When visiting restaurants abroad, it’s always worth trying something you don’t recognize on the menu, because it could be really good. The same is true when working through a micropalaeontological sample… there could be bits and pieces in there that you don’t recognize straight away, but they could prove to be very useful. Items listed as incertae sedis always leave you wondering what they might be and how they might fit in to the story.
For several years micropalaeontologists logged ‘pink, calcareous spheres’ from North Sea Upper Cretaceous Chalk sections. Persistence established that these were calcitized Cenosphaera radiolaria, occurring within one of three isochronous red chalk events (Early Campanian, Mid Santonian, or Early Coniacian). Each of these represents a warm, oxygenated water mass influx into the basin on the eastern margin of the opening North Atlantic. Each event has an associated abundance of planktonic foraminifera and age-constrained calcareous nannoplankton allowing precise age assignment as well as a palaeoenvironmental interpretation. A fourth red chalk event occurs at the top of the Late Campanian interval, characterized by a distinctive planktonic foraminifera traceable around the North Atlantic and into Tethyan sections, including the Campanian–Maastrichtian stage boundary type section at Tercis, France.
When logging through thin sections of carbonates, it’s easy to ignore fragments of bioclastic debris which are not immediately recognizable. A typical overlooked item could be fragmented echinoderm debris. It’s easy to recognize an echinoid spine in thin section because of the distinctive ribbing or floral cross section; however it’s not so easy to pick out more irregular rhomboid echinoid plate fragments. After a search you start to recognize that they frequently reveal their internal calcite crystal structure by having minute crystals of pyrite on crystal boundaries. They will vary in the amount of pyrite growth, but it’s always the echinoids which show pyritization first, allowing them to be distinguished from other bioclastic debris.
It’s questionable why you might want to count echinoid debris in thin sections, but in very shallow-water carbonates, where there is often a restricted microfauna, pulses of echinoid debris appear to coincide with minor flooding events and these may provide a key to the environmental controls on sea level in the area. It’s difficult to know accurately what sort of echinoid numbers may be present in a sample when you consider that a single Ophiura albida Forbes (brittle star) comprises a total of 4966 individual calcite plates (yes — they have been counted), but when you’re dealing with regular pulses in abundance then these may be significant.
The most critical ability for a micropalaeontologist today is the power of observation, followed closely by the need to record accurately what has been observed. Recognition and identification are secondary as long as you can recall what you found and where you found it.