Styloolithus

Styloolithusis anoogenusof highly distinctivefossil eggfrom theUpper CretaceousDjadokhta Formationand theBarun Goyot Formationin Mongolia.

Styloolithus Temporal range:
Two partial cluthces
Egg fossil classification
Oofamily:	incertae sedis
Oogenus:	†Styloolithus
Oospecies
†S. sabathi

History

The eggs ofStyloolithuswere first discovered in1991byKarol Sabathin an assemblage of fossil eggs in theGobi Desert,and were described as "larger avian eggs".They were considered classified asGobioolithus majorin 1996 by Mikhailov.In 2015, Varracchio and Barta redefinedG. major,and reclassified the "larger avian eggs" into an entirely new oogenus,Styloolithus.

Description

Styloolithusegg next to twoGobioolithus

Thetype specimenofStyloolithusis a fossilclutchof 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, likePrismatoolithusandTroodontideggs, in contrast to the loosely scattered nests ofenantiornithinebirds. However, the eggs themselves are quite different from Troodontids, and the associated skeletal remains are similar to the enantiornithineGobipteryx minuta(=Nanantius valifanovi).

S. sabathieggs are quite different from all other known fossil eggs of theCretaceous.At 70 mm long and 32 mm across, they are larger than both oospecies ofGobioolithus,and more elongated. It can be distinguished from all non-aviandinosaureggs by the thick third layer of the shell (possibly representing an external zone). It lacks shell ornamentation, unlikeElongatoolithusandMacroelongatoolithus.

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.If correctly interpreted, this would prove thatStyloolithusin fact represents bird eggs. However, it cannot be proven to have an external layer until its shell is examined byscanning electron microscopy.

Parataxonomy

When they were first discovered in 1991,Styloolithuseggs (then unnamed) were described as bird eggs.The threephylogenetic analysesperformed by Varracchio and Barta (2015) were inconclusive as to whether it represented a bird or a non-aviantheropod.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 withStyloolithuseggs suggests intense parental care.The tight egg clutches show that the incubation methods differed from that ofGobipteryx.Like Troodontids, the parents ofStyloolithusprobably sat on top of largely buried eggs. It is unknown whetherS. sabathiwas incubated by the father or the mother of the eggs.