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Continuoolithus

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Continuoolithusis anoogenus(fossil egg genus) ofdinosaur eggfound in the lateCretaceousofNorth America.It is most commonly known from the lateCampanianofAlbertaandMontana,but specimens have also been found dating to the olderSantonianand the youngerMaastrichtian.It was laid by an unknown type oftheropod.These small eggs (measuring 77–123 mm (3.0–4.8 in) long) are similar to the eggs ofoviraptoriddinosaurs (oofamilyElongatoolithidae), but have a distinctive type of ornamentation.

Continuoolithus
Temporal range:LateSantonian-Maastrichtian
Egg fossil classificationEdit this classification
Basic shell type: Ornithoid
Morphotype: Ornithoid-ratite
Oogenus: †Continuoolithus
Zelenitsky, Hills & Currie, 1996
Oospecies
  • †C. canadensisZelenitsky, Hills & Currie, 1996
Synonyms

Spongioolithus hirschiBray 1999

Continuoolithusnests would have been incubated under vegetation and sediment, unlike nests ofTroodonand oviraptorids, which were incubated by brooding adults. Adaptations in the eggshell, such as high porosity and prominent ornamentation, would have helped the embryo breathe while buried. One fossil egg contains a tiny embryonic skeleton at an exceptionally young stage of development (perhaps eight to ten days old) showing the earliest stages of bone development.

Description

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Complete eggs range from 95 by 60 mm (3.7 by 2.4 in) to 123 by 77 mm (4.8 by 3.0 in) in size. They are elongated and ovoid shaped (i.e., with one blunt end and one pointed end).[1]Known nesting traces contain from three to six eggs arranged parallel to each-other in linear rows.[1][2]The outer surface of the egg is ornamented with coarse ornamentation, accounting for one fifth the total thickness of the shell. Unlike elongatoolithids,Continuoolithus'sornamentation pattern consists of randomly dispersed nodes (dispersituberculate ornamentation).[3]The pores follow the angusticanaliculate type (i.e. narrow and straight pores).[3][2]Continuoolithushad a remarkably high porosity and therefore a high rate of gas exchange, which is associated with incubation of eggs in covered nests.[2][4]

Continuoolithus canadensis'seggshell was 0.94–1.28 mm (0.037–0.050 in) thick.[3][2]Other specimens differ in shell thickness: some fragments referred toC.sp. have a slightly thinner shell,[5]C.cf.canadensisfragments from Willow Creek have a thicker shell,[6]andC.cf.canadensisfrom Milk River are thinner.[7]Similar to most theropod eggs, its shell consists of two layers of calcite crystals.[8]The inner layer, called the mammillary layer, is made of tightly packed cones called mammillae. Overlying this layer is the continuous layer, which is four to eight times thicker than the mammillary layer. Inelongatoolithidsand inContinuoolithus,this layer is distinctive because it is not subdivided into well-defined crystal units (hence the name continuous layer).[3]While some division into prisms can be observed near the outer surface of the shell, this is mostly obscured by scale-like squamatic ultrastructure.[2]

Two specimens ofContinuoolithuspreserve theshell membrane,a layer of fibrous proteins found in extantarchosaur(birdandcrocodylian) eggs beneath the hard crystalline shell. The original protein is not preserved, but the specimens do show networks of tubular fibers anchoring the mammillae.[9]

Paleobiology and parenting

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Continuoolithuswas most likely laid by a non-avian theropod dinosaur. Its microstructure is very similar to that of theropods; it differs from avian eggs in its relative size, its lack of a third eggshell layer, and its prominent ornamentation.[2][9]Like many other types of non-avian theropod eggs,Continuoolithuseggs are typically found paired;[10]this is because the parent dinosaurs had two functionaloviducts,each of which would produce an egg simultaneously.[11]

Comparing the Maastrictian-aged specimens to the older Campanian specimens ofContinuoolithusand other types of theropod eggshells shows a trend of increasing eggshell thickness, which may be correlated with some theropod taxa increasing in body size in the late Cretaceous.[7][12]

Nesting and incubation

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The known nests ofContinuoolithuseggs consist of three to six eggs arranged parallel to each other in a linear row.[1][2]Multiple lines of evidence show that mother of the eggs would, after excavating the nest and laying a clutch of eggs, bury them in a thin layer of mud and vegetation. One nest is preserved with a carbonaceous covering, representing sediment or vegetation that covered the nest.[2][10]Also, the eggs have a remarkably high rate of gas conductance,[2][10]which correlates strongly with burial of nests because eggs covered in sediment cannot as readily exchange air and water with their environment as those left exposed.[4]Also, the prominent nodes on the surface of the eggshell may have functioned to prevent debris from clogging the pores when the egg was buried.[10]Thus, unlikeTroodoneggs and elongatoolithids (the eggs of oviraptorosaurs),[13]Continuoolithuswould have been incubated in substrate rather than by a brooding adult.[2]The heat from the decaying vegetation may have aided the incubation.[10]

Embryo

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OneContinuoolithusegg contains embryonic remains representing a relatively early stage of development so that the skeleton was almost entirelycartilaginous,which has been largely replaced in the fossil by an amorphous calcite mass. Two long skeletal elements are recognizable, however. Both of them appear to be in the very earliest stages of bone formation (ossification). The shorter of the two (measuring 9 mm (0.35 in) long) is thought to be a femur because of its shape. The longer element (15.5 mm (0.61 in) long) is not developed enough to identify, but may be a tibia. The taxonomic identity of the embryo is impossible to determine, but based on comparisons toTroodon,Orodromeus,andMaiasaura,it is estimated to have been 60–70 mm (2.4–2.8 in) long. It clearly represents a very early stage of development (in fact, it is the youngest vertebrate skeleton ever discovered), both because of the lack of ossification and because of its tiny size relative to the egg; based on comparisons to the developmental patterns of modern birds, Horner (1997) estimated it may have died eight to ten days after fertilization.[14]

Classification

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Only one oospecies ofContinuoolithushas been named:C. canadensis.The microstructure of its eggshell closely resembles that of elongatoolithids, so it was classified in Elongatoolithidae by Wanget al.(2010).[15]However, most authors do not include it in Elongatoolithidae, considering it to be ofuncertain placementbecause it has different ornamentation[3]and also shows similarity toPrismatoolithidae.[2]Carpenter (1999) suggested thatContinuoolithusis different enough to warrant its own oofamily.[16]It belongs to the ornithoid-ratite morphotype, a grouping which primarily includespaleognathousbirds and non-avian theropods.[3]

Continuoolithus canadensishas one junior synonym,Spongioolithus hirschi,which was originally classified as a distinct oogenus and oospecies of Elongatoolithidae.[17][12]

History

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Eggs have been known from the Two Medicine Formation in Montana since 1979. In 1990,Continuoolithusspecimens, found at the Egg Mountain locality, were first described in detail by paleontologistsKarl HirschandBetty Quinn,but they did not give them a parataxonomic name. At that time, prominent American paleontologistJack Hornerbelieved them to be eggs ofTroodonbased on the appearance of the embryonic remains.[1]However, after further analysis of the embryo, Horner concluded that it could not be taxonomically identified.[14]The eggs were conclusively shown not to beTroodonwhen the structurally quite distinctPrismatoolithus(previously thought to be eggs ofOrodromeus) were shown to beTroodonby more thorough study of the preserved embryo.[18][19]

In 1996, Canadian paleontologistsDarla Zelenitsky,L.V. Hills and Phillip Currie namedContinuoolithusbased on newly discovered remains in Alberta.[3]They noted similarity between the new specimens and the?Troodoneggs of Two Medicine, but the Two Medicine eggs would not be formally assigned toContinuoolithusuntil Zelenitsky and Sloboda (2005), at which time they also reported the first occurrence ofContinuoolithusin theDinosaur Park Formation.[3][9]

A nesting trace ofContinuoolithuswas excavated in 1994 at the Flaming Cliffs locality in Two Medicine. It was not prepared and described until 2012, when Rebecca Joy Schaff analyzed this nest and otherContinuoolithusspecimens extensively in her masters thesis atMontana State University.[10]In 2015, she, and her advisor Frankie Jackson, along with David Varricchio and James Schmitt published these results in the journalPALAIOS.[2]

In 2008, Ed Welsh and Julia Sankey published the first report of fossil dinosaur eggs from Texas, discovered in theAguja Formation.They described several eggshell fragments, including some that were comparable toContinuoolithus,but perhaps more similar to the elongatoolithidMacroelongatoolithusin their ornamentation.[20]In 2011, Kohei Tanakaet al.described numerous eggshell fragments from theFruitland FormationinNew Mexico,including a few fragments referable toContinuoolithussp.[5]

In 2017, a team of Canadian paleontologists led by Darla Zelenitsky reported the discovery of a pair ofContinuoolithusshell fragments at theWillow Creek Formationin Alberta, representing the first fossils of the oogenus found in theMaastrichtian.[6]The same year, Zelenitskyet al.also discovered the firstContinuoolithusspecimens in theSantonian,found at theMilk River Formation,also in Alberta.[7]

The oogenus and oospeciesSpongioolithus hirschiwas first named in 1999 by Emily Bray, based on numerous eggshell fragments discovered at theNorth Horn Formation.She classified it as a distinct type within Elongatoolithus.[17]However, this oospecies is indistinguishable fromC. canadensis,so in 2018, Jared Voris, Zelenitsky, Therrien, and Tanaka synonymized the oospecies.[12]

Distribution and paleoecology

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Continuoolithus canadensisis known from the Flaming Cliffs and the Egg Mountain localities (and possibly Sevenmile Hill too) of the Two Medicine Formation inMontana,from Devil's Coulee in theOldman FormationinAlberta,and from theDinosaur Park Formationin Alberta, all of which date to the Late Cretaceous (Campanian).[3][2][21][9][22]

The Two Medicine Formation represents the coastal plains along the western side of theWestern Interior Seaway.The Flaming Cliff locality formed in a well-drainedoverbankof analluvialfloodplain.[2]The Egg Mountain locality also represents a floodplain overbank.[23]The formation has a diverse assemblage of dinosaurs including theropods such asTroodon,Albertosaurus,ornithomimidsanddromaeosaurs,as well as several types ofhadrosaurs,ceratopsians,ankylosaurs,and smallerornithischianssuch asOrodromeus.It also includespterodactyloidpterosaurs,Champsosaurus,turtles,lizards,andmammals.[24]Other types of eggs from Two Medicine includeMontanoolithus,[25]Prismatoolithus levis(the eggs ofTroodon formosus), some small unidentified theropod eggs,[26]P. hirschi,Triprismatoolithus,Tubercuoolithus,Spheroolithusalbertensis(eggs ofMaiasaura),S. choteauensis,eggs ofHypacrosaurus,andKrokolithes.[21]

The Oldman Formation was formed by ephemeral rivers in a semi-arid environment characterized by seasonal precipitation.[22]Like the Two Medicine Formation, the Oldman Formation is also known for its diversity of fossil eggs; in addition toContinuoolithus,eggs ofHypacrosaurus,Spheroolithus,[27]Prismatoolithus,[28]Porituberoolithus,Tristaguloolithus,andDispersituberoolithusare also known.[3]Dinosaurian skeletal remains includeTroodon,tyrannosaurids,ankylosaurids,hadrosaurids,ceratopsids,andornithomimids.[29]Footprintsof hadrosaurs are also known.[22]The formation was also populated bymultituberculatemammals,[30]numerous types of turtles,Champsosaurus,sturgeons,and pterosaurs (including the giantQuetzalcoatlus).[29]

Artist's restoration of some megafaunal dinosaurs of the Dinosaur Park Formation.

The Dinosaur Park Formation is time-equivalent to the Oldman Formation, and both formations are part of theBelly River Group.It represents the deposits of a perennial, sinuous river system andparalicenvironments.[22]It is widely known for its incredible diversity of dinosaurian fauna, representing over 50 valid taxa including theropods such asdromaeosaurs,caenagnathids,troodontids,ornithomimids,andtyrannosaurids,as well as ornithischians such aspachycephalosaurs,hadrosaurs,ceratopsians,ankylosaurs,andthescelosaurs.[31]Other types of fossil eggs from the formation includeReticuoolithus,Porituberoolithus,Prismatoolithus,Spheroolithus,andKrokolithes.[9]

OtherContinuoolithusspecimens, not classified into an oospecies, are known from the late Campanian of the Fruitland Formation (representing a well-drained river delta plain) in New Mexico, along withPorituberoolithus,Prismatoolithus,indeterminate theropod eggshells,Testudoolithus,and krokolithids.[20]Also, fragments ofC.cf.canadensisare known from the lateMaastrichtianWillow Creek Formationin Alberta. This formation has relatively low dinosaurian diversity; eggs from the formation predominately belong to the ornithopod oogenusSpheroolithus,but some types of theropod eggs (Continuoolithus,Montanoolithus,Porituberoolithus,andPrismatoolithus) are known.[6]C.cf.canadensisfragments were also found in the lateSantonianMilk River Formation, wlong withPorituberoolithus,Prismatoolithus,Spheroolithus,andTriprismatoolithus.[7]Maastrictian-agedContinuoolithusspecimens have also been discovered in theNorth Horn FormationinUtah,[12]a formation rich in dinosaur eggs, includingSpheruprismatoolithus,Prismatoolithus,Ovaloolithus,andSpheroolithus.[17]

See also

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References

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  1. ^abcdHirsch, Karl F.; Quinn, Betty (1990). "Eggs and Eggshell Fragments from the Upper Cretaceous Two Medicine Formation of Montana".Journal of Vertebrate Paleontology.10(4): 491–511.Bibcode:1990JVPal..10..491H.doi:10.1080/02724634.1990.10011832.
  2. ^abcdefghijklmnJackson, Frankie D.; Schaff, Rebecca J.; Varricchio, David J.; Schmitt, James G. (2015)."A theropod nesting trace with eggs from the upper cretaceous (Campanian) Two Medicine Formation of Montana".PALAIOS.30(5): 362–372.Bibcode:2015Palai..30..362J.doi:10.2110/palo.2014.052.S2CID131229105.
  3. ^abcdefghijZelenitsky, Darla K.; Hills, L.V.; Currie, Philip J. (1996). "Parataxonomic classification of ornithoid eggshell fragments from the Oldman Formation (Judith River Group; Upper Cretaceous), southern Alberta".Canadian Journal of Earth Sciences.33(12): 1655–1667.Bibcode:1996CaJES..33.1655Z.doi:10.1139/e96-126.
  4. ^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.
  5. ^abTanaka, Kohei; Zelenitsky, Darla K.; Williamson, Thomas; Weil, Anne; Therrien, Francois (2011). "Fossil eggshells from the Upper Cretaceous (Campanian) Fruitland Formation, New Mexico".Historical Biology.23(1): 41–55.Bibcode:2011HBio...23...41T.doi:10.1080/08912963.2010.499171.S2CID85213812.
  6. ^abcZelenitsky, Darla K.; Therrien, François; Tanaka, Kohei; Currie, Phillip J.; DeBuhr, Christopher L. (2017). "Latest Cretaceous eggshell assemblage from the Willow Creek Formation (upper Maastrichtian – lower Paleocene) of Alberta, Canada, reveals higher dinosaur diversity than represented by skeletal remains".Canadian Journal of Earth Sciences.54(2): 134–140.Bibcode:2017CaJES..54..134Z.doi:10.1139/cjes-2016-0080.hdl:1807/75326.
  7. ^abcdZelenitsky, Darla K.; Therrien, François; Tanaka, Kohei; Kobatashi, Yoshitsugu; DebBuhr, Christopher L. (2017). "Dinosaur eggshells from the Santonian Milk River Formation of Alberta, Canada".Cretaceous Research.74:181–187.Bibcode:2017CrRes..74..181Z.doi:10.1016/j.cretres.2017.02.016.
  8. ^Laura E. Wilson, Karen Chin, Frankie D. Jackson, and Emily S. Bray.II. Eggshell morphology and structure.UCMP Online Exhibits: Fossil Eggshell
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  10. ^abcdefSchaff, Rebecca J. (2012).Incubation ofContinuoolithus canadensiseggs from the late Cretaceous Two Medicine Formation of Montana(PDF)(M.Sc.). Montana State University.
  11. ^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.
  12. ^abcdVoris, Jared T.; Zelenitsky, Darla K.; Therrien, Francois; Tanaka, Kohei (2018). "Dinosaur eggshells from the lower Maastrichtian St. Mary River Formation of southern Alberta, Canada".Canadian Journal of Earth Sciences.55(3): 272–282.Bibcode:2018CaJES..55..272V.doi:10.1139/cjes-2017-0195.hdl:1807/81388.[permanent dead link‍]
  13. ^Varricchio, David J.; Moore, Jason R.; Erickson, Gregory M.; Norell, Mark A.; Jackson, Frankie D.; Borkowski, John J. (2008)."Avian Paternal Care Had Dinosaur Origin"(PDF).Science.322(5909): 1826–1828.Bibcode:2008Sci...322.1826V.doi:10.1126/science.1163245.PMID19095938.
  14. ^abHorner, John R. (1997). "Rare Preservation of an Incompletely Ossified Fossil Embryo".Journal of Vertebrate Paleontology.17(2): 431–434.Bibcode:1997JVPal..17..431H.doi:10.1080/02724634.1997.10010987.
  15. ^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.
  16. ^Carpenter, Kenneth (1999). "Appendix II. Dinosaur Eggs Types".Eggs, Nests, and Baby Dinosaurs.Bloomington and Indianapolis: Indiana University Press. pp.299–300.ISBN978-0-253-33497-8.The eggs are so different from any other ornithoid egg that they belong to their own family.
  17. ^abcBray, E. S. (1999). "Eggs and eggshell from the Upper Cretaceous North Horn Formation, central Utah".Vertebrate Paleontology in Utah, Utah Geological Survey Miscellaneous Publication.99(1): 361–375.
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  19. ^Varricchio, David J.; Horner, John R.; Jackson, Frankie D. (2002). "Embryos and Eggs for the Cretaceous Theropod Dinosaur Troodon formosus".Journal of Vertebrate Paleontology.22(3): 564–576.doi:10.1671/0272-4634(2002)022[0564:eaeftc]2.0.co;2.S2CID85728452.
  20. ^abWelsh, Ed; Sankey, Julia T. (2008). "Chapter 11. First dinosaur eggshells from Texas: Aguja Formation (late Campanian), Big Bend National Park". In Sankey, J.T.; Baszio, S. (eds.).Vertebrate Microfossil Assemblages Their Role in Paleoecology and Paleobiogeography.Bloomington: Indiana University Press. pp. 166–177.
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  24. ^Rogers, Raymond R. (1990)."Taphonomy of three dinosaur bone beds in the Upper Cretaceous Two Medicine Formation of northwestern Montana: evidence for drought-related mortality".PALAIOS.5(5): 394–413.Bibcode:1990Palai...5..394R.doi:10.2307/3514834.JSTOR3514834.
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  27. ^Zelenitsky, Darla K.; Hills, L.V. (1997). "Normal and pathological eggshells ofSpheroolithus albertensis,oosp. nov. from the Oldman Formation (Judith River Group, Late Campanian), Southern Alberta ".Journal of Vertebrate Paleontology.17(1): 167–171.Bibcode:1997JVPal..17..167Z.doi:10.1080/02724634.1997.10010960.
  28. ^Zelenitsky, Darla K.; Hills, L.V. (1996). "An egg clutch of Prismatoolithus levis oosp. nov. from the Oldman Formation (Upper Cretaceous), Devil's Coulee, southern Alberta".Canadian Journal of Earth Sciences.33(8): 1127–1131.Bibcode:1996CaJES..33.1127Z.doi:10.1139/e96-085.
  29. ^abCurrie, Philip J.; Russell, Dale A. (1982). "A giant pterosaur (Reptilia: Archosauria) from the Judith River (Oldman) Formation of Alberta".Canadian Journal of Earth Sciences.19(4): 894.Bibcode:1982CaJES..19..894C.doi:10.1139/e82-074.
  30. ^Fox, Richard C. (1980). "Mammals from the Upper Cretaceous Oldman Formation, Alberta. IV. Meniscoessus Cope (Multituberculata)".Canadian Journal of Earth Sciences.17(11): 1480–1488.Bibcode:1980CaJES..17.1480F.doi:10.1139/e80-155.
  31. ^Brown, Caleb Marshall; Evans, David C.; Champione, Nicolas E.; O'Brien, Lorna J.; Eberth, David A. (2013). "Evidence for taphonomic size bias in the Dinosaur Park Formation (Campanian, Alberta), a model Mesozoic terrestrial alluvial-paralic system".Palaeogeography, Palaeoclimatology, Palaeoecology.372:108–122.Bibcode:2013PPP...372..108B.doi:10.1016/j.palaeo.2012.06.027.
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