美國猶他州中寒武紀Wheeler Formation球接子蟲Peronopsis interstricta的個體發育

Abstract

三葉蟲與球接子蟲類群的親緣關係長久以來一直飽受爭議，個體發育的研究是近幾十年來被視為解決此爭議的重要方法。本研究針對一件來自美國猶他州中寒武紀Wheeler Formation的頁岩，頁岩上有145個球接子蟲原地死亡的標本進行個體發育研究，其中鑑定為Peronopsis interstricta的標本有129個，期望將研究結果作為未來研究球接子蟲類群親緣關係的基礎。 經量測頭甲的長度與寬度、頭鞍的長度、尾甲的長度與寬度、尾甲軸部的長度後，依據散佈圖數據點的分布，本研究將P. interstricta的個體發育過程區分為M0、M1a、M1b、Ha、Hb、Hc、Hd、He、Hf等九期蛻期。九期蛻期在形態上無太大的變化，僅有兩部分有較明顯的改變：1. 在蛻期M1b時，頭鞍尾端開始明顯變寬。2. 在蛻期M0及M1a時，尾甲軸葉呈三角錐狀；自M1b開始，尾甲軸葉的形貌開始改變，自蛻期Ha開始，尾甲軸葉的外形就固定下來。在完整個體中，有部分個體出現蟲體彎曲、體節錯位的現象，且在分離頭甲與分離尾甲中，亦發現部分個體保留有1-2節胸節。 綜合此類較特殊的產狀，本研究提出P. interstricta兩種可能的蛻殼模式：模式一：舊殼沿尾甲與胸甲間的連接部分分離；模式二：舊殼亦可能沿任一體節連接處分離。此外，另有兩點是本研究的不同處，本研究描述了P. interstricta的尾甲軸部在中甲期至成甲期的變化；並觀察到P. interstricta的M0階段，確認軸後中溝於M0時就已發育完成。

The phylogenetic relationship between agnostids and trilobites is a subject of intense scientific debate, and ontogenetic studies of both groups are thought to be the key to resolve the issue. This study focuses on one slab containing 145 agnostid specimens from the Wheeler Formation, Utah. Among them, 129 specimens are identified here as Peronopsis interstricta. It serves as a stepping stone to understand fully the evolutionary development of agnostids. This study shows that individuals of P. interstricta once lived, molted, died and were buried in situ. Measurements of cephalon length and width, glabella length, pygidium length and width, and length of pygidial axial lobe were recorded and plotted. A total of nine instars, including M0, M1a, M1b, Ha, Hb, Hc, Hd, He and Hf, can be distinguished based on the bivariate plots of cephalon width versus length, cephalon with versus glabella length, pygidial width versus length, and pygidial width versus length of pygidial axial lobe. Although there are obvious size changes during instars, two morphologic changes are noticed. First, during stages M0 & M1a, pygidial axial lobe are cone-shaped. Second, since the onset of the stage M1b axial lobe in the pygidium becomes wider until stage Ha. No morphologic change is noticed after stage Ha. Among complete specimens, some specimens are partially disarticulated or preserved with dislocated thoracic segments. In addition, a few disarticulated cephalons or pygidia are found with one or two intact thoracic segments. These rare but valuable specimens provide clues about the molting process of agnostids. Two plausible molting behaviors are proposed here. Model one: molting suture occurs along the connection region between pygidium and chephalon and then the agnostid crawls upward and out of the exoskeleton. Model two: it can occur in any thoracic segments. Compared with previous studies, new contribution includes the following: 1) description of the transformation of axial lobe in the pygidium of P. interstricta from meraspid to holaspid stages; and 2) new observation on development of postaxial median furrow during the M0 stage.