Thursday, February 28, 2019

Quill knobs; on a carpometacarpus?

“Believe nothing you hear, and only one half that you see.” A quote from the great American poet and author Edgar Allan Poe.

I couldn’t help but to think back to those exact words the other morning while reading Sankar Chatterjee’s book; The Rise of Birds, 225 Million Years of Evolution (2015).

Here, Chatterjee nicely illustrates (and labels) the anatomical similarities and differences between forelimbs of a dromaeosaur (Deinonychus), and a Golden Eagle (Aquila chrysaetos). 

Dromaeosaur and Golden Eagle (Aquila chrysaetos) forelimb comparisons
Image from: Rise of Birds by Sankar Chatterjee (2016)
Informative in so many ways, the diagram did however present one peculiar and curious detail. Highlighted on the carpometacarpus of the Golden Eagle were six conspicuously drawn quill knobs. Quill knobs, I questioned, on a carpometacarpus? Have I ever seen quill knobs on a carpometacarpus before?

Golden Eagle (Aquila chrysaetos) carpometacarpus 
Image from: The Rise of Birds by Sankar Chatterjee (2016)
Quill knobs are typically shown on the ulna, where they serve as attachment points where secondary flight feathers are affixed to the bone with ligaments.

Northern Flicker (Colaptes auratus) ulna quill knobs
Photo Credit: Paul Cianfaglione
Taking Poe’s words to heart, I started to second guess my own knowledge of avian wing topography. Was I just plain careless over the years to have missed such an obvious feature? Or, was Chatterjee’s illustration a gross exaggeration.

As soon as I arrived home that evening, I pulled out a few (sixteen to be exact) bird bone specimens, systematically examining each and every one. What I discovered under the microscope was that most of the carpometacarpus’s lacked quill knobs altogether, with one bone holding a potential attachment point.

The carpometacarpus is unique to birds. It is the result of fusion of carpals and metacarpals. A carpometacarpus is reminiscent of a violin bow. The grip is composed of metacarpal I and has a pronounced pisiform process in the middle of its palmar surface and a metacarpal process extending radially. The bow is metacarpal II, and the much thinner metacarpal III represents the strings (source: Avian Osteology. Gilbert, Martin, Savage.1996).

Avian carpometacarpus
Image from: Avian Osteology, Gilbert, Martin, Savage. (1996)
Equally confusing is how the carpometacarpus is depicted in print while supporting most of the primary remiges. Some Ornithology manuals place the primary attachment directly on the slimmer metacarpal III, with the feather quills held firmly in place by the postpatagium. 

Rock Dove (Columba livia) wing anatomy
Image from; Manual of Ornithology, N.S. Proctor (1993)
Others, including a recent research paper (2016) on the flight feather attachment in rock pigeons, clearly shows the primaries attaching onto metacarpal II of the carpometacarpus.

Carpometacarpus primary feather attachments
Image from; Tobin L. Heironymus

So, who do we believe here? Where do the primaries actually attach? Is Chatterjee’s illustration correct? If so, where are all the quill knobs?

As I had mentioned earlier, I did detect what looks to be an attachment scar on metacarpal II of the carpometacarpus on a Pleistocene-aged American Coot (Fulica Americana). 

Possible American Coot (Fulica americana) carpometacarpus quill scar
Photo Credit: Paul Cianfaglione
If quill knobs did exist on the carpometacarpus, it makes more sense that they would be found on the wider and stronger metacarpal II. Chatterjee’s placement of the quill knobs (on metacarpal II) would therefore be correct, but to what extent.

After examining sixteen specimens myself, I still dispute the idea that quill knobs on a birds carpometacarpus are clear or apparent. Specimens that I inspected ranged in size from the large bodied Southern Screamer (Chauna torquata) and American Crow (Corvus brachyrhynchos), to the diminutive wood warblers and sparrows.

American Crow (Corvus brachyrhynchus), Northern Flicker (Colaptes auratus), American Coot (Fulica Americana)
carpometacarpus's (from top to bottom)
Photo Credit: Paul Cianfaglione
Granted, sixteen specimens does not qualify me as an expert on this subject, nor prove anything either way. Maybe Golden Eagles, and other species, do have prominent quill knobs on their carpometacarpus. At this moment, I wouldn’t know.

My hope is to see more illustrations, like the one below, that show the correct position of the primaries in relation to the carpometacarpus.

Bird wing feather attachments
Image from; Tobin L. Heironymus

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