Extinct genus of ceratopsian dinosaur
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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, but coexisted with the herbivorous "Ankylosaurus", "Edmontosaurus", "Pachycephalosaurus", "Triceratops" and "Thescelosaurus" among other ornithischians, and theropods including dromaeosaurids, troodontids, "Ornithomimus", "Elmisaurus", an alvarezsaurid and "Tyrannosaurus". Mammals are known from diverse forms that lived alongside "Leptoceratops", and there are also fishes, amphibians, turtles, crocodilians, pterosaurs, and birds known.
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 ;dfn id="" ;presacral ;/dfn ;, three ;dfn id="" ;sacral ;/dfn ;, 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 ;dfn id="" ;frill ;/dfn ;, a more forwards center of mass suggesting quadrupedality, and a higher number of vertebrae in front of the ;dfn id="" ;pelvis ;/dfn ;. 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.
The skull of "Leptoceratops" is similar to "Protoceratops" in general form, but is lower and longer and uniquely lacks a crest at the back of the skull to make a prominent frill. The complete skulls known range from long in CMN 8889 to long in CMN 8887, with respective mandibles and long. Extrapolating the complete CMN skulls to the preserved material of AMNH 5205 and UWGM-200 gives lengths of and respectively. The skull bones of CMN 8889 are fused suggesting it is an adult, while those of CMN 8887 and UWGM-200 are unfused indicating they may not yet be fully grown. There is no horn on the ;dfn id="" ;nasal ;/dfn ; unlike more derived ceratopsians, nor on the ;dfn id="" ;postorbital ;/dfn ;. Unlike "Protoceratops" there are no teeth in the ;dfn id="" ;premarillary bone ;/dfn ; of the snout, though there are 17 teeth in both the ;dfn id="" ;maxilla ;/dfn ; and ;dfn id="" ;dentary ;/dfn ;. There is a large ridge above the teeth on the maxilla, which would have supported a large muscled cheek pouch to assist in shredding large amounts of vegetation. As in other ceratopsians, the ;dfn id="" ;jugal bones ;/dfn ; of the cheek are flared to the side and are capped by an ossification, the ;dfn id="" ;epijugal ;/dfn ;, which is unique for the group. The ;dfn id="" ;palpebral ;/dfn ; bone, which forms an eyebrow in ornithischians, is only small and loosely positioned within the eye socket. The ;dfn id="" ;parietal ;/dfn ; bones of the rear skull, which in "Protoceratops" are elongated to form a frill and bear fenestrae, are reduced and solid in "Leptoceratops". The anatomy of the braincase of "Leptoceratops" is similar to that of "Montanoceratops" with minor differences, including a restricted neck to the ;dfn id="" ;occipital condyle ;/dfn ;, an unsplit ridge on the ;dfn id="" ;supraoccipital ;/dfn ;, and an elevated articulation for the palate on the ;dfn id="" ;exoccipital ;/dfn ;. The cranial nerves of "Leptoceratops" have the same arrangement patterns as those of "Montanoceratops", "Triceratops" and "Chasmosaurus mariscalensis". ;dfn id="" ;Hyoid ;/dfn