Brocklehurst and Field 2021: Tooth loss in birds

Brocklehurst and Field 2021 report,
“The origin of crown bird edentulism has been discussed in terms of a broad-scale selective pressure or trend toward toothlessness, although this has never been quantitatively tested. Here [Fig. 1], we find no evidence for models whereby iterative acquisitions of toothlessness among Mesozoic Avialae were driven by an overarching selective trend. Instead, our results support modularity among jaw regions underlying heterogeneous tooth loss patterns, and indicate a substantially later transition to complete crown bird edentulism than previously hypothesized (∼ 90 MYA). We show that patterns of avialan tooth loss adhere to Dollo’s law and suggest that the exclusive survival of toothless birds to the present represents lineage-specific selective pressures, irreversibility of tooth loss, and the filter of the K–Pg mass extinction.”

Never? Not true and more quantitively than in Brocklehurst and Field. According to the LRT a clade of Cretaceous toothed birds arose from a series of toothless taxa, including Megapodius (Figs. 2, 3). Brocklehurst and Field could have found this, too, but their taxon list is too small. Taxon exclusion is the #2 problem in paleontology.

Figure 1. Cladogram from Brocklehurst and Field 2021. Note the paucity of cherry-picked taxa compared to the LRT.

Figure 1. Cladogram from Brocklehurst and Field 2021. Colors added to clades. Note the paucity of cherry-picked taxa compared to the LRT. Entire clades of extinct birds are missing here due to taxon exclusion.

When you minimize taxon exclusion,
as in the LRT (subset Fig. 2) the actual patterns of evolution start to emerge. When you cherry-pick taxa (Fig. 1), you risk missing the important nodes and steps that Brocklehurst and Field missed.

Figure 4. Subset of the LRT focusing on birds. Chongmingia is highlighted in yellow in the Scansoriopterygidae.

Figure 4. Subset of the LRT focusing on birds. The amber box are the toothed Cretaceous birds, descendants of toothless taxa like Megapodius.

The Brocklehurst and Field 2021 study
depends on a valid phylogenetic context, but suffers from taxon exclusion. Only one ‘Archaeopteryx‘ taxon was used. A competing online cladogram (the LRT, subset Fig. 2) finds that nine of thirteen Solnhofen birds are needed to flesh out the origins of various succeeding bird clades, each with a few Solnhofen birds at their base.

Figure 1. Click to enlarge. Toothed birds of the Cretaceous to scale.

Figure 1. Click to enlarge. Toothed birds of the Cretaceous to scale.

Several toothless extant birds
that phylogenetically precede the Eogranivora to Ichthyornis and Yanornis clade (Figs. 2, 3) were excluded from this analysis. Missing taxa include Apteryx, Megapodius, and all members of the Palaeognathae, both living and extinct. Brocklehurst and Field missed a great opportunity due to taxon exclusion.


References
Brocklehurst N and Field DJ 2021. Macroevolutionary dynamics of dentition in Mesozoic birds reveal no long-term selection towards tooth loss, ISCIENCE (2021), doi: https://doi.org/10.1016/j.isci.2021.102243.

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