New Specimens of Nemegtomaia from Mongolia Figure 10. Eggs of Nemegtomaia barsboldi. Details of eggs preserved in the upper layer of eggs in specimen MPC-D 107/15. Scale bar A–E 5 cm. doi:10.1371/journal.pone.0031330.g010 or variation from the equatorial to apical region. Eggshell microstructure from a total of eight fragments collected from the nest were studied in tangential and radial thin sections, using optical and polarizing light microscopy, and scanning electron microscopy. Unfortunately, all analyzed fragments were heavily altered and re-crystallized calcite, and all histostructures, including pore canals, were obliterated. Furthermore, no pores were observed on the external surfaces of the eggshells. Discussion The Nemegt locality provides excellent exposures of the transitional interval between the Baruungoyot and Nemegt formations, and in particular the gradual southeastward prograda- tion of the Nemegt Formation depositional system (characterized by channel-fill and sheet-flood deposits) on top of the Baruungoyot Formation (dominated by semi-arid or arid environments with recurrent aeolian beds) [33]. This interval is also the most fossil- rich vertebrate stratigraphic section in the area, hosting most of the vertebrates observed in the Nemegt beds [33]. This assumption draws attention to the palaeobiology of faunal assemblages recovered from this interval. In particular, oviraptor- ids of Mongolia and China have been collected primarily from deposits representing semi-arid or xeric environments [1,4–7,10– 12,20,39,67–70]. The sole exception was represented by the type specimen of Nemegtomaia, which was collected from the fluvially- dominated Nemegt Formation. Insufficient data are available at present for the type specimen of Heyuannia huangi [68,71], recovered from the? Maastrichtian Zhutian Formation in southern China. However, this study suggests that Nemegtomaia is found in the typical upper Baruungoyot Formation facies, in the Baruun- goyot-Nemegt interfingering interval, and also in the Nemegt Formation. Therefore, this study provides additional evidence that both environments co-existed laterally, and that the generally distinctive faunas from the two units do overlap as in the case of Avimimus, Ingenia, Nemegtomaia, and Tarchia [30,33,36,72,73]. The occurrence of a Nemegtomaia nest in this stratigraphic interval may indicate that oviraptorid nesting grounds were chosen near permanent or seasonal streams that provided soft and sandy substrate as well as a source of food within an otherwise xeric environment. The large number of adult oviraptorids found in brooding positions may indicate that they brooded their clutches for relatively long periods of time. This is similar to some present day birds from arid regions of Africa and Australia (such as the ostrich [Struthio camelus], the emu [Dromaius novahollandiae], and the black-breasted buzzard [Hamirostra melanosternon]), which incubate PLoS ONE | www.plosone.org 13 February 2012 | Volume 7 | Issue 2 | e31330