Wednesday, June 25, 2014

Fishing in the Oligocene: catching a big teleost in the Naseby Greensand

I haven't had much opportunity for field work in the past year or so. Our lab was fairly active during my first year and a half of studies here, but we got to the point where too many plaster jackets had accumulated in various places, and we didn't have any room to add any more. During my first year, on one single visit to a quarry in South Canterbury, in less than two hours we found a penguin skeleton, two dolphin skeletons, a fragmentary dolphin mandible, and two (or three?) very partial baleen whale skeletons. We had to return two more times to successfully excavate and jacket everything. The two dolphin skeletons have finally - in the last month - been exposed in the lab, and the less spectacular of the two has a partial skull with a more or less complete braincase, associated teeth, and postcrania; the other specimen is a virtually complete skeleton with skull, mandibles, teeth, tympanic bullae, periotic, most of the vertebral column, complete left and right forelimbs, and ribs. In comparison with much of the other material we have here, it is one of the most complete odontocetes we have - and probably one of the most (if not the most) complete fossil odontocetes from the Oligocene, anywhere. My adviser, Ewan Fordyce, recently received funding for a preparation "triage" for unopened plaster jackets; this recently collected material has all been dealt with, and now our preparators are beginning to open jackets from 2010. One enormous, ~2.5 meter long jacket with much of a baleen whale skull inside remains to be opened, but it is very promising progress on a rather large backlog of unprepared material.

So, when I got the invitation from undergraduate Marcus Richards (B.Sc. Honors) to go out to his field area in North Otago in the Kyeburn area (with the prospect of finding some Oligocene shark teeth and to help dig out a big teleost skeleton he had found) - I jumped at the opportunity. Marcus is doing a study of the stratigraphy, age, and paleontology of the Naseby Greensand - a stratigraphic unit of probable late Oligocene age that is thought to be a lateral equivalent of the Kokoamu Greensand a bit further north. The Kokoamu Greensand is generally mapped north of the Kakanui Range - an east/west oriented mountain range that meets the sea about an hour's drive north of Dunedin, near the sleepy town of Palmerston. On the south side of the Kakanui Range, similar greensand occurs but without the overlying Otekaike Limestone, and it is thought to be closer to shore; here it is mapped as the Naseby Greensand, it is considerably less well understood, probably because the outcrops are fewer, less spectacular, and generally more difficult to access than those in the Waitaki Valley region. A few weeks prior, Marcus had brought in a handful of fragments he had collected, including a couple of flat bones, a couple vertebrae, and a large dentary with well-preserved needle-like teeth. He indicated there was more left in the hillside, and that it could be quite large. The vertebrae were similar in size to modern fish approaching two meters in length - and this fish could easily be the size of a modern enormous tuna or small swordfish.

We unfortunately don't have many photos from the first day, so a quick synopsis is necessary. Marcus, postdoctoral teaching fellow Uwe Kaulfuss (a paleobotanist/paleoecologist), my wife Sarah, and I drove out to the locality, stopping three different times for coffee at Uwe's insistence (I thought I was a coffee freak). It was forecasted to be one of the last nice days before winter really set in, and it was in fact the official first day of winter in NZ. In the sun, it was about 10-12C (50-54 F for American readers back home), so not frigid, but not toasty by any means either. I generally have a difficult time in any weather over 75 F (~24C) anyway, so this was actually optimal. Marcus showed us the spot where the fossil fish was: up on a hillside, with a small cliff below. The cliff was steep enough to prevent us from walking directly uphill to it, requiring us to hike down to it from above through tussock grass, which is a bit awkward to walk through since you can't see the ground where your feet are planting - a bit alarming when you're within a few feet of a ten foot drop. We spent most of the day collecting the few bits that were exposed on the surface, digging a trench the fossil, uncovering more bones (by accident) in the trench and bagging up those, and eventually getting the fossil into a condition ready to jacket. During the excavation we bagged up another half dozen vertebrae and what looks to be a partial skull. At 4pm the sun went down, and my hands went into icy water to start mixing the plaster (by the way, if you are ever digging in a spot where you need to get water up a steep incline and don't want to risk hauling it up in a bucket that can spill - just pour the water into a large freezer ziplock bag, it works great). The water was straight out of the river below and was so unbelievably cold. As the sun went down over the hill, we felt the air temperature drop to about 0C (32F) within five minutes. Marcus, Uwe, and myself worked quickly to apply the jacket, clean up, and leave. A return to the locality would be necessary - soon - in order to remove the jacket before it just got too damn cold to do anything about it.

Four days later Marcus, Tsai, and I returned to the locality to finish the excavation. In this photo of Marcus and I you can see the vertical face just a few feet behind Marcus, which originally was an even slope until we chopped a big ledge into the hillside.

There was a pretty nice view looking north towards Dansey's Pass. And yes, that's snow. There should even be more of it this time of year. Matter of fact, the first day, the road was still closed because of a snowstorm that blocked the pass (further up the road, of course) so we had to park and walk up about two kilometers. When we returned, the road was opened again and Marcus drove to within about 100 meters of the excavation site.

Marcus eventually got fed up with having to hike way around the hill, so he carved in a handful of precarious footholds for a more direct ascent from the spot at the bottom of the hill where we kept most of our gear.

Within a half hour we had flipped the jacket over and began trimming it down. Prior to trimming, the jacket weighed well over 150 pounds or so - not enormous, but far larger than it had to be. As it turned out, the bedding here was also near vertical or perhaps around a 60 to 70 degree angle - so the bedding plane with the fossils was actually exposed on the side of the pedestal. We began "carefully" scraping rock out of the opposite side, working our way down to the ~4-6" thick zone where disarticulated fish bones were known to be present. Here Tsai and Marcus are working on the final trimming. We had to cut away about 10" of empty plaster jacket siding after removing rock from the jacket. The rock, by the way, was this wonderfully compliant lightly consolidated muddy greensand and very easy to dig through. Although the others were a bit skeptical and thought we risked damaging the bone, I insisted that we scrape it down until we get to the bone layer, leaving a 4" thick "surfboard" of rock inside a relatively light and easy to remove jacket. There is nothing worse than having a jacket that is too difficult to safely lift. Also, there was another safety concern nagging at the back of my mind...

Generally it's best to use paper towel as a separator. Without a separator, the plaster will stick directly to the rock and bone alike and become a complete pain to remove. Newspaper works fine, but occasionally ink can be transferred onto the specimen. The first day we had forgotten to bring anything for wrapping small fossils or as a separator, so Marcus had to run back to the Dansey's Pass Coach Inn and see if they could spare some toilet paper and newspaper (which they did, and we were very greatful for it). In the past decade, I've also learned that a camelback not only keeps you hydrated, but is an excellent way to get a nice coat of water onto a separator like paper towel or newsprint. If you put on the separator dry, the wind will often just blow it away; it's also easier to get it to conform to the shape of the rock pedestal if it's damp.

That's some cold river water. This time, we started jacketing at about 2pm.

Marcus and I putting the final touches onto the opposite side of the jacket.

And that about does it - after a bit of waiting, we had to figure out how to remove the jacket. That morning I inquired with various geology staff whether or not anyone had a cargo net - an odd question, but we have so much random equipment here you never know and might as well ask. Unfortunately, that was one piece of equipment nobody seemed to have. I wanted to lower the fossil down the hillside rather than haul it up and over - which would be a nightmare. The rock is soft enough that unless considerable effort went into carving steps, that much weight would just result in someone sliding. Watching a carefully made jacket with a priceless specimen inside bouncing down a hillside and bursting apart is one of my worst nightmares.

While we were unable to find a cargo net, I did find an alternative...

In the paleo equipment storage room, we've got an unusually thick burlap sack (or hessian sacks as they call them here) that's usually used as something to put the chainsaw on top of when out in the field. I figured that if we could put the jacket inside of it, we could tie ropes to it and carefully lower it down the hillside.

Fortunately, the jacket was just small enough to be slipped inside the burlap sack. Unfortunately, the burlap sack (like most) does not have brass grommets, so in order to tie ropes to the sack, I used an old trick I learned in my days in boy scouts: find a small rock from the river below, stick it into the corner at the opening, pull the burlap tight around the rock, and then tie the rope around the neck of the "pouch" that the rock is sitting in: the rock is wider than the rope knot, and cannot leave the small pouch you've made.

It worked like a charm.

I was actually surprised at how light the jacket was - it took barely any effort at all to control it as it went down the hillside.

And, the best part is, we made it into the car at around 4pm just around when the sun went behind the hill. Here's Marcus with his prized catch inside, ready for the two hour trip back to town. We're really looking forward to seeing what is inside the jacket, and for having the other fish bits that we collected during the trenching process prepared.

Next up: more of Marcus' treasures from the Naseby Greensand.

No comments: