extinction event, which affected some taxa (e.g., brachiopods) more severely than others (e.g., bivalves). However, recovery was also differential between taxa. Some survivors became extinct some million years after the extinction event without having rediversified (dead clade walking, e.g. the snail family Bellerophontidae), whereas others rose to dominance over geologic times (e.g., bivalves). Marine ecosystems Marine post-extinction faunas were mostly species-poor and dominated by few disaster species such as the bivalves Claraia and Unionites. At least some marine communities however thrived as early as one million years after the extinction event, as demonstrated by the Guiyang biota. Seafloor communities maintained a comparatively low diversity until the end of the Early Triassic, approximately 4 million years after the extinction event. This slow recovery stands in remarkable contrast with the quick recovery seen in nektonic organisms such as ammonoids, which exceeded pre-extinction diversities already two million years after the crisis. The relative delay in the recovery of benthic organisms has been attributed to widespread anoxia, but high abundances of benthic species contradict this explanation. More recent work suggests that the pace of recovery was intrinsically driven by the intensity of competition among species, which drives rates of niche differentiation and speciation. Accordingly, low levels of interspecific competition in seafloor communities that are dominated by primary consumers correspond to slow rates of diversification and high levels of interspecific competition among nektonic secondary and tertiary consumers to high diversification rates. Other studies point to the recurrent episodes of extremely hot climatic conditions during the Early Triassic for the delayed recovery of oceanic life. A 2019 study attributed the dissimilarity of recovery times between different ecological communities to differences in local environmental stress during the biotic recovery interval, with regions experiencing persistent environmental stress post-extinction recovering more slowly. Early Triassic environmental stresses also acted as a ceiling limiting the maximum ecological complexity of marine ecosystems until the Spathian. Whereas most marine communities were fully recovered by the Middle Triassic, global marine diversity reached pre-extinction values no earlier than the Middle Jurassic, approximately 75 million years after the extinction event. Prior to the extinction, about two-thirds of marine animals were sessile and attached to the seafloor. During the Mesozoic, only about half of the marine animals were sessile while the rest were free-living. Analysis of marine fossils from the period indicated a decrease in the abundance of sessile epifaunal suspension feeders such as brachiopods and sea