Cambrian Period In Texas

Cambrian Period In Texas

Texas laid under a shallow sea during the Cambrian Period. Land lying to the northwest contained sediments comprising of mostly sands that were carried into the sea. Other sediments, such as limestones and dolostones were deposited further out to sea. Those rocks, not visible on the map above, are found in the Llano Uplift area of Texas.  Fossils of sponges, gastropods, trilobites, bivalves and bryozoans are found within those deposits.

North American Continent during the Cambrian Period showing most of Texas under a shallow sea

 Gastropods are a large group of molluscs. Most have a coiled or conical shell, a muscular foot, eyes, tentacles and a feeding organ or radula. They first appeared in the Cambrian period and colonised all marine habitats. Some lived on the seabed, others burrowed into the mud or sand or attached themselves to rocks. 

  

Trilobites are an extinct group of animals that lived in the worlds oceans millions of years ago. They get their name from the way that their bodies are divided into three sections. Most trilobites are thought to have scavenged for food either under, or on the surface of the sediment of the sea floor although some would have been able to swim. They ranged in size from just one mm to about one metre in length.

These bryozoans are also known as moss-animals and are filter feeders.  Mineralized skeletons of bryozoans appear in rocks from Early Ordovician period, about 480 million years ago, while all other fossilized phyla appear in earlier periods. This has led researchers to suspect that bryozoans had arisen earlier but were initially unmineralized, and may have differed significantly from fossilized and modern forms.

Cambrian Explosion

Most major animal groups appear for the first time in the fossil record some 545 million years ago on the geological time scale in a relatively short period of time known as the Cambrian explosion. Of great worry to Darwin, the explanation of this sudden, apparent explosion persists as a sources of numerous major debates in paleobiology. While some scientists believe there was indeed an explosion of diversity (the so-called punctuated equilibrium theory elaborated by Nils Eldredge the late Stephen J. Gould - Models In Paleobiology, 1972, others believe that such rapid acceleration of evolution is not possible; they posit that there was an extended period of evolutionary progression of all the animal groups, the evidence for which is lost in the all but nonexistent precambrian fossil record. Modern molecular technologies (genomics and other omics), through comparing nucleic acid and amino acid sequences across living species, are enabling the identification of genetic components and patterns stingily conserved by evolution, from which times of evolutionary branching of the tree of life can be inferred.

The theory of the Cambrian Explosion holds that, beginning some 545 million years ago, an explosion of diversity led to the appearance over a relatively short period of 5 million to 10 million years of a huge number of complex, multi-celled organisms. Moreover, this burst of animal forms led to most of the major animal groups we know today, that is, every extant Phylum. It is also postulated that many forms that would rightfully deserve the rank of Phylum both appeared in the Cambrian only to rapidly disappear. Natural selection is generally believed to have favored larger size, and consequently the need for hard skeletons to provide structural support - hence, the Cambrian gave rise to the first shelled animals and animals with exoskeletons (e.g., the trilobites). The early Cambrian and the size of many animals also "exploded".

The Cambrian Explosion is the outcome of changes in environmental factors leading to changes in selective pressures in turn leading to adaptive diversification on a vast scale. By the start of the Cambrian, the large supercontinent Gondwana, comprising all land on Earth, was breaking up into smaller land masses. This increased the area of continental shelf, produced shallow seas, and expanded diversity of environmental niches in which animals could specialize and speciate.

The debate persists today about whether the evolutionary "explosion" of the Cambrian was as sudden and spontaneous as it appears in the fossil record. The discovery of new pre-Cambrian and Cambrian fossils help, as these transitional forms support the hypothesis that diversification was well underway before the Cambrian began. More recently, the sequencing of the genomes of thousands of life forms is revealing just how many and what genes and the proteins they encode have been conserved from the Precambrian. The explosion of external form in the fossil record is what we see, but more gradual adaptation was taking place at the molecular level. Wang et. al. (1999) for example, recently conducted phylogenetic studies divergences among animal phyla, plants, animals and fungi. These researchers estimated Arthropods diverged from more primitive chordates more than 900 million years ago, and Nematodes from that lineage almost 1200 million years ago. They furthermore estimated that the plant, animal and fungi Kingdoms might have split almost 1600 million years ago. Finally, they conjecture that the basal animal phyla (Porifera, Cnidaria, Ctenophora) diverged between about 1200 and 1500 million years ago. If their research is valid, at least six major metazoan phyla appeared deep in the Precambrian, hundreds of millions of years before the oldest fossils in the fossil record. 

(http://www.fossilmuseum.net/Paleobiology/CambrianExplosion.htm)

 

 


For more information:
http://www.paleoportal.org/index.php?globalnav=time_space&sectionnav=state&name=texas
http://ediacaran.blogspot.com/2010/10/evolution-among-trilobites-part-2.html
http://www.shropshire.gov.uk/legacy/llmrc.nsf/open-exhib/Poleumita%20discors%20%284%29
http://tapestry.usgs.gov/ages/ages.html

Paleontology and geology of Texas

  

Time Period:
	Quaternary Tertiary Cretaceous Jurassic Triassic Permian Carboniferous Devonian Silurian Ordovician Cambrian Precambrian
Dates (mya)

THE PRECAMBRIAN ERA 

(from 4.6 billion to 570 million years ago)

From the formation of its crust over 4.5 billion years ago, the Earth was covered by ocean. Within a billion years, simple one-celled bacterial organisms had appeared. The formation of Gondwanaland during the Late Precambrian (uniting the modern continents of the Southern Hemisphere) provided warm, shallow, off-shore environments where complex living organisms first appeared.

Precambrian Texas

Geologically speaking, Texas began to form about one billion years ago. Large amounts of coarse to fine sediment were deposited in an adjacent sea. An episode of mountain building buried and metamorphosed these sediments into schists and gneisses, and igneous processes produced magmas that cooled to form granites. By the end of the Precambrian, the mountain system produced by this tectonic event had eroded to near sea level.

Precambrian rocks can be seen today in the area commonly referred to as the Llano Uplift and in the Van Horn and El Paso areas of far west Texas. As the rocks are primarily igneous, fossils are rare to non-existent in the Precambrian rocks of Texas.

Llano Uplift showing fault-lines

The rocks exposed in the Llano uplift have been deformed by late Paleozoic extentional tectonics that was primarily accommodated by horst and graben structures that trend southwest to northeast. Fault-line escarpments mark the large-displacement faults that juxtapose Precambrian crystalline rocks against early Paleozoic carbonate rocks. In the semi-arid climate of the Llano, carbonate rocks are more resistant to erosion, therefore, the topographic ridges are held up by the grabens whereas the valleys are underlain by Precambrian crystalline rocks. In the photograph below, an outcrop-scale horst and graben structure is exposed within the Lion Mt. member of the Riley Formation.  Topographically high areas of the Llano uplift

Llano Uplif

are held up by the more resistant Paleozoic cover sequence. The exposure below overlooks Lake LBJ near Marble Falls, Texas, and is used as a type locality for several members of the Cambrian Riley and Wilberns Formations. In the photograph below the distinctive glauconitic Lion Mt. member of the Upper Cambrian Riley Formation is exposed along the roadside, and is overlain by the Welge Sandstone and basal portion of the Morgan Creek members, both being stratigraphic components of the Wilberns Formation. Mapping of these stratigraphic units is a major component of the projects conducted in Texas.

Red Rock Ranch, Van Horn, TX

                                                                                       

Red RockRanch has one of largest natural Precambrian sandstone exposures in North America, according to their website. The Ranch is located outside of Van Horn.

Exposures of Precambrian metamorphic rocks occur in the Van Horn Mountains of west Texas. These exposures are part of the igneous-metamorphic platform on which Paleozoic and younger sedimentary rocks were deposited in the southeastern United States. Knowledge about their history comes basically from these isolated exposures and from samples taken from wells drilled through the sedimentary cover. The youngest of these rocks appear to be about 1 billion years old based on radiometric dates. The oldest Cambrian sedimentary rocks are only 570 million years old. This suggests that a long period of weathering and erosion strongly modified the Precambrian surface after emplacement of the intrusive igneous rocks and eruption of volcanic rocks. A substantial amount of rock and historical record was probably removed before the first sediments were deposited in Cambrian seas.

Precambrian Animal Life

It is not known when life originated, but carbon in 3800 million year old rocks from islands off western Greenland may be of organic origin. Well-preserved bacteria older than 3460 million years have been found in Western Australia. Probable fossils 100 million years older have been found in the same area. There is a fairly solid record of bacterial life throughout the remainder of the Precambrian.

Excepting a few contested reports of much older forms from Texas and India, the first complex multicelled life forms seem to have appeared roughly 600 Ma. A quite diverse collection of soft-bodied forms is known from a variety of locations worldwide between 542 and 600 Ma. These are referred to as Ediacaran or Vendian biota. Hard-shelled creatures appeared toward the end of that timespan. So, very little evidence of life did exist in present day Texas.

For more information, visit 

http://www.paleoportal.org/index.php?globalnav=time_space&sectionnav=state&name=Texas

http://www.richardancheta.com/html/encyclopedia/Animal,Plants,History&Painting/Animal,Plants,History&Painting-Precambrian.htm

http://soilphysics.okstate.edu/S257/book/geology/
http://www.southalabama.edu/geography/allison/gy480tour.htm