''Montanoolithus'' is an

oogenus Egg fossils are the fossilized remains of eggs laid by ancient animals. As evidence of the physiological processes of an animal, egg fossils are considered a type of trace fossil. Under rare circumstances a fossil egg may preserve the remains ...

of fossil egg found in

Montana Montana ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Idaho to the west, North Dakota to the east, South Dakota to the southeast, Wyoming to the south, an ...

and

Alberta Alberta is a Provinces and territories of Canada, province in Canada. It is a part of Western Canada and is one of the three Canadian Prairies, prairie provinces. Alberta is bordered by British Columbia to its west, Saskatchewan to its east, t ...

. They were probably laid by a

dromaeosaur Dromaeosauridae () is a family of feathered coelurosaurian theropod dinosaurs. They were generally small to medium-sized feathered carnivores that flourished in the Cretaceous Period. The name Dromaeosauridae means 'running lizards', from Gr ...

or a caenagnathid.D. K. Zelenitsky and F. Therrien. (2008) "Unique maniraptoran egg clutch from the Upper Cretaceous Two Medicine Formation of Montana reveals theropod nesting behaviour." ''Palaeontology'' 51(6):1253–1259

Distribution

The type specimen of ''Montanoolithus'' was found in the

Two Medicine Formation The Two Medicine Formation is a geological formation, or rock body, in northwestern Montana and southern Alberta that was deposited between 82.4 Ma and 74.4 Ma, during Campanian (Late Cretaceous) time. It crops out to the east of the Rocky Mountai ...

on the

Blackfeet Reservation The Blackfeet Nation (, ), officially named the Blackfeet Tribe of the Blackfeet Indian Reservation of Montana, is a List of federally recognized tribes in the United States, federally recognized tribe of Blackfoot Confederacy, Siksikaitsitap ...

. Other specimens are known from the Oldman Formation in Alberta. All ''Montanoolithus'' fossils yet discovered date to the

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cre ...

Description

''Montanoolithus strongorum'' is known from several eggshell fragments and a partial egg clutch (with five preserved eggs). When complete, this clutch likely had at least twelve eggs, arrayed in pairs in a ring, similar to the Asian oviraptorid clutches. The most complete egg is elongated, measuring long by wide, and slightly asymmetrical. The outer surface of its shell is ornamented with anastomosing ridges. ''Montanoolithuss eggshell ranges from 0.70 to 0.85 mm thick, and is composed of two layers. The outer layer, called the columnar layer (or squamatic zone, so named because of the peculiar texture of the layer),Laura E. Wilson, Karen Chin, Frankie D. Jackson, and Emily S. Bray
II. Eggshell morphology and structure
''UCMP Online Exhibits: Fossil Eggshell'' is twice as thick as the inner mammillary layer. The two layers are divided by a gradual boundary. The mammillae (the cone-shaped structures in the mammillary layer which make up the base of each eggshell unit) are formed by wedge-shaped crystals.

Palaeobiology

Cladistic analysis shows ''Montanoolithus'' to be maniraptoran eggs, more basal than troodontids, but more derived than

oviraptorids Oviraptoridae is a group of bird-like, herbivorous and omnivorous maniraptoran dinosaurs. Oviraptorids are characterized by their toothless, parrot-like beaks and, in some cases, elaborate crests. They were generally small, measuring between one ...

. Maniraptorans are only represented at the Two Medicine formation by ''

Troodon ''Troodon'' ( ; ''Troödon'' in older sources) is a controversial genus of relatively small, bird-like theropod dinosaurs definitively known from the Campanian age of the Late Cretaceous period (about 77 million years ago). It includes at l ...

'' (whose eggs are already known),

dromaeosaurs Dromaeosauridae () is a family (biology), family of feathered coelurosaurian Theropoda, theropod dinosaurs. They were generally small to medium-sized feathered carnivores that flourished in the Cretaceous period (geology), Period. The name Drom ...

, and caenagnathids. Therefore, the parent of ''Montanoolithus'' was probably a dromaeosaur or a caenagnathid. The mother of the ''Montanoolithus'' eggs made a mound-shaped nest out of sand, and laid the eggs in a ring around the top. The nest was made from freshly deposited sand (perhaps near to a river), or in a poorly vegetated area. Even though no parent was found with the eggs, it is likely based on its identification as a maniraptoran that the eggs were incubated, since this behavior has been observed in both troodontids and oviraptorids. The pairing of the eggs suggests that, like other maniraptorans, the egg-layer of ''Montanoolithus'' had two functioning

oviducts The oviduct in vertebrates is the passageway from an ovary. In human females, this is more usually known as the fallopian tube. The eggs travel along the oviduct. These eggs will either be fertilized by spermatozoa to become a zygote, or will degen ...

which would each form an egg simultaneously.

Parataxonomy

''Montanoolithus'' is classified in its own oofamily, Montanoolithidae, which is related to the eggs of oviraptorids, troodontids, and birds. It contains a single oospecies: ''M. strongorum''.

References

{{Taxonbar, from=Q16919269 Egg fossils Fossil parataxa described in 2008 Dinosaur reproduction Coelurosauria