Hallopus was a prehistoric reptile, named in 1877 as a species of Nanosaurus and classified as a genus by O. C. Marsh in 1881 from the Late Jurassic Morrison Formation.

[2] However Marsh was vague in his description of where the specimen originated, writing that it was "probably Jurassic" and assigning it to the lower part of the Dakota Group, despite detailed accounts of the locality being sent to him by Baldwin.

He retains Hallopoda within Theropoda regardless and it was in 1890 that Hallopus received a full, detailed description figuring the type material, which was later expanded upon and revised throughout the 1890s.

[3] In 1939, Charles Schuchert suggested that Marsh was correct in his first assumption, which proposed that Hallopus was found in the upper members of the Morrison Formation.

In his work, Schuchert presumes a rocky hill known as "The Nipple" to be where Hallopus originated, but his research contains a series of contradictions that cast doubt over the specifics.

Regardless of the mystery surrounding the exact locality, the description offered by Baldwin and the highly specialised skeletal adaptations both support an assignment to the late Jurassic.

The reason Marsh continuously revised the age of Hallopus may have been that he was simply unaware of any Upper Morrison sediments matching the given description, which were later confirmed to exist around "The Nipple" by Ague, Carpenter & Ostrom in 1995.

[5] Overall Hallopus was a relatively small and gracile animal with strongly elongated limbs, five fingers on each hand and effectively tridactyl hindlimbs.

[4] The classification of Hallopus has a long history dominated by two main hypothesis that placed it on vastly different branches of the archosaur family tree.

Erpetosuchus Saltoposuchus Terrestrisuchus Litargosuchus Hesperosuchus Dromicosuchus Kayentasuchus Sphenosuchus Dibothrosuchus Junggarsuchus Almadasuchus Macelognathus Hallopus Hemiprotosuchus Protosuchus Orthosuchus Gobiosuchus Sichuanosuchus Zosuchus Mesoeucrocodylia Postosuchus Carnufex Hesperosuchus Pseudhesperosuchus Trialestes Kayentasuchus Dromicosuchus Terrestrisuchus Redondavenator Litargosuchus Sphenosuchus Dibothrosuchus Hallopus victor Phyllodontosuchus Junggarsuchus Macelognathus Almadasuchus Crocodyliformes

They furthermore analysed traits used in prior analysis critically and built a data matrix based on previously established phylogenies including that of Leardi (2017).

Leardi and colleagues conclude that the two may be synonymous, however closer inspection of Hallopus holotype or additional material clearly referrable to the taxon would be required to be sure.

Walker proposes that while running the forelimbs would have performed a kind of strut, with the anterior part of the body bouncing off to allow for a greater stridelength.

Walker further argues that this might compensate for the much longer hindlimbs by raising the glenoids, and proposed that Hallopus would have been capable of moving in ways similar to hares and greyhounds.

However, the closely related, if not synonymous Macelognathus does preserve pieces of the jaw, displaying a unique toothless mandible tip, possibly covered by a keratinous rhamphotheca.

The teeth of Macelognathus meanwhile are unserrated and mediolaterally flattened, not matching a carnivorous diet and instead being better suited for consuming insects and plant material.