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 ''Macroelongatoolithus'', however they are all now considered to be a single oospecies: ''M. carlylensis''.

History of discovery

''"Oolithes" carlylensis'' was described in 1970 by James Jensen.Jensen J. 1970. Fossil eggs in the lower cretaceous of Utah. ''Brigham Young University research studies. Geology Series. Geology Studies'' 17: 51-65. In 1998, ''O. carlylensis'' was moved to a new oogenus, ''Boletuoolithus'', which was then classified as a Spheroolithid.Bray, E. S. 1998. Dinosaur eggshell ''Boletuoolithus carlylensis'', oogen. nov. from the Lower Cretaceous Cedar Mountain Formation of Utah; pp. 221–223 in S. G. Lucas, J. I. Kirkland, and J. W. Estep
(eds.), Lower and Middle Cretaceous Terrestrial Ecosystems. New Mexico Museum of Natural History and Science Bulletin No. 14. ''Macroelongatoolithus'' was first described in 1995 by Li ''et al.'', with a single oospecies: ''M. xixiaensis''. It was classified in the oofamily Elongatoolithidae.Li Y, Yin Z, Liu Y. (1995) "The discovery of a new genus of dinosaur egg from Xixia, Henan, China." Journal of Wuhan Institute of Chemical Technology 17(1): 38-41. (In Chinese) Later in the same year, Wang and Zhou described ''Longiteresoolithus xixiaensis'', which they placed into a new oofamily, Macroelongatoolithidae. These three oospecies are now considered synonymous, and generally classified in Elongatoolithidae. Since ''M. carlylensis'' was named first, it has priority, but the name was corrected to ''M.'' ''carlylei'' to follow ICZN provisions.Zelenitsky DK, Carpenter K, Currie PJ. (2000) "First record of Elongatoolithid theropod eggshell from North America: the Asian oogenus Macroelongatoolithus from the lower Cretaceous of Utah." ''Journal of Vertebrate Paleontology'' 20(1): 130-138.

Description

''Macroelongatoolithus'' eggs are most notable for their large size. They are at least long, but the largest specimens are over long.Carpenter, K. 1999. Eggs, Nests, and Baby Dinosaurs: A Look at Dinosaur Reproduction (Life of the Past). Indiana University Press, Bloomington, Indiana. They are also very elongated, usually roughly three times longer than they are wide. They are often found in large clutches of up to 26 eggs, with the eggs forming a ring in diameter. The shell is typically between 1.38 mm and 4.75 mm thick.

Like other elongatoolithids, ''Macroelongatoolithuss eggshell is divided up into two structural layers, and the outer layer (called the continuous layer) is not divided up into distinct shell units, unlike other oofamilies. In ''Macroelongatoolithus'', the boundary between the continuous layer and the mammillary layer (the inner layer of the eggshell, also called the cone layer) is wavy, but clearly defined. The ratio of the thickness of the two layers varies from 2:1 to 8:1.

The surface ornamentation of the eggshells is variable, even on a single egg. It is usually lineartuberculate (nodes forming linear ridges), ramotuberculate (nodes forming irregular, meandering chains), or dispersituberculate (scattered nodes).

''Macroelongatoolithus'' specimens are extremely variable in size, shape, and microstructure, even in eggs laid by a single individual. For example, within a single clutch, the egg lengths can vary by several centimeters. The high amount of variability is probably due to their large size.

Classification

''Macroelongatoolithus'' has a convoluted parataxonomic history. Several oospecies have been described, but currently they are all considered synonyms of ''M. carlylei''. While it is occasionally classified into a separate oofamily, Macroelongatoolithidae, the general consensus is that it is a member of Elongatoolithidae. Two other oospecies have also been described: ''M. zhangi'' and ''M. goseongensis'', but both are now considered synonymous with ''M. carlylensis''. The oogenus and oospecies ''Megafusoolithus qiaoxiaensis'' is also a junior synonym of ''Macroelongatoolithus carlylei''.

Paleobiology

While they were once considered to be the eggs of large tyrannosaurids (like ''Tarbosaurus'') on the basis of their huge size,Qian, M. P., Jiang, Y., Jiang, Y. G., Zhang, Y. J., Chen, R., Li, L. M., and Xing, G. F. (2008). "New evidence on fossil eggs of Cretaceous Tyrannosaurs in eastern China." ''Jiangsu Geology'', 32: 86-97. egg shape and microstructural evidence suggests they are eggs of gigantic oviraptorosaurs (like '' Beibeilong'' or ''Gigantoraptor''). ''Macroelongatoolithus'' eggs closely resemble those of oviraptorids (Elongatoolithidae), which are largely known for their brooding adult-nest associations, embryonic remains, and unique nests. In 2017 the caenagnathid ''Beibeilong'' was described, based on an embryonic specimen (named "Baby Louie") associated with 6 to 8 ''Macroelongatoolithus'' eggs and partial nest. The discovery of ''Beibeilong'' further concludes that the oogenus ''Macroelongatoolithus'' was laid by large oviraptorosaurs, in this case caenagnathids.

The gas conductance of the eggshells indicate that ''Macroelongatoolithus'' nests were buried in vegetation or sediments. Like other elongatoolithids, ''Macroelongatoolithus'' eggs are laid in pairs because the parents had two functional oviducts and thus laid two eggs simultaneously. The nests have unusually large volume of eggs compared to the body size of the parents, which could mean that multiple females would contribute to a single nest. Association of other oviraptorids with their eggs suggests extensive parental care was typical for elongatoolithids.

In 2018, Kohei Tanaka and team examined the egg clutches of numerous oviraptorosaur specimens, including egg clutches of ''Macroelongatoolithus'', in order to correlate the nest configuration and body size to incubation behaviour. Their results showed that eggshell porosity indicates that the eggs of almost certainly all oviraptorosaurs were exposed in the nest without an external covering. Though most oviraptorosaur nests have eggs arranged in a circular fashion, the morphology of the nest is different in smaller and larger species in that the center of the nest is highly reduced in the former species, and becomes significantly larger in the latter species. This nest configuration suggest that whereas smallest oviraptorosaurs probably sat directly on the eggs, a large, ''Gigantoraptor''-sized animal likely sat on the area devoid of eggs. Tanaka and colleagues pointed out that this adaption was beneficial to avoid egg-crushing and could have allowed some body-contact during incubation in these giant oviraptorosaurs.

Paleoecology

Distribution

''Macroelongatoolithus'' eggs have been found in the United States, China, and South Korea, ranging in age from Early Cretaceous to Maastrichtian. More specifically, it is known in North America from the Cedar Mountain, Dakota, Kelvin, and Naturita Formations of Utah, the Wayan Formation of Idaho, the Blackleaf Formation of Montana, the Thomas Fork Formation of Wyoming, and the Willow Tank Formation of Nevada. In China, it is known from the Liangtoutang and Chichengshan Formations in Tiantai County, Zhejiang Province,X. Jin, Y. Azuma, F. D. Jackson and D. J. Varricchio. (2007) "Giant dinosaur eggs from the Tiantai basin, Zhejiang province, China." ''Canadian Journal of Earth Sciences'' 44:81-88.Wang Qiang, Zhao Zikui, Wang Xiaolin, Jiang Yangen, and Zhang Shukang. (2010) "A New Oogenus Of Macroelongatoolithid Eggs From The Upper Cretaceous Chichengshan Formation Of The Tiantai Basin, Zhejiang Province And A Revision Of The Macroelongatoolithids" '' Acta Paleontologica Sinica'' 49(1)：73-86. from the Gaogou, Sigou, Majiacun, and Zoumagang Formations in Henan Province, China.Grellet-Tinner, G., Chiappe, L., Norell, M., and Bottjer, D. (2006).
Dinosaur eggs and nesting behaviors: a paleobiological investigation.
''Palaeogeography, Palaeoclimatology, Palaeoecology,'' 232(2): 294-321.Li, G., Chen, P., Wang, D., & Batten, D. J. (2009).
The spinicaudatan Tylestheria and biostratigraphic significance for the age of dinosaur eggs in the Upper Cretaceous Majiacun Formation, Xixia Basin, Henan Province, China.
''Cretaceous Research'', 30(2): 477-482. It is known in South Korea from the Goseong Formation near Tongyeong and from Aphae-do in Shinan-gun, Jeollanam-do Province.Huh, M., Kim, B. S., Woo, Y., Simon, D. J., Paik, I. S., & Kim, H. J. (2014). First record of a complete giant theropod egg clutch from Upper Cretaceous deposits, South Korea. Historical Biology, 26(2), 218–228. https://doi.org/10.1080/08912963.2014.894998

The presence of ''Macroelongatoolithus'' in the United States indicates that there was likely a giant oviraptorosaur present during the Late Cretaceous in North America. Also, the fact that they have been found in both Asia and North America is evidence of an Albian-Cenomanian interchange of fauna between the two continents.

See also

* List of dinosaur oogenera
* Elephant bird, the avian dinosaur that laid the largest egg.

References

External links

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{{Taxonbar, from=Q6725267

Fossil parataxa described in 1995
Elongatoolithidae
Cedar Mountain Formation