When true birds, pre-birds and pseudo-birds first started flapping

Figure 1. Xiaotingia with new pectoral interpretation. See figure 3 for new tracing.

Figure 1. Xiaotingia with new pectoral interpretation. See figure 3 for new tracing. Based on the height of the coracoids, comparable to the height of the furcula, Xiaotingia was an early flapper. Based on the shorter tail length those taller coracoids represent yet another convergence with true birds. 

According to the cladogram
of the large reptile tree the proximal outgroup to Archaeopteryx and the flapping birds includes Eosinopteryx (Godefroit et al. 2013, Middle-Late Jurassic, Tiaojishan Formation, YFGP-T5197, 30 cm, 12 in long) and Xiaotingia (Figs. 1-4; Xu et al. 2011, STM 27-2).  This nesting has not changed despite the addition of several very bird-like theropod taxa recently (some listed below) to the large reptile tree.

Although they both had large wing feathers,
only Xiaotingia had tall coracoids. Coracoids were narrow, but short in Eosinopteryx. Tall coracoids are morphological signs that an extinct taxon was flapping.

Figure 2. Eosinopteryx with new pectoral interpretation. See figure 4 for in situ tracings.

Figure 2. Eosinopteryx with new pectoral interpretation. See figure 4 for in situ tracings. This taxon had smaller coracoids than in Xiaotingia. Based on tail length, this is the plesiomorphic condition.

Tall coracoids first appear
in the true bird lineage with the basalmost Archaeopteryx, the Thermopolis specimen (Fig. 5).

Figure 3. GIF animation of Xiaotingia pectorals showing new interpretations for the coracoid and sternum. Reconstruction in figure 1.

Figure 3. GIF animation of Xiaotingia pectorals showing new interpretations for the coracoid and sternum. Reconstruction in figure 1. The fuzzy yellow and gray drawing is the original published interpretation.  Outlying areas are low rez surrounding higher resolution central area. The difficult to see left coracoid is in green, crushed and scattered. The ventral rim of the right coracoid might be peeking beneath the vertebrae, angled toward the sternum. 

By convergence
and along with Xiaotingia, tall-ish coracoids also appear in the unrelated pseudo bird-like taxa Microraptor + Sinornithosaurus and Velociraptor + Balaur. Evidently they were flapping too.

Figure 4. GIF animation for new interpretation of Eosinopteryx pectoral region. The coracoids appear to be half as long but just as tall as previously interpreted. This is a reduction, as in Cosesaurus, rather than an elongation.

Figure 4. GIF animation for new interpretation of Eosinopteryx pectoral region. The coracoids appear to be half as long but just as tall as previously interpreted. This is a reduction, as in Cosesaurus, rather than an elongation. Reconstructed in figure 2. There were two clavicles hidden in their. The dark green areas may be dermal in origin. 

The Thermopolis specimen
of Archaeopteryx (Fig. 5) has the shortest and smallest coracoids of the Solnhofen birds. Note the basal troodontid (Fig. 7) proportions of the small skull and long tail, distinct from the larger skulls and shorter tails in the clade that includes Xiaotingia and Eosinopteryx.

Figure 1. The six tested Solnhofen birds currently named Archaeopteryx, Jurapteryx and Wellnhoferia.

Figure 1. The six tested Solnhofen birds currently named Archaeopteryx, Jurapteryx and Wellnhoferia.

By contrast and convergence,
and based on the reduction of their coracoids to struts, prevolant pterosaur ancestors, like Cosesaurus (Fig. 6), were flapping millions of generations before this clade had anything resembling wings,

Figure 1. Cosesaurus flapping - fast. There should be a difference in the two speeds. If not, apologies. Also, there should be some bounce in the tail and neck, but that would involve more effort and physics.

Figure 6. Click to enlarge and animate. Cosesaurus flapping – fast. There should be a difference in the two speeds. If not, apologies. Also, there should be some bounce in the tail and neck, but that would involve more effort and physics.

Many workers nest
microraptors and velociraptors closer to birds. At least part of that nesting includes the presence of feathers and tall narrow coracoids, ideal for flapping. Unfortunately these alternate nestings cannot be confirmed by the large reptile tree that nest small troodontids, like Xiaotiingia and Eosinopteryx, basal to birds. At least one prior analysis was riddled with errors. I have not examined others yet.

Figure 1. Sinornithoides youngi figure modified from Russell and Dong 1993.

Figure 7. Sinornithoides youngi figure modified from Russell and Dong 1993. Compare these proportions to the basal Archaeopteryx specimens with their small skulls, short torsos and long tails. 

This is where software comes in handy,
finding most parsimonious trees based on a long list of traits despite convergence in a few traits and making every attempt to keep paradigm and tradition out of every computation. These taxa were reexamined and discovered because the the coracoids did not match while so many other characters do match and nest them together. The coracoids still do not match on sisters Xiaotingia and Eosinopteryx, but several errors were repaired.

References
Godefroit P, Demuynck H, Dyke G, Hu D, Escuillié FO and Claeys P. 2013. Reduced plumage and flight ability of a new Jurassic paravian theropod from China. Nature Communications 4: 1394. doi:10.1038/ncomms2389
Xu X, You H, Du K and HanF-L 2011. An Archaeopteryx-like theropod from China and the origin of Avialae. Nature 475 (7357): 465–470.

 

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