Fusulinids: Jumbo Forams

Foraminifera are among the most abundant and best known kinds of microfossils (and they aren't exactly confined to the past, either). While it's true that many aren't microscopically microscopic, they're still smaller than is comfortable for practically any human being to study without some kind of magnification at hand. Without such, you'd be stuck squinting at sand-grain-sized things and making such helpful observations as "okay, this one looks like a stack of spheres, and this one is coiled up, and this one is... no, wait, that's an itty-bitty fish vertebra or something." And then there are some that don't require quite as much eye strain to spot.

Like these: every grain-looking thing is a foram.

Among these giant microfossils are the fusulinids (or "fusies" if you're lazy like me), the kings of the realm of single-celled organisms during the late Paleozoic. If you can see something the size of a rice grain or a grass seed, you can see a fusulinid. In fact, the comparison will give you the idea of what to look for: fusulinids tend to look like fat, whitish seeds. The Latin word "fusus" means "spindle-shaped", or "something that's long, widest in the middle, and tapering at the ends". There's your fusulinid.

And there's another, and another... These are getting to around 3 mm long, or a bit more than a tenth of an inch. (Note also what looks to be a sliver of a trilobite pygidium near the center.)

During their heyday, fusulinids could be so abundant that their tests (shells, basically) could more or less make up sediment beds. The resulting rocks are a bit monotonously fossiliferous, as fusulinids tended to look the same on the outside. The interior architecture of chambers is how different species are distinguished, so even though you can see them without needing a microscope, you're going to need one to tell them apart. (Plus the grinding and polishing and all that jazz.) "Why bother?", you may ask. Well, it turns out that the geologically rapid turnover of fusulinid species makes them great biostratigraphic indicators in some places where people are keenly interested in subterranean resources (like the Permian oil fields of Oklahoma and Texas). Know the fusulinids, know the rock; know the rock, know the resources.

Zooming down to an even closer look, we can see that many of the fusulinids have a "perforate" appearance, which is the result of weathering exposing some of the internal chambers. Although most of the forams are seen lengthwise, there are some cross-sections scattered throughout, showing rings of chambers.

The particular examples in these photos come not from in situ outcrops, but building stone. The rock is Cottonwood Limestone used in historic structures in Kansas, and although there are other fossils, the fusulinids are by far the most abundant. The Cottonwood is early Permian in age (Wolfcampian stage in the grand old North American series), so at this point the fusulinids had a few tens of millions of years left to flourish before bowing out at the end of the Permian. According to my old copy of "Invertebrate Fossils" by Moore, Lalicker, and Fischer (1952), this would most likely represent the Pseudoschwagerina zone.

The stairs are made of forams! (No, it doesn't have the same ring as "The floor is lava!", but you *are* walking on the bodies of millions of fossilized amoeba things...)

Mitchell Caverns

Back in the fall of 2021, I made a work visit to Mojave National Preserve, located logically enough within the Mojave Desert of southern California. While there, I had the opportunity to tour Mitchell Caverns. Mitchell Caverns is in the unusual position of being part of a state land parcel (Mitchell Caverns Natural Preserve or State Natural Preserve, depending on the source), entirely surrounded by another parcel of state land (Providence Mountains State Recreation Area), which is itself surrounded by a National Park Service unit (Mojave National Preserve). For good measure, the cave system is also a National Natural Landmark. It's parks all the way down in the Providence Mountains. (To be fair, the natural preserve designation is kind of a map artifact; it's not really distinct from the state recreation area.)

Regarding forams

Life started out microscopic (at least to humans) and most of it has stayed that way. Of course, many microscopic organisms have poor fossil records, due to factors like lack of hard parts and the whole "microscopic" thing (finding and studying microfossils takes special equipment and expertise that aren't used for collecting, say, brachiopods). However, a subset of microscopic organisms have very significant fossil records. We saw the ostracodes a few years ago, but there are also a number of groups of single-celled organisms that produce hard parts suitable for fossilization. Among the most important are: coccolithophores, phytoplankton which form skeletons of scale-like objects known as coccoliths, micron-scale structures that make up chalk (and which are sometimes called nannofossils because they're so darn small); diatoms, phytoplankton with cell walls made of silica; dinoflagellates, which form organic-walled cysts; radiolarians, protozoans that form body structures of silica; and the subjects of today's entry, the foraminifera, which can be described glibly as "amoebas with shells".

A living foram, the brackish-water benthic calcareous species Ammonia tepida, showing strands of pseudopodia surrounding the coiled test. What do all these terms mean? Read on! (Photo from Wikimedia Commons; unfortunately, no scale, but you'll get an idea of the size of what we're dealing with in the photos to come.)