Ojo Alamo Formation

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.

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 69-68 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.Sullivan, R.M., and Lucas, S.G. 2006.
The Kirtlandian land-vertebrate "age" – faunal composition, temporal position and biostratigraphic correlation in the nonmarine Upper Cretaceous of western North America
" New Mexico Museum of Natural History and Science, Bulletin 35:7-29. All dinosaur fossils probably come from this unit.

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.

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.Williamson, T.E. and Weil, A. (2008). "Metatherian mammals from the Naashoibito Member, Kirtland Formation, San Juan Basin, New Mexico and their biochronologic and paleobiogeographic significance. ''Journal of Vertebrate Paleontology'', 28: 803-815.

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".Jasinski, S. E., Sullivan, R. M., & Lucas, S. G. (2011). Taxonomic composition of the Alamo Wash local fauna from the Upper Cretaceous Ojo Alamo Formation (Naashoibito Member) San Juan Basin, New Mexico. Bulletin, 53, 216-271.

* Lizards
** '' Peneteius''
* Fish
** '' Myledaphus'' sp.
** '' Squatirhina'' sp.
** cf. '' Lepisosteus'' sp.
** ?Osteichthyes indet.
* Amphibians
** Caudata indet.
* Turtles
** '' Aspideretoides''
** ''Compsemys''
** cf. ''Hoplochelys'' sp.
** '' Neurankylus''
** '' Plastomenus''
** '' Adocus''?
** ''Basilemys''?
* Theropods
** '' Dineobellator notohesperus''
**'' Ojoraptorsaurus boerei''
** Caenagnathidae indet.
** Ornithomimidae indet.
** '' Richardoestesia'' sp.
** Troodontidae indet.
** cf. '' Tyrannosaurus'' sp.
* Sauropods
** ''Alamosaurus sanjuanensis''
* Ankylosaurs
** '' Glyptodontopelta mimus''
** Ankylosauridae indet. similar to '' Euoplocephalus'' and '' Ankylosaurus''
** Ankylosauria indet.
* Hadrosaurids
** Hadrosauridae indet.
** Lambeosaurini indet. similar to '' Corythosaurus'' and '' Hypacrosaurus altispinus''
* Ceratopsians
** Ceratopsidae indet.
**'' Ojoceratops fowleri''
* Mammals
** cf. ''Alphadon marshi''
** Alphadontinae indet.
** ''Essonodon browni''
** cf. '' Glasbius'' sp.
** '' Mesodoma formosa''
** cf. ''Meniscoessus'' sp.
** Multituberculata indet.
** cf. Pediomyidae indet.

History of investigation

The formation was named by Barnum Brown in 1910 for exposures near Ojo Alamo springs in the San Juan Basin.Brown, Barnum, 1910, The Cretaceous Ojo Alamo beds of New Mexico with description of the new dinosaur genus Kritosaurus: American Museum of Natural History Bulletin, v. 28, art. 24, p. 267-274.
Baltz ''et al.'' reassigned the lower beds to the Kirtland Formation in 1966, but this has not been generally accepted.Baltz, E.H., Ash, S.R., and Anderson, R.Y., 1966, History of nomenclature and stratigraphy of rocks adjacent to the Cretaceous-Tertiary boundary, western San Juan basin, New Mexico, IN Shorter contributions to general geology, 1965: U.S. Geological Survey Professional Paper, 524-D, p. D1-D23.

See also

* List of dinosaur-bearing rock formations

References

Further reading

*

{{Chronostratigraphy of Colorado, Cenozoic state=expanded

Geologic formations of the United States
Cretaceous geology of New Mexico
Paleogene geology of New Mexico
Natural history of New Mexico
Fossiliferous stratigraphic units of North America
Paleontology in the United States