2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 1
Presentation Time: 8:10 AM

Aftermath and Recovery from the Valdemiedes Extinction Event (Lower/Middle Cambrian)


, gamez@unizar.es

The Valdemiedes Extinction Event (VEE) has been postulated for the Lower/Middle Cambrian boundary. An extinction event of probably global scale was listed already by Cambrian workers in the XIX century among the most important crises in the Earth history and located the boundary between the traditional “Protolenus” (or Lower) and “Paradoxides” (or Middle) Cambrian. In its Spanish stratotype area (Rambla de Valdemiedes, near Murero), the VEE is expressed by the disappearance of all (but one) trilobite species of the uppermost Cambrian Stage 4 (Bilbilian Stage, Protolenus jilocanus Zone), belonging to Protoleninae, Ellipsocephalinae, and Resseropinae, and their following replacement by a new trilobite assemblage of the Leonian Stage including paradoxidid “invaders” (Acadoparadoxides mureroensis Zone), as well as by a progressive dwarfing of brachiopods, strong decrease in trace fossil diversity, and pronounced anomalous geochemical (major and minor elements) and mineralogical signals. Particularly, beds recording the VEE and those immediately below show quick oscillations in d13C values between -3.3‰ and -27.5‰ (ROECE excursion?).

During the event aftermath (or survival interval), the low-diversity community of surviving taxa (or relic assemblage) consisted of scarce individuals of one trilobite species (Alueva undulata) and abundant specimens of a few inarticulate brachiopod taxa, notably Trematobolus simplex of smaller body size than before the event (“Lilliput effect”). Size was progressively recovered towards the end of the survival interval. This is the first documented case of the “Lilliput effect” in Cambrian times.

Only one protolenid genus (Alueva) passed the event beds (survival interval), reaching basal Middle Cambrian strata (post survival interval) to eventually disappear within the first Middle Cambrian zone (Acadoparadoxides mureroensis Zone). Similarly, the relic olenellid trilobite Cambropallas telesto reaches the basal Middle Cambrian in Morocco, just as the protolenid Protolenus pisidianus in Turkey.

Shortly after, a second exotic trilobites invasion (conocoryphids) coincided with the extinction of relic protolenids.