Earlier I attempted a color tracing of the skull of the common chicken (Gallus gallus), and I got some flak for it, but, unfortunately, no corrective illustrations.
Figure 1. Chicken skull from around 1906. Note this illustration has only a smidgen of a maxilla above the anterior jugal. Sadly only a few bones and openings are identified here.
So I went to Digimorph.org and also found several online illustrations of bird skulls (Fig. 1; anatomical studies are quite rare and typically very old) to try to figure out where I went wrong.
Here (Fig. 2) are the results. The skulls of Archaeopteryx (courtesy of Greg Paul), an Early Cretaceous enathiornithine (Sanz et al. 1997), Struthio (the ostrich), Anser (the goose) and Gallus (the chicken) are compared (Fig. 1, click to enlarge).
Figure 2. Bird skulls compared. Here are Archaeopteryx, Struthio (ostrich), Gallus (chicken) , Anser (goose) and an unnamed Early Cretaceous enantiornithine nestling as large as Archaeopteryx (Sanz et al. 1997). Archaeopteryx by Greg Paul used with permission. Click to enlarge.
The similarities and variations among the bird skulls are interesting. I have avoided birds for too long (probably the reason for earlier errors). Let’s discuss them in brief.
You’ll notice first off that the skulls of Archaeopteryx, Struthio and the LP specimen are quite similar overall. By comparison, the more derived Anser and Gallus skulls are less similar.
Figure 3. The Struthio toothless rostrum in palatal view has regularly spaced pockets where teeth once appeared in its ancestry. Now these provide nutrients to the ever-growing beak. Click to enlarge.
Archaeopteryx and the LP specimen both had teeth, but extant birds do not. The last clues left by the teeth can still be seen in the primitive bird, Struthio (Fig. 3), which has living tissues, lips and gums and a beak in place of teeth. Those are not tooth roots, but nutrient foramina.
Upper temporal fenestrae disappearance
Archaeopteryx and the LP specimen have an upper temporal fenestrae bordered laterally by the postorbital and squamosal. In extant birds, the situation is not so clear. The postorbital is reduced to a vestige and the squamosal is incorporated into the brain case. That leaves the quadrate (jaw joint bone) less connected to the skull.
Archaeopteryx and the LP specimen have a lacrimal (stem-like bone between the antorbital fenestra and orbit) but it becomes a vestige in extant birds, still connected to the prefrontal above it.
Archaeopteryx and the LP specimen have a typical premaxilla tipping the snout. The maxilla extends halfway beneath the naris. In Struthio and Gallus the premaxilla extends further posteriorly, crowding out the maxilla. In Anser the premaxilla extends further posteriorly, and the maxilla is distinctly robust, both traits strengthening the rostrum.
In most tetrapods and most dinos the ectoptergoid (a palate bone) arises from the lateral flange of the pterygoid and contacts the cheek bones, typically the maxilla, sometimes the maxilla and jugal. In Archaeopteryx it contacts only the jugal, and then just barely. In a late Cretaceous flightless bird, Hesperornithis, the ectopterygoid is a vestige not in contact with the cheek. Ultimately the ectopterygoid is not present in the extant Struthio, Gallus or Anser. So evidently the ectopterygoid doesn’t fuse to the pterygoid and become part of it. Rather the ectopterygoid becomes a vestige and disappears.
Loss of maxilla ascending process
Archaeopteryx and the LP specimen have a typical maxilla with an ascending process arising to meet the nasal and lacrimal. But note it is much more gracile in the LP specimen. In extant birds the ascending process is not visible in lateral view, either replaced or covered by the descending process of the nasal.
Archaeopteryx has a tall quadrate. The others have a short quadrate, The others also have a larger cranium, housing a larger brain. That also lowers the ventral squamosal.
If I made any errors this time,
don’t be shy about providing illustrated corrections. I’ve only studied comparative birds skulls for one day so far. I’m sure there’s more to learn.
The animated Longisquama post had about twice as many hits as on a typical day. Thank you for your interest.
Sanz et al. 1997. A Nestling Bird from the Lower Cretaceous of Spain: Implications for Avian Skull and Neck Evolution. Science 276:1543-1546.