Reading Practice 19

[1] About 542 million years ago, a large diversity of animal phyla appeared. This spurt of evolution is called The Cambrian Explosion and is backed up by further fossil evidence. Over the course of the first 10 million years, marine animals developed the basic body forms now seen in modern groups. The record includes fossils of crustaceans, mollusks, sponges, algae, and starfish. However, animal fossils from the Precambrian era have been found, indicating that many animal phyla actually developed prior to the Cambrian explosion. In fact, plant fossils have been found dating back to 1400 million years ago in Montana, USA, and China. These fossils represent organisms that do not resemble any modern organism yet prove that animal life did exist.

 

[2] The Cambrian Explosion lasted for about 20 to 25 million years, resulting in the divergence of most modern metazoan phyla. Prior to the Cambrian Explosion, many animals were unicellular organisms. Over the course of the explosion, the rate of diversification accelerated, resulting in many of the phyla we have today.

 

[3] The appearance of Cambrian fauna provokes several questions. These questions include “why was there was such a huge diversification of complex organisms over a specific period of time?” and “What is the source of this change and what does it suggest about the origin of animal life?”. Trilobites were the first fossils of the Cambrian Explosion to be discovered. In his book, On the Origin of Species, Charles Darwin names this appearance of trilobites as troubling to his theory of natural selection, because this group has no apparent antecedents.

 

[4] Paleontologist Charles Walcott suggests that an interval of time – he calls it the ‘Lipalian’ Era – actually existed but is not proven by a fossil record, and the predecessors of Cambrian animals lived during this time. Harry B. Whittington did a new analysis on Burgess Shale fossils in the 1970s with the conclusion that many were complex but different to modern animals. For example, the spiny slug-like Wiwaxia was different from any modern animal. Whittington and other researchers suggested that modern animal phyla developed at the same time during a specific period of time. This idea led to the theory of punctuated equilibrium. This theory develops Darwin’s theory of evolution as a series of long intervals, punctuated by short periods of fast change. Another theory suggests these unique animals – such as the Wiwaxia – evolved prior to the Cambrian era.

 

[5] When examining fossils, it’s important to remember that two-thirds of fossils of living fossils have never been found; therefore, the fossil record of animal phyla is incomplete. It is rare to find fossils of soft-tissued animals. However, the Cambrian fossil record includes many lagerstatten fossils, which preserved soft tissues. These permit paleontologists to study the internal anatomy of animals. Trace fossils are made up of tracks and burrows and marks left by feeding. These types of fossils represent another source that is not limited to animals with hard parts. We can also study the behavior of organisms from these fossils. These trace fossils might supply the earliest physical proof of the appearance of complex animals.

 

[6] Around the start of the Cambrian Explosions, many chemical markers point to a huge change in the environment. These markers are consistent with a mass extinction or warming resulting from the release of methane ice. Indeed, this sudden change could have been a cause of the Cambrian Explosion. Undeterred by the uncertainties, the chemical evidence encourages scientists to concentrate on theories consistent with environmental change.

 

[7] Cladistics is a technique in biological classification in which animals are grouped together based on whether or not they have one or more shared, unique sets of traits that are inherited from the group’s last common ancestor. This technique results in a set or group of clades. This technique helps to ascertain the date that these lineages first appeared. We can also guess how long two living clades had diverged from the last ancestor. This results in an approximate dating of the lineages.