Gerd E. G. Westermann

Ammonoid Life and Habitat

Since the last review of Jurassic—Cretaceous ammonoid ecology (Westermann, 1990), much additional work has been done on ammonoid autecology (architecture or macrostructure) as well as on the associations and occurrences of ammonoids in the field (synecology). Important works on Paleozoic through Triassic ammonoids, dispersed in the literature, have not been reviewed previously. Quantitative autecological studies, begun in the mid-1980s, concerned buoyancy and orientation. Electron and light microscopic studies of the shells have also contributed to an understanding of the soft parts. Research on shell fabrication, strength, and hydrodynamics has increased greatly, also contributing to ammonoid autecology. Intraspecific morphological variation has been studied intensively but remains poorly understood ecologically; most authors still fail to consider variation in the functional interpretation of shell shape. Ammonoid synecology was significantly advanced in recent years by the renewed interest in Paleozoic and Mesozoic dysoxic black-shale facies and their relation to eustasy and orbitally enforced cycles. Other recent studies in synecology have emphasized the interrelations among sediment, eustasy, and biofacies. Finally, ammonoid taxonomy has been summarized in The Ammonoidea (Special Volume 18, The Systematics Association, 1981).

Marine faunal realms of the Mesozoic: review and revision under the new guidelines for biogeographic classification and nomenclature

Consensus among the Friends of Paleobiogeography produced tentative guidelines to the classification and nomenclature of marine biogeographic units, i.e. biochores. Based on these loose ‘rules’, the present paper is a critical review of all published Mesozoic marine Realms, Subrealms, and ?Superrealms (formerly called ‘climatic belts’).Biochores are defined by the overall endemism within geographic and time envelopes that include choro- and chrono-types, without consideration of facies or climate. The hierarchy of ranks or tiers is: ?Superrealm, Realm, Subrealm, Province, Subprovince (only ranks in italics are obligatory); ‘region’ is informal. These ranks scale with endemism, geographic range, and persistence in time. Biochores are named geographically, following nomenclatural guidelines (not strict rules) of synonymy and homonymy, with priority beginning with Uhligs paper of 1911. Uhligs two ‘climatic belts’ are tentatively accepted as optional, top-ranking ‘Superrealms’ for times of exceptional global endemism; available names for them are ‘Euroboreal’ (or ‘Panboreal’, new) and ‘Tethys–Panthalassa’. The validity of the 29 existing names for realm-group biochores (Realms, Subrealms and Superrealms) in the marine Mesozoic is discussed; 19 names are rejected for a variety of reasons, including synonymy, homonymy and usefulness. The most important realm-group biochores are: Arctic and Boreal–Atlantic in the Boreal/?Euroboreal Realm or ?Superrealm; and Tethyan, Mediterran–Caucasian, Indo-Pacific (Jurassic–Early Cretaceous) and Austral (Middle–Late Cretaceous) in the ?Tethys–Panthalassa ?Superrealm.

Model for origin, function and fabrication of fluted cephalopod septa

The model supposes that sub-hemispheric septa of deep-water cephalopods evolved transverse pillars by meridional fluting in order to support flat and therefore weak areas of the shell wall. Several trends toward reduction of sutural spacing for improved wall support resulted in rise of fluting and finally, marginal crenulation. During ontogeny, the function of the ammonite septum as complex vault system against ”normal“ pressure from the body was succeeded by compound-pillar function for wall support. Fabrication of ammonitic septa probably involved positive pressure differential of new ”cameral“ liquid, orientation of mantle fibers along stress lines, and successively affixed tie-points which lay on an aponeurosislike mantle structure.ZusammenfassungDas Modell nimmt an, daß subhemisphärische Septen von Tiefsee-Cephalopoden phylogenetisch transversale Pfeiler durch meridionale »Faltung« (fluting) entwickelten, um flache und daher schwache Teile der Schalenwand zu unterstützen. Mehrere Trends zur Verminderung des Suturabstandes für verbesserte Wandabstützungen resultieren in vergrößerter Faltungshöhe und schließlich randlicher Krenulierung des Septums. Das ammonitisene Septum funktionierte ontogenetisch zuerst als komplexes Gewölbe gegen Körperdruck, dann ais Bündelpfeiler für Wandabstützung. Die Fabrizierung des Ammonitenseptums wurde wahrscheinlich durch positiven Druckunterschied der neuen Kammerflüssigkeit, Ausrichtung der Mantelfasern nach den Drucklinien und nacheinander befestigte Haftpunkte entlang einer Aponeurosis-ähnlichen Mantel-Struktur unterstützt.

Biochore classification and nomenclature in paleobiogeography: an attempt at order

Abstract Paleobiogeographic terminology has increased dramatically in recent decades, but the absence of ‘rules’ or even a guide has resulted in confusion and misunderstanding. The Friends of Paleobiogeography are attempting to solve problems related to classification, definition and nomenclature of the biogeographic units (biochores), in conjunction with neobiogeographers. Historic and current developments are discussed. Biochores are highly dynamic units that not only expand and shrink in range, but also change in rank (tier) through time. They should be based on as many higher taxa as possible representing the biota, but from a single major biome, e.g. pelagic versus benthic; different biomes may need distinct sets of biochores. The following tentative ‘rules’ are proposed for a prospective guide for the distinction, ranking and naming of biochores, with emphasis on stability. (1) Definition: a biochore is defined by the overall endemism of its biota (not particular taxa) within a geographic envelope around a core area, whereas biochore boundaries are defined by the temporary range limits of their constituent endemic taxa. (2) Ranks (tiers): the biochores ranks are, with ‘bold’ for obligatory use and an additional tentative rank for exceptional conditions in brackets: [Superrealm], Realm, Subrealm, Province, Subprovince; ‘region’ is for informal use. Ranks scale with the degree of endemism as well as duration and biota distribution (range). (3) A typical region or chorotype and a typical stage (age) or chronotype are designated for each biochore. (4) Nomenclature: biochore names are geographic and related terms, not taxa-based. Nomenclatural priority begins with Uhlig [Mitt. Geol. Ges. Wien 4 (3) (1911) 229–448]. ‘Rules of Homology’ and ‘Synonymy’ apply in guide form only, e.g. long-term disuse of name or poor definition of biota invalidates synonym or homonym even if senior.

Ammonoid habitats and habits in the Western Interior Seaway: a case study from the Upper Cretaceous Bearpaw Formation of southern Alberta, Canada

Abstract Multidisciplinary research on the habitats and life-habits of ammonoids of the Upper Cretaceous Bearpaw Formation is based on hydrostatics and hydrodynamic aspects of shell shapes, occurrence in lithofacies, evidence of predation, and stable-isotopic data. The large and well-streamlined, oxyconic Placenticeras , with large chitinous anaptychus, had the weakest shells known, and suffered mosasaur tooth perforations at minimal ambient pressure. Anomalously light δ 18 O signatures preserved in their aragonitic shells suggest that they grew in brachyhaline water. Juveniles and some adults of orthoconic Baculites had a vertical life orientation and primarily resided in the lower to middle levels of the water column, flourishing during oxygenation events. Some adults, however, may have changed to a mainly horizontal swimming habit in mid-water. Attitudinal change is indicated by relative shortening of the body-chamber during late ontogeny. Neutral buoyancy at maturity could have been achieved only with a partially liquid-filled phragmocone, implying the exceptional use of cameral liquid as an apical counterweight. Juvenile scaphitids were planktic or sluggishly nektic, as implied by sphaeroconic and discoconic immature whorls, respectively. The adults had different habits, as indicated by their strongly modified body-chambers, which gave high stability suited to vertical swimming. Adult Hoploscaphites and most Jeletzkytes , with open-hooked (scaphitoconic) body-chambers, were demersal and lived close to the sea floor during oxygenation events, whereas adult Rhaeboceras , with an elliptical, contracted body-chamber (elliptosphaerocone), lived in mid-water.

Notes on animal weight, cameral fluids, swimming speed, and color polymorphism of the cephalopod Nautilus pompilius in the Fiji Islands

Forty-six specimens of Nautilus pompilius Linnaeus were captured in depths varying between 100 and 500 m outside of the fringing reef near Suva, Fiji Islands. Thirty- eight of the specimens were male. Air weight per individual varied between 347 and 630 g. Sexual dimorphism in size is indicated, since mature shell modifications (approximated septa, blackened aperture) were present in two females weighing about 350 g (soft parts plus shell) and one weighing slightly over 400 g; the smallest male showing mature shell mod- ifications weighed 496 g. All newly captured specimens were heavier than seawater, with mean weight in seawater of 1.87 g determined for twenty-five specimens. Total volumes of cameral liquid ranged between 13.5 and 0 ml. Thirteen of twenty-five sampled specimens showed less than 1.0 ml of cameral liquid from all chambers. Average cameral liquid os- molarity was lower than that observed in sampled populations of N. macromphalus from New Caledonia and N. pompilius from the Philippine Islands. Maximum swimming rates were 0.25 m/sec. N. pompilius exhibits two common color polymorphs.

Mesozoic Tethyan strata of Thakkhola, Nepal: evidence for the drift and breakup of Gondwana

The Thakkhola region of central Nepal contains at least 1.5 km of coastal to neritic and (upper) slope deposits of Late Triassic to mid-Cretaceous (latest Albian) age. New paleomagnetic, paleobiogeographic and paleoflow data confirm that the strata were deposited on the northern Gondwana margin, bordering Tethys while Thakkhola lay at mid-latitudes (28–41° S). Late Triassic coastal deposits are overlain by Early-Mid Jurassic shelf units; thick Early Jurassic carbonates correspond with the most northerly paleolatitude recorded, probably reflecting Thakkholas rapid drift into the subtropics. The Late Jurassic to Early Cretaceous drift again in higher latitudes resulted in deposition of terrigenous clastics. A hiatus spanning late Early Callovian through Early Oxfordian time corresponds to a global transgression that may reflect accelerated sea-floor spreading in both the Atlantic and Pacific Oceans. Late Jurassic dark shales, correlative with the regionally extensive Spiti Shale, are deep shelf to slope deposits; septal strength indices for cephalopods suggest depths > 250 m. The shales contain an Indo-SW Pacific ammonite assemblage but also a diversified agglutinated foraminiferal assemblage of Boreal affinity. Such deep-water foraminiferal assemblages probably had a large cosmopolitan component. The shale is intensely deformed and its overmature condition probably reflects in part Cenozoic orogenic events. Lower Cretaceous (Berrisian through Aptian) deltaic deposits overlying the shales contain volcanoclastic material that probably was derived from coeval volcanics of the Lesser Himalayas to the south, based on compositional, paleoflow and facies analysis. Volcanism reflects the onset of sea-floor spreading between Greater India and northwestern Australia in Late Valanginian time. At the top of the Thakkhola Mesozoic sequence are hemipelagic, slope, carbonates of latest Albian age. The Thakkhola succession generally correlates well with that observed on a formerly contiguous continental margin, offshore northwestern Australia.

Finite-element analysis of simulated ammonoid septa (extinct Cephalopoda): septal and sutural complexities do not reduce strength

Abstract Finite-element analysis of circular septum models indicates that (1) anticlastic fluting weakened the last septa of the same radius of curvature by a factor of about 2.5 relative to the tensile stresses in a sphere of nacre, (2) septa with ammonitic sutures were stronger than those with goniatitic sutures of the same thickness, and (3) septa with more “complex” ammonitic sutures were stronger at the edge between lobes and saddles than “simple” ones. These results contradict recent claims that ammonoid septa became weaker as sutural complexity increased from goniatitic through ammonitic, so that the most complex sutures were limited to the shallowest habitats. The smaller marginal flutes of complex septa were relatively strong, allowing them to be thinner than the central septum and still act as elastic wall supports. Many Mesozoic ammonoids with highly sinuous sutures occurred in deep epeiric and open-ocean habitats, whereas it is those with secondarily reduced, ceratitic sutures that were typically associated with restricted shallow basins.

Hydrodynamic properties of cephalopod shell ornament

We investigated the hydrodynamic properties of cephalopod shell sculpture in two ways: 1) flow visualization experiments with sculptured shells; and 2) application of drag coefficient data for simple geometric bodies to cephalopod shells. Results of this work suggest: 1) the hydrodynamic effect of shell sculpture depends primarily on the size of the sculptural elements relative to the size of the shell and on the positions of sculpture elements on the shell and relative to each other. 2) sculpture is detrimental to swimming (reduces hydrodynamic efficiency) if it exceeds the height of the lower part of the shells boundary layer. 3) sculpture is advantageous to swimming (increases efficiency) if it remains immersed in the boundary layer and induces premature conversion to turbulent boundary layer flow. To be hydrodynamically optimal, small shells (diam 10 cm) must have rough (sculptured) surfaces, whereas large shells (diam 100 cm) require smooth surfaces. Thus, in order to maintain maximum efficiency throughout life, the ontogeny of small individuals, or species, should be characterized by progressive roughening of the shell, while large forms should be- come increasingly smooth. Such allometries are observed among many ammonoids. 4) sculpture always has an effect on the flow around a cephalopod shell. In some species this effect was probably negligible, while in others, those with compressed shells especially, it was probably of major importance. In these species, sculpture appears to have functioned primarily to increase swimming ability.

Were limpets or mosasaurs responsible for the perforations in the ammonite Placenticeras

Abstract We present evidence defending the hypothesis that specimens of the ammonite Placenticeras from the Upper Cretaceous Bearpaw Formation of southern Alberta were preyed upon by mosasaurs. Perforations linked to diagenetic modification of limpet home scars are anomalous features that are rare in comparison to true tooth marks. Point loading of the ammonite phragmocone wall by mosasaur teeth produced distinct perforations without the development of long, angular fractures. This was due to low pressure and rate of loading permitted by the unusually loose articulation of the lower jaw of mosasaurs, combined with the nearly uniform internal support of the shell wall by complexly fluted septa and their sutures. In contrast, the body-chamber, which lacked septal support (except at its posterior end), tended to shatter. Remnants of the body-chamber preserve distinctive notches that resemble tooth marks and sometimes show evidence of healing. Thorough reassessment of the perforations and associated features indicates that the large majority of perforations in shells of Placenticeras , at least those from southern Alberta, are best explained by predatory activities of mosasaurs. The currently popular hypothesis that all perforations preserved in shells of Cretaceous ammonites were produced by sediment loading and collapse of limpet home scars fails to explain many diagnostic features of the perforations. These include the shape, siting and arrangements of perforations as well as the equal puncturing of all size classes of alleged limpet home scars. The limpet hypothesis also fails to consider structural differences between shells of ammonoids and Nautilus and erroneously assumes point loading by sediment compaction. Furthermore, the extreme rarity of limpets that had the potential of excavating home scars in Placenticeras from the Upper Cretaceous Bearpaw Formation of Alberta, Canada, cannot be attributed to preservational bias and is at odds with the large number of perforated specimens known from this formation. The mosasaur-bite hypothesis, on the other hand, accommodates all of these features.