Ojo Alamo Formation

The Ojo Alamo Formation is divided into two subunits separated by a large unconformity—a gap in the geologic record. The lower Naashoibito member (sometimes considered part of the Kirtland Formation) was deposited during the Maastrichtian age of the Cretaceous period, specifically between about 70-68 million years ago. It overlies the De-na-zin member of the Kirtland formation, though the two are separated by another large unconformity that spans a period of geologic time equivalent to 73-69 million years ago. All dinosaur fossils probably come from this unit. In 2025, Flynn et al. determined the strata of the Naashoibito Member preserving non-avian dinosaur fossils to be latest Maastrichtian in age, yielding maximum depositional ages of 66.87 ± 0.04 Ma and 66.38 ± 0.08 Ma based on direct argon dating on two dinosaur fossil–bearing samples around and above the base of the member respectively. They interpreted that high diversity of North American dinosaurs lived before the Cretaceous–Paleogene extinction event based on this finding, and that Maastrichtian dinosaur faunas from Laramidia were not uniform in the entire continent. == Fossils ==

Dinosaur remains are among the fossils that have been recovered from the formation, though all dinosaur remains come from the lowest part of the formation, the Naashoibito member (sometimes considered part of the Kirtland Formation, which dates to the late Maastrichtian stage of the Cretaceous period). Some researchers have claimed to find isolated non-avian dinosaur remains in the younger Kimbeto Member. If this is the case, it would represent the only known instance of a non-avian dinosaur population persisting after the Cretaceous–Paleogene extinction event. However, most scientists consider these to have been stratigraphically misinterpreted or reworked from the older Naashoibito member. Alamo Wash fauna The following species are known to be present in the Naashoibito Member "Alamo Wash Fauna". Fish Amphibians Squamates Turtles Crocodylians Dinosaurs A bite-and-drag mark on a juvenile tyrannosaurid vertebra (NMMNH P-40953) is deep and narrow, suggesting the tooth that made it was strongly compressed and blade-like, unlike those of T. rex. This supports the presence of a distinct southern North American tyrannosaurid during the Maastrichtian. Mammals == History of investigation ==

The formation was named by Barnum Brown in 1910 for exposures near Ojo Alamo springs in the San Juan Basin. Baltz et al. reassigned the lower beds to the Kirtland Formation in 1966, but this has not been generally accepted. == See also ==