''Incognitoolithus'' is an oogenus of medioolithid fossil bird egg. It is notable for bearing evidence of predation, possibly from a bird pecking the eggshell.

Distribution

''Incognitoolithus'' fragments are found at two localities in the

DeBeque Formation The DeBeque Formation is a geologic formation in Colorado's Piceance Basin,Bown & Kihm, 1981 preserving fossils which date back to the Late Paleocene to Early Eocene period (Clarkforkian to Wasatchian in the NALMA classification.

in Garfield County, Colorado. Both localities are dated to the

Early Eocene In the geologic timescale the Ypresian is the oldest age or lowest stratigraphic stage of the Eocene. It spans the time between , is preceded by the Thanetian Age (part of the Paleocene) and is followed by the Eocene Lutetian Age. The Ypresian i ...

History

Eggshell fragments of ''Incognitoolithus'' were discovered in 1977 and 1979 during two separate field seasons. The oospecies ''I. ramotubulus'' was first described in 1997 by

Karl Hirsch Karl T. Hirsch (born October 17, 1970 in Phoenix, Arizona) is an American film director and producer. He retired from filmmaking in 2015. Awards *''Audience Favourite'' at the Victoria Independent Film & Video Festival (1998, won for ''Green'') * ...

, Allen Kihm and

Darla Zelenitsky Darla K. Zelenitsky (born 1968) is a Canadian paleontologist most notable for her research on dinosaur reproductive biology and fossils. She was a part of a team that first found evidence of feathered dinosaurs in North America, and since then has ...

Description

''I. ramotubulus'' is known from over 200 eggshell fragments which may represent a single disintegrated egg. When complete, the egg is estimated to have been long and , making it significantly larger than the next largest known Eocene bird egg in North America, which measured only 6.4 x 8.9 cm. The eggshell is 1.27–1.43 mm thick and consists of two structural layers, unlike most modern

ratite A ratite () is any of a diverse group of flightless, large, long-necked, and long-legged birds of the infraclass Palaeognathae. Kiwi, the exception, are much smaller and shorter-legged and are the only nocturnal extant ratites. The systematics ...

eggs (which have a third layer, the external layer). The outer layer, called the continuous layer, is three times the thickness of the inner layer (the mammillary layer). It is one of the few fossil eggs to have only two layers and a smooth eggshell. ''Incognitoolithus'' has a distinctive pore system. It has both branching and unbranching pores. The openings of these pores on the surface of the eggshell matches the " aepyornithid" type: Pores open in small pits or in grooves which may accommodate multiple pore openings.

Paleobiology

Parentage

The pore arrangement and structure of ''Incognitoolithus'' is similar to that of '' Aepyornis''. The microstructure and lack of ornamentation are similar to modern

eggs. However, since no embryonic remains or parental remains were found with ''Incognitoolithus'', it is impossible to infer what kind of bird laid the eggs.

Predation

Multiple fragments of ''Incognitoolithus'' have been found with holes interpreted as peck marks. While it is possible that these were made by a parent assisting the hatching, this behavior is very rare in modern birds. More likely, they are the result of predation. Experiments on modern chicken eggs by Hirsch '' et al.'' in 1997 found that ''Incognitoolithus'' holes are most consistent with those from a bird's peck. However, the dynamics of a thick eggshell like ''I. ramotubulus'' differ from those of chicken eggs, so the possibility that the holes are tooth marks of a mammal or reptile cannot be ruled out.

References

{{Palaeognathae, P. Fossil parataxa described in 1997 Paleogene birds of North America Egg fossils