VOL. 32 - N° 1, 2013 Revue De Paléobiologie, Genève (Juin 2013) 32 (1): 249-265 ISSN 0253-6730

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 area, South Dakota, in one of the richest the required shell proportions (brevidomes). mono-specific ammonite assemblages known with The first investigator to consider the extension masses of the as yet unnamed ‘Baculites sp. (smooth)’ of this flooded-chambers ‘Orthoceras’ model for Gill & Cobban, 1966 (Larson, 2012). orthoconic ammonoids was the South African Baculites The propulsion of baculitids, like that of all or most expert Klinger (1981). At the 1979 “Ammonoids” ammonoids, has almost universally been considered to symposium of the Systematic Association in York, resemble that of all living cephalopods: by the water jet U.K., he suggested that my flooded-chambers model exiting from a single hyponome. A remarkable exception for ‘Orthoceras’ brevidomes might be applicable to – but as good as forgotten – was the supposition of Herman orthoconic ammonites, in particular the Cretaceous Schmidt (1930, p. 202, fig. 8b; fide Helmut Keupp, Baculites. But he presented neither examples nor data on pers. com.) that in ammonoids with ventral rostrum the shell dimension and proportions, while pointing out that hyponome was probably divided, placing the force of the Hydrostatics, propulsion and life-habits of the Cretaceous ammonoid Baculites 251

BCL & BCV BCL & BCV centre of buoyancy A A’ body gas body 42% 80% 45% 85%

liquid centre of mass B B’

14% 36% 23% 54% liquid

liquid C C’ 24% 56% 30% 60%

CaCO3

Fig. 2: Computer models of 3 hydrostatic systems of Paleozoic ‘Orthoceras’ with neutral buoyancy and thin-walled phragmocones (Tw/R = 0.04), but different body-chamber walls. Left column: thickness growth remains isometric; right column: thickness growth stops, i. e. Tw/R reduces (all modified from Westermann, 1977, figs. 17, 18). A, bipartite with phragmocone ‘empty’, but last chamber flooded during growth: no significant difference in body-chamber length. B, hypothetical, tripartite ‘flooded- chambers’ model featuring brevidomic Baculites: body-chamber length (BCL) increases greatly while Tw/R reduces, and the last chamber is empty at maturity. C, tripartite models of orthoceratids with calcareous deposits; this efficient system has large body-chambers, especially when the wall thins out. Note: the body-chamber wall probably thinned throughout growth, which would make the left-side models entirely hypothetical. jet closer to the center of the shell preventing rotation based on synchrotron x-ray microtomography of the during backward swimming. But, unfortunately, his buccal complex and food remains in a Baculites from chosen, illustrated example was a ceratite with ‘simple’ South Dakota (Kruta et al., 2011). The splendidly aperture. Forwards swimming of Mesozoic ammonoids preserved radula was delicate and chitinous, useful for was rarely, if ever, discussed and ‘streamlined’ predators consumption of microfauna and soft-bodied organisms were supposedly pursuing their prey backwards with only. Probable food remains in the buccal mass consisted extended tentacles (Westermann & Tsujta, 1996). of a gastropod larva and unidentified isopods, juvenile Possibly the only other author supporting or even and adult. Living isopods include planktic juveniles, discussing this double-hyponome (Funnel, Trichter) but their adults are demersal, emerging diurnally from hypothesis was Frentzen (1937; fide Helmut Keupp, the substrate to feed on epibenthos. Kruta et al. (2011; pers. com.) in his description of Amaltheus margaritatus Kruta, pers. com.) conclude that this Baculites [here (Montfort, 1808), another brevidomic, Lower Jurassic identified with ‘B. sp. (smooth)’] was a vertically oxycone with long ventral rostrum. His arguments for a mobile migrant that fed on the plankton throughout the double hyponome and its function preventing rotation, water-column of the shallow Western Interior Seaway. presumably also during forward swimming, mirror those Earlier reconstruction by Westermann & Tsujita (1999, of Schmidt (1930). Not surprisingly, this poorly formed fig. 21.18) showed the juveniles of similar species (B. speculation of two hyponomes or a divided hyponome gr. compressus) with closely similar planktic life-habit, with two nozzles has not been mentioned again (cf. but the adults as nekton in the water-column. Kruta et Keupp & Weitschat, 2000). al. (2011) may have over-generalized their proposition Slender orthocones like baculitids, especially when that all aptychophora were planktivorous in mid-water. curved, are exceptional because their shape obviously Yet, the most common food remains of ammonites were prevents backward swimming and their long ventral demersal crinoids, planktic or pseudoplanktic ostracodes rostrum is in the way of a backwards bent hyponome. and minute ammonites, including ammonitellas from Nevertheless, Klug et al. (2012, fig. 12) illustrate egg masses floating freely in gelatinous masses at mid- two forward swimming baculitids, both with single water and the hatchlings (Westermann & Tsujita, 1999). hyponome, but in one it bends over the long rostrum, in Isotope analysis by Landman et al. (2012a) identified a the other it projects through the arms and over the dorsal surprising, previously unknown habitat for Baculites, i. e. lip; both are improbable. methane-seeping seamounts of the U.S. Western Interior Recently, progress has been made on several fronts. seaway. The feeding habits of Baculites have been postulated, Soft-parts also have become better known. Larson (2012, 252 G. Westermann

figs. 8, 10) illustrated large ventral and dorsal muscle scars mass coincide, so that the animal reaches the state of from ‘Baculites sp. (smooth)’ and interpreted the ventral neutral equilibrium losing static stability. To determine ones as indicators of intensive body motion relating to volumes for buoyancy was simple, but calculation of swimming, presumably resembling the ‘piston-pump’ of neutral equilibrium for the different tripartite hydrostatic Nautilus. Klug et al. (2012) described an amazing variety systems of Paleozoic orthoconic nautiloids proved to of uniquely preserved, black remnants of apparent soft- be complex (cf. differential equations solved without tissues in unidentified, small “baculitids”. Their Late modern software or special programs by Heinig & Cenomanian age and illustrated reconstructions clearly Behrens in Westermann, 1977, appendix). I therefore indicate that they belong to the Baculites-ancestor depend on the theoretical ‘Orthoceras’ models and the Sciponocerasn Hyatt, 1894. The richly illustrated radulae new aquarium experiments with test models. closely resemble those of Baculites (Kruta et al., 2011). Three types of shell bauplan, which would have produce The black remnants included large, paired structures major differences in the animal’s life-habits, are here that were only tentatively interpreted in the Conclusions proposed, called Hydrostatic Systems. They were either as “eye capsules (?)”. They nevertheless stated that the constant throughout life (holoplanktic) or changed large “ocular sinuses” (their term for the lateral sinuses) (meroplanktic). harbored large eyes that adapted the baculites to deep 1. The Bipartite Hydrostatic System was the basic water. The illustrations of Klug et al. (2012, fig. 6) show bauplan: empty (liquid-free) phragmocone + body- the long (mesodomic to longidomic) body-chamber chamber with body (Fig. 2a). Neutral buoyancy resembling my earlier reconstruction (Westermann, was maintained by equilibrating the body-chamber/ 1996, fig. 5) and as typical for vertical orientation, but phragmocone proportions with shell thickness. The their reconstructions (fig. 12) show them as horizontal viable combinations of wall thickness (Tw/R) and swimmers, based mainly on the dubious evidence from a body-chamber length (BCL) for isometric growth single case of a barnacle attached to a Sciponoceras shell extended to the common body-chamber modification (Hauschke et al., 2011). Even Baculites of this small size by shell thinning (Tw/R negative). In this modification would have ‘empty’ phragmocones and vertical posture; nacre growth was retarded relative to aperture growth, their planktic habitat permitting extensive passive reducing weight and permitting chamber lengthening. dispersal, including the crossing of oceans (Tsujita & As in the living Nautilus (Westermann, 1971), this Westermann, 1998). growth phenomenon probably applied to the entire The purpose of this study is fourfold: growth sequence of an individual, not only the adult. 1. To analyze the morphology and architecture of Starting at the beginning of the body-chamber, the Baculitidae in hydrostatic terms. wall ceased to grow in thickness and finally thinned 2. To demonstrate experimentally the validity of a at the aperture. Another common, but uniquely adult hypothetical computer model, the ‘Orthoceras’ modification, was the reduction in the taper of the model, developed 35 years ago (Westermann, 1977) body-chamber, which may have allowed the addition that uses only flooded chambers to produce neutral of another 10-20% of growth (Ward, 1976). equilibrium, making horizontal swimming possible. 2. Tripartite Hydrostatic System (Fig. 2b): adapical 3. To interpret Baculites life-habits and habitats based chambers of the phragmocone were re-flooded as on hydrostatic and lithofacies. ballast to balance the body-chamber, thus achieving 4. To discuss the morphology and role of the hyponome neutral equilibrium. in propulsion. 3. Quadripartite Hydrostatic System: in this entirely hypothetical modification of the last system, a fourth hydrostatic unit was added by re-emptying PARADIGMS, MATERIALS & EXPERIMENTS the most apical chambers of the dorsally concave phragmocone, creating a modest second buoyant Hydrostatic paradigms force that functioned for stabilizing the animal in the The principle types of architecture (bauplan) of the venter-down position and prevented rolling. orthoconic Baculitidae shell are slender cones divided longitudinally into two or three, possibly even four, Materials alternately positively and negatively buoyant hydrostatic Dimensions and parameters (Fig. 1). Lengths and unites (Figs. 1, 2). When the sum of the units equals zero, volumes of body-chamber and phragmocone, relative the organism is neutrally buoyant; i.e. the submerged to the entire shell are standardized in terms of body- weight of an object must equal the weight of the chamber length (BCL/L). The three major types with displaced water. If the animal is to potentially attain a functional distinction are: horizontal posture for swimming, a large apical part of brevidomes < 30 %; the phragmocone has to become the ‘counterweight’ to mesodomes = 30-40 %; the body-chamber with body. The proportions of the three longidomes > 40 %. units are critical in order for the centers of buoyancy and The models are designed for isometric phragmocone Hydrostatics, propulsion and life-habits of the Cretaceous ammonoid Baculites 253

growth, although in reality it may be anisometric. Body- phragmocone and negative buoyancy (weight) for the chamber growth patterns are commonly reduced in cone body-chamber. At neutral buoyancy, their sum becomes expansion and wall thickness and are treated here in zero. isometric and anisometric variants. However, in light of the considerable ontogenetic The overall densities of phragmocone (and constituent variations of the shell and the unknown parameters parts) (PHp) and body-chamber (BCp) used in the for cameral fluids and soft-body, a few percentages of experiments are primarily the products of the relative volume or density differences are insignificant. wall thickness (Tw/R) and nacre density (2.65). Densities: PHp = 2 Tw/R [rel. wall volume] x 2.65 [nacre Additional components for PHp are mainly the septa, density] x 1.1 [septa etc.] [e. g., if Tw/R = but also poorly preserved organic tissues and unknown 0.04; PHp = 0.08 x 2.65 x 1.1 = 0.23] quantities of cameral water. Based on measurements BCp = 2 Tw/R x 2.65 + 1.04 [BC fill] [e. g., of septum and wall areas in longitudinal sections of if Tw/R = 0.04; BCp = 0.08 x 2.65 + 1.04 = Paleozoic ammonoids, Saunders & Shapiro (1986) 1.025] calculated the relative volumes of septa to wall, by which Volumes: PHV = PHL3 wall volume has to be multiplied to arrive at total relative BCV = BCL3 – PHL3 shell volume. This ‘septal factor’ (SF) ranges from 1.05 Neutral buoyancy (bipartites): PHV x PHp = BCV x BCp to 1.17 with a mean of 1.09, which they applied to all Taper Index : TI = 100 (2 R1 –2 R2) / LX , where R = (DW taxa. However, this factor excludes the siphuncle and the + DH) / 4 potentially significant (but unknown) volumes of cameral membranes and, especially, cameral water. I use a value of Fossil material. There are two species: Baculites grandis SF = 1.1 in my experiments, but considering that they are and ‘B. sp. (smooth)’. made in freshwater, the value in saltwater would increase 1. Baculites grandis my SF to 1.125, which would allow for, e. g., cameral a. An approximately 155 cm long composite specimen water needed during growth and potentially trapped in from South Dakota, which is on display in the folioles. Ammonoid septa thin out radially compensating American Museum of Ancient Life in Lehi, S. Dakota for the surface increase by intensive ‘fluting’ (lobes and (see below). saddles, lobules and folioles, etc.), so that their total mass b. The dimensions required for modeling the Baculites approximates a cross-sectional plate with the thickness models were available from the closely similar, of the septum center (Westermann, 1975a). Therefore also reconstructed specimen BHI-5058 from the the more direct method to determine the constituent Pierre Shale of the Black Hills in Weston Co., of the septa to overall density formerly applied to the Wyoming, housed in the Black Hills Institute of spherically curved septa of orthoconic nautiloids can be Geological Research in Hill City, South Dakota. used for ammonoids, simply by measuring the ratio of The measurements were made by Neal Larson. Most central shell thickness to spacing. Including the cameral specimens were body-chambers with incomplete water required during growth of each new septum, the phragmocones and required reconstruction. In order optimum thickness/spacing ratio of 1:30 was calculated; to determine the lengths of the missing septate it is indeed commonly present in the fossils (Westermann, portions, specimens and fragments of different sizes 1975b). The optimal “septation density” (septa + empty from the same assemblage were measured and the camerae) is therefore 2.65 [aragonite density]/30 = 0.09 taper determined for the different size classes of or SF = 1.0, i. e. the mean value of Saunders & Shapiro the phragmocone. The dimensions and parameters (1986). of this brevidome are listed in Table 1. Note that Additional constituents of BCp are the soft-body with phragmocone wall thickness is the same as in the a relatively heavy buccal mass bearing an aptychus ‘Orthoceras’ models, but with the adapical half of the (Kruta et al., 1910) and a density of about 1.065, i. e. phragmocone (possibly due to post-mortem collision or 4% higher than seawater (Ward, 1988). The body with the substrate). filled perhaps two-thirds of the body-chamber lumen 2. ‘Baculites sp. (smooth)’. A large collection from the and the remainder was seawater. To compensate for the Lower Campanian ‘B. sp. (smooth) Range Zone’ of the reduced uplift of freshwater, a density of 1.04 is used Pierre Shale near Belle Fourche, South Dakota (Gill & her for body-chamber fill. Volume calculations are based Cobban, 1966), made by Larson (2012, table 1; note: the on simple equations for cones and truncated cones. The BCL used here ends at the dorsal lip, not the lateral sinus) taper of a shell segment is measured by the Taper Index appears to belong to a single species. Juveniles have not (TI) (Matsumoto & Obata, 1962), which is the increase been found here, indicating that this deposit is an event of the mean diameter per segment length (LX) times 100. horizon (Tsujita & Westermann, 1998). The 11 measured The cone angle or vertex (A) is its tangent. specimens vary greatly in size and probably include male The test models are submerged in a freshwater aquarium. and female shells, but dimorphism cannot be proven due Deduction of freshwater density (p = 1) from PHp to substantial size overlap. All are brevidomes (Table 1) and BCp results in positive buoyancy (uplift) for the and have body-chambers with negligible degree of taper, 254 G. Westermann

Table 1: Dimensions and parameters for bipartites

L cm PH-Tw/R PHp BCp BCL/L BCV/Vt

Fossils B. grandis [BHI 5058] 155 0.04 0.25 1.19→1.23 22% 53% B. sp. (smooth), N=11 55-100 c. 0.04 c. 0.25 c. 1.20 c. 21-25% c. 51-59%

Test models Fig. 4, left 15.5 0.09 0.50 1.50 23% 50% Fig. 4, right 23.5 0.04 0.25 1.22 45% 82% Fig. 5a 20.0 0.04 0.25 1.25 22% 48%

establishing adulthood. The taper index (TI) reduces in 2. The Baculites test models are slender cones of 25 cm larger phragmocones and approaches zero in all body- length and 2 cm base, comprising an apex angle of 5°, chambers. The volume change caused by the reducing approaching the dimensions of the fossil material. taper would have permitted additional growth of 15-20% in these extremely slender cones. Bipartite hydrostatic system. ‘Orthoceras’ models – Two models (Fig. 4, Table 1) test my computer models Experiments (Westermann, 1977, figs. 17, 18). Test models. The basic components of the test models 1. Extremely ’thick-shelled’ (high density) and are plexiglass (density 1.19) and styrofoam (density brevidomic, with equal volumes of body-chamber ca. 0.01). The densities of the model units representing and phragmocone (50%) (Fig. 4 left). phragmocones (PHp) and body-chambers (BCp) are produced by adding internal weights, small pieces of iron symmetrically distributed. The models have been designed to replicate the different structural units required by orthocones to achieve neutral buoyancy in bipartite systems and, in addition, neutral equilibrium in tripartite systems (Westermann, 1977). The test models are proportioned according to different specifications. Some are designed to test the theoretical limits of neutrally buoyant cephalopods with orthoconic shells using ‘realistic’ taper and shell thickness (Figs. 2b, 3-4). The main aim is to produce differently shaped models to test the validity of the ‘flooded-chambers’ computer model for brevidomic Baculites, i. e. does the flooding of adapical chambers have the potential to produce neutral equilibrium? All test models are immersed in a small freshwater aquarium – and surprisingly close to the required buoyancy and equilibrium conditions. ‘Fine- tuning’ of the test models involves trimming thin slices from the adapical end of the ‘body-chamber’ and from the ‘(empty) phragmocone’. Two basic types of test models are produced, the ‘Orthoceras’ models resembling the original illustrations (Westermann, 1977, figs. 17b, 18b) of the computer Fig. 3: Comparison of the tripartite, theoretical ‘Orthoceras’ models for testing the theoretical models (note that cross- models with the brevidomic Baculites grandis: a, section shape does not matter for experiments with vertical ‘Orthoceras’ model with isometric growth and last chamber still flooded: body-chamber is extremely short; orientations), and the Baculites models resembling the b, more ‘realistic’ model with thinning body-chamber fossils for testing the hydrostatic properties of Baculites. wall and last chamber empty: body-chamber is much 1. The ‘Orthoceras’ test models are pyramids of ca. longer; c, B. grandis with proportions intermediate 20 cm length and 4 cm base, comprising an apex between a and b: body-chamber thinning is reversed at angle (nexus) of 13°. aperture. Hydrostatics, propulsion and life-habits of the Cretaceous ammonoid Baculites 255

2. Moderately ‘thin-shelled’ (moderate density) 1. ‘Orthoceras’ models – When the bipartite, brevidomic and longidomic, with body-chamber four times test model 3 above is modified by re-flooding 81% of phragmocone volume (80%) (Fig. 4 right). phragmocone length, equaling 52% of its volume, the Both are neutrally buoyant, in stable vertical now tripartite model (Fig. 5b) becomes neutrally buoy- positions and plot near the opposite ends of the curve ancy and is in neutral equilibrium. Significantly, the of potential density/volume proportions (Fig. 4). flooded portion of the phragmocone is 8% longer than 3. A third model, representing a hypothetical brevidome the emergent part of the bipartite model, and approxi- with empty phragmocone and thin shell (cf. Figs. mates the 81% featured in the hypothetical ‘Orthoceras’ 3c, 5b), floats with 75% of phragmocone length and model (Figs. 2B right). Furthermore, the volume ratio of 42% of volume emerging from the water (Fig. 5a) – a flooded chambers to emergent chambers approximates most improbable habit for a cephalopod. This model 1,25, i. e. the ratio of in-air -weight/buoyancy of the plots far away from the line of neutral buoyancy (Fig. phragmocone. 4). But it fits the tripartite hydrostatic system, as 2. Baculites models – Consistent with the morphology explained below. of the B. grandis BHI-5058, the three models feature the same body-chamber length (BCL/L) and reduction Tripartite and quadripartite hydrostatic systems. of body-chamber taper. They test the effects of shell Two major types of models were cast (see above) in the ‘curvature’ (modeled by the hinge in the flooded proportions of Baculites grandis (Figs. 3-6): phragmocone that permitted the apex to tilt slightly

Fig. 4: Curve showing the theoretical shell proportions of bipartite Baculites shells, confined by neutral buoyancy balancing weight and uplift. Relative body-chamber length and volume are plotted against wall thickness and phragmocone density. Light-grey indicates the estimated distribution of entirely isometric hypothetical shells; dark-grey, the estimated distribution of ‘realistic’ shells with the body-chamber reducing in relative wall thickness (Tw/R) throughout growth. This resulted in a 20-30% reduction in body-chamber density, so that it could grow longer. Two extreme ‘Orthoceras’ models are: left, equal volumes of body-chamber and phragmocone; right, 4 times higher body-chamber volume. * is for the quadripartite B. grandis (cf. Fig. 6a); o…..o is for the North Pacific B. inornatus (Ward, 1996, table 7.3), with thick-walled phragmocone followed by thin-walled body-chamber. . 256 G. Westermann

c. Quadripartite ‘curved’ with ‘empty’ adapical chambers comprising less than 2% of shell volume: stable in the natural, dorsum-up orientation.

Results. The aquarium experiments with test models made of plexiglass and styrofoam and weighted to realistic densities, affirm the correctness of my computer models designed some time ago for Paleozoic longiconic nautiloids, the ‘Orthoceras’ models (Westermann, 1977). 1. The bipartite ’Orthoceras’ test models are neutrally buoyant and in stable vertical position. 2. The tripartite ‘Orthoceras’ and Baculites test models with B. grandis proportions: a. Straight: neutrally buoyant and in neutral equilibrium; the animal could attain horizontal posture effortlessly but was unstable against rolling. b. Gently ‘curved’: neutrally buoyant, but the center of mass now lies above the center of buoyancy: becoming stable in ‘upside-down’ position (ventral rostrum above). 3. The quadripartite Baculites model with empty tilted-up apex (to represent the curvature of B. grandis), is stable in the natural, venter-down position. (This fourth unit is entirely hypothetical.)

DISCUSSION

Baculitidae shell architecture and functions Orthocerida analogies. The diverse calcareous deposits employed by Paleozoic orthocerids for ballast were mentioned in the Introduction. Many orthocerids, however, lacked these deposits and lost them entirely in the Mesozoic (Doguzhaeva, 1994), thus resembling the baculitids in architecture and orientation. Although data on the relative length of the body-chamber are missing, at least the majority of Mesozoic orthoconic nautiloids had bipartite shells (without deposits or flooded chambers) and vertical posture. They closely resembled the vertical Fig. 5: Two ‘Orthoceras’ models of Baculites grandis with migrants among the baculitid plankters. same body-chamber length in aquarium: a, the bipartite model with empty ‘phragmocone’ (in white) floats with most of it emerging above the water (photo from Shell structure and life-habits. The evolution of the above water surface); b, the tripartite model, with most Cretaceous heteromorph ammonoids remains poorly ‘chambers flooded’ (cf. Fig. 2), is submerged in the understood in terms of functional morphology. Why state of neutral equilibrium and neutral buoyancy. Note did they evolve from planispiral ammonites, thereby that the ’flooded part’ approximates the emerged part of sacrificing the compactness and the protection of the the bipartite pattern (photo through aquarium wall). fragile juvenile shell? Many gained shell strength against ambient pressure with their circular cross-sections, but upwards) and the stabilizing effect of empty apical their habitats were probably limited to the plankton. chambers: Among the heteromorphs were the hamiticones, probable a. Tripartite straight (‘ortho-longicone’) (Fig. 6c): in ancestors to the Baculitidae. Their parallel shafts with neutral equilibrium, but rolling in an entirely unstable upwards-directed aperture indicate that they were mid- state water vertical migrants (Westermann & Tsujita, 1999, fig. b. Tripartite ‘curved’ (‘curvi-longicone’) (Fig. 6d): 21.14 A). Exceptional longevity, possibly even briefly turning upside-down, ventral rostrum above. (The surviving the Cretaceous/Tertiary Boundary (Landman raised ‘heavy’ apex positions the center of mass et al., 2012b) and global distribution of the Baculitidae below the center of buoyancy). (Klinger & Kennedy, 2001) substantiate the adaptive Hydrostatics, propulsion and life-habits of the Cretaceous ammonoid Baculites 257

PH L 78% V 47% PH L 22% V 53%

L 20% V 1% L 44% V 26% L 14% V 20% CB Tw/R 0.04

Fig. 6: Baculites grandis: a, display specimen at the American Museum of Ancient Life, length 155 cm (“File: Baculites grandis shell”, 2009); b, design for the tripartite test models of # 5058, Black Hills Institute, with dimensions and estimated centers of mass (CM) and buoyancy (CB); c, straight, tripartite test model; d, ‘curved’ tripartite test model; e, ‘curved’ quadripartite test model with ‘re-emptied apical chambers’, submerged in aquarium closely above ammonite-strews bottom. 258 G. Westermann

advantages of the orthoconic shells of the Baculitidae. B. compressus, more compressed than would appear Survival of what appears to be the C/T- boundary bolide advantageous considering that hydrodynamic efficiency impact may be due to the unique, methane-vent habitats needed to be balanced not only against the decreasing of several Maastrichtian species, i. e. resembling the shell volume to weight ratio, but also the diminishing survival of Nautilus, they did not depend on the plankton, strength of the phragmocone to withstand ambient which was eliminated by the impact (Landman et al., pressure (Jacobs, 1990)? A plausible answer can be given 2012a). for horizontal swimmers with straight shells (tripartites) In order to understand baculitid life-habits, the most that lacked the apical buoyancy unit of the curved significant structural features of the shell are interpreted quadripartites: they gained stability against rolling, with regard to their probable functions. Our knowledge pitching and yawing by the opposing forces of the of ammonoid behavior in general will remain tentative, massive ventral rostrum and the proposed gas bubbles in however, without considering the propulsion mechanism, dorsal folioles and lobules. The static righting moment especially the hyponome (last chapter). Let me put this in (torque) would have increased with distance from the the format of presumed fact followed by question and cone axis, i. e. the relative height of the cross-section. answer. This explanation again concerns quasi-demersal, i.e. The Baculitidae family includes the cosmopolitan, epidemersal (new term, explained below) habits – any mid- to late Cretaceous lineage Lechites Novak, 1908 – possible advantage for vertical orientations would appear Sciponoceras [or a parallel development?] – Baculites; all to have been insignificant. orthoconic to slightly curved. Did their long, thin shells Both circular and elliptical cross-sections were present give them any functional advantage over planispiral, in Sciponoceras, direct ancestor of Baculites. Its especially involute discoconic or oxyconic shells? What species were small, with pronounced ribbing and deep immediately comes to mind is hydrodynamic efficiency constrictions producing high drag, and commonly with (Klinger, 1981). The common, mesodomic baculites adult apertures that were modified into heavily collared with vertical posture, including all juveniles, would or even hood-like structures. These features would have have gained hydrodynamic efficiency for the vertical been defenses against predation. Long body-chambers excursions they employed to actively trap mainly (Westermann, 1996, fig. 5) and delicate radulae (Klug microfauna with their ‘plankton-nets’. (Slight negative et al., 2012), global distribution and independence from buoyancy causes sinking, jet action is needed for rising; lithofacies suggest a poorly mobile planktite, whose Ward, pers. com.) But they lost the efficient circular cross-section had no significant function. The prominent cross-section of the hamiticones. sculpture (ribs and constrictions) of these early The earliest (Albian) baculitids included some species Baculitidae contrasts with the low-profile ribs of most with laterally compressed (oval or elliptical) shells, later baculitids and would have affected swimming only while others retained the circular cross-sections of their slightly, while strengthening the flattish flanks against holoplanktic ancestors. The moderate lateral compression the point load of carnivore teeth (not against hydrostatic of many Lechites and especially Turonian Sciponoceras load) (Westermann, 1996). For comparison and contrast species would have restricted the animal to shallower to the largely planktic, widely dispersed and lithofacies- water than the species with circular sections or required independent baculitids, the mid-Jurassic Spiroceratidae more advanced septal support. The Maastrichtian Hyatt, 1900 are of interest. Their occurrence is localized Fresvillia Kennedy, 1986 and Trachybaculites Cobban in still-water facies and single species may comprise & Kennedy, 1995, on the other hand, all had circular cyrtocones of varying curvature as well as torticones. sections (Ifrim, pers. com.) indicating that they were Their probable habitat was peudo-planktic in algal mats, vertically mobile plankters. in which they would have fed on the small fry attached to What would have been the advantage of laterally the seaweeds (Dietl, 1978). compressed shells, which are weaker against ambient The cross-sections of some baculitid species vary pressure and less space-efficient than circular ones? from elliptical to oval, but there would have been no The answer may lie again in hydrodynamics. Lateral functional advantage of one over the other in immobile compression would improve any sideways mobility of plankters. And why did some species have consistently sub-vertically oriented baculites during feeding from elliptical and others oval cross-sections (beside other, the seafloor (Westermann, 1990) and prevent them from less common shapes)? A plausible answer is that oval being swept away by currents (Jacobs & Chamberlain, (tear-drop) shapes produced lower drag than elliptical, 1996). Both explanations imply benthos-dependency, but equally compressed shapes during lateral (cross-axial) not necessarily feeding on epibenthos, but alternatively movement while foraging. scooping up demersal microfauna in the water just The early growth stages are commonly more strongly above the substrate. For the Campanian-Maastrichtian, tapered (higher apex angle) than later growth stages of swimmers, on the other hand, the difference in drag the phragmocone. Why did the ‘slenderness’ of the cone between circular and compressed shells was probably change during growth by reducing taper (apex angle) not insignificant. Why then are many species, e. g. only in the adult body-chamber, but commonly also mid- Hydrostatics, propulsion and life-habits of the Cretaceous ammonoid Baculites 259

way through the phragmocone (from ca. 2 to 3 degrees), orthoconic nautiloids with calcareous deposits .When and why were some species extremely slender? This is the last few grams of cameral water had been added at a complex question with multiple answers. (a) Adult about mid-length of the shell, the baculite gained neutral body-chambers could extend in length if they ceased equilibrium. Now it could effortlessly assume any posture expansion and thickness growth as in the most common – but had lost all stability required for foraging and/ adult modification; (b) in the always vertically oriented, or locomotion (Fig. 6c). The gentle, apex-up curvature planktic juveniles, only phragmocone volume and shell (Fig. 6d) would only turn the animal upside-down unless thickness, not shape, were the essential parameters, and the most adapical chambers were re-emptied, resulting thicker, shorter phragmocones were less fragile than in a quadripartite hydrostatic system (Fig. 6e). This not thin, long ones; (c) the brevidomic, neutral-equilibrium only provided sufficient stability against rolling, but shells of the nektonic adult swimmers, however, needed also introduced a useful stability effect on a horizontal to optimize overall buoyancy to keep a body-chamber of posture. It appears that this was the principal function of feasible size. They did this by maximizing the moment the shell curvature. (torque) produced by the flooded chambers that balanced the body-chamber, i. e. they maximized the distance Aperture shape and implication for hyponome(s) of this ‘counter-weight’ from the center of buoyancy Most ammonitologists assume that all ammonoids – resulting in extremely slender phragmocones of the could move actively by jet propulsions. But this was B. compressus group, ‘B. sp. (smooth)’ and B. grandis. certainly not true for serpenticonic to spheroconic, Baculites grandis and several other species had a gently entirely unstable longidomes, which presumably were curved, dorsally concave shell (apex-up; Larson, pers. largely passively drifting megaplankters. For limited com.). What was the advantage of curved over straight locomotion, however, they may have used modified shells? The answer regarding its possible stability arms or webbing with medusa-like movements. Because function as the fourth hydrostatic unit in the quadripartite baculitids are well ‘streamlined’, I will limit myself to jet system was given above (see Hydrostatic systems). propulsion systems. The curvature would also have obvious disadvantages. Pumping systems. The cephalopod jet-propulsion Backward escape from benthic predators would have been system consists of pump and directable spout, the more difficult because the baculite would have moved in hyponome. The low-stroke piston-pump system that large circles – on the other hand, this might have made makes the extremely brevidomic, stable Nautilus it more difficult for the predator to catch it. During the pitch (‘rock’) with each jet thrust could not have been frequent but slow ascents (by hyponome) of the juveniles employed by the common, mesodomic and unstable the effect of the curvature was presumably compensated ammonoids. Similar jet thrusts would have rotated the for by jet direction or arm positioning; during forward ammonoid, especially when, during forward swimming, swimming, the up-turn of the apex would have produced the thrust vector and the movement of the center of mass minimal drag, while preventing its collision with the (due to the withdrawing cephalic complex) coincided. substrate during near-bottom feeding. Therefore, with the possible exception of brevidomes, an ammonoid pumping system must have been entirely Bi-, tri- and quadripartite Baculites hydrostatic different. It could, for example, have been analogous systems and ontogeny. The proposed changes during to a primitive ‘heart-pump’ consisting of the rapidly shell growth of these hydrostatic systems required a contracting internal extension of the hyponome, greatly seemingly complex sequence of structural changes – yet expanded and with in- and exit-valves to the mantle even more complex morphogeneses must have occurred cavity and external hyponome, respectively. Its beat in orthoconic nautiloids with calcareous counterweights. would have been weaker but much faster than the slow While the phragmocone grew disproportionately stroke frequency (ca. 1/sec.) of Nautilus, producing less replacing posterior body-chamber space with new ‘rocking’. This would also have supplied the required chambers, aperture growth had to continue just to retain continuous ventilation of the gills, placed much further body-chamber volume and shell length increased by back from the aperture than in Nautilus. There is, possibly another third. however, no fossil record of any such system and one can The slow change from bi- to tripartite came with a only speculate until more soft-tissues are found fossilized. great energy burden, because the beneficial effect I will here discuss only the external propulsion system, of the re-orientation would not be realized for many based on limited ’circumstantial evidence’. months. During this process the animal dragged around Baculitidae peristome. The aperture (peristome) of a steadily growing phragmocone without substantially Baculites and Eubaculites is highly oblique and sinuous, increasing body size, because it had to maintain neutral with a ventral rostrum up to ¼ as long as the body- buoyancy. Allowing water (in addition to that required chamber, a broad lateral sinus on either flank and a short to compensate for the episodic chamber growth) to seep dorsal lip (Fig. 6a). The primary function of the massive back into adapical chambers, on the other hand, was a ventral rostrum was perhaps twofold: (a) to produce much simpler process than that required by Paleozoic stability against rolling when in non-vertical orientation. 260 G. Westermann

Whereas preadult rostra thinned out in thickness (Tsujita, While existing arguments for two hyponomes are poorly pers. com.) and so would have been of limited use for supported, a strongly modified version of Schmidt’s stability and defense, late-adult thickening (Ward, pers. hypothesis is presented here, based mainly on the com.) would have offered significant advantages by baculitid aperture as outlined above, i. e. the twin-nozzle the time sexual maturity arrived; (b) to protect the head hyponome hypothesis (Fig. 7) – a dual jet system in complex from benthic predators during forward feeding which each nozzle could be directed independently on near the seafloor. either side of the head, passing the peristome at the lateral But the ventral rostrum posed propulsion problems, if sinuses, and allowing forward, upward, downward and one accepts the traditionally held assumption that all sideward jetting. Neither the closed-tube hyponome of the ammonoids possessed a single tubular hyponome and coleoids nor the loosely scrolled, single-sheet hyponome nozzle as in the coleoids. The rostrum, especially when of Nautilus could have produced the long twin-nozzles bent inwards, would have prevented the hyponome required for forward-swimming ammonoids. Two long from flexing backwards for swimming forwards. But sheets at the outer end of the hyponome tube are required. swimming backwards would have been hazardous and The hyponome-type here proposed fulfills the highly improbable for baculites, especially when the requirements of both, functioning alternately as single shell was curved as in B. grandis. or paired external tube with nozzles of sufficient length The important implication to be drawn from the peristome for forward- and backward propulsion. The internal part shape of these heteromorphs with limited steerage differs of the hyponome is a coleoid-like tube; the external part significantly from the previous hypothesis that attempted is modified into a pair of long, muscular sheets, which to explain only backward swimming of planispiral forms. can be either pressed or scrolled together to form either a Schmidt (1930), followed only by Frentzen (1937), single nozzle or separate to form two nozzles by muscle proposed that some streamlined Mesozoic ammonoids contraction; water flow rate and jet direction can be with ventral rostrum had two hyponomes, one on either controlled separately by each nozzle. side of the body. Schmidt (1930, p. 202, fig. 8b) wrote [my One major advantage of the double-sheet nozzle would be translation from German and my comments brackets]: functional plasticity – only a single, modest modification “We commonly find forms, e. g. Harpoceras [i. e. a of the external part (nozzle) of a coleoid-type hyponome Lower Jurassic oxycone], with two sinuses [per side], was required. The nozzle could have been changed one for the arms and the other for the funnel; between rapidly from its single state to a twin-nozzle and back, for the right and left funnel sinus is a rostrum of variable example between the juvenile vertical-migrant and adult length. If the funnel would have run along the rostrum, horizontal-swimmer stages of Baculites gr. compressus, the jet would have rotated the animal. Since the jet force as well as between taxa and habitats. The fact that the had to be applied to the center [of rotation] of the animal, opening area of the single nozzle is twice that of the twin- the thrust must have been equal to the right and the left nozzles combined, would make the former more efficient of the rostrum.” However, Schmidt’s illustrated example in certain circumstances (e. g. pumping rate and speed, of a Ceratites Haan, 1825 has no proper rostrum; the both unknown). Perhaps the nozzle would have been hyponomes appear to originate at mid-flank from the single for backward (upwards) migration in the water mantle cavity; the ammonite would have swum steeply column and changed to its twin state during forward back-upwards. The validity of his hypothesis hinged on movement. the aperture angle, which he probably chose intuitively as very steep because the cephalic complex was in forward position, centered at the aperture. All Ceratites species a b had about ½ whorl body-chambers (Urlichs, 2009) that were highly stable with the resting angle of the aperture, about level with the centers of mass and buoyancy (Raup & Chamberlain, 1967, text-fig. 3). This would have enabled the ceratite to swim horizontally backward, even with a single hyponome. However, the aperture would have rotated downward by the forward shift of body mass to approximately the illustrated angle. Furthermore, backward-swimming may have been possible for a brevidomic ‘Harpoceras’ with a single hyponome Fig. 7: The hypothesized twin-nozzle hyponome: the internal part (cut off short) leading to the mantle cavity is a running along the ventral rostrum. It would not require cylindrical (coleoid-like) tube, the external part a pair pairing as outlined in the text (Schmidt, 1930) – the more of hemi-cylindrical sheets: a, in the single-nozzle state critical requirements for forward-swimming were not both sheets are pressed (or scrolled) together into one mentioned. It is not surprising that ammonite researchers tube; b, in the twin-nozzle state, the two sheets are generally ignored Schmidt’s hypothesis. coiled or scrolled independently. Hydrostatics, propulsion and life-habits of the Cretaceous ammonoid Baculites 261

Additional support for the twin-nozzle hyponome Tertiary boundary tentatively postulated by Landman hypothesis comes from planispiral ammonites and et al. (2012b), in contrast to the overwhelming host of assumes forward swimming during foraging and food planktivores that were starved out. capture of nektonic species. The tentative taxonomic distribution of the basic bauplans 1. The ventral rostrum commonly found in oxycones and habits is as follows. prevents a single hyponome from bending backward 1. Megaplanktic drifters (holoplanktic): small, coarse- beneath the shell. A pair of nozzles flexing over ribbed bipartites; the mid-Cretaceous Sciponoceras both flanks would solve the problem. Examples are and Lechites. Probably the eggs and hatchlings of all the Harpoceras and Amaltheus mentioned above. baculitids. Backward swimming may have been common in 2. Megaplanktic vertical migrants: planispiral, brevidomic ammonoids, but catching a. In water-column only (holoplanktic – Smooth prey with a pair of tentacles that extended beyond and with circular cross-section; the Maastrichtian the shell (Westermann, 1996, fig. 3.3.1 a) appears Fresvillia; the juveniles of some Baculites species. unlikely. According to Roger Hanson (pers. com.), b. Vertical migrants occationally extended to present-day squid usually hunt for- or backwards; but epidemersal skimming of demersal microplankton they approach and capture their prey only forward. (holoplanktic) with sluggish transverse mobility; 2. The small rostra in the ventral openings of some possibly also capable of diving rapidly in and lappet-bearing microconchs (male shell) suggest out of bottom dysoxia that lay 50-100 m below that these were the outlets of a pair of hyponomes or the surface (Westermann, 1996), and of rapidly nozzles; their allomorphs are ‘streamlined’ indicating escaping benthic predators when feeding near the a nektonic habit including forward-swimming while seafloor (Klinger, 1981; Westermann, 1990) – mostly mesodomic bipartites; probably the majority of Late foraging (Westermann, 1996, p. 674). Examples Campanian-Maastrichtian Baculites, Eubaculites and are the mid-Jurassic haploceratoid ‘Pelecodites’ Trachybaculites and all of their juveniles. Buckman, 1923 [m], allomorph Witchellia Buckmann, 3. Nektonic at mid-water and epidemersal 1889 [M]; Cadomoceras Munier-Chalmas, 1892 [m] (meroplankton); commonly curved quadripartite or and Strigoceras Buckman, 1924 [M]. This contrasts tripartite brevidomes propelled forward by twin-nozzle with the most common platyconic (‘planulate’) hyponome – large adults of the Baculites compressus ammonites, which have undivided mid-ventral group and ‘B. sp. (smooth)’ of the Western Interior openings or sinuses. Seaway. Their maximum habitat depths, based on septum strength, ranged from approximately 60 m to 100 m Behavioral patterns of baculitids (Tsujita & Westermann, 1998). While B. cuneatus had The functional explanations of the shell architectural septa that, quite exceptionally, grew stronger toward ‘designs’ listed above indicate several basic behavioral maturity (Hewitt & Westermann, 1996), most species patterns, which may have occurred sequentially in of the B. compressus group had stronger juvenile septa; individual life-cycles as well as life-long in other the young presumably descended and ascended deeper Baculitidae species. At least the great majority were than the adults. Planktivorous at mid-water to the planktic and planktivorous. The feeding apparatuses bottom-waters where they were epidemersal microvores, of baculitids, however, remain entirely hypothetical, perhaps rapidly penetrating bottom dysoxia (Tsujita & although muscular arms as common in coleoids are Westermann, 1998) or micrivorous around seamounts. improbable because ammonoid feeding crowns were too It is worth mentioning that preliminary investigations fragile to fossilize. Possible feeding structures include by Ward (1976 and pers. com.) suggest that the pelagic ‘plankton-nets’ (last chapter) or ‘umbrella-webs’. I have Pacific baculitids were mostly mesodomic, thick-shelled here chosen the former without particular reason. This plankters that presumably could dive deeper than the proposed filter-feeding habit would have been inherited neritic species of the shallow North American Western from non-orthoconic heteromorphs (hamiticones?). Interior seaway. Habits would have been either holoplanktic (life-long) or meroplanktic, i. e. vertically migrating plankters as juveniles, followed by nektonic adults foraging SUMMARY: LIFE, DEATH AND AFTERMATH OF throughout the middle and lower water column. Some BACULITES GRANDIS adults were epidemersal (nov.) foraging on demersal microfauna of the seafloor or seamounts that the baculite I am here describing the possible events surrounding the scooped up from the water after flushing it from the life, death and biostratinomy (pre-burial taphonomy) substrate, possibly by fanning with their aptychus as of B. grandis. This story is told in finite terms, but envisioned by Lehmann (1976, p. 105). It was perhaps naturally remains largely hypothetical even if based on this seafloor-dependent feeding habit that saved them the experiments and discussion given on previous pages. from extinction from the bolide impact at the Cretaceous/ 262 G. Westermann

Life history positively buoyant and rose apex-up to the surface Growing up, the baculite followed in its ancestors’ of their shallow sea (Figs. 9-1a.b). Since orientation habitat spending about three years among the plankton would remain vertical, decomposition gases were in the water-column. Life began in a free-floating egg trapped inside the body-chamber, keeping the mass in a neutrally buoyant, gelatinous matrix followed shell floating with part of its ‘empty’ phragmocone by a minute, self-efficient hatchling (Fig. 8-1a) in the protruding into the air, allowing for extensive surface plankton (Westermann, 1990, fig. 8; 1996, figs. 15, 16). drifting. Current transport increased still more if part The juvenile baculite grew into a megaplankter (> 20 mm, of the aperture was also broken away (Fig. 9-1d). The Rhode, 1974; Fig. 8-1b) trapping micro-organisms chambers flooded and the shell sank to the seafloor, its with a wide-stretched ‘plankton net’ (Fig. 8-3) while orientation depending on the location of the flooded sinking and rising (Fig. 8-1b), and sometimes diving to chambers – but probably mostly still apex-up (Figs. the seafloor at maximally 100 m depth or seeking out a 9-1c). When the sea was deep enough (>100 m?), seamount produced by a methane vent, to skim for small any ‘empty’ chambers that had remained untouched demersal fauna flushed from the surface, a behavior here by post-mortem re-filling, would have imploded called epidemersal (Figs. 8-1c, d) – still all in sub-vertical (Fig. 9-1e), just like orthocerids (Westermann, 1985, posture. During the roughly year-long adolescence fig. 7). Presumably this occurred in the pelagic waters stage, the internal shell architecture changed greatly, along the Pacific coast. the phragmocone growing disproportionately while its 2. Horizontally oriented adults, after natural or violent adapical chambers re-flooded, all the time keeping the death and losing at least their tissues (Fig. 9-2a), also animal at neutral buoyancy. During epidemersal feeding became positively buoyant, but in addition lost their episodes, it could recline its long shell by the forward finely calibrated neutral equilibrium and turned apex- jet forces of its twin-nozzles, aided by its ‘streamlined’ down while rising to the surface and beginning to cross-section. Neutral equilibrium occurred only when drift (Fig. 9-2b). Their relatives in oceanic habitats over half of phragmocone was flooded. To achieve some probably lived deeper, and after death their empty stability against rolling and minimally against pitching chambers would have flooded more rapidly under and yawing, its shell had grown slightly dorsally curved the higher ambient pressure, so that most of them and the most adapical chambers were re-emptied to sank promptly, apex-down to the bottom. This was produce a secondary buoyancy unit (Fig. 6d). Now it confirmed by Olivero (2007, fig. 12 F-G), who could swim and feed anywhere in the upper 100 m of the described vertically embedded Baculites in apex- water-column, including epidemersally (Figs. 8-2a, b). down orientation from a Santonian deep-water The entire growth to maturity took perhaps five years, delta system of Antarctica. This seemingly wrong relatively short for such a large shell but due to the thin orientation can be explained only by assuming that septa and rapid emptying rate in a shallow habitat. In this baculite had been in neutral equilibrium, i. e. comparison, estimates for large deep-water ammonoids, a horizontal swimmer – consistent with Olivero’s e. g. mesopelagic lytoceratids, range to at least 50 years illustrations of short body-chambers. Returning to the (Westermann, 1996). adult shells continuing to drift apex-down at or near Soon, a female and somewhat smaller male would mate the surface, they would finally encounter and collide at mid-water. After its eggs were fertilized internally, with the seafloor, causing a good-sized, adapical part the female would produce a large number of them in a of the fragile, thin-shelled phragmocone to break neutrally buoyant gelatinous mass, before releasing them off (Fig. 9-2b, c). After rotating to an apex-upward into the well oxygenated water. orientation, it became a ‘race’ between water flooding through the siphuncle open at both ends and the Death and aftermath uplift produced by removing water-filled chambers, The pre-burial taphonomy or biostratinomy of the shell if the remaining shell should resurface or sink down included a variety of probable events that differed immediately. The shell would finally land, probably from those of a bipartite orthocone as conventionally under an angle but perhaps with the broken end of the reconstructed and are therefore added here. Baculites phragmocone up more often than down (Figs 9-d, e). are commonly found in calcareous concretions within bituminous shale (Tsujita & Westermann, 1998; Landman et al., 2010). Adults may occur alone, commonly ACKNOWLEDGMENTS incomplete, or are mixed with juveniles, or juveniles form the majority of the baculite fauna – all indicating I am grateful for the most generous support I received, strong biostratinomic bias. I envision the processes from the generous e-mailing of essential literature and of post-mortem shell transport and deposition to have thorough scientific and editorial advice to technical proceeded the following way: help with computer problems – at this stage in my life 1. The vertically oriented juveniles, after death naturally all has certainly been needed, when all work has to be or by predator and removal of tissues, became carried out at home. Most important to mention here Hydrostatics, propulsion and life-habits of the Cretaceous ammonoid Baculites 263

Fig. 8: Life history of a Baculites grandis. 1a, free-floating egg mass in gel and hatchlings; 1b, juvenile vertical migrants and drifter; 1c, juvenile sweeping up demersal microfauna; 1d, juvenile rapidly penetrating dysoxic bottom-water; 2a, adult swimming near seafloor; 2b, mid-water adult swimmer. 3, frontal view of the possible ‘plankton-net’, with thin arms carrying long cirri (m, mouth; r, rostrum). [Artwork by C. Tsujita].

Fig. 9: Death and pre-burial aftermath of a Baculites grandis. 1a-c, fish eats body and rostrum of a juvenile: the vertically stable shell floats upwards, then sinks after slow re-flooding, and finally implodes in deep water; 1d, alternatively, the damaged shell continues to float with air trapped in its body chamber and drifts with the current to the shore; 2a, Mosasaur kills an adult; 2b-d, the unbalanced shell turns apex-down and during drift hits bottom losing part of the phragmocone – making it top-heavy once more; 1e, the shell fragments are deposited. [Artwork by J. Tsujita] 264 G. Westermann

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Accepté mars 2013