Ultraviolet light illuminates the avian nature of the Berlin Archaeopteryx skeleton

Abstract

The question of whether the iconic avialan Archaeopteryx was capable of active flapping flight or only passive gliding is still unresolved. This study contributes to this debate by reporting on two key aspects of this fossil that are visible under ultraviolet (UV) light. In contrast to previous studies, we show that most of the vertebral column of the Berlin Archaeopteryx possesses intraosseous pneumaticity, and that pneumatic structures also extend beyond the anterior thoracic vertebrae in other specimens of Archaeopteryx. With a minimum Pneumaticity Index (PI) of 0.39, Archaeopteryx had a much more lightweight skeleton than has been previously reported, comprising an air sac-driven respiratory system with the potential for a bird-like, high-performance metabolism. The neural spines of the 16^th to 22^nd presacral vertebrae in the Berlin Archaeopteryx are bridged by interspinal ossifications, and form a rigid notarium-like structure similar to the condition seen in modern birds. This reinforced vertebral column, combined with the extensive development of air sacs, suggests that Archaeopteryx was capable of flapping its wings for cursorial and/or aerial locomotion.

Figure 1

Photographs and interpretative drawing of cervical vertebrae of the Berlin specimen of Archaeopteryx, MB.Av.101. (a) UV photograph of the neck and skull, (b) photograph of the neck under visual light, structures previously described and figured are marked, (c) magnified frame of (a) showing details of pneumatic structures in the neck, (d) interpretative drawing of (c) with recognized pneumatic structures. Anatomical abbreviations used: abs, area of abraded bone surface with regular spongy internal structure; amf, area with unidentified pores or putative camellae; cam, pneumatic camellae; CR, cervical rib; for, foramen; fos, fossa; nc, neural canal; pf, pneumatic foramen. Pneumatic foramina and camellae are unambiguous pneumatic structures and labeled with bold face. Foramina and pneumatic foramina are filled with black; fossae are filled with grey but without margin. Scale bars are 10 mm.

Figure 2

Photographs and interpretative drawing of presacral (thoracic) vertebrae with pneumatic structures of the Berlin specimen of Archaeopteryx, MB.Av.101. (a) UV photograph, (b) magnification of 16^th presacral vertebra under UV light, and (c) the same vertebra under visible light, (d) interpretative drawing of region a. Anatomical abbreviations used: abs, area of abraded bone surface with regular spongy internal structure; amf, area with unidentified pores or putative camellae; cam, pneumatic camellae; for, foramen; fos, fossa; ipc, internal pneumatic camerae; nc, neural canal; pf, pneumatic foramen. Pneumatic foramina, internal camerae and camellae are unambiguous pneumatic structures and labeled with bold face. Foramina and pneumatic foramina are filled with black; fossae are filled with grey but without margin. Scale bars are 10 mm.

Figure 3

UV photographs and interpretative drawings of the pelvis and sacral and caudal vertebrae with pneumatic structures of the Berlin specimen of Archaeopteryx, MB.Av.101. (a) Pelvic region in lateral view, with the last (22^nd) presacral vertebra, (b) 1^st to 5^th caudal vertebra, (c) 5^th to 9^th caudal vertebrae, (d) 10^th to 15^th caudal vertebrae, (e) 14^th to 21^st (last) caudal vertebrae. Anatomical abbreviations and filling of structures as in Figs 1 and 2. Scale bars are 10 mm.

Figure 4

UV photographs of the Berlin specimen of Archaeopteryx, MB.Av.101, with magnified region of the fused presacral neural spines forming a notarium-like structure. (a) Axial skeleton of Archaeopteryx, white frame demarks area of fused neural spines; scale bar is 20 mm. (b) Magnification of the 16^th to 22^nd presacral vertebrae, white arrows mark ossifications between the neural spines and remnants of broken spinal processes; scale bar is 10 mm.

Figure 5

Skeleton of Archaeopteryx (Berlin specimen, MB.Av.101) with schematic representation of air sacs and pneumatic postcranial elements as deduced from this study. Skeleton is shown from left lateral side and paired elements are represented only from the left body side. Black arrows indicate the path of pneumatization within air sacs. Light blue = cervical air sac (a), red = lung (b), dark blue = abdominal air sac (c). Grey = clavicular (d), anterior thoracic (e), and posterior thoracic (f) air sacs; these have no osteological evidence in Archaeopteryx, making their presence uncertain. Skeleton redrawn from Wellnhofer.