For today's blog, I wanted to talk a bit about the collection and preparation of one of the spinier New York trilobites; Bellacartwrightia. First off, I have heard many, many times that new collectors struggle with identifying this particular bug. More to the point, they have trouble with distinguishing Bellas from Greenops (which are also found in the same formations). Both belong to the asteropyge group of trilobites which generally have long, sweeping genal spines, pygidial lappets and holochroal eyes. The differences between Bellas and Greenops can seem minor at first glance but with practice, are quite different indeed. On to the differences!
Bellacartwrightia tends to be larger - often considerably larger than Greenops. The pleural tips are very long, thin and sharp, sweeping away from the body. Greenops pleural tips are triangular, short and somewhat blunt in comparison. The dead giveaway on wether you have found a Bella? Axial nodes. Axial nodes run down the middle of the trilobite thorax and are quite delicate. You'll see the small black 'dots' of the spines locked into the negative if you have indeed found a Bella. The most prominent of these spines/nodes is the occipital spine which is attached to the head/cephalon. Longer than the rest, you'll see a round base where this spine attaches to occipital ring on the head/cephalon. There is also a pygidial node on the very first section of the tail/pygidium which can be easy to miss. The eyes are quite a bit taller and larger than their Greenops cousins however, these are often flattened by the incredible geological pressures at play in high energy deposits. Combine this with a differently shaped set of genal spines (Bella genals tend to have an indentation where Greenops are smooth with granulations). Finally, the pygidial lappets (tail spines) are considerably longer and sharper than your general Greenops trilobites. I hope this helps in your identifications! Photos can also be sent to my instagram account and I'd be happy to ID your find for you. Bella's are often referred to being "a Greenops on steroids". I concur with this statement.
On collecting Bellacartwrightia: These can be difficult bugs to extract under the best conditions. They tend to be much thinner, delicate and fragile than say, Eldredgeops trilobites. I've yet to hear of a perfectly split Bella in the smoke creek, mostly due to their eyes and axial spines. If you're lucky enough to find one of these trilobites, you're going to have shell stuck between the positive and negative slabs. A secret weapon to ensure your best outcome is a good field glue. Personally I like Starbond ultra-thin, it has low viscosity which means it will penetrate and stabilize the rock better than a thick goopy glue that will be difficult (or a downright nuisance) to remove in prep. In particular, the technique of "wicking" the glue in will save you shell and agony in the long run. I like to travel with toothpicks as they will suck up some glue when dipped into it, then gently tap beside your trilobite shell in the negative. If done correctly, the glue will get wicked under the shell without making contact with the shell (the latter outcome being an instant swearing situation for me). This adds stability not only for transport- but also for stability if you have to cut your prize out of the formation. No matter how badly your bug breaks, there's still a good chance that a professional fossil preparator can put it back together. This is especially true if you use a thin-set glue in the field and use good sense like collecting all loose pieces in a baggie or tinfoil packet.
Preparing Bellacartwrightia: I fear this post has already become tubby and overly specialized so I'll keep this short! Now that your Bella has made it home safely, it's time to plan out your prep. If you're not confident of your skills yet, set him aside! One of the beautiful things about fossils is that they don't rot. At least not in the basement where they're kept dry and free of the weather. Also, you have the option of sending your bug to a professional preparator. I'll put my basic process here in case anyone can benefit from it.
Step 1: Make sure all loose bits have been gently removed and labelled. This can be as easy as having sandwich bags labelled "right side, mid-thorax" or "left genal spine" etc. It keeps your shards tidy and organized. Next, I may wick in a little more glue for extra stability.
Step 2: Clean both positive and negative with gentle air abrasion for a tight fit. I usually crank my system down to about 15PSI for this part. Just in case, I don't want to blow any bits away in the blast box. Using a soft blast media like dolomite or sodium bicarb, I gently go over the entirety of both pos/neg surfaces to remove any dust, dirt, loose flakes or crud that will interfere with a tight fit.
Step 3: test fit. It should be tight with the minimum of gaps possible. This is so very important for a good final result.
Step 4: plan your prep. Do you want the trilobite as the highest point? Do you want a fancy bowl in the rock like so many Cambrian pieces? Use this time to make a plan and visualize the final orientation. Perhaps most importantly, use a ruler or straight edge to draw a grid in sharpie on the top of the negative to mark out where your trilobite is situated. It's very frustrating to get your piece glued only to realize you didn't mark where the bug was or even which side is up. It has happened too many times and it sucks. Don't rush your markings. I like to mark out the axial line of the trilobite, then I mark where it should start and where it should end with head and tail marked off as well. This way, the trilobite won't surprise you when you accidentally blast a hole through it where you weren't expecting it. On most other trilobites I prepare, I go straight for the axials. They tend to be thicker shell and it's easy once you find the bug in the rock. On a Bella, you want to avoid this because of the axial nodes. I save these for last by making a 'matrix mohawk' that will protect the spines until the final stages and so I won't do something stupid like brush the spines off by accident. Plan out your prep and it will go well. I would suggest, if you're new to this, have a picture printed off that more-or-less matches your specimens orientation so you have a visual reference in real time. This is great to do with ANY bug you've never prepped before. Less heartbreak.
Step 5: Glue. Get some good clamps, set your fit and clamp hard. the seam should be near invisible and tight. Use multiple clamps for best results. Once clamped, use your low viscosity glue on the seams. You won't need a ton but a moderate amount is good. The glue will wick into the seam via capillary action. This is why low-viscosity glue is your fiend. Let sit for 24 hours to dry.
Step 6: So anyway, I started blasting. That line is funnier from Danny DeVito. Start your air abrasion. To find the bug, you can start with a little more pressure, like 40psi. Choose a section away from the head or tail (I like to start on the right side of the thorax). The tail and head skin is very thin on Bellacartwrightia and you don't want to start blasting on those to start. The angles will be too severe at first for quality blasting. Making small, ever-widening test holes to find the bug is a great technique. You can also use your scribes to assist widening the holes while you don't see shell. Be careful where you're scribing though, the spines and sometimes the eyes can sit higher than you may think. Once you get down to the bug, slow down! I like to switch to about 20psi for this species, though there's no stone tablet of commandments for prep. Every job is an adventure. Mixing between scribe and blaster, work away until you've uncovered the main body. Plan out your landscaping and finish the job! Fine tips and low pressures will be needed to clean out the tiny folds and features but that's about it. Plan your prep and prep your plan. It's like that joke about carving an owl out of a block of wood. Just blast away the parts that aren't the trilobite and you'll be fine.
I hope my unsolicited advice helps or at least illuminates some of the more challenging elements to preparing one of these awesome trilobites. They are impressive and beautiful when done right. As always, I take prep commissions and I'm also happy to help if you run into a snag. Good luck! Below are some photos of a recent Bella recovery and prep. Thanks for reading! -Mike
The contact zone between the Smoke Creek and Bayview formations. A Bella sits below my shirt and splitting wedge! The negative had infilled with water so the shirt is acting as a sponge while also holding fragile bits of shell in place during the rocks portion.
Apparently I'm missing several photos of this sequence; Step 0: Be organized!
He appears! Sporting his stone mohawk we are getting close to the final stages
All finished. A little shell was lost in the split and had to be restored. Otherwise, a rather nice example of this undescribed trilobite!
Comments