Squirrel Resurrection

Three years ago my family discovered a dead squirrel in our basement.  It was unclear how the squirrel got in, or exactly how the squirrel died, but it was in good condition when we found it.  So I convinced my dad to help me bury the squirrel so  that I could get a complete squirrel skeleton.  We wrapped the body of the squirrel in the black mesh that you put down under gardens to prevent weed growth so that the little bones would not get lost.  Next we dug a small hole out behind the shed and put the make shift mesh bag in.  After covering the body with dirt, we covered the burial with a large paving stone.

Today I dug up that squirrel!

Can you feel the anticipation??

Although the squirrel was probably fully skeletonized two years ago, I had completely forgotten about it, and usually when I am home for winter break the ground it frozen so it would be very difficult to recover the squirrel.  This winter has been surprisingly mild, so I was actually able to dig up the squirrel and sit outside for several hours sorting the skeletal remains out of the mass of dirt that has accumulated in the mesh. 

Big reveal!!! ;)

Ooooo, teeny tiny squirrel bones

After sorting out the bones from the dirt clods and roots, I was left with a nearly complete squirrel skeleton.  I was even able to collect most of the phalanges and a good number of carpals and tarsals.  I will need to look for a squirrel reference book, since I am not familiar with how many carpals and tarsals a squirrel should have.  Also, I learned that squirrels only have four toes on their front paws, though they do have five on their hind paws.  I was wondering why I only recovered four sets of metacarpals even after extensive searching so it was exciting to discover that I had actually collected them all.

Fingers and toes!!!

Ribs and verts

Box-o-squirrel

One interesting thing about this particular squirrel is that it was not fully grown.  I can determine this by looking at the epiphyseal fusion.  Epiphyses are the ends of the bones and form separately from the diaphysis or shaft of the element.  The epiphyses form via secondary ossification centers that are disconnected from the shaft.  As an individual grows, the shaft undergoes longitudinal growth (increase in length) reducing the space between the shaft and ends.  Eventually the portions meet along the metaphysis or growth plate and fusion occurs.  When the diaphysis and epiphysis initially come together a line can be seem at the point of union.  Over time this line is obliterated and it is impossible to see where the portions joined.  In this case, the distal femur (top of knee joint), proximal tibia (bottom of knee) and proximal humerus (at shoulder) are all unfused.  Furthermore, the fusion lines are still visible on the distal radius and ulna (wrist). 

After a little more cleaning, and research into squirrel skeletal development I will hopefully have a better idea of what happened to this little guy.  :)

You've got a Lizard in your Ear

The mammalian skull offers a unique opportunity to discuss evolutionary adaptation and and the difference between homologous and analogous features.  When examining the fossil record, the separation of modern "mammals" is demarcated by a change in joint connecting the mandible to the skull.  Prior to this point the connection was formed by two separate bones, the quadrate and the articular.  In birds and lizards and other non-mammalian genera these bone are separate from the body of the mandible and the cranium.  In mammals, the mandible articulates directly with the temporal bone of the cranium, without any intermediary bones.

Meow?

So, are the quadrate and articticular bones present in mammalian skulls?  In fact these bones are conserved, but they serve a different function in the mammalian skull.  Homologous features are the same physical structure between different taxa.  While the features are morphologically the same, they may preform different functions in different organisms.  On the other hand, analogous features have the same functional uses, but arise from different anatomical structures.  In mammals, the incus and the maleus bones of the ear are homologous to the quadrate and articular, respectively.  Although the bones are the same anatomical structure, in mammals, the bones preform a major role in hearing rather than jaw movement.  Instead, the articulation between the temporal and the mandible is analogous to the quadrate/ articular connection in non-mammals, since it has taken on the functional job of connecting the mandible to the cranium.

Transitional fossils have been found showing organisms with a jaw articulation between the temporal and mandible as well as the quadrate and articular.  The change in the jaw articulation is likely due to several different factors.  First of all, reduction in size of the articular and quadrate made them more sensitive to vibrations and improved the hearing capabilities of these organisms.  Organisms with better hearing were better able to avoid predators and reproduce.  Thus, the continued reduction of the ear bones was adaptively advantageous.  Alternatively, the reduction of the jaw joint limited the size of the jaw and the bite strength of the organism.  In this case, a larger joint was more advantageous because it allowed the organism to consume larger prey and also subdue the prey more efficiently.  Thus, decoupling these two functions, allowed mammals to benefit from both increased hearing and jaw size.

Be Aware, It's a Bear!!!

If life ever gets you down, just remember, you have a tiny little lizard jaw joint working in your ear. :)