertical migrants. (2) The test models also show that flooding of most adapical chambers of thin-walled orthocones (tripartite hydrostatic system) could achieve neutral equilibrium; the body-chamber was reduced (brevidomic) but the unstable orientation permitted horizontal swimming. Scale models of adult Baculites from the North American Western Interior seaway replicate this brevidomic computer model with unstable orientation. Modified to represent the gentle, apex-up curvature of B. grandis, the model turns venter-up; but the added buoyancy of a few adapical chambers (quadripartite hydrostatic system) keeps the model in stable life position. I propose that the curvature functioned to stabilize the adult shell against rolling and pitching during forward swimming. Preadult and most adult baculitids were bipartite megaplankters with quasi-vertical posture and probably long-range vertical migrants, whereas adult B. grandis and other species with tri- or quadripartite shells were probably nektonic with relatively efficient forward-swimming potential; backward swimming was impossible. It appears that all species and growth stages were planktivorous vertical migrants in the middle to basal water column, preying on planktic and demersal microfauna above the seafloor and around methane-seeping seamounts that protruded above bottom dysoxia. This feeding habit extending normal vertical migration is here termed epidemersal. Forward swimming by the jet propulsion of a conventional hyponome curved backwards below the aperture would have been impeded by the long ventral rostrum. Therefore, Hermann Schmidt’s (1930) proposition of a double hyponome is revived with modification and named the twin-nozzle hyponome hypothesis. Keywords Ammonoids, orthocones, hydrostatics, functional morphology, hyponome, Baculites. INTRODUCTION these experimentally in planispiral and heteromorphic ammonoids was Trueman (1941). Most ammonitologists Amateur and specialist collectors alike have always been accepted his interpretations and viewed the Cretaceous fascinated with heteromorph ammonites because of their orthoconic Baculitidae as vertically mobile plankters (e. unusual, often bizarre shapes that were difficult to explain g. Reyment, 1973; Ward, 1976; Batt, 1989; Westermann, in terms of functional morphology and habitat. For 1990; Westermann & Tsujita, 1999; Kruta et al., 2011). example, the seemingly irregularly grown vermeticones Some authors conjectured a horizontal posture without were confidently interpreted as worm-like benthos, but considering the required shell proportions (Fig. 1). even they proved to have been neutrally buoyant (Ward Klinger (1981) based his argument for horizontal posture & Westermann, 1977). Like planispirals, heteromorphs on the combination of compressed section, long ventral presumably were adapted to multiple functions and rostrum and large size of some Baculites Lamarck, different life-habits: housing the body and protecting it 1799. Hauschke et al. (2011), on the basis of a single against predators; withstanding ambient water pressure; case of a gooseneck barnacle attached to a Sciponoceras ‘streamlining’ for a variety of locomotions, e. g. vertical shell, concluded that baculitids swam horizontally. This migration of planktic juveniles and adults, horizontal dubious interpretation was accepted by later authors swimming and sluggish lateral motion in some nektonic (Klug et al., 2012), although the long body-chamber species, and to remain stationary at the feeding ground in indicated a vertical posture. The same type of cirripedes currents. Jaws and radulae are found fossilized indicating have been described abundantly colonizing the vertically feeding habits and food source, but nothing is known oriented flanks of contemporary planispiral ammonites, of the arms or the mantle and hyponome required for but never from baculitids (Henderson et al., 2002); they locomotion. occur as epibionts on shell surfaces of all orientations The basic features related to the life-habits of (Ifrim et al., 2011). ectocochliate cephalopods are neutral buoyancy and Among ancient cephalopods were almost perfect orientation in their marine habitats. The first to examine analogues to the baculitids, i.e. the Ordovician-Triassic 1 144 Secord Lane, Burlington ON, Canada L7L 2H7. Email: [email protected] 250 G. WESTERMANN “the implications of this possibility on current concepts of floating position and buoyancy control in ammonites need not be stressed!”, but “for the present, a horizontal swimming position for orthoconic ammonites must be viewed with extreme caution”. His alternative model for Fig. 1: The basic units and terms of orthocones, with brevidomic Baculites, the phragmocone empty of water, phragmocone units 1 and 2 for tripartites. Dimensions is depicted lying flat on the water surface (an error to be are relative to total length (L) and volume (Vt) in corrected below). Following his presentation, I rejected percentages (%); R, mean radius; Tw/R, relative wall his application of my hypothetical flooded-chambers thickness. ‘Orthoceras’ model to ammonites, because it was not possible to apply it even to orthocerids; baculitids with the required proportions were also unknown; and the liquid orthocerids (which astoundingly recurred briefly in the transport rates known from Nautilus would not allow middle Cretaceous – an amazing longevity of a simple the required re-flooding (Westermann, 1990). Thirtyfive ‘bauplan’; Doguzhaeva, 1994). Some had the same years later, I not only vindicate Klinger’s application slender cones with ‘empty’ chambers and presumably of my old flooded-chambers model to ammonites, but similar planktic life habits, confined to vertical support it with calculations and experiments. excursions for catching micro-organisms, possibly with In the early nineties, while examining the collections plankton-net or umbrella-net arm crowns (Westermann, at the Royal Tyrrell Museum of Alberta, I noticed 1996, fig. 5; Westermann & Tsujita, 1999, fig. 20.9). In that the body-chambers of some giant Baculites were contrast to ammonoids, most ortho-longiconic nautiloids exceptionally short, accounting for only 20-25% of balanced the body-chamber much more efficiently with total shell length (brevidomes) – much shorter than in calcareous cameral and/or endosiphuncular deposits in previously measured species from the North American the apical regions of their phragmocones, especially on West coast (Ward, 1976) that at the time were believed the ventral side, producing anti-rolling stability (Fig. 2c). typical for the genus. Simple calculations showed that As concluded long ago (Schmidt, 1930), these nektonic the large phragmocone would have produced buoyancy nautiloids were limited to forward swimming. in great excess if ‘empty’ (water-free). Drawing from my Vertical posture of Baculites was confirmed experimen- earlier work on the hydrostatics of Silurian Orthocerida tally with test models in my laboratory by Ward (1976); from Bohemia and their computer modeling (Westermann, all models were neutrally buoyant when an insignificant 1977), I determined that these Baculites brevidomes fit part (4%) of the phragmocone was represented as ‘flood- the hypothetical model with flooded adapical chambers ed’. All species collected by him from the North Ameri- producing neutral equilibrium. This raised the possibility can West Coast supposedly had body-chambers of about that brevidomic baculitids living in the shallow waters 1/3 of the total shell length (mesodomes) and about twice of the Cretaceous Western Interior Seaway possessed the the volume of the phragmocone. Concurrently investigat- ability to swim horizontally (Westermann, 1996; Tsujita ing the hydrostatics of nautiloid orthocones, I designed & Westermann, 1998; Westermann & Tsujita, 1999, figs. computer models, here called ‘Orthoceras’ computer 21, 18). [Note: in these papers, body-chamber length was models that focused on the basic bauplan (architectural based on ventral length; dorsal length used here is 15- pattern) with neutral equilibrium produced by calcareous 20% less.] Tsujita & Westermann (1998) placed these deposits (Fig. 2; Westermann, 1977). As an alternative, brevidomic Upper Campanian – Lower Maastrichtian I then added an entirely hypothetical computer model brevidomic species with flooded-chambers equilibrium in which flooding of most chambers provided the only in the Baculites compressus group: B. compressus Say, counterweight balancing the body-chamber – possible 1820, B. cuneatus Cobban, 1962 and B. reesidei Elias, only if the body-chamber decreased its weight by reduc- 1933; added here is B. grandis Hall & Meek, 1856. ing wall thickness (Fig. 2b). There were no known ex- Similar brevidomic shells occur in the Campanian of the amples at that time, however, of orthocerids possessing Belle Fourche