P HONE O PTIONS

Gámez Vintaned, J.A. and Liñán, E.

Área y Museo de Paleontología, Departamento de Ciencias de la Tierra, Facultad de Ciencias, Universidad de Zaragoza, C/ Pedro Cerbuna, 12, 50009 Zaragoza, España; [email protected]

The best successions spanning the Precambrian/Cambrian boundary in western Europe are located in Spain. Liñán et al. (1984) established the basis for their palaeontological analysis and correlation. In several areas, successions straddling this transition are interrupted by hiatuses originated by terminal Cadomian deformation, but this is not the case in central Spain. Trace fossils from late Vendian to early Cambrian successions in central Spain record evolutionary patterns in primitive metazoan behaviour and the incoming of new animal groups, which do not appear to be directly related with environmental facies changes because relevant ichnological innovations occur in monofacial successions. Among these, perhaps the FAD of the Monomorphichnus lineatus + Phycodes pedum assemblage is the best tool to delineate the Precambrian/Cambrian boundary (Gámez Vintaned & Liñán, 1996), since both ichnospecies appear at very similar stratigraphic positions in numerous sections in the world (Crimes, 1987).

At the río Huso section in the Valdelacasa anticline, fine siliciclastics of the Pusa shale crop out providing a fairly rich record of ichnofossils (Brasier et al., 1979; Palacios Medrano, 1989). Small shelly fossils, trilobites and other groups are also present in neighbouring localities of the Pusa shale. Cambrian-diagnostic trace fossils first appear in basal greenish shales (a monofacial succession deposited under sublittoral marine conditions) of the lower member, including Phycodes pedum, Monomorphichnus lineatus and small specimens of Psammichnites ichnosp., among others (Gámez Vintaned, 1996). Starting well below this level, Phycodes aff. P. pedum (a series of short “probe” burrows about 1 mm in diameter, connected to each other but lacking a true main burrow) is abundant, reaching the basal Cambrian rocks where it occurs together with Monomorphichnus lineatus and Phycodes pedum (which, in contrast, consists of a master burrow bifurcating successively into shorter distributary burrows, all directed first to the same side and then upwards). Therefore, Phycodes pedum differs substantially from Neoproterozoic-type burrows and represents a behaviour more complex and evolved than that of Phycodes aff. P. pedum. Phycodes pedum also differs from ichnospecies of the Cambrian ichnogenus Treptichnus –with which it is confused in some recent literature– in having a master burrow. The architecture of P. pedum reflects a novel strategy of deposit feeders to exploit the benthic marine habitat at the beginning of the Cambrian explosion.

This ethological evolution leading to novel strategies (perhaps entailing also the appearance of new animal groups) was paralleled by the appearance of arthropods (provided with stiff appendages capable of a wide range of behaviours), as evidenced in rocks of the río Huso section, first by simple raking traces (Monomorphichnus) and shortly after by more complex raking (Dimorphichnus) and walking traces (Diplichnites).

The widespread distribution of selected trace fossils throughout late Vendian-early Cambrian successions in the Iberian Peninsula allows establishing the following ichnofossil zonation, from bottom to top:

– Torrowangea rosei Zone (uppermost Vendian). It records the ichnofossils Bergaueria? ichnosp., Cochlichnus ichnosp. A and B, Gordia ichnosp., Helminthopsis ichnosp., cf. Megagrapton, Neonereites aff. N. uniserialis, Planolites ichnosp., Phycodes aff. P. pedum,

Phycodes? n. ichnosp. and Torrowangea rosei, as well as the body fossil Nimbia occlusa. – Phycodes pedum + Monomorphichnus lineatus Zone (Lower Corduban; lowermost Cambrian). Its beginning coincides with the Precambrian/Cambrian boundary event (Liñán et al., 2006). It records the ichnofossils Belorhaphe ichnosp., Bilinichnus ichnosp., Cochlichnus ichnosp. A and B, Dactyloidites cabanasi, Dimorphichnus ichnosp., Diplichnites ichnosp., Gordia ichnosp., Helminthopsis ichnosp., Megagrapton cf. M. irregulare, Monomorphichnus lineatus, Neonereites uniserialis, N. aff. N. uniserialis, Phycodes pedum, Phycodes aff. P. pedum, Planolites montanus, Psammichnites gigas, Psammichnites ichnosp., gr. Scolicia, Torrowangea rosei, Treptichnus bifurcus and Treptichnus ichnosp.

– Rusophycus avalonensis + Rusophycus bonnarensis Zone (Upper Corduban). Its beginning coincides with the Lower/Upper Corduban event (Liñán et al., 2006). It records the ichnofossils Arenicolites ichnosp., Cruziana cantabrica, Dimorphichnus obliquus, Diplocraterion ichnosp., Gordia ichnosp., Monocraterion ichnosp., Monomorphichnus ichnosp., Phycodes ichnosp., Rusophycus avalonensis (= R. fasciculatus), R. bonnarensis (= R. cantabricus) and Teichichnus ichnosp., among others.

This zonation may be correlated with similar records in other areas of the world, and it can be compared with the zonation made by Crimes (1987), our two first zones corresponding respectively to Crimes’ Zone I (Upper Vendian) and Zone II (Nemakit- Daldynian to Lower Tommotian). The Rusophycus avalonensis + Rusophycus bonnarensis Zone correlates only with the lower part of Crimes’ Zone III (Upper Tommotian-Lower Atdabanian), being the upper part of the latter correlatable with the Astropolichnus hispanicus Zone of Spain, which is Lower Ovetian (= Lower Atdabanian) in age.

Acknowledgements

This is a contribution to the projects: IGCP 493 (“Rise and fall of the Vendian biota”; IUGS- UNESCO), Consolíder CGL2006-12975/BTE ("MURERO"; Spanish Ministerio de Educación y Ciencia-FEDER-EU, and Grupo Consolidado E-17 (“Patrimonio y Museo Paleontológico”; Gobierno de Aragón). Thanks are due to Universidad Complutense of Madrid and to Prof. M. D. Brasier (University of Oxford) for providing access to their ichnological collections.

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