贵州台江凯里生物群中遗迹化石(Gordia)与水母状化石(Pararotadiscus)的关系及其意义

Relationship between Trace Fossil Gordia and Medusiform Fossils Pararotadiscus from the Kaili Biota, Taijiang, Guizhou, and Its Significance

贵州台江凯里生物群中的遗迹化石 Gordia marina Emmons 常被保存在水母状化石Pararotadiscus guizhouensis Zhao and Zhu 盘体的上下,构成凯里生物群生态特色之一。笔者认为,G.marina 可能系蠕形类造迹动物在层面或近层面觅食或吞食富含有机物的沉积物所形成。生活于清澈陆棚环境一定水域的 P.guizhouensis,由于突发性快速沉积事件的发生,改变了海水的物化条件,而被“毒死”,并随沉积物安静地沉落于弱氧化—还原环境的海底。在较为缓慢的腐烂过程中,营食沉积物的 G.marina 造迹生物觅食于 P.guizhouensis 盘体上下富含有机物的沉积物中。随着沉积物的增厚,P.guizhouensis 生物尸体周围的微环境还原性增强,腐烂过程很快被终止,P.guizhouensis 生物软体得以保存,浅层或层面觅食性造迹生物亦随即离去,留下其生活活动的潜穴。同时通过研究,还认为 G.molassica(Heer)与G.marina 为同义名,可能为 G.marina 造迹生物的幼体。

Trace fossil Gordia marina Emmons (1844) is commonly associated with the surface and the underside of medusiform fossil Pararotadiscus guizhouensis Zhao and Zhu, which is one of the core-element of the Kaili Biota. We suggested that variations of G. marina represent the grazing trails of vermiform animals. P. guizhouensis was living in a clear, open-shelf environment, and then 'poisoned' to death by the sudden changes of the ocean chemistry due to devastating, rapid sedimentation events. After P. guizhouensis was deposited on the seafloor in the dsyaerobic zone where decomposition process is very slow, makers of G. marina grazed on the surface and the underside of medusiform fossils for organic matters. Due to later sedimentation that promotes subtle changes in pH, the decaying process ceased; thus, the non-mineralizing body of P. guizhouensis and the associated trace fossil G. marina were preserved. G. molassica (Heer) is regarded by the authors as the junior synonym of G. marina. Description of Trace Fossil Ichnogenus Gordia Emmons, 1844 Gordia marina Emmons, 1844 (PL I -1 - 6; PI. II-1 - 6; Fig. 2) Description: Smooth, irregularly curving burrow without ornamentation, commonly found to be associated with the disc of Pararotadiscus guizhouensis. Burrow can extend outside of the medusiform animals, and these end portions are long (up to 6mm), nearly straight and angular to bedding surface. On the part of the outer and middle ring of P. guizhouensis, the burrow is curve, crossing irregularly, or circling around the disc. On the parts of the center and inner ring, the burrow is commonly substraight or subcurve. Sometimes burrow enters the other layers of the disc subobliquely. Mud infillings of burrow are cylindrical to subcylindrical, structureless and differ visibly from the host rock (PL I -1, 1-1; Fig. 2). The length of the burrow ranges from several millimeters to several ten centimeters, and the width varies between 0. 8 and 2. 2 mm in diameter. But burrows made by the same maker are consistent in width (PL I -1, 3, 5; PL II -6). There are some pellet-like ornaments formed by the makers of burrows on the surface of the disc (PL I -2; Pl. II -2) , but the discs show no sign of destruction. Discussion: The most characteristic feature of Gordia is that the width diameter of burrow is consistent. G. marina on the P. guizhouensis sometimes show slight changes in width diameter due to compression of original relief. Some burrows' diameter is as small as G. molassica (Heer) (PL I -1,5; PL II -5, Fig. 2); thus, authors regarded that G. molassica and G. marina are synonym. In addition, Fillion and Pickerill (1990) suggested that G. molassica and G. hanyangensis are junior synonyms of G. marina. G. maeandria Jiang differs from G. marina by having wider a diameter and sub-triangle shape in its cross-section. G. marina has similarities with G. simplex Luo et Hu in the width diameter of burrow that varies somewhat, but differ in that there is a shallow-furrow on the latter's surface. The ichnospecies described here is probably formed by vermiform animal while it was devouring organic-rich sediments on the shallow subsurface or on the bedding surface. The pellet-size ornaments, commonly found on the surface of the disc of Pararotadiscus guizhouensis, are interpreted here as resulting from the head movement of vermiform animals pressing against the disc while they were searching for food. Occurrence: Kaili Formation (Middle Cambrian) in Taijiang County, Guizhou Province, China.