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.

Ojo Alamo Formation
Stratigraphic range: Maastrichtian-Danian ~69–64 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
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 / 36.3305764; -108.0350723
Region	San Juan Basin, New Mexico
Country	USA
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 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.^[1] All dinosaur fossils probably come from this unit.^[2] Although argon dating suggest the naashoibito member would be dated to 66.5 mya. ^[3] 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]

Fossils

A restoration of the environment, featuring Dineobellator (center front), Ojoceratops (right), Tyrannosaurus (far left), and Alamosaurus (center back), taxa all known from the formation.

Ojo Alamo Formation showing conglomerate lens with petrified wood, De-Na-Zin Wilderness

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).^[4]

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".^[5]

Fish

Genus	Species	Material	Notes	Images
Myledaphus[5]	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.
?Squatirhina[5]	?S. sp.	An isolated tooth	An orectolobid elasmobranch. May represent the first and only record of this taxon from the Naashoibito Member.
Lepisosteidae[5]	Indeterminate	Multiple isolated scales	A gar

Amphibians

Genus	Species	Material	Notes	Images
?Batrachosauroididae[5]	Indeterminate	An incomplete trunk vertebra	An indeterminate salamander. Tentatively assigned to Batrachosauroididae based on general similarities to Opisthotrition-like specimens.

Testudines

Genus	Species	Material	Notes	Images
Compsemys[5]	C. sp.	Multiple shell fragments	A pleurosternid testudine. Species-level identification is not possible, but similarities have been noted with Compsemys vafer.
Hoplochelys[5]	H. sp.	A complete plastron and a few fragments
Plastomenus[5]	cf. P. sp.	Nearly complete right parietal	A trionychid testudine. Compares readily to Plastomenus thomasi.
Adocidae[5]	Indeterminate	Mostly carapace fragments	An adocid testudine. Among the attributed material is the holotype of Adocus vigoratus, now considered a nomen dubium.
Basilemys[5]	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[5]	Indeterminate	An incomplete carapace fragment	An indeterminate paracryptodiran. Similar in shell morphology to Compsemys.
Baenidae[5]	Indeterminate	Carapace and skull fragments	An indeterminate baenid.
Trionychidae[5]	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[5]	Indeterminate	Indeterminate shell and carapace fragments.	Indeterminate testudine. Not identifiable to any lower taxonomic rank due to their fragmentary nature.

Squamates

Genus	Species	Material	Notes	Images
Peneteius[5]	P. sp.	A jaw fragment and multiple teeth	Fossil material requires proper documentation.
?Chamops[5]	?C. sp.	A tooth	A teiid lizard. The tooth was originally identified as that of a cyprinid fish. Its presence in the Alamo Wash fauna is questionable.

Crocodylians

Genus	Species	Material	Notes	Images
Brachychampsa[5]	cf. B. sp.	A single tooth	An alligatorid. The tooth was previously attributed to Allognathosuchus.
Crocodylidae[5]	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

Genus	Species	Material	Notes	Images
Dineobellator[6]	D. notohesperus
Ojoraptorsaurus[7]	O. boerei		Possibly a nomen dubium[8]
Caenagnathidae	Indeterminate
Ornithomimidae	Indeterminate
Richardoestesia	R. sp.
Troodontidae	Indeterminate
Tyrannosaurus	cf. T. sp.
Alamosaurus[9]	A. sanjuanensis
Glyptodontopelta[10]	G. mimus
Ankylosauridae	Indeterminate		Noted to be similar to Euoplocephalus and Ankylosaurus
Ankylosauria	Indeterminate
Hadrosauridae	Indeterminate
Lambeosaurini[11]	Indeterminate		Noted as being similar to Corythosaurus and Hypacrosaurus
Ceratopsidae	Indeterminate
Ojoceratops[12]	O. fowleri

Mammals

Genus	Species	Material	Notes	Images
Alphadon	cf. A. marshi
Alphadontinae	Indeterminate
Essonodon[13]	E. browni
Glasbius	cf. G. sp.
Mesodma	M. formosa
Meniscoessus	cf. M. sp.
Multituberculata	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.^[14] Baltz et al. reassigned the lower beds to the Kirtland Formation in 1966, but this has not been generally accepted.^[15]

See also

• Earth sciences portal
• Paleontology portal
• Dinosaurs portal

• List of dinosaur-bearing rock formations