The Cambrian Explosion and Representative Early Life

Author: John Smith
Reviewed by: Dongda

A Few Words Before We Begin

  As its name suggests, paleontology studies life from its earliest origins through to the recent past. Geologists have divided Earth's history into many units according to the order of rock strata and the fossils they contain. Much as biology classifies species into kingdom, phylum, class, order, family, genus, and species, the geologic time scale is divided into eons, eras, periods, epochs, ages, and chrons. Every smaller unit belongs within a larger one. The relationship is like that of the Zhìdé era name (583–587), which belongs to the Chen dynasty, itself part of the Northern and Southern Dynasties. Broadly speaking, units larger than a period span too much time, while smaller units are too fine-grained for our purposes. Familiar names such as the Jurassic, Triassic, and Cretaceous are all periods.
  The word “Cambrian” originally came from studies of a rock stratum in Cambria, Wales. A Japanese scholar later represented its sounds with the Chinese characters 寒 (kan), 武 (bu), and 纪 (ki)—kanbuki—and the term then entered Chinese. Unlike the names of other periods that were translated according to their meaning—for example, the Cretaceous was named after accumulations of chalk—this term is purely phonetic and does not derive its meaning from the Chinese characters.

Main Text

  As the title suggests, we focus on the Cambrian because the period is distinguished by an explosion of life. Before the Cambrian, life consisted mainly of algae and bacteria, along with a very small number of invertebrates. We can study this early life only through the relatively few surviving fossils and microfossils (Figures 1 and 2).
Figure 1: Graptolite
Figure 2: Algae
  During the Cambrian, large numbers of marine invertebrates appeared, with levels of complexity far beyond those of earlier organisms. Consider one of our old friends: the trilobite.
  Trilobites are probably among the best-known and most ancient forms of life. As arthropods, their bodies were covered by segmented, calcified exoskeletons, giving them an unusually high rate of fossilization. Trilobites are among the most frequently unearthed fossils from this period. On the one hand, however, this may be because their calcified shells helped them fossilize. Algae clearly do not leave fossils as readily as trilobites.
  When we study the fossil of an organism, what do we hope to learn?
  First, we want to understand what makes it distinctive. Might it actually be an organism we already know? How should it be classified? How did it behave when it was alive? What environment did it inhabit? Did it live in symbiosis with other organisms? What did it eat? To begin answering these questions, let us first look at a classic arthropod.
  Its body is divided into a head, thorax, and abdomen; it has many segments, paired symmetrical appendages, and a hard outer covering.
  • Head, thorax, and abdomen √
  • Body segments √
  • Symmetry √
  We clearly cannot reconstruct the hard shell of an organism from hundreds of millions of years ago, but several approaches allow us to infer what it was like. Start with the fact that we have its fossil. Hard bones and exoskeletons fossilize more readily, while soft tissue almost entirely disappears. This also explains why mollusk fossils are rare. The survival of large numbers of fossils like these to the present day further suggests that the structures were skeletal. Because they do not look like endoskeletons, we can still conclude that they were exoskeletons.
  Once we have concluded that the organism was an arthropod, the stratum in which its fossils were excavated and the other fossils found in that stratum allow us to infer that it was a marine arthropod. Because it had no obvious swimming or buoyancy organs, we believe it lived on the seafloor.
  What, then, were its habits? Was it a predator, or did it merely filter nutrients from sand and seawater? At this point, we can examine other fossils that have been discovered:
Figure 3
Figure 4
  In Figure 3, we see a fossil of a trilobite attached to another organism. The enlarged area of the pattern shows a clear boundary, which we can take to indicate that they were two separate organisms. Now consider the fossil in Figure 4. Within the impression left by the larger trilobite, we can clearly see the trace of a smaller organism. Both specimens suggest that trilobites—or at least some species of trilobite—were predators.
  These observations represent only a preliminary analysis of the fossils. A competent paleontologist studies different kinds of fossils from an organism to build a complete picture of the species. In the vast flow of history, every living thing will eventually become part of the past. Studying that past can therefore guide us toward a better future.

Image Sources

https://www.fossilguy.com/species/invert/trilo/utah/agnostid_trilobite_fossil3b.jpg
http://www.evolution-biologique.org/echelle-du-temps/echelle-des-temps-geologiques/ordovicien/la-locomotion-des-trilobites.html