Cretaceous CSI: Footprints of Death

Here is a Cretaceous mystery.

Three different species of dinosaur are found in one filled-in pit. They are well preserved, although the method by which it happened is unknown. It’s not a “whodunit”, but a “what-dunit.” There are clues as to the case, though. And they can be unlocked.

First a quick question: Where do fossils come from? For the most part, the recipe for a fossil is fairly simple. Take one or more dead animals (or, more rarely, traces of it such as skin impressions or footprints.) Place the carcasses in conditions that allow for good preservation. Silt is good, if you can get it. So are volcanic eruptions or poorly oxygenated sea-floors. Let the sediment harden, let the bones mineralize and turn to rock. Allow for a bake time of anywhere from 1 million to 4.5 billion years. Et Viola!

It may sound cut-and-dried, but it’s not. The necessary conditions for fossilization can arise in strange places. Clearly something like that has happened here. In the spirit of a good detective novel, lets put forth all of the known evidence before we begin.

Fact 1: The dinosaurs found are Limusaurus, a turkey-like herbivore, and Gaunlong, a wolf sized predator. Both are bipeds. There are two of each in the bone bed.

Fact 2: The bodies are stacked one on top of another in the bone bed. Their carcasses are contorted and wonderfully preserved. Clearly, all did not die at once.

Fact 3: The rock in which they were found is a mix of volcanic mudstone, ash, and sandstone. The sandstone indicates a wet, marshy environment. The volcanic ash and mudstone indicates hard crusts of ash and ash churned mud.

Fact 4: The bone-bed itself is shaped in a column-like pit.

Fact 5: There are sauropods in this ecosystem. Big ones.

Taking these facts together, what scenario can be extrapolated? David Eberth and colleagues think that they’ve found the answer, and they’re publishing it in the February edition of PALEOS. Analyzing the facts on hand, they’ve come up with a very likely scenario.

Let’s set the scene. It’s the late Jurassic, over 160 million years ago. The stage is a floodplain that’s slipping into marshland. Puddles and bogs dot the landscape in a clogged mess of horsetail and ferns. Volcanic activity is fairly common; this particular plain has been showered with ash at some point in the past. Now the ash has hardened and plants are growing once more.

A Mamenchiosaurus moves across the bog. It’s a massive sauropod dinosaur, 69 feet long, weighing several tons, with a staggeringly long neck and whip-like tail. As it moves the ground gives way with bubbling squelches under its massive feet. It sinks up to its knees in soupy muck. But it’s a strong animal, and it’s able to pull itself along through the marsh until it reaches firmer ground. Behind it, something rather interesting has happened. The volcanic mudstone has broken under the beast’s weight. The moist soil underneath is saturated with water, and the footprints backfill with quicksand. For all intents and purposes, the marsh looks undisturbed.

Some time passes. A flock of Limusaurus wander through the floodplain, grazing. They’re much smaller then the Mamenchiosaurus. In fact, they’re under 40 pounds and not much longer then a big dog: far too light to break the thick crust of ash. As they nibble, one of them slips and becomes mired in the thick mud. Like most theropods, it uses its hind legs to move and has small arms. When it falls in the mud, it is trapped. Eventually its weight pulls it down and it drowns. Another Limusaurus is trapped as well. A few days later, a Gaunlong arrives, attracted by the scent of putrefying meat, and it too is trapped. Another, larger Gaunlong investigates. Into the pit it goes.

Rinse, Stir, Repeat.

Eventually, the pit trap is filled with dead animals. Those that stumble in now can get out. Quadrapedal herbivores don’t get caught; they can use both forelimbs and hind limbs to clamber out without much trouble.

Time passes. The mud and carcasses congeal and are covered by successive floods and ash falls. Eventually, they become fossils.

This kind of analysis is part and parcel of Paleontology. Using clues preserved at the scene, scientists can make fairly plausible guesses about the nature of their finds. Of course, we can rarely know for certain exactly what happened. This entire scenario might well be wrong, and perhaps some new bit of evidence will arise to throw it out. But for now, David Eberth and co. have done a first class bit of paleontological detective work. And they have provided something more magical: A spark for the imagination.

And that, of course, is what the best fossils (and detective stories) do.

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