Leptoceratops (meaning 'small horn face') is a genus of ceratopsian dinosaur from the Late Cretaceous of North America. First found in Alberta in 1910, the type species Leptoceratops gracilis was named in 1914 by Barnum Brown for a partial skull and skeleton of two individuals found in the Scollard Formation of Alberta. Additional specimens found in the Scollard include one complete and two mostly complete skeletons together, uncovered in 1947 by Charles M. Sternberg. Specimens from Montana that were among the earliest referred to Leptoceratops have since been moved to their own genera Montanoceratops and Cerasinops, while new specimens of L. gracilis include bonebed remains from the Hell Creek Formation of Montana and a partial skeleton from the Lance Formation of Wyoming. Together with related taxa, Leptoceratops is the eponymous genus of the family Leptoceratopsidae. Leptoceratops is known from more than ten individuals, all from Maastrichtian deposits of Alberta, Montana and Wyoming, representing the entire skeleton.
Multiple unusual features can be seen in the skeleton of Leptoceratops, which has a mixture of primitive and derived ceratopsian features and is around long. The head is very large with a strong jaw, but lacks horns and has a very reduced frill. The forelimbs and hindlimbs are robustly built, and Leptoceratops was likely bipedal when moving at speed and quadrupedal when moving slowly. The vertebrae of the tail were high-spined, though not as high as those of its relative Montanoceratops, and the pectoral and pelvis girdle bones were slender and more like earlier ceratopsians. The teeth of Leptoceratops are unique among dinosaurs, showing tooth wear in a fashion that must have been driven by mammal-like rotation of the jaw while chewing. This, along with the handling of stress in the jaws, show that Leptoceratops had an efficient bite allowing it to be adaptable to different food types, such as the angiosperms, conifers, or cycads found in its environment.
The environment inhabited by Leptoceratops was a semi-humid floodplain region with regular braided streams and small-treed forests. The climate was cool in the foothills of the mountainous cordillera, but the range of Leptoceratops also extended into coastal plains where it lived alongside much larger herbivorous dinosaurs. It is possible that within these environments, Leptoceratops dug and lived in multi-generational burrows. Leptoceratops was not a common component of the dinosaur fauna it lived in, but coexisted with various dinosaurs and other animals.
Discovery and species
A 1910 expedition by the American Museum of Natural History (AMNH) discovered two ceratopsian specimens from the Maastrichtian age 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 described in 1914 by American palaeontologist Barnum Brown. The name translates as "slender small horn face", with the generic name derived from the Ancient Greek word leptos, 'small', and Ceratops, 'horned face', though Brown did not explain its etymology. The specimen preserves parts of the skull and jaws, a complete right forelimb, partial hindlimbs, 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 , three , and one caudal vertebrae. Both individuals are catalogued under the specimen number AMNH 5205, with the second individual being slightly larger. The 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 above the base of the Upper Edmonton member, and CMN 8889 was an additional above them. 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. 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). 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.
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. 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.
American palaeontologist Charles W. Gilmore led three expeditions of the United States Geological Survey to the Two Medicine Formation of Montana in 1913, 1928 and 1935, discovering two specimens in the third expedition that he later referred to Leptoceratops. The material, which is fragmentary but clearly similar to other Leptoceratops remains, also included a complete foot for the first time, along with other elements not represented in other specimens. The older age of the Two Medicine material than that from Scollard or St. Mary River suggested that Gilmore's specimens, United States National Museum numbers 13863 and 13864, could belong to a distinct species, but the fragmentary nature meant he did not give it a name and left it as indeterminate Leptoceratops species. These specimens were later removed from Leptoceratops by Sternberg in 1951, and then later referred to its close relative Cerasinops when it was named in 2007 by American palaeontologists Brenda J. Chinnery and John R. Horner based on an 80% complete skeleton also found in the Two Medicine Formation.
A Leptoceratops locality was found in 1962 within the Lance Formation of Wyoming by Michael Ramus, where a single individual was collected. This specimen, Princeton University (now Yale Peabody Museum) number 18133, preserves a nearly complete hindlimb, pelvis, and tail alongside some isolated teeth. Described in 1978 by American palaeontologist John Ostrom, the skeleton is very similar to the material assigned to Leptoceratops from Alberta, while also displaying differences to both Montanoceratops and other former Leptoceratops material from Montana. An isolated tooth (AMNH 2571) from the Pinyon Conglomerate of Teton County, Wyoming was also referred to Leptoceratops, but not L. gracilis, by Ostrom. The assignment of YPM VPPU 018133 to Leptoceratops was questioned by Chinnery in 2004, as the jugal is similar to that described for Prenoceratops and the locality may be part of the Meeteetse Formation instead.
Leptoceratops was first found in the Hell Creek Formation of Montana, equivalent in age to the Scollard and Lance Formations of Alberta and Wyoming, in 1992 with the discovery of a rear part of a skull by Brig Konecke. The skull, University of Wisconsin-Madison Geology Museum number 200, was prepared in 1998 and shows the first complete braincase yet described. An isolated tooth, UWGM-201, was also found. UWGM-200 was found within a thick cross-bedded sandstone containing unionid bivalve shells from the upper Hell Creek, and UWGM-201 was found below the Cretaceous-Paleogene boundary in the middle Hell Creek.
An isolated dentary of a protoceratopsian, TMP 95.12.6, was found in 1995 by Canadian palaeontologist Philip J. Currie in the middle Dinosaur Park Formation of Alberta, which is middle Campanian in age. It was first described as Leptoceratops sp. by Michael J. Ryan and Currie in 1998 despite being substantially older than other Leptoceratops material then known, as the only other North American protoceratopsian named, Montanoceratops, showed clear differences in anatomy from the Dinosaur Park specimen. Additional descriptions of other new leptoceratopsids following 1998 led Ryan, Currie and colleagues to redescribe TMP 95.12.6 in 2012 as the new taxon Unescoceratops.
Description
Leptoceratops was an unusual early ceratopsian, displaying a very large head but minimal , a more forwards center of mass suggesting quadrupedality, and a higher number of vertebrae in front of the . All the described specimens of Leptoceratops are of a similar size, but the type (AMNH 5205) and one referred specimen (YPM VPPU 18133) appear to be larger than the three CMN specimens, with CMN 8887 being the smallest. The complete skeleton of CMN 8889, with the tail of CMN 8887 added and scaled up to match the other bones, measures in length. The largest individuals of Leptoceratops were likely around long and weighed . This is similar in size to some other early ceratopsians like Cerasinops, Zhuchengceratops, Prenoceratops, Montanoceratops and Protoceratops, but smaller than the large Udanoceratops which was around long and .
Skull