Devonian Period: The Age of Fishes & Ammonites - Research Paper

Introduction

In Geologic time, the Devonian Period occurred around 419.2 million and 358.9 million years ago. It happened after the depletion of marine creatures during the Silurian Period. It is also referred to as the "Age of Fishes" because of the many and various types of marine creatures that were swimming in the Devonian seas (Becker, Konigshof & Brett, 2016). Moreover, forests and ammonites (coiled shell-bearing seas creatures) were also noticed during the Devonian Period. The ammonites appeared in the early period while the four-legged amphibians appeared later. This demonstrated that the vertebrates had colonized the land. The period led to the expansion introduction of the first harks, amphibians and plants with stomata, roots, and leaves. Although these were noticeable changes, the period is popular for its mass and diverse marine life. There was the appearance of fish with not only cartilage but also bony skeletons. These primitive ancestors of fish comprise the majority of modern fish in every aquatic environment.

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The Devonian period is named after a country called Devon, southern England. The discussions about the rock structure and age spread all over the country in the Devonian timescale. Many significant changes on the earth geography occurred during the period. It occurred when the land was divided into Euramerica and Gondwana, the two largest continents. However, during the period tectonic activities occurred drawing closer the two continents. The collision of Baltica and Laurentia formed the continent of Laurussia (Euramerica) that revolved to the dry zone beside the Tropic of Capricorn. It resulted in Ferrel and Hadley cells which the current large air-masses. Although they were in different hemispheres, they were close to each other. The other world was covered by a large uninterrupted ocean. Also, the continents were bounded by many subduction zones. The zone surrounding Euramerica and Gondwana drew them together to create Pangaea, a massive single continent. The Devonian period is subdivided into three, early, middle, and late. The Early Devonian occurred 419 million years ago (Becker at al., 2016). It is the period that ammonoids first appeared on earth. The middle Devonian was the period associated with the extinction of the jawless fishes in the aquatic environment. Earth fish began to appear in the oxygenated water due to the existence of plants. The late Devonian mark when plants took shape on earth.

Climate

The climate during the Devonian was warm and without glaciers. The temperature slope from the equator was shorter than today making the weather cold. Moreover, the equator was the hottest region and weather was arid for normal creatures to survive. This is evident because of the Northern Hemisphere's extensive spreading of evaporating basins coal deposits in Spitsbergen and Arctic Canada. It is demonstrated by the extensive carbonate reefs and desert conditions. The high temperatures caused high evaporation of salt deposits which mostly found in Australia. The levels of carbon dioxide dropped gradually to approximately 5 degree Celsius as new forests evolved (Becker at al., 2016). The temperature destroyed the organisms in reefs and later it led to the extinction of stromatoporoid.

Devonian Life

In addition to global shifts, numerous significant transformations occurred. For instance, Europe and North America continents collided causing the formation of large granite. The Appalachian Mountains that are found on the eastern side of the North American were also formed. Devonian reefs were also formed due to carbonate production creatures that make wave resistance structures near the sea level. Kimberley Basin in Australia is one of the current barrier reefs that extended some miles to fringe the Devonian continent. The Devonian reefs were constructed by stromatoporoids, tabulate corals, calcareous algae among others while the current reefs are composed of calcareous and corals algae. In the worldwide, sea levels were relatively high and tropic reefs organisms stayed on land that lay on shallow seas. The rest on the earth was covered by the universal ocean, the Panthalassa. Most of the planet's largest reef complexes were made through extensive reef building. They were formed in equatorial seas of the two previous continents. Large seas in central Asia, North America, and Australia serve as basins storing gypsum, rock salt, and other minerals triggered (House, 2019).

Marine Life

Vertebrates and invertebrates animals lived during the Devonian because sea levels in the Devonian were relatively high. Aquatic faunas were dominated by mass and different brachiopods and bryozoa. Other faunas that were found in marine include microconchids, crinoids, corals, trilobites, and hederellids. Small invertebrate called dacryoconarids occupied surface water in the early while ostracods in the late period. Protozoa such as sponges, Radiolaria and Foraminifera also existed. Bryozoans can be defined as aquatic moss faunas that resemble corals. They dominated the aquatic environment during the Devonian period, particularly in shallow shelf seas. They appeared in both stony netted and netted form. Other marine species that dominated the aquatic environment during the Devonian period were the brachiopods. They belong to the group of filter-feeding species similar to clams and existed in a group of different forms. The most common type is the spore-bearing spiriferoids which have been utilized as index fossils. The spiny mud-dwelling productids and loop-bearing terebratulids are the two significant types of brachiopods that appeared. However, some groups such as pentamers and orthids became extinct. They were small species (2.5cm) in width or length (Becker at al., 2016).

Among vertebrates, the Devonian fishes are found in the fossil record because they were armored. The earliest fishes such agnathans were jawless and probably hunted in the mud. They belong to the class of fish called ostracoderms and include the osteostracan cephalaspids, anapsids, and arthrodires had a brain and nervous system. Their bodies were protected by plate-like amour. The anaspids also were sheltered by scales to protect their bodies. Most of these groups depleted in the early Devonian and the gnathostomes (jawed fish) emerged in both fresh and seawater. Moreover, the ammonites emerged during or before the early stage about 400 Mya. Armored placoderms were many during the early and middle stages but, became extinct in the Late Devonian, due to competition for food against the other jawed fish species. Fish boney fish with cartilage such as Osteichthyes and Chondrichthyes dominated the Devonian seas. In the middle stage, sharklike fishes, the chondrichthyans and osteichthyans appeared.

In the late stage, numerous diverse fish emerged such as the rhipidistians, Dipnoi (lungfishes), and the coelacanths. In the late period, the four-footed amphibians and the remaining higher groups of vertebrates also appeared in seas.

Moreover, the first numerous genus of shark, Cladoselache, also emerged during this period. The Devonian period is called the "The Age of Fish" because of the great diversity of fish that appeared around at the time.

Early shark Cladoselache, several lobe-finned fishes, including Eusthenopteron that was an early marine tetrapod, and the placoderm Bothriolepis in a painting from 1905

The early plants had no leaves or roots similar to extant plants, but they have vascular tissue. The premature land plants spread by spores and vegetative growth for instance Drepanophycus.The Cooksonia also belong to the early terrestrial plants contained leafless terminal sporangia and dichotomous axes. These plants were short-statured because they grew for a few centimeters in length. The enigmatic Prototaxites is known to be the longest terrestrial plant during the Devonian period. It was the fruit part of an enormous fungus bowled liverwort mat or additional creature of inexact affinities. They grew more than eight meters long and surpassed over the low, carpet-like plants. The middle stage was associated with shrub-like forestry of primitive vegetation such as horsetails, progymnosperms, ferns, and lycophytes had advanced. The vegetation had leaves and roots that were visible and most of them were relatively tall. The last stage of the Devonian period was the time the first trees, from the genus Wattieza which appeared about 385 Mya. Other primitive trees that emerged during the period include Progymnosperm Archaeopteris and cladoxylopsids that consisted of conifer-like wood and fern-like vegetation. They are the earliest known trees in the world history of forests. At the end of the late Devonian period, seed-forming vegetation began to appear. The rapid emergence of plants and trees in the Devonian period resulted to be era being called the "Devonian Explosion" (Murck, Skinner & Mackenzie, 2008). The green plants acted as a carbon sink in the continents, therefore, reducing the amount of carbon dioxide concentrations. The process reduces the temperature which could have caused massive extinction.

Animals and the First Soils

The diverse earthly plants lead to the evolution of primitive arthropods. It also led to the evolution of seed-plants and insects that associated a modern planet in the beginning in the late stage. Soil sedimentation also occurred due to the rapid development of plant roots system. The late Devonian period also introduced vertebrates because there were rapid changes in the earthly ecosystem. The marine animals created an opportunity for terrestrial animals. At the end of the period, arthropods were firmly introduced on the land. In the middle stage, the land surface was a thin microbial protocol or bedrock. The evolution of has been changing the rock to support substantial foliage. Late Devonian extinction occurred in the late stage of the period (372.2 Mya) which is also known as Famennian faunal stage. At this stage agnathan fishes suddenly became extinct which greatly affected the marine life (Murck, Skinner & Mackenzie, 2008).

Conclusion

In conclusion, the Devonian period lasted around sixty years where contents changed. The period was characterized by the massive evolution of both marine and terrestrial life. It led to the expansion introduction of the first harks, amphibians and plants with stomata, roots, and leaves. The rapid emergence of plants and trees occurred in the Devonian period. Moreover, forests and ammonites were also noticed during the period. The ammonites appeared in the early period while the four-legged amphibians appeared later. It demonstrates that the vertebrates had colonized the land after the raid appearance of green plants dropped off the level of carbon dioxide hence cooling the temperature.

References

Murck, B. W., Skinner, B. J., & Mackenzie, D. (2008). Visualizing geology. Hoboken, NJ: John Wiley & Sons.

House M., (2019). Devonian Period | geochronology". Encyclopedia Britannica. Retrieved from https://www.britannica.com/science/Devonian-Period#ref69886 Feb 27, 2019.

Becker, R. T., Konigshof, P., & Brett, C. E. (2016). Devonian climate, sea level, and evolutionary events: an introduction. Geological Society, London, Special Publications, 423(1), 1-10.