ZOMBIES OF THE PHANEROZOIC: HOW PREVALENT ARE DEAD CLADES WALKING?

Dead Clades Walking (DCWs) are fossil orders that experience a sharp reduction in diversity but subsequently linger without recovery for millions of years or longer before extinction. DCWs have previously been considered an exclusive product of mass extinction events, but many properties of DCWs remain unknown. Drawing from the Paleobiology Database of fossil occurrences, we explore the following questions: 1) are DCWs uniquely created by mass extinctions, or can DCWs form during background intervals? 2) are some biological or ecological clades more prone to becoming DCWs? and 3) does DCW status affect the longevity of a clade?

The distribution and span of DCWs were identified from order-level diversity data with a suite of knickpoint analyses (e.g., Bayesian change point analysis). We observe a general anatomy of DCW diversity patterns which includes the stage for the initial drop in diversity (death sentences) and the final stage at which the clade goes extinct (death knells). Whereas we find mass extinctions generally initiate death sentences, the time that a DCW persists until its death knell is highly variable. The relative timing of these steps may therefore influence estimates of taxonomic loss and recovery in the aftermath of mass extinctions, e.g., end-Ordovician DCWs which extend into the Late Devonian and Carboniferous. These lengthy delays in death knells suggest that earlier crises leave behind vulnerable clades that are more susceptible to later mass extinctions, thereby increasing the events’ severity. Ultimately, the prevalence and distribution of order-level DCWs signify that their identification is vital to interpreting the timing and environmental drivers of biodiversity loss through the Phanerozoic.