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==Discovery and species==
[[File:Leptoceratops gracilis.jpg|thumb|upright|left|Forelimb of ''Leptoceratops'' type AMNH 5205]]
A 1910 expedition by the [[American Museum of Natural History]] (AMNH) discovered two ceratopsian specimens from the [[EdmontonianMaastrichtian]] bedsage of the [[Cretaceous]] along the [[Red Deer River]] in [[Alberta]], Canada. The two individuals were found together in a weathered-out cow trail that had destroyed and fragmented some of the material, so that only parts of each skeleton were recovered. One of these individuals preserves a partial skull and was designated as the [[holotype]] of the new [[taxon]] ''Leptoceratops gracilis'' when it was [[species description|described]] in 1914 by American palaeontologist [[Barnum Brown]].<ref name="brown1914"/> The name translates as "slender small horn face", with the [[generic name (biology)|generic name]] derived from the [[Ancient Greek]] word ''leptos'', 'small', and ''[[Ceratops]]'', 'horned face', though Brown did not explain its etymology.<ref name="lucas2016">{{cite book|last=Lucas|first=S.G.|year=2016|title=Dinosaurs: The Textbook|edition=6th|publisher=Columbia University Press|pages=357|isbn=978-0-231-54184-8}}</ref><ref name="dodson1996">{{cite book|last=Dodson|first=P. |year=1996|title=The Horned Dinosaurs|publisher=Princeton University Press|pages=346|isbn=978-0-691-02882-8}}</ref> The specimen preserves parts of the skull and [[jaw]]s, a complete right [[forelimb]], partial [[hindlimb]]s, a series of 24 [[caudal vertebrae]], as well as two other isolated [[vertebrae]]. The second individual also preserves part of the forelimb, as well as four {{dinogloss|presacral}}, three {{dinogloss|sacral}}, and one caudal vertebrae.<ref name="brown1914"/> Both individuals are catalogued under the specimen number AMNH 5205, with the second individual being slightly larger. The [[type locality (biology)|locality]] that ''Leptoceratops'' was first found in could not be relocated by Canadian palaeontologist [[Charles M. Sternberg]], but the description of it provided by Brown was sufficient for Sternberg to conclude that it was from within the Upper Edmonton member of the [[Edmonton Formation]]. Sternberg described three additional skeletons of ''Leptoceratops'' from the Edmonton Formation in 1951, including one complete individual. The specimens were discovered towards the end of the 1947 field season, where Sternberg located a skull, jaw, and most of a skeleton of an individual ([[Canadian Museum of Nature]] number 8889), student assistant T. P. Chammery found a smaller individual missing most of the head and a partial left hand (CMN 8888), and excavation of CMN 8888 uncovered a third individual (CMN 8887) directly beside it. CMN 8888 and 8887 were {{convert|50|ft|m|abbr=on|order=flip}} above the base of the Upper Edmonton member, and CMN 8889 was an additional {{convert|5|ft|m|abbr=on|order=flip}} above them.<ref name="sternberg1951">{{cite journal|last=Sternberg|first=C.M.|year=1951|title=Complete skeleton of ''Leptoceratops gracilis'' from the Upper Edmonton member on Red Deer River, Alberta|journal=National Museum of Canada Bulletin|volume=123|pages=225–255}}</ref> These specimens were complete enough to allow Canadian palaeontologist [[Dale A. Russell]] to publish a completely reconstructed skeleton of ''Leptoceratops'' based only on the CMN individuals.<ref name="russell1969">{{cite journal|last=Russell|first=D.A.|year=1969|title=A skeletal reconstruction of ''Leptoceratops gracilis'' from the upper Edmonton Formation (Cretaceous) of Alberta|journal=Canadian Journal of Earth Sciences|volume=7|pages=181–184|doi=10.1139/e70-014 }}</ref> Newer geology has separated the Edmonton Formation into four formations as the [[Edmonton Group]], with ''Leptoceratops'' known from the uppermost named the [[Scollard Formation]], which was deposited at the end of the [[Maastrichtian]]. Within the Scollard Formation, ''Leptoceratops'' was known in 2013 from nine specimens found in the lower portion, with an estimated range of 65.5-66.1 million years old (0.5 my older with recalibration).<ref name="eberth2013">{{cite journal|last1=Eberth|first1=D.A.|last2=Evans|first2=D.C.|last3=Brinkman|first3=D.M.|last4=Therrien|first4=F.|last5=Tanke|first5=D.H.|last6=Russell|first6=L.S.|year=2013|title=Dinosaur biostratigraphy of the Edmonton Group (Upper Cretaceous), Alberta, Canada: evidence for climate influence|journal=Canadian Journal of Earth Sciences|volume=50|issue=7|pages=701–726|doi=10.1139/cjes-2012-0185|bibcode=2013CaJES..50..701E }}</ref> Along with the described types and CMN specimens, known ''Leptoceratops'' specimens from the Scollard also include the partial skeletons [[Royal Tyrrell Museum of Palaeontology]] 93.95.1 and 95.86.1.<ref name="ryan1998">{{cite journal|last1=Ryan|first1=M.J.|last2=Currie|first2=P.J.|year=1998|title=First report of protoceratopsians (Neoceratopsia) from the Late Cretaceous Judith River Group, Alberta, Canada|journal=Canadian Journal of Earth Sciences|volume=35|issue=7|pages=820–826|doi=10.1139/e98-033|bibcode=1998CaJES..35..820R }}</ref>
In 1916, Brown collected an additional specimen, AMNH 5464, from the [[St. Mary River Formation]] of [[Montana]]. This specimen, including a well-preserved skeleton with a fragmentary skull, was prepared in 1918 at the AMNH and then mounted in 1935 to be put on display; the skull of this mount was completely modelled out of plaster. Differences in the skull from the first ''Leptoceratops'' specimens lead Brown and Erich M. Schlaijker to describe the specimen in 1942 as a new species, ''Leptoceratops cerorhynchus''. Brown and Schlaijker concluded that ''Leptoceratops'' was a very close relative of ''[[Protoceratops]]'', and that both genera should be united within the family [[Protoceratopsidae]].<ref name="brown1942">{{cite journal|last1=Brown|first1=B.|last2=Schlaikjer|first2=E.M.|year=1942|title=The skeleton of ''Leptoceratops'' with the description of a new species|journal=American Museum Novitates|issue=1169|pages=1–15}}</ref> Comparisons between the St. Mary River specimen and the skeletons collected by Sternberg in Alberta showed that ''L. cerorhynchus'' should be given its own genus, which Sternberg named ''[[Montanoceratops]]'' in 1951.<ref name="sternberg1951"/>
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