Cambrian Stage 2


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Cambrian Stage 2
The "Tommotian" - 528 to 521 million years ago

The second age of the Cambrian period (the time when the second stage of the Cambrian system, ie. succession of rock, was laid down) does not yet have an official (ICS) name, but is widely known as the Tommotian after the corresponding Siberian regional stage.



"The wave of discoveries that rewrote the story of the earliest Cambrian began when the former Soviet Union mustered sizable teams of scientists to explore geological resources in Siberia after the end of World War II. There, above thick sequences of Precambrian sedimentary rocks, lie thinner formations of early Cambrian sediments undisturbed by later mountain-building events (unlike the folded Cambrian of Wales). These rocks are beautifully exposed along the Lena and Aldan rivers, as well as in other parts of that vast and sparsely populated region. A team headed by Alexi Rozanov of the Paleontological Institute in Moscow discovered that the oldest limestones of Cambrian age contained a whole assortment of small and unfamiliar skeletons and skeletal components, few bigger than 1/2 in (1 cm) long. These fossils have been wrapped in strings of Latin syllables but have been more plainly baptized in English as the "small shelly fossils" (SSFs for short).
In 1969, a 380-page monograph was published in Russian describing the unknown fossils, and paleontologists who now knew what to look for began to discover parallel sequences scattered - in sites from Meishucun in southern China, and India to Newfoundland and Nova Scotia, and from Shropshire to southern Australia."
J. John Sepkoski Jr, "Foundations - Life in the Oceans" in The Book of Life, ed. by Stephen Jay Gould, 1993, Ebury Hutchison, London, pp.48-49

Global events such as a major rise in the level of marine carbonates and phosphates triggered the sudden appearance of phosphate rich shelly-organisms. "[A] high reading of negative carbon isotope indicates geological events starting with a preceding oceanic anoxic event (OAE). The re-establishment of a well-mixed oxic Late Proterozoic ocean was apparently coupled to the breakup of the Precambrian megacontinent(s) that consequently generated an array of epicontinental seas and bays, notably in the realm of the newly-formed paleotethys. In the wake of restored marine circulation patterns, these shelf areas were inundated by previously stagnant, phosphate-rich seawater, turning the shelves into regions of high primary productivity". ("Carbon Isotope Stratigraphy of the Phosphorite-Bearing Precambrian-Cambrian Transition Series of the Lesser Himalaya (Uttar Pradesh, India)")

The Tommotian was an important period which saw the rise of diversified metazoans with skeletons. This represented the earliest abundant and diverse small skeletonized assemblage stage stratotype in Siberia, and was characterized by a great diversity of SSFs, the first archaeocyaths, primitive molluscs, and inarticulate brachiopods.

The Small Shelly Fauna (SSFs) consists of various calcareous (also some silica, some calcium phosphate) fossils some 1-3 mm long. They represented a variety of organisms: sponges, molluscs, annelids, lobopods, and other forms that do not seem to belong to any recent phylum.

The archaeocyaths were the main Early Cambrian reef formers. They had a distinctive cone inside a cone structure and a calcareous skeleton. Most were no more than a few centimeters in height


The term "procoelomate" was coined by the Swedish paleontologist Jan Bergström to slug-like bilaterian animals that evolved from ancestral aschelminthes and gave rise to the various coelomate phyla during the Ediacaran-Cambrian transition. Genetic (RNA) analysis indicates that the various groups of animals based on the coelomate body plan evolved rapidly from this hypothetical "pro-coelomate" ancestor. This agrees with the paleontological evidence of a "Cambrian explosion". Dr Bergström goes on to suggest that groups of late surviving procoelomates became the strange armoured animals that lived in early Cambrian times.

Tommotian Reefs

The earliest metazoan reefs at the base of the Tommotian, as exposed on the Aldan River, Siberia, were already ecologically complex. While possessing low diversity, they had erect framework elements of branching archaeocyaths, with a cryptic biota of archaeocyaths and calcified cyanobacteria. These reefs were associated with skeletal debris of a diverse associated fauna; microburrowing deposit-feeders continued to proliferate within the sheltered areas of the framework.

By the middle Tommotian, archaeocyath - cyanobacterial reefs became more diverse and ecologically complex due to the appearance of other sessile, calcified organisms inferred to have been suspension- or filter-feeders. These organisms included radiocyaths, a variety of simple cup-shaped forms known as "coralomorphs," globally rare but locally abundant large skeletal tabulate corals and other cnidarians, and stromatoporoid sponges. Possible calcarean sponges appeared in the early mid-Tommotian; probable sponge borings have been noted within coralomorph skeletons from the Canadian Rocky Mountains, and silt-sized microspar grains resembling "chips" from clionid-type sponges have been identified within Lower Cambrian reef cavities.


page uploaded 15 May 2002
checked ATW080219
unless otherwise indicated, content © M. Alan Kazlev 2002
wikified HAJ090212
this material may be freely used for non-commercial purposes

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