The end-Cretaceous mass extinction event at the impact area: A rapid macrobenthic diversification and stabilization

The Cretaceous-Paleogene (K-Pg) mass extinction, 66.0 Ma (Renne et al., 2013), was one of the most important events in the Phanerozoic, severely altering the evolutionary and ecological history of biotas (Schulte et al., 2010). This extinction was caused by paleoenvironmental changes associated with the impact of an asteroid (Alvarez et al., 1980) on the Yucatán carbonate-evaporite platform in the southern Gulf of Mexico, which formed the Chicxulub impact crater (Hildebrand et al., 1991). Prolonged impact winter resulting in global darkness and cessation of photosynthesis, and acid rain have been considered as major killing mechanisms on land and in the oceans. Major animal groups disappeared across the boundary (e.g., the nonavian dinosaurs, marine and flying reptiles, ammonites, and rudists), and other groups suffered severe species level (but not total) extinction, including planktic foraminifera, and calcareous nannofossils. Other groups, including many deep sea benthic organisms, did not experience extinctions but did undergo observable changes in abundance, diversity and composition (Schulte et al., 2010). Thus, the end-Cretaceous impact event had a major importance in the evolution of life in the Earth from the Paleogene.
To evaluate the significance of the asteroid impact in the K-Pg mass extinction it is important to study the impact crater itself. On this challenge, in April and May 2016, a joint expedition of the International Ocean Discovery Program and the International Continental Scientific Drilling Program Expedition 364 drilled into the Chicxulub peak ring and recovered ~130 m of impact deposits which provide a record of the recovery of life in a sterile zone. Analysis of trace fossils reveals the effect of impact-driven paleoenvironmental changes on the macrobenthic community, a group comparatively poorly known. Trace fossils, as records of macrobenthic tracemakers, are closely related to paleoenvironmental conditions; ichnological research is being increasingly used as a tool to study the “Big Five” mass extinctions, with special attention to the K-Pg impact mass extinction event (Lavandeira et al., 2016).
Ichnological data, integrated with planktic foraminifera and calcareous nannoplankton datasets, revealed that life reappeared in the basin just years after the impact. Clear, discrete trace fossils, including Planolites and Chondrites, are registered in the sediments deposited just immediately after the event (Lowery et al., 2018). Thus, proximity to the impact did not delay recovery and that there was therefore no impact-related environmental control on recovery (Lowery et al., 2018). To follow up on this study, ichnological research has been conducted to investigate the initial diversification, evolution, restructuring, and stabilization of the macrobenthic community following the impact event (Rodríguez-Tovar et al., 2020). After the initial recovery a first phase of diversification is recognized, extended to ~45 k.y. after the K-Pg impact event, characterized by the increase in the abundance and size of the trace fossils and the development of an initial community with Planolites, Chondrites, and Palaeophycus, as well as a shallow indeterminate infauna. Subsequently, a phase of stabilization is registered in the infaunal community, with changes only in relative abundance between ichnotaxa, until ~640–700 k.y. into the Paleocene. At this time, following the prolonged phase of stabilization, a second phase of diversification is observed, characterized by the appearance of well-developed Zoophycos. This diversification marks the beginning of the highest diversity, abundance, and size of traces, with a community of Zoophycos, Chondrites, Planolites, and Palaeophycus representing the establishment of a well-developed tiered assemblage within ~700 k.y. This community is maintained during the phase of consolidation/dominance, through at least ~1.25 m.y. after the K-Pg boundary. 
These data support the fast progression of recovery in the macrobenthic tracemaker community in the impact area, with a total reestablishment ~700 k.y. after the impact event. This is rapid in comparison with other mass extinction events, as that occurred at the end-Permian, which took millions of years (Twitchett, 2006). Such rapid recovery demonstrates the ephemeral nature of environmental change at the K-Pg boundary compared to earlier mass extinctions driven by fundamentally slower mechanisms. 

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