Screamers: the return of digit ‘0’

Screamers are extant birds
in the family Anhimae. They include the genera Chauna (Oken 1816; Southern screamer; up to 90 cm in length) and Anhima (Brisson 1760; horned screamer). The clade lacks uncinate processes on the ribs. but has large spurs on the metacarpus (Figs. 1, 2). The young are precocial (able to run soon after hatching). This is a rather primitive and very vocal clade.

The Anhimae clade was considered closest
to ducks (Anatidae) based on DNA, but the rostrum, as you can see (Fig. 1) lacks many duck-like traits.

By contrast,
the large reptile tree (LRT, 1065 taxa), grounded on morphology, nests Chauna at the base of the chicken, sparrow and parrot clade.

Fig. 1. Anhima skeleton and skull.

Fig. 1. Anhima skeleton and skull.

The manus of screamers is atypical
(Fig. 2) in that large spurs arise from the distal metacarpus (as a new ossification) and from the proximal metacarpus (as the return of digit ‘0’, a digit first brought to light with the Limusaurus discovery and misinterpretation). Along with the return of digit ‘0’ we also find fused vestiges of digits 4 and 5.

Fig. 2. Screameer manus showing the full expression of digit 0 at the base of the metacarpals producing a large anteriorly-directed spur.

Fig. 2. Screameer manus showing the full expression of digit 0 at the base of the metacarpals producing a large anteriorly-directed spur. Faint vestiges of digits 4 and 5 are also present.  Note how easy color explains things by clearing segregating one bone from another, even when they fuse.

You won’t find any references to digit ‘0″
in the academic literature. That reversal in theropods and birds was first hypothesized here a few years ago, and well documented above (Fig. 2).

References
Brisson MJ 1760. Ornithologie, ou, Méthode contenant la division des oiseaux en ordres, sections, genres, especes & leurs variétés : a laquelle on a joint une description exacte de chaque espece, avec les citations des auteurs qui en ont traité, les noms quils leur ont donnés, ceux que leur ont donnés les différentes nations, & les noms vulgaires
Oken L 1816. Lehrbuch der Zoologie (or Lehrbuch der Naturgeschichte 1–3. Theil. Zoologie ; 2. Abt. Fleischthiere) Jena.

wiki/Anhima
wiki/Chauna

 

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