fossil embryos of various species during development. The presence of two shelled eggs within the birth canal shows that oviraptorosaurs were intermediate between the reproductive biology of crocodilians and modern birds. Like crocodilians, they had two oviducts. However, crocodilians produce multiple shelled eggs per oviduct at a time, whereas oviraptorosaurs, like birds, produced only one egg per oviduct at a time. Relationship to birds Oviraptorosaurs, like deinonychosaurs, are so bird-like that several scientists consider them to be true birds, more advanced than Archaeopteryx. Gregory S. Paul has written extensively on this possibility, and Teresa Maryańska and colleagues published a technical paper detailing this idea in 2002. Michael Benton, in his widely respected text Vertebrate Paleontology, wrote placement of oviraptorosaurs among birds is highly controversial . However, a number of researchers have disagreed with this classification, retaining oviraptorosaurs as non-avialan maniraptorans slightly more primitive than the deinonychosaurs. Classification The internal classification of the oviraptorosaurs has also been controversial. Most studies divide them into two primary sub-groups, the Caenagnathidae and the Oviraptoridae. The Oviraptoridae is further divided into the small, short-armed, and crestless subfamily Ingeniinae, and the larger, crested, long-armed Oviraptorinae. However, some phylogenetic studies have suggested that many traditional members of the Caenagnathidae were more closely related to the crested oviraptorids. Phylogeny The 2007 cladistic analysis of Turner and colleagues recovered the Oviraptorosauria as a maniraptoran clade (natural grouping) of maniraptorans more primitive than true birds. They found that the oviraptorosaurs are the sister group to the Therizinosauria and that the two, together, are more basal than any member of Paraves. However, a more recent study by Zanno and colleagues challenged that finding, showing therizinosaurs to be more primitive and not closely related to oviraptorosaurs. The following cladogram was found by an analysis published with the description of the caenagnathid Anzu. See also Timeline of oviraptorosaur research References Early Cretaceous first appearances Maastrichtian extinctions