The World Turned Upside Down
One of my prized possessions is an original reprint of the seminal 1972 paper on punctuated equilibrium by Niles Eldredge and Stephen Jay Gould. Eldredge handed it to me early in 1973, when I started as a freshman work-study student at the American Museum of Natural History. It was the first paleontology paper, indeed the first scientific paper, that I ever read. Its influence remains with me fifty years later.
One of the key sections of the paper is “The Cloven Hoofprint of Theory.” In it, Eldredge and Gould argue persuasively that “We do not recognize that all our perceptions and descriptions are made in the light of theory” and that “Science progresses more by the introduction of new worldviews or ‘pictures’ than by the steady accumulation of new information.” In a footnote, they add that by “picture” they mean “alternate ways of seeing the world that render the same facts in different ways” (emphasis theirs). To put it simply, no scientist simply gathers facts; facts are gathered within an existing theoretical framework that guides us to which facts are relevant. Eldredge and Gould then go on to argue that the prevailing theoretical view of the evolutionary process at geological time scales, which they labeled phyletic gradualism, was at odds with the actual patterns seen in the fossil record. They then proposed a new concept, punctuated equilibrium, which was more in keeping both with known biological processes and with the data. Same data, different picture.
I recently experienced “an alternate way of seeing the world” that rendered long-known facts in a different way. Not far from where I live are the collecting sites for the world-renowned Mazon Creek biota, a Pennsylvanian age delta deposit which has yielded abundant plants and an array of terrestrial, freshwater, and brackish/marine water animals. Mazon Creek is a Lagerstätten, a place where unusual conditions preserve the remains of soft-bodied animals. The most famous of these is Tullimonstrum gregarium, aka the Tully monster, whose affinities are still being vigorously argued. In the same marine influenced sites that produced Tullimonstrum, the most abundant animals are the fossils that collectors have long called “blobs.” So common were they that collectors would often leave them behind.
Blobs are an apt name. Highly variable in appearance, these fossils don’t immediately bring to mind the clearly identifiable shrimp, scallops, or horseshoe crabs that are among the large variety of other animals we see at Mazon Creek. But looked at in one way, investigators saw a compelling resemblance to jellyfish (Figure 1). In 1979, Bradley University professor Merrill Foster published a formal description, gave them the taxonomic name Essexella asherae, and assigned them to the Scyphozoa, the true jellyfish. Foster also recognized two distinct regions, an upper smooth one, which he termed the bell, by comparison with modern scyphozoans, and the skirt, a “membranous curtain of rather resistant material that hangs below the bell.” The skirt region is irregular, with various “pustules and ridges” which he suggested “represent one or more of the following objects: oral lobes, tentacles, clusters of tentacles, tentacle-like structures from the oral lobes, folds in the oral lobes, and gas bubbles from decay of structures such as the oral lobes and tentacles. ”
Figure 2 shows an artist’s reconstruction of Essexella based on Foster’s interpretation.
One key piece of evidence given by Foster was the presence of a small snail, Strobeus, on the skirt of some specimens. He analogized this with the modern snail Janthina, which feeds on modern planktonic animals such as Portuguese man-of-war. “The great similarities between Strobeus and Janthina in morphology and apparent position on the prey suggests that… Strobeus was a planktic predator, that the “blobs” were probably planktic coelenterates, and that the skirt represents the lower surface of the animal.” Since the paper by Foster, every paper on Mazon Creek listed Essexella as a jellyfish.
Our department’s teaching collection had a small number of Essexella, which I dutifully labeled and taught as jellyfish. But something nagged at me; in detail, they really did not look like jellyfish. The skirt made no functional sense; why have something that would interfere with feeding, defense, and swimming? And I started to notice that every specimen had almost exactly the same outline, which would also be weird for a floppy animal like a jellyfish (as my colleague James Hagadorn puts it, you should get something like when you plop a string mop on the floor).
In early 2016, I voiced my concerns to Hagadorn and Graham Young, experts on the fossil record of jellyfish, who had earlier expressed some doubts about Essexella. We quickly agreed that whatever it was, it was not a jellyfish, but that it deserved more study. We played with other gelatinous marine organisms, such as salps and siphonophores, but were quickly dissuaded from that interpretation. Then in late 2016, James Hagadorn visited me, and we went to the Field Museum to look at and photograph their huge collection of Essexella, the largest in the world (most of these were collected and donated by enthusiastic amateurs). Staring at the specimens, a thought began to bubble at that back of my brain. I soon wrote to James and Graham: “Just a wild guess, can we rule out these things are anemones?” Not long after, that wild guess became a certainty.
Jellyfish and anemones are closely related. Both are cnidarians, animals with stinging cells, that also includes corals and siphonophores. There are two fundamental body forms for cnidarians: the typically sessile polyps and the usually swimming medusas (“jellyfish”). In some groups, such as Scyphozoa, they spend their entire lives as medusas. In others, such as corals and anemones (Actiniaria), they are always polyps. And there are some groups that alternate between polyps and medusa. In either case, the basic body plan is the same: outer and inner layers of tissues, separated by the jelly like mesoglea, and a ring of tentacles with stinging cells surrounding a single aperture (“mouth,” which is also the “anus”) leading to a central hollow space. To put it simply, in the jellyfish the mouth points down and in a polyp, it points up. So, by saying that Essexella was an anemone and not a jellyfish, we were taking the same fossils and turning them upside-down (Figure 3).
When the Essexella animal died, it rotted away for different amounts of time before burial and fossilization. As a result, some specimens were quite detailed (Figure 4), whereas othaers were just faint outlines (Figure 5).
Many specimens were a mix of internal and external features compressed upon each other. To make our case, we thus had to look at thousands of specimens. We also had to become familiar with the anatomy and ecology of modern anemones. Eventually, we ended up with some general features of Essexella, which supported our new interpretation. These included:
· Compared to most other animals at Mazon Creek, Essexella had substantial relief. Like many modern anemones, it had to be tough and firm.
· It did have two regions, one smooth and one textured, but the smooth region could be either much smaller or much bigger than the textured one. This suggested that it could expand and contract. Some modern anemones have a base, the pedal disk, which can expand to help it burrow (a physa).
· The overall shape of nearly every specimen was the same, with textured region having a nearly flat truncated top margin (we suspect this similarity in shape is why Foster decided there was a “skirt”). We interpret the textured region as the column of the anemone, with the top margin being the mouth area, which contracts to close in modern anemones. In some specimens, the surface texture of the column is preserved.
· There are six-fold linear bands running from the top of the smooth area to the top of the column (Figure 1). We interpret these as contracted muscles that are used in living anemones to retract the column.
· There are other bands that run in an angle from the top of the smooth area to the sides of the column. These we interpreted as equivalent to muscles in modern anemones that bend the stalk.
What about the gastropod Strobeus? Our study indicated that when found they were tiny, poorly preserved specimens, often in small groups, and always near the midline. Other paleontologists had suggested that Strobeus was a scavenger. We agree, and believe these scavenged partly buried specimens, with the middle sticking up just above the sediment.
The final piece of the puzzle came when we looked at the bottom of modern anemones; the bottom of the pedal disk has a radiating pattern. This looked very similar to another “jellyfish” that Foster described from Mazon Creek, Reticulomedusa. Examining those species revealed we were looking at the underside of Essexella, preserved with the top down. We also realized that this looked very much like a well-known trace fossil of the same age, Conostichus, long thought to be made by a burrowing anemone. We suggest that Essexella, or something very like it, was the trace maker.
Putting it all together, we envision Essexella as an anemone that burrowed into the sea floor using expansions and contractions of its pedal disk. We are not certain whether it was partly or fully buried.
This artwork (top), by the amazing Julius Csotonyi, gives an interpretation of how Essexella may have looked in life. Note that there are jellyfish swimming above them, Octomedusa and Anthracomedusa. These were also described by Foster, and we think his interpretation in this case is correct.
Foster and many other scientists before and since looked at specimens of Essexella and saw a jellyfish. They fit what they saw into the picture they had. We looked at the same specimens, and at one point we turned the picture upside down to make a new one. We saw an anemone. Same data, different picture. But that is sometimes how science works.
For more details, see our paper: Plotnick, R. E., G. A. Young, and J. W. Hagadorn. 2023. An abundant sea anemone from the Carboniferous Mazon Creek Lagerstӓtte, USA. Papers in Palaeontology 9(2):e1479.
