脊椎動物化石 哺乳動物 概論
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後設資料
- 資料識別：
- 系統識別碼:0b00000180ab8d8d
- 資料類型：
- 型式:合集
- 紀錄類別:脊椎動物化石知識單元
- 主題與關鍵字：
- 學域-大分類:地質-化石類
- 分類:脊椎動物化石
- 拉丁界名:Animalia
- 中文界名:動物界
- 拉丁門名:Chordata
- 中文門名:脊索動物門
- 拉丁亞門名:Vertebrata
- 描述：
- 簡介:最早的哺乳動物，出現在三疊紀，距今約二億二千萬年，牠的祖先則可追溯至出現在石炭紀末至三疊紀間的一群稱為似哺乳爬行類動物。牠們屬於爬行動物綱中的單弓亞綱，可說是連接原始爬行類與哺乳類的橋樑。從化石證據中得知，在中生代早期的哺乳動物體型小數量又少，完全被當時幾乎無所不在的大型爬行動物－恐龍所壓倒。然而，儘管中生代的哺乳動物是如此渺小，牠們卻對生命演化史做了特殊重要的貢獻，因為在此時哺乳動物正逐步發展，各項基本特徵也逐漸生成，似乎是為將來的發展打下基礎，爾後終於在恐龍滅絕之後，哺乳動物得到幾乎全然空曠的發展空間，得以大肆分歧繁衍，造成新生代哺乳動物的新世紀。
- 現在的哺乳動物（包括有袋類與胎盤類）皆起源於中生代白堊紀的獸亞綱，其頰齒為三尖構造，意即在上、下前臼齒與臼齒都有三個明顯的尖突。食蟲目動物至今仍然保留臼齒三尖構造的原始型態，即為居中的原尖、位於前方的前尖與後方的後尖，而下頜臼齒除了三尖之外，更衍生出下內尖與下次尖，增加上下臼齒咬合與對食物咀嚼的功能。在哺乳動物的演化過程中，臼齒的型態由具有三尖突起的原始雛形開始發展，隨著食物的不同，適切地產生改變。例如肉食性動物的臼齒齒尖突起更為發達，以利切割食物；植食性動物的臼齒齒尖趨於平整，擴大了咀嚼面，以利研磨食物；雜食性動物則臼齒齒尖呈高低交錯的圓丘狀突起，使得其咀嚼面能搗碎、割斷食物。
- 隨著哺乳動物的繁衍發展，其牙齒結構也由爬行動物的「同型齒」發展成「異型齒」，即牙齒可區分為門齒、犬齒、前臼齒與臼齒。牙齒是哺乳動物最堅硬也最容易保存形成化石的部位，許多已經滅絕的哺乳動物，都是經由牙齒化石得以辨認，經由牙齒化石的線索，使得我們能夠進一步探究哺乳動物食性的多樣化與環境變異的適應。Mammals Overview哺乳動物化石概論The earliest mammals appeared about 220 million years during the Triassic Period. Their ancestors can be traced back to a group of mammal-like reptiles that emerged between the late Carboniferous Period and the Triassic Period. They belong to the Synapsida Subfamily of the Reptilia Family and can be considered the link between primitive reptiles and mammals. From the fossil evidence we know that during the Mesozoic Era the early mammals were very small insize and few in number. They were no match for dinosaurs, the large reptiles that ruled the surface of the planet. Despite the weakness of the mammals during the Mesozoic Era, they made an important contribution to the history of evolution. During this period, the basic characteristics that came to define mammals gradually emerged, as if preparing for the development to come. When dinosaurs became extinct, mammals inherited a wide open space for development. There was a veritable explosion in the number of mammalian species, making the Cenozoic Era their age.
- The living mammals (including marsupials and placentals) belong to the Theria subclass that originated during the Cretaceous Period of the Mesozoic Era. Their cheek teeth are tribosphenic, meaning the upper and lower premolars and molars all have a distinct three-pointed structure. Animals in the Insectivora Order still retain the original tribosphenic molars consisting of the protocone in the middle, the paracone at the front and the metacone at the back. The lower molars also evolved the hypoconid and entoconid for an improved biting and chewing function. As mammals evolved, the primitive three-cusped molar adapted as well to suit dietary needs. For example, carnivores developed pointed cusps to shear meat, herbivores flattened cusps to expand the grinding surface, and omnivores rounded cusps of varying heights to help cut and grind up food.
- As mammals developed and evolved, their dentition developed from the "homodonts" of reptiles into "heterodonts". This meant that teeth were specialized into incisors, canines, premolars and molars. Teeth are the hardest and most easily preserved part of the mammal, so they have the best chance of becoming a fossil. Many now extinct mammals were all identified through their fossilized teeth. Using their fossilized teeth as clues, we can explore the diversity in mammalian diets and how they adapted to changes in their environment.
- 出版者：
- 數位化執行單位:國立自然科學博物館：地質學組脊椎動物化石學
- 管理權：
- 典藏單位:國立自然科學博物館
- 使用聲明:館內公開瀏覽
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- 鍾舜丞
04-23226940*762
chungnmns@nmns.edu.tw
