Øyvind Hammer

The Ordovician Period

Abstract: Rapid and sustained biotic diversification reached its highest levels in the Paleozoic. A prolonged “hot-house” climate through Early Ordovician, cooling through Middle Ordovician and changing to “ice-house” conditions in Late Ordovician, global glaciation, oceanic turnover and mass extinction at end of period, strong fluctuations in eustatic sea level, appearance and diversification of pandemic planktonic graptolites and conodonts important for correlation, moderate to strong benthic faunal provincialism, re-organization and rapid migration of tectonic plates surrounding the Iapetus Ocean and migration of the South Pole from North Africa to central Africa all characterize the Ordovician Period. All seven Ordovician stages have formalized GSSPs.

Biodiversity curves for the Ordovician of Baltoscandia

Biodiversity curves for the Ordovician of Baltoscandia show a substantial increase in taxonomical diversity through the period, as seen also in global data sets. A database of 10,340 records of first and last appearances of species at different localities in the region has been analysed using simple species counts, and partly validated with resampling methods. While the biodiversity curve for all fossil groups combined is probably reasonably accurate except for an unknown scaling constant, taxonomical or geographical subsets may not be sufficiently well sampled to allow precise measurement of their species counts through time. The analysis shows a major diversification event commencing in the middle Arenig (Ibex-Whiterock boundary), and more limited diversification events in the Llanvirn, Caradoc and Ashgill. The Scandinavian (Norwegian and Swedish) biodiversity curves are broadly correlated with major changes in sea level, with low biodiversity at highstands and high biodiversity at lowstands, although this pattern is not clear for all fossil groups. In the Arenig, graptolites and trilobites appear to have higher diversities at high sea levels, while the brachiopods and ostracodes show higher diversities at low sea level. As a consequence, the Arenig diversification is delayed for the latter two groups until the upper end of the interval.

Buckman's first law of covariation–a case of proportionality

Buckmans first law of covariation states that in ammonoids, coarseness of ribs and other ornamentation correlates negatively with degree of compression (whorl height/width ratio) and involution. This ‘law’ has escaped several attempts at functional or physiological explanation. However, Buckmans first law becomes self-evident, and almost collapses to a tautology, when we reword it as follows: Size of lateral and ventral ornamentation correlates with size of the aperture (its width and height, respectively). In a shell that is strongly compressed laterally, the diameter of the aperture, and hence the soft parts within, is small in the lateral direction. For such a shell, coarse lateral ribbing would imply impossibly strong relative compression and expansion of the soft body through the formation of a rib. Conversely, if the shell is depressed (compressed dorsoventrally), the diameter of the soft parts is small in the dorsoventral direction, and the ventral ribbing correspondingly small. Buckmans first law is thus simply a statement of proportionality that needs no special explanation. On the contrary, any departure from this ‘law’ would imply strong deformations of the soft body as ornamentation was formed. Measurements on the Triassic ammonite Pseudodanubites halli show that ventral ribs get relatively stronger on compressed specimens, in accordance with this view of Buckmans law. ? Ammonoids, intraspecific variation, morphometry, ornamentation.

Models for the morphogenesis of the molluscan shell

We give a review of current theories of morphogenesis of both the general coiling and the ornamentation of molluscan shells. These two aspects of shell growth are closely connected, as ornamentation is primarily due to local perturbations of the general apertural growth field controlling coiling. Also, a new, generalized, free-form apertural growth map model is presented in this paper, illustrating some aspects of the regulation of logarithmic spiral growth. This model is used to simulate the formation of megastriae in ammonoids. We emphasize the importance of damaged specimens and how they regenerated, as illustrated with examples from ammonoids. The phenomenon of ornamental compensation can be explained by a mechanism involving a pre-pattern in the mantle. However, simple reaction-diffusion models for ornamental pattern formation should be regarded only as useful abstractions.

Reaction-diffusion processes: Application to the morphogenesis of Ammonoid ornamentation

Ammonoid ribs, tubercles, keels and other ornamental features are organized in a great diversity of patterns. From the viewpoint of developmental biology, these patterns can only be understood as the product of a dynamical system controlled by genes but also dependent upon biochemical and mechanical processes. Here, an attempt is made to view the development of ammonoid ornamentation in the light of reaction-diffusion systems, though it is emphasized that diffusion of biochemical morphogens is only one of several possible interpretations of this class of theoretical models. Also, morphogenetical gradients and threshold functions are presented as partial explanations for some specific ornamental patterns.

Experimental investigation of the hydrodynamics in pockmarks using particle tracking velocimetry

Water tank experiments were performed in order to investigate the behaviour of currents in pockmarks. A particle-seeded flow was visualised and quantified with the aid of the particle tracking velocimetry technique. The employed analogue pockmark is a 1:100 idealised scale model of a natural pockmark, while the highest Reynolds number in the experiments was one order of magnitude smaller than in nature. Interaction of the flow with the pockmark geometry resulted in an upwelling current downstream of the pockmark centre, along with enhanced water turbulence in the depression. Scaling-up the experimental measurements, it is found that the upwelling would be capable of preventing the settling of particles as large as very fine sand. Furthermore, the increased turbulence would support the suspended fine material, which can thus be transported away before settling. The net effect for a variable-direction near-bed current over long periods of time would be to winnow the settling sediments and reduce the sedimentation rate in pockmarks. These mechanisms may be responsible for the observed lack of sediment infill and the typical presence of relatively coarser sediments inside pockmarks compared to the surrounding bed. In contrast, sediments transported as bedload are likely to be deposited in pockmarks because of the weakening of near-bed currents as well as lateral flow convergence associated with the upwelling. Bedload, however, may not be the dominant mode of sediment transport in areas covered by cohesive sediments, where pockmarks are found.

The development of ammonoid septa: An epithelial invagination process controlled by morphogens?

Suture lines are important in ammonoid taxonomy. Their complex morphologies, caused by iterated invaginations of the posterior body wall, can be explained using a mechano‐chemical model inspired by modem developmental models for epithelial folding in kidneys, lungs, teeth, mammary glands and other organs. A morphogen, organized to form a regular spacing pattern by reaction‐diffusion dynamics or similar processes, induces changes in cell shape and/or rate of proliferation, causing invagination and the formation of lobes. Interactions between mechanical and chemical effects, combined with expanding size due to overall growth, produce a “fractal”; pattern of smaller (secondary) invaginations superimposed on larger (primary) ones. The pattern of increasing sutural complexity that is observed in ammonoid evolution may be a simple heterochronic effect, that allowed iterated invagination to be extended to higher and higher levels of folding. In contrast with some earlier theories, the present model is based on t...

Norwegian Offshore Stratigraphic Lexicon (NORLEX)

The Norwegian Offshore Stratigraphic Lexicon (NORLEX) provides a relational stratigraphic database for the North Sea, Norwegian Sea, Barents Sea and Svalbard. Both regional lithostratigraphy and biostratigraphy are being substantially updated, following guidelines laid out in the International Stratigraphic Guide. The main body of information developed is available as a petroleum consortium (oracle-style) database, and the new lithostratigraphic definitions as a public domain (paper) document. NORLEX is presented as a browsing website via the internet at http://www.nhm.uio.no/norlex. Seismic cross-sections, core photographs, well logs, field outcrops, microfossil occurrences and other vital attributes are relationally cross-linked. In addition, there are menus for instantly finding updated formation and member tops or microfossil events in all wells, plus a map contouring routine for unit thicknesses and depths.

Generalized ammonoid hydrostatics modelling, with application to Intornites and intraspecific variation in Amaltheus

ABSTRACT A numerical procedure for calculating the buoyancy, apertural orientation and rotational stability of ammonoids can accommodate noncircular apertures and allometries. The hydrostatic properties of the Triassic oxyconic ammonite Intornites nevadanus were estimated from measured coiling parameters, body chamber length and digitized whorl section. The calculations indicate close to neutral buoyancy. The orientation of the aperture is 33 degrees from the horizontal, and the stability index relatively high at 0.054. Unlike for many other ammonites, these values are comparable to those of Nautilus, suggesting a similar nektobenthic lifestyle. An apertural interior shell callus does not contribute to stability in this genus, and its function remains unknown. In Amaltheus margaritatus, hydrostatic properties and body chamber length vary significantly as a result of intraspecific variation in shell shape. Covariation between septal spacing and whorl shape can be given a functional explanation in terms of hydrostatic properties. Buckmans second law of covariation between ribbing and sutural complexity can possibly be thought of in terms of heterochrony.

Travertine terracing: patterns and mechanisms

Abstract Travertine terracing is one of the most eye-catching phenomena in limestone caves and around hydrothermal springs, but remains fairly poorly understood. The interactions between water chemistry, precipitation kinetics, topography, hydrodynamics, carbon dioxide degassing, biology, erosion and sedimentation constitute a complex, dynamic pattern formation process. The processes can be described and modeled at a range of abstraction levels. At the detailed level concerning the physical and chemical mechanisms responsible for precipitation localization at rims, a single explanation is probably insufficient. Instead, a multitude of effects are likely to contribute, of varying importance depending on scale, flux and other parameters.