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 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 unlike more derived ceratopsians, nor on the . Unlike
Protoceratops there are no teeth in the of the snout, though there are 17 teeth in both the and . 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 of the cheek are flared to the side and are capped by an ossification, the , which is unique for the group. The bone, which forms an eyebrow in ornithischians, is only small and loosely positioned within the eye socket. The 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 , an unsplit ridge on the , and an elevated articulation for the palate on the . The
cranial nerves of
Leptoceratops have the same arrangement patterns as those of
Montanoceratops,
Triceratops and
Chasmosaurus mariscalensis. bones are known in
Leptoceratops, only otherwise preserved in specimens of
Centrosaurus and
Protoceratops among ceratopsians. The of CMN 8889 is well preserved and has been separated from the rest of the skull by the removal of
matrix during preparation so its anatomy can be more easily seen. The , which forms a lower beak, is shorter than in any more advanced ceratopsian, though the union with the dentary is strong. The dentary is short and deep in
Leptoceratops, similar to
Protoceratops and most other leptoceratopsids except for
Montanoceratops, in which it is longer and straighter. There is a smooth beveling on the dentary to form the matching surface of the cheek to the maxilla above. When articulated, the mandibles of
Leptoceratops are similar to other basal neoceratopsians in being very wide with a lowered articular surface for the skull, reflective of different feeding adaptations during the evolution of ceratopsians where the closing of the jaws brings all the teeth into contact at the same time. The teeth in
Leptoceratops are some of the most distinct in Ceratopsia, showing a unique type of
tooth-on-tooth wear only shared with taxa such as
Udanoceratops and
Archaeoceratops.
Leptoceratops teeth are also the largest in absolute size of any ceratopsian with single-rooted teeth; the tooth roots of ceratopsids are split to be double-rooted. The facet formed by 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. 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 .
Dental enamel is present on both sides of the crown.
Postcranial skeleton The entire skeleton of
Leptoceratops is known thanks to the preservation of multiple specimens. Complete vertebral columns of CMN 8888 and 8887 are and long respectively, with nine , thirteen , and six vertebrae in the . The number of caudal vertebrae varies, with 38 in CMN 8887, 48 in CMN 8888, and 48 or 49 in YPM VPPU 18133. The first three cervicals are not fused together into a until adulthood, which is distinctly more primitive than
Protoceratops and ceratopsids. in cervical vertebrae are more slender and longer in
Leptoceratops than
Protoceratops, but the vertebrae are otherwise similar. The dorsal vertebrae of
Leptoceratops are more distinct, as the neural spines are far more slanted and, together with the , overhanging the . The rib articulations are also longer than in
Protoceratops. The number of fused sacral vertebrae is variable between individuals, but there are six sacral vertebrae based on the anatomy of rib articulations of the last dorsal and first caudal. The neural spines of the caudal vertebrae are very elongate, reaching two to four times the height of the centrum; when
Leptoceratops was first described, they were the longest of any ceratopsian known at that time. However, the neural spines of the more recently discovered
Montanoceratops exceed five to eight times their centrum height. Neural spines are present on all but the last five caudals, and are increasingly sloped towards the end of the tail. The number of caudal ribs varies between specimens, with ribs persisting to the 18th caudal in the CMN specimens, but only as far as the seventh caudal in YPM VPPU 18133. All vertebral bodies but the last eight are taller than long. and of the pectoral region are known in
Leptoceratops; the sternals are comparable to those of
Centrosaurus and
Chasmosaurus in shape, but clavicles are absent in ceratopsids and non-ceratopsian ornithischians. 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 is more slender than in ceratopsids but similar to
Protoceratops while lacking the ridge that extends diagonally across the scapular blade found in the other genera. The , , and are intermediate between
Protoceratops, where they are more slender, and ceratopsids, where they are more robust. The humeri, long in AMNH 5205, long in CMN 8889, long in CMN 8888, and long in CMN 8887, are shorter proportionally and more robust than in
Protoceratops. 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. The hand is complete, displaying two rows of , and three digits with hooves on the first three. The two proximal carpals, the and , which articulate with the ulna and radius, respectively, are large, while the three distal carpals are small and irregular. The are more compact than those of
Triceratops, but show the ability for free movement of the fingers, unlike in ceratopsids. The manus has a phalangeal formula of 2-3-4-3-1. The 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 substantially and is unexpanded, and the posterior process narrows gradually, being much narrower than that of
Montanoceratops. The is similar but proportionally smaller than in
Protoceratops, with a short and narrow anterior process and a curved and tapered posterior process that is only in length. The is also similar to that of
Protoceratops, but does not show any expansion towards its end or internal groove near the base. The ischia 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 primitive. The , which is long in CMN 8889, long in CMN 8888, long in CMN 8887, and over long in YPM VPPU 18133, is always slightly shorter than the , which is unlike the heavier ceratopsids. The trochanters for muscle articulations are pronounced. There are two distal in
Leptoceratops, one capping the third and one capping the fourth. Overall the foot is longer proportionally than in
Protoceratops. 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. ==Classification==