LIFE IN THE CAMBRIAN

Ediacara “fauna”

The first metazoans

The first shelly faunas

Cambrian macrofauna

The fossil archives

Chengjiang

Orsten

Other fossil lagerstätten

Once more: The Cambrian Explosion

Other than in most other periods of earth history, the Cambrian fossil record indicates a distinct development from simple organisms to organisms comparable in morphology and organization to the present-day animals. This rapid phylogenetic development started in the latest Proterozoic and was more-or-less finished at the end of the Early Cambrian. The development is documented by faunal assemblages represented by (1) the Ediacara fauna, (2) the first complex trace fossils, (3) the earliest shelly faunas, and (4) the onset of the typical Cambrian macrofaunas. Additional important data come from fossil archives. It is amazing that this rapid evolution took place in an interval of less than 25 m.y., and the evolution from the first hard-part animals to the presence of most of the present-day phyla was restricted to an interval of probably less than 10 m.y. Multicellular life evolved at an incredible supersonic speed, and for this reason this part of organismal evolution is termed the "Cambrian Explosion", or "Evolution's Big Bang."

The oldest fossils of metazoan aspect are remains that belong to the so-called “Ediacara fauna”. The peculiar character of this fauna was first recognized in the Pound Quartzite, Ediacaran Hills, South Australia, although the same type of fauna was earlier discovered in the present-day southern Namibia. Ediacaran-type fossils are now known from numerous sites worldwide (such as Namibia, Ireland, England, northwestern Russia, South Australia, Newfoundland and the Canadian Northwest Territories) in rocks dated between 610 and 510 m.y., thus ranging from the Late Proterozoic (Vendian or late Neoproterozoic) to the Middle Cambrian. The typical Ediacaran fauna of Late Proterozoic age includes organismal remains that look like feathery fronds, pouches or disk. Frond-like remains usually show delicate branches, and none of these organisms had heads or obvious circulatory, nervous or digestive systems. Earlier suggested affinities include sea pens, molluscs, jellyfish, or worms, but a new interpretation by A. Seilacher now assigns them as single-celled organisms with hydraulic architecture. This previously unknown kingdom Vendobionta of organisms is characterized by flattened, quilt-like anatomy, and is seemingly an experiment in life with fluid-filled, air-mattress-type bauplans.

Coexisting with latest Neoproterozoic Ediacaran faunas are simple trace fossils ("worm burrows") that were created by multicellular animals, the first evident Metazoa. Indeed, the stratigraphical occurrence of trace-fossils depicts an evolution to more complicated traces, which, in turn, proves the progressive evolution to more anatomically complicated animals that were able to perform a progressively complex behaviour. The first trace with a somewhat complicate pattern is Trichophycus pedum (formerly known as "Phycodes pedum"). It occurs nearly worldwide, and its first occurrence is with late fossils of typical Ediacaran aspect or, usually, in strata above them, whereas the first shelly fossils appeared clearly later. Hence, the ichnofossil assemblage with Trichophycus pedum marks the first occurrence of well-developed, fairly complex metazoan animals, and this is today regarded as the most useful landmark to characterize the boundary between the Precambrian and the Phanerozoic and, synchronously, the Proterozoic and the Cambrian. Accordingly, the International Subcommission on Cambrian Stratigraphy (through its Working Group on the Precambrian-Cambrian Boundary) made the official decision in 1991 to draw the base on the Cambrian at the first appearance date (FAD) of Trichophycus pedum in the reference section at Fortune Head, southeastern Newfoundland. Other characterisitic assemblages with more and more complicate trace fossils occur later that than Trichophycus pedum but still before the first hard-part fossils.

Trichophycus pedum, characteristic of the

lowermost Cambrian ichnofossil assemblage.

Rosenhof Member, Fish River Subgroup, Nama Group.

Kosis Farm, Namibia.

Giant traces of Psammichnites gigas

Lower Cambrian Hardeberga Sandstone.

Brantevik harbor, Scania, Sweden.

No animals are known from the very base of Cambrian that had hard parts, either as an external skeleton or simply spicules. The first shelled metazoas which are then characteristic for the Cambrian occur well above the earliest complex trace fossils. This suggests that hard part production evolved later, and the typical Cambrian faunas (such as trilobites, archaeocyaths, and small shelly fossils) are unknown before the middle part of the Early Cambrian. The evolution of shelled metazoan is reflected by the appearence of successively more advanced shelly fossils. The typical small shelly fossils (SSFs, or early shelly fossils, ESFs) are tiny (generally 1 to 5 mm) tubes, spines, cones and plates that are not clearly allied with modern groups. Many of these organisms were recognized either as of unknown affinity or as representatives or groups that became extinct before the end of the Cambrian. The most "primitive" stage is marked by characteristic elements, such as anabaritids, tommotiids, and hyolithellids, known as the "Tommotian fauna." Later SSFs have been identified as sclerites of worm-like animals or as early representatives of the major fossil groups.

Insoluble residue of limestone samples with phosphatic tubes

and sclerites of so-called "small shelly fossils".

Late Tommotian, Malykan, Lena River, Siberia

Skeletonized organisms and organisms with distinct limbs became more and more abundant as the Early Cambrian progressed. The first macroscopic faunas occur at the end of the Siberian Tommotian Stage, when major reefal complexes were formed by archaeocyaths. Archaeocyaths are sponges with a simple morphology. Their calcareous skeleton consists of an inner and an outer wall that are variably connected.

Arthropods are apparently the most diverse of the Cambrian animal groups. Numerous enigmatic forms with strongly differing cephalic appendages are known, especially from the various fossil archives. Bivalved arthropods of ostracod aspect but with phosphatic shell are important fossils in several regions.

Brachiopods are relatively frequent but rarely abundant fossils. The first articulate brachiopod groups occur at the end of the Early Cambrian. Conodonts are important are biostratigraphic tools in the Upper Cambrian. Other characteristic Cambrian invertebrates include early mollusc groups (helcionellids, pelagiellids), hyoliths, and echinderms (e.g., helicoplacoids, eocrinoids, cinctans, edrioasteroids).

SEM micrograph of a

hesslandonid phosphatocopine.

Oblique view into partly open shield shows

excellently preserved limbs.

By courtesy of D. Waloßek, Ulm University

The Cambrian period is surprisingly rich in beds that yield well-preserved fossils. Due to general decay, fossils are usually remains of only hard parts that have enough potential to be preserved. However, unusual circumstances may serve for a preservation of soft-parts. This does not only give much more insight into the morphology and anatomy of the hard-part animals but usually enlarges the spectrum of the known animals because only between 5 and 10 percent of the fossils in those fossil archives have a skeleton that would be preserved under normal circumstances.

Burgess Shale

The most prolific example for such a fossil archive is the Middle Cambrian Burgess Shale from the Burgess Pass, Yoho National Park, British Columbia. It represents a so-called Lagerstätten deposit, where shallow dwelling marine animals were swept to a deeper site of the shelf and rapidly buried by slumps. The Burgess Shale fauna was discovered in 1909 by Charles Walcott, who interpreted the fauna as belonging entirely to modern phyla. However, recent studies have shown that many of the organisms had strange and previously unknown morphologies and have no modern analogues. They are now recognized to represent a large number of higher taxa that are only known from the Cambrian fossil archives and probably became extinct before the end of the Cambrian.

Even more beautifully preserved are faunas from the Early Cambrian of Chengjiang, Yunnan Province, on the Yangtze Platform, South China. The Chengjiang locality yielded a number of fossils that were only known from the Burgess Shale, but also another number of strange creatures. Description of the Chengjiang fossils is in progress.

Another type of fossil archive was discovered in the Upper Cambrian of Sweden, where typical deposits are smelly alum shales with fossil-rich calcareous nodules ("orsten"). Dissolution of these nodules by organic acids revealed a marvellous fauna of microscopic arthropods in an amazingly good, three-dimensional preservation. The same “orsten-type” preservation has been found meanwhile in numerous other regions with organic-rich, dark rocks, including Australia and Siberia. Most of these fossils represent early and often previously unknown arthropod groups, but probably also the earliest known tardigrades and pentastomids (which are extant groups but otherwise unknown from the fossil record).

Additional fossil archives with soft-part preservation are known from the Early Cambrian Sirius Passet, Greenland, the Early Cambrian Mount Cap Formation of the Canadian Northwest Territories, the Early Cambrian Emu Bay Shale, South Australia, and the Middle Cambrian of Siberia, and we can be ceratin that there are many more to be discovered...

Thin section of archaeocyath "bioherm" showing cross-sections

of archaeocyaths and intergrowing calcimicrobes.

Lower Cambrian Lemdad Formation,

Lemdad syncline, High Atlas, Morocco

Copyright (c) G. Geyer, 1997

The major Cambrian animal group in the fossil record are the trilobites. Trilobites are arthropods with a characteristic longitudinal and transverse tripartition that was name giving. They are not only abundant in various shelf deposits but are helpful as index fossils and to characterize biofacies and paleobiogeography. Despite the relatively strong faunal provincialism, Cambrian trilobites constitute the biostratigraphical framework that allows to compare rock successions from regions on different Cambrian continents.

Cambropallas telesto, the latest known olenelloid

from the level wit the "Giant Paradoxides",

Middle Cambrian of Morocco,

a searched collectors' item, now in many

exhibitions of natural history museums.

Cephalopyge notabilis Zone,

Jbel Warmast Formation,

Tarhoucht section, eastern Anti-Atlas.

Copyright (c) 1997 by G. Geyer.

Olenoides serratus (Rominger, 1887), a corynexochoid trilobite.

Almost complete dorsal exoskeleton (left free cheek absent).

Middle Cambrian Stephen Formation,

Burgess Shale at Walcott Quarry, Yoho N.P., Canada.

Copyright (c) 1995 by Andrew MacRae.

The latest Neoproterozoic to Early Cambrian fossil record indicates that multicellular life evolved into a large number of possible bauplans as soon as it got a foothold. These bauplans, or types of organization, characterize high-ranked taxa such as phyla. Although life developed to a huge diversity as seen today, probably no new phyla developed in post-Cambrian times and the number of phyla has actually decreased since. The Middle Cambrian may thus represent the time with the organizational diversity at a maximum. What are the reasons of the Cambrian Explosion? This is a question that nobody can answer with enough certainty in the moment.

Physical examination of latest Proterozoic and Cambrian rocks indicate that there was

(1) a distinct fluctuation of carbon isotopes around the Proterozoic-Cambrian,

(2) a dramatic increase of the d³⁴S curve,

(3) an increase of the global sea-level,

(4) a distinct rise of the phosphorite production, and

(5) a slow increase of oxygen in the atmosphere from late Proterozoic to early Phanerozoic times.

These facts form the frame of a probably complex scenario, which ecologically equals the filling of an ecological barrel. However, we only hypothesize factors that may be responsible for a dramatic increase of phylogenetic development, such as possibly simpler Cambrian genomes or a more direct translation of gene to product, which may have enabled early diversification. Other hypotheses are needed to explain the rapid evolution and diversification of hard parts. Most of those hypothesis focus on changes in the physico-chemical environment and ecological stimuli (such as the evolution of the first predators). Regardless of the reasons, the novelty of hard parts led to more efficiency and improvements in the performance of animals and so is directly related to "advanced" animal groups such as arthropods and the group to which we belong, the chordates.

To learn more about the Cambrian Explosion you may want to browse the relevant pages of the Hooper Virtual Natural History Museum.

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