Anisian (Middle Triassic) marine ichnocoenoses from the eastern and western margins of the Kamdian Continent, Yunnan Province, SW China: Implications for the Triassic biotic recovery

Abstract

Two Anisian (Middle Triassic) marine ichnocoenoses are reported from the Boyun and Junmachang (JMC) sections located along the eastern and western margins of the Kamdian Continent, Yunnan Province, Southwest China, respectively. The Boyun ichnoassemblage is middle Anisian in age and is dominated by robust Rhizocorallium, while the JMC ichnoassemblage is of an early Anisian age and is characterized by the presence of Zoophycos. The ichnoassemblage horizons of the Boyun section represent an inner ramp environment, while the JMC section was likely situated in a mid-ramp setting near storm wave base as indicated by the presence of tempestites. The ichnofossil-bearing successions are usually highly bioturbated in both the Boyun (BI 3–5, BPBI 5) and JMC (BI 3–4, BPBI 3–4) sections. Three large, morphologically complicated ichnogenera: 1) Rhizocorallium; 2) Thalassinoides; and, 3) Zoophycos characterize the Anisian ichnocoenoses. Of these, Rhizocorallium has mean and maximum tube diameters up to 20.4mm and 28mm, respectively, while Thalassinoides mean and maximum tube diameters are 14.2mm and 22mm, respectively. Zoophycos is present in the early Anisian strata of the JMC section, and represents the oldest known occurrence of this ichnogenus following the latest Permian mass extinction. Similar to coeval ichnoassemblages elsewhere in the world, the Yunnan ichnocoenoses embrace a relatively low ichnodiversity, but their burrows usually penetrate deeply into the sediment, and include large and complex Rhizocorallium and Thalassinoides. All of these ichnologic features are indicative of recovery stage 4 after the latest Permian crisis. Anisian ichnoassemblages occur globally in six different habitat settings, and all show similar ecologic characteristics except for slightly different degrees of ichnotaxonomic richness, indicating that depositional environment is not a crucial factor shaping the recovery of the trace-makers, but may have an impact on their ichnodiversity. When compared with some important Early Triassic (mainly Spathian) ichnoassemblages worldwide, the Anisian ichnocoenoses examined for this study are slightly less diversified, and possess more or less the same maximum burrow sizes, but the penetration depth of burrows and the distribution of burrow sizes are much larger than those from the Early Triassic. It is worthy of note that the lower ichnodiversity of the Anisian ichnocoenoses may have resulted from intense bioturbation by deeper tiers, representing a taphonomic product that is totally unrelated to environmental stress. In addition, Anisian Rhizocorallium and Thalassinoides have much larger burrow sizes than the same ichnotaxa from the Lower Triassic, implying that ichnocoenoses may have recovered in Spathian, but did not stabilize until the Anisian.