The Ornithopoda was one of the longest-lived groups of dinosaurs.  Members of this group had a great range in size from 1-2 meters to more that 12 meters.  The discovery of the teeth of Iguanodon in 1822 is often considered the start of dinosaur discoveries in the western world.  Iguanodon was named in 1825 and became one the first members of the newly named “dinosaurs” in 1842.  In 1878, 31 complete Iguanodon skeletons were found.  Some hadrosaurs have provided tremendous fossil finds including small delicate bones, ossified tendons, mummified skin, and even stomach contents.  Ornithopods are known from every continent including Antarctica and from both the North Slope of Alaska and the southern coast of Australia.
  The Heterodontosauridae, “different teeth lizards”, are represented by the turkey-sized Early Jurassic Heterodontosaurus and Lanasaurus.  They had different types of teeth: cutting incisors, grinding cheek teeth, and, at least in males, canine-like teeth or tusks (in both upper and lower jaws).  Given these derived tooth characters, they were probably not ancestral to later groups but they retained many primitive features of the basal ornithischians.  They had grasping hands, their tail had not yet stiffened, and they were capable of both bipedal and quadrupedal locomotion.  Their carpus (wrist) more ossified than in other groups and they possessed an additional wrist bone (the pisiform) which is interesting since most other archosauromorphs had lost it (or at least it wasn’t ossified so that it didn’t preserve well) (Lambert, 1990, Fastovsky, 1996).

      The species of the family Hypsilophodontidae are grouped into 7 genera lasting from the Middle Jurassic to the Late Cretaceous.  Hypsilophodonts measured 2-4 meters and are known from 4 continents. They descended from fabrosaurs and are ancestral to hadrosaurs and iguanodontids.  They still had the primitive feature of teeth in premaxillary bone and had a joint in upper jaw between maxillary and premaxillary bones allowing sides of upper jaws to rotate outwards when jaws were closed.  They possessed a horny beak for cropping plants and teeth were regularly replaced.  Leaellynasauria was a hysilophodont that lived close to the South Pole of the Mesozoic.  It had large optic lobes in its brain, suggesting that it had enhanced vision to allow it to see in the long polar nights (Lambert, 1990, Fastovsky, 1996).

      The dentition of Hypsilophodon is very similar to that of Lesothosaurus except that the cheek teeth can interlock to some degree (Norman, 1984). Hypsilophodon possessed 5 premaxillary teeth, 10 or 11 maxillary teeth, 13 or 14 dentary teeth.  Its fifth metatarsal was no more than a splint of a bone (Weishampel).

hypsilophodon hypsilophodon skeleton
     Thescalosaurus “wonderful lizards”appeared in the Late Cretaceous, and was among the last dinosaurs.  Thescelosaurs is a primitive ornithopod, a little more advanced than Hypsilophodon. (Weishampel, 1992).
  In the family Dryosauridae, “oak lizards”, the front of the upper jaw was toothless.  These were transitional forms linking earlier ornithopods to iguanodontids.  Tenontosaurus was a little more advanced than Hypsilophodon and a little more primitive than Dryosaurus  (Weishampel, 1992).
  In the family Camptosauridae, the toes and fingers ended with small hooflike claws.  Some had the beginnings of the spiked thumbs that became better developed in the iguanodontids.  Their teeth were densely packed.  Camptosaurus, “bent lizard”, possessed a sharp, horny, toothless beak (Lambert, 1990, Fastovsky, 1996).
camptosaurus camptosaurus skull
     The Iguanodontidae include primitive forms plus the hadrosaurs.  They had long snouts and toothless beaks.  Their arms were larger than in camptosaurids, and large species depended more on quadrupedal locomotion.  They may have had a prehensile tongue.  In the iguanodonts, the premaxillary and predentary were wider than in the more primitive ornithopods.  There are many more teeth as well, with 20-29 maxillary teeth and 19-25 dentary teeth (Norman, 1984).
Iguanodon skeleton igunaodon skeleton 2
iguanodon skull
     Iguanodon “iguana teeth”, was one of the first recognized dinosaurs.  The first fossil finds were thought to be pre-Flood monsters.  Originally, Iguanodon was represented as standing and leaning on tail but this would have caused tail to break.  Tracks indicate that could walk on all fours and mass accumulations of bones indicate that iguanodonts could travel in groups.  Hooves were located on the fingers in addition to a grasping 5th finger (see illustration below).  The spike thumb served as a weapon (in early representations, this was thought to be a nose horn) (Lambert, 1990, Fastovsky, 1996).
iguanodon hand iguanodon skull 2
One species assigned to the genus Iguanodon differs considerably in its snout.
     A number of the primitive iguanodonts had fused bones in their thumbs which formed spikes that could have been used as weapons.  Iguanodon, Ouranosaurus, and (to a degree) Camptosaurus had opposable fingers in addition to their pointed thumb.  Ouranosaurus and Probactrosaurus had back sails whose function is unknown.  It is possible that the sail of males was larger and was used in display.
ouranosaurus Probactrosaurus

       The more advanced ornithopods were better adapted for eating plants than were the earlier groups with a number of jaw modifications.   In primitive ornithopods, the dentary bone starts just posterior to the predentary (a condition retained in Iguanodon, Bactrosaurus, Probactrosaurus, and Telmatosaurus) while there was a space or diastema between these two in Ouranosaurus, Protohadrosaurus, and the higher hadrosaurs.  The number of tooth positions gradually increased.  The dentary tooth positions numbered 10-16 in primitive ornithopods,  21-25 in Iguanodon species, 23 in Ouranosaurus, 30-31 in Telmatosaurus (the most primitive hadrosaur),  30-41 in Lambeosaurinae (hadrosaurs) and up to 50 in some members of Hadrosaurinae (hadrosaurs) (Casanovas, 1999).  Maxillary tooth counts varied in iguanodonts according to size, age, and species.  The maximum maxillary tooth counts was 23 in Iguanodon atherfieldensis, 29 in Iguanodon bernissartensis, 14 in Camptosaurus, 22 in Ouranosaurus, and 23 in Probactrosaurus (Weishampel).

     Not only was there an increase in the number of tooth positions along the jaw, but also in the number of replacement teeth which were present at any one time.  The number of teeth per position was 2 in lower ornithopods (1 tooth and 1 replacement); 2 in the front teeth and 3 in the back teeth of the iguanodont Altirhinus, 3 in Bactrosaurus, and  3-5 in hadrosaurs.  In ancestral ornithopods there was no dental battery.  In iguanodontids, the teeth support each other but were not cemented together.  Hadrosaurs added cement to their tooth rows (Casanovas, 1999).