Carpenter
et al. (2005, pp. 43–44) determined that the holotype of
Tanycolagreus represents a subadult individual which measured approximately long in life. However, one of the referred fossils, the premaxilla from the Cleveland-Lloyd Quarry, would have belonged to a larger individual, measuring long. In 2010
Gregory S. Paul estimated the weight of a four-meter-long animal at . It cannot be determined whether or not the Cleveland-Lloyd specimen represents a fully mature adult, so the upper size limit for the
taxon remains unknown. The head of
Tanycolagreus is large, elongated and rectangular in profile due to a blunt snout. The leg is rather long and lightly built. Carpenter
et al. (2005; pp. 27 & 29) diagnosed
Tanycolagreus topwilsoni as follows: "Medium-sized
tetanuran having short, deep-bodied premaxilla pierced by narial foramen at base of nasal process, orbital process on postorbital, T-shaped quadratojugal, centrodiapophyseal lamina on dorsals. Differs from
Coelurus in the absence of pleurocoel on anterior dorsals; posterior caudal prezygapophyses elongated to one-third centrum length, rather than short; straight, rather than sigmoidal, humeral shaft; bowed, rather than straight, radius; flat-bottomed rather than arced pubic foot; straight rather than sigmoidal femoral shaft;
metatarsal length
subequal to humeral length, rather than 1.75 times humeral length. Differs from
Ornitholestes in straight anterior margin of premaxilla, rather than rounded; T-shaped rather than L-shaped quadratojugal; elongate neural spine; posterior caudal prezygapophyses only one-third centrum length, rather than one-half centrum length; bowed, slender radius, rather than straight, robust radius". The single premaxillary tooth preserved with the holotype is badly damaged, but does exhibit the asymmetrical cross-section typical in theropod teeth; the cheek teeth are too poorly preserved to show any detail. In the foot the second toe is slightly hyperextendable but does not carry an enlarged claw. ==Classification==