The Geirud Formation consists mainly of clastic sediments which are interbedded with limestone stringers. This rock unit is well-exposed along the Central Alborz Range in northern Iran. The Geirud Formation rests unconformably on the Mila Formation (Upper Cambrian) and is conformably overlain by the Mobarak Formation (Lower Carboniferous). This study was undertaken to determine the geologi...

Almost all of the modern animal phyla appear to originate in a relatively compressed interval of time known as the Cambrian Explosion. Although several factors have been identified that may have enabled the Cambrian Explosion, there remains the question of the final trigger. The development of cellular differentiation, the capacity of a cellular lineage to become specialized permanently in resp...

The discovery of fossilized brains and ventral nerve cords in lower and mid-Cambrian arthropods has led to crucial insights about the evolution of their central nervous system, the segmental identity of head appendages and the early evolution of eyes and their underlying visual systems. Fundamental ground patterns of lower Cambrian arthropod brains and nervous systems correspond to the ground p...

We report 87Sr/86Sr measurements on a suite of well preserved sedimentary carbonates from Lower Cambrian strata of the Lena River region of Siberia. Stable isotopes and major and trace element chemistry have been used to identify potentially unaltered samples for Sr isotopic measurements. The Sr data define a smooth curve of paleoseawater 87Sr/86Sr values from the Tommotian through to the early...

The evolutionary divergence of cnidarian and bilaterian lineages from their remote metazoan ancestor occurred at an unknown depth in time before the Cambrian, since crown group representatives of each are found in Lower Cambrian fossil assemblages. We report here a variety of putative embryonic, larval, and adult microfossils deriving from Precambrian phosphorite deposits of Southwest China, wh...

Beautifully preserved organisms from the Lower Cambrian Maotianshan Shale in central Yunnan, southern China, document the sudden appearance of diverse metazoan body plans at phylum or subphylum levels, which were either short-lived or have continued to the present day. These 530 million year old fossil representatives of living animal groups provide us with unique insight into the foundations o...