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|There are two great groups of dinosaurs: saurischians (which include theropods and sauropods) and ornithischians. Ornithischians represent a monophyletic clade descended from basal forms of the Triassic. One of their most obvious features is the opisthopubic condition of the hip–the pubis is parallel to ischium and angles backward. This modification of the pelvis provided additional space for abdominal organs. A larger digestive tract would have allowed more area for the fermentation of plant matter and this was important for herbivores which did not reach the great sizes observed in sauropods. A few later genera developed a process on the pubic bone that faces forward. Note the differences between the pelves of the Saurischians (Tyrannosaurus and Barapsaurus) and the 2 Ornithischians in the images below (Lambert, 1990, Fastovsky, 1996).|
Ornithischians possessed an additional palpebral bone in the vicinity of the eye, probably on the eyelid. Crocodiles and many primitive tetrapods also had bones associated with eyelids or orbits. In primitive ornithischians, the supraorbital bone attaches other skull bones of the orbit. Some groups (iguanodontids and pachycephalosaurs) possessed 2 supraorbital bones.
Ossified tendons existed in the sacral region in most groups and stiffened the vertebral column (except, for example, in stegosaurs which had to swing their tails in defense). The tail was held horizontally and used for balance. Ossified tendons were not completely rigid and were capable of some stretching (Lambert, 1990, Fastovsky, 1996).
|Ornithischians had short necks and long tails. All were herbivorous and all had compressed tooth crowns of cheek teeth for grinding plant. In later groups these cheek teeth interlocked and the exposure of several rows of teeth at once formed “dental batteries” for grinding or slicing food. Ornithischians also possessed fleshy cheeks to hold plant material and large chewing muscles. In the image below, the tooth rows are inset, rather than being positioned at the lateral edges of the skull, creating cheeks outside the tooth row.|
All ornithischians had an unpaired predentary bone in the front of the lower jaw. The roughened surfaces of jaws indicate that later groups which lacked teeth in the front of their jaws possessed horny beaks on both their upper and lower jaws. Ornithischians cropped vegetation with the horny bill which was secured in the lower jaw by the predentary bone. The predentary also permitted the dentary bones to rotate while chewing (Sereno, 1999; Sereno, 1986). While sauropods fed on soft fern leaves or the newest leaves at the tops of trees, ornithischians were prepared to chew coarser material.
Note the multiple rows of interlocked cheek teeth in the dental batteries pictured below. Also note in Kritosaurus that there are no teeth in the front of the jaw and that the lower jaw ends with a single unpaired bone, the predentary.
Many ornithischians had dermal armor as did many of the earlier thecodonts and some sauropods. It is possible that some dermal armor was an ancestral condition for dinosaurs which was lost in many lineages.
From the basal ornithischians evolved a number of ornithischian groups. The first to diverge from the other lineages were the thyrephorans, or armored dinosaurs, composed of stegosaurs, nodosaurs, and ankylosaurs. The next group was the very successful group of ornithopods, composed of small primitive groups, iguanodonts, and hadrosaurs (“duck bill” dinosaurs). Finally the ornitischians produced the pachycephalosaurs and the ceratopsians or horned dinosaurs (Sereno, 1986).
The family “Fabrosauridae”, “Fabre’s lizards”, are the primitive, basal ornithischians from which other groups evolved. The dinosaurs included in the group “Fabrosauridae” has been revised since some were more closely related to other ornithischian groups than to each other.
These dinosaurs were bipedal and under 1 meter long. Bony
tendons stiffened the tail and the cheek teeth possessed ridges.
They did not have fleshy cheeks, unlike all higher ornithischians.
They lived from the early late Triassic to early Jurassic and are
possessed horn covering the tip of its snout and could rotate its lower
jaw while eating. Its forelimbs
are short, only about 40% the length of the hindlimbs (Sereno,
1991). Lesothosaurus had
a heterodont dentition (a dentition with different
kinds of teeth) with simple teeth in the premaxillary
bone and shorter serrated teeth in the maxillary (
Other basal ornithischians included Pisanosaurus and Technosaurus, both of which were also 1-2 meter bipedal herbivores. Pisanosaurus possessed 15 teeth in its lower jaw, two fewer than Lesothosaurus (Weishampel).
Pisanosaurus is known from the Triassic (Sereno,
ornithischians from the Late Triassic such as Fabrosaurus, Heterodontosaurus,
show similiarities with prosauropods (Bonaparte,