Leptoceratops

Leptoceratops Temporal range: Late Cretaceous (Maastrichtian), 67.2–66.2 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Fossils CMN 8888 and CMN 8887 at the Canadian Museum of Nature, Ottawa
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	†Ornithischia
Clade:	†Neornithischia
Clade:	†Ceratopsia
Family:	†Leptoceratopsidae
Genus:	†Leptoceratops Brown, 1914[1]
Species:	†L. gracilis
Binomial name
†Leptoceratops gracilis Brown, 1914[1]

Leptoceratops (meaning 'small horn face') is a genus of leptoceratopsid 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. Additional specimens found in the Scollard include a fully complete skeleton and two other very complete individuals alongside it, uncovered in 1947 by Charles M. Sternberg. Specimens from Montana that were among some of the earliest material referred to Leptoceratops have since been moved to their own genera Montanoceratops and Cerasinops, while new material including bonebed remains from the Hell Creek Formation of Montana, and a partial skeleton from the Lance Formation of Wyoming, have been referred to L. gracilis. Combined, Leptoceratops is known from more than ten individuals all from the Maastrichtian deposits of Alberta, Montana and Wyoming, with the complete skeleton represented.

Multiple unusual features can be seen in the skeleton of Leptoceratops, which has a mixture of primitive and derived ceratopsian features and is around 2 m (6.6 ft) long. The head is very large with a strong jaw, but lacks horns and a frill. The forelimbs and hindlimbs are robustly built, and Leptoceratops was likely bipedal when moving at speed, or quadrupedal when moving slowly. The vertebrae of the tail were high-spined, though not as high as its relative Montanoceratops, and the pectoral and pelvis girdle bones were slender and more like earlier ceratopsians. The teeth of Leptoceratops are especially unique, 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.

A 1910 expedition by the American Museum of Natural History discovered two specimens of ceratopsian from the Edmontonian 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. The individual preserving a partial skull, collected and accessioned as AMNH specimen 5205, was designated as the holotype of the new taxon Leptoceratops gracilis when it was described in 1914 by American palaeontologist Barnum Brown.^[1] The name translates as "slender small horn face", though Brown did not explain its etymology.^[2] AMNH 5205 preserved parts of the skull and jaws, a complete right forelimb, partial hindlimbs, and a series of 24 vertebrae from the tail, as well as two other isolated vertebrae. The second individual, also included under AMNH 5205 despite being slightly larger, also preserved part of the forelimb, as well as four presacral, three sacral, and one caudal vertebrae.^[1] The locality that Leptoceratops was first found in, originally described as the Edmonton Formation only, was identified in 1951 by Canadian palaeontologist Charles M. Sternberg as being from the Upper Edmonton member based on location information given, as the exact quarry was lost. Sternberg in the same publication described three skeletons of Leptoceratops from the Edmonton Formation, including one fully 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 a first specimen, 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 a third complete individual directly beside the latter was uncovered during excavation (CMN 8887). CMN 8888 and 8887 were 15 m (50 ft) above the base of the Upper Edmonton member, and CMN 8889 was an additional 1.5 m (5 ft) above them.^[3] These specimens were complete enough to allow Canadian palaeontologist Dale A. Russell to describe and published a complete skeleton of Leptoceratops based only on the CMN individuals.^[4] 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).^[5] Along with the described types and plesiotypes, known Leptoceratops specimens from the Scollard also includes the partial skeletons Royal Tyrrell Museum of Palaeontology 93.95.1 and 95.86.1.^[6]

Brown collected additional remains referred to Leptoceratops in 1916 from the St. Mary River Formation of Montana, AMNH 5464. This specimen, including a very good skeleton and a fragmentary skull, was prepared in 1918 at the AMNH and then mounted in 1935 to be put on display, with the fragmentary skull being completely modelled out of plaster for the mount. 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. The description of the material alongside the anatomy of the other specimens known lead to Brown and Schlaijker concluding that Leptoceratops was a very close relative of Protoceratops that together should be united within Protoceratopsidae.^[7] 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 separate from Leptoceratops, which Sternberg named Montanoceratops in 1951.^[3]

American palaeontologist Charles W. Gilmore led three expeditions of the US Geological Survey to the Two Medicine Formation of Montana in 1913, 1928 and 1935, discovering two specimens in the third expedition he described as Leptoceratops. The material, fragmentary but showing clear similarities to the earlier-known remains, also included a complete foot for the first time, along with other materials that supplemented the known anatomical regions. The separate stratigraphic position from both the Alberta and St. Mary River material 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.^[8] These specimens were later removed from Leptoceratops by Sternberg in 1951,^[3] 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 for an 80% complete skeleton also found in the Two Medicine Formation.^[9]

A Leptoceratops locality was found in 1962 within the Lance Formation of Wyoming by Michael Ramus, where a single individual was collected, Princeton University (now Yale Peabody Museum) number 18133. This specimen preserves a nearly complete hindlimb, pelvis, and tail alongside some isolated teeth. Described in 1978 by American palaeontologist John Ostrom, the skeleton is extremely 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 from the Pinyon Conglomerate of Teton County, Wyoming was also referred to Leptoceratops, but not L. gracilis, by Ostrom.^[10] The assignment of YPM VPPU 018133 to Leptoceratops was questioned by Chinnery in 2004, as the jugal is very similar to that described for Prenoceratops and the locality may be part of the Meeteetse Formation instead.^[11]

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 partial rear skull by Brig Konecke. The skull, University of Wisconsin-Madison Geology Museum number 200, was prepared in 1998 to show the first complete unfused braincase to be described. An isolated tooth, UWGM-201, was also found. UWGM-200 was found within a 1 m (3.3 ft) cross-bedded sandstone containing unionid bivalve shells from the upper Hell Creek, and UWGM-201 was found 75 m (246 ft) below the K-T boundary in the middle Hell Creek.^[12]

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.^[6] Additional descriptions of other new leptoceratopsids following the description of the specimen led Ryan, Currie and colleagues to redescribe TMP 95.12.6 in 2012 as the new taxon Unescoceratops.^[13]

Leptoceratops was an unusual ceratopsian, displaying a very large head but minimal frill, with a forwards center of mass that may suggest quadrupedality, and a higher number of vertebrae in front of the pelvis than other early ceratopsians.^[4] All the described specimens of Leptoceratops are of a similar size, but the type AMNH 5205 and referred PU 18133 specimens appear to be slightly larger than the three CMN specimens, with CMN 8887 being the smallest.^[3][10] The complete skeleton of CMN 8889, with the tail of CMN 8887 added and scaled up to match the other bones, measures 1.81 m (5.9 ft) in length,^[4] so Leptoceratops was likely around 2 m (6.6 ft) long and weighed 100 kg (220 lb). 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 4 m (13 ft) long and 700 kg (1,500 lb).^[14]

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 frill. The complete skulls known range from 450 mm (18 in) long in CMN 8889 to 330 mm (13 in) long in CMN 8887, with respective mandibles 345 mm (13.6 in) and 275 mm (10.8 in) long.^[3] Extrapolating the complete CMN skulls to the preserved material of AMNH 5205 and UWGM-200 gives lengths of 529 mm (20.8 in) and 468–540 mm (18.4–21.3 in) 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.^[12] There is no horn on the nasal unlike more derived ceratopsians, nor on the postorbital. Unlike Protoceratops there are no teeth in the premarillary bone of the snout, though there are 17 teeth in both the maxilla and dentary. 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 jugal bones of the cheek are flared to the side, and the unique epijugal ossification is present. The palpebral bone, which forms an eyebrow in ornithischians, is only small and loosely positioned within the eye socket. The parietal bones of the rear skull, which in Protoceratops are elongated to form a frill and bear fenestrae, are reduced and solid in Leptoceratops.^[3] The anatomy of the braincase of Leptoceratops is similar to that of Montanoceratops with minor differences, including a restricted neck to the occipital condyle, an unsplit ridge on the supraoccipital, and an elevated articulation for the palate on the exoccipital. The cranial nerves of Leptoceratops have the same arrangement patterns as those of Montanoceratops, Triceratops and Chasmosaurus mariscalensis.^[12] Hyoid bones are known in Leptoceratops, only otherwise preserved in specimens of Centrosaurus and Protoceratops.^[3]

The mandible of CMN 8889 is very well preserved, and has been separated from the rest of the skull. The predentary, which forms a lower beak, is shorter than in any more advanced ceratopsian, though the union with the dentary is strong.^[3] The dentary, which in Montanoceratops is longer and straighter, is very short and deep in Leptoceratops, similar to Protoceratops and most other leptoceratopsids.^[3][13] There is a smooth beveling on the dentary to form the matching surface of the cheek to the maxilla above.^[3] When articulated, the mandibles of Leptoceratops are similar to other basal neoceratopsians in being very wide, with a lowered articular surface for the skull to being the tooth rows more in line, reflective of different feeding adaptations during the evolution of ceratopsians.^[15] The teeth in Leptoceratops are some of the most distinct in Ceratopsia, being both the largest single-rooted ceratopsian teeth as well as showing a unique type of tooth wear only shared with taxa such as Udanoceratops and Archaeoceratops. The facet formed by tooth-on-tooth wear is nearly or fully vertical in the teeth of Leptoceratops, but only extends partway down the tooth so that there is a sharp and distinct shelf.^[3][16] The unworn surface of teeth of Leptoceratops shows a strong ridge down along the crown, which is slightly offset from central and often follows a curved path. Two to three weaker ridges are present parallel to that primary ridge in the teeth, with the entire ornamented region of the crown surrounded by a strong ridge called the cingulum. Dental enamel is present on both sides of the crown^[16]

The complete skeleton of Leptoceratops is known, with all anatomical regions represented by multiple specimens. Complete vertebral columns of CMN 8888 and 8887 are 1.66 m (5.4 ft) and 1.35 m (4.4 ft) long respectively, with nine cervical vertebrae, 13 dorsal vertebrae, six vertebrae in the sacrum, and 38 vertebrae of the tail in CMN 8887, 48 in CMN 8888, and 48 or 49 in YPM VPPU 18133.^[3][10] The first three cervicals of Leptoceratops are not fused together into a syncervical in all specimens but the adult, which is distinctly more primitive than Protoceratops and ceratopsids. Neural spines are more slender and longer in Leptoceratops than Protoceratops, but the cervical vertebrae are otherwise similar. Dorsal vertebrae of Leptoceratops show greater difference, as the neural spines are far more slanted, with the neural spines as well as the zygapophyses overhanging the main vertebral body. The rib articulations in Leptoceratops are also longer than in Protoceratops. The fusion of the vertebrae in the sacrum of Leptoceratops is inconsistent, but while vertebral fusion is variable the count of six vertebrae agrees with the fusion and rib articulation forms of both the last dorsal and first caudal.^[3] The caudal vertebrae of Leptoceratops show elongate neural spines, when it was first named they were the longest of any ceratopsian at two to four times the height of the centrum, but vertebrae known in Montanoceratops have been found that exceed five to eight times their centrum height. Neural spines are present on all but the last five caudals, and gradually slope more towards the end of the tail.^[3] Variability in the presence of ribs on the caudal vertebrae is present in Leptoceratops, with ribs persisting to the 18th caudal in CMN specimens, but only as far as the seventh caudal in YPM 18133. All vertebral bodies but the last eight have are taller than the are long.^[10]

Sternal plates and clavicles of the pectoral region are known in Leptoceratops, the former of which are comparable to those of Centrosaurus and Chasmosaurus in shape, while the latter were probably lost evolutionarily as they have not been found. The remainder of the forelimb is similar to other ceratopsians in general anatomy, showing similar areas for muscle attachment despite differences in bone form. The scapula is more slender than in ceratopsids but similar to Protoceratops while lacking the ridge found in the other genera. The humerus, radius, and ulna are intermediate between Protoceratops, where they are more slender, and ceratopsids, where they are more robust. The humeri of all specimens from Alberta can be compared: AMNH 5205 is 290 mm (11 in) long, CMN 8889 is 242 mm (9.5 in) long, CMN 8888 is 255 mm (10.0 in) long, and CMN 8887 is 185 mm (7.3 in) long. The radius of Leptoceratops is far more similar to ceratopsids, differing very little from Triceratops in form and strength of the ridge along its length.^[1][3] The hand is complete, displaying two rows of carpals, and three digits with hooves on the first three. The two proximal carpals, the ulnare and radiale which articulate with the ulna and radius respectively, are large, while the three distal carpals are small and irregular. Metacarpals are more compact than Triceratops, but show the ability for free movement of the fingers unlike ceratopsids. Leptoceratops has the manual phalangeal formula of 2-3-4-3-1.^[1]

The ilium of Leptoceratops is primitive for a ceratopsian, being similar to Protoceratops but thinner and without any expansion of its margin found in later ceratopsians. The anterior process of the ilium does not curve greatly and is unexpanded, and the posterior process narrows gradually, being much narrower than Montanoceratops.^[3] The pubis is similar but proportionally smaller than Protoceratops, with a short and narrow anterior process and a curved and tapered posterior process that is only 50 mm (2.0 in) in length. The ischium is also similar to Protoceratops, but does not show any expansion towards its end or internal groove near the base. They show a pronounced curvature along their length and are more slender than in Montanoceratops. As with the forelimb, the hindlimb is more robust than in Protoceratops, though the structure of the foot is equally as primitive. The femur, 270 mm (11 in) long in CMN 8889, 265 mm (10.4 in) long in CMN 8888, 230 mm (9.1 in) long in CMN 8887, and over 257 mm (10.1 in) long in YPM VPPU 18133, is always slightly shorter than the tibia, which is unlike the heavier ceratopsids.^[3][10] The trochanters for muscle articulations are distinct. There are two distal tarsals in Leptoceratops, one capping the third metatarsal and one capping the fourth. Overall the foot is longer proportionally than in Protoceratops.^[3] The first four digits are fully developed and bear claws, with the first being slightly outturned from the rest, while the fifth digit is reduced to just a splint-like metatarsal. The third metatarsal is the longest, followed by the second, fourth, and first, and the pes has a phalangeal formula of 2-3-4-5-0.^[3][10]

Leptoceratops was first named as a primitive member of Ceratopsia outside the family Ceratopsidae, with Brown suggesting that it and Brachyceratops may form their own family.^[1] This was not followed by Hungarian palaeontologist Franz Nopcsa, who instead placed Leptoceratops within its own subfamily of Ceratopsidae separate from Brachyceratops but joining Protoceratops, Leptoceratopsinae.^[17][18][19] A similar classification was supported by American palaeontologist Alfred Romer, who placed Leptoceratops separately within the new family Leptoceratopsidae in 1927, which joined Ceratopsidae and Protoceratopsidae within Ceratopsia.^[20] The description of new material of Leptoceratops led a similarity to Protoceratops being recognized, and the two genera, as well as Montanoceratops after its separation, were included within Protoceratopsidae.^[3][7][8]

Recognition that Leptoceratops and other early ceratopsians were not all united within Protoceratopsidae began with the descriptions of new genera from Asia since 1975 and the introduction of phylogenetics to ceratopsian classification.^[21] American palaeontologist Paul C. Sereno suggested in 1986 that Protoceratopsidae as traditionally used is paraphyletic, instead forming a successive acquisition of features closer to Ceratopsidae.^[22] This was supported by multiple phylogenetic analyses, where Leptoceratops was either unrelated to other forms,^[23] within Protoceratopsidae,^[24] or outside Protoceratopsidae but forming a clade with taxa like Montanoceratops, Udanoceratops, or Asiaceratops.^[25][26] The latter hypothesis gained support as studies progressed, with the name Leptoceratopsidae adopted for the group and its taxonomic content expanding to include a diverse variety of Late Cretaceous early ceratopsians, including both existing and newly described genera.^{[9][13][21][27][28][29]}

All previously published neoceratopsian phylogenetic analyses were incorporated into the analysis of Eric M. Morschhauser and colleagues in 2019, along with all previously published diagnostic species excluding the incomplete juvenile Archaeoceratops yujingziensis and the problematic genera Bainoceratops, Lamaceratops, Platyceratops and Gobiceratops that are very closely related to and potentially synonymous with Bagaceratops. While there were many unresolved areas of the strict consensus, including all of Leptoceratopsidae, a single most parsimonious tree was found that was most consistent with the relative ages of the taxa included, which is shown below, restricted to Leptoceratopsidae.^[30] Further phylogenetic analysis also adding the new leptoceratopsid taxa Ferrisaurus and Gremlin slobodorum have also been conducted, but the inclusion of these taxa and the fragmentary nature of many early ceratopsians results in there being very little resolution of leptoceratopsid relationships.^[31][32]

As Ceratopsidae evolved to be fully quadrupedal, the transition between bipedality and quadrupedality occurs within early ceratopsians. Interpretations have been that Leptoceratops was quadrupedal,^[1][4] but the first investigation specifically into the function of neoceratopsian forelimbs was published by Phil Senter in 2007, who looked at Psittacosaurus, Leptoceratops and Protoceratops. The forelimbs of Leptoceratops are long enough to reach the ground upright, allowing for quadrupedality. While in Protoceratops the radius could rotate around the forearm to allow pronation of the hand, this is not possible in Leptoceratops, limiting quadrupedal posture to when the hands spread to the sides. The forelimb would be held straight with the elbow flexed anteriorly and the thumb and second finger pointing forwards while the other digits splayed to the side, although a more lateral sprawl was possible. Leptoceratops lacks opposable digits to allow for one-handed prehension, but would have been able to hold with two hands when moving bipedally. Proportions of the fore and hindlimbs suggest that both Leptoceratops and Protoceratops could have been bipedal when moving at high speeds but reverted to quadrupedality when slowly moving, with Leptoceratops tending to be slightly more bipedal than Protoceratops.^[33] Scratch-digging is also believe to have been possible in Leptoceratops based on its forelimb anatomy,^[33] which is supported by fossils from the Hell Creek Formation suggesting burrowing. Three fossil bonebeds were discovered in the Hell Creek Formation containing large amounts of Leptoceratops specimens of various ages. Originally believed to represent a miring event, Denver Fowler and colleagues presented in 2019 the belief that the sites instead represent adults buried during burrow collapses in a multi-generational burrow. All three bonebeds are from the lower third of Hell Creek, where the environment was better-drained than the swampy upper Hell Creek, which lacks Leptoceratops.^[34]

Leptoceratops, like other ceratopsians, would have been a herbivore. Nutritious angiosperms were not widespread in the Early Cretaceous, and despite their diversity by the end of the Cretaceous they may not have been ecologically abundant, with conifers and ferns instead providing vegetation. The foliage of Equisetum provides large amounts of energy, Ginkgo contains high protein, and cycads and palms may have provided starch. Basal ceratopsians were obligate low browsers due to their smaller size, though large basal neoceratopsians could have consumed seeds and fruits not available to smaller counterparts. Ceratopsids have been found with cycad material in their teeth, showing that they did consume the plants, and large neoceratopsians could have used their strong bites to consume them as well.^[35]

The unique wear pattern on teeth of Leptoceratops is inconsistent with the standard chewing of neoceratopsians, indicating that the bite included an arcing path to create the curved tooth wear seen. The consistency of tooth wear shows that a powerful chewing stroke was used, with the power initiated by the same jaw adductor muscles in other ceratopsians, but with a smooth transition of power between the external and posterior adductor muscles, which was given the name circumpalinal by Frank Varriale in 2016 when he described the jaw motion of Leptoceratops. The mastication of Leptoceratops is different from all other dinosaurs, with the jaw undergoing an orbital motion that is mammal-like, despite the very large differences in jaw joint anatomy between Leptoceratops and analogous mammals.^[36] Biomechanical studies of the jaws of ceratopsians have also identified that Leptoceratops, as well as other leptoceratopsids, had efficient bites allowing them to be adaptable to different food types, not simply showing improvements along the evolution of ceratopsids.^[37][35]

Leptoceratops has been used as an indicator of faunal provinciality during the late Maastrichtian of North America, with Thomas Lehman identifying the "Leptoceratops fauna" in the Scollard, Lance and Pinyon Canyon Formations of Alberta and Wyoming, where Leptoceratops occurs, along with Triceratops and Tyrannosaurus, but Edmontosaurus and Alamosaurus are absent. Lehman first separated the fauna including Leptoceratops from the other Triceratops-bearing locations in 1987, identifying that while Triceratops overlaps in range with Leptoceratops, where the latter is found the former is less abundant than elsewhere. The sediments of these regions were alluvial plain and foothill, with the Leptoceratops fauna consequently inhabiting cool environments flanking the mountainous cordillera.^[38] However, Leptoceratops has since been found within the Hell Creek Formation that is part of the Triceratops/Edmontosaurus fauna, and its extremely small sample size of specimens (6% of the Scollard dinosaur fauna) instead preclude Leptoceratops from being a suitable diagnostic animal for a faunal province. With the Triceratops/Edmontosaurus fauna being from coastal plains, Leptoceratops is not as ecologically restricted as previously thought.^[12]

The Scollard Formation from which Leptoceratops is known was deposited from 66.88 million years ago until the end of the Maastrichtian and Cretaceous at 66.043 mya, with differences in sediment deposits suggesting that the lower Scollard Formation (Cretaceous deposits) lasted around 0.68 million years and the upper 0.16. The assemblage including Leptoceratops, Triceratops, and the ankylosaur Ankylosaurus is from the lower Scollard, while the large theropod Tyrannosaurus is from the upper. The Hell Creek Formation in contrast lasts from earlier in the Maastrichtian, approximately 67.20 mya, to the end. Middle and lower sections of Hell Creek would be older than the entire Scollard Formation and instead be equivalent to the underlying Battle Formation of southern Alberta.^[39]

Palaeoenvironments of the Scollard and Hell Creek formation show that the very end of the Cretaceous was intermediate between semi-arid and humid, with both formations showing braided streams and floodplains and meandering channels, that shifted to become more humid and wetland following the Cretaceous-Paleogene boundary.^[40] The formations where Leptoceratops fossils have been found represent different sections of the western shore of the Western Interior Seaway dividing western and eastern North America during the Cretaceous, a broad coastal plain extending westward from the seaway to the newly formed Rocky Mountains. These formations are composed largely of sandstone and mudstone, which have been attributed to floodplain environments.^[41][42][43] While slightly older floras were codominated by cycad-palm-fern meadows, by the time of the Hell Creek angiosperms were dominant in a forested landscape of small trees.^[44] Many fossil vertebrates are found in the Scollard Formation alongside Leptoceratops, including Chondrichthyes and Osteichthyes such as Palaeospinax, Myledaphus, Lepisosteus and Cyclurus, amphibians like Scapherpeton, turtles including Compsemys, indeterminate champsosaurs, crocodilians, pterosaurs and birds, a variety of theropod groups including troodontids, ornithomimids, the tyrannosaurid Tyrannosaurus, and ornithischians including Thescelosaurus, pachycephalosaurids, Triceratops and Ankylosaurus. Mammals are also very diverse, with multituberculates, deltatheridiids, the marsupials Alphadon, Pediomys, Didelphodon and Eodelphis, and the insectivorans Gypsonictops, Cimolestes and Batodon.^[43] Within the Hell Creek Formation of Montana, Leptoceratops lived alongside the dinosaurs including Thescelosaurus, pachycephalosaurids Pachycephalosaurus, Stygimoloch and Sphaerotholus, the hadrosaurid Edmontosaurus and possibly Parasaurolophus, ceratopsians like Triceratops and Torosaurus, the nodosaurid Edmontonia and ankylosaurid Ankylosaurus, multiple dromaeosaurids and troodontids, the ornithomimid Ornithomimus, the caenagnathid Elmisaurus, tyrannosaurids including Tyrannosaurus, an alvarezsaurid, and the bird Avisaurus.^[45]

• Media related to Leptoceratops at Wikimedia Commons
• Timeline of ceratopsian research