Guegoolithus

Guegoolithus is an oogenus of fossil egg from the Barremian of Spain. It is classified in the oofamily Spheroolithidae, and was probably laid by an ornithopod dinosaur.

Distribution
Guegoolithus is so far known exclusively from the Maestrazgo Basin, dating to the lower Barremian. Fossils have been found in the Blesa, El Castellar, Camarillas, and Mirambel Formations.

Description
Guegoolithus is known from over 400 fossil eggshell fragments, but no complete eggs have been found. They are very thin for spheroolithids, ranging from 0.42 to 1.5 mm in thickness. The wide range of eggshell thickness is partially due to erosion on the inner and outer surfaces of the fragments. The size and shape of the complete egg is unknown.

The original description of "Macroolithus" turolensis said it had a two-layered eggshell with linearituberculate ornamentation and an angusticanaliculate pore system. However, the reanalysis by Moreno-Azanza et al. (2014) found all of these to be misinterpretations: in fact, it has a single-layered eggshell, and sagenotuberculate ornamentation (nodes and ridges forming a net-like pattern, with pits and grooves in between ) and a prolatocanaliculate pore system, similar to Spheroolithus. Also like Spheroolithus, it has a prolatospherulitic morphotype, with eggshell units fused together and horizontal growth lines on the lower portion of the eggshell. Guegoolithus has much more prominent ornamentation that Spheroolithus, and a thinner eggshell than most Spheroolithus species.

History
Guegoolithus was first discovered in 2000 by Spanish paleontologists Olga Amo-Sanjuán, José Ignacio Canudo, and Gloria Cuenca-Bescós, though it was described as an oospecies of Macroolithus. In 2008, a paper suggested that ''"M." turolensis'' be used as an index fossil for fossil sites in the Sistema Ibérico, because they are easily identifiable and always found in rocks of Hauterivian-Barremian age. An analysis in 2014 by Miguel Moreno-Azanza, José Ignacio Canudo, and José Manuel Gasca found that in fact, ''"M." turolensis is not an elongatoolithid, but instead belongs in Spheroolithidae, and named a new oogenus for it: Guegoolithus''.

Classification
While it was previously considered an oospecies of Macroolithus in the oofamily Elongatoolithidae, Guegoolithus is now considered a spheroolithid because of the similarities of microstructure, ultrastructure, and ornamentation. It contains a single oospecies: G. turolensis. Unfortunately, relationships within Speroolithidae are not well known because of the unclear definitions of Spheroolithus and Speroolithidae.

Paleobiology
Guegoolithus is classified as a spheroolithid, which would indicate it was laid by an ornithopod dinosaur. It is very similar to the eggs of Maiasaura peeblesorum and other hadrosaurs, but these were not common in Europe during the Early Cretaceous. G. turolensis is most likely laid by a closely related kind of ornithopod, like Gideonmantellia, Mantellisaurus, Iguanodon, Delapparentia, or Proa.

Its reclassification outside of Elongatoolithidae implies that no elongatoolithids are known from the early Cretaceous of Europe. However, elongatoolithids have been reported from the Late Cretaceous of France.