楯形目海膽生長模式與井骨微結構：以臺灣西部更新世沙錢海膽為例

Abstract

海膽依其形態可大致區分為兩大類，分別為正形海膽與歪形海膽。正形海膽有著近乎完美的五輻對稱形態，而歪形海膽則屬於兩側對稱，本文研究的楯形目(Clypeasteroida)則屬於後者，主要棲息於淺海軟底質的沙質環境，因其形狀大致為圓盤狀如硬幣般而俗稱「沙錢」。本篇研究材料—奇異掘海膽(Scaphechinus mirabilis) 的成體大小約3~7公分長，其現生種在日本、韓國、中國、俄羅斯的沿海地區皆有紀錄，但在台灣僅在苗栗地區頭嵙山層有化石紀錄，儘管產地非常局限，但化石產量卻相當豐富，且保存情況也相當良好。本研究的目的為了解此沙錢的個體發育、骨片的生長模式和細微結構—井骨的多樣性。 本研究一共針對971件收藏於臺灣大學地質科學系的奇異掘海膽化石以及另外593件樣本做研究。在形貌計量中，一共使用92件樣本進行殼長及殼寬的測量研究，86件樣本進行頂系變化的測量，在肛口位置變化上也使用了67件樣本進行研究。為了更好的研究奇異掘海膽的井骨構造及個體發育，我們針對其中65件大小約0.7至6公分且保存良好的化石標本及一件產自日本的現生標本進行薄片製作，一共製作出84張化石沙錢薄片及1張現生沙錢薄片。其中65張是海膽縱切面，以研究其殼緣骨片的生長模式。另外19張則是海膽殼體各部位的薄片，包含頂系、瓣帶、口面等區域，以研究其井骨形態。藉由測量井骨孔大小與井骨條的寬度有助於了解井骨孔可能附著的肌肉纖維或膠原纖維。 研究結果顯示在生長過程中，殼長與殼寬維持一定的比例、頂系位置無明顯的變化但肛口位置有向殼緣移動的趨勢。在殼緣骨片生長模式結果上，藉由偏振光顯微鏡來觀察化石薄片能清楚的判斷骨片的邊界，在生長過程中殼緣骨片的寬度及厚度皆有增大的趨勢，而兩者的比例維持固定不變。而井骨方面，柱廊型與迷宮型為最主要的井骨型態，分別於口面處及殼疣處有此兩種井骨形態的發現，在瓣帶表面則主要是由直線型井骨構成。瓣帶處的井骨孔直徑平均約14.6微米，而井骨條的平均厚度約為12.2微米；殼疣的區域Ⅳ (boss)的井骨孔直徑平均約10.9微米，而井骨條的平均厚度約為12.3微米；殼疣的區域Ⅴ (areole)的井骨孔直徑平均約18.6微米，而井骨條的平均厚度約為10.2微米；口面骨片的井骨孔直徑平均約19.2微米，而井骨條的平均厚度約為12.2微米。此外，本研究也發現了清晰的拱柱生長紋，幫助我們瞭解其生長方向。

Morphologically, sea urchins can be subdivided into two principal groups – regular and irregular echinoids. While regular echinoids exhibit an almost perfect penta-radial symmetry, irregular ones bear a bilateral symmetry. The subject of this thesis is one fossil member of Clyeasteroida, which belongs to the latter group. The main habitat for this Order collectively is living in the soft, sandy substrate in the shallow sea. Most of them are circular in outline and the average adult size is approximately the same as an old silver coin (~38 mm); thus, the nickname is called “sand dollar.” This study focuses on the taxon Scaphechinus mirabilis, and its adult size ranges from 3 cm to 7 cm in size. The living distributions of this species occur along the coastal and shallow subtidal areas in Japan, Korea, China and Russia. Although there is no reported occurrence of the living population and the fossil occurrences of S. mirabilis are limited to the Miaoli region, Taiwan, fossils are well-preserved and occur abundantly in several sites. The main goals of this study are to have a better understanding about S. mirabilis fossil assemblage by studying its later phase of ontogenetic development, growth pattern of the marginal plates and the diversity of three-dimensional microstructures known as stereom. A total of 971 specimens deposited at the Department of Geosciences, National Taiwan University (NTUG) with additional 593 specimens collected by colleagues were studied. Among them, 92 specimens were used for obtaining morphologic data, including the length and width measurements; 86 specimens for measuring the changes in apical system; 67 specimens for measuring the changes in periproct position. In order to study better the ontogenetic development of stereom, 65 well-preserved specimens, ranging from 0.7 cm to 6 cm, and one modern analogue from Japan were selected for making thin-sections. A total of 84 thin-sections derived from fossil specimens and one from the living specimen were made. Among them, 65 thin-sections contain longitudinal profiles for studying the growth patterns of marginal plates and the other 19 sections are plan-views of various parts of the test, including apical system, petaloid and basicoronal plates, for studying the diversity of stereom types. We can distinguish the muscle fibers from collagen fibers attached to the pores in between stereom by measuring both the sizes of interconnecting pores and the trabecular thickness. Results show that the test length and width grow proportionally at a stable rate during later ontogeny. While there is no obvious change for the position of apical system, there is a subtle trend of shifting the periproct position from aboral side to the margin. On the growth of marginal plates, the boundaries of individual plates can be distinguished under a polarized microscope. The width and thickness of marginal plates both increase, and the ratio of these two factors is relatively constant during growth. Among stereom types, galleried and labyrinthic stereoms are the most common types. While they are associated commonly with the oral plates and tubercles, and the rectilinear stereom is associated with the aboral surface of petaloids. The average size of interconnecting pores is 14.6 μm, and the average thickness of trabeculae is 12.2 μm in the petaloid region; average size of interconnecting pores is 10.9 μm, and average thickness of trabeculae is 12.3 μm in the boss region (IV) of a tubercle; average size of interconnecting pores is 18.6 μm, and average thickness of trabeculae is 10.2 μm in the areole region (V) of a tubercle; average size of interconnecting pores is 19.2 μm, and average thickness of trabeculae is 12.2 μm among oral plates. Besides, we report here the discovery of growth lines of internal pillars preserved in fossil specimens of S. mirabilis, allowing us to interpret their growth directions.