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Elongatoolithus

Elongatoolithusis anoogenusofdinosaur eggsfound in the Late Cretaceous formations of China and Mongolia. Like otherelongatoolithids,they were laid by small theropods (probablyoviraptorosaurs), and were cared for and incubated by their parents until hatching. They are often found in nests arranged in multiple layers of concentric rings. As its name suggests,Elongatoolithuswas a highly elongated form of egg. It is historically significant for being among the first fossil eggs given aparataxonomicname.

Elongatoolithus
Temporal range:Late Cretaceous
Elongatoolithus excellensnest inCosmoCaixa Barcelona.
Egg fossil classificationEdit this classification
Basic shell type: Ornithoid
Morphotype: Ornithoid-ratite
Oofamily: †Elongatoolithidae
Oogenus: †Elongatoolithus
Zhao, 1975
Type oospecies
†Elongatoolithus andrewsi
Zhao, 1975
Oospecies

See text

Description

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Elongatoolithuseggs are small to midsized eggs, with a maximum size of about 17 cm (6.7 in). The eggs are highly elongated, typically being 2 to 2.2 times longer than they are wide, and slightly asymmetric (with one end pointier than the other). The outer surface of their shells is ornamented with a fine pattern of nodes and ridges, either aligned parallel to the egg's long axis (lineartuberculate ornamentation) or in irregular chains (ramotuberculare ornamentation). At the ends of the egg, the ornamentation may become randomly dispersed nodes (dispersituberculate ornamentation).[1][2]: 8–10 This ornamentation is less pronounced than that of the closely relatedMacroolithus.[3]

In general, the shell of elongatoolithids is made up of two structural layers: the inner layer, called the mammillary (or cone) layer, is made up of cone-shaped structures called mammillae, and the outer layer, called the continuous layer, is made up of shell units fused together into a continuum with a scale-like, or squamatic, structure.[2]Elongatoolithushas an abrupt boundary between these two layers, and the continuous layer is two to six times thicker than the mammillary layer. Wavy growth lines can be observed in the continuous layer parallel to the egg's surface.[4]They have an angusticanalicute pore system, meaning that the eggshells have narrow, straight pores.[1]

Oospecies

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Pair ofE. elongatuseggs,Paleozoological Museum of China

Several oospecies ofElongatoolithusare known. They can be broadly divided into two classes based on ornamentation: most oospecies have linear ridges parallel to the long axis of the egg, but some (notablyE. sigillariusandE. excellens) a rippled pattern of reoriented ridges transverse to the egg's long axis.[1]Within these two groups, oospecies vary primarily by eggshell thickness, ratio of continuous to mammillary layer, and overall size of the eggs.[2]Identification of oospecies based on fragments (not whole eggs) can be problematic.[1]

  • E. andrewsi(type) - this oospecies was named afterProtoceratops andrewsi,in reference to the now-outdated hypothesis that it was laid by a protoceratopsid dinosaur. Their eggshells range from 1.10 to 1.49 mm (0.043 to 0.059 in) thick, of which one fourth is the mammillary layer.[5]They range in size from 138 to 151 mm (5.4 to 5.9 in) long by 63 to 77 mm (2.5 to 3.0 in) wide.[6][4]
  • E. excellens,meaning "graceful elongated egg", measures only 9–11 cm (3.5–4.3 in) long by 4 cm (1.6 in) across, making it the smallestElongatoolithusoospecies, and possibly the smallest elongatoolithid.[7]It has a correspondingly thin eggshell, measuring 0.3–0.9 mm (0.012–0.035 in) thick, predominantly ranging between 0.4 and 0.7 mm (0.016 and 0.028 in). It has a distinctive, rippling pattern of ridges around the equator of the egg (similar toE. sigillarius).[3]
  • E. frustrabilis,meaning "deceptive elongated egg", is similar toE. elongatusandE. andrewsi,but is larger, measuring 150–170 mm (5.9–6.7 in) long by 60–70 mm (2.4–2.8 in) wide.[3]The shell thickness ranges from 0.8 to 1.5 mm (0.031 to 0.059 in), being thickest at the poles of the egg.[3]It is the most common type ofElongatoolithusin theDjadokhta FormationofMongolia.[8]
  • E. elongatusis an oospecies widespread in China.[9]Its shell averages 0.91 mm (0.036 in) in thickness, of which one sixth is the mammillary layer.[5]The complete eggs are 14 to 15 cm (5.5 to 5.9 in) long and 6.1 to 6.7 cm (2.4 to 2.6 in) wide.[4]
  • E. magnusis an oospecies native toHunan.As the name (which means "large elongated egg" ) suggests,E. magnuseggs are large, measuring 162–172 mm (6.4–6.8 in) long by 63–82 mm (2.5–3.2 in) across. The shell is 0.68–0.90 mm (0.027–0.035 in) thick, about 1/6 of which is the mammillary layer. The boundary between the two layers is indistinct.[10]
  • E. sigillariusis known from a single, poorly preserved clutch from Mongolia (Nemegt Formation). Its name means "ornamented" in Latin, referring to the distinctive rippling or speckling of its eggshell. The eggshell thickness is 0.4–0.8 mm (0.016–0.031 in) in the equatorial region, but can range from 0.3 to 1.1 mm (0.012 to 0.043 in) over the whole egg. The complete eggs are 15–17 cm (5.9–6.7 in) long and 6–7 cm (2.4–2.8 in) wide.[3]
  • E. subtitectorius,meaning "thin-ribbed elongated egg", is a Mongolian oospecies with very well-defined striations on its eggshell. Its shell is relatively thin, only about 0.7–0.8 mm (0.028–0.031 in) thick (excluding ornamentation). The oospecies has only been discovered in fragments, so the size of the whole egg is unknown.[4][3]
  • E. taipinghuensisis named forTaipinghu,a lake inAnhui,Chinanear to where it was discovered. It measures 17 cm (6.7 in) long by 6–7 cm (2.4–2.8 in) across.[11]

Paleobiology and parenting

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Parenting

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Fossil ofCitipatiparent sitting on its nest

Elongatoolithuswere most likely laid byoviraptorosaurs.While most eggs are preserved without any trace of embryo or parent, several elongatoolithid nests have been found in association with skeletons of adult oviraptorosaurs. The parents apparently brooded on the nest to incubate the eggs.[2]An adultCitipatiskeleton was discovered in the Upper Cretaceous of Mongolia associated with a nest of eggs most likely referable toE. frustrabilis.[8]Like many types of modern birds,Elongatoolithusnests would have been cared for by the father of a communal nest, which would contain eggs of several females.[12]

Nests

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Several nests ofElongatoolithuseggs are known.[4][5][10]They follow the typical elongatoolithid pattern of arranging eggs in multi-layered circular nests.[2]The type specimen ofE. andrewsiconsists of a well preserved nest, containing eleven eggs arranged in at least two circular layers.[6][5]The type specimen ofE. elongatusis a similarly structured, well preserved nest of 13 eggs. These eggs are broken, apparently because they had hatched before fossilizing.[13]Like other elongatoolithids, the eggs are found paired because the parents would lay two eggs at a time. Unlike modern birds, dinosaurs had two functioning oviducts, so they could produce two eggs simultaneously, as shown by a fossil oviraptorosaur discovered in China with a pair of elongatoolithid eggs preserved in its pelvis.[14]

Most, if not all, elongatoolithid nests would not have been buried by their parents.[15]However, according to a statistical analysis of eggshell pore density by Tanakaet al.(2015), it is possible thatE. elongatuseggs were buried.[15]

Classification

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See also:Egg fossil § Classification

Fossilized eggs are classified in their own, parataxonomic system parallel toLinnaean taxonomy.Elongatoolithusand the closely related oogeneraMacroolithusandNanhsiungoolithuswere the first oogenera described in this system. These oogenera are all classified in the oofamilyElongatoolithidae.[5]

Elongatoolithuscontains eight oospecies. The type oospecies was designated by Zhao 1975 asE. andrewsi,[5]but some later publications listE. elongatusas the type.[3][1]Five oospecies originally described asElongatoolithusin fact belong toPrismatoolithidae:"E." chichengshanensis,"E." chimeiensis,"E." jiangchangensis,and"E." laijiaensis,and"E." tiantaiensis(which has been recombined asPrismatoolithustiantaiensis).[7][16]

History

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In 1954, Chinese paleontologistYang Zhongjiandescribed several fossil eggs from Laiyang, including an elongated type which he named"Oolithes"elongatus.He noted their similarity to Mongolian fossil eggs supposedly laid byProtoceratops,and therefore conjectured"O" elongatuswas laid by a similar type of dinosaur.[13]Later, in 1965, Yang discovered a comparable type of egg inNanxiong.[6]In 1975, the Chinese paleontologist Zhao Zikui published the prototype of the modern parataxonomic system for eggshell classification, dividing fossil eggs into oospecies, oogenera, and oofamilies. He named the oogenusElongatoolithusto contain the majority Yang's"O." elongatusspecimens, splitting it into two separate oospecies:E. andrewsi,based on a well-preserved nest described by Yang (1965), andE. elongatusfor most of the rest of the"O." elongatusspecimens. Two partially preserved"O." elongatusnests were reclassified into their own oogenus,Nanhsiungoolithus.He also noted that several types of fossil eggs from Mongolia and North America were also probably attributable toElongatoolithus.[5]

In 1979 another oospecies,E. magnus,was discovered and named by the Chinese paleontologists Zeng Demin and Zhang Jinjian, based on a nest of nine eggs uncovered inHunan.They also described otherElongatoolithusspecimens of an indeterminate oospecies.[10]

In 1991, the Russian paleontologist Konstantin Mikhailov created the modern, formal classification scheme for fossil eggs. He used Zhao's naming conventions, keepingElongatoolithus,Macroolithus,andNanhsiungoolithusin Elongatoolithidae. He mentioned that several Mongolian eggs were referable to Elongatoolithidae orElongatoolithus.He also recognized elongatoolithids to be theropod eggs, demonstrating their numerous structural differences from the supposedProtoceratopseggs.[17]In 1994, when Mikhailov described and reviewed the fossil elongated eggs from Mongolia, he named four new oospecies ofElongatoolithus(E. excellens,E. frustrabilis,E. subtitectorius,andE. sigillarius) as well as mentioning two other unnamed forms.[3]Also in 1994, the theropod parentage ofElongatoolithuswas confirmed when an oviraptorid embryo was discovered preserved inside an elongatoolithid egg,[18]which Mikhailov (1997) tentatively referred toElongatoolithus.[1]

The Chinese paleontologist Yu Xinqi namedE. taipinghuensis,based on newly discovered remains from Anhui, in 1998.[11]

In 2000, the Chinese paleontologists Fang Xiaosi, Wang Yaozhong, and Jiang Yan'gen described several ootaxa from the egg-rich Tiantai Basin, including the oospeciesE. tiantaiensis.[19]In 2003, Fang and his colleagues also describedE. chichengshanensisandE. laijiaensis.[20]However, Wanget al.2010 questioned the referral of these oospecies toElongatoolithus.They argued thatE. chichengshanensisshould belong to a new group,E. laijiaensisrequired further study to determine its affinities, andE. tiantaiensisshould be moved to Prismatoolithidae.[21]In 2011, Wanget al.formally recombinedE. tiantaiensisasPrismatoolithus tiantaiensis.[16]In 2016, Tanakaet al.also movedE. chichengensisandE. laijiaensis,as well asE. chimeiensisandE. jianchangensis(which had been named by Fanget al.in 2007[22]) to Prismatoolithidae.[7]

References

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  1. ^abcdefMikhailov, Konstantin (1997)."Fossil and recent eggshell in amniotic vertebrates: fine structure, comparative morphology and classification".Special Papers in Palaeontology.56.London: The Palaeontological Association: 5–80.
  2. ^abcdeSimon, D. J. (2014). "Giant Dinosaur (theropod) Eggs of the Oogenus Macroelongatoolithus (Elongatoolithidae) from Southeastern Idaho: Taxonomic, Paleobiogeographic, and Reproductive Implications."(Doctoral dissertation, Montana State University, Bozeman).
  3. ^abcdefghMikhailov, Konstantin K.E. (1994). "Theropod and protoceratopsian dinosaur eggs from the Cretaceous of Mongolia and Kazakhstan".Paleontological Journal.28(2): 101–120.
  4. ^abcdeCarpenter, Kenneth (1999).Eggs, Nests, and Baby Dinosaurs: A Look at Dinosaur Reproduction (Life of the Past),Indiana University Press;ISBN0-253-33497-7.
  5. ^abcdefgZhao, Zikui (1975)."The microstructure of the dinosaurian eggshells of Nanxiong, Guangdong Province—on the classification of dinosaur eggs"(PDF).Vertebrata PalAsiatica.13(2): 105–117.
  6. ^abcYang, Zhongjian (1965)."Fossil eggs from Nanhsiung, Kwangtun and Kanchou, Kiangsi"(PDF).Vertebrata PalAsiatica.9(2): 141–170.
  7. ^abcTanaka, Kohei; Zelenitsky, Darla; Saegusa, Haruo; Ikeda, Tadahiro; DeBuhr, Christopher; Therrien, François (2016). "Dinosaur eggshell assemblage from Japan reveals unknown diversity of small theropods".Cretaceous Research.57:350–363.Bibcode:2016CrRes..57..350T.doi:10.1016/j.cretres.2015.06.002.
  8. ^abMikhailov, Konstantin E. (2013)."Eggshell structure, parataxonomy and phylogenetic analysis: some notes on articles published from 2002 to 2011".Historical Biology.26(2): 144–154.doi:10.1080/08912963.2013.829824.S2CID84878973.
  9. ^Zhao, Zikui (1979)."Progress in the Research of Dinosaur Eggs"(PDF).Mesozoic and Cenozoic Red Beds of South China.Selected Papers from the "Cretaceous-Tertiary Workshop," Nanxiong, Guangdong Province. Institute of Vertebrate Paleontology, Paleoanthropology & Nanjing Institute of Paleontology Science Press: 330–340.
  10. ^abcZeng, Demin; Zhang, Jinjian (1979)."On the dinosaurian eggs from the western Dongting Basin, Hunan"(PDF).Vertebrata PalAsiatica.17(2): 131–136.
  11. ^abYu, Xinqi (1998). "Characteristics of Dinosaur Fossil from Southern Anhui and their significance for stratigraphic division".Regional Geology of China(in Chinese).17(3): 278–284.
  12. ^Varricchio, David; Moore, Jason; Erickson, Gregory; Norell, Mark; Jackson, Frankie; Borkowski, John (2008)."Avian Paternal Care Had Dinosaur Origin".Science.322(5909): 1826–1828.Bibcode:2008Sci...322.1826V.doi:10.1126/science.1163245.PMID19095938.S2CID8718747.
  13. ^abYang, Zhongjian (1954)."Fossil Reptilian Eggs from Laiyang, Shantung, China".Acta Scientia Sinica.3:505–522.doi:10.1360/ya1954-3-4-505(inactive 1 November 2024).{{cite journal}}:CS1 maint: DOI inactive as of November 2024 (link)
  14. ^Sato, Tamaki; Cheng, Yen-nien; Wu, Xiao-chun; Zelenitsky, Darla; Hsiao, Yu-fu (2005)."A Pair of Shelled Eggs Inside A Female Dinosaur"(PDF).Science.308(5720): 375.doi:10.1126/science.1110578.PMID15831749.S2CID19470371.
  15. ^abTanaka, Kohei; Zelenitsky, Darla K.; Therrien, François (2015)."Eggshell Porosity Provides Insight on Evolution of Nesting in Dinosaurs".PLOS ONE.10(11): e0142829.Bibcode:2015PLoSO..1042829T.doi:10.1371/journal.pone.0142829.PMC4659668.PMID26605799.
  16. ^abWang, Qiang; Zhao, Zikui; Wang, Xiaolin; Jiang, Yan'gen (2011)."New ootypes of dinosaur eggs from the Late Cretaceous in Tiantai Basin, Zhejiang Province, China"(PDF).Vertebrata PalAsiatica.49(4): 446–449.
  17. ^Mikhailov, Konstantin (1991)."Classification of fossil eggshells of amniotic vertebrates"(PDF).Acta Palaeontologica Polonica.36(2): 193–238.
  18. ^Norell, Mark A.; Clark, James M.; Demberelyin, Dashzeveg; Rhinchen, Barsbold; Chiappe, Luis M.; Davidson, Amy R.; McKenna, Malcolm C.; Altangerel, Perle; Novacek, Michael J. (1994). "A Theropod Dinosaur Embryo and the Affinities of the Flaming Cliffs Dinosaur Eggs".Science.266(5186): 779–782.Bibcode:1994Sci...266..779N.doi:10.1126/science.266.5186.779.JSTOR2885545.PMID17730398.S2CID22333224.
  19. ^Fang, Xiaosi; Wang, Yaozhong; Jiang, Yang'en (2000)."On the Late Cretaceous Fossil Eggs of Tiantai, Zhejiang".Geological Review.46(1): 105–112.
  20. ^Fang, Xiaosi; Lu, Liwu; Jiang, Yan'gen; Yang, Liangfeng (2003). "Cretaceous fossil eggs from the Tiantai basin of Zhejiang, with a discussion on the extinction of dinosaurs".Geological Bulletin of China.22(7): 512–520.
  21. ^Wang, Qiang; Wang, Xiaolin; Zhao, Zikui; Jiang, Yan'gen (2010)."A new oogenus of Elongatoolithidae from the Upper Cretaceous Chichengshan Formation of Tiantai Basin, Zhejiang Province"(PDF).Vertebrata PalAsiatica.48(2): 111–118.
  22. ^Fang, Xiaosi; Zhang, Zhijun; Pang, Qiqing; Li, Peixian; Han, Yingjian; Xie, Honliang; Yan, Ronghao; Pang, Fengjiu; Lu, J; Cheng, Zhengwu (2007). "Cretaceous strata and egg fossils in Xixia, Henan Province".Acta Geoscientia Sinica.28(2): 123–142.
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