Size , Lewisuchus, and Silesaurus'', with human silhouettes for scale. '' In general, silesaurids were small compared to most Jurassic and Cretaceous dinosaurs.
Silesaurus opolensis, a species with well-understood skeletal anatomy, could reach a total length of around . Considering indeterminate fossils, some silesaurids were the largest dinosauriforms of their time. and
Zambia By the late Carnian, the carnivorous dinosaur
Herrerasaurus reached a similar size, and
theropod and
sauropodomorph dinosaurs achieved even greater sizes later on in the Late Triassic. Early Jurassic ornithischians were smaller than most silesaurids, hinting at a
miniaturization event across the
Triassic–Jurassic mass extinction.
Shoulder and forelimb '', showing preserved bones.|left Silesaurids are unusual in the sheer length of their forelimbs, which are equally as long as the hindlimbs in well-preserved taxa such as
Silesaurus and
Asilisaurus. This has led many paleontologists to conclude that silesaurids spent most of their time on all fours, a
quadrupedal stance. In contrast, all theropods were certainly
bipedal (walking on only their hindlimbs), which was also the case for
herrerasaurians, early
sauropodomorphs, and the early dinosauriform
Lagosuchus. Ornithischians were traditionally assumed to have a bipedal ancestor, since many small Jurassic representatives were bipeds with short forelimbs. If silesaurids are Triassic ornithischians, they hint at a more complex series of shifts between bipedal and quadrupedal mobility over the course of ornithischian evolution. The bones of the forelimb are slender, and projections for muscle attachment are small. Muscle reconstructions in
Silesaurus suggest that the forelimb had weak muscles but a rigid stance, more useful as a source of stability rather than mobility. All silesaurids are
propubic, meaning that the
pubis (front lower bone of the pelvis) points down and forwards. This is a standard reptile-like arrangement, also inherited by saurischian dinosaurs. It is a major difference from "traditional" ornithischians, which are
opisthopubic, meaning that the pubis is bent back to meet the
ischium (rear lower bone of the pelvis). ''|left The
ilium (upper bone of the pelvis, above the hip socket) has a thick ridge along the upper lip of the
hip socket. Additional ridges extend to the upper front and rear corners of the bone. The upper edge is the thinnest part of the ilium, and silesaurids are sometimes described as having a saddle-shaped ilium. There is a small blunt projection at the upper front corner of the ilium. This projection (known as a
preacetabular process) is much longer and narrower in "traditional" ornithischians. The lower portion of the ilium forms a wedge-shaped inner wall of the hip socket. In contrast, nearly all dinosaurs have a perforated hip socket with an open space instead of an inner wall. Among silesaurids, the ilium of
Kwanasaurus is an intermediate state, with a thin medium-sized preacetabular process and a concave lower ilium, leaving a small gap in the wall of the hip socket. The
head of the femur (the upper end which connects to the hip socket) is slightly offset from the shaft by an obtuse, straight-edged notch. In sulcimentisaurian silesaurids, the surface which directly fits into the hip socket is flattened. Silesaurids have a "pillar-erect" hip joint, where the femur supports the thickened upper lip of the hip socket. A similar hip structure is found in "
rauisuchians" and
aetosaurs, large Triassic reptiles more closely related to
crocodilians. On the other hand, most dinosaurs have an erect hip structure based on a
ball-and-socket hip joint, where the head of the femur is rounded, deeply embedded in the hip socket, and sharply offset from the shaft at a curved right angle. Both erect and pillar-erect hip structures allow for a narrow mammal-like gait, unlike the sprawling posture of many other reptiles. '', including an upper right femur (a-e) and lower left femur (f-i)|left Like other dinosauriforms, the upper half of the femur has a set of bony mounds and crests which provide muscle leverage. The
fourth trochanter, on the inner surface of the shaft, connects to muscles which pull the leg back. Silesaurids usually have a small fourth trochanter with a symmetrical ridge-like form. This is unlike the large hook-shaped fourth trochanter of "traditional" ornithischians. The
anterior trochanter, on the front surface of the femoral head, connects to muscles which splay the legs. Early silesaurids have a mound-like anterior trochanter connected to a
trochanteric shelf, a scar which wraps around the shaft of the femur. In later species the trochanteric shelf may disappear while the anterior trochanter is offset from the shaft by a cleft, similar to "traditional" ornithischians and most theropods. When seen from behind, the lower end of the femur has a long and wide groove separating its condyles (the two knobs which contribute to the knee joint) and continuing up the lower third of the shaft. The
tibia (shin bone) is shorter or equal in length to the femur, while in other early dinosaurs it is longer. Like other dinosauriforms, the ankle is simple and hinge-like. The foot is symmetrical with five toes, the middle toe (III) as the largest and the first and last toes (I and V) as the smallest. The
unguals (toe claws) are straight and slightly flattened.
Atreipus footprints only include impressions of the middle three toes of the foot, nearly identical to theropod footprints such as
Grallator. ==Classification==