Gobioolithusis anoogenusof fossilbird eggnative toMongolia.They are small, smooth-shelled, and elongated eggs that were first discovered in the 1960s and early 70s during a series of fossil-hunting expeditions in theGobi Desert.Two oospecies have been described:Gobioolithus minorandG. major.The eggs were probably laid in colonial nesting sites on the banks of rivers and lakes.
| Gobioolithus Temporal range:Campanian
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| Gobioolithus major(left) compared toG. minor(right) | |
Egg fossil classification
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| Basic shell type: | Ornithoid |
| Morphotype: | †Ornithoid-prismatic |
| Oofamily: | †Gobioolithidae |
| Oogenus: | †Gobioolithus Mikhailov,1996a |
| Oospecies | |
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G. minoris unusual because it frequently is found with embryonic skeletons of theenantiornithinebirdGobipipus.These embryos have well-developed wings, which suggest they would be able to fly very soon after hatching, unlike most modern birds.
Description
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Gobioolithuseggs are small and smooth-shelled. They are asymmetrically shaped, similar to many modern bird eggs, with one end pointier than the other. The two oospecies are distinguished mainly by their size:G. majorranges from 50 to 53.5 mm long and 25 to 32 mm across, with an eggshell thickness of 0.2-0.4 mm,[1]whereasG. minoris only 30–46 mm by 20–24 mm and 0.1-0.2 mm thick.[2]
The microstructure ofGobioolithus'eggshell has not been thoroughly studied, and heavyrecrystallizationof most specimens makes it difficult to examine the eggshell structure or pore system.[1][3]The eggshell consists of two (or possibly three)structural layers.The inner layer, called the mammillary layer, is about half the thickness of the outer, or continuous, layer. On the outside, many specimens have a recrystallized outer layer. This could simply due todiagenesisor it could be a true external zone,[1]which is a third layer present in most bird eggs but is rare in non-avian dinosaurs.[4]However, a few specimens are unaffected by recrystallization. These do not have a third layer, but this does not rule out the possibility that the eggshell originally had a three layers since the external layer can easily separate from the rest of the eggshell.[5]These specimens also reveal an angusticanaliculate pore system, which means that the pores have a low density, and are long, narrow, and straight.[3][6]
Styloolithus,another fossil enantiornithine egg from the Gobi, differs fromGobioolithusin that it is larger and has a thicker eggshell with a proportionately smaller mammillary layer.[1]Laevisoolithids,which are also eggs of enantiornithines, are also larger thanGobioolithus,but they have a much thicker mammillary layer.[7]
Embryos
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ManyGobioolithus minorspecimens contain embryonic remains of the enantiornithine genusGobipipus.[7]The embryos have well-ossifiedskeletons, implying that they were at a late stage in development when they died. Their wings and shoulders are especially well-developed. Only the modernmegapodesand thelittle ternexhibit a comparable degree of embryonic ossification in the arm and shoulder bones. It is likely thatGobipipushatchlings, like megapodes and little terns, would be able to fly very soon after hatching.[8][7]
No embryos are known fromG. majoreggs,[9]but they are usually assumed to have been laid by a similar type of bird.[7]
Nests
editGobioolithuseggs were probably laid in opennestson the banks ofephemeralrivers or lakes, which could frequently flood the nesting areas and bury the eggs.[3][7]The distribution of the eggs suggests that they had a long-term colonial nesting site at the Khermeen Tsav locality in the Barun Goyot Formation.[10][7]At the Bayn-Dzak locality, the eggs are typically arranged in clutches, whereas elsewhere they are scattered randomly, each oriented nearly vertically in the substrate.[3]The solitary eggs may have been laid and buried individually, similar to the nesting habits of modern megapodes. It is also possible that they were originally laid in clutches, but flooding separated them and deposited them vertically as the water level dropped. Water damage would also explain whyGobioolithusshells are frequently heavily recrystallized.[3][7]
Classification
editAccording to theparataxonomicsystem used to classify fossil eggs,Gobioolithusis classified in the oofamily Gobioolithidae, which, in turn, is classified in the prismatic morphotype (also called theneognathmorphotype) of the ornithoid basic type.[10]A cladistic analysis performed by Varricchio and Barta (2015) (pictured below) foundGobioolithusto be a sister taxon toStyloolithus.However, they consideredStyloolithusdifferent enough fromGobioolithusto warrant its exclusion from Gobioolithidae.[1]
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Distribution
editGobioolithusis found in the Gobi Desert inMongolia.More specifically, the fossils are found in theBarun Goyotand theDjadokhta Formationsof theNemegt Basin,which is dated to theUpper Cretaceous.[2][1]
History
editNumerous fossils, includingGobioolithusspecimens, were discovered and collected by the Polish-Mongolian fossil-hunting expeditions in theGobi Desertfrom 1963 to 1971 and by the Soviet-Mongolian expeditions between 1969 and 1996. These fossils were brought to the Institute of Paleobiology of thePolish Academy of SciencesinWarsawand to thePaleontological Institute of the Russian Academy of Sciencesin Moscow.[3][11]Eggs now classified asGobioolithuswere first analyzed in 1981 by thePolishpaleontologistAndrzej Elżanowski,who described several well-developed bird embryos within some of the fossil eggs housed in Warsaw. He identified them as most similar toGobipteryx minuta,[8]so scientists began referring to them as "Gobipteryxeggs ".[2]
In 1991, theRussianpaleontologistKonstantin Mikhailovintroduced the modern parataxonomic system used to classify fossil eggs. While he did not give the "Gobipteryx"eggs a formal name under this classification scheme, he did assign them to the prismatic morphotype in the ornithoid basic type. He believed that they were eggs ofvolantpaleognaths,but probably notGobipteryx(which was then considered to be a paleognath).[10]In the same year, the Polish paleontologistKarol Sabathreviewed the entire collection of fossil eggs discovered on the Polish expeditions, including the not-yet-namedGobioolithuseggs. Following Elżanowski, he referred them toGobipteryx,though only tentatively because at the time ongoing studies of similar eggs found on the Soviet expeditions cast doubt on this classification.[11]
In 1994, Mikhailov, working with Sabath and Kurzanov, divided the Mongolian "Gobipteryx"eggs into two informal groups: G1, containing the smaller eggs (including the ones with embryos described by Elżanowski), and G2 for the larger eggs.[3]Two years later, Mikhailov went on to classify these eggs parataxonomically as a new oofamily, Gobioolithidae, containing the single oogenusGobioolithus,with two oospecies:G. minorandG. major,corresponding to G1 and G2, respectively.[2]In 2013,Kurochkin,Chatterjee,and Mikhailov described a new genus and species of bird,Gobipipusreshetovi,based on the embryos withinGobioolithuseggs. They classifiedGobipipusas anenantiornithine.[7]In 2015, some of the larger egg specimens previously assigned toG. majorwere moved into their own new oogenus and oospecies,Styloolithussabathi.[1]
References
edit- ^abcdefgVarricchio, David J.; Barta, Daniel E. (2015)."Revistiting Sabath's" Larger Avian Eggs "from the Gobi Cretaceous".Acta Palaeontologica Polonica.60(1): 11–25.doi:10.4202/app.00085.2014.
- ^abcdMikhailov, Konstantin (1996)."Bird Eggs in the Upper Cretaceous of Mongolia".Paleontological Journal.30(1): 114–116.
- ^abcdefgMikhailov, Konstantin; Sabath, Karol; Kurzanov, Sergey (1994). "Eggs and nests from the Cretaceous of Mongolia". In Carpenter, Kenneth; Hirsch, Karl F.; Horner, John R. (eds.).Dinosaur eggs and babies.Cambridge University Press. pp. 88–115.ISBN978-0-521-44342-5.
- ^Wilson, Laura E.; Chin, Karen; Jackson, Frankie D.; Bray, Emily S."II. Eggshell morphology and structure".UCMP Online Exhibits: Fossil Eggshell.Retrieved16 April2016.
- ^Mikhailov, Konstantin (2014). "Eggshell structure, parataxonomy, and phylogenetic analysis: some notes on articles published from 2002 to 2011".Historical Biology.26(2): 144–154.Bibcode:2014HBio...26..144M.doi:10.1080/08912963.2013.829824.S2CID84878973.
- ^Carpenter, Kenneth (1999). "How to Study a Fossil Egg".Eggs, Nests, and Baby Dinosaurs: A Look at Dinosaur Reproduction (Life of the Past).Bloomington, Indiana: Indiana University Press. pp.122–144.ISBN978-0-253-33497-8.
- ^abcdefghKurochkin, Evgeny; Chatterjee, Sankar; Mikhailov, Konstantin (2013). "An embryonic enantiornithine bird and associated eggs from the Cretaceous of Mongolia".Paleontological Journal.47(11): 1252–1269.Bibcode:2013PalJ...47.1252K.doi:10.1134/s0031030113110087.S2CID86747842.
- ^abElżanowski, Andrzej (1981)."Embryonic Bird Skeletons From the Late Cretaceous of Mongolia"(PDF).Palaeontologia Polonica.42:147–179. Archived fromthe original(PDF)on 2016-04-12.Retrieved2016-04-10.
- ^Varricchio, D.J.; Jackson, F.D. (2004)."A phylogenetic assessment of prismatic dinosaur eggs from the Cretaceous Two Medicine Formation of Montana".Journal of Vertebrate Paleontology.24(4): 931–937.doi:10.1671/0272-4634(2004)024[0931:apaopd]2.0.co;2.S2CID85987939.
- ^abcMikhailov, Konstantin E. (1991)."Classification of fossil eggshells of amniotic vertebrates".Acta Palaeontologica Polonica.36(2): 193–238.
- ^abSabath, Karol (1991)."Upper Cretaceous amniotic eggs from the Gobi Desert"(PDF).Acta Palaeontologica Polonica.36(2): 151–192.