Continuoolithus

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Continuoolithus
Temporal range: Late Santonian-Maastrichtian
Egg fossil classification OOjs UI icon edit-ltr.svg
Basic shell type: Ornithoid
Morphotype: Ornithoid-ratite
Oogenus:† Continuoolithus
Zelenitsky, Hills & Currie, 1996
Oospecies
  • †C. canadensisZelenitsky, Hills & Currie, 1996
Synonyms

Spongioolithus hirschiBray 1999

Continuoolithus is an oogenus (fossil egg genus) of dinosaur egg found in the late Cretaceous of North America. It is most commonly known from the late Campanian of Alberta and Montana, but specimens have also been found dating to the older Santonian and the younger Maastrichtian. It was laid by an unknown type of theropod. These small eggs (measuring 77–123 mm (3.0–4.8 in) long) are similar to the eggs of oviraptorid dinosaurs (oofamily Elongatoolithidae), but have a distinctive type of ornamentation.

Continuoolithus nests would have been incubated under vegetation and sediment, unlike nests of Troodon and 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

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's ornamentation pattern consists of randomly dispersed nodes (dispersituberculate ornamentation). [3] The pores follow the angusticanaliculate type (i.e. narrow and straight pores). [3] [2] Continuoolithus had 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's eggshell was 0.94–1.28 mm (0.037–0.050 in) thick. [3] [2] Other specimens differ in shell thickness: some fragments referred to C. sp. have a slightly thinner shell, [5] C. cf. canadensis fragments from Willow Creek have a thicker shell, [6] and C. cf. canadensis from 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. In elongatoolithids and in Continuoolithus, 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 of Continuoolithus preserve the shell membrane, a layer of fibrous proteins found in extant archosaur (bird and crocodylian) 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

Continuoolithus was 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, Continuoolithus eggs are typically found paired; [10] this is because the parent dinosaurs had two functional oviducts, each of which would produce an egg simultaneously. [11]

Comparing the Maastrictian-aged specimens to the older Campanian specimens of Continuoolithus and 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

The known nests of Continuoolithus eggs 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, unlike Troodon eggs and elongatoolithids (the eggs of oviraptorosaurs), [13] Continuoolithus would 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

One Continuoolithus egg contains embryonic remains representing a relatively early stage of development so that the skeleton was almost entirely cartilaginous, 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 to Troodon, Orodromeus, and Maiasaura, 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

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

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

History

Eggs have been known from the Two Medicine Formation in Montana since 1979. In 1990, Continuoolithus specimens, found at the Egg Mountain locality, were first described in detail by paleontologists Karl Hirsch and Betty Quinn, but they did not give them a parataxonomic name. At that time, prominent American paleontologist Jack Horner believed them to be eggs of Troodon based 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 be Troodon when the structurally quite distinct Prismatoolithus (previously thought to be eggs of Orodromeus ) were shown to be Troodon by more thorough study of the preserved embryo. [18] [19]

In 1996, Canadian paleontologists Darla Zelenitsky, L.V. Hills and Phillip Currie named Continuoolithus based on newly discovered remains in Alberta. [3] They noted similarity between the new specimens and the ?Troodon eggs of Two Medicine, but the Two Medicine eggs would not be formally assigned to Continuoolithus until Zelenitsky and Sloboda (2005), at which time they also reported the first occurrence of Continuoolithus in the Dinosaur Park Formation. [3] [9]

A nesting trace of Continuoolithus was 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 other Continuoolithus specimens extensively in her masters thesis at Montana State University. [10] In 2015, she, and her advisor Frankie Jackson, along with David Varricchio and James Schmitt published these results in the journal PALAIOS . [2]

In 2008, Ed Welsh and Julia Sankey published the first report of fossil dinosaur eggs from Texas, discovered in the Aguja Formation. They described several eggshell fragments, including some that were comparable to Continuoolithus, but perhaps more similar to the elongatoolithid Macroelongatoolithus in their ornamentation. [20] In 2011, Kohei Tanaka et al. described numerous eggshell fragments from the Fruitland Formation in New Mexico, including a few fragments referable to Continuoolithus sp. [5]

In 2017, a team of Canadian paleontologists led by Darla Zelenitsky reported the discovery of a pair of Continuoolithus shell fragments at the Willow Creek Formation in Alberta, representing the first fossils of the oogenus found in the Maastrichtian. [6] The same year, Zelenitsky et al. also discovered the first Continuoolithus specimens in the Santonian, found at the Milk River Formation, also in Alberta. [7]

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

Distribution and paleoecology

Continuoolithus canadensis is known from the Flaming Cliffs and the Egg Mountain localities (and possibly Sevenmile Hill too) of the Two Medicine Formation in Montana, from Devil's Coulee in the Oldman Formation in Alberta, and from the Dinosaur Park Formation in 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 the Western Interior Seaway. The Flaming Cliff locality formed in a well-drained overbank of an alluvial floodplain. [2] The Egg Mountain locality also represents a floodplain overbank. [23] The formation has a diverse assemblage of dinosaurs including theropods such as Troodon, Albertosaurus , ornithomimids and dromaeosaurs, as well as several types of hadrosaurs, ceratopsians, ankylosaurs, and smaller ornithischians such as Orodromeus . It also includes pterodactyloid pterosaurs, Champsosaurus , turtles, lizards, and mammals. [24] Other types of eggs from Two Medicine include Montanoolithus , [25] Prismatoolithus levis (the eggs of Troodon formosus), some small unidentified theropod eggs, [26] P. hirschi, Triprismatoolithus , Tubercuoolithus , Spheroolithus albertensis (eggs of Maiasaura ), S. choteauensis, eggs of Hypacrosaurus , and Krokolithes . [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 to Continuoolithus, eggs of Hypacrosaurus, Spheroolithus, [27] Prismatoolithus, [28] Porituberoolithus , Tristaguloolithus , and Dispersituberoolithus are also known. [3] Dinosaurian skeletal remains include Troodon, tyrannosaurids, ankylosaurids, hadrosaurids, ceratopsids, and ornithomimids. [29] Footprints of hadrosaurs are also known. [22] The formation was also populated by multituberculate mammals, [30] numerous types of turtles, Champsosaurus, sturgeons, and pterosaurs (including the giant Quetzalcoatlus ). [29]

Artist's restoration of some megafaunal dinosaurs of the Dinosaur Park Formation. Dinosaur park formation fauna.png
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 the Belly River Group. It represents the deposits of a perennial, sinuous river system and paralic environments. [22] It is widely known for its incredible diversity of dinosaurian fauna, representing over 50 valid taxa including theropods such as dromaeosaurs, caenagnathids, troodontids, ornithomimids, and tyrannosaurids, as well as ornithischians such as pachycephalosaurs, hadrosaurs, ceratopsians, ankylosaurs, and thescelosaurs. [31] Other types of fossil eggs from the formation include Reticuoolithus , Porituberoolithus, Prismatoolithus, Spheroolithus, and Krokolithes. [9]

Other Continuoolithus specimens, 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 with Porituberoolithus, Prismatoolithus, indeterminate theropod eggshells, Testudoolithus, and krokolithids. [20] Also, fragments of C. cf. canadensis are known from the late Maastrichtian Willow Creek Formation in Alberta. This formation has relatively low dinosaurian diversity; eggs from the formation predominately belong to the ornithopod oogenus Spheroolithus, but some types of theropod eggs (Continuoolithus, Montanoolithus, Porituberoolithus, and Prismatoolithus) are known. [6] C. cf. canadensis fragments were also found in the late Santonian Milk River Formation, wlong with Porituberoolithus, Prismatoolithus, Spheroolithus, and Triprismatoolithus. [7] Maastrictian-aged Continuoolithus specimens have also been discovered in the North Horn Formation in Utah, [12] a formation rich in dinosaur eggs, including Spheruprismatoolithus , Prismatoolithus, Ovaloolithus, and Spheroolithus. [17]

See also

Related Research Articles

<i>Troodon</i> Extinct genus of dinosaurs

Troodon is a former wastebasket taxon and a potentially dubious genus of relatively small, bird-like theropod dinosaurs definitively known from the Campanian age of the Late Cretaceous period. It includes at least one species, Troodon formosus, known from Montana. Discovered in October 1855, T. formosus was among the first dinosaurs found in North America, although it was thought to be a lizard until 1877. Several well-known troodontid specimens from the Dinosaur Park Formation in Alberta were once believed to be members of this genus. However, recent analyses in 2017 have found this genus to be undiagnostic and referred some of these specimens to the genus Stenonychosaurus some to the genus Latenivenatrix, and some to the genus Pectinodon. The genus name is Ancient Greek for "wounding tooth", referring to the teeth, which were different from those of most other theropods known at the time of their discovery. The teeth bear prominent, apically oriented serrations. These "wounding" serrations, however, are morphometrically more similar to those of herbivorous reptiles, and suggest a possibly omnivorous diet.

<i>Maiasaura</i> Extinct genus of reptiles

Maiasaura is a large herbivorous saurolophine hadrosaurid ("duck-billed") dinosaur genus that lived in the area currently covered by the state of Montana and the province of Alberta, Canada, in the Upper Cretaceous Period, about 76.7 million years ago. Maiasaura peeblesorum is the state fossil of Montana.

<span class="mw-page-title-main">Two Medicine Formation</span> Geological formation in Montana, United States and Alberta, Canada

The Two Medicine Formation is a geological formation, or rock body, in northwestern Montana and southern Alberta that was deposited between 83.5 ± 0.7 Ma and 70.6 ± 3.4 Ma, during Campanian time. It crops out to the east of the Rocky Mountain Overthrust Belt, and the western portion of this formation is folded and faulted while the eastern part, which thins out into the Sweetgrass Arch, is mostly undeformed plains. Below the formation are the nearshore deposits of the Virgelle Sandstone, and above it is the marine Bearpaw Shale. Throughout the Campanian, the Two Medicine Formation was deposited between the western shoreline of the Late Cretaceous Interior Seaway and the eastward advancing margin of the Cordilleran Overthrust Belt. The Two Medicine Formation is mostly sandstone, deposited by rivers and deltas.

<i>Elongatoolithus</i> Fossil dinosaur eggs

Elongatoolithus is an oogenus of dinosaur eggs found in the Late Cretaceous formations of China and Mongolia. Like other elongatoolithids, they were laid by small theropods, 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, Elongatoolithus was a highly elongated form of egg. It is historically significant for being among the first fossil eggs given a parataxonomic name.

<i>Cairanoolithus</i> Oogenus of dinosaur egg

Cairanoolithus is an oogenus of dinosaur egg which is found in Southwestern Europe. The eggs are large and spherical. Their outer surface is either smooth, or covered with a subdued pattern of ridges interspersed with pits and grooves. Multiple fossil egg clutches are known but the nest structure is unclear.

Protoceratopsidovum is an oogenus of dinosaur egg from Mongolia. Despite its name, it does not represent the eggs of a protoceratopsid, but rather the eggs of maniraptoran theropods.

<i>Macroelongatoolithus</i> Oogenus of dinosaur egg

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.

<i>Macroolithus</i> Oogenus of dinosaur egg

Macroolithus is an oogenus of dinosaur egg belonging to the oofamily Elongatoolithidae. The type oospecies, M. rugustus, was originally described under the now-defunct oogenus name Oolithes. Three other oospecies are known: M. yaotunensis, M. mutabilis, and M. lashuyuanensis. They are relatively large, elongated eggs with a two-layered eggshell. Their nests consist of large, concentric rings of paired eggs. There is evidence of blue-green pigmentation in its shell, which may have helped camouflage the nests.

Dispersituberoolithus is an oogenus of fossil egg, which may have been laid by a bird or non-avian theropod.

<span class="mw-page-title-main">Egg fossil</span> Fossilized remains of eggs laid by ancient animals

Egg fossils are the fossilized remains of eggs laid by ancient animals. As evidence of the physiological processes of an animal, egg fossils are considered a type of trace fossil. Under rare circumstances a fossil egg may preserve the remains of the once-developing embryo inside, in which case it also contains body fossils. A wide variety of different animal groups laid eggs that are now preserved in the fossil record beginning in the Paleozoic. Examples include invertebrates like ammonoids as well as vertebrates like fishes, possible amphibians, and reptiles. The latter group includes the many dinosaur eggs that have been recovered from Mesozoic strata. Since the organism responsible for laying any given egg fossil is frequently unknown, scientists classify eggs using a parallel system of taxonomy separate from but modeled after the Linnaean system. This "parataxonomy" is called veterovata.

Montanoolithus is an oogenus of fossil egg found in Montana and Alberta. They were probably laid by a dromaeosaur or a caenagnathid.

Wendy Sloboda is a Canadian fossil hunter from Warner, Alberta. She has made fossil discoveries of dinosaurs and other extinct animals on several continents, with finds in Canada, Argentina, Mongolia, France, and Greenland. She is commemorated in name of the horned dinosaur Wendiceratops, remains of which she discovered in 2010, as well as the fossil footprint Barrosopus slobodai which she discovered in 2003.

Coralloidoolithus is an oogenus of dinosaur egg from the Tiantai Basin in Zhejiang Province, containing a single known oospecies C. shizuiwanensis. Formerly, it was classified in the oogenus Paraspheroolithus; however, it was considered sufficiently different to be classified in its own genus. C. shizuiwanensis is similar to Stalicoolithus, leading to their classification in the same family, Stalicoolithidae.

<span class="mw-page-title-main">Elongatoolithidae</span> Oofamily of dinosaur eggs

Elongatoolithidae is an oofamily of fossil eggs, representing the eggs of oviraptorosaurs. They are known for their highly elongated shape. Elongatoolithids have been found in Europe, Asia, and both North and South America.

Dictyoolithidae is an oofamily of dinosaur eggs which have a distinctive reticulate organization of their eggshell units. They are so far known only from Cretaceous formations in China.

Triprismatoolithus is an oogenus of dinosaur egg native to Teton County, Montana. It is classified in the oofamily Arriagadoolithidae, the eggs of alvarezsaurs.

Nipponoolithus is an oogenus of fossil egg native to Japan. It is one of the smallest known dinosaur eggs, and was probably laid by some kind of non-avian maniraptor.

Trigonoolithus is an oogenus of dinosaur egg, representing a basal prismatoolithid. Its eggshell, like avian eggs, is composed of three structural layers, but cladistic analysis suggests that its parent was a non-avian theropod.

Incognitoolithus is an oogenus of medioolithid fossil bird egg. It is notable for bearing evidence of predation, possibly from a bird pecking the eggshell.

Nanhsiungoolithus is an oogenus of dinosaur egg from the late Cretaceous of China. It belongs to the oofamily Elongatoolithidae, which means that it was probably laid by an oviraptorosaur, though so far no skeletal remains have been discovered in association with Nanhsiungoolithus. The oogenus contains only a single described oospecies, N. chuetienensis. It is fairly rare, only being know from two partially preserved nests and a few eggshell fragments.

References

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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. S2CID   85728452.
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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. JSTOR   3514834.
  25. ↑ Zelenitsky, Darla K.; Therrien, Francois (2008). "Unique maniraptoran egg clutch from the Upper Cretaceous Two Medicine Formation of Montana reveals theropod nesting behaviour". Palaeontology. 51 (6): 1253–1259. Bibcode:2008Palgy..51.1253Z. doi: 10.1111/j.1475-4983.2008.00815.x . S2CID   129364937.
  26. ↑ Varricchio, David J.; Jackson, Frankie D. (2004). "A phylogenetic assessment of prismatic dinosaur eggs from the Cretaceous Two Medine Formation of Montana". Journal of Vertebrate Paleontology. 24 (4): 931–937. doi:10.1671/0272-4634(2004)024[0931:APAOPD]2.0.CO;2. S2CID   85987939.
  27. ↑ Zelenitsky, Darla K.; Hills, L.V. (1997). "Normal and pathological eggshells of Spheroolithus 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. 1 2 Currie, 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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