Psittacosaurus Sibiricus
Beginning in the 1950s, Russian paleontologists began excavating Psittacosaurus remains at a locality near the village of Shestakovo in the oblast of Kemerovo in Siberia. Two other nearby localities were explored in the 1990s, one of which produced several complete skeletons. This species was named Psittacosaurus sibiricus in 2000 in a scientific paper written by five Russian paleontologists, but credit for the name is officially given to two of those authors, Alexei Voronkevich and Alexander Averianov. The remains were not completely described until 2006. Two nearly complete, articulated skeletons and a variety of disarticulated material from other individuals of all ages are known from the Ilek Formation of Siberia, which ranges from the Aptian to Albian stages of the Early Cretaceous.
P. sibiricus is the largest known species of Psittacosaurus. The skull of the type specimen is 20.7 centimeters long (8.25 in), and the femur is 22.3 cm (8.75 in) in length. It is also distinguished by its neck frill, which is longer than any other species, at 15 to 18% of skull length. A very striking feature of P. sibiricus is the number of 'horns' around the eyes, with three prominences on each postorbital, and one in front of each eye, on the palpebral bones. Similar horns found on the postorbital of P. sinensis are not as pronounced but may be homologous. The jugal has extremely prominent 'horns' and may contact the premaxilla, both features also seen in the possibly related P. sinensis. There is a flange on the dentary of the lower jaw, similar to P. mongoliensis, P. meileyingensis, and P. sattayaraki. It can be told apart from the other species of Psittacosaurus by its combination of 32 anatomical features, including six that are unique to the species. Most of these are skull details, but one unusual feature is the presence of 23 vertebrae between the skull and pelvis, unlike the 21 or 22 in the other species where the vertebrae are known.
Read more about this topic: List Of Psittacosaurus Species