I thought it would be fitting to highlight an unusual fossil from this region. The star of this particular entry is the following handsome fellow, known as Rutgersella.
|Rutgersella says "hi". Image from Wikimedia Commons. This also features as Figure 4E in Retallack (2015), where the scale indicates it is about 4 cm across, or about an inch and a half.
Rutgersella is represented by flattened oval objects on the order of a couple of inches across, with a slit-like central depression and radiating surficial features. In the region I was visiting, examples have been found in the Lizard Creek Member of the Lower Silurian Shawangunk Formation. The productive beds have been interpreted as intertidal to estuarine or lagoonal, with sediment coming off of eroding topography imposed on the area by one of North America's numerous Paleozoic collisions with wayward crustal bits. The associated fossils include a variety of invertebrate burrows and trails, several species of eurypterids (the famous "sea scorpions", which we haven't had occasion to meet because Minnesota isn't exactly a hotbed of the things), inarticulate brachiopods, bivalves, cephalopods, an ostracoderm fish (armored jawless fish), and early plant-like things. All of these together suggest a setting with terrestrial, freshwater, and marine influences (Retallack 2015).
As you may have guessed from the photo, Rutgersella isn't obvious about what it was. It was first described by Johnson and Fox (1968) as a dipleurozoan jellyfish similar to Dickinsonia, a classic Precambrian fossil. Times have changed, and point-of-comparison Dickinsonia is no longer thought to be a jellyfish. Instead, Dickinsonia is thought to be... Dickinsonia, which although accurate is also deeply unhelpful. (Well, it's an Ediacaran fossil, aka vendobiont, which is more of a state of mind than a solid biological classification, but there you go.) In more practical terms, there is a lot that is not known about how Dickinsonia lived and what it was related to; structurally it seems to have been something like a quilted air mattress, and it may have consumed microbes, somehow (no word on whether all Dickinsonia were called Zem, or if they flolloped around from microbe mat to microbe mat).
Anyway, Rutgersella bears a striking resemblance to Dickinsonia, albeit with a much shorter midline groove and a much smaller body size. It is also not unlike a pyrite sun, which is a natural, non-biological mineralization, as Cloud (1973) pointed out. Retallack (2009, 2015) came to the defense of Rutgersella, finding: although there is pyritization of some Rutgersella, pyrite is not always associated; cross-sections of Rutgersella show it to have had internal non-pyrite chambers rather than being layers of pyrite; and various other mineralogical and morphological differences. Retallack (2015) described Rutgersella as an immobile tidal-flat organism with a Dickinsonia-like structure, living more than one hundred million years after the heyday of Dickinsonia. This may not be immediately impressive, but the brief latest Precambrian fad for mattress creatures ended more or less the same time that hard parts and limbs showed up. Were such organisms making like stromatolites and taking to harsher, underpopulated settings like tidal flats where they could (temporarily, as it turned out) avoid the ravages of trilobites and worms and such? Retallack (2015) further compared Rutgersella to lichen, which makes things even more complicated because a lichen, by definition, is made up of more than one kind of organism, defeating traditional notions of classification (and for that matter calling it a singular organism).
One final thought: Rutgersella, as an oval object a couple of inches long with radial corrugations, has serious cookie potential. (Of course, I'm biased; I think there should be more dessert fossils.)
|Think this, except the fossils are chocolate.
Cloud, P. 1973. Pseudofossils: a plea for caution. Geology 1:123–127.
Johnson, H., and S. K. Fox. 1968. Dipleurozoa from Lower Silurian of North America. Science 162(3849):119–120.
Retallack, G. J. 2009. Cambrian-Ordovician non-marine fossils from South Australia. Alcheringa 33(4):355–391.
Retallack, G. J. 2015. Reassessment of the problematic fossil Rutgersella as another post-Ediacaran vendobiont. Alcheringa 39(4):573–588.