Ojo Alamo Formation
Geologic formation in New Mexico
From Wikipedia, the free encyclopedia
The Ojo Alamo Formation is a geologic formation in New Mexico spanning the Mesozoic/Cenozoic boundary. Non-avian dinosaur fossils have controversially been identified in beds of this formation dating from after the Cretaceous–Paleogene extinction event, but these have been explained as either misidentification of the beds in question or as reworked fossils, fossils eroded from older beds and redeposited in the younger beds.
| Ojo Alamo Formation | |
|---|---|
| Stratigraphic range: Maastrichtian-Danian ~ | |
Ojo Alamo Formation in the Bisti/De-Na-Zin Wilderness | |
| Type | Geological formation |
| Sub-units | Naashoibito Member, Kimbeto Member |
| Underlies | Nacimiento Formation |
| Overlies | Kirtland Formation |
| Lithology | |
| Primary | Conglomerate, sandstone, shale |
| Location | |
| Coordinates | 36.3305764°N 108.0350723°W |
| Region | San Juan Basin, New Mexico |
| Country | United States |
| Type section | |
| Named for | Ojo Alamo Spring |
| Named by | B. Brown |
| Year defined | 1910 |
Description
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.[1] All dinosaur fossils probably come from this unit.[2] The upper unit of the Ojo Alamo Formation is the Kimbeto Member, which was deposited mainly during the earliest Cenozoic (Danian age of the Paleogene period), between 66 and 64 million years ago.[1] Argon dating indicates the base of the Naashoibito Member is 66.5 million years old.[3][4]
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 5 metres (16 ft) and 3.5 metres (11 ft) 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.[5]
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).[6]
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.[2]
Alamo Wash fauna
The following species are known to be present in the Naashoibito Member "Alamo Wash Fauna".[7]
Fish
| Genus | Species | Material | Notes | Images |
|---|---|---|---|---|
|
M. sp. |
A single tooth | An anacoracid elasmobranch. A single tooth (NMMNH P-44485), consisting of a six-sided crown with double roots. Only occurrence of the genus in the Naashoibito Member, but miight actually be from the underlying De-na-zin Member of the Kirtland Formation. | ||
|
?S. sp. |
An isolated tooth | An orectolobid elasmobranch. May represent the first and only record of this taxon from the Naashoibito Member. | ||
| Lepisosteidae[7] | Indeterminate | Multiple isolated scales | A gar |
Amphibians
| Genus | Species | Material | Notes | Images |
|---|---|---|---|---|
|
Indeterminate |
An incomplete trunk vertebra | An indeterminate salamander. Tentatively assigned to Batrachosauroididae based on general similarities to Opisthotrition-like specimens. | ||
Squamates
Turtles
| Genus | Species | Material | Notes | Images |
|---|---|---|---|---|
|
C. sp. |
Multiple shell fragments | A pleurosternid testudine. Species-level identification is not possible, but similarities have been noted with Compsemys vafer. | ||
|
H. sp. |
A complete plastron and a few fragments | |||
|
cf. P. sp. |
Nearly complete right parietal | A trionychid testudine. Compares readily to Plastomenus thomasi. | ||
|
Indeterminate |
Mostly carapace fragments | An adocid testudine. Among the attributed material is the holotype of Adocus vigoratus, now considered a nomen dubium. | ||
|
B. sp. |
Fragments of carapace and plastron | A nanhsiungchelyid testudine. Previously Basilemys nobilis, it is now considered a nomen dubium which isn't assignable at the species level. | ||
| Paracryptodira[7] | Indeterminate | An incomplete carapace fragment | An indeterminate paracryptodiran. Similar in shell morphology to Compsemys. | |
| Baenidae[7] | Indeterminate | Carapace and skull fragments | An indeterminate baenid. | |
| Trionychidae[7] | Indeterminate | Dozens of specimens | An indeterminate trionychid. A lot of material has been recovered, but it is fragmentary and not able to be defined to the genus level. | |
| Testudinidae[7] | Indeterminate | Indeterminate shell and carapace fragments. | Indeterminate testudine. Not identifiable to any lower taxonomic rank due to their fragmentary nature. |
Crocodylians
| Genus | Species | Material | Notes | Images |
|---|---|---|---|---|
| Brachychampsa[7] | cf. B. sp. | A single tooth | An alligatorid. The tooth was previously attributed to Allognathosuchus. | |
| Crocodylidae[7] | Indeterminate | A mandibular fragment, multiple teeth and osteoderms. | An indeterminate crocodillian of which material has been referred to multiple genera in the past, including Leidyosuchus and Denazinosuchus. |
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.[8]
| Genus | Species | Material | Notes | Images |
|---|---|---|---|---|
|
D. notohesperus |
 | |||
|
O. boerei |
Possibly a nomen dubium[11] |  | ||
|
Indeterminate |
||||
|
Indeterminate |
 | |||
|
R. sp. |
 | |||
|
Indeterminate |
 | |||
|
cf. T. rex[12] |
||||
|
A. sanjuanensis |
 | |||
|
G. mimus |
 | |||
|
Indeterminate |
Noted to be similar to Euoplocephalus and Ankylosaurus |
 | ||
|
Indeterminate |
||||
|
Indeterminate |
||||
|
Indeterminate |
Noted as being similar to Corythosaurus and Hypacrosaurus |
|||
|
Indeterminate |
||||
|
O. fowleri |
 | |||
| Torosaurus | T. latus | Known from EKM 0001, a nearly complete skull and postcrania. [17] |
Mammals
| Genus | Species | Material | Notes | Images |
|---|---|---|---|---|
|
cf. A. marshi |
||||
|
Alphadontinae |
Indeterminate |
|||
|
E. browni |
||||
|
Glasbius |
cf. G. sp. |
|||
|
M. formosa |
||||
|
cf. M. sp. |
||||
|
Indeterminate |
||||
|
Pediomyidae |
cf. Indeterminate |
|||
History of investigation
The formation was named by Barnum Brown in 1910 for exposures near Ojo Alamo springs in the San Juan Basin.[19] Baltz et al. reassigned the lower beds to the Kirtland Formation in 1966, but this has not been generally accepted.[20]
