[1] The specimen was discovered in 2001 by the Sino-American field expedition, but it was not described until 2010 when Jonah Choiniere, James Clark, Catherine Forester, and Xu Xing published a full analysis of the bones.

[2] Choiniere and colleagues noted that, at the time of its description, Zuolong was one of the oldest coelurosaurs known to science, but that the implications of its discovery cannot be fully understood until more fossil material is discovered.

[2] Later, Gregory S. Paul suggested that the holotype is a juvenile and estimates a total adult length of 3 metres (9.8 ft) and a mass of 50 kilograms (110 lb).

[2] They also describe several autapomorphies of the post-cranial skeleton including: a centrum of the fifth sacral vertebra with an obliquely angled posterior articulation, a large fovea capitis, a large distal condyle of the third metatarsal, a short post-acetabular wing of the ilium, and a lack of a pubic tubercle, a straight ulna and radius, a ridge on the head of the tibia, a lack of paired lateral foramina on the vertebrae, a lack of lateral fossae on the vertebral centra, a straight humeral and femoral shaft, and a high and rounded ilium.

There are very few primitive coelurosaurs known from complete remains, however the authors are able to draw numerous distinctions between Zuolong and other Late Jurassic small theropods such as Guanlong, Coelurus, and Tanycolagreus.

It has very large orbits which face laterally and a pronounced anterior process of the lacrimal, which gives the appearance of a small crest above the eyes, a trait very common among theropods.

[2] The preserved alveoli of the tooth positions are relatively well-preserved, which led Choiniere and colleagues to estimate that in life, Zuolong likely had a total of four premaxillary and twelve maxillary teeth.

The dorsal vertebrae are not as well-preserved as the cervicals, but they are complete enough to determine that they have twin pleurocoels and lack both fossae and foramina on their lateral surfaces.

The caudal vertebrae, like the dorsals, lack both fossae and formania on their lateral surface, and they also preserve sharply inclined neural spines, though these are not as pronounced as they are in allosauroids.

One of the autapomorphies for Zuolong, the enlarged fovea capitis, is known to be a pathological condition in male turkeys, so the authors say they cannot rule this out as a possibility, although they think it is relatively unlikely and cannot be proven until a second specimen is found.

The species is also from a part of the Mesozoic where several theropod groups are known to have originated, so the affinities of Zuolong needed to be tested against a broad range of taxa, including coelophysoids and ceratosaurs in addition to avetheropods.

[2] The resulting analysis recovered Zuolong within a monophyletic coelurosauria, but did not resolve any more specific relationships between basal coelurosaurs outside of well-established groups like maniraptora and tyrannosauroidea.

Their placement of Zuolong as a coelurosaur was based on the following synapomorphies: a maxillary fenestra behind the antorbital fossa, a dorsal ridge of the antorbital fossa formed by the nasal and lacrimal bones, d-shaped premaxillary teeth, maxillary teeth with non-uniform serrations, cervical vertebrae with multiple pleurocoels, a femur shorter than the tibia, a cnemial crest level with the posterior proximal condyle of the tibia, a groove on the ascending process of the astragalus, and a lack of a horizontal groove on the astragalar condyles.

[2] Tugulusaurus Proceratosaurus Tanycolagreus Xinjiangovenator Coelurus Ornitholestes Zuolong Ornithomimosauria Tyrannosauroidea In 2020, a group of several authors led by the Brazilian paleontologist Juliana Manso Sayão described a new genus of coelurosaur from the Romualdo Formation, Aratasaurus.

Both taxa were recovered as basal coelurosaurs, similar to Choiniere and colleagues, based on the following synapomorphies: an antorbital fossa with a dorsal border in lateral view, a medial opening on the ectopterygoid bones of the palate, a d-shaped cross-section of the premaxillary teeth, a rounded surface on the bottom of the caudal vertebrae, a shelf-like fossa on the ilium, and a lack of an anterior process on the pubic boot.

[8] Other novel results of this analysis included finding a paraphyletic proceratosauridae and recovering Bicentenaria as a relatively derived stem-maniraptoran and the sister taxon of Ornitholestes.

Although the discussion of their results focused most heavily on its implications for megaraptoran taxonomy, both of their analyses resolved the same relationships for Zuolong and other basal coelurosaurs.

megalosauroidea) Zuolong Nqwebasaurus Aorun Ornitholestes Coelurus Tyrannosauroidea Megaraptora Ornithomimosauria A similar result was recovered by Andrea Cau in his study of theropod phylogeny and ontogeny in 2024.

[9] Tugulusaurus Bicentenaria Tyrannosauroidea Zuolong Aniksosaurus Ornithomimosauria Ornitholestes In 2020, a group of scientists led by Lida Xing from the Chinese Academy of Sciences published their description of a compsognathid, Xunmenglong.

Tyrannosauroidea Tanycolagreus Kileskus Scipionyx Tugulusaurus Coelurus Ornitholestes Compsognathidae Zuolong Aorun Ornithomimosauria Zuolong Tugulusaurus Coelurus Tanycolagreus Tyrannosauroidea Ornitholestes Compsognathidae Scipionyx Ornithomimosauria Andrea Cau conducted a landmark phylogenetic analysis in 2018 which included hundreds of taxa and sought to resolve the evolution of the avian body plan from the base of archosauria to the evolution of crown birds.

[16] Cau's analysis was assembled over the course of a decade using over 1,400 discrete characters and showed support for the controversial "Ornithoscelida hypothesis", which groups theropods and ornithischians as sister taxa to the exclusion of sauropodomorphs.

[10][17] Cau's analysis differs substantially from those conducted by Matthew Baron in 2017[17] and Paul Dieudonné and colleagues in 2020[18] by recovering the enigmatic Chilesaurus as a basal member of tetanurae.

Synapomorphies recovered for tetanurae in this analysis include: the loss of the lacrimal shelf which overhangs the antorbital fossa, a contact between the lateral ridge and condyle of the quadrate bone, a vertically compressed cervical vertebrae, a reduced shelf over the acetabulum, a perforation of the pubic apron, a medially-facing head of the femur, and a reduction of the femoral trochlea.

[21] This region is inland and arid today, but in the Late Jurassic, it formed a coastal basin on the northern shores of the Tethys Ocean.

[22] This pattern of rainfall led to the prominence of seasonal mires, possibly exacerbated by substrate liquefaction by the footfalls of massive sauropods which created "death pits" that trapped and buried small animals.