Siliceous and Delicious

Another slow start to the day (aren’t they all, these days).  The call is for rad lineages, that most detested of projects. Last week, I had finally managed to download the radiolarian data from Neptune, after a wasted day of failed attempts. So, today, let’s take a look at that in R.

So far, so good—Didymocyrtis (in red) and Diartus (in blue), the first species I looked at, is extremely well represented in Neptune, with 1657 and 595 occurrences respectively:

Now, unfortunately, it seems that this particular download of Neptune data did not include information about preservation, which I would also like to consider when making my sample selection. Perhaps I can find that out by cross-referencing with the MRC database when the time comes. Onwards to the other lineages. Lophocyrtis has 252 occurrences and looks like this:

Artophormis looks like this:

And Stichocorys like this:

Centrobotrys like this:

And, finally, Phormocyrtis:

OK, so the first hurdle is cleared; clearly species from within these lineages have been reported on in the DSDP/ODP literature. Presumably, this means there are also corresponding slides available from the MRC. One hopes. What next? Let’s start with the last of these lineages, Phormocyrtis, which consists of two species: P. striata exquisita and P. striata striata.

Most of the Neptune samples matching this genus name are from samples aged 28–55 Ma, with only a handful of younger ones (only four younger than 27 Ma). Limiting this search further to matches for P. striata only, yields 101 samples distributed like this (and yes, I spent a little bit of time making a nice timescale to go with it—but I think it’ll really help see things relative to the Cenozoic stages):

Now, this is a little bit surprising, since the Bolli volume describes the range of this lineage as extending only to the Middle Eocene. I imagine it must mean that it was discovered in younger sediments more recently. Only a few of these occurrences are of P. striata exquisita (highlighted in red), the older species:

This is mighty confusing. According to the Bolli volume, P. striata exquisita ranges up until the end of the Early Eocene, then gives rise to P. striata striata. The Neptune data show something quite different! Only one Early Eocene occurrence, and then a range up through the Oligocene. OK, so this is also just five occurrences (which is, in and of itself, a substantial problem), but this is a radically different picture from what I would have expected. Where, then, did the authors of the chapter in the Bolli volume (which includes Riedel and Sanfilippo) see all of these occurrences of P. s. striata, and why aren’t they in Neptune?

Perhaps a literature search specific to this lineage will throw up something helpful. After a solid half hour of scouring Google scholar, I’m not convinced that will help. Riedel and Sanfillipo (1978) say that one turns into the other—they even use this transition to mark the base of a radiolarian biozone. Deeply confusing.

To cheer myself up, spent the last sliver of the working day looking at something different. Since I had the radiolarian data loaded up in R, I thought I’d try running the scripts I’d written for the diatom data to see what the diversity—at least for the simplest (sampled in-bin) reading of it—looked like:

It looks nothing like the diatom curve—no sign of that E/O drop, and really just a monotonic increase. Perhaps that’s because rads are more resistant to erosion, and so perceived diversity won’t drop so much in the face of a degradation in taphonomic conditions? Or perhaps the diatom signal is real after all, and has nothing to do with preservation at all?