Mesozoic mammaliaform diversity: The effect of sampling corrections on reconstructions of evolutionary dynamics

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Recent years have witnessed an explosion of new fossil discoveries and analyses documenting the unappreciated ecological and morphological diversity of Mesozoic Mammaliaformes. In contrast, the taxonomic diversity dynamics through the first 165 million years of mammal evolution have not yet been rigorously analysed, leaving patterns of diversification during this important period open to conjecture. Here, we present a comprehensive statistical analysis of global mammaliaform diversity spanning from the Late Triassic appearance of mammaliaforms (~230 million years ago [hereafter, mya]) to the end Cretaceous mass extinction (66mya). We analysed 691 occurrences representing 367 genera and 550 species in standard time bins of approximately 10 million years in duration. Significant correlations between diversity and sampling proxies suggest sampling biases in the mammaliaform fossil record. Shareholder quorum subsampling and model-based approaches were used to mediate these biases. After applying these methods, the following patterns were supported: low standing diversity during the Late Triassic–Early Jurassic evolution of early Mammaliaformes (e.g., morganucodonts) was followed by high standing diversity during the Late Jurassic due primarily to the diversification of Eutriconodonta, Multituberculata and Cladotheria. This peak was followed by a fall in diversity during the middle of the Hauterivian–Barremian interval, suggesting that extinctions typically associated with the Jurassic–Cretaceous boundary may instead have occurred later, during the Early Cretaceous. Standing diversity recovered through several fluctuations during the ‘mid’ Cretaceous (approximately Barremian to Albian), leading to a second peak in the Campanian that reflects the diversification of key clades, including therians. Analyses of geographically restricted datasets illustrate a significant spatial heterogeneity in sampling, with several intervals dominated by North American occurrences. Uneven sampling effort and geographic heterogeneities in the fossil record are significant factors affecting reconstructions of Mesozoic mammaliaform diversity, and correcting these biases can markedly alter observed patterns and their interpretation.