The Cambrian explosion is a remarkable period in world fossils history. It is the period when all the varieties of animal phyla were first noted. (Darroch et al., 2019, p. 659).
Scientists have tried to review the whole concept to get a proper explanation of the Cambrian explosion. Studies have developed environmental evolution, genetically, and ecological reasons for the process by linking each aspect to its role in phyla animal origin. Considering the onset of the radiation can be related to environmental evolution (Zhuravlev & Wood 2018, p. 8). In terms of the period it took, the explosion seems to be controlled by developmental innovation. The uniqueness of the process is related to ecological saturation.
Charles Walcott, a paleontologist, was the first to identify the unique fossil discoveries in the rocks. Over sixty thousand specimens were discovered in the proceeding research, and about one hundred and seventy species were detected. It is believed that before fossilization, the species stayed under warm and well-oxygenated water, making them be preserved well. Later on, these animals were swept into deeper waters and buried under sediments (Steele et al., 2018, p.18). Burring them in regions of insufficient oxygen supply protected them against predators. Then there emerged new animals with diverse body forms that could bio-mineralize substantial body parts such as exoskeleton.
Many scientific fields have now majored in exploring the massive diversification of animals, which has improved the process. Recent discoveries have proven that the radiation resulted majorly due to environmental evolution. Increase oxygenation of the ocean, changes in the chemical composition of the sea, and deposition of methane into the environment caused ecological hostility. In the attempt to adapt, the new environmental diversification intensified. The then available species tried to modify their bodies to have defense mechanisms. Massive extinction of many species was experienced because they failed to adapt.
Developmental genes primarily regulate animal diversification. Research done on the modern metazoan reveals that their developmental genes have about twenty genes which account for a fraction of all the genes (Daley et al., 2018, p. 5328). It is suggested that the bilateral developmental system and Hox genes expansion led to the Cambrian explosion and animal diversification. The Hox genes are the earliest discovered developmental genes, with each ancestral metazoan having two Hox genes.
Molecular clock and fossil proof show that the bilaterians’ developmental system was in existence before phyla’s first appearance. Therefore, the ecological explanation of the Cambrian explosion is becoming more common. The predator-prey development is the most explained ecological factor of animal diversification. This led to new adaptive strategies such as skeletonization, camouflaging, and venous attacks. Consequently, the animals’ morphological function diversified to help the animal defend itself against aggression (Cai et al., 2018, p. 388). The ecological explanation, however, is questionable because of the confound repercussions with causes. It is argued that feedback mechanisms to environmental factors can lead to biodiversity but not the Cambrian radiation.
Currently, trilobites exist only in fossil forms because of their extinction after the Cambrian period. They are easily recognizable because of their characteristic body that had a three-segmented form (Paterson, Edgecombe & Lee 2019, p. 4397). During the Paleozoic Era, trilobites were a diverse and very essential class of water invertebrates. They exclusively existed in the marine environments and varied in length from a centimeter to about one meter. Once during the Ordovician, Devonian, and Cambrian periods, the trilobites thrived as the most successful of all species. The focus on the trilobites is because of their adaptive body forms made up of a strong skeleton than the shells that were easy to fossilize.
Though arthropods have a rigid skeleton, it is non-mineralized. The primary fossil of arthropods is the trilobites. Trilobites are the ancient form of arthropods that went extinct after the Cambrian period. The body forms and patterns of arthropods can be connected to those of trilobites. Studies done on the Hox gene patterns expressed by arthropods can be related to the Hox gene in the trilobites. Arthropod existence began from trilobite fossils with modifications to adapt to the new environment. The improvements included a grouping of body segments to form distinctive body parts.
The Mollusca is one of the largest groups of phylum class of invertebrates. The group consists of gastropods, bivalves, cephalopods, and polyplacophorans. It is believed that the ancient Mollusca had eyes, but there is no fossil proof. Scientific suggestions argue that gastropods, cephalopods, and polyplacophorans had eyespots that had patches of sensitive photoreceptor proteins that react in the presence of light but would not distinguish between dark and light (Cai et al., 2018, p. 390). As most animals turned to predation during the Cambrian period, the eyespots evolved by forming depressions that allowed them to determine the direction of light. The pits deepened and photoreceptive cells such as pigmented retina cells developed allowing them to perceive the visual details.
The eyes of gastropods and cephalopods are situated located on the head and occasionally on the tails. Bivalves and polyplacophorans have eyes at random places, such as edges of the shell. Gastropods and cephalopods have paired eyes, while polyplacophorans have multiple eyes. Most bivalves lacked eyes, while others had simple eyes consisting of photosensory cells. In terms of shape, gastropods had cup-shaped eyes. Polyplacophorans had flat oval-shaped eyes, and cephalopods had spherical-shaped eyes.
Cephalopods’ eyes size was about 2 centimeters those of gastropods range between two micrometers to eleven inches, and polyplacophorans eyes size varies between sixty-five to eighty micrometers (Daley et al., 2018, p. 5330). The Mollusca group’s eyes complexity differed as per the group. Gastropods had pit eyes, cephalopods eyes were lensed, and those of polyplacophorans were multi-valved. Paleontologists are stilling conducting research on the Cambrian radiation and with time it will be understood well.
References
Cai, Y. et al. (2019). Diverse biomineralizing animals in the terminal Ediacaran Period herald the Cambrian explosion. Geology, 47(4), pp. 380-394.
Daley, A. et al. (2018). Early fossil record of Euarthropoda and the Cambrian Explosion. Proceedings of the National Academy of Sciences, 115(21), pp. 5323-5331.
Darroch, S. et al. (2018). Ediacaran extinction and Cambrian explosion. Trends in Ecology & Evolution, 33(9), pp. 653-663.
Paterson, J.R., Edgecombe, G.D. and Lee, M.S., 2019. Trilobite evolutionary rates constrain the duration of the Cambrian explosion. Proceedings of the National Academy of Sciences, 116(10), pp. 4394-4399.
Steele, E.J. et al. (2018). Cause of Cambrian explosion-terrestrial or cosmic? Progress in Biophysics and Molecular Biology, 136, pp. 3-23.
Zhuravlev, A.Y. and Wood, R.A., 2018. The two phases of the Cambrian Explosion. Scientific Reports, 8(1), pp. 1-10.