Relation between trace fossil Gordia and medusiform fossils Pararotadiscus from the kaili biota, Taijiang, Guizhou, and its significance

Abstract

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.