DATING THE EXTINCTION OF PALEOCENE LAZARUS DINOSAURS BASED ON MAGNETOCHRONOLOGY, SAN JUAN BASIN, NEW MEXICO

Recent studies show that a diverse assemblage of dinosaur fossils in the San Juan Basin of New Mexico are Paleocene in age, thus some dinosaurs survived the asteroid-impact “extinction” event that is generally accepted as marking the end of the Cretaceous Period throughout the world. It is suggested that these “Lazarus” dinosaurs may have survived the short period of maximum devastation, immediately following the impact, as eggs laid shortly before the impact occurred. Even though all mature dinosaurs were probably killed by the impact and the ensuing period of global darkness, their recently laid eggs would have provided a survival sanctuary for some of the developing dinosaur embryos for from one to two years. The San Juan Basin's Paleocene dinosaur fauna is named the Alamoan fauna for the Ojo Alamo Sandstone, the formation in which these dinosaur fossils have been found.

Revised ages for the global, geomagnetic time scale indicate that the age of the Cretaceous-Tertiary boundary is 65.5 Ma (as determined by Obradovich). The boundary is in magnetochron C29r. Published data from the Hunter Wash area of the southern San Juan Basin (where most of the Lazarus dinosaurs have been discovered)show that the lower (dinosaur-bearing)part of the Paleocene Ojo Alamo Sandstone contains a thin interval of reversed polarity, identified as the uppermost part of magnetic-polarity chron (magnetochron) C29r, overlain by a thicker interval of normal magnetic polarity, magnetochron C29n, The thin interval of C29r at the base of the Ojo Alamo, is estimated to represent about 0.170 m.y. Published estimates give the duration of magnetochron C29n as 0.769 m.y., thus, the duration of the Alamoan Lazarus dinosaur fauna would have been about 0.939 m.y. - virtually 1 million years - and the time of extinction of this Paleocene dinosaur fauna would have been about 64.5 Ma (Obradovich time scale).