Mesozoic Marine Reptiles - Term Paper Example

Mesozoic Reptiles The term Mesozoic was coined in reference to these reptiles has focused on predation in the seas during the era of dinosaurs. Existing evidence suggests that marine reptiles invaded the seas and oceans, with minimal predation completion as mammalian predators were non-existent. In the strict sense applied in this paper, the term refers to the reptiles whose feeding takes place in the seas. On the other hand, the term ‘reptile’ refers to the cold-blooded ones. Evidently, more than twelve groups of Mesozoic reptiles do exist. Mesozoic reptiles survived during the era of the dinosaurs, and exhibited the adaptations of marine reptiles, with the pertinent adaptations of tolerating the water conditions. This paper will focus on the sauropterygia, mosasauridae and ichthyopterygia. Most of them were predators, attacking fish, soft-bodied invertebrates, and cephalopods. Evidence from fossils indicates that not all the Mesozoic reptiles exhibited a complete aquatic lifestyle. Sauropterygia is the most outstanding group of Mesozoic reptiles, with two unique lineages. The lineages have been highlighted as placodonts and eosauropterygians. The second group is the ichthyopterygia, which, according to many biologists, exhibits multiple similarities to the fish. This has prompted the development of the term ‘fish lizards’ to define the group. The third group is the mosasauridae, which, in common reference, biologists have labeled the organisms in this group as seagoing monitor lizards because of the evident resemblance to the monitor lizards (Borden, Varanus, and Biawak, 2007). Introduction According to evolutionary history, the era that saw the existence of dinosaurs was unique. This is because it exhibited the presence of vast oceans in comparison to land. Existing evidence suggests that marine reptiles invaded the seas and oceans, with minimal predation completion as mammalian predators were non-existent. The term Mesozoic was coined in reference to the reptiles that focussed on predation in the seas during the era of dinosaurs. Research that seeks to understand marine reptiles suggests that four groups existed. Although different groups of the Mesozoic reptiles existed, this paper will focus on only three, namely sauropterygia, ichthyopterygia, and mosasaurs (Dutchak, 2005). The discussion in this paper will be developed from an analysis of the different features exhibited by these reptiles, and their pertinent ancestries. Definition of Marine Reptiles In a strict sense that does not lead to confusion, the terms ‘marine reptiles’ need a specific definition in the context of this paper. Evidently, the term ‘marine’ is in use in different contexts in a general sense. Its use commonly refers to the residing in the water. However, in the strict sense applied in this paper, the term refers to the reptiles whose feeding takes place in the seas. On the other hand, the term ‘reptile’ refers to the cold blooded ones, excluding the warm blooded birds that qualify as reptiles in accordance with the ancestral lines. In a bid to understand the adaptation of Mesozoic reptiles, there is salient need of bringing into focus some features exhibited by reptile prior to the age of the dinosaurs. About 70 million years, the emergence of reptile from vertebrates with limbs had taken place. Although the ancestral vertebrates exhibited the presence of gills that enabled them to embark on effective breathing in water, reptile lacked such gills (Druckenmiller and Russell, 2008).. This placed a compulsion on reptiles to resurface each time they needed to breathe, as they were unable to exhibit proper breathing while in the water. However, analysis of the emergence of marine reptile indicates that the lack of gills did not serve as a barrier for the invasion of the sea by reptiles. Despite this obvious disadvantage, reptiles exhibit other salient features that made it easier for them to survive in water (Benton, 2007). Notably, reptiles fall under the category of cold-blooded organisms that require an external source of heat. Scientists have used the term exothermic to definitely need for the reptile to rely on an external source of heat. Although many would opine that this is an obvious disadvantage to reptiles as the water lowers their body temperatures, but reptiles prove to adapt to the situation, as long water on the surface exhibits a higher temperature. Moreover, the fact that reptiles have the capacity to maintain low body temperatures serves as an additional advantage unlike mammals that face the compulsion of exhibiting higher metabolic rates in a bid to generate additional heat to counter the low temperatures in water (Bell & Polcyn, 2005). Notably, reptiles exhibit a lower need for oxygen each second compared to mammals, and hence face a minimal compulsion of emerging onto the surface to breathe. Research also reveals that marine reptiles have the capacity to tolerate anoxia during times of oxygen insufficiency. Mesozoic Marine Reptiles There is evidence pointing to the existence of more than twelve groups of Mesozoic reptiles. Mesozoic reptiles survived during the era of the dinosaurs, and exhibited the adaptations of marine reptiles as discussed above. Among the over a dozen groups, this paper will focus on the sauropterygia, mosasauridae and ichthyopterygia. Evidence from fossils indicates that not all the Mesozoic reptiles exhibited a fully aquatic lifestyle. However, each of the groups exhibited a mode of swimming that facilitated their motion in water, with some using limbs, tails, and the body axis (Benton, 2007). A close analysis of the teeth indicates that they depended on a diverse range of diets as they do not exhibit a similar dentition. However, fossil evidence highlights that most of them were predators, attacking fish, soft-bodied invertebrates, and cephalopods. Although the efforts to understand the reproductive biology of Mesozoic reptiles has only yielded a partial understanding, some fossils have indicated that live birth was a common phenomenon among them. However, there is limited understanding of the significance of live births or the prevalence of its occurrence. Moreover, some Mesozoic reptiles also exhibited the pattern of laying eggs on land. Studying the lineages that led to the emergence of the different groups has been one of the common strategies adopted by biologists in a bid to understand the features they exhibited alongside their significance (Brischoux, Cook, and Shine, 2008). Sauropterygia This forms one of the outstanding groups of Mesozoic reptiles, with two unique lineages. The lineages have been highlighted as placodonts and eosauropterygians. Although these lineages define the largest group of the Mesozoic reptiles, they have unique anatomical designs. According to the in-depth analysis of the placodonts of the available fossils, it is evident that they exhibited short necks with saliently robust skulls, but that did not prove to belong. On the other hand, the eosauropterygians exhibited the presence of long necks in accordance to the available fossils (Benton, 2007). However, the skulls of this lineage proved to be narrow and of a smaller proportion when compared to the entire length of the body. Despite these pronounced differences of the two lineages, it is evident that they belong to a similar group because they exhibit a range of similarities. One of the outstanding similarities is the design of the shoulder girdle, which was a unique adaptation of the group. Moreover, the presence of clavicles placed in the medial position in comparison to the scapula. In both lineages, the lacrimal bone has proved to be absent from the available fossil records. Evidently, both lineages have exhibited the presence of reduced tarsal and carpals (Borden, Varanus, and Biawak, 2007). Placodonts From the analysis of the available fossil records, this lineage did not exhibit an even geographical distribution, as the available fossils have only linked their occurrence to the Tethys Sea. Therefore, fossils have only been characterized in areas that exhibit a close proximity to the Tethys Sea. According to the chronological placement of fossils, there is evidence suggesting that this lineage belonged to the Triassic period. Some of the older fossils of this lineage were found in a place in Germany. From the analysis of the dentition of placodonts, it is evident that they relied on prey that had hard bodies. This is supported by the presence of flat teeth on the palate and the jaw margin. Moreover, analysis of the available fossil has revealed that there are two distinct groups of the placodonts, characterized by different body armors. Moreover, this lineage has several bones on the skin axis. Some biologists have highlighted the existence of a connection between this lineage of Mesozoic reptiles with turtles, although the idea has been highly refuted (Caldwel, 2006). Eosauropterygians Unlike the placodonts, this group exhibits a high level of geographical variation with its presence in almost each continent. Its occurrence has been highlighted as sometime during the conclusion of the early Triassic, a period whose end reached upon the extinction of dinosaurs. The plesiosaurs define the natural group of this lineage, and exhibits certain unique characteristics that are not evident in the other groups. In this group, some of the reptile exhibited long necks while others had short ones. The significance of the long necks is an aspect that is yet to be understood. According to the implication derived from the existing fossils, there is the lack of any reason supporting the use of the elongated necks. Notably, there were intermediaries between the long-necked and short-necked. There is evidence pointing to the presence of live births in this lineage as well. Ichthyoptetygia This group defines the second largest group described as belonging to the melozoic reptiles. The group had some of the individuals exhibiting a large body size. The appearance of this group, according to the chronological evidence is the early Triassic, with the pertinent extinction of the group occurring in the early Cretaceous. According to many biologists, this group exhibits multiple similarities to the fish. This has prompted the development of the term ‘fish lizards’ to define the organisms. The evident resemblance to the fish family is not a mere occurrence, as the existing theories have suggested that the group that originally Tetrapod anatomy eventually underwent a rigorous change to adopt a fish like appearance. The fish like appearance is exhibited by the presence of a crescent-shaped cradle lake. Moreover, the eyes of the organisms in this group were remarkably large, an aspect common in fish, an additional evidence of a rigorous evolution from fish. In terms of finger count, this group exhibited the highest number (Borden, Varanus, and Biawak, 2007). There is limited understanding of the exact process through which organisms in this group adopted the fish-like appearance. However, further analysis of fossil records has been done in a bid to analyze the significance of the changes that emerged. Unlike the case with the sauropterygians, there is a level of consistency in the occurrence of fossils in this group. As in the case with other groups, the fish, lizards exhibited the presence of live births (Fernández, 2006). Mosasauridae This is the third group under the analysis of the Mesozoic reptiles. According to the chronological placement of these organisms, their existence occurred in the late cretaceous period. In other words, these organisms resided in the sea about 98 million years ago in accordance to the fossil record calculations. This group occurred during a similar period with the turtles and the plesiosaurs. However, evidence shows that the renowned K/T mass extinction led to their fading away, leaving only their fossils, which have formed the basis for understanding their pertinent adaptations. According to the collected fossils, these organisms inhabited different continents, and were not limited to one region. In common reference, biologists have labeled the organisms in this group as seagoing monitor lizards because of the evident resemblance to the monitor lizards. Evidently, not all organisms from this group were fully aquatic, with some spending limited time in the sea. From the analysis of their pertinent dentition, the mosasauridae exhibited a general feeding pattern with minimal specifications (Benton, 2007). Conclusion Evidently, the marine reptiles that lived in the Mesozoic period had unique characteristics that have prompted modern research on the existing fossils in a bid to understand the different adaptations of the organisms. As highlighted above, three main groups, namely the sauropterygia, ichthyopterygia, and mosasuridae have formed the core of recent research on these organisms. As highlighted above, the groups had the pertinent capacity to invade the water and predate different types of prey. References Bell, G. L. Jr, & Polcyn, M. J., (2005). Dallasaurus turneri, a new primitive mosasauroids from the Middle Turonian of Texas and comments on the phylogeny of Mosasauridae (Squamata). Neth J Geosci. 84:177–94. Benton M. J., (2007). Vertebrate paleontology. 3rd ed. New York: Wiley. Borden, R., Varanus, S., Biawak (2007). Proceedings of the 11th Annual Meeting on Health, Science and Technology. Université de Tours. Brischoux, F., Cook T., and Shine, R. (2008). Allometry of diving capacities: ectothermy vs. endothermy. J Evol Biol. 21:324–32. Caldwel, T. (2006). On the aquatic squamate Dolichosaurus longicollis Owen 1850 (Cenomanian, Upper Cretaceous), and the evolution of elongate necks in squamates. J Vertebr Paleontol. 20:720–35. doi:10.1671/0272-4634(2000)020[0720: OTASDL]2.0.CO;2. Druckenmiller, P., and Russell, A. (2008). A phylogeny of Plesiosauria (Sauropterygia) and its bearing on the systematic status of Leptocleidus Andrews,1922. Zootaxa, 1863:3–120. Dutchak, D. (2005). A review of the taxonomy and systematics of aigialosaurs. Neth J Geosci 84:221–9. Fernández, M. (2006). Dorsal or ventral? Homologies of the forefin of Caypullisaurus (Ichthyosauria: Ophthalmosauria). J Vertebr Paleontol. 22:515–20. Read More