A NEW HOLOCEPHALAN TOOTH PLATE FROM THE BANGOR LIMESTONE (MISSISSIPPIAN, CHESTERIAN) OF NORTHERN ALABAMA

A holocephalan tooth plate having nearly perfect bilateral symmetry is reported from the Mississippian Bangor Limestone of Franklin County, Alabama. The symmetry indicates that it occupied a symphyseal, or possibly parasymphyseal, position. Symphyseal dentitions have rarely been reported in holocephalan chondrichthyans, but are not unknown, e.g., the tooth whorl of Helodus coxanus, which is also known from the Bangor Limestone. Presuming that the tooth plate occupied a symphyseal position, it is elongated along the anterior-posterior axis and is compressed laterally. Viewed from the sides, the margin of the occlusal surface forms a moderately curved, convex arc with six low cusps. Viewed from the anterior or posterior ends, the occlusal surface appears as a sharply pointed, angular ridge. The occlusal surface is coarsely punctate. The tooth plate resembles Deltodopsis bialveatus. However, D. bialveatus is not bilaterally symmetric, has a smooth, convex occlusal surface, and likely occupied a median position on either side of one of the jaws. Aside from its bilateral symmetry, the most remarkable feature of the new tooth plate is the sharp, multicusped median ridge. The ridge might have had a grasping function and formed an element of a dentition that also included broader, flatter tooth plates, like those of Deltodus or Cochliodus, that would have had a crushing function. The tooth plate is dated to the Chesterian Stage (late Mississippian) by conodonts found in the same bed.