''Microolithus'' is an oogenus of

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

bird egg Bird eggs are laid by the females and range in quantity from one (as in condors) to up to seventeen (the grey partridge). Clutch size may vary latitudinally within a species. Some birds lay eggs even when the eggs have not been fertilized; it i ...

from

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, with preserved embryonic remains inside some of its specimens.

History

Fossil bird eggs from the

Paleogene The Paleogene ( ; also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period million years ago (Mya) to the beginning of ...

are rare in North America, and have only occasionally been examined microstructurally and compared to modern birds. The specimens that would later be named ''Microolithus'' were collected in 1940 by the paleontologist R. W. Wilson. In

2013 File:2013 Events Collage V2.png, From left, clockwise: Edward Snowden becomes internationally famous for leaking classified NSA wiretapping information; Typhoon Haiyan kills over 6,000 in the Philippines and Southeast Asia; The Dhaka garment ...

, paleontologists from

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Frankie D. Jackson and David J. Varricchio teamed up with Joseph A. Corsini from Eastern Oregon University to analyze these specimens, which they assigned to a new oogenus and oospecies, ''Microolithus wilsoni''. This would be only the second oogenus of Paleogene bird egg to be named in North America (the first being ''

Incognitoolithus ''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 DeB ...

'').

Distribution

The known specimens of ''Microolithus'' were found in the

Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', " ...

Willwood Formation in

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Description

''Microolithus'' is, as its name suggests, a small egg; the smallest specimen is by . Like all medioolithids, it is roughly spherical. The eggshell is 600 μm thick, and its outer surface is smooth and glossy, but this may be due to erosion. Its eggshell is composed of three structural layers. The outermost layer (the external layer) contacts the middle layer (the continuous layer), abruptly. The continuous layer has squamatic texture obscuring the prism-shaped eggshell units. The innermost layer (the mammillary layer) makes up one-fourth of the eggshell's thickness, and has an abrupt but wavy boundary with the continuous layer. The eggshell has circular pores with a density of 1.08 per square millimeter. Some ''Mircoolithus'' specimens have embryonic remains of long bones and possibly vertebra, but no useful characteristics for determining which bones or to what bird group they belong are preserved. The

periosteum The periosteum is a membrane that covers the outer surface of all bones, except at the articular surfaces (i.e. the parts within a joint space) of long bones. Endosteum lines the inner surface of the medullary cavity of all long bones. Struc ...

of these bones is somewhat porous. ''Microolithus'' is very similar to modern bird eggs, even more so than ''

Metoolithus ''Metoolithus'' is an oogenus of fossil bird egg from Nebraska. It is known from a single, near-complete egg, as well as several eggshell fragments. They are small, (60 mm x 44 mm) and spheroid. They show a mix of avian Avian may refer to: *Bir ...

'' and the other known

bird eggs. It closely resembles modern

neognaths Neognathae (; ) is a infraclass of birds, called neognaths, within the class Aves of the clade Archosauria. Neognathae includes the majority of living birds; the exceptions being the tinamous and the flightless ratites, which belong instead t ...

, especially the

sandhill crane The sandhill crane (''Antigone canadensis'') is a species of large crane of North America and extreme northeastern Siberia. The common name of this bird refers to habitat like that at the Platte River, on the edge of Nebraska's Sandhills on th ...

Parataxonomy

Because of the numerous similarities of ''Microolithus wilsoni'' to the

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Medioolithus ''Medioolithus'' is an oogenus of fossil egg laid by a paleognath.Kohring, R., & Hirsch, K. F. (1996). "Crocodilian and avian eggshells from the Middle Eocene of the Geiseltal, Eastern Germany." ''Journal of Vertebrate Paleontology'', 16(1), 67 ...

'', they are classified together (along with ''Incognitoolithus'') in the oofamily Medioolithidae.

References

{{Reflist Fossil parataxa described in 2013 Egg fossils Neognathae