Although most silesaurids are inferred to have been herbivorous based on the shape of their teeth, coprolites (fossil dung) assigned to Silesaurus contain beetles, which shows they were not strictly plant-eaters.

The first fossils of this taxon were discovered by a team from Harvard University in the northeastern Argana Basin, 2.9 km (1.8 mi) east of Imziln, Morocco, with support from the National Geographic Society and permission from the Moroccan Ministry of Energy and Mines.

The remains were found in a quarry at the Irohalene Mudstone Member of the Timezgadiouine Formation, as part of a layer of disarticulated specimens that included fossils of phytosaurs, prolacertiforms, fish, and temnospondyls.

[1] In 2012, the paleontologists Christian F. Kammerer, Sterling J. Nesbitt, and Neil H. Shubin scientifically described the remains, and identified them as representing the first skeletal fossil record of the group Silesauridae from North Africa.

The specific name is ancient Greek for 'leathery arm', in reference to the possible integument (external tissue) of the animal, and also honors Dionysius Scytobrachion, a classical mythographer who chronicled the mythical history of North Africa.

Although these elements were not found associated with each other, and probably represent different individuals, the describers assigned them all to Diodorus based on comparison with the holotype (in the case of the isolated teeth), or on the silesaurid or dinosauriform features of the bones.

[1] Features thought to be shared by most silesaurs include a beak-like front of the lower jaw, leaf-shaped (or folidont) teeth, long limbs, and a quadrupedal posture.

[3][4] Small-bodied, ancestral ornithodirans (the group that includes dinosauromorphs and pterosaurs) may have had filamentous (fuzzy) integument covering their bodies to retain heat.

This ridge, running parallel to the tooth socket margin, is only known from Diodorus among silesaurids, and is therefore considered an autapomorphy (a distinguishing or diagnostic feature) of this genus.

[1] The roots of the four preserved teeth are firmly fused to their sockets (the ankylothecodont condition), like in all silesaurids except possibly Lewisuchus and in non-archosauriform archosauromorphs, but unlike other members of Archosauria.

The three preserved tooth crowns are triangular with denticles (serrations) on the front and back edges, as in all silesaurids except Lewisuchus and Asilisaurus, narrow, and tilted (or canted) forwards.

Only fragmentary specimens of non−dinosaurian dinosauromorphs (basal or early diverging members of the clade that includes dinosaurs) from Argentina were recognized until the 21st century, when their larger taxonomic diversity, and geographic and stratigraphic range was realized.

They inferred that the various lineages within Ornithodira (such as dinosaurs and silesaurids) must have diverged from each other by the late Anisian stage of the early Middle Triassic, about 242 million years ago.

Although fossilized footprints had earlier indicated the presence of dinosauromorphs in the Timezgadiouine Formation, Diodorus is the first definitive silesaurid record, which supports the idea that this group had a cosmopolitan distribution in the Middle−to−Late Triassic.

They suggested that basal dinosauromorphs were widespread, temporally long-ranging, and common rather than rare and restricted in time and space in Triassic fossil assemblages.

They speculated that this pattern had only been recently recognized due to specimens being misidentified as true dinosaurs and the rather low potential of these small-bodied, delicate animals being preserved.

[1][9] In 2014, the paleontologists Max C. Langer and Jorge Ferigolo fully described the anatomy of Sacisaurus and reanalyzed earlier phylogenetic studies of silesaurids.

[4] The following cladogram shows the placement of Diodorus among Silesauridae according to Martz and Small, 2019:[4] Lagerpeton Dromomeron Marasuchus Dinosauria Lewisuchus Asilisaurus Silesaurus Sacisaurus Lutungutali Diodorus Eucoelophysis Kwanasaurus In 2020 the paleontologists Rodrigo Temp Müller and Maurício Silva Garcia found silesaurids to be a stem-group leading to "core" ornithischian dinosaurs, which would make silesaurids themselves a paraphyletic (unnatural) group, consisting of basal ornithischians instead of a sister group to all of Dinosauria.

[12] A 2022 study by the paleontologist David B. Norman and colleagues expanded on the dataset of Müller and Garcia's 2020 analysis (by for example including early Jurassic ornithischians) and also found silesaurids to be a paraphyletic group on the branch leading to traditional Ornithischia.

[13] The cladogram below is based on the 2022 study by Norman and colleagues and shows Diodorus as an ornithischian dinosaur:[13] Herrerasauridae Daemonosaurus Chindesaurus Tawa Eodromaeus Sauropodomorpha Theropoda Lewisuchus Soumyasaurus Asilisaurus Diodorus Technosaurus Ignotosaurus Silesaurus Sacisaurus Lutungutali Kwanasaurus Eucoelophysis Pisanosaurus Laquintasaura Scutellosaurus Emausaurus Scelidosaurus Eocursor Lesothosaurus Heterodontosauridae Agilisaurus Hexinlusaurus Herbivory has been suggested for silesaurids in general and Silesaurus in particular based on tooth shape, and a 2014 study by the paleontologists Tai Kubo and Mugino O. Kubo of microwear on its teeth found it consistent with herbivory, though omnivory could not be ruled out.

[14] A 2019 study by paleontologist Martin Qvarnström and colleagues examining coprolites (fossil dung) that contained beetles attributed them to Silesaurus based on size and other factors.

In 2010, the paleontologists Rafał Piechowski and Jerzy Dzik considered such proportions typical of fast-running, quadrupedal animals, but noted that the long tail of Silesaurus which would have acted as a counterweight to the body, as well as the very gracile forelimbs, indicates it retained the ability for fast bipedal running.