Tupuxuara’s shoulders: the scapulocoracoid

In humans, the bone that connects the scapula to the sternum is the clavicle. Most other vertebrates have coracoids connecting scapula to stern (although they may also have articulating clavicles). Humans, marsupials, and mammals that don’t lay eggs do not have coracoids (but they do have a small hook in the upper scapula which is called the coracoid process). Birds and pterosaurs which require a strong shoulder girdle to support the wings have the scapula and coracoid bones fused together. This bone is called the scapulocoracoid.

The Tupuxuara specimen at the Iwaki Museum in Japan has one well preserved scapulocoracoid (as far as I know: I only have the pictures), so I used it as a model for both left and right scapulocoracoids.

cut

My sources are only four views of the scapulocoracoid (two sides, inner and outer views). It would be best to have at least six, and some photos in different angles as well, but from these sources I could infer the shape of the bone to start making it and later make any necessary adjustments. From the outer and inner views it seems that the scapulocoracoid has the twisted shape below, when seen from the side that connects to the arm.

twist

The place where the two flat long bones are fused together is full of details which aren’t so easy to see from the photos. I cut out the general shape in thick foam using the side views as a reference.

connection

Then I cut it again (since the base is not flat), trimmed the parts so they would fit well and pasted it onto the scapulocoracoid shaped foam strip. The next step is to shape the coracoid and scapula bones. I started adding strips of foam (since I will make it hollow.)

foam1

I reinforced both long bones with a thin (3mm) somewhat flexible wooden stick inside, along the length of the bone. I didn’t photograph it. The photo below shows both scapulocoracoids almost finished.

foam2

This is the right scapulocoracoid just before finishing with fire.

right_foam

And this is the left scapulocoracoid after the fire sculpting.

left_foam

Here are both of them:

finished_foam

testing_foam

After that, I added a layer of acrylic resin (modelling paste) and tried to mount the pectoral girdle. But… which side is the coracoid and which side is the scapula? The part I thought was the coracoid seemed too short, so maybe it’s the other way around.

AorB

I lack experience in vertebrate anatomy, so I needed to do some research. I discovered that, unlike Guidraco, Anhanguera and other Ornitocheirids, Tupuxuara has a shorter coracoid (like Tapejara and Azhdarchids). In Anhanguera the shoulders would be placed near the back, where in Tupuxuara they should be closer to the front. The pictures above are not precise since I still had to adjust all the parts, but it seems that the correct placement is the second. The first assembly places the scapular bones almost perpendicular to the notarium (that’s why in ornitocheirids there sometimes is a depression on the the vertebra’s spinal process.) For the second one to work, the scapulas have to move back more and barely touch the spine at an angle (45 degrees or more). I am still not sure about all of this and will do more research before attempting to assemble it.

Here are the pictures of the finished scapulocoracoids.

back

bottom

front

top

Since this is a very complex bone to grasp from photos, it’s good to have views in other angles.

angle1

angle2

outside

I still can’t assemble the full pectoral girdle connecting the stern and notarium. I made the first vertebra too wide and will have to remove at least 1 cm from each side. But I was able to test the connection with the stern. Here are some pictures (I didn’t double check and don’t really know if this is the right way to do it, but it seems to fit nicely).

girdle_back

girdle_bottom

girdle_front

girdle_side

girdle_top

The two pieces below should fit together nicely, but they don’t because of a problem in the larger vertebra. I will have to fix that before assembling the pectoral girdle.

girdle_notarium

But I tried anyway (it’s made of foam, so I can always squish it a bit :)) The pectoral girdle will look something like this when finished:

neck_mount

head_mount

Tupuxuara now has shoulders. Now we need to connect some arms to them. Tomorrow we will make two humeri.

side_head

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1 Comment

Filed under Pterosaur #5: Tupuxuara

One response to “Tupuxuara’s shoulders: the scapulocoracoid

  1. Pingback: Making of a Tupuxuara | The Imaginary Pterosaur

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