Appearance Of Skeletons Research Articles

Cambrian Explosion: A Complex Analysis of Facts

Most researchers attribute the appearance of skeletons to some arbitrarily chosen factors. Many aspects of the phenomenon (the diversity of the composition of the remains, the mass nature of the phenomenon, geological immediacy, the role of geological and biotic factors, etc.) remain unexplained in this case. A comprehensive analysis of facts from different branches of science (lithology, tectonics, chemistry, biology, paleontology) allows us to explain (in addition to the listed) the smallness of Cambrian organisms, the replacement of chemical precipitation by biological, as well as the widespread development of bilaterality, the emergence of new taxa of high rank, and the morphological gap between the Ediacaran and Cambrian faunas. Both abiotic and biotic factors were important: Without the active participation of the living in the precipitation of salts, the formation of skeletons would not have been possible.

Early skeletal fossils

The Precambrian-Cambrian transition saw the burgeoning of diverse skeletal organisms (“small shelly fossils”), represented in the fossil record by spicules, tubes, tests, conchs, shells, and a variety of sclerites and ossicles. Whereas calcareous biomineralization as such may have been facilitated by changes in ocean chemistry at this time, the utilization of biominerals in mineralized skeletons is a different process. The massive appearance of skeletons is most likely an epiphenomenon of the general radiation of body plans and tissues. The “choice” of biominerals (mainly calcium carbonates, calcium phosphates, and silica) may reflect the environmental conditions under which the particular skeleton first evolved.

Environmental Changes and Evolution of Marine Invertebrates from the Late Precambrian to the Cambrian

The rapid appearance of skeletons in various marine invertebrates in the beginning of the Cambrian produced the sudden explosive increase of the animal fossils. This dramatic evolutionary event was not performed by the explosive diversification of marine life but simply by the physiological changes of the organisms in increasing oxygen pressure in the earth's atmosphere. According to the recent physiological studies, various marine animals can survive in low dissolved oxygen conditions of rather long period with an anaerobic metabolism, suggesting the respiratory mechanisms of the primitive forms in the Precambrian. The anaerobic metabolic activities imposed some physiological restrictions on the animals through the acidic end-products (eg, inhibition of skeleton formation, small body size, and low metabolic efficiency etc.). The skeleton formation in the Cambrian life was initiated by the advent of aerobic metabolism, which indicated the release from the physiological restrictions by the anaerobic metabolism.

An Ecological Theory for the Sudden Origin of Multicellular Life in the Late Precambrian

According to modern ecological theory, high diversity at any trophic level of a community is possible only under the influence of cropping. Until herbivores evolved, single-celled algae of the Precambrain were resource-limited, and a small number of species saturated aquatic environments. In the near-absence of vacant niches, life diversified slowly. Because the changes required to produce the first algae-eating heterotrophs were therefore delayed, the entire system was self-limiting. When the "heterotroph barrier" was finally crossed in the late Precambrian, herbivorous and carnivorous protists arose almost simultaneously, for no major biological differences separate the two groups. These events automatically triggered the formation of a series of self-propagating feedback systems of diversification between adjacent trophic levels. Comparable systems arose among multi-cellular groups, which radiated rapidly from the newly diversifying protist taxa. The sudden proliferation of complex food webs formed by taxa invading previously vacant adaptive zones produced an explosive diversification of life over a period of a few tens of millions of years. The rapid appearance of skeletons in various groups, though of special geological importance, was no more dramatic than other aspects of the radiation. The overall rate of diversification was comparable to rates for less-extensive adaptive radiations of the Phanerozoic.