Chinese Strata Yield a Gigantic Bird-like Dinosaur

June 13th, 2007 Placozoan Posted in News |

NATURE REPORTS today the discovery of a bizarre 1,400 kg, surprisingly gracile bird-like dinosaur, named Gigantoraptor erlianensis. The animal is an oviraptosaur, one of a group of bird-like theropod dinosaurs. It has been placed basally in the Oviraptoridae clade, and no doubt will cause some perplexity as it has a variety of more avian traits that are missing in other early oviraptosaurs.

However, Gigantoraptor exhibits many features that are not seen in other oviraptorosaurians but do occur in more derived maniraptorans^{2, 3, 18, 19}. The anterior caudal centra are wider transversely than deep dorsoventrally. The scapular blade is shallower distally without an expanded distal end. The forelimbs are proportionally long, with a humerus+radius+metacarpal II to femur+tibiotarsus+metatarsal III ratio of 0.60. The radius bears a sub-spherical distal end. Metacarpal I is less than one-third the length of metacarpal II and the latter is much more robust than the former (Fig. 1n). The pubis is laterally compressed and straplike for most of its length, though a long pubic symphysis is present. The femur is avian-like in having a spherical femoral head with a distinct neck, a high and prominent trochanteric crest much thicker and higher anteriorly than posteriorly (Fig. 1q), a shallow popliteal fossa distally bounded by a low shelf, and lacks a fourth trochanter. The tibia has a lateral margin sub-equal in depth to the medial margin on the distal end. The astragalar main body extends laterally to hide the small calcaneum from anterior view (Fig. 1s). A distinct proximal projection is present on the lateral margin of tarsal IV and metatatarsal III is ginglymoid distally.

While it has many avian traits, it may not have feathers. Some smaller members of this group are known to have had symmetric vaned feathers, and feathers may be characteristic of this group (unfortunately evidence of feathers is rarely preserved). The authors hypothesize that with its large size Gigantoraptor may have lost feathers since insulation would not have been necessary with its greater body mass. Feathers may have been retained on the tail and forelimbs, since these are not used primarily for insulation (fossil evidence suggests some oviraptosaurs used their feathered forelimbs to cloak their eggs while brooding), but determining this will require future research.

As is expected due to its large size compared to other members of its clade, Gigantoraptor also has unique specializations.

Gigantoraptor possesses a large number of autapomorphies (see ‘Diagnosis’ section), indicating that it represents a highly specialized lineage in oviraptorosaurian evolution. In particular the presence of many features unknown in any other dinosaur, such as the highly specialized caudal vertebrae and the bizarre humerus, suggests that morphological diversity among the dinosaurs is higher than previously known. This is further indicated by Gigantoraptor’s unusual limb allometry. Among theropods, the tibiotarsus and metatarsus show negative allometry relative to the femur when size increases^{9, 28} and limb circumference scales to limb length¹⁰, though other factors, including phylogeny, contribute to the relative proportions and robustness of the limb elements as well⁹. A comparison of the femur circumference-to-length ratio, tibiotarsus-to-femur length ratio and metatarsus-to-femur length ratio values in Gigantoraptor and oviraptorids indicates that Gigantoraptor has values similar to those in its much smaller relatives (Table 1) and significantly different from theoretically predicted ones^{9, 10, 28}. This is inconsistent with a general pattern that large-sized members of non-avian theropod sub-groups have proportionally stouter limbs and shorter lower legs than their small-sized relatives⁹. Compared with other similar-sized theropods including tyrannosauroids, Gigantoraptor has much more slender limbs and longer lower legs as indicated by the femur circumference-to-length ratio, tibiotarsus-to-femur length ratio and metatarsus-to-femur length ratio values (Table 1), suggesting that it might be the most cursorial theropod of comparable size.

Gigantoraptor, like other oviraptosaurs, had a beak, but the authors make no speculations as to its diet. It was likely a fast runner compared to similarly sized dinosaurs.

In recent years Chinese strata have produced many ancient birds and theropod dinosaurs, and will continue to do so, helping to further delineate the evolutionary relationship of these groups.

Xu, X.; Tan, Q.; Wang, J.; Zhao, X.; Tan, L. “A gigantic bird-like dinosaur from the Late Cretaceous of China.” Nature 2007, advanced online publication 13 June 2007 (doi:10.1038/nature05849)