Handout I

Fig. 3. Skeletal innovation in the three major clades of dinosaurs (Ornithischia, Theropoda, and Sauropodomorpha) as shown by contemporaneous species from the Upper Jurassic (Kimmeridgian) Morrison Formation of North America. Labeled features evolved at various nodes as described in the text. Scalebar, 1m. (A) Camptosaurus dispar, an ornithischian. (B) Allosaurus fragilis, a theropod. (C) Camarasaurus lentus, a sauropodomorph [after (44)]. Skeletal innovations are as follows:

1. horny beak for cropping;
2. predentary bone for lower bill support;
3. cheek depression for oral processing of plant matter;
4. leaf-shaped crowns with wear facets and a symmetrical enamel for shearing plant matter;
5. coronoid process for attachment of robust jaw-closing muscles;
6. opisthocoelous cervicals with reduced neural spines for flexibility;
7. ossified tendons to stiffen trunk;
8. rigid digit I with subconical ungual for defense;
9. pubis with prepubic process and posteroventrally directed postpubic process opening posterior trunk;
10. pendant fourth trochanter for enhanced caudal hindlimb retractors;
11. intramandibular joint for flexible bite;
12. metacarpal extensor depressions for manual raking;
13. hollow skeleton to reduce bone weight;
14. semilunate carpal simplifying wrist action to maneuver large hands;
15. manual digit II longest, emphasizing inner digits;
16. long penultimate phalanges enhancing grasping capability;
17. pubic foot for body support at rest;
18. astragalar ascending process uniting tibia and tarsus;
19. elongate prezygapophyses unite distal tail forming a dynamic stabilizer;
20. crowns with regular V-shaped wear facets indicate precise occlusion for slicing vegetation;
21. nares enlarged and retracted;
22. columnarlimbpostureforweightsupportatlargebodysize;
23. 12 or more opisthocoelous cervical vertebrae composing a longer neck;
24. 11 or fewer dorsal vertebrae shortening the trunk;
25. bifurcate neural spines accommodating a robust median elastic ligament;
26. arched ligament-bound metacarpus for digitigrade manual posture;
27. manual/pedal phalanges reduced in number for a more fleshy foot pad;
28. manual digits I and V weight-bearing to broaden support;
29. manual digit I ungual enlarged possibly for intraspecific rivalry;
30. distal tarsals unossiÞed increasing shock-absorbing cartilage in joints;
31. elephantine pes for weight support at large body size.

Credit: Reprinted with permission from “The Evolution of Dinosaurs” by Paul Sereno, Science 284, 25 JUNE 1999, 2137–2147. Copyright 1999 AAAS. Readers may view, browse, and/or download the material on this page for temporary copying purposes only, provided these uses are for noncommercial personal purposes. Except as provided by law, this material may not be further reproduced, distributed, transmitted, modified, adapted, performed, displayed, published, or sold in whole or in part, without prior written permission from the publisher (http://www.sciencemag.org).