Morphological Analysis and Palaeoenvironmental Significance of Rhizocorallium from the Middle Triassic Guanling Formation in the Xingyi Area

Key words: 
• Guanling Formation /
• Rhizocorallium /
• trace fossils /
• trace-making organisms /
• burrows

Abstract: Abstract: [Objective] To study a large number of Rhizocorallium in Guanling Formation in Xingyi area, and analyze the morphological and structural characteristics of its burrows, so as to reveal the behavioral habits of trace-making organisms and their indicative significance to the ancient sedimentary environment. [Methods] The cross-sectional diameter of the relict tube and the distance between the two wing tubes of Rhizocorallium U-shaped burrows were measured. The correlation of parameters was analyzed by linear regression, and the morphological structure and trace-making process were analyzed. The disturbance intensity was evaluated by bioturbation index (BI), and the sedimentary environment was revealed by combining lithology and other characteristics. [Results](1) The basic structure of Rhizocorallium is composed of U-shaped and linear burrows, and its formation mechanism is the mutual transformation of foraging-crawling behavior of trace-making organisms. (2) The cross-sectional diameter (0.1~2.6 cm) of the U-shaped trace tube was highly positively correlated with the distance between the two wing tubes (0.5~11.9 cm) (R2=0.92688), indicating that the behavior patterns of trace-building organisms from larvae to adults were highly consistent, which was consistent with the survival strategy characteristics of opportunistic species (r-selected).The bioclastic limestone and micritic limestone in the lower Guanling Formation are characterized by high-density Rhizocorallium ( BI = 3 ~ 5 ), mainly distributed in low-energy open platforms. The bioclastic limestone in the upper section of the is characterized by sparse Rhizocorallium ( BI = 1 ~ 3 ), which is mainly distributed in the high-energy intertidal zone. [ Conclusion ] In this study, the morphological identification and identification of Rhizocorallium revealed the characteristics and formation mechanism of its burrow structure, and further confirmed the consistency of behavior and its r-selection strategy in the process of individual development. More importantly, the study clarified the behavioral response and adaptive characteristics of Rhizocorallium to different sedimentary environments. These findings together indicate that Rhizocorallium can be used as a reliable ichnological marker for the reconstruction of paleoecology and paleosedimentary environment.