User:Ashorocetus/sandbox/Elongatoolithidae

Ashorocetus/sandbox/Elongatoolithidae Temporal range:
Elongatoolithid eggs in the Academy of Natural Sciences, Philadelphia
Egg fossil classification
Basic shell type:	Ornithoid
Morphotype:	Ornithoid-ratite
Oofamily:	†Elongatoolithidae
Oogenera
Ellipsoolithus Elongatoolithus (Type) Heishanoolithus Macroelongatoolithus Macroolithus Nanhsiungoolithus Paraelongatoolithus Rodolphoolithus Spongioolithus Trachoolithus Undulatoolithus

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

Distribution

Elongatoolithids have a very broad distribution. They have been found across Asia and the US, as well as in Spain, France, and Argentina, with ages ranging from lower Cretaceous to the Paleocene.

Description

Elongatoolithids are, as their name suggests, highly elongated eggs; they are at least twice as long as they are wide. They vary widely in size, ranging from the 7 cm long Elongatoolithus chichengshanensis to the gigantic 60 cm Macroelongatoolithus. All known clutches are laid in concentric circles of paired eggs, sometimes in up to three superimposed layers.

Eggshell

Elongatoolithid eggshells are made up of two layers: The inner layer, called the mammillary layer or the cone layer, is made up of radiating calcite crystals. The outer layer is distinctive for not being divided into well-defined shell units, and hence it is called the continuous layer or the cryptoprismatic layer. The boundary between the two layers is abrupt, but wavy. Typically, elongatoolithid eggs have an angusticanaliculate pore system, meaning the pores are thin, straight, and unbranching. The surface ornamentation of the eggshells is varies from scattered nodes (dispersituberculate) to linear ridges (lineartuberculate), occasionally with nodes in long irregular chains (ramotuberculate).

Coloration

The presence of pigments in the eggshells of three specimens of the elongatoolithid Macroolithus yaotunensis indicates that these eggs were a blue-green color. The preserved fossil pigments (biliverdin and protoporphyrin) are the same pigments found in modern bird eggs, strongly suggesting homology. In modern birds, the coloration can act as camouflauge or, in some species, as a post-mating sexual signal. Eggshell coloration is also correlated with paternal care.

History

See also: Timeline of egg fossil research

An elongatoolithid with embryo preserved inside

The first elongatoolithid eggs were discovered in the 1920s, and were thought to belong to Protoceratops. Oviraptor was first discovered in 1924 with a nest of elongatoolithid eggs, and it was conjectured to have been caught in the act of raiding a Protoceratops nest. In 1991, Russian paleontologist Konstantin Mikhailov first suggested that elongatoolithids were the eggs of theropods, because they were quite distinct from the known hadrosaur eggs and eggs supposed to be of protoceratopsids. Also, he noted their similarity to eggs tentatively assigned to Troodon that had been discovered in 1990 (which are now classified in the oogenus Continuoolithus. In 1994, his hypothesis was confirmed when Norell et al. discovered embryonic remains of an Oviraptorosaur inside an elongatoolithid egg. It was then hypothesized that Oviraptor was in fact a brooding mother, not an egg thief. Since then, several discoveries of embryos and association of adults with eggs have shown that elongatoolithids are the eggs of Oviraptorosaurs.

Prior to the advent of modern eggshell parataxonomy, the Chinese paleontologist Yang Zhongjian named two egg-species now classified in Elongatoolithidae. In 1954, he named "Oolithes" elongatus based on remains from Laiyang in Shandong. He noted their similarity to the supposed Protoceratops eggs of Mongolia, and therefore thought it likely that they were laid by a similar type of dinosaur. In 1965, Yang described and named several types of eggs from Nanxiong in Guangdong, including more specimens of "O." elongatus and a new type, "O." rugustus. Yang noted that these two types were clearly similar, and postulated that both were eggs of ornithischians. In 1975, Chinese paleontologist Zhao Zikui devised a formal classification system for fossil eggs. He created the oogenus Macroolithus for Yang's O. rugustus, Elongatoolithus for most O. elongatus specimens, and Nanhsiungoolithus for two other specimens of "O." elongatus

Nesting and Parental Care

Citipati parent sitting on its nest of elongatoolithid eggs

Elongatoolithids are known to be the eggs of oviraptorosaurs (except for the avian Ornitholithus, which is occasionally considered an elongatoolithid). Several oviraptorosaurs have been found in association with elongatoolithid eggs, including some specimens still inside the mother. Fossil embryos found inside elongatoolithid eggs have also been identified as oviraptorosaurian.

Several oviraptorosaurs, like Nemegtia, Citipati, Oviraptor, and cf. Machairasaurus, have been found sitting on top of their nests. All of them have their arms spread out over the eggs in a bird-like posture, and the parent's body would effectively cover the entire nest. This indicates intensive parental care of the eggs. It is not certain whether the specimens found brooding are male or female, but the examined limb bones of a brooding Citipati show none of the evidence of egg-laying that would be expected if theropods took phosphorus and calcium from long bone tissues (like crocodylians) or medullary bone (like birds), suggesting it was a male. Also, the clutches were proportionally large compared to the size of the adult, which suggests a polygamous system, similar to modern paleognaths, in which multiple females contribute eggs to a single nest which is then cared for by the father. Smaller oviraptorosaurs would have sat directly on the eggs, similar to modern birds. Larger species, on the other hand, show a unique adaptation to place most of their weight on the raised center of the nest, rather than on the eggs.

The eggs are laid in pairs, as shown by the discovery of two Macroolithus eggs simultaneously within the mother, and the pairing of eggs within nests. This shows that oviraptorosaurs had two functional oviducts (unlike birds, which have only one), and would produce two eggs at a time. Also, the relatively large size of the eggs indicates that a female could not lay more than one pair at a time.

Elongatoolithid nests were probably typically incubated above ground, instead of being buried under substrate or vegetation like some other types of dinosaur eggs. Some authors in the past have argued that elongatoolithids such as Elongatoolithus and Macroolithus were buried, which would explain the high porosity of the eggshells. Deeming (2006) noticed the incongruence of this hypothesis with the evidence of brooding in Oviraptor and Troodon, and suggested that these dinosaurs merely sat atop a buried nest. However, a statistical analysis of eggshell porosity in 2015 by Tanaka et al. found that elongatoolithids would have been laid in open nests, with the possible exception of Elongatoolithus elongatus.

Parataxonomy

In the basic-type and morphotype scheme for eggshell classification (which is now typically disused), elongatoolithids are of the Ornithoid basic type and Ornithoid-Ratite morphotype. They are similar to the Troodon eggshells, which are now classified in the oofamily Prismatoolithidae.

Elongatoolithidae contains the oogenera Nanhsiungoolithus, Elongatoolithus, Macroolithus, Macroelongatoolithus, Ellipsoolithus, Trachoolithus, Heishanoolithus, Ornitholithus, Paraelongatoolithus, Undulatoolithus, Rodolphoolithus, and Spongioolithus. Also, Porituberoolithus and Continuoolithus are occasionally included in Elongatoolithidae.