Dumb as a rock? A lumpy bit of earth reveals a geological timeline in Seymour

Editor’s note: Today’s blog comes to us from paleontologist and field volunteer Neal Immega.

You all know by now that the museum has a dig in Seymour where we are finding fabulous Permian fossils, including the toothy Dimetrodon and the weird boomerang-headed salamander Diplocaulus. We don’t dig in just one place; lots of people go prospecting for new sites (or maybe they are just looking for a private spot to do their business).

A previous blog topic was on a weird lump that turned out to be a caliche ball. Today we have another lumpy rock to look at. This specimen was collected by geologist Gretchen Sparks, who brought it in just to plague me.  Let’s see just how much information we can squeeze out of it:

CrossBeds

At first glance, it’s just a rock showing cross-bedded sandstone with low-angled bedding, doubtless caused by water deposition in the Permian creek that crossed our digging area. This is pretty normal stuff. We see cross-bedding everywhere at the dig site, because the sandstone is durable and stands in relief.

But why is it lumpy? The bulge in this picture is not exactly standard:

Profile

It gets better. The rock is too heavy to be only quartz. A heavy, light-colored sandstone is likely to be cemented by barite (barium sulfate). Let’s cut the rock in half and polish the face.

cutLumpLabeledThis is turning out better than I expected. You can see a seam of barite cutting the nodule vertically in half. The sandstone shows horizontal layers which correspond to the cross-bedding.

What about the red-colored area? The area we are working in North Texas is called the Permian Red Beds because everything got oxidized from prolonged exposure to the atmosphere. It was a really dry time, and the critters stayed close to the Permian creek which deposited these sediments. It is good for us because the fossil remains are concentrated in a small area (our dig site is just about the size of a tennis court).

Let’s consider this possible sequence of events.

1.    250 million years ago, sandstone was deposited in a creek. It is all cross-bedded.

2.    The sand grains were likely coated with iron oxide and thus turned red.

3.    The sandstone was buried by maybe 1000-plus feet of additional rock.

4.    Shales deeper in the geological section were heated by the normal geothermal gradient to hundreds of degrees and adsorbed water was squeezed out, taking with it the barium that was also adsorbed (from the ocean) on to the clay surfaces. The water moved vertically along cracks in the rock.

5.    When the barium reached the rocks we are digging in, the barium precipitated because the pore water is very “hard” with dissolved gypsum. The barium reacts with the local sulfate, producing barium sulfate which is essentially insoluble in water. It is the ultimate “hard water” scum.

6.    The barite precipitated as the vertical seam and filled the pore space in the red rock.

7.    A whole lot of rock was eroded in the next 250 million years to bring us to the present and the rocks back to the surface.

8.    10 to 100-thousand years ago or so, North Texas was in an Ice Age, and it was really wet with lots of vegetation. Decomposing vegetation created a reducing environment which dissolved iron right out of the rock. Barite is very chemically resistant, and this lump could have been at the surface through part of the ice age. The iron could have partially leached out of the lump at that time. You can see that the leaching went deeper into the lump where there are horizontal fractures in the rock.

All this from a lump of rock. Thanks, Gretchen!

The Seymour Blob: Putting something in your head from the ground beneath your feet

As you may already know, the Houston Museum of Natural Science has long been digging up wonderful Permian fossils in Seymour, Texas. Curator of Paleontology Dr. Robert T. Bakker and his team of hot, tired and pink (from the dirt) volunteers have made major finds, but sometimes it’s the little things that count — like finding little amphibians, such as the boomerang-headed Diplocaulus and the snake-like Lysorophus, too.

lysorophus 2

The rock stars of the Seymour dig are people like Chris Flis, who finds bones everywhere. There are also geologists on the team, like Gretchen Sparks, who are interested in sedimentology (how the dirt got there) and who pick up interesting bits and wonder what they are.

This is a warty blob (that’s a technical term) that she found. It sort of looks like a bone or a burrow dug by something. I tested it and found that it is made of calcium carbonate.

CalichiSmall

To see more, I ground and polished the end. Now we can see that this is not a bone because it does not have a marrow cavity or bone lamellae. Warty surfaces like this are frequently found on the outsides of arthropod burrows because the animal lines the burrow with spit and sediments balls — but the warty parts of these structures are inside and on the exterior, so this is not a shrimp burrow.

The flowery appearance of the growth indicates that this is caliche, one of those sedimentary features that often get little attention.

Caliche is a hard-water deposit on steroids. Caliche forms in dry areas like North Texas when more water evaporates from the surface of the ground than falls as rain. Ground water dissolves minerals like calcium carbonate and gypsum from the soil and rock. When the water evaporates from the surface, these minerals are deposited in flowery growths called “efflorescences”.

The climate in Seymour is so dry that caliche is growing right now. During the Permian (about 250 million years ago), the climate must have been even dryer, because we find bands of caliche in the soil. Each band represents an ancient soil layer.

CalichiCloseupc Annotated

We can even tell the history of the caliche deposition. The interpretation is based on what covers what:

A.    White layer was deposited in a nearly flat crack in the dirt. Note the white flowers.
B.    The pink layer covers the white and has the same shape of flowers but contains more iron.
C.    Layer B was partially covered by darker pink laminations. The laminations indicate that crystal growth was much slower.
D.    The blob fractured and the dark red layer of sediment was deposited along with fragments of Layers A and B. This is a mini sedimentary dike.
E.    Since Layer D is made of sediment and not hard caliche, it shrank and cracked in the process of drying. This crack was filled with a quickly-deposited rind of fine grained white material followed by very slow growing clear crystals, making the darker band inside the white. This looks like an agate filling but is still carbonate.

Were all these layers deposited 250 million years ago? That actually would be easy to tell, because young carbonate has a carbon delta C13 signature well within the 50,000 year sensitivity range for the method. It just costs money to have the analysis done, and there are probably better uses for the resources right now – like having more specimens mounted!

I conclude that the warty bone-looking thing is really an inorganic crystallization of carbonate and possibly gypsum. Did you know you could learn so much from gravel?

Hittin’ the road with the HMNS Paleo crew!

BB describing boomerhead

I got the chance to travel from Houston to Seymour, TX and explore the Texas Redbeds in search of fossils with David and the HMNS Paleo Program. HMNS staff and volunteers have been making these trips for four years now. They have found several excellent specimens and brought them here to prepare for our new and improved Paleontology Hall. I’d had some experience looking at the bones and things that the crew had been bringing back to the Museum but this was my first experience actually in the field – and I was pretty excited!

Drawing of a Diplocaulus

The first morning we arrived at the site and looked around at a few different locations before settling down in the “pit” to dig. I got to spend a little time training my eyes to see fossilized bone, teeth, cartilage and coprolites among the rocks at the “spoil pile” which is a great experience because the ratio of fossils to rocks on the surface is such that you have a pretty good chance of closing your eyes and picking up a fossil! Then we moved over to learn the digging technique where fossils were a bit more hidden in the pit; it took a few minutes to get the hang of how to hold the tools and make sure that you are using enough force to move the dirt but not so much that you break a hidden bone. All and all it was really enjoyable first day at the site.

Over the next two days after Dr. Bakker arrived we visited several other sites on the property and I got a chance to work on excavating a dimetrodon spine, map some dig sites (here’s a fun school dig site mapping activity), learn about other findings like the diplocaulus or “boomerang head” skull we’re looking at in the photo above. I enjoyed the opportunity to work alongside the experts and learn about all of the preparation work that is required for each and every specimen that will be in the new Paleontology hall (coming soon!) here at the Houston Museum of Natural Science. I can’t wait to see everything on display in the new wing of the Museum – it’s going to be so exciting!

For more information about what fossils are found at the dig site in Seymour check out some of the entries on the Prehistoric CSI blog, you can also find some really awesome illustrations on that site to bring the animals to life!

Diplocaulus: The Boomerang-Head Amphibian

 Super-sized “Boomerang-head”
amphibian from
290 million years ago

The Houston Museum of Natural Science has just excavated the complete skull of one of the most bizarre animals that ever lived – the amphibian Diplocaulus. With a head shaped like an armor-plated banana, or an Australian boomerang, this distant kin of today’s salamander is so famous that it stars in most kids’ books on dinosaurs – and in college textbooks as well.

The Boomerang-Head (my favorite nickname for the Diplocaulus) was only of modest size – twenty pounds live weight would be an average adult. But since the first discovery in 1878, the extraordinary cranial design has flummoxed the best paleontological minds. Baby Boomer-Heads had a normal salamander-oid shape, with a rounded snout lined with needle-sharp teeth ideal for snapping up worms on the bottom of ponds.

Weirdness entered the growth cycle as Diplocaulus approached adolescence. The rear corners of the skull grew much faster than the rest of the head, so when adulthood was achieved the head was three times wider than long. And the skull corners became pointed horn-like devices composed of thick, dense, armor-like bone material.

No species alive today comes even close.

What did Boomerang-Heads do with their strange skulls?  Theories abound. Perhaps they plowed up crustaceans hiding in freshwater ponds. Perhaps they used the heads as hydrofoils for flying in river currents. Or for staying put on the bottom during floods. The notion I favor is that the adults whacked each other during courtship battles.

More mysteries surrounded the biggest adults. Heads a foot across are common – but a few incomplete specimens showed creatures 30% bigger. Did the giants represent old males who hid in specialized habitats? Or Boomerang-Head matriarchs?

When HMNS began its long-term field survey of Red Beds near Seymour, Texas, getting  Boomerang Heads for the new Fossil Hall was a top priority. Museum crews did find many parts of mid-sized specimens. Many had evidence of being chewed up and dismembered by predators. Who ate Boomerang Heads? Teeth shed during feeding identified the culprit. It was the Dimetrodon,  a reptile close to the direct ancestry of warm-blooded mammals, including us.

No one in the museum field party hoped for a complete Boomerang Head skull of record size, until……

……Kathleen Zoehfeld, long-time museum volunteer and award-winning author of kids’ science books, scouted a shallow arroyo cut into brick-red pond deposits. Boomerang Head bones were everywhere – including neck vertebrae of gigantic size. Then Zoehfeld spotted the front edge of a skull poking out of the rock. Not just a partial specimen of the sort found elsewhere but the entire head, complete from eye-sockets to horn tips.

Zoehfeld christened the specimen “Geoff” in honor of her son, a sophomore at Columbia University.

Geoff’s head was 16 inches or more wide, as big or bigger than any other noted in paleontological journals. And beautifully preserved.

As soon as I saw it, my mind jumped…I could see how Geoff’s skull would star in the Red Beds tableau of our new exhibit. It would make everyone, kids and adults, stop dead in their tracks and stare.

I took charge of cleaning the specimen personally. It’s 90% done. Our friends at the Black Hills Institute will make casts to share with other museums.

And about those mysteries regarding giant Boomerang heads: HMNS is gathering more clues. Parts of several other giant skeletons were secured near Geoff’s site, suggesting that a sort of “old Boomerang men’s club” might have existed in Red Beds time. Or, alternatively, an amphibian-matriarch society.  Skulls were accompanied by evidence from the other end of the animals – beds full of coprolites (fossilized feces) that may well have been produced by big Boomerang-Heads.

We don’t have the final answers. But the new finds will help. Maybe we’re getting closer to understanding these wonderful critters. And the exhibit of bones and coprolites will delight the scientific imagination of museum visitors.

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