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2 Acta Geologica Polonica

2 2010

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Patterns of occurrence and distribution of crustacean ichnofossils in the Lower Jurassic-Upper Cretaceous of Atlantic occidental margin basins, Portugal

Carvalho C., Baucon A., Rodrigues N., Santos V., Viegas P.

Acta Geologica Polonica

2010

Vol. 60, no. 1

19-28

Crustacean ichnofossils are most abundant in the stratigraphic record of Portugal. In this paper is presented a study on crustacean ichnoassemblages from the Sinemurian to middle Cenomanian, during the opening and subsequent filling of Atlantic occidental margin basins (Lusitanian Basin and western part of Algarve Basin). Thalassinoides dominates the lagoonal and inner shelf facies from the late Sinemurian at least to the Turonian, generally defining all of the carbonate sequence with its dense, "nodular" ichnofabric since the Late Jurassic. Asequence is described in the lower Barremian of Cabo Espichel in which burrow mazes of Thalassinoides suevicus occur, containing hundreds of Mecochirus rapax as an obrution lagerstatte. The Cretaceous of the Lusitanian Basin is rich in marly limestones mottled with Thalassinoides but almost devoid of Rhizocorallium. Fairly common in dark marls and biomicrites from the Jurassic, Rhizocorallium irregulare locally occurs in dense monospecific fabrics. Slipper-shaped and oblique forms (Rhizocorallium jenense) as well as spiral and lobate forms are rhizocoralliid foraging modifications usually developed in deeper tiers than Thalassinoides and sometimes evidencing bioimprints in Glossifungites preservation. Siliciclastic facies related to rifting subsidence along major fault scarps and fan deltas/braided river depositional systems show pervasive bioturbation with Psilonichnus tubiformis.Another crab-style behaviour ascribed to Macanopsis plataniformis is described for the first time in Portugal, associated with a coarse-grained sandstone episode in a tidal flat setting from the Kimmeridgian, where monospecific brachyuran burrows were developed in a firmground.

Da Vinci's Paleodictyon : the fractal beauty of traces

Baucon A.

Acta Geologica Polonica

2010

Vol. 60, no. 1

3-17

The origins of ichnology are located in a land of convergence between Art and Science, in a historical period - the Renaissance - during which the scientific method had its birth. Trace fossils were studied and graphically represented by preeminent naturalists such as Leonardo da Vinci, Konrad Gesner, Johann Bauhin and UlisseAldrovandi - who defined ichnofossils as "exceptionally beautiful". In this study, the representation of trace fossils in the Renaissance is explored by employing a method widely used in studying visual perception - fractal geometry. In particular, this paper focuses on the reasons for the aesthetic appeal of traces and proves that (1) the aesthetic perception of traces is closely tied to their fractal dimension, and (2) many traces are aesthetically appealing because they have fractal behaviour. In particular, graphoglyptids and chondritids display significant fractal-like features that are linked with their constructional program and function. Such fractal traces are hierarchically structured and their whole geometric structure can be regarded as an expression of self-organization processes producing correlations between different orders of scale.