Bird Skull and Pelvis vs Mammal Skull and Pelvis

When IDing bones, one of the most common mistakes I come across is confusion between bird and mammal skulls and pelvises. Not just confusing a bird pelvis for a mammal one, but often a bird pelvis for a mammal skull. Since this is so common, I decided to have a post with a more in depth explanation.

I will be using a Domestic Muscovy Duck (Cairina moschata var. domestica) and Gray Fox (Urocyon cinereoargenteus) to demonstrate.

From left to right: Duck pelvis, duck skull, fox skull, fox pelvis.
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First I will start with the most common mistake of this order confusion, mistaking a bird pelvis as a mammal skull. Most of the confusion seems to come from the bird pelvis' acetabulum, circled below.
(see also, Parts of a bird pelvis: https://www.inaturalist.org/journal/lizardking/30724-parts-of-a-bird-pelvis)

(Left, duck pelvis, right, fox skull.)
This is where the femur articulates into the pelvis. However, its perfectly circle shape is often mistaken as an eye socket. As you can see on the fox skull, eye sockets aren't actually that clean and circle. It is a very common mistake though, and I will also detail some ways to tell the bone is a pelvis, and not a skull.

(Left, duck pelvis, right, fox skull.)
Both the duck pelvis and fox skull have a similar form, a wider posterior (back) and then a more narrow anterior (front), the fox's snout seeming analogous to the duck's anterior. That, however, when turned upside-down, is revealed to be not dental and nasal structures, but fused vertebra.

(Left, duck pelvis, right, fox skull.)
Bird bones have to be very efficient. Strong, supportive, but light weight. They have a lot of fusion, and the pelvic girdle is a great example of this. Much of the lower spine is fused to be part of the pelvis. So while the anterior of a mammal skull should have teeth or holes for teeth, the anterior of a bird pelvis -and its entire length- has fused vertebra.
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Second is a less obvious, but still common, confusion.
When bird skulls have their beaks, they are typically easily distinguishable from mammal skulls. I have seen confusion when the bill is broken off, and also on occasion rabbit skulls being confused for avian. The easiest way to tell an avian skull from a mammalian one is the rounded "bump", or occipital condyle, on the skull, around the foramen magnum, or the opening at the back of the skull the spinal cord runs through. Birds have one, right under the center of the foramen magnum, and mammals have two, on either side of the opening.

Left, duck skull, single arrow pointing to it's single occipital condyle. Right, fox skull, arrows pointing to both occipital condyles.

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And last is a less common, but still frequent cause of confusion, mistaking bird and mammal pelvis.

(Left, duck pelvis, right, fox pelvis)
As said above, bird bones have incredible efficiency of strength and lightness, and fusion is one mechanism that helps achieve this. Mammals don't have this same need, and the structure of their pelvises appear far more simplistic. Bird pelvic girdles have a lot of spine fused to the pelvic bones, where mammals lack any apparent vertebra in their structure*.

(Left, duck pelvis, right, fox pelvis)
Also discussed earlier, the circle opening confused for an eye socket is actually the acetabulum, where the femur attaches to the pelvis. In mammals, this is less of an opening and more of a crater or indent. The spherical indent on the side of the pelvis is the fox's acetabulum.

*The exception to this is the sacrum, which all mammal pelvises have, but often falls off as an animal decays, and isn't always found with the pelvis when exploring in nature. My fox pelvis didn't have the sacrum still on it when found, but here is what one would look like: https://www.boneid.net/product/anterior-view-grey-fox-sacrum/
It sits in between in the pelvis.
It isn't as large or thing as typical of avian pelvic structures, and mammal sacrums in general tend to be bulkier than anything typical of avians.

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This is an overview, a general explanation to help educate and also to explain my own personal ID corrections, but I can gladly explain aspects more in depth, and as always, I am always open to help with anything bird bone related. Lee Post has an excellent book that covers how to differentiate the entire avian skeleton from mammalian, The Bird Building Book.

Publicado el junio 18, 2021 06:31 TARDE por lizardking