Finding Nemo

In the 2003 Disney Pixar Movie Finding Nemo, a clownfish named Nemo has to navigate the hazards of both open ocean and captive aquarium life with a reduced front flipper.   SPOILER: Nemo does fine, but actual organisms in the wild would likely not fare as well.

Case in point, SDSM 32720, a partial skeleton of the mosasaur Platecarpus tympaniticus from the Niobrara Chalk from Tom Conger’s ranch on the southeastern flank of the Black Hills.  The humerus of this specimen is figured below.

IMG_2216Outlined

Left: humerus of SDSM 32720.

Right: Same humerus outlined to indicate displacement of anatomical features.  Arrows indicate dorsoventral axis from an “unaltered” element.  Dashed lines outline calloused area.

 

Anyone somewhat familiar with mosasaur anatomy will immediately notice something amiss.  In most vertebrates, the humerus is an elongate bone in the upper arm.  Because mosasaurs evolutionarily convert their limbs to shortened “flippers”, all of the arm bones are, correspondingly, compressed, giving the humerus an hourglass shape.  This humerus is unique.

Below is the humerus of 32720 compared with a drawing of a “normal” humerus of the mosasaur Prognathodon.  You’ll note that 32720 is, to put it gently, “messed up.”  The distal, or bottom portion of the humerus is rotated approximately 30-40 degrees off center.  This is due to a pathology, or evidence of a disease or injury.  It appears as if this individual suffered a pretty severe break to one of its forelimbs.  But, as evidenced by the thickly calloused area (dotted outline above), this animal actually managed to survive long enough to heal up.  Additional images of the opposite side of the humerus are included below with comparison with a “normal” one.  This poor guy endured quite the trauma.  Additional elements currently being prepped, including the coracoid and scapula, show similar evidence of injury.  It’s difficult to say what caused this injury, but a larger mosasaur or short-necked plesiosaur would be a good bet.

lateralcomp
SDSM 32720 compared to a drawing of a “normal” humerus from Prognathodon.
medialcomp
Opposite side of SDSM 32720 compared with a drawing of a “normal” humerus from Prognathodon.

This specimen is unique for two reasons.  First, it shows firsthand how difficult life in the wild can be.  Animals routinely suffer extremely brutal injuries.  Despite these setbacks, many heal and survive long after the trauma.  Pathologic evidence is common in vertebrate fossils, and has been reported in stegosaurs, tyrannosaurs, mammoths, saber-toothed cats, and may other taxa.

Additionally, SDSM 32720 had already been partially curated.  Several vertebrae and ribs were already in the collections.  However, because this specimen was removed from multiple jackets, we only came across additional portions recently, several years after the original material was prepared and moved to collections storage.  Proper documentation, from discovery to excavation to prep, has allowed us to reunite this unique specimen.

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