The pre or post K-T origin of extant birds (Neornithes)

we’re going to add Apsaravis (Norell and Clarke 2001, Late Cretaceous, Figs. 1–3) to the large reptile tree.

Figure 1. Yanornis, Apsaravis and Pseudocrypturus to scale. The origin of all extant birds starts here.

Figure 1. This is not a phylogenetic order. Apsaravis is an offshoot. Here are Yanornis (Early Cretaceous), Apsaravis (Late Cretaceous) and Pseudocrypturus (early Eocene) to scale. The origin of all extant birds starts here. We’re still missing fossils in the small morphological but large chronological gap between Yanornis and Pseudocrtypturus (Late Cretaceous + Paleocene), But in the meantime, Paleocene birds show great variation (see yesterday’s post).

Norell and Clarke 2001
introduced us to Apsaraviis (Fig. 1), a Late Cretaceous volant and probably toothed bird from Mongolia. Clarke and Norell 2001 nested Apasaravis between Hesperornis and Ichthyornis + Aves (Crypturellus, a genus of extant tinamous).

Clarke and Norell 2002 reported:
“Phylogenetic placement of Apsaravis ukhaana as the sister taxon of Hesperornithes + Aves resulted from analysis of 202 characters scored for 17 avialan ingroup taxa.”

Largely in agreement with both original papers,
the large reptile tree (LRT, 1063 taxa) nests Apsaravis with Ichthyornis and Hesperornis. Yanornis martini (Zhou and Zhang 2001) nested basal to all three and basal to Pseudocrypturus, at the base of extant birds. Neither Norell and Clarke 2001 nor Clarke and Norell 2002 mention Yanornis in their papers that came out at about the same time as the publication of Yanornis. Note: The LRT did not employ 202 bird traits, but 231 generalized traits, only a few of which were needed to nest Apsaravis exactly where Norell and Clarke did (sans Yanornis).

This is why taxa are more important than characters.
You need taxa. They are the puzzle pieces that create the big picture. You need a long list of characters, but you don’t need a really long list of characters if the long list is sufficient to lump and separate all the pertinent taxa, even if does so just barely.

Figure 1. Apsaravis in situ. Here the poster end of the scapulae hide among the ribs, but a very simple case of DGS clarifies the situation.

Figure 1. Apsaravis in situ I’m ventral exposure from Norell and Clarke 2001. Here the posterior end of the scapulae hide among the ribs, but a very simple coloring (DGS) clarifies the situation. See Figure 3 for a reconstruction of these elements.

Sometimes reconstructions are presented rough
(Fig. 3) which keeps the fidelity of the original tracing at the expense of creating a clunky in vivo pose.

Figure 3. Apsaravis reconstructed from original drawings in Norell and Clarke 2001.

Figure 3. Apsaravis reconstructed from original drawings in Norell and Clarke 2001. The rostrum was probably long, as in Ichthyornis.

Wikipedia reports:

“Apsaravis is important in avian paleontology. It has provided evidence that is directly relevant to at least four issues:”

  1. Evidence against a clade Sauriurae, which includes Archaeopteryx, Confuciusornis and Enantiornithes and is separate from modern birds. The LRT nests several Solnhofen birds (= in this sense Archaeopteryx) at the bases of all the higher bird clades, including modern birds. Adding more Solnhofen birds needs to be done in all future analyses. The LRT does not recover the clade Sauriurae separate from modern birds.
  2. Apsaravis erodes the monophyly of the clade Enantiornithes. The LRT supports the monophyly of the Enantiornithes with or without Apsaravis.
  3. Found in desert dry sediments Apsaravis proves that not all early members of Ornithurae (extant birds + Ichthyornis + Hesperornis) were shore birds. True. But we know that volant birds can fly and die anywhere. Additionally deserts often include oases.
  4. Asparavis is the most basal bird with an extensor process, a bony projection medial to metacarpal 1. This automates extension of the manus during extension of the forelimb. Here that extensor process is homologous to digit 0. I thought the non-rotating radius bone, working like a parallelogram, was responsible for automatic extension of the manus during forelimb extension (as in bats and pterosaurs). Let’s compromise and say all the parts work together.

Today’s examination of Apsaravis
brings up a similar genus, Ambiortus (Kurochkin 1985; Late Cretaceous). Unfortunately it is only known from an incomplete anterior torso and a few feathers.

And then, there’s Gansus.
Gansus (Hou & Liu 1984, You et al. 2006) is now known from several incomplete skeletons, all lacking a skull. It has been described as an Early Cretaceous (Aptian-Albian), duck-like ornithurine. You et al. nested it between Hesperornis and Ichthyornis, so, according to their studies, it is not a member of the clade of modern toothless birds, the Neornithes. That is confirmed by a recent reconstruction and nesting in the LRT that nests Gansus as a basal hesperornithid. We’ll look at that taxon in more detail soon.

We’re still missing Late Cretaceous and Paleocene fossils
fromn the small morphological gap, but large chronological gap between Yanornis and Pseudocrtypturus. In the meantime, Paleocene birds show great taxonomic variation (see yesterday’s post). So my guess/prediction is we’re going to someday see more pre-Tertiary euornithine birds discovered that survive the K-T event and continue to radiate in the Paleocene. Pseudocrtypturus changed relatively little during that time, resembling extant tinamous.

Norell MA and Clarke JA 2001. Fossil that fills a critical gap in avian evolution. Nature 409:181–184.
Clarke JA and Norell MA 2002. The morphology and phylogenetic position of Apsaravis ukhaana from the Late Cretaceous of Mongolia. American Museum Novitates 3387:46 pp.



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