As a stegosaur,
Baiyinosaurus would have been a quadrupedal herbivore with a paired row of a combination of large plates and spines running along the top of the animal from the neck to the tail tip. While the
Baiyinosaurus fossil material generally resembles stegosaurs in morphology, several features are also reminiscent of basal
thyreophorans such as
Scelidosaurus and
Emausaurus, indicating that
Baiyinosaurus had both
plesiomorphic thyreophoran traits as well as
derived stegosaurian traits. The holotype specimen of
Baiyinosaurus belongs likely belongs to an adult individual. This can be determined as the
neurocentral sutures on the dorsal vertebrae are fused and not visible, while they would be open and unfused in a juvenile.
Skull Baiyinosaurus is known from several skull bones. The
posterior subnarial process of the left
premaxilla (upper jaw bone at the tip of the snout) is preserved and has an expanded surface where it would contact the
nasal. The front and middle part of the left
maxilla (primary tooth-bearing bone of the upper jaw) is known, and indicates that this tooth row is inset
medially (toward the middle of the skull), a feature expected in most ornithischian dinosaurs. A partial bone—likely the right
jugal—is known, and has a triradiate form with a very expanded
dorsal process. Another fragment—tentatively identified as the dorsal part of the right
squamosal—includes visible sutures where it would be overlapped by the
parietal and
postorbital. The paired
frontals (large bones of the skull roof) are generally broad and shorter from front to back when compared to other stegosaurs besides
Tuojiangosaurus. This shape is also similar to the basal non-stegosaurian thyreophorans
Scelidosaurus and
Emausaurus. The describing authors identified this structure of the frontal—being wider than long and contributing significantly to both the medial and anterior margins of the
supratemporal fenestrae—as an
autapomorphy (unique derived trait) of
Baiyinosaurus. The left
dentary (tooth-bearing bone of the lower jaw) is largely complete. It curves down and inward at the front. While a
predentary (lower beak bone) is not preserved, there are visible facets for it on both sides of the tip of the dentary. Similar to many basally-branching stegosaurs like
Huayangosaurus,
Gigantspinosaurus, and
Kentrosaurus, the first dentary tooth is immediately behind the predentary, without the wide
diastema (gap) present in later taxa like
Stegosaurus and
Jiangjunosaurus. The dentary preserves eighteen
alveoli (tooth sockets), some of which have partially erupted teeth visible. For both of the two teeth that are entirely preserved, the crown shows two
denticles at the tip and seven denticles on both the front and back edges. This might be the condition for all of the dentary teeth, although the number of denticles for some other stegosaur species varies depending on the side.
Postcranial skeleton The postcrania of
Baiyinosaurus comprises only bones from the
axial skeleton. No
osteoderms are known. The intercentrum of the
atlas (first
cervical vertebra) is the only cervical vertebra preserved, and it notably lacks the two ridges seen on the underside in
Stegosaurus. The seven dorsal vertebrae (two anterior, five mid-posterior) have a typical combination of features. In most stegosaurs, the
neural arches are very expanded dorsally, but like the more basal taxa
Huayangosaurus and
Gigantspinosaurus the arches are not significantly elongated. In the middle/posterior dorsals, the
parapophyses (processes that articulate with the ribs) are elevated on short stalks, similar to the anatomy seen in
Bashanosaurus and
Scelidosaurus. The
neural spines are comparatively broad from front to back in
lateral view compared to other stegosaurs, but generally similar to
Gigantspinosaurus and the non-stegosaurs
Scelidosaurus,
Laquintasaura and
Lesothosaurus. The
neural canal is ovate in cross-section. One anterior caudal vertebra is known. It is missing the
zygapophyses (articular processes) and neural spine. There is a facet for a
chevron at the base of the
centrum. The neural canal is round in cross-section. == Classification ==