Teasing out overlooked details from an Early Cretaceous bird, Zhongjianornis

In their description of a new Early Cretaceous birds ‘without teeth’,
Zhou and Li 2010 traced Zhongjianornis and did a great job on the skeleton overall (Fig. 1).

Figure 1. Zhongjianornis in situ from Zhou and Li 2010.
Figure 1. Zhongjianornis in situ from Zhou and Li 2010.

The Zhongjianornis skull was another matter
as the authors made several errors (Fig. 2). They just did not put in the effort to pull out details, but were content to trace broad areas.

Figure 2. Zhongjianornis skull from Zhou and Li 2010, plus their tracing (gray diagram). Colors and reconstruction added here.
Figure 2. Zhongjianornis skull from Zhou and Li 2010, plus their tracing (gray diagram). Colors and reconstruction added here. The reconstruction is the puzzle you put together based on the color tracings. If all the pieces fit, that’s a good sign, but mistakes are still possible.

Here, without looking at the fossil first-hand,
and only using DGS methods to trace their published photo, a more accurate identification of skull parts is offered. The skull parts can be moved (as they are without the unconscious bias of freehand) to a reconstruction of the skull that not only fits together like the puzzle it is, but also matches patterns and shapes in closely related taxa, in this case, Confuciusornis (Fig. 4). Only the beak is relatively longer.

Figure 3. Confuciusornis skull traced and reconstructed. Compare to Zhongjianornis in figure 2.
Figure 3. Confuciusornis skull traced and reconstructed. Compare to Zhongjianornis in figure 2.

Zhongjianornis yangi
(Zhou and Li 2010; Early Cretaceous, IVPP V15900, pigeon-sized, scale bar on skull image is 1cm) nests with Confuciusornis in the LRT, but has a longer beak, shorter hands and smaller sternum. Pedal digit 4 is as long as 3. The present tracing is more precise than the original. Confuciusornithids had only 8 cervicals, one less than most coeval birds.

Zhou and Li reported in their abstract:
“The new taxon is characterized by possessing the following combination of features: upper and lower jaws toothless, snout pointed, humerus with large and robust deltopectoral crest, second phalanx of the major manual digit longer than the first phalanx, unguals of the alular and major digits of similar length and significantly shorter than the corresponding penultimate phalanges, tibiotarsus slender and more than twice the length of the tarsometatarsus, and metatarsal IV longer than the other metatarsals.”

Several of these traits are common to Confuciusornis. The short metatarsus relative to the tibia is not found in related taxa. Zhou and Li’s own tracing indicates mt 4 = mt3 and digits 3 and 4 extend subequally measured from the tarsus (Fig. 1).

Phylogenetic analysis indicates that Zhongjianornis is phylogenetically basal to Confuciusornis and the dominant Mesozoic avian groups, Enantiornithes and Ornithurae, and therefore provides significant new information regarding the diversification of birds in the Early Cretaceous.”

By contrast, in the LRT (Fig. 3) Zhongianornis nests with the GMV-2132 specimen of Confuciusornis and these two nest with MBAv1168 specimen, so Zhongianornis is not basal to the genus, but perhaps congeneric. Confuciusornithids are derived members of a larger clade that nests basal to Enantiornithes and Ornithurae in the LRT, agreeing with Zhou and Li.

It also represents the most basal bird that completely lacks teeth,”

Not the most basal bird in the LRT where it shares that credit (Fig. 2) See above.

suggesting that tooth loss was more common than expected in early avian evolution and that the avian beak appeared independently in several avian lineages, most probably as a response to selective pressure for weight reduction.”

This is a common trope, or recurrent theme that might be a myth. Think about it. Which would weigh more: a set of small teeth or a swallowed mouse? If herbivorous, substitute a pile of seeds or a drink of water.

Finally, the presence of a significantly enlarged humeral deltopectoral crest suggests that Zhongjianornis shares with other basal birds such as Jeholornis, Sapeornis and Confuciusornis a distinctive mode of adaptation for flight contrasting with that seen in more advanced birds, which instead possess an elongated sternum and a prominent keel.”

Note that Jeholornis, Sapeornis and Confuciusornis
are members of related clades in the LRT (Fig. 2).

Figure 3. Subset of the LRT focusing on basal birds. Blue taxa are all Solnhofen birds (= traditional Archaeopteryx). Note: Archaeopteryx (Wellnhoferia) grandis) nests at the base of the Confuciusornis clade.

Members of the Confuciusornis clade were not restricted to China.
A basal member, Archaeopteryx (= Wellnhoferia) grandis (Fig. 4) is from the Late Jurassic of the Solnhofen Formation. Whenever taxa are added, unexpected interrelationships tend to appear.

Figure 4. Confuciusornithiformes to scale. Note the lack of a pygostyle in the majority of taxa.
Figure 4. Confuciusornithiformes to scale. Note the lack of a pygostyle in the majority of taxa.

Recent housekeeping
on the bird and fish subsets of the LRT resulted in several details and scores. Corrections were made. Now each of those subsets is completely resolved. Based on the number of taxa, it still takes a few minutes for the software to crunch the numbers on my old IBook that still runs System 9, but a single tree was recovered. PAUP can only handle a maximum of 1500 taxa, which is the reason for the splitting up of the 2052 taxa into three overlapping MacClade files.

Your cladogram can be your best friend.
If it is not fully resolved, those poorly resolved nodes are telling you where some of the mistakes are. The DGS method of coloring digital images of bones mentally simplifies comparative anatomy. It is also helpful to have all the data digitized or online for ready and rapid access on a pair of monitors. Gone are the days of rifling through a file cabinet full of photocopies, thank goodness.

I’m not saying the scoring is perfect now.
It never is. I am saying the scoring is much improved at the nodes that needed it.

References
Zhou Z and Li FZZ 2010. A new Lower Cretaceous bird from Chna and tooth reduction in early avian evolution. Proceedings of the Royal Society B 277:219–227.

wiki/Zhongjianornis

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