MAMMALS, ARCHOSAURS, AND THE EARLY TO LATE CRETACEOUS TRANSITION IN NORTH-CENTRAL TEXAS

The middle portion of the Cretaceous Period is marked by a profound transformation of terrestrial ecosystems, most notably the ascendancy of flowering plants, the origin of modern groups of mammals, amphibians, and lizards, apparently high extinction rates, and the appearance of new forms of dinosaurs, particularly hadrosaurids. The stratigraphic section of north-central Texas comprises a sequence of intercalated terrestrial and marine formations extending from the Aptian through the Cenomanian stages, and thereby documents the major features of this 27 my interval. From 121 Ma to 107 Ma, the archosaur and mammalian fauna in Texas appears to have been stable in composition and diversity. By 100 Ma, dramatic change had occurred with the apparent local extirpation of sauropods, basal iguanodontians, triconodontids, and primitive tribosphenids, and with the introduction of the nodosaurid Pawpawsaurus, followed by hadrosaurids and metatherians. A seven million year interval of marine deposition in Texas disrupts the terrestrial record, but correlations with other sections suggest that the interval of faunal stability documented in Texas may have lasted until 102 Ma elsewhere in North America, followed by an interval of faunal turnover perhaps as short as 2 Ma. This pattern of stability followed by relatively rapid change with little commingling of earlier and later faunal components suggests a concurrence of biotic and abiotic events culminating in completion of the Western Interior Seaway, faunal interchange with Asia, high levels of extinction and origination, rapid evolutionary diversification, and the growing dominance of flowering plants.