- 學位論文
新屬種深居彎型生痕化石Pennichnus formosae 於臺灣東北海岸中新世大寮層的訂立與介紹
Pennichnus formosae igen. nov. isp. nov. – A deep-reaching L-shaped burrow in the Miocene Taliao Formation, northeastern coast of Taiwan.
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摘要
在臺灣東北角野柳及八斗子區域廣泛出露的早中新世大寮層中，集中保存了許多難以分類歸屬的Ｌ型穴居生痕化石於下部野柳砂岩段，該種生痕化石的主要特徵為其特殊的羽毛狀構造。由於在以往研究中並無任何紀錄描述此樣貌的生痕化石，故本研究依此型態提出了新屬種生痕化石Pennichnus formosae。 為了取得Pennichnus formosae的外觀模型並分析其分布環境，本研究在野柳及八斗子區域的下部野柳砂岩段中，測量了數量豐富的樣本並加以分析。此區共計有319個樣本，各樣本的管直徑、垂直深度、水平延展長度以及周圍羽毛狀構造的擾動寬度皆被量測建檔。為了能夠有更細部的型態外觀剖析，本研究對部分樣本進行成分及內構造的分析，包含岩礦分析、能量色散X-射線光譜分析 (EDS）、X射線電腦斷層掃描 (CT scanning) 和X射線螢光光譜分析 (XRF)。 本研究目前命為Pennichnus formosae的生痕化石，其外觀為一L形的管狀生痕，此管狀構造先以垂直層面方向向下延伸超過70公分，再轉往水平，最長的水平延伸可達一公尺。此生痕化石的管壁為薄襯裡，直徑2–3公分，管內則由均質的沉積物填充，而特殊的羽毛狀構造則集中在垂直豎管的上部。值得注意的是，在某些樣本的管內含有一根較細的內管。結合環境分析結果，本研究推斷Pennichnus formosae為分布在遠濱過度帶到下部濱面帶的居住構造。 本研究提出了六種可能的生痕製造者，包含甲殼類動物、雙殼類動物、海葵、腕足動物、蠕蟲狀的無脊椎動物及脊椎動物。此六類生物都沒有辦法解釋所有Pennichnus formosae所涵蓋的特徵。最有可能的生痕製造者為蠕蟲狀動物，因為其細長的身軀能夠直接支撐管壁防止崩塌，不過蠕蟲狀動物無法製造內管。由於內管只出現在部分樣本，因此有可能是由另一隻小型生物，在前一位生痕製造者(如前述所提的蠕蟲狀動物)死亡並分解後，再次進入管中所留下的痕跡。抑或是後期成岩作用導致管中心和近管壁區域產生顏色差異，使管中心形成類似內管的樣貌。
並列摘要
At the northeastern coast of Taiwan, peculiar, more than one meter long, L-shaped burrows surrounded by distinct feather-like structures were observed within certain strata of the Miocene Taliao Formation. Because this kind of trace fossil has not been described previously, a new ichnospecies, Pennichnus formosae, is proposed. To decode the secret of Pennichnus, a morphological model had to be constructed, and the depositional environment needed to be assessed. At the Yehliu and Badouzi study areas, 319 specimens were photographed and several morphological parameters were measured. In addition, selected samples were studied in detail using petrographic thin sections, SEM, EDS, CT scanning, and XRF analysis, to address the subtle details in the morphology of different parts of Pennichnus. Pennichnus consists of a large (vertical penetration > 70 cm; horizontal extension >1m), L-shaped, thinly lined, passively filled burrow, 2–3 cm in diameter, with distinct cone-in-cone structures appearing around the upper shaft, and an inner tube sometimes occurring inside the burrow. The trace fossil is distributed from proximal lower shoreface to lower offshore, and the morphological observations suggest that it was used as a dwelling burrow inhabited for a longer period of time. Six possible producers are discussed in this study, crustaceans, bivalves, sea anemones, brachiopods, vermiform invertebrates, and vertebrates. The most probable producer of Pennichnus is a vermiform organism, such as giant annelids or eels, who use their slender body to support the burrow. One argument against this ethological model is the inner tube, which can’t be satisfactory explained by a vermiform producer. However, it is likely that the inner tube is produced by an unrelated, smaller organism which entered the burrow after the vermiform producer of Pennichnus died or migrated out of the burrow.
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