Lesothosaurus diagnostics (Galton 1978, Early Jurassic, 2.2m) was originally considered a basal ornithopod. Sereno (1991) suggested it may be one of the most primitive of all ornithischian dinosaurs. Butler et al. (2008) proposed that Lesothosaurus was a basal member of Neornithischia (pachycephalosaurs, ceratopsians and ornithopods), or Thyreophora (stegosaurs + ankylosaurs). The large reptile tree nests Lesothosaurus with quadrupedal Scutellosaurus and stegosaurs, not ankylosaurs. Galton, Sereno, Butler et al. published before the discoveries of more basal ornithischians like Daemonosaurus and Chilesaurus, but Wikipeidia needs another excuse for these deletions.
Just slight differences here (Fig. 1)
from traditional imagery and interpretations, chiefly those of Sereno 1991.
This new interpretation clears up
some of the problems in the large reptile tree (now 694 taxa) that were preventing high bootstrap scores at certain nodes. A bit of posterior palpebral appears above the fused postorbital/postfrontal. You also find that in Stegosaurus. The naris is close to the ventral rim, atypical for basal phytodinosaurs and basal ornithischians, but convergently appearing in certain derived ornithischians. Likewise a toothless premaxilla developed several times in the Ornithischia.
Butler RJ, Upchurch P, Norman, DB 2008. The phylogeny of the ornithischian dinosaurs. Journal of Systematic Palaeontology 6 (1): 1–40. doi:10.1017/S1477201907002271.
Galton PM 1978. Fabrosauridae, the basal family of ornithischian dinosaurs (Reptilia:
Ornithopoda). Paläontolgische. Zeitschrift 52:138–59.
Knoll F, Padian K and de Ricqles A 2009. Ontogenetic change and adult body size of the early ornithischian dinosaur Lesothosaurus diagnosticus: implications for basal ornithischian taxonomy”. Gondwana Research online preprint: 171. doi:10.1016/j.gr.2009.03.010.
Sereno PC 1991. Lesothosaurus, “fabrosaurids,” and the early evolution of Ornithischia. Journal of Vertebrate Paleontology 11(2):168-197