Tracing back Paleozoic taxa to study the origin and structure of jaws and teeth is one of the common means of studying the evolutionary relationships of early vertebrates. It can analyze the oral structure from an anatomical perspective, and the evolution of predatory relationships can also be inferred through such information. Species in different habitats have adapted to specific ecological niches, foraging strategies and defense mechanisms. Therefore, comparing the jaw bones and the arrangement of the teeth is an excellent access point for studying the evolutionary relationships of taxa. From a morphological point of view, one of the main differences between the oral structures of the two crown-groups of jawed vertebrates (gnathostomes) osteichthyans and chondrichthyans is the arrangement of their teeth and the morphological structure of their lower jaw. According to Dearden and Giles (2021), the mandibles of bony fishes and tetrapods (Osteichthyes) will absorb part of the tooth base and after the teeth are lost, they will be replaced to the outside of the dermal bone and above the endoskeletal jaw cartilage, becoming part of the external tooth arc and the internal tooth arc. Meanwhile, in cartilaginous fish such as sharks, rays and chimaeras, the teeth would grow on the jaw cartilage in a direction that is parallel to the labial tooth and their teeth would also shed and be replaced in this same pattern (Dearden and Giles, 2021). Acanthodians, possibly the earliest gnathostome group, have some shark features and some bony fish features. Recent study indicate that the belong to the chondrichyan stem (Brazeau, 2009), thereby being important for understanding chondrichthyan evolution. However, due to the fact that their unmineralized endoskeletons are rarely fossilized, their evolutionary relationship with modern gnathostomes still leaves a lot of unresolved research to be done, and the structure of their oral structure is one of them. From the fossil record, it can be tentatively concluded that teeth and jaw bones first appeared in the late Silurian period (approximately 425 million years ago) (Choo, 2014). The subsequent Devonian period (approximately 420-360 million years ago) was the main period of evolution of the growth and replacement mechanism of the mandibular dentition. This period is also when species diversity and functional differentiation of gnathostomes began. Both of these time periods have high research value in the evolutionary history of gnathostomes. There are many potential reasons for the evolution and diversification of gnathostomes, such as changes in habitat, improvement of defense mechanisms, simplification or complication of food webs, or changes in species' feeding habits (herbivory, carnivory, omnivory, etc.). Comparing and analyzing the changes in the oral structures of gnathostomes in different periods can provide certain explanations for some of the hypotheses (Friedman, 2021). The existence of computed tomography technology has brought great advancement to the study of anatomical models of jawed vertebrate tooth structures. We can continue to address many questions such as the arrangement of teeth, cartilage structure, and the degree of vascularization of bones through the combination of computed tomography and 3D modeling (Wang, 2019). This study was based on samples of fossil dental jaw bones from the Silurian collected from Ohessaare cliffs on the island of Saaremaa, Estonia. One specimen was selected and scanned using computed tomography. It was 3D-modelled and compared with available data from younger dental jaw bones from the Devonian-Permian. The anatomical similarities and differences were explored and compared with several jaw bones from different periods (Dearden and Giles, 2021. Rücklin et al., 2021). Placed in a phylogenetic context, this new data was verified to be consistent with the actual results from the previous research on stem-chondrichthyan jaw bones.
| Identifer | oai:union.ndltd.org:UPSALLA1/oai:DiVA.org:uu-532505 |
| Date | January 2024 |
| Creators | Yang, Qianrui |
| Publisher | Uppsala universitet, Institutionen för biologisk grundutbildning |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Student thesis, info:eu-repo/semantics/bachelorThesis, text |
| Format | application/pdf, application/pdf |
| Rights | info:eu-repo/semantics/openAccess, info:eu-repo/semantics/openAccess |
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