Sunday, July 31, 2011

Preparing an auk bone from the Purisima Formation, part 2

In order to properly prepare this bird bone (see previous post) , I decided to pour two part epoxy onto the eroded surface of the bone. In the field, I carved a block out of the rock with the bird bone in it, and wrapped it in tin foil. Weeks later, I unwrapped it and began the preparation process.

Beginning steps of preparation. A - the collected block prior to preparation. B - supplies needed. C - application of vinac using a paintbrush. D - application of thin superglue to stabilize parts of the bone.

First off, the bone had to be stabilized. Vinac and Butvar are two acetone-based consolidant glues which are very thin (i.e. have a low viscosity) and soak into porous bone well. In this case, I was not satisfied with vinac alone, so I began by dripping superglue into the most poorly preserved parts of the bone; superglue is also thin enough for this task, although it is substantially more difficult to reverse if you screw something up. I followed this by a liberal application of vinac onto the bone. I also painted vinac onto the sandstone where I would later pour epoxy. I did not want the outermost layer of the sandstone to flake off of the cured epoxy and take the bone with it, or alternatively, have only part of the bone stick onto the epoxy plate and the rest fragment off.

Epoxy application onto the fossil. A - a rolled up cylinder of paper serves as a convenient and cheap tool to drip epoxy with (as opposed to popsicle sticks or tongue depressors). B and C - the rolled up paper tube is used to drip epoxy. D - the epoxy sheet is allowed to cure overnight.

Part two of the preparation process was to apply a generous layer of epoxy, once the vinac had cured. Two part epoxy comes in paired tubes, and has to be mixed - I usually just mix it onto a piece of scratch paper. I tear off one side, and roll it up to use as a "honey dripper" (you know, like the thing you see in honey nut cheerios commercials and cereal boxes) to collect and drip epoxy from. It is imperative to try and mix it 1-1 - it can be difficult, because sometimes pushing on the plunger results in one tube being pushed more than the other, and you get something more like 1.3-1, which will take longer to cure and may not cure ever, which is a really bad problem if you're working on an important specimen. Don't screw that up. Sometimes, there is also more of a bubble in one tube, making one of the component parts come out more than the other; you can mitigate this by pushing back on the sides of the plunger, making the other tube extrude more epoxy (maybe I should do a post on beginner epoxy tips).

A second coat of epoxy was applied along the bone in order to strengthen it, and
was allowed several days to cure.
Once it was cured, the soft sandstone matrix was wetted for easy removal, and the block was picked away and carved down to size.

The initial coat came out sort of thin in places, which I thought would be too flimsy. I decided to mix some more epoxy and add a second, thicker coat right up along the bone. The first coat was still sticky to touch even after 24 hours, so after I added this second coat, I allowed it to cure for about four or five days while my fiance and I went on vacation. When we returned, I soaked the sandstone block in warm water and allowed the sandstone to become saturated; Purisima Formation sediment is much easier to prepare and separates from bone much more easily when wet - I estimate it makes preparation take 2/3 to 1/2 less time than it would if the sandstone were dry. The block was carved down to a size roughly equal with the epoxy plate.

Preparation of the sandstone off of the bone and epoxy plate. A - one centimeter of sandstone left. B - a small bit of the ventral tubercle (a small dark spot of bone down and to the left of the 'B') is exposed. C - it doesn't take more than another ten minutes to expose nearly the entire proximal end of the bone. D - after some more work with a dental pick and a wet toothbrush, the preparation is finished. E - the finished product, after vinac application.

The above picture shows the progress of preparation. The final product shows the important bits of the cranial/anterior surface of the bone. Additionally, where the middle of the shaft had broken away it left a slight mold that more or less shows the shape and curvature of the shaft - the epoxy filled in this as a small cast, and the original shape of the bone can be seen. The only unfortunate thing about this is that the mancalline "scar" that allows identification of various species is not really accessible - it is probably preserved, and the pneumatic foramen is filled in with sediment; preparation of this would require removal of some of the epoxy around the ventral tubercle. It does, however, preserve a slight muscle attachment crest on the bicipital crest, which suggests this specimen may belong to Mancalla lucasi as well. Either way, this specimen indicates that this part of the Purisima is Pliocene in age.

Saturday, July 30, 2011

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.

Saturday, July 16, 2011

A bony toothed bird from the Purisima Formation, part 2

After I got the bone identified as a bird, I emailed pictures to my colleague N. Adam Smith, who at the time was a Ph.D. student at University of Texas at Austin, studying with Julia Clarke. We both agreed on the identification of the specimen as a pelagornithid, and also agreed that it could be written up quickly, and we immediately began working on the manuscript. This was the fastest paper I ever wrote - partially because each of us did about 50% of the work, and we went through a bazillion drafts (a new draft every 24-36 hours for four weeks in September and October), and we finally submitted it on Halloween.

We were pretty anal about properly figuring and labeling all the anatomic structures of this specimen in our paper; we were wholly dissatisfied with previous papers which had not labeled the relatively divergent anatomical structures of pelagornithid humeri sufficiently. Many anatomical features are fairly modified relative to non-pelagornithids, and without adequately labeled (or unlabeled) figures, it is difficult for the non-specialist to interpret their morphology; we thought we'd do everyone a favor and do it properly. One exception is Bourdon et al. (2010), who studied Eocene pelagornithids from Morocco (whose paper was also not out yet when we submitted our article).

Figure 2 from our paper, highlighting the anatomical structures of UCMP 219007.

A comparison of humeri of pelagornithids and modern pelecaniform and procellariiform birds. A- Pelagornis sp. (UCMP 219007); B- Pelagornis chilensis, late Miocene, Bahia Inglesa, Chile (holotype, from Mayr and Rubilar-Rogers 2010); C- Pelagornis miocaena, Miocene, France; D- Pelagornis sp., Pisco Formation, Peru (Pliocene); E- Morus bassanus, Gannet, extant (CAS specimen); F- Phoebastria irrorata, Waved Albatross, extant (CAS specimen); G - cf. Macrodontopteryx (synonymized with Dasornis iby Bourdon, 2010), Eocene, Belgium; H- Pelagornis mauretanicus, Pliocene, Morocco.

Many features of pelagornithids (and specifically, Pelagornis spp.) are highly divergent relative to other pelecaniformes (as you can see above, compared with Morus). Additionally, as you can see, our specimen is one of the most well preserved pelagornithid humeri now known, even more so than the beautiful holotype of Pelagornis chilensis, the humerus of which still has quite a bit of matrix encrusted and glued onto the bone, obscuring some of the fossae. One of the pneumatic fossae is actually pneumatized, and bears a pneumatic foramen that appears to connect to the internal cavity of the bone, a condition we had not read about or observed in any fossil or modern relative.

Shaded drawing I made and used for figure 3 of our paper, showing the pneumatic foramen.

Lastly, for our paper I thought I would try a skeletal reconstruction of Pelagornis. It was on my to-do list for the paper, and we already had a couple of drafts sent back and forth when Pelagornis chilensis graced the cover of JVP: it couldn't have been more timely. Although not in the original paper, associated with the press release were several skeletal reconstructions. I emailed the artist Carlos Anzures in Chile to ask for permission to modify it for our study. Well, I didn't just modify it; I redrew it by hand, reposed it, inked the drawing on vellum, and then edited the image in adobe illustrator, to eventually get something like what you see below:

Part of Figure 1 of our paper, showing the skeletal reconstruction of Pelagornis.

Next time: more on pelagornithid evolution and ecology, as well as biogeography and the implications of our find.

Bourdon, E., Amaghzaz, M., and Bouya, Baadi. 2010. Pseudotoothed birds (Aves, Odontopterygiformes) from the Early Tertiary of Morocco. American Museum Novitates 3704:1-71.

Mayr, G., and D. Rubilar-Rogers. 2010. Osteology of a new giant bonytoothed bird from the Miocene of Chile, with a revision of the taxonomy of Neogene Pelagornithidae. Journal of Vertebrate Paleontology 30:1313–1330.

Tuesday, July 12, 2011

A bony toothed bird from the Purisima Formation, part 1

One foggy morning while doing fieldwork in the Purisima Formation, I spotted a cylindrical bone in the base of a cliff. It initially appeared hollow, but at the time of discovery, I wasn't so sure - sometimes mud and weathering products can obscure certain details of an exposed fossil. Eventually, I decided that it was most likely hollow - I scratched the inside of the bone, and had grains of ancient sand in my palm rather than bone fragments. The first step of excavating a fossil is trying to identify it: if you are positive about A) what bone you have discovered, B) what taxon it belongs to, and C) how it is oriented, you may excavate the fossil in a large block and not damage it. Sometimes you cannot limit your identification to one taxon, and this case was an example.

The end of the bone as it was exposed in the field.
Given the size of the bone, and how common marine mammals are, I assumed the most likely possibility was some sort of a fossil odontocete (toothed whale) jaw: the posterior lower jaws of odontocetes are hollow, and walled with thin bone. The only problem with this identification was that in cross section, odontocete jaws are flattened and sometimes nearly kidney-shaped - while this specimen had a more oval cross section. Nevertheless, it was my best guess at the time. When confronted by a situation like this- where you are uncertain of points A and B above, the best option is to carefully expose as much as possible until you can positively identify it. This is sometimes called "field prepping" (i.e. preparation), and sometimes may result in fragile bits of bones being broken off if you screw up or make a mistake while excavating (because field tools are less precise than lab tools, among other reasons). Aside from potentially resulting in breakage, field prepping takes time - time you may not have, if for example, you are working at low tide within the intertidal zone and have two hours left to finish.
The two sides of the unidentified bone.

Towards the end of the excavation, it was becoming clear that whatever I had found was something strange. I still thought I had an odontocete jaw, and at the time it was my best guess (I'll explain why later). I joked to myself in the field "Perhaps you've found the world's first Pliocene pterosaur!" Little did I know, I was more right about that joke than I realized at the time.

It had a slight curve to it, but it did not fan out at the posterior end like it should have if it were a jaw. I collected it in three big pieces, and upon these coming out, I saw that the sediment inside the bone was cemented - explaining why it was so well preserved, and not crushed. When I began preparing it at home, I was surprised to see that there was no enlarged mandibular foramen - again, a large hole should have been there - but instead, there was no opening in the bone.
A dentary of the bottlenose dolphin Tursiops; the enlarged posterior end and mandibular foramen can be seen on the right side of the picture (from Mead and Fordyce, 2009).

So what the hell was it? Upon leaving the field, I thought all I had was some weird odontocete with a strangely shaped jaw - perhaps a small sperm whale; they often have skinny lower jaws. However, I was once again surprised (and frustrated) by my lack of an identification, now that I had prepared the end of it (whichever end it was!). So I took a guess: some sort of a large bird bone. I did not take my thought too seriously, but the bone was in fact hollow, so I humored myself and opened up my copy of Lee Creek Volume III, and flipped to the article on the Yorktown Fm. bird assemblage.

One side of the complete end of the mystery bone.
The other end of the mystery bone.

Lo and behold, I had a match! The proximal end was a nearly exact match with the proximal fragment of a Pelagornis humerus figured by Olson and Rasmussen (2001); Pelagornis is a gigantic extinct species of bony toothed bird. I couldn't believe it: there was a very specific reason that I had not considered a bird as the owner of the unidentified bone: it was too large to represent any bird already known from the Purisima Formation, even a pelican or an albatross (in fact, it was over twice the size). I did not consider a pelagornithid simply because there are no documented occurrences of pelagornithids in younger than early Late Miocene rocks from the eastern North Pacific: there are plenty of Middle Miocene records of the bird Osteodontornis,
and a couple of records of it from the Monterey Formation. I had always assumed that they had gone extinct in the NE Pacific before the Pliocene; not only that, but this was a late Pliocene fossil. There are some Tortonian stage-like critters from the lowermost Purisima - a possible record of Imagotaria, as well as Megaptera miocaena, a Nannocetus-like cetotheriid, a possible record of Dusisiren, and some odds and ends - but this bird was far, far younger than this assemblage.Comparison of the fossil pelagornithid humerus (A, C) with the fragment from the Pliocene Yorktown Formation of North Carolina figured by Olson and Rasmussen (2001).

Check back for part 2, soon.


Boessenecker, R.W. and N.A. Smith. 2011. Latest Pacific basin record of a bony-toothed bird (Aves, Pelagornithidae) from the Pliocene Purisima Formation of California, U.S.A. Journal of Vertebrate Paleontology 31(3):652-657.

Mead, J. G., and R. E. Fordyce. 2009. The therian skull: a lexicon with emphasis on the odontocetes. Smithsonian Contributions to Zoology 627:1-248.

Olson, S. L., and P. C. Rasmussen. 2001. Miocene and Pliocene birds from
the Lee Creek Mine, North Carolina. Smithsonian Contributions to
Paleobiology 90:233–365.