''Triprismatoolithus'' 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 t ...

dinosaur egg Dinosaur eggs are the organic vessels in which a dinosaur embryo develops. When the first scientifically documented remains of non-avian dinosaurs were being described in England during the 1820s, it was presumed that dinosaurs had laid eggs bec ...

native to

Teton County, Montana Teton County is a county in the U.S. state of Montana. As of the 2020 census, the population was 6,226. Its county seat is Choteau. The county was founded in 1893. Geography According to the United States Census Bureau, the county has a total ...

. It is classified in the oofamily

Arriagadoolithidae Arriagadoolithidae is an oofamily of fossil eggs, representing the eggs of Alvarezsaurs. Description The eggs are well stratified with three layers: the external, the prismatic, and the mammillary. The connection between prismatic and external la ...

, the eggs of

alvarezsaur Alvarezsauroidea is a group of small maniraptoran dinosaurs. Alvarezsauroidea, Alvarezsauridae, and Alvarezsauria are named for the historian Gregorio Álvarez, not the more familiar physicist Luis Alvarez, or his son geologist Walter Alvarez w ...

Distribution

''Triprismatoolithus'' is known exclusively from Sevenmile Hill, a fossil site at 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 and (million years ago), during Campanian (Late Cretaceous) time. It crops out to the east of the Rocky M ...

in Teton County,

Montana Montana () is a state in the Mountain West division of the Western United States. It is bordered by Idaho to the west, North Dakota and South Dakota to the east, Wyoming to the south, and the Canadian provinces of Alberta, British Columbi ...

. It is the oldest known fossil egg site at that formation, dating to 80 million years ago, the early part of the

Campanian The Campanian is the fifth of six ages of the Late Cretaceous Epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campanian s ...

History

Fossil eggs are common from the upper part of the Two Medicine formation, but it was not until

2010 File:2010 Events Collage New.png, From top left, clockwise: The 2010 Chile earthquake was one of the strongest recorded in history; The Eruption of Eyjafjallajökull in Iceland disrupts air travel in Europe; A scene from the opening ceremony of ...

that paleontologists discovered eggs in the lower part, including ''Triprismatoolithus''. They were discovered by two paleontologists from the

University of Montana The University of Montana (UM) is a public research university in Missoula, Montana. UM is a flagship institution of the Montana University System and its second largest campus. UM reported 10,962 undergraduate and graduate students in the fal ...

: Frankie D. Jackson and David J. Varricchio. It remained of uncertain classification (because no similar eggs were known) until

2012 File:2012 Events Collage V3.png, From left, clockwise: The passenger cruise ship Costa Concordia lies capsized after the Costa Concordia disaster; Damage to Casino Pier in Seaside Heights, New Jersey as a result of Hurricane Sandy; People gather ...

when Agnolin '' et al.'' discovered similar eggs associated with ''

Bonapartenykus ''Bonapartenykus'' (meaning "José F. Bonaparte's claw") is a monospecific genus of alvarezsauroid dinosaur from Argentina that lived during the Late Cretaceous (Campanian-Maastrichtian) in what is now the upper Allen Formation of the Río Negro ...

'', which they classified with ''Triprismatoolithus'' in a new oofamily,

Description

The

type specimen In biology, a type is a particular wiktionary:en:specimen, specimen (or in some cases a group of specimens) of an organism to which the scientific name of that organism is formally attached. In other words, a type is an example that serves to a ...

of ''Triprismatoolithus'' is a complete egg measuring 30 mm by 75 mm, but several other incomplete eggs and eggshell fragments are known. The eggs were paired, similar to '' Continuoolithus''. The eggshell is between ranges from 525 to 850 μm thick. ''Triprismatoolithus'' is notable for having three structural layers, similar to birds and unlike most other non-

avian Avian may refer to: *Birds or Aves, winged animals *Avian (given name) (russian: Авиа́н, link=no), a male forename Aviation *Avro Avian, a series of light aircraft made by Avro in the 1920s and 1930s *Avian Limited, a hang glider manufacture ...

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

eggs.Laura E. Wilson, Karen Chin, Frankie D. Jackson, and Emily S. Bray. (2012).
Fossil eggshell: Fragments from the past
UCMP's online fossil egg exhibit. The second layer, known as the prismatic layer, is made up of closely packed calcite crystals, and exhibits irregular squamatic texture.F. L. Agnolin, J. E. Powell, F. E. Novas and M. Kundrát. 2012. "New alvarezsaurid (Dinosauria, Theropoda) from uppermost Cretaceous of north-western Patagonia with associated eggs." ''Cretaceous Research'' 35(1):33–56 It is five times thicker than the mammillary layer. The surface of the eggshell of ''Triprismatoolithus'' is covered with low, rounded tubercles, similar to ''

Arriagadoolithus ''Arriagadoolithus'' is the oogenus of fossil eggs which includes the eggs of the alvarezsaurid ''Bonapartenykus''. Fossils of the creature were found in Patagonia; it may have brooded on its eggs. Description ''Arriagadoolithus patagoniensis'' i ...

''.

Classification

''Triprismatoolithus'' shares many unique characteristics with ''Arriagadoolithus'', so they are classified together in the oofamily Arriagadoolithidae. ''Triprismatoolithus'' contains a single oospecies: ''T. stephensi''.

Paleobiology

''Triprismatoolithus'' has not been found associated with embryos or adult remains, so it is unknown precisely what kind of dinosaur laid the eggs. However, the closely related oogenus, ''Arriagadoolithus'', was discovered closely associated with the

alvarezsaurid Alvarezsauridae is a family of small, long-legged dinosaurs. Although originally thought to represent the earliest known flightless birds, they are now thought to be an early diverging branch of maniraptoran theropods. Alvarezsaurids were highly ...

''Bonapartenykus'', so it is likely that ''T. stephensi'' was also laid by an unknown alvarezaur or a closely related dinosaur. Arriagadoolithids, like elongatoolithids, prismatoolithids, and '' Continuoolithus'', are laid in pairs, which is interpreted as evidence that

oviraptorosaurs Oviraptorosaurs ("egg thief lizards") are a group of feathered maniraptoran dinosaurs from the Cretaceous Period of what are now Asia and North America. They are distinct for their characteristically short, beaked, parrot-like skulls, with or wit ...

troodontids Troodontidae is a clade of bird-like theropod dinosaurs. During most of the 20th century, troodontid fossils were few and incomplete and they have therefore been allied, at various times, with many dinosaurian lineages. More recent fossil discov ...

, and alvarezsaurs had two functioning

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

, unlike birds which have only one.

References

{{Taxonbar, from=Q24913691 Fossil parataxa described in 2010 Late Cretaceous dinosaurs of North America Arriagadoolithids Dinosaur reproduction Paleontology in Montana Milk River Formation Campanian genus first appearances Campanian genus extinctions