The Origin of Archaeopteryx – Illustrated

Added March 15, 2014: Be sure to also visit Aurornis here.

An Old Debate, Now Clearly Settled
The origin of birds has been long debated, but the debate has been over for awhile. Birds arose from theropod dinosaurs and recent finds from China have presented us with a wide variety of feathered theropods — all part of the big evolutionary bush that ultimately begat the modern birds that fill our skies, waddle across Antarctica and sprint across the plains of the southern continents.

There have been several phylogenetic analyses of bird origins and diversification. Today’s blog simplifies and focuses the process by eliminating most of the side branches, like the tyrannosaurids, the alvarezsaurids, etc.

The Old View
In the old days paleontologists hauled out Compsognathus, Archaeopteryx and Gallus (the chicken). These were all essentially correct and told the right story, but now we know more of the details.

Figure 1. Old yet essentially correct view of bird evolution featuring Compsognathus, Archaeopteryx and Gallus, the chicken. 

The Current View
The current view of bird evolution (below) gets a little more detailed.

Figure 2. Theropods to birds step by step. Purple taxa should not have been included. They are not related to dinosaurs. Yellow highlights refer to taxa shown and discussed below. 

The Current View and The List of 23 Increasingly Bird-Like Characters
The current view of bird relations (above) nests many more taxa along the lineage of pre-bird theropods. Each one adds at least one more bird-like character to the list. The only serious problems are pterosaurs and Lagerpeton (in purple) are not related to dinosaurs.

1. Mesotarsal ankle – the Ornithodira (includes pterosaurs)
2. Functional tridactyl foot – Lagerpeton 
3. Fully perforated acetabulum – Herrerasaurus
4. Loss of digit 5 on hand – Tawa
5. Postaxial vertebral pneumaticity – Tawa
6. Pedal digit 1 loses contact with ankle – Coelophysis
7. Furcula – Coelophysis
8. Maxillary fenestra – Spinosaurus
9. Strap-like scapula – Allosaurus
10. Dorsal astragalus tall/broad – Allosaurus
11. Expanded pneumatic ectopterygoid -Tyrannosaurus
12. Three tympanic systems in ear region – Tyrannosaurus
13. Promaxillary fenestra – Tyrannosaurus
14. Fused semilunate carpal – Alvarezsauria
15. Enlarged sternum – Alvarezsauria
16. Ossified sternal ribs – Therizinosauria* + Oviraptosauria* + Dromaeosauridae + Troodontidae
17. Shortened tail – Dromaeosauridae + Troodontidae + Archaeopteryx
18. Subdivided ulna – (not sure what this means, unless these are quill locations)  Dromaeosauridae + Troodontidae + Archaeopteryx
19. Retroverted pubes – Dromaeosauridae + Troodontidae + Archaeopteryx
20. Asymmetric flight feathers – Dromaeosauridae + Troodontidae + Archaeopteryx
21. Sickle claw on foot – Dromaeosauridae + Troodontidae
22. Forelimb longer than hindlimb – Archaeopteryx (doesn’t appear to be true)
23. Pygostyle – Jeholornis and higher birds

* Derived taxa had a shortened tail (#17) and retroverted pubis (#19)

But wait there’s more…

Figure 3. Sample taxa in the lineage of birds sans all the cousins and offshoots. From top to bottom: Tawa, Juravenator, Sinocalliopteryx, Archaeopteryx, Cathayornis, Sinornis plus enlarged skulls.

The Revised View and List of Characters
This simplified view of bird relations notes that pterosaurs and Lagerpeton were not related to dinosaurs and birds. It also takes the view that dromaeosaurids and oviraptorids were probably derived from Archaeopteryx due to the shared trait of an elongated coracoid (analysis not done yet). Oviraptorids appear to have reshifted the pubis forward (but that’s for another blog). Here’s a new list of bird characters as they appeared in the above taxa (Fig. 3). Tyrannosaurids and Alvarezsaurids, among others, were skipped because, although their stage of evolution did add characters, in both cases the forelimbs became reduced, representing offshoots that did not ultimately evolve into birds.

Here, in this simplified and focused account, Tawa, Juravenator and Sinocalliopteryx precede Archaeopteryx. Sinornis and Cathayornis succeed Archaeopteryx. The following list of 29 characters are offered to replace the list of 23 (above).

1. Mesotarsal ankle – Gracilisuchus at the base of the Archosauria
2. Functional tridactyl foot – Trialestes (but note sauropods had a functionally pentadactyl foot, so reversals are possible)
3. Fully perforated acetabulum – Herrerasaurus

4. Loss of digit 5 on hand – Tawa
5. Postaxial vertebral pneumaticity – Tawa
6. Strap-like scapula – Tawa

7. Finger 3 shorter than 2. – Juravenator
8. Loss of digit 4 on hand – Juravenator
9. Pedal digit 1 loses contact with ankle – Juravenator
10. Furcula (fused clavicles) – Juravenator
11. Maxillary fenestra – Juravenator
12. Pubis rotated beneath anterior ilium – Juravenator 

13. Dorsal astragalus tall/broad – Sinocalliopteryx
14. Promaxillary fenestra – Sinocalliopteryx
15. Fused semilunate carpal – Sinocalliopteryx
16. Ossified sternal ribs – Sinocalliopteryx
17. Protofeathers – Sinocalliopteryx
18. Smaller teeth – Sinocalliopteryx 

19. Pubes beneath or behind acetabulum – Archaeopteryx
20 Asymmetric flight feathers – Archaeopteryx
21. Forelimb nearly as long as hindlimb – Archaeopteryx
22. Elongated coracoid locked onto sternum – Archaeopteryx
23. Reduced cervical ribs – Archaeopteryx
24. Chevrons parallel centra – Archaeopteryx
25. Toes beneath shoulder glenoid – Archaeopteryx
26. Anterior skull half the height of posterior skull – Archaeopteryx
27. Pedal digit 1 retroverted for perching – Archaeopteryx 

28. Sickle toe claw – Dromaeosaurids and troodontids (neoflightless birds)
29. Pygostyle – Cathayornis and higher birds, including oviraptorids by convergence
30. Toothless – Certain higher birds, including oviraptorids by convergence

The elongated and immobile coracoids of dromaeosaurids and oviraptorids indicate they were secondarily flightless, following a sister to Archaeopteryx. Alvarezasaurids and therizinosaurids appear to have been derived from sisters to Juravenator or Sinocalliopteryx (among listed taxa) and developed their bird-like characters (short tail, retroverted pubis) by convergence.

Sinocalliopteryx was an Early Cretaceous and too large to be in the lineage of Archaeopteryx, but an earlier, smaller sister would have been a suitable candidate. The development of longer forelimbs and longer coracoids was initiated here. Along with protofeathers, some early flapping may have accompanied running.

Archaeopteryx had larger forelimbs and relatively smaller hips, indicating a transition to forelimb locomotion, which included vigorous flapping assisted by flight feathers. The center of balance had shifted forward to the shoulder glenoid, which is where it is located in flying birds (and bats and pterosaurs). This was accomplished by shortening the torso. The neck could be pulled back further and the forelimbs were elongated and adorned with larger feathers.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Chiappe LM 2009. “Downsized Dinosaurs: The Evolutionary Transition to Modern Birds”. Evolution: Education and Outreach: 248–256.
Heilmann G 1926. The Origin of Birds. London: Witherby. 208 pp
Ji Q and Ji S-A 1996. On the discovery of the earliest bird fossil in China and the origin of birds (PDF). Chinese Geology 233: 30–33.
von Meyer CEH 1861. Archaeopteryx lithographica (Vogel-Feder) undPterodactylus von Solnhofen (in German). Neues Jahrbuch für Mineralogie, Geologie und Paläontologie 1861: 678–679.
Ostrom JH 1973. The ancestry of birds. Nature 242 (5393): 136–136.Bibcode 1973Natur.242..136O. doi:10.1038/242136a0.
Paul GS 2002. Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds. Baltimore: Johns Hopkins University Press. p. 472p

wiki/Origin_of_birds