Saturday, December 2, 2023

Sampling microvertebrate fossils from the early Oligocene Ashley Formation (Rupelian)

One of our sites we visit for Charleston Fossil Adventures has proven time and again to be full of surprises - mostly little surprises. Well, big surprises, science-wise, that happen to be physically small. Most of the large fossil shark teeth are from a small minority of species but make up the bulk of the fossils that are found - which is called collection bias. Simply put, it's easier to spot large fossils. Small fossils - especially small shark teeth - frequently get damaged more rapidly and perhaps are more difficult to find in a condition that is well-suited for scientific study. This is preservation bias. Critically, the majority of sharks in any fauna are rather small with tiny teeth - and there are a few gentle giants out there with absurdly tiny teeth (e.g. whale, basking, and megamouth sharks, manta/mobula rays) that you will never find if you're only going after the big trophy specimens. You're probably missing out on over 75% of a fauna unless you're looking for small teeth.

At this locality, there are loads of tiny teeth that have weathered out of relatively friable dredge spoils of the Ashley Formation - normally this rock unit is quite hard (indurated) and teeth often break as they are weathering out of limestone blocks. But at this spot, the dredging dug into what must have been an unusually soft and crumbly exposure - we do find teeth with matrix attached, but they're typically in good shape, and we find loads of small teeth amongst crumbly carbonate sand. That solves the preservation bias issue. To solve collections bias, we go out to this locality with knee pads and stare at it from a few inches away - as well as screen the matrix and pick through it for microvertebrate remains. As we've done this, we've added a bunch of species to the faunal list for the Ashley Formation - and possibly discovered some fossil shark species that are new, or the first occurrence of the genus in the fossil record ever.

Ashby, Sarah, and I heading on down the river to the spot. 

 It started off as a chilly morning - hence the chest waders - I was in a tank top by lunchtime. 20 mph wind, but blocked that day by the copse of trees. Given the strong, direct sunlight, Ashby tried collecting for a while in the shade underneath an umbrella. Sarah and I went for the direct approach. Collecting micros from the surface on a cloudy day is a bit easier, since there's no harsh shadows. At some quarries, the only way to find certain fossils is to hunt in your shadow.

A beautiful blood red tooth of the extinct devil ray Plinthicus stenodon.

That same tooth of Plinthicus. Incredible colors!

A teensy, tiny little burrfish (Chilomycterus) tooth plate. Not sure if this is Pliocene contamination.

 A tiny little orange tooth! Let's take a closer look.

It's a tooth of
Dasyatis - a very large tooth (roughly 2mm wide) of a large adult female stingray. Male teeth are frequently smaller and have a pointed crown, whereas the crown is blunt and low in females. 

Sarah inching along the sandbar - and being followed closely by the advancing tide.

A rostral spine of Pristiophorus - a sawshark! Not to be confused with sawfish, which are also known (but, as yet, unreported) from Oligocene rocks of the Charleston Embayment. Pristiophorus is known from a single oral tooth, but we've found loads of the rostral spines from this location - at least 20, and ALL found this year! Sawsharks are extraordinarily rare in the Atlantic coastal plain fossil record.

Screening matrix from the very top of the sand bar - convenient at high tide.

Sarah joined Ashby, Mike, and I for a staff day at Ashby's picking through all the matrix we had screened.

More picking - we picked hundreds of teeth, bones, and denticles out of about 60 lbs of screened concentrate.

We returned the following week and found this very large "fish otolith" - actually a roasted pumpkinseed left on the beach by Sarah the prior week. She said "I told you guys I accidentally left at least a couple behind!"

We decided to take the coarser screened fraction of the sample and just dry it out on the paddleboard, since we could pick through it in a matter of minutes - negating the need to haul all this back home.

And more picking from the coarse sample.


Al Mannering said...

Hi Bobby,
What screen size do you use? I find that a 1/4 inch is suitable for the coarse fraction and 1/16 inch catches most of the rest.

Robert Boessenecker said...

Hey Al! Great question. This is a set that comes with 1/2, 1/4, 1/8, 1/16, and possibly a 1/32 screen. Depending upon if we are screening matrix from the sandiest part of the sandbar or one of the zones that is a bit rockier, we might leave out the 1/2" screen and just use the 1/4" and 1/8". We tend not to keep the 1/16" fraction since none of us really have time to go through all that. The 1/4" fraction is what we dried out on the surfboard.