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Dinosaur Gizzard


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So, many years ago when my son was small, he found this "rock" in one of our dry washes in Winona County, MN. I took it to a geologist prof at Winona State University and she said it was a dinosaur gizzard.

Well, this summer I ran across a paleontologist from the U of M and asked him about dino gizzards, described it, he confirmed from the discription and said that they were so common around here he didn't even have an interest in looking at it.

What do you guys think?

Bev :)

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Dinosaur gastroliths (gizzard stones) are only found where dinosaur fossils are found, and then are of dubious identity unless found among the bones where the stomach would have been. If this were found in the Morrison Formation in Wyoming, or the Hell Creek Formation in Montana/North Dakota, I'd say "it's possible"; in Minnesota, it is vanishingly unlikely. :(

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It is a very unusual sedimentary formation. Any guesses?

Bev :)

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Hi Bev

That looks like a sandstone concretion of the kind known as Moqui Marbles, Kansas Pop Rocks and various other names, depending on where found. See one of the examples towards the bottom of the page, here:


Gastroliths are normally extremely smoothly polished. Unnaturally so, and smoother than you could get from river polishing. They frequently feel really odd in the hand.

Here’s one from Late Jurassic deposits in Colorado, found as part of a group and in association with unidentified rib bones. It’s completely and utterly smooth on all surfaces, apart from a small fracture and one or two long, streak-like scratches. Except, that is, where there are depressions in the rock that are below the rest of its surface (as in the pics below).




Gastroliths (literally “stomach-stones”) were swallowed to assist in the digestion of tough plant material. Herbivourous animals without proper grinding teeth retained them in the upper reaches of the gastrointestinal tract (the crop, or gizzard) where they acted like a rock tumbler, grinding down the plant tissue to improve the release of its nutrient value. The highly acidic nature of gastric juices results in a combination of abrasive and chemical polishing.

In this case, the animal was an unidentified large sauropod dinosaur. This particular gastrolith is also sometimes called a “Morrison stone” since they are commonly found in the Morrison Formation, west of Denver.

Stones like this may have been swallowed accidentally along with root and soil material, but since irregularly-shaped ones weighing several pounds are occasionally found, they must also have been swallowed deliberately. An animal would continue swallowing stones through its lifetime since they may eventually have worn away to nothing or “passed through” (ouch!). Probably animals also acquired more and larger stones as they grew up.

In the most studied case (a Cedarosaurus dinosaur found in Utah) which seemed to have a complete set of gastroliths, 115 stones with a total weight of 15 lb were recovered. More than half of the stones were less than half an inch across, but the largest weighed 25 ounces. There’s no firm evidence to suggest that dinosaurs were selective about the nature of the stones with respect to type, size, shape or colour. The Utah Cedarosaurus set included chert (some with fossils!), sandstone, siltstone and quartzite in a variety of colours. Occasionally, petrified wood examples can be found.

Modern birds with a mainly herbivorous diet – notably ostriches - swallow stones for the same reason. That’s also why chickens need grit. Some aquatic animals in prehistoric times swallowed stones as ballast to help control their buoyancy in water… plesiosaurs for example. Modern crocodiles, alligators and seals still do this.

The key identifiers are:

- Unlike other rocks found in the vicinity.

- Typically found in clusters occupying a very small area.

- Found in association with appropriate fossils.

- Rounded and polished, even if they started life as being angular (note that stones from birds and aquatic buoyancy stones may not show any polishing at all).

- Highly polished on higher surfaces with little or no polish in depressions or crevices (similar to the surfaces of worn teeth) which is not the case for water-polished rocks.

- Sometimes, long microscopic hairline scratches on highly polished surfaces (caused by the sharp edge of a freshly swallowed stone acting on one that’s been in situ for a while).

- High surface reflectance value (greater than 50%, versus water-polished rocks typically at less than 35%)… ie they are very shiny.

- Colour tends to be dull, not bright or garish.

- Size depends on the animal concerned but no bigger than your fist would be typical for a large sauropod.

Edited by painshill
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Lots of info!


Have you heard of these "moqui marbles" in MN?

Bev :)

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I agree with Painshill that it is a sandstone croncretion. They are fairly common in the bluffs around the Mississippi river.

Edited by Caleb
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What is a "sandstone concretion"?

Caleb, have I got a deal for you :)

We are fairly close. Next time you are in Fillmore County I would like to invite you (wife and kids :) over to see my deck and help me to identify things that I just can't get decent photos of. Also, I just got given access to the quarry where these were gotten (off Hwy. 16 near Wykoff), so we could go fossil hunting (after hours or weekend)--with or without the wife and kids, my place is awesome for kids and relaxing (swings, fairy gardens, trout stream, raspberries, horses and donkey, cats, collie dog, chickens...) AND I can grill you and yours probably the best Ribeye steak you have ever eaten :)

Have I sweetened the pot enough for you to consider coming to Spring Valley?

Bev :)

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Read it! Believe you are right. Interesting that it was found (with two others) in '85 or '86 just as work was being published about these things. So the prof was working off old info...

One busted in half and had rings in it like a tree, but I do not remember a fossil inside. The other one just disappeared...

Neat rock though!

Bev :)

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