title: A new mass mortality of juvenile Protoceratops and size-segregated aggregation behaviour in juvenile non-avian dinosaurs.
abstract: BACKGROUND
  Monodominant bonebeds are a relatively common occurrence for non-avian dinosaurs, and have been used to infer associative, and potentially genuinely social, behavior. Previously known assemblages are characterized as either mixed size-classes (juvenile and adult-sized specimens together) or single size-classes of individuals (only juveniles or only adult-sized individuals within the assemblage). In the latter case, it is generally unknown if these kinds of size-segregated aggregations characterize only a particular size stage or represent aggregations that happened at all size stages. Ceratopsians ("horned dinosaurs") are known from both types of assemblages.
  
  
  RESULTS
  Here we describe a new specimen of the ceratopsian dinosaur Protoceratops andrewsi, Granger and Gregory 1923 from Mongolia representing an aggregation of four mid-sized juvenile animals. In conjunction with existing specimens of groups of P. andrewsi that includes size-clustered aggregations of young juveniles and adult-sized specimens, this new material provides evidence for some degree of size-clustered aggregation behaviour in Protoceratops throughout ontogeny. This continuity of size-segregated (and presumably age-clustered) aggregation is previously undocumented in non-avian dinosaurs.
  
  
  CONCLUSIONS
  The juvenile group fills a key gap in the available information on aggregations in younger ceratopsians. Although we support the general hypothesis that many non-avian dinosaurs were gregarious and even social animals, we caution that evidence for sociality has been overstated and advocate a more conservative interpretation of some data of 'sociality' in dinosaurs.
authors.affiliation: School of Earth Sciences, University of Bristol, Bristol, United Kingdom; School of Biological & Chemical Sciences, Queen Mary University of London, London, United Kingdom.
authors.forename: David W E
authors.initials: DW
authors.lastname: Hone
authors.affiliation: Raymond M. Alf Museum of Paleontology, Claremont, California, United States of America.
authors.forename: Andrew A
authors.initials: AA
authors.lastname: Farke
authors.affiliation: Hayashibara Museum of Natural Sciences, Okayama, Japan.
authors.forename: Mahito
authors.initials: M
authors.lastname: Watabe
authors.affiliation: Hayashibara Museum of Natural Sciences, Okayama, Japan.
authors.forename: Suzuki
authors.initials: S
authors.lastname: Shigeru
authors.affiliation: Paleontological Center, Mongolian Academy of Sciences, Ulaanbaatar, Mongolia.
authors.forename: Khishigjav
authors.initials: K
authors.lastname: Tsogtbaatar
mesh_terms: D000367:Age Factors; D000818:Animals; D025061:Dinosaurs / Q000033:anatomy & histology* / Q000145:classification / Q000502:physiology; D053476:Extinction, Biological; D005580:Fossils; D006733:Horns / Q000033:anatomy & histology* / Q000502:physiology; D008986:Mongolia; D010802:Phylogeny; D012886:Skull / Q000033:anatomy & histology* / Q000502:physiology; D012919:Social Behavior