For the past few decades, the theropod hypothesis of bird origin has been strongly corroborated by fossil evidence1-9 and systematic work10-15. Dromaeosaurids, one of the most distinctive theropod groups, have attracted particular attention since the description of Deinonychus1, owing to its pivotal role in supporting a theropod origin of birds. They, either by themselves or with troodontids7, 10, 11, 13-16, have been regarded as most closely related to birds. A better understanding of these animals is therefore crucial in reconstructing the evolutionary transition towards birds. Recent discoveries from the Jehol Group of western Liaoning, China, suggest that basal dromaeosaurs are small, feathered animals with forelimbs similar to those of Archaeopteryx, and feet showing features comparable to those of arboreal birds7, 8, 17. In 2001 and 2002, we obtained six new basal dromaeosaurid specimens from the Lower Cretaceous Jehol Group at a few localities in Chaoyang Basin, western Liaoning, China. IVPP (The Institute of Vertebrate Paleontology and Paleoanthropology) V13352 and V13320 were identified as a new species of Microraptor, TNP00996 (Tianjin Museum of Natural History), IVPP V13351 and IVPP V13476 as Microraptor sp., and IVPP V13477 as Dromaeosauridae gen. et sp. indet. These specimens provide new information on the morphology and distribution of feathers on non-avian dromaeosaurids. Here we describe the new dromaeosaurid species and in particular the morphology and distribution of feathers on the newly collected dromaeosaurid specimens.
Description. Microraptor gui is a small animal, the holotype being approximately 77 cm in total length . Little can be said about the cranial morphology but a tri-radiate postorbital is identifiable. As in M. zhaoianus8, the basal troodontid Sinovenator15, and the basal oviraptorosaur Caudipteryx19, M. gui has a relatively short trunk length, which is 44–50% of hindlimb length (according to the method of ref. 19). The tail is long but has relatively few vertebrae (approximately 26). The middle and posterior caudals are significantly elongate as in other basal dromaeosaurs, basal troodontids and Archaeopteryx20. The sternum is a single, flat and large bone, different from the condition in other dromaeosaurids7, 21 where two unfused sternal plates are present. At least seven pairs of slender uncinate processes are present. The anterior uncinate processes cover three ribs and the posteriormost one is short and does not reach the succeeding rib. The fused scapula and coracoid are similar to those of Sinornithosaurus and M. zhaoianus in the following features: scapula shorter than humerus; glenoid fossa laterally faced; angle between scapula and coracoid less than 90°; and large supracoracoid fenestra present on coracoid20. The forelimb is approximately 2.7 times the femoral length. The ulna is bowed and the radius is much thinner than the ulna . Metacarpal I is about one-quarter of metacarpal II in length and metacarpal III is slender and bowed laterally. Manual digit II, particularly phalanx II-1, is thick. The pelvis displays the following derived features as in M. zhaoianus, Sinornithosaurus, Sinovenator and basal birds5, 7, 15: postacetabular process of the ilium tapered; pubis retroverted; and ischium short, with a distally located obturator process and two dorsal processes. The tibia is variably bowed, more so in the referred specimen than the holotype. The pes is similar to that of M. zhaoianus in showing a sub-arctometatarsalian condition and in having slender, strongly curved claws8, 20. |
