A Velociraptor wishbone

Abstract

The ‘wishbone’ of birds comprises two clavicles fused into a structure¹ known as a furcula. In an influential 1926 book on bird origins by Heilmann², the furcula's supposed absence in dinosaurs was considered powerful evidence barring them from bird ancestry. A furcula has now been found in several theropod dinosaurs³,⁴, but its absence in other theropods, and the uncertainty of whether this absence is real or an artefact of preservation, obscures the evolutionary history of this structure⁵. Here we report the discovery of a furcula in Dromaeosauridae, a group posited to be the closest relative of birds⁶,⁷.

Main

Among close bird relatives (non-avialian Maniraptora), a furcula is known only in Oviraptoridae⁸. Although a furcula was tentatively reported to be present in an articulated skeleton of Velociraptor mongoliensis preserved embracing a Protoceratops skeleton³,⁶, the only firsthand description of this specimen maintained that it is not present⁸, and our observations of that specimen confirm this. A furcula has not been identified in any of the other five described species of Dromaeosauridae. Other taxa either lack any evidence of clavicles or, in one troodontid specimen⁹, clavicles are present but not fused. But the rarity of articulated specimens, the similarity of the furcula to ribs and gastralia, and incomplete preservation conspire to obscure whether this absence is real.

A partial skeleton of Velociraptor mongoliensis (specimen IGM 100/976) was discovered at Tugrugeen Shireh, Mongolia, during the 1991 expedition of the Mongolian Academy of Sciences-American Museum of Natural History Expedition. The specimen consists of a nearly complete skull and the anterior part of the skeleton, preserving derived characters that allow definitive reference to Velociraptor mongoliensis¹⁰. The bones are heavily bioturbated, and many elements are punctured by borings similar to those often present in Djadokhta Formation sediments¹¹, but the specimen is otherwise very well preserved and articulated. The block that holds the furcula also preserves a pair of sternal plates, posterior cervical vertebrae, the proximal ends of both humeri, the coracoids and the right scapula. The specimen is large relative to others of this species, and the cervical ribs are nearly fused to the vertebral centra, indicating that the specimen is near full-size.

The furcula is in articulation between the scapulocoracoids anterior to the sternal plates (Fig. 1), in the same position as in oviraptorids and birds. The bone is ‘V’-shaped and very slender, much thinner than in oviraptorids and Archaeopteryx. In cross-section it is nearly circular. A short midline process, the hypocleidium, extends posteroventrally. The proximal, or epicleideal, process sweeps posterodorsally to articulate with the acromion of the scapulocoracoid. The proximal process of the furcula tapers to a point where it contacts the scapulocoracoid, and the posterior articulating surface contacting the acromion is smooth.

Figure 1: V. mongoliensis specimen IGM 100/976 with ‘V’-shaped furcula.

a, Ventral view of anterior trunk region. c, posterior cervical vertebrae; f, furcula; lc, left coracoid; vr, ventral ribs; lst, left sternal plate; rst, right sternal plate; rh, right humerus; rs, right scapula; rc, right coracoid. b, Detail of same region.

The broad distribution of a furcula among the non-flying relatives of birds⁴,⁵,⁸,¹² indicates that its origin is not tied to the origin of flight. The furcula's use in powered avian flight is therefore a co-option of a structure already present before flight evolved in the lineage leading to modern birds⁴. The discovery of a furcula in a taxon previously known from an articulated specimen underscores the difficulty of assessing whether the apparent absence of this feature is real or an artefact of preservation or ontogenetic sampling⁵.