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.[1]
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.[1]
Elongatoolithids are, as their name suggests, highly elongated eggs; they are at least twice as long as they are wide.[2] 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.[1]
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 varies from scattered nodes (dispersituberculate) to linear ridges (lineartuberculate), occasionally with nodes in long irregular chains (ramotuberculate).[1]
The first elongatoolithid eggs were discovered in the 1920s, and were thought to belong to Protoceratops.[3] 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.[4] Following the discovery of Troodon eggs in 1990, their close resemblance to elongatoolithids lead Russian paleontologist Konstantin Mikhailov to believe they were actually theropod eggs, not Protoceratops eggs.[2] 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.[5] Since then, several discoveries of embryos and association of adults with eggs have shown that elongatoolithids are the eggs of Oviraptorosaurs.[1]
The first oospecies formally described was "Oolithes" elongatus (Young, 1954), from China. They were thought to belong to Protoceratops or a related dinosaur because of their similarity to the supposed Protoceratops eggs found in Mongolia.[6] In 1975, Chinese paleontologist Zhao Zikui devised a formal classification system for fossil eggs, in which he created a new oogenus for "Oolithes" elongatus: Elongatoolithus. He classified Elongatoolithus and another new oogenus Macroolithus into a new oofamily, Elongatoolithidae.[7]