CONTROLLING FOR THE SPECIES-AREA EFFECT REMOVES LONG-TERM DIVERSITY TRENDS IN MESOZOIC TERRESTRIAL VERTEBRATES

Almost half of extant vertebrate species live on land. Whether this spectacular diversity arose via bounded or unbounded diversification processes remains controversial, with the dominant expansionist model positing exponential increases in standing diversity from the origin of tetrapods ~360 Ma to the present day. Recent rigorous analyses of fossil occurrences using sample-standardisation techniques strongly contradict this view, however, suggesting nearly static long-term diversity in Mesozoic tetrapods. These analyses also highlight the strong relationship between level of spatial sampling and both raw and subsampled diversity estimates. The species-area effect is one of the most pronounced and best-known macroecological scaling patterns, but as variability in spatial sampling affects the underlying species pool, coverage-based sampling-standardisation techniques cannot correct for these biases. Here, we present a global diversity curve for non-flying, non-marine Mesozoic–early Paleogene tetrapods that directly addresses biases introduced by the species-area effect. Minimum-spanning trees (MSTs) are used to subsample fossil collections at fixed levels of palaeogeographic spread, ensuring that the scale of spatial sampling remains constant over time and between geographic regions. We also construct species-area relationships for the end-Cretaceous and early Paleogene. This reveals an even flatter long-term Mesozoic trend than previously recovered, with standing diversity almost unchanged between the start of the Triassic and end of the Cretaceous. A previously-noted jump in diversity across the K-Pg (representing an apparent tripling or quadrupling of species) remains, although this increase may be an artefact of the greater diagnosability of Cenozoic mammalian taxa. Our results strongly support bounded diversification models during the Mesozoic, and suggest that equilibrium levels were reset at a higher level in the aftermath of the end-Cretaceous extinction.