What happened to the postfrontal and postorbital in birds?

Fauth and Rauhut 2020 bring us
“A short overview of the evolution of the skull of birds.”

From the first paragraph (Google translated from German)
“There are a number of advantages to being able to fly, be it the possibility of rapid geographical expansion, the settlement of trees, the escape from predators or the development of new feed sources, including prey capture. However, it cannot be regarded as the sole factor for the success of birds.”

Thereafter
the authors discuss and show (Fig. 1) skull traits, but make a traditional mistake based on a lack of attention to detail. Foth and Rauhut provide only one figure (Fig. 1), in which the postorbital is identified (in orange) only in Allosaurus (B) Archaeopteryx (C) and the enanthiornine, Shenqiornis (D). The postorbital is deemed absent in the extant Crax (A) and the extinct Ichthyornis (E) despite its presence in their diagram.

Figure 1. Theropod and bird skulls from Foth and Rauhut 2020. Postorbital is highlighted in orange, but not the same vestigial postorbital is not highlighted in bird skulls.

Figure 1. Theropod and bird skulls from Foth and Rauhut 2020. Postorbital is highlighted in orange, but not the same vestigial postorbital is not highlighted in bird skulls. Note: ‘Archaeopteryx’ is a wastebasket taxon with variation among the 13 known specimens.

Unfortunately
Foth and Rauhut took the easy way out by using previously provided oversimplified diagrams that lack the data needed to create a valid figure. They also followed paleontological tradition, which, at times like this, fail to provide valid data in the details.

Here are the missing details
in an actual Crax skull (Fig. 2) colorized using DGS methods. It shows a descending postfrontal (orange) and a vestigial postorbital (yelllow splint, but see caption for one more option). The postfrontal is largely fused to the frontal, but that does not negate its presence. No unfused frontal descends beyond mid depth in any vertebrate skull. We should label and score with reason, not with invalid traditions.

Figure 1. Crax tuberosa skull in three views.

Figure 2. Crax tuberosa skull in three views. Note the splint-like post0rbital (yellow). Alternate hypothesis: the splint is the postorbital process of the jugal (cyan, separate ossification from the base below the quadratojugal (olive). That would make the lumpy orange postfrontal the postfrontal + fused postorbital. Time to look at some embryos to see what is happening here: another great PhD dissertation.

The Eichstätt specimen of Archaeopteryx (= Jurapteryx)
shows the separation of the postfrontal (orange) from the frontal and the postorbital (in yellow) disarticulated and shifted slightly posteriorly in situ. This is the specimen basal to extant birds.

Figure 3. The Eichstätt specimen, Jurapteryx recurva, nests with the living ostrich, Struthio, presently in the LRT.

Figure 3. The Eichstätt specimen, Jurapteryx recurva, nests with the living ostrich, Struthio, presently in the LRT.

The tiny Early Cretaceous theropod, Scipionyx
(Fig. 4), demonstrates the separation of the frontal (blue), postfrontal (yellow-green) and postorbital (orange) in non-avian theropods. These elements tend to fuse with size. Phylogenetic miniaturization (= neotony) tends to separate the original elements. When dealing with shrinking taxa, like birds, try to keep this in mind.

Figure 1. Scipionyx skull and overall. The tail and feet are restored.

Figure 4. Scipionyx skull and overall. The tail and feet are restored.

The enantiornithine, Shenqiornis,
will be considered in greater detail In future blogposts.


References
Foth C and OWM Rauhut 2020. Eine kurze Betrachtung der Evolution des Vogelschädels [A short overview on the evolution of the skull of birds]. Jahresbericht 2019 und Mitteilungen 48. ISSN 0942-5845 ISBN 978-3-89937-253-3

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