Here we've listed those basal ornithischians which do not belong to any of the main groups of bird-hipped dinosaurs. Scelidosaurus, Heterodontosaurus, Lesothosaurus, and (less-frequent) Scutellosaurus are the most common in dino-books; in older works you'll frequently also read the name "Fabrosaurus".
Size doesn't matter (just for once): Lesothosaurus
- When talking about Ornithischians, we can find the same issues of Saurischians: in the Triassic/Early Jurassic they were all so-similar each other, its hard task to classify them accurately. Nonetheless, they are extremely important animals for scientists, no matter their often tiny size. Other than the scelidosaurians and the heterodontosaurians (which make two distinct groups on their own), we have several other examples, among them Lesothosaurus, Eocursor, and Pisanosaurus. Let's start with Lesothosaurus diagnosticus, which was once considered the forerunner of all bird-hipped dinos, and thought not to belong to any great ornithischian group; recent research suggest it could be a very basal Thyreophoran, thus ancestor of Scelidosaurs, Stegosaurs, and Ankylosaurs. From Early Jurassic Southern Africa like the famous "three-kinds-of-teeth" Heterodontosaurus, its name derives from the Kingdom of Lesotho, a small South African enclave (once called Basutoland) where its remains were dug out in 1978; the species name diagnosticus underlines its importance to understand early ornithischians evolution. Merely 3 ft long, even smaller than the already-small Heterodontosaurus, and with a more gracile frame with smaller head and forelimbs, Lesothosaurus was about the bulk of a Compsognathus weighing only 3-4 kg. Unlike the scelidosaurians and the heterodontosaurians, it seems not to have any specialization in its anatomy. Its mouth had simple teeth not apt for proper grinding but only to tear vegetation off to the plants (just like the contemporaneous prosauropods); it probably had only small cheeks, but had already the lower toothless bill (technically, the "predental bone") which is actually the main hallmark of every ornithischian (other than the shape of the pelvic bones of course). Its forelimbs were short and five-digited, its hindlimbs apt for running, its tail long and flexible (its vertebrae lacked the bony tendons of more evolved birdhipped dinos) and it not shows signs of armor on its body. Finally, its pelvis lacked the "prepubis", a forward-pointing prominence of the pubis typical of all the main/most evolved ornithischian lineages (the scelidosaurs and heterodontosaurs also lacked it). Because of the body-shape popular dino-books often give to it, it was said that Lesothosaurus "resembles a lizard more than any other dinosaur", but in other illustrations Lesothosaurus looks more like an undersized Hypsilophodon than to a long-legged lizard. Interestingly, it's also hypothized that Lesothosaurus underwent long "hibernations" to survive the harsh desertical conditions of the habitat it lived within, but this is not yet demonstrated.
The same animal?: Fabrosaurus
- Fragmentary remains from Lesotho that have been named Fabrosaurus australis ("Southern Fabre's lizard") may be synonymous with Lesothosaurus diagnosticus; since they were named slightly before Lesothosaurus (in the sixties), Fabrosaurus would be the valid genus name for this dinosaur. Other possible Lesothosaurus remains have been classified in 2005 in another genus, Stormbergia. In old textbooks, the "fabrosaur" was often shown as the prototypical basal ornithischian; since the 1980s, Lesothosaurus took over this role. Once, the "fabrosaurid" family was recognized by scientists as a catch-all grouping for undetermined basal ornithischians, but modern cladistic science do not accept artificial assemblages like this, and "fabrosaurid" has mostly disappeared in literature. Alleged "fabrosaurids" included also some animals now considered ornithopods or near-ornithopods, like the Late Jurassic Nanosaurus or Alocodon.
- Scutellosaurus ("lizard with small shields", not to be confused with the near-reptile Scutosaurus) has traditionally been the most primitive thyreophoran, variably classified in the Scelidosaurids or in its own family, Scutellosaurids. Discovered only in the 1980s, was also a small bipedal animal with a similar look, but slighty bigger, longer-tailed, more robustly-built than the lesothosaur, and with longer forelimbs (some think was partially quadruped). More importantly, it had a light armor made by small bony plates placed in rows upon its torso, similar to that of the bigger Scelidosaurus. Like the scelidosaur, Scutellosaurus lived in Early Jurassic, but was found not in Europe like the former but in Arizona, where the popular double-crested Dilophosaurus lived: some portrayals have shown the scutellosaur as that dinosaur's prey, but this is not confirmed. Also found in the last decades of the XX century, the Argentinian Pisanosaurus mertii lived in the Middle Triassic (well before Coelophysis and Plateosaurus) and shared its habitat with the alleged first theropods" Herrerasaurus & Eoraptor and many non-dinosaur reptiles such as rhynchosaurs, "thecodonts", and mammal-ancestors (all these were much more common at the time than dinosaurs, never forget this). The pisanosaur still remains the most ancient ornithischian known to science, but sadly, is known only from one incomplete fossil. It was arguably similar to Lesothosaurus in shape and size, and with no armor like the latter. One significative thing is that some Triassic non-dinosaurian archosaurs were once considered basal ornithischians as well (often put in the "fabrosaurid" assemblage): Technosaurus from Texas is one example, sometimes mentioned as "the most ancient North American ornithischian"; other two examples are Revueltosaurus and chinese Dianchungosaurus (the latter was believed a heterodontosaur). The evocative name Technosaurus comes from the Texas Tech University; interestingly, another basal ornithischian, the European Emausaurus (known only from a skull) also derives its name from an university, the German EMAU. It is usually believed in the middle between Scutellosaurus and Scelidosaurus, but some think it's a very primitive stegosaurian. Other three animals are usually considered closer to Scelidosaurus than to Scutellosaurus: Portuguese Lusitanosaurus ("lizard from Portugal") and Chinese Bienosaurus and Tatisaurus.
- Heterodontosaurians were originally thought ornithopods, then ancient relatives of ceratopsians and pachycephalosaurs; today they are generally regarded as very basal ornithischians. Despite their primitiveness, heterodontosaurs not only flourished in the Early Jurassic, but also managed to survive until the Late Jurassic and even the Early Cretaceous: English Echinodon lived alongside Iguanodon and Hypsilophodon! Half the length of Heterodontosaurus tucki and with small tusks only in the upper jaws, Echinodon ("hedgehog tooth") is known to science since the middle XIX century, but its classification as a heterodontosaurian has been confirmed only after the discovery of the namesake of the group (it was also briefly believed a Scutellosaurus relative in the nineties). Geranosaurus ("crane lizard") and Lycorhinus ("wolf nose") were both found in South Africa at the start of the XX century, and also were originally not classified as heterodontosaurs because Heterodontosaurus was not known yet: Lycorhinus, with its typically heterodontosaurian mammal-like dentition, was initially believed a non-dinosaurian therapsid like Cynognathus. Also South-African and Early-Jurassic, Abrictosaurus and Lanasaurus (the latter often synonimized with Lycorhinus) were found about in the same time of Heterodontosaurus; the former's name, "awake lizard", is actually ironical, because it was hypothized that Abrictosaurus underwent "hibernations" (just like what has been proposed for Lesothosaurus, but again, this is not demonstrated). Curiously for a heterodontosaur, Abrictosaurus was totally tusk-less, and because of this was once believed a possible female Heterodontosaurus (this originated from a confrontation with the modern musk-deers, whose males only bear tusks). Some important dinosaur discoveries that have been made since the 2009 regard the heterodontosaurian group. For example, Fruitadens ("Fruita's tooth" from the geological formation that preserved it) lived in the Late Jurassic North America alongside the famous jurassic Stock Dinosaurs; in opposite to the "younger" Echinodon, Fruitadens has tusks only in its lower jaw. With only two feet of length (the same size of a Microraptor), Fruitadens is currently the smallest known North American dinosaur; it and Echinodon are among the smallest bird-hipped dinosaurs ever discovered, only equalled by some marginocephalians (ceratopsians & pachycephalosaurs) like Microceratus, and some "hypsilophodont" ornithopods.
- Like the basal saurischians, basal ornithischians as a whole are mostly known only since the 1960s (not counting Scelidosaurus, which has been known since the XIX century but has recently re-classified as an extremely basal ankylosaurian), and still arent well-understood. So, every recent discover could be very significative. Eocursor and Tianyulong in particular, have fairly gained much consideration in scientific field because of their objective importance. Found in 2007, Eocursor parvus (small dawn-runner) was discovered in South Africa like Heterodontosaurus and Lesothosaurus, and its name recalls that of the famous Eoraptor (dawn robber). Its relevance is due to the fact that its the only Triassic ornithischian known so far from a complete skeleton (while the remain of the even earlier Pisanosaurus is only partial); this gives us precious information about the deepest ornithischian roots, and also could better explain the relationship between bird-hipped dinosaurs and the saurischians. According to the most accepted classification, ornithischians are divided in two main lineages: Thyreophorans and Cerapods. The former are, as is known, Stegosaurs+Ankylosaurs+some basal forms (Scutellosaurus, Emausaurus, and maybe Lesothosaurus). Cerapods include almost all the other ornithischians, furthermorely divided in Ornithopods (duckbills, Iguanodon, Hypsilophodon etc) and Marginocephalians (ceratopsians+pachycephalosaurs). Indeed, Cerapods is just a Portmanteau made of Cera(topsian) and (Ornitho)pod. About Tianyulong confuciusi: this is a heterodontosaurid from the Late Jurassic found in 2009 in the same Liaoning site from which the Jurassic near-bird Anchiornis was discovered. Tianyulong, like the latter, has preserved some sort of proto-feathers around its body. The thing is, this is the first time that unequivocally feather-like structures have been found in a non-theropod dinosaur (not counting the quills of Psittacosaurus). See the useful notes about dinosaurs in general to understand the revolutionary implications of this discovery.