SVP abstracts – Ichthyornis and the origin of extant birds

Benito et al. 2019 dive into bird phylogeny
with a study of the Late Cretaceous toothed bird, Ichthyornis (Fig. 1).

Figure 1. Skull of Ichthyornis in 3 views from Field et al. 2018 and overall skeleton.

From their abstract
“The origin of crown birds is poorly understood

By contrast, in the large reptile tree (LRT, 1592 taxa) the origin of crown birds is well understood back to Silurian jawless fish. Ichthyornis is a member of the clade of crown birds in the LRT, not an ancestor to it.

“…and the study of their early evolution must incorporate data from their closest relatives among Mesozoic stem birds. The postcranial morphology of the Late Cretaceous toothed bird Ichthyornis dispar may be more representative of the ancestral condition of crown birds than that of any other known Mesozoic avialan, and its study has crucial implications for understanding morphological evolution prior to the great radiation of the avian crown group.”

By contrast, in the LRT Vegavis (Latest Cretaceous) is basal to all extant birds including Mesozoic toothed birds like Icthyornis. It was a late survivor from an earlier genesis.

“Here we present high resolution scans of new, exquisitely preserved three dimensional specimens of Ichthyornis from the Late Cretaceous of Kansas. These correspond to a partial skeleton from a single individual, more complete and in better condition than the classic material known since the 19th Century. The new material includes a complete sternum and shoulder girdle with evidence of extensive pneumatization. This new skeleton shows certain morphological differences from the classic material, including the absence of some previously proposed autapomorphies of I. dispar. Thus, the new material may represent a previously unknown species, or it could indicate that morphological variation within I. dispar was greater than previously appreciated.”

Good to have these new data.

Figure 2. Subset of the LRT focusing on early birds, including Ichthyornis.

Benito et al. continue:
“Phylogenetic analyses incorporating our new morphological data corroborate recent results and recover a grade of predominantly marine taxa close to the origin of crown birds. I. dispar is recovered stemward of Hesperornithes and Iaceornis marshi, which is recovered as the sister taxon to all crown birds. Additional information on the crownward-most portion of the avian stem group will help confirm these results and provide critical information on the ancestral ecology of the crown bird radiation.”

I don’t know if Benito et al. employed all the taxa shown here (Fig. 2) in this subset of the LRT, but you can see Ichthyornis nests in the LRT within the clade of extant/crown birds. Here Ichthyornis is a highly derived member of its own small clade of toothed birds, within extant birds between megapodes and seriemas. Other taxa closer to the main line of Cretaceous bird evolution would probably make a better model for studies like this.

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References
Benito J et al. 2019. New Ichthyornis specimens: shedding new light on modern bird origins. Journal of Vertebrate Paleontology abstracts.