Continuoolithus

Complete eggs range from to in size. They are elongated and ovoid shaped (i.e., with one blunt end and one pointed end). Known nesting traces contain from three to six eggs arranged parallel to each-other in linear rows. Continuoolithus canadensis eggshell was thick. and C. cf. canadensis from Milk River are thinner. 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). 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. ==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. Like many other types of non-avian theropod eggs, Continuoolithus eggs are typically found paired; 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. Continuoolithus would have been incubated in substrate rather than by a brooding adult. ==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). However, most authors do not include it in Elongatoolithidae, considering it to be of uncertain placement because it has different ornamentation It belongs to the ornithoid-ratite morphotype, a grouping which primarily includes paleognathous birds and non-avian theropods. Continuoolithus canadensis has one junior synonym, Spongioolithus hirschi, which was originally classified as a distinct oogenus and oospecies of Elongatoolithidae. ==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. In 1996, Canadian paleontologists Darla Zelenitsky, L.V. Hills and Phillip Currie named Continuoolithus based on newly discovered remains in Alberta. 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. In 2015, she, and her advisor Frankie Jackson, along with David Varricchio and James Schmitt published these results in the journal PALAIOS. 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. 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. 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. However, this oospecies is indistinguishable from C. canadensis, so in 2018, Jared Voris, Zelenitsky, Therrien, and Tanaka synonymized the oospecies. ==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). 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. Other types of eggs from Two Medicine include Montanoolithus, Prismatoolithus levis (the eggs of Troodon formosus), some small unidentified theropod eggs, P. hirschi, Triprismatoolithus, Tubercuoolithus, Spheroolithus albertensis (eggs of Maiasaura), S. choteauensis, eggs of Hypacrosaurus, and Krokolithes. 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, Prismatoolithus, Porituberoolithus, Tristaguloolithus, and Dispersituberoolithus are also known. Footprints of hadrosaurs are also known. numerous types of turtles, Champsosaurus, sturgeons, and pterosaurs (including the giant Quetzalcoatlus). Other types of fossil eggs from the formation include Reticuoolithus, Porituberoolithus, Prismatoolithus, Spheroolithus, and Krokolithes. 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. 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. C. cf. canadensis fragments were also found in the late Santonian Milk River Formation, wlong with Porituberoolithus, Prismatoolithus, Spheroolithus, and Triprismatoolithus. Maastrictian-aged Continuoolithus specimens have also been discovered in the North Horn Formation in Utah, a formation rich in dinosaur eggs, including Spheruprismatoolithus, Prismatoolithus, Ovaloolithus, and Spheroolithus. ==See also==