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The word fossil could evoke the bones of a creature itself – an imposing T. rex, a tiny trilobite, a medium-sized giant sloth. But life can be immortalized in other, more oblique ways: in the traces that an organism leaves behind during its lifetime. Some fossil traces are almost poetic. Footprints left by a dinosaur or a burrow dug by an ancient worm begs the question: who left this behind? Other fossil traces are less poetic but even more mysterious. A rounded fragment or small piece of bone raises not one but two questions: who left it behind, and from which end did it come? In other words: poo or vomit?

All fossils require some deciphering, but anything excreted or regurgitated by an animal millions of years ago can be a real puzzle. The former, called coprolites, are much more common, and they often look exactly what you’d expect: grainy brown lumps. But just as modern poop is a thing of many splendours, ancient poop can take many forms. Some wavy brown fossils that might look like poo are actually imposters, also called pseudocoprolites. (The Wilkes Formation in southwest Washington is a treasure trove of these pseudo-poops, formed inorganically when silt and clay filled hollow wood fragments.)

Even rarer than coprolites are regurgitalites, or fossilized vomit. “It’s pretty rare to find direct evidence of who eats who, or who vomits who, in the fossil record,” said paleoartist and filmmaker Brian Engh. Although the soft, plant-like vomit of an ancient herbivore has less of a chance of geological immortality, a predator’s yakking might at least contain some bones, according to John Foster, curator at the Utah Field House of Natural History State Park Museum. .

Foster and colleagues described a new fossil regurgitation in a paper recently published in the journal Palaios. The newly described regurgitation is tiny – about as long as a staple – but contains the scattered remains of at least two frogs and a salamander fragment

When Foster’s team first dug the rock, they paid little attention to it. They were working in Utah’s famous Morrison Formation, an Upper Jurassic site containing hordes of dinosaur bones, including the sauropod Diplodocus, whose cruciferous barf was unlikely to survive the ravages of time. But Foster and his colleagues focused on a lesser-known strip of the Morrison whose abundance of fossilized plants earned it the nickname “Salad Bar”. “There’s still a lot to find in this formation, and some of it is going to be thrown up,” Engh said.

Credit: Utah Field House of Natural History State Park Museum
The regurgitation specimen.

The researchers took the specimen back to the museum, where it sat for a year among an assortment of mysteries — “things that we just can’t tell what they are,” Foster said. Some of these mysteries require two or three turns under a microscope until their identity becomes clear. For example, one of the Salad Bar’s mysteries turned out to be a fossil water bug, whose veiny wings initially appeared to be the veins of a leaf.

With a microscope, Foster realized that what he was looking at was not a plant at all, but a jumble of amphibian bones, some of which were only three millimeters long. And the bones weren’t from a single tragically disassembled amphibian, but rather a bunch of different amphibians. The frogs were tiny – an inch or two long at most. “We knew we had at least two frogs,” Foster said. “We found at least a single salamander bone.”

But then came the real issue of the specimen. “To find out,” Foster trailed off, “whether the thing was vomited or pooped, basically.”

There were a few visual cues. “Most of the coprolites you find are basically little ovals or little tubes or something like that,” Foster said. “They retain a sort of three-dimensional character.” But the jumble of bones was flat, without the soil mass typical of coprolites, and the stone surrounding it had several stratifications – accumulations of sediment that probably accumulated each year around the small heap. But to be certain, the researchers had to do a geochemical analysis. X-ray fluorescence analysis of the specimen revealed that the specimen did not have elevated levels of phosphorus, which is usually indicative of coprolite. The only elevated phosphorus was found in the bones, demonstrating that the phosphorus had not been removed from the fossil in the process of turning it into rock.

The specimen shows several spots of a mysterious gray mass, which also contained no phosphorus. Foster hopes to scan the fossil with a more precise machine at the University of Utah, which would allow him to focus on specific areas. “That might give us a pretty good indication of what this unidentified material is,” Foster said.

But if the fossil was vomit, who vomited? For now, the identity of the culprit is lost in time. The researchers’ best guess is a fish, possibly similar to a modern bow fin, due to the scales they found around the site. Other predators, such as turtles and semi-aquatic mammals, are also a possibility, but they have not yet been discovered near the site, Foster said. Anything bigger, like a prehistoric crocodile, wouldn’t have bothered to munch on frogs the size of limes. “The barf gives us a window into what else was going on in the ecosystem,” Engh said.

Engh, the paleoartist responsible for illustrating what regurgitation might look like in real life, had a challenge ahead of him. At first, he says, he simply planned to illustrate a vomiting fish. “But then all the questions would be why is this fish throwing up? And it won’t show what the fish was eating either. To preemptively answer this question, Engh tried another sketch of a fish bitten by a crocodilian and vomiting defensively – a way to distract the predator. But with no evidence of crocodilians in that corner of the Morrison, that version was also abandoned.

The final illustration was inspired by the Jaws poster: a bowhead approaching the unconscious frog from below, ready to chew. “I realized I still wanted to show the puke, so I added another arc fin puke in the background,” Engh said. When Engh’s wife, an evolutionary biologist who studies fish, mentioned that lungfish chew by encapsulating everything they eat in a ball of mucus and sucking it all up, Engh added a layer slime of mucus to his regurgitated frog.

an illustration of a prehistoric fish approaching an unconscious frog on the surface of the water, and another fish regurgitating a frog in the backgroundCredit: Brian Eng
Engh is full, Jaws– inspired scene: two fish and two doomed frogs.

The bones inside the regurgitalite weren’t super fragmented, indicating that they may have only been partially digested by the predator. It’s possible the predator vomited up the swallowed frogs defensively or after digesting frog meat in an effort to purge the frog bones, Foster said. A drop of mucus would have helped the bones stay together and be preserved, possibly isolating the bones from scavengers or microbes, he added.

Foster is amazed that the fragile pile of tiny, mostly hollow bones has been preserved. But his favorite part of the fossil is how it captures the interactions of modern-looking animals that lived 150 million years ago. “It kind of helps illustrate how everything in the dinosaur era wasn’t really weird and wacky,” Foster said. “Some of them would have been very familiar to us.” Had we been sitting on the banks of this Jurassic pond, we would likely have heard a chorus of frogs, Foster said, and perhaps even the distinct, timeless sound of a carnivore ejecting a frog that was no longer singing.

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