''Styloolithus'' is an oogenus of highly distinctive

fossil egg 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 ...

from the

Upper Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger 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 ''creta'', the ...

Djadokhta Formation and the Barun Goyot Formation in

Mongolia Mongolia; Mongolian script: , , ; lit. "Mongol Nation" or "State of Mongolia" () is a landlocked country in East Asia, bordered by Russia to the north and China to the south. It covers an area of , with a population of just 3.3 million, ...

.Varricchio, D.J. and D.E. Barta (2015).
Revisiting Sabath's "Larger Avian Eggs" from the Gobi Cretaceous
''Acta Palaeontologica Polonica'' 60(1):11-25.

History

The eggs of ''Styloolithus'' were first discovered in

1991 File:1991 Events Collage.png, From left, clockwise: Boris Yeltsin, elected as Russia's first president, waves the new flag of Russia after the 1991 Soviet coup d'état attempt, orchestrated by Soviet hardliners; Mount Pinatubo erupts in the Phil ...

Karol Sabath Karol Sabath (April 24, 1963 – October 10, 2007) was a Polish biologist, paleontologist and paleoartist. He was employed by the Instytut Paleobiologii of the Polska Akademia Nauk - Institute of Paleobiology, Polish Academy of Sciences, Warsaw. ...

in an assemblage of fossil eggs in the

Gobi desert The Gobi Desert (Chinese: 戈壁 (沙漠), Mongolian: Говь (ᠭᠣᠪᠢ)) () is a large desert or brushland region in East Asia, and is the sixth largest desert in the world. Geography The Gobi measures from southwest to northeast an ...

, and were described as "larger avian eggs".Sabath, K. (1991).
Upper Cretaceous amniotic eggs from the Gobi Desert
. '' Acta Palaeontologica Polonica'' 36: 151–192. They were considered classified as '' Gobioolithus major'' in 1996 by Mikhailov. In 2015, Varracchio and Barta redefined ''G. major'', and reclassified the "larger avian eggs" into an entirely new oogenus, ''Styloolithus''.

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 ''Styloolithus'' is a fossil

clutch A clutch is a mechanical device that engages and disengages power transmission, especially from a drive shaft to a driven shaft. In the simplest application, clutches connect and disconnect two rotating shafts (drive shafts or line shafts). ...

of at least four eggs and associated adult remains (probably representing the parent). Several other fossil eggs are also known. The egg clutches are tightly packed together, like ''

Prismatoolithus ''Prismatoolithus'' is an oogenus of dinosaur egg from the Cretaceous (Hauterivian-Maastrichtian) and possibly also the earliest Paleocene. They likely belonged to troodontids Troodontidae is a clade of bird-like theropod dinosaurs. During m ...

'' and Troodontid eggs, in contrast to the loosely scattered nests of

enantiornithine The Enantiornithes, also known as enantiornithines or enantiornitheans in literature, are a group of extinct avialans ("birds" in the broad sense), the most abundant and diverse group known from the Mesozoic era. Almost all retained teeth and cla ...

birds. However, the eggs themselves are quite different from Troodontids, and the associated skeletal remains are similar to the enantiornithine ''

Gobipteryx minuta ''Gobipteryx'' (from Gobi eferring to the Gobi Desert where it was first discovered and Greek pteryx “wing”) is a genus of prehistoric bird from the Campanian Age of the Late Cretaceous Period.Elżanowski, A. (1974): Preliminary note on th ...

'' (=''Nanantius valifanovi''). ''S. sabathi'' eggs are quite different from all other known fossil eggs of the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...

. At 70 mm long and 32 mm across, they are larger than both oospecies of ''

Gobioolithus ''Gobioolithus'' is an oogenus of fossil bird egg native to Mongolia. They are small, smooth-shelled, and elongated eggs that were first discovered in the 1960s and early 70s during a series of fossil-hunting expeditions in the Gobi desert. Two ...

'', and more elongated. It can be distinguished from all non-avian

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 by the thick third layer of the shell (possibly representing an external zone). It lacks shell ornamentation, unlike '' Elongatoolithus'' and ''

Macroelongatoolithus ''Macroelongatoolithus'' is an oogenus of large theropod dinosaur eggs, representing the eggs of giant caenagnathid oviraptorosaurs. They are known from Asia and from North America. Historically, several oospecies have been assigned to ''Macroelo ...

''. The eggs are interesting because of the possible presence of an external zone, the outer layer of the eggshell which is rare outside of bird eggs. Laura E. Wilson, Karen Chin, Frankie D. Jackson, and Emily S. Bray
II. Eggshell morphology and structure
''UCMP Online Exhibits: Fossil Eggshell'' If correctly interpreted, this would prove that ''Styloolithus'' in fact represents bird eggs. However, it cannot be proven to have an external layer until its shell is examined by scanning electron microscopy.

Parataxonomy

When they were first discovered in 1991, ''Styloolithus'' eggs (then unnamed) were described as bird eggs. The three phylogenetic analyses performed by Varracchio and Barta (2015) were inconclusive as to whether it represented a bird or a non-avian theropod. However, it is most likely avian because it has an unornamented, thin shell that probably has three structural layers, a combination of characters unique to bird eggs. Also, the bones found associated with the eggs seem to be avian.

Palaeobiology

The association of adult bones with ''Styloolithus'' eggs suggests intense parental care. The tight egg clutches show that the incubation methods differed from that of ''Gobipteryx''. Like Troodontids, the parents of ''Styloolithus'' probably sat on top of largely buried eggs. It is unknown whether ''S. sabathi'' was incubated by the father or the mother of the eggs.

References

{{Reflist Egg fossils Campanian life Late Cretaceous birds of Asia Cretaceous Mongolia Fossils of Mongolia Barun Goyot Formation Djadochta fauna Fossil parataxa described in 2015