Thesis on "Environmental Settings of the Cambrian Explosion"

Thesis 10 pages (3368 words) Sources: 18

[EXCERPT] . . . .

Environmental Settings of the Cambrian Explosion

The objective of this work is to examine the development of natural environments alongside the evolution of life throughout the Cambrian explosion. This work will focus on beginnings of life, their natural environments and their evolution in changing environments from the beginning to the end of the Cambrian explosion. The key theme of this essay is the history of life during the Cambrian explosion. This work will attempt to deduce patterns of evolution alongside the development of natural environments.

The Cambrian explosion has the appearance of being a dramatic and sudden appearance of various complex animals approximately 540 million years ago however; it is know that this was not the origin of complex life forms. (Morris, 1985) There are two views on the Cambrian explosion. The first view is that it was indeed a true explosion and that an organism, yet unidentified, was the inventor of biomineralization which resulted in rapid radiation of shelled creatures over a period of ten million years. The other view is that there was a diversification of the phyla earlier and that these developed evolving with biomineralized shells at approximately the same time which is attributed to mineral availability at appropriate levels, rising O2 levels or being triggered by predation appearing. The focus of this study is the problem that is noted in the rapid and mass evolution of animal forms which occurred during the Cambrian explosion.

I. The Burgess Shale - 505 MYA - British Columbia, Canada

The Burgess shale is a Cambrian location in British Columbia which claims fossils of preserved soft-bodied specimens and as well contains species in large number generally not found preserved in the fossil record. This site was discovered in 1907 by Walcott and the fossils were reanalyzed in the 1970s by Whittington and a group that utilized more modern type equipment. After years of analysis they disagreed with Walcott, who claims that the fossils were Metazoan phyla and classes and stated that they represented Metazoan phyla of a novel type. The reconstruction of these fossils resulted in strange creatures with unusual patterns of body. The contemporary understanding of evolution is stated by Fenchel (2002) to explain directional evolution as the replacement of alleles in species populations driven by natural selection." (2002) Fenchel states that there is "an overwhelming amount of theoretical and experimental documentation as well as data deriving from paleontology and natural history" that provides evidence that this "description is fundamentally correct." (2002) it has been suggested by some in their writings that the Cambrian explosion resulted from a rise in the oxygen in the atmosphere

The base of the Cambrian Period as well as the base of the Cambrian system of classification is located in Newfoundland and is stated to be "where a particular trace fossil, known as Trichophycus pedum" (the Cambrian Explosion, nd) first appeared. There are stated to be only a few fossil animals dating back 543 million years ago such as these and the Ediacaran assemblage has for the most part disappeared with only a few very small smelly organisms or shelly fauna appearing until the Cambrian Explosion. Bowring et al. (1993) states of the Cambrian Explosion: "This explosion is perhaps the most striking single event documented by the fossil record. In the strict sense, the explosion refers to a geologically abrupt appearance of fossils representing all except two of the living [animal] phyla that had durable (easily fossilizable) skeletons. One of those two phyla is the Porifera (sponges), which was present in the fossil record at an earlier time. The other is the Bryozoa, a phylum that contains some soft-bodied groups and may well have been present but not yet skeletonized. A number of enigmatic organisms of obscure relationships also appear during the explosion, enriching the early Cambrian fauna. Precision dating indicates that the explosion began at 530 Ma (million years ago) and ended before 520 Ma." (Bowring, et al., 1993)

Documented in the Cambrian fossil record is a radiation of skeletal morphologies of a very real and dramatic nature representing the skeletonization of many lineages that were previously soft-bodied as well as an accelerated diversification within lineages that were already skeletonized. Resulting is stated to be a "disparity - as measured by the number of major animal types - was at least as great as the present." (Gon, 2005)

II. Maotianshan shales (Chengjiang), 525 MAY, Yunnan Province, China

The Chengjiang biota was discovered by Hou Xianguang in 1984. These are soft-bodied animals which have been preserved in fine mudstones with four species recorded including: (1) Eoredlichia intermedia; (2) Kuanyangia pustulosa; (3) Yunnanocephalus yunnanesis; and (4) Wutingaspis tingi. (Gon, 2005)

III. Emu Bay shale, 525 MYA, South Australia

The Emu Ban Shale formation is located on Kangaroo Island in South Australia and is inclusive of faunal elements including "Anomalocaris, Tuzoia, Isoxys, Xandarella, and Primicaris." This site also features wonderful specimens of trilobites including: (1) Redlichia takooensis; (2) Emuella polymera, (3) Balcoracania dailyi, and (4) Estaingia (=Hsuaspis) bilobata. (Gon, 2007) the Emu Bay shale is found in shallow water deposition which indicates that preservation of soft tissue "occurred in a range of environmental settings during the Cambrian." (Gon, 2007) Gon writes that the Emu Bay Shale was at first thought to be late in the Cambrian Explosion however calibration reveal that the "occurrence of R. takooensis and species of Hsuaspis matches the Tsanglangpuian in the Chinese sequence and contemporary South Australian faunas correlate with the Botomina of Siberia. (Gon, 2007) Therefore, Gon states that the Emu Bay Shale age lay between the "Middle Cambrian Burgess Shale, and the upper Atdabanian Chengjian of China." (Gon, 2007)

IV. Sirius Passet, 518 MYA, Greenland

The work of James W. Valentine (2002) entitled: "Prelude to the Cambrian Explosion" relates that the Sirius Passet fossil located in Greenland dates "from or immediately after the explosion interval" and has "yielded fossils that were preserved under exceptional circumstances that many details of their soft-bodied anatomy can be observed. Many of these softbodied forms belong to phyla that lack durable skeletons altogether and would not be known from those early times except for the unusually preserved fossil assemblages." (Valentine, 2002) Valentine states that it can be "said with confidence...that the biological factors necessary to produce the Cambrian explosion were evolved during late Neoproterozoic and earliest Cambrian times, an interval which forms a Prelude to that remarkable event." (2002) Valentine states that the Early Cambrian fossil metazoan "...provides the best indication as to what must have been accomplished during the evolutionary Prelude to the Cambrian explosion. The base of the Cambrian is marked by the appearance of larger penetrating burrows." (2002) the boundary is stated to be drawn "at the earliest appearance of the trace fossil Trepnichnus pedum in the Chapel Island Formation, Burin Peninsula, Newfoundland." (Valentine, 2002) Valentine relates that T. pedum is "an arcuate horizontal burrow from which branches rise to probe toward the surface. The earliest Cambrian stage, the Manyiaian or Nemakit-Daldyn, began approximately 443 mya and lasted until the explosion, a period that is at least 13 Ma long and may be as long as 23 Ma" according to Grotzinger et al. (1995) and Landing et al. (1998). Valentine additionally relates that lower Cambrian trace fossils "are generally larger than those of the Neoproterozoic." (2002) Valentine states that there are in existence "...a number of ecologically-based hypotheses that speak to the taxonomic richness of the Cambrian explosion..." which are of the nature that may "imply conditions during the prelude..." (2002)

The work of PV Sukumaran (2004) entitled: "Cambrian Explosion of Life: The 'Big Bang' in Metazoan Evolution" Recent advances in genetics and molecular biology have shed new light on genetic controls of body plan development in metazoan phyla. New fossil discoveries and molecular techniques have also brought in controversies: do the metazoans have deep evolutionary roots in the Precambrian or do the paleontological data confirm a major evolutionary mile- post in the early Cambrian? Besides, molecular evidence shows that regulatory genes that control development of morphology in animals are fairly similar in all phyla, but give rise to very disparate body plans. These advances are providing new lines of evidence to look into the origin of metazoans and thereby into the mystery of the Cambrian explosion." (Sukumaran, 2004) According to Sukumaran (2004) the 'neutral theory of molecular evolution "postulated by Motoo Kimura" holds that the majority of the nucleotide substitutions in genes arising out of mutation are selectively neutral or of little functional consequence to the organisms." However, it is related that there was a slight modification of this theory later on for accommodation of the "observation that most molecular evolutions involves slightly deleterious substitutions rather than strictly neutral ones." (Sukumaran, 2004) Sukumaran states that molecular data is comprised of "long sequences of nucleotide bases in the nucleic acids" forming the genetic text in DNA. Sections of nucleotide sequences which "code for a particular protein are called genes" which are nucleotide sequences yet the product of their protein are amino acid sequences."… READ MORE

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