Dead clades walking are a pervasive macroevolutionary pattern

Abstract

D. Jablonski [Proc. Natl. Acad. Sci. U.S.A. 99, 8139-8144 (2002)] coined the term "dead clades walking" (DCWs) to describe marine fossil orders that experience significant drops in genus richness during mass extinction events and never rediversify to previous levels. This phenomenon is generally interpreted as further evidence that the macroevolutionary consequences of mass extinctions can continue well past the formal boundary. It is unclear, however, exactly how long DCWs are expected to persist after extinction events and to what degree they impact broader trends in Phanerozoic biodiversity. Here we analyze the fossil occurrences of 134 skeletonized marine invertebrate orders in the Paleobiology Database (paleobiodb.org) using a Bayesian method to identify significant change points in genus richness. Our analysis identifies 70 orders that experience major diversity losses without recovery. Most of these taxa, however, do not fit the popular conception of DCWs as clades that narrowly survive a mass extinction event and linger for only a few stages before succumbing to extinction. The median postdrop duration of these DCW orders is long (>30 Myr), suggesting that previous studies may have underestimated the long-term taxonomic impact of mass extinction events. More importantly, many drops in diversity without recovery are not associated with mass extinction events and occur during background extinction stages. The prevalence of DCW orders throughout both mass and background extinction intervals and across phyla (>50% of all marine invertebrate orders) suggests that the DCW pattern is a major component of macroevolutionary turnover.

Keywords: biodiversity; macroevolution; mass extinction; recovery.

Fig. 1.

The anatomy of a DCW in brachiopod order Spiriferida. (A) Spiriferid range-through genus richness trends. Orange and red bars mark the DS and LAD stages, respectively (Induan–Rhaetian). (B) The posterior probability of a change within the genus richness time series in A, calculated with a Bayesian change point algorithm (Materials and Methods). Probabilities falling below the 0.8 cutoff were excluded as significant change points.

Fig. 2.

Ranges of 70 order-level DCWs. Thick, light lines represent the pre-DS life span of the order; thin, dark lines extend from each clade’s DS to its LAD, each plotted using the lower stage boundary age. Gray bars mark the stages associated with Big Five ME events in this study.

Fig. 3.

Histogram of DCW duration following their DS. Dashed line marks the median (30.4 Myr). (Inset) Density plot of randomized DCW span durations (dashed line indicates 111 Myr median range).

Fig. 4.

Distribution of the DS and LAD occurrences of DCWs. (A) Number of non-DCW orders compared to DCW orders featuring a DS at either an ME or background stage (BG). (B) Distribution of DS at ME orders throughout the Big Five ME events. Hatched region of the end-Ordovician corresponds to the potentially associated Aeronian DS (DSs in ME vs. Background Stages). (C) Number of DCWs with their LAD associated with either ME or BG stages, compared to extant examples. (D) Distribution of LAD at ME orders throughout the Big Five ME events.