The so-called ” Cambrian explosion ” was a great event of diversification for living beings, a great explosion of life, where many of the large groups of animals that we know today appear for the first time in the fossil record. And this, this sudden appearance in the fossil record, is partly a consequence of the appearance of the first “hard parts” in their bodies: the first skeletons and shells facilitated the fossilization process. The appearance of these large groups of animals could have been rapid in geological terms, a process that could have lasted about 10 million years.
But to know how such a revolution took place, we must first go back a few million years, to the end of the Proterozoic eon , about 600 million years ago . Until this time, the continents had been grouped into a supercontinent, a first Pangea. At this time they began to fragment, volcanic activity increased, and the atmosphere was enriched in gases of volcanic origin, such as carbon dioxide, which could lead to higher temperatures.
Thus began a warm period on Earth at the start of the new eon, the Phanerozoic , whose first geological era is the Paleozoic. This era, in turn, is divided into six periods. And precisely the first of them is the Cambrian. That is why we usually speak of the Cambrian as the moment when so many things begin.
The disintegration of this first Pangea gave rise to four large continents. The largest, moving south, is called Gondwana . Three minors moved north, and are called Laurentia, Baltica and Siberia . In addition to being a warmer period, the appearance of more coastlines and shallow seas between these continents were ideal for life, and evolution, to make their way.
The first explosion of new forms of living things was in the form of small shells and shells, which is why it is known as the Small Shell Fauna or SSF (from English, Small Shelly Fauna ). This is the first time in the geological record that we see fossils of hard, mineralized skeletons: shells, spicules, capsules, tubes… Some interpreted as individual shells or shells of small organisms, others as small parts of larger organisms. And because of the similarity of these small elements with those of current animals, we can suggest that some of these animals could have resembled sponges or mollusks, but most of them are of unknown affinities .
But without a doubt, the main deposit responsible for the fame of the Cambrian explosion is the Burgess Shale , whose fossils date from a little later, from the Middle Cambrian , about 530 million years ago. It is one of those sites that paleontologists call “exceptional conservation” because they preserve impressions of the soft parts of animals, and even complete bodies. Among the slabs of this site, a large number of fossils have been recovered. Some of them are identifiable as groups that continue to live today, such as molluscs, arthropods, annelids or worms, sponges, or chordates (a group in which we include vertebrates). In fact, the first chordate found in the fossil record, a distant relative of ours, is a small eel-like animal called Pikaia , which would already have a kind of rod called the notochord (the ancestor of our spine) and muscle bundles on both sides. of it with which to swim efficiently.
But if we always talk about the evolutionary radiation of this period being unprecedented, it is because of the number of new forms that appeared at this time, especially those that have not reached our days. Groups of molluscs or arthropods appeared that later became extinct without leaving descendants. And even entire groups of animals that we have only been able to know from the fossil record. For example, speaking only of arthropods (a group to which insects or crustaceans belong, among others), this is the time when the first trilobites appear (arthropods that dominated marine fauna throughout the Paleozoic era) but also much stranger forms. , like the predator Anomalocaris , provided with fins, two large eyes, two large jointed appendages with which to catch its prey, and a circular mouth.
But this explosion of animal diversity was not the only great Cambrian revolution. Until the appearance of these new animals, life forms were limited to living on the sand or mud substrate of the seabed. In all the geological record prior to this moment, the sedimentation shows this: the seabed consisted of a harder substrate, well stratified and covered with mats or bacterial films. But with these new, more complex and active animals, many with shells and shells, this changed forever. They began to actively move through the substrate, to move the particles, to dig and make their burrows in it… and the result is a greater use of these “soils”, which is also reflected in the fossil record with the appearance of dozens of footprints of their activity, such as excavations and burrows. This change in the seabed has been given the name “ Cambrian Substrate Revolution” or “ Cambrian Agronomic Revolution ”.
It is partly to blame for this Cambrian revolution and explosion (fault or merit, depending on how you look at it) that the paleontologists Stephen Jay Gould and Niles Eldredge proposed a mechanism for the evolutionary processes, the so-called “ punctuated equilibrium ” or “ interrupted equilibrium ” ( of the English “ Punctuated Equilibrium ”), according to which the changes over time in living beings can be almost negligible until, from time to time and due to changes in their environment, the evolutionary rate shoots up. And it is that in the Cambrian it did change our world forever, despite the fact that this period is popularly unknown.
References :
Eldredge, N.; Gould, S.J. 1972. Punctuated equilibria: an alternative to phyletic gradualism. In: Models in Paleobiology. Freeman Cooper, 82-115.
Gould, SJ 1999. The Wonderful Life . Review.
Seilacher, A.; Pflüger, F. 1994. From biomats to benthic agriculture: A biohistoric revolution. In: Biostabilization of Sediments. Universität Oldenburg (BIS)-Verlag, Oldenburg, 97-105.
