Thomas Farm Volunteer Dig--April 2016

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The last interesting bone of the day (and I'm glad there some interesting bones today) was found just at quitting time 4:30pm. I pulled this out and cleaned it off but Dr. Hulbert had said he needed to get back to the university to take care of some things and had taken off. Again, it seemed likely that it was somehow part of a Parahippus leg but I couldn't think of where (and had no fossil anatomy books with me to compare). Luckily, Dr. Hulbert hadn't left yet and had to come back down to the digging area to pick up something before leaving. I was able to go to the oracle one last time and learn one last lesson for the day. The bone, it turned out, was indeed from a horse leg and its unusual shape and texture could be explained by its identity--a tibial epiphysis. Epiphyseal plates are ingenious structures at the end of our long bones that allow the bones to continue to grow behind the plate while the articulating surface of the plate remains fixed. In adults, the epiphyses fuse to the ends of the bones and are no longer found separately. This indicates that this bone came from a pre-adult individual. You can see that one face of this epiphyseal plate (with the wavy articulating surface) has a more smooth and solid surface texture while the reverse side is more spongy (easily seen by the clay infilling the holes).

This was my last little learning opportunity for the day. Begrudgingly, I put away my tools and headed back to my hotel room. When I'm having fun and finding new and interesting bones I wish I could stay till the sun set and darkness descended--in fact, I wish I had a headlamp and could dig through the evening. But all good days must come to an end--and there must be regular infusions of ibuprofen so that I'll make it out of bed for my firth and final day tomorrow. Probably won't get a chance to post an update tomorrow as I'll be leaving for home directly from the Thomas Farm site and its at least a six hour drive back home. I'll sleep in well tomorrow night and post a final chapter for this trip report sometime when I'm reasonably coherent on Saturday. Fingers crossed for some more interesting (and informative) finds tomorrow.

Cheers.

-Ken

[ynot]

Way to go Ken! Glad that there were more fossils in the new square.

Hope that You do better still on day 5 !!!

Tony

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So here is how the last day at the Thomas Farm dig site wrapped up for me. Just as I was cleaning up and getting ready to go on the previous afternoon, I encountered something flat and smooth. It was rather thin and crumbly (Dr. Hulbert calls these 'pudding bones' a flippant terminology that I've adopted). I initially suspected that it might be another Parahippus scapula but as I uncovered more the shape and flatness didn't seem to match the scapula I'd uncovered earlier in this grid square. As I started following the left edge it started making a curve and I started to worry that this could be a Parahippus jaw (though it still didn't seem to match). The bottom edge of the bone where I first encountered it in the matrix was very crumbly and pieces broke off and crumbled to a brown powder. I was actually hoping it wasn't a mandible as I uncovered more since I would have wanted a better specimen if it was a more valuable specimen. I finally found the borders and the maximum extent of this piece and decided that I should do no more till Dr. Hulbert had a chance to see this but Dr. Hulbert had just left having to tend to some things back at the university. I laid a plastic collecting bag over the specimen and weighed it down with a handful of loose matrix to keep it protected and prevent it from drying out too much.

So when met with Dr. Hulbert on Friday morning I mentioned that I had found something that I could not identify (a common occurrence) and was waiting for his input before proceeding. While I was gathering my digging equipment and setting up a new bone bag and sand bag for the day's digging, Dr. Hulbert lifted off the plastic bag to see what was hidden beneath. The answer (as usual) was obvious once you know it--the specimen was a partial tortoise carapace--Hesperotestudo tedwhitei (a species described from and only known from the Thomas Farm site). Dr. Hulbert said I should work around it and make a pedestal so it could be jacketed. This was the first piece I had found all week that was jacket-worthy so the morning was off to a good start.

Around 10am a handful of other volunteers showed up and were given a briefing on the site--its discovery in the 1930s and the history of the excavations to the present day. I've heard the story before but it was always good to hear it told again by Dr. Hulbert--catching a few forgotten details here and there. The new group of volunteers were assigned squares to work. Since the excavation this season is limited to the higher ground area that for the last five years has been under an aluminum car port and buried under a pile of matrix-filled sandbags, only a small number of squares forming an L-shape along the edge of the higher plateau were being worked--a total of 9 squares (1m x 1m in size). I had to snug-up a bit and pull my bucket containing my sandbag and my water bottle and sunscreen within my square to make room for several people digging in the squares either side of mine. I found a position that I could sit comfortably in my now limited space and continued to focus on removing material from around my tortoise.

One of the first pieces I pulled out can be seen in the lower right of the image above. It was pretty clear even when only half excavated that it was a horse astragalus--likely Parahippus (always a safe bet).

This square--unlike my rather depauperate squares of the first few days--was pretty packed with smaller bones, especially some of the well-preserved leg and toe bones that were turning up in this corner of the square. There were also several larger pieces of bone that (not being as well calcified as the weight-bearing leg bones) tended to be very fragile and came out in several pieces--which were all stored together in their own small plastic zip-top bag before being placed in the larger plastic bone bag. I uncovered the nice shiny phalanx (toe bone) shown at the bottom below just before lunch and bagged it (so it wouldn't get lost) and brought it up to show Dr. Hulbert during our lunch break. It was too thin to be the sturdy middle toe (number 3) of either species of horse and didn't appear laterally flattened like the phalanges in toes 2 or 4. For anatomical reasons as horses shed their digits over evolutionary time, the toes "dropped off" from the outside leaving the 3-toed horses with digits 2-4. This one didn't seem to match others I'd seen--or the resin casts I'd studied the previous day and so I suspected it was something different. Dr. Hulbert confirmed this notion stating that it was a toe bone of one of the carnivores. The exact species won't likely be known till it is pulled out of the bone bag and classified before being accessioned into the collection. As carnivores are a much smaller portion of the fossil record at Thomas Farm (or anywhere I suspect) I was happy with my little pre-lunch find.

After a quick bite to eat I was eager to get back to my square to complete the removal of material around the turtle carapace. It took some time to remove the surrounding material but for a good reason--the matrix in this area was relatively packed with some smaller finds (both solid bones that could go directly into the larger plastic bone bag and more crumbly pieces that required individual smaller bags to keep their parts together). At one point I had to get up to get some additional small zip-top bags for one of the fragmented finds I'd just uncovered. When I returned to my square I knelt down on my blue boat cushion that I'd been using to keep my backside from the hard ground. As I was rather pinned in with people all around I had let my cushion get too near the tortoise I was working on. With my attention diverted to the piece I was going to place in the little zip-top bag I didn't notice that my cushion had slid forward when I knelt on it and just clipped the bottom of my emerging tortoise pedestal. In a split second of inattention the bottom of the pedestal had crumbled taking some of the loose pieces of the tortoise shell with it. This was (needless to say) devastating. While this wasn't a very rare specimen it was valuable enough to jacket--at least till I turned it into a puzzle for a preparator. Dr. Hulbert gave me the squeeze bottle of the consolidant glue B72 and told me to saturate the pieces so they wouldn't further fragment. In hindsight, it likely would have been a good idea to have treated this specimen with the consolidant when I'd first discovered it as it was apparent that it was fragile and riddled with fine cracks. Highly disappointed, I waited for the B72 to dry so it was no longer tacky and then got a larger zip-top bag and chunked out the specimen. I put in the loose pieces and used the trowel to lift off the bulk of the specimen quite easily as it was sitting on a soft and easily cleavable sand layer.

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With the tortoise carapace removed from the square I continued to work towards the back of my square removing material and finding several interesting items. I soon uncovered a larger bone that seemed quite solid. As I worked around it to determine its extent I could see the overall shape and a telltale bump on the lower edge that made me think calcaneum. I had found a Parahippus calcaneum at Thomas Farm almost two years ago to the date and so was familiar with them. The calcaneum is in humans what we would call the heel bone. I had not often thought about the bones of the foot (unless I had stubbed a toe or strained an Achilles tendon) so when I first encountered one in 2014 Dr. Hulbert was gracious enough to explain the anatomy to me. These bones seem to be well calcified and quite strong so they seem to preserve well. I asked Dr. Hulbert if most of the specimens of this bone in the museum's collection were complete and wondered how many they might have. He confirmed that nearly all of the calcaneum in their collection from Thomas Farm were complete or nearly so and estimated that they probably have around 300 of them. I said, "three hundred and one" as I dropped mine into the bone bag. It is amazing that after 18 million years that this bone looked in such great shape.

I had to include a photo of one of the rarer bones that turned up in my friend Cindy's square as she continued to work her way through the cluster of bones that littered her square. This is a rhino humerus just before it vanished again under the blanket of a plaster jacket.

Rhino wasn't the only interesting species to come to light that day. Mary, a long time volunteer digger, had been working the corner square in the L-shaped arrangement of grid squares that we were working. She'd found a few smaller bones and fragments but nothing notable till she saw the glint of grayish enamel when she uncovered an Amphicyon (bear-dog) molar. It has missing one root and had a crack separating it into two halves but still it won the enviable fossil of the day award from the rest of the volunteers.

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While secretly hoping that a little more of the bear-dog had spilled over into my square I continued to work through the material that was left in my grid. The abundance of small fossils (mostly Parahippus and Archaeohippus as expected) was a pleasant change from the barren clay removal of my first three days at Thomas Farm. It was always fun when while uncovering one item you chanced upon another in close proximity and then another--fossil overload can be a fun thing. I spotted a Parahippus medial phalanx for the larger (third) toe and was pleased when I could identify it after it was only half exposed. This mean--at least for the most common fossils--that I was developing something of a search image and a small cache of locally topical knowledge. While nowhere approaching the encyclopedic knowledge of Dr. Hulbert who has worked with Thomas Farm fossils since the 80s, it is nice to know that I seem to know more than I did a week ago (and that's a start). Here are the two Parahippus toe bones--a larger medial phalanx from the weight-bearing toe and likely a smaller proximal phalanx from one of the smaller side toes. I also found another beautifully preserved gator osteoderm from Alligator olseni.

As I continued to clear material I was slowly working my way around an outcrop of badly broken bone material near the center of the square that was revealed when we first pulled off the carpet squares when I started working on this section. I decided I'd work around this deposit of bone and then work into it from the sides in case it was part of something larger--best not to start the middle where it is visible but to stealthily approach from the sides. I came across another piece of bone that wasn't a small isolated chip or a crumbly 'pudding bone'. I had removed enough of it that, when Dr. Hulbert walked by while checking to see what the volunteers had uncovered, he peered at the specimen and said that it appeared to be a peripheral edge of tortoise plastron. I figured it was an isolated piece but as I worked around it to determine its extent it continued. Once it looked like there was a more complete specimen Dr. Hulbert said that I had a chance of redeem myself with this new piece of Hesperotestudo tedwhitei. I believe in the adage "once bitten, twice shy" and so when Dr. Hulbert asked if I wanted the B72 to consolidate what I'd uncovered so far I heartily welcomed the suggestion. I noticed that the smooth texture of what I was uncovering seemed similar to the badly crushed bone that was hiding at the surface under layers of toilet paper when we uncovered the square. I asked if it could be related and Dr. Hulbert said it was unlikely. The more I progressed and uncovered the more interesting it got. The line of plastron bone angled upward till it connected with the fragmented bone at the surface. I wetted the surface a bit and peeled off more of the toilet paper revealing bone that seemed to be similar in shape and texture to what I was uncovering. At this point we decided that it would be best to jacket the whole thing and let the preparators find out what was what--I would find a specimen to jacket after all. I worked hard to quickly but cautiously cut a trench around the entire specimen to leave a pedestal and room around it to place the jacket.

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So just under the wire in the last minutes of the last day of volunteer work on the Thomas Farm site for 2016 I managed to find a specimen worthy of a jacket (two if you don't count my earlier mini-tragedy). The winds were picking up and the layers of toilet paper that we were laying on top of the fossils to protect them from the jacket's plaster were blowing off unless I held them down till they were saturated with water. The top of the pedestal was well padded with "paleo tissue" and Dr. Hulbert quickly applied the plaster gauze strips to construct the jacket for the tortoise plastron. By the time we had to leave the jacket was still drying and the afternoon shade was finally reaching the square so we didn't stick around for the jacket to solidify enough to disconnect the pedestal from its mount. I figured I'd leave that for the vertebrate paleontology class that was going to hold a two-day fieldwork outing over the weekend.

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So another year of Thomas Farm volunteer opportunity has come to a close for me. The Thomas Farm site will soon tarped and sandbagged to keep it safe from the summer rains till it comes to life again next spring with a short burst of activity with a new crop of volunteers (and some regulars) finding new and exciting discoveries (and lots more common horse toe bones to add to the ever growing collection). My Hesperotestudo tedwhitei caparace and plastron pieces will require a fair bit of preparation work before they receive an accession number and enter the collection. Many of the bones collected in the bone bags each day will get sorted by Dr. Hulbert. The rarer bones and those in good condition will be separated out, classified, and added to the collection. The lower quality specimens--badly damaged common bones are returned to Thomas Farm and dumped on the spoil pile out back. When kids groups come through for a tour of the site the digging method is demonstrated but as they are too young to dig in the active portion of the sinkhole, they are allowed to hunt for fossils out back and keep whatever they find as the museum has already deemed them of no scientific value for the collection.

Here is my final specimen all jacketed up and ready for its trip to the museum to wait its turn for preparation. I nearly completed the leveling of my square but this will probably be finished off by some paleo student over the weekend. This was my final view of the site in 2016--last one out of the pit as usual.

While looking back at the photos I took two years ago when I was last here, I didn't take any photos specifically of the metal car port that stood over the mountain of sandbags and the equipment where we were now digging this year but I did catch a couple of glimpses. It is interesting to look back at these photos with 20/20 hindsight and know a few of the fossils that lay underneath the gray tarps waiting another two years for their moment in the spotlight.

Thanks for making it to the end of this verbose and highly illustrated topic.

Cheers.

-Ken

[Auspex]

You've filled my imagination with things at once marvelous and mundane; I will dream of knee pads...

[ynot]

Nice Report!

Glad that You found something good before it ended!

Tony

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I came across this webpage from the National Park Service website as part of their National Fossil Day overview. It gives a good history of the Thomas Farm site and some great photos. I'm including a link here for those who are interested in learning more about Thomas Farm:

https://nature.nps.gov/geology/nationalfossilday/cenozoic_thomasfarm.cfm

Cheers.

-Ken

[fossilized6s]

Great report, Ken. Sounds like fun.

Too bad about the tortoise, but now you've learned a very valuable lesson for next time! Stabilize, stabilize, stabilize.