111 / 2022-02-20 12:43:13

The youngest Dictyodora from the Late Permian deep sea in West Qinling, central China

Trace fossil; Ichnostratigraphy; Turbidite; Evolution; end-Permian mass extinction

Dictyodora has been generally considered an ichnogenus with restricted temporal ranges from the Cambrian to the Mississippian. Up till now, there have been no reports of Dictyodora after the Mississippian. Here we report on well-preserved and abundant Dictyodora from deep-sea sediments of the Lopingian Maomaolong Formation in West Qinling, central China, which is hitherto the youngest occurrence of Dictyodora and extends the stratigraphic range of Dictyodora to the Late Permian. Two ichnospecies of Dictyodora are distinguished: D. zimmermanni and D. cf. scotica. Unlike the small-sized Dictyodora in the pre-Pennsylvanian, three-dimensional morphology of Dictyodora studied here shows larger amplitudes and longer limbs with clear inclined walls and basal burrows, which is interpreted to be made by a soft-bodied worm-like organism with a snorkel-like organ as similarly documented in previous studies. Detailed systematic ichnological features and SEM and EDS results indicate that Dictyodora here is not simply a respiration structure made during the locomotion of the trace maker, but results from active filling by the trace maker. Dictyodora is probably not a good indicator of the deep-marine environment in the Paleozoic. From our dataset, Dictyodora originated from the shallow-marine environment in the early Cambrian, then migrated into the deep sea in the Ordovician and the marginal-marine setting in the Wenlock, and was finally restricted to the deep marine environment in the Carboniferous and Permian. By detailed comparison with global biotic-environmental changes, we conclude that the evolutionary trend of Dictyodora is multi-affected by the Great Ordovician Biodiversification Event, the close of the Iapetus and Rheic oceans, and the Late Devonian and end-Permian mass extinctions. Both Psammichnites and Dictyodora are structures made by organisms with snorkel-like organs, and both were eliminated during the end-Permian mass extinction.