Preparing an auk bone from the Purisima Formation, part 1

I know I had promised another post on pelagornithids (and sooner than this), but I just finished a preparation project on a small bird bone, and I think it is too cool not to share the method I used for this. Dick Hilton and I were recently doing some fieldwork together in the Purisima Formation, and I jumped up on a ledge and found a bird humerus exposed in a horizontal exposure.Dick Hilton near a new baleen whale find in the Purisima Formation.

The new bird bone, not entirely unsalvageable.

As soon as I saw it, I thought it was unfortunate that so much was missing; nearly the entire caudal/posterior face was missing, and eroded parallel to the long axis of the bone. Additionally, the middle of the bone was completely gone; enough of each end were missing that it would be very difficult to collect each end separately and not have them fragment into a million pieces during preparation. Bird bones are not exactly rare in the Purisima Formation, but since the length of bird bones has historically been used as a taxonomic tool along with other morphologic features, I thought it better to excavate it as a block - and to be honest, nearly the moment I saw it, the gears were already turning and quickly formulated a preparation solution.

The new fossil specimen compared with a more complete humerus of Mancalla lucasi (formerly Mancalla diegensis). The cross-hatched area indicates what was missing of the new specimen.

Additionally, the curvature of the shaft and the distinctive proximal end (even in cross section) made the bone very easy to identify - it is transversely flattened and curved, which identifies it as the flightless auk Mancalla; this bird happens to be the most common bird taxon in the Purisima Formation - auks and puffins (Alcidae) happen to be relatively common in general. In a forthcoming paper, I describe a fossil humerus identifiable as the species Mancalla diegensis; in a recent paper on mancalline auks, my colleague (and coauthor on the Pelagornis article) Adam Smith sunk Mancalla diegensis and erected a new taxon, Mancalla lucasi (I have another post lined up summarizing mancallines and Smith's new monograph). This specimen is very similar to Mancalla diegensis in terms of size, but could just as easily belong to Mancalla cedrosensis, Mancalla calforniensis, or Mancalla vegrandis.

I thought an easy way to deal with this specimen would be to liberally apply a large sheet of two-part epoxy onto the exposed broken surface of the bone and sediment, and prepare it down from the other side. This I hoped would result in a sheet of plastic with the caudal surface of the bone exposed on the other side. The next post will describe the preparation process, and hopefully give you ideas on how to tackle similar problems when dealing with fragmentary fossils.