SVP 2018: Stages in the origin of avian flight

Hartman, Mortimer and Lovelace 2018
reconstruct the origin of avian flight in a series of stages:

  1. Acquisition of theropod characters unrelated to avian flight, including bipedalism, three fingered hands, a furcula, and filamentous epidermal structures.
  2. Acquisition of characters directly exapted for flight such as enlarged forelimbs, pennaceous feathers on the forelimbs and tail, increased angle between scapula and distal coracoid, and laterally facing glenoid fossae.
  3. Characters acquired due to aerial locomotion, including tertial feathers, expansion of the flight stroke and associated muscles, and in more derived taxa an alula and reduction of the distal caudal series to a pygostyle.
  4. Characters associated with higher endurance crown avian-style flight including enlarged keeled sterna, hinged sternal ribs, loss of gastralia, and well-developed caudal air sacs.
Figure 1. Xiaotingia, the proximal outgroup to the Thermopolis specimen of Solnhofen birds, the basalmost bird.

Figure 1. Xiaotingia, the proximal outgroup to the Thermopolis specimen of Solnhofen birds, the basalmost bird.

Based on the outgroup taxon, Xiaotingia
(Fig. 1) and the basalmost Solnhofen bird, the Thermopolis specimen, missing from the above list of traits are:

  1. phylogenetic miniaturization
  2. more gracile bones overall
  3. a smaller skull
  4. a more gracile neck
  5. a longer tail
  6. an elongate coronoid, which signals the start of flapping
  7. a larger olecranon process
  8. avian-style wrist
  9. a more robust retro pedal digit 1 with a larger ungual

Hartman, Mortimer and Lovelace conclude:
“Stage 2 taxa with small body size and enlarged forelimbs may have utilized wing assisted incline running (WAIR) to access trees despite lacking unambiguously arboreal characters, breaking the ground-up/trees-down dichotomy.” Yes, but this seems like old news as Ken Dial published the same conclusion in 2003. Where in the author’s list is the elongate coracoid common to all flapping tetrapods? …and found in non-avian convergent micro raptors and sinornithosaurs?

…and the authors continue:
“Several Stage 2 taxa independently approached Stage 3 conditions, including some
microraptorians, Rahonavis, Archaeopteryx and scansoriopterygids; this suggests that
WAIR enabled several parallel experiments with aerial locomotion.” Good points first noted in the LRT, except that scansoriopterygids are birds when more Solnhofen birds are added to the taxon list. (Need to consider all Solnhofen birds as taxa, not just have one and label it Archaeopteryx). T-rex ancestor, Zhenyuanlong might also be added to this list, given its large wing feathers.

References
Dial KP 2003. Wing-assisted incline running and the evolution of flight.  Science 299:402-404.
Hartman S, Mortimer M and Lovelace DM 2018. A testable macroevolutionary framework for character acquisition in the origin of avian flight. SVP abstracts.

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