Early Cretaceous

The Early Cretaceous is an important interval for several reasons. It is the longest epoch of the Mesozoic, spanning more than 45 Ma. Terrestrial life, both plants and animals, changed radically over the course of the Early Cretaceous, both taxonomically and morphologically. In part, this has been attributed to reciprocal biotic interactions involving the appearance and radiation of flowering plants, or angiosperms (see Wing and Tiffney, 1987; and papers in Friis et al., 1987), but other factors such as climatic change, sea level, and intercontinental dispersal (e.g., Cifelli, Kirkland, et al., 1997) are probably involved as well. Many "modern" groups—cimolodontan multitubercu-lates, marsupials, and placentals among mammals, for example—first appear or are suspected to have originated during the Early Cretaceous (Lillegraven et al., 1987; Kielan-Jaworowska and Dashzeveg, 1989; Cifelli, 1993b; Kielan-Jaworowska and Hurum, 2001). On northern continents, archaic groups, such as "triconodonts," "sym-metrodontans," and "eupantotherians," dwindled in importance or became extinct. Significant separation of Laurasia and Gondwana occurred, with the result that their respective faunas became more distinct than they had been earlier in the Mesozoic.

When Simpson summarized the known record of Early Cretaceous mammals in 1928, only three specimens, all isolated teeth from the Wealden of England, were known from this 45-Ma interval (Simpson, 1928a). Given the near absence of Early Cretaceous fossils, the taxonomic and morphologic differences between mammals of the Late Jurassic and Late Cretaceous were magnified— particularly because the early Late Cretaceous was poorly known as well. In this context, discoveries in Lower Cretaceous rocks during the second half of the twentieth century collectively rank as one of the major advances in understanding the early history of mammals. Early Cretaceous mammals are now known from Western Europe, Russia (Siberia), various parts of Asia including Japan, Africa, South America, Australia, and North America— every major landmass except Antarctica. To be sure, many of these are incompletely known, but some are represented by magnificently complete skulls and skeletons.

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