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Tracking The Glen Rose

JohnJ

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January 2, 2010

The Lower Cretaceous Glen Rose Formation (Kgr) of Central Texas is roughly 110 million years old. Its classic exposures look like man-made steps or solid blocks that are occasionally interrupted with softer rock or marl. The formation is typically divided into upper and lower units by a layer of Corbula fossil clams. Just below this layer was the destination I wanted to find for my first fossil hunt of the year. It takes its name from the isolated occurrence of an ornate fossil sea urchin - the Salenia texana zone!

A bright dawn had not yet thawed the frost when I headed to meet my friend, Bob. He was excited to show me a new quarry where he had found echinoids the previous month. When we arrived at the site, he oriented me to the most productive layers in the formation, and we started hunting the youngest strata. I immediately began to find fossils. Erosion of the shelf, we were searching, left fragments of 'heart' urchins, gastropods, and bivalves everywhere. I was trying to be selective, looking for the better preserved specimens, but it was hard to pass up an unusual oyster or clam.

Oyster (Ceratostreon weatherfordensis ?) with the partial mold of the shell where it was attached

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Juvenile Arctica sp. clam

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Soon, Bob was calling out, "Spiny urchin!" with periodic repetition. He wryly commented, "I just seem to be a magnet for those things." Meanwhile, I gouged my elbow on rock as I crawled along the ground. Glancing to check the damage, I spied one of the small, prickly echinoids. It was just one of those small moments...that capture your love of the outdoors. The late morning light was perfect, and when I reached for the camera, a little heart urchin caught my attention. Even better. So, I digitally captured the two 'echies' before putting them in my box. We finished the morning and the rest of the layer with several more echinoids and a partial crab claw.

Loriolia texana echinoid with Orbitolina texana foram

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Heteraster obliquatus echinoid among Orbitolina texana forams

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Loriolia texana echinoid

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Some finds after a little cleaning

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From this area, we moved down into the "zone". A hard limestone bench capped a six foot thick layer of softer rock. It weathered into chunky clay before a transition back to solid stone. Even within this bracketed strata, I noted some subtle differences in the coloration and hardness. But meanwhile, Bob had started finding echinoids while I was "getting the lay of the land". The marble-sized Leptosalenia texana were eroding with regularity from the top half of the zone. A small, disk-like foram, known as Orbitolina texana, littered the ground. Scattered among them were a variety of different gastropods and a non-fossil caterpillar.

Leptosalenia texana with forams and gastropods

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Caterpillar

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Leptosalenia texana echinoids

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Bob previously mentioned that he had found a couple of plates (a part of an urchin's shell) from a very uncommon echinoid on his last visit. So, as we leaned against the wall of the formation, I asked him what else he remembered. He described them as being more whitish in their preservation than some of the other finds we were making; and when he said it, I thought of the variation in the rock I had seen earlier. We had already found fragments of the spines which the 'Salenia' urchins used to protect their shells; but I was not tracking them - we were tracking a cidarid echinoid! In the Glen Rose Formation, two species have been described: Phyllacanthus texanus and P. tysoni. So, I grinned when I saw part of a larger, bumpy spine sticking out of the rock.

About that time, Bob suggested that we move over a short distance to a fresh spot. Hunting anything, with success, requires identifying and following certain clues. In the new spot, I put my suspicions to the test. A few feet below the caprock, I found a lighter layer that was somewhat hidden by runoff from layers above. I flaked away the debris to get a better look and immediately started to find several spine fragments! I announced my excitement, "Cidarid spines!"

Echinoid spines

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5 cm echinoid spine in matrix

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A slightly elevated heart rate accompanied the anticipation of following signs in the rock. Then, I had an adrenalin spike when Bob called out, "You need to look at this." He walked toward me, and in his hand were 3 connected plates of our cidarid urchin quarry. I showed him some of the spines and explained the "hidden" layer we could focus on. I thought we were close to our treasure, and he asked if I had "covered" the area just to my right. I told him, "No, go ahead" as I knelt down for a look at some of the spines eroding from the ground.

"JOHN!" I turned to see him stand up beside me with a golf ball-sized, knobby echinoid in his palm!

"You did it!" I yelled. "You really...did it! Way to go!" We stood a moment, looking at the rare urchin with a range of emotions. Then, he handed me his prize while he went back to get his camera. I put it back in the spot he picked it from and took a few photos. When he came back, more photos ensued...it was an amazing Texas find! Although I know quite a few cidarid urchins have been found through the years, I am personally aware of just five...including Bob's - certainly not a common discovery. Checking a few references later indicated he had found a Phyllacanthus texanus!

Bob's discovery

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Phyllacanthus texanus echinoid

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Well, as you can imagine, the adrenaline of discovery had us quickly back in search mode. More spines were found. I ravenously scanned the layer we isolated. Then, my "heart jumped in my throat" when I spotted the partial test (shell) of another Phyllacanthus! So close...but not this time.

My Phyllacanthus texanus partial test

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Late into the fading light, we searched to no avail. The cool wind and darkness ended our efforts, and we congratulated each other with our goodbyes. Hopefully, with some weathering and heavy rain, we will get another chance to track the rare fossil urchins of the Glen Rose.



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wonderful report. it's really great that you have the knowledge and skills to appreciate your finds at the time you make them, yet still get the adrenalin rush and happiness from them.

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John--aren't we forgetting a certain statement that was made when the Phylly thingamabob was found? It had to do with Tetragrammas...

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tracer and MikeD: I always appreciate your comments. Thank you.

BobC: Are you referring to me denegrating the so-called rarity of Tetragrammas vs. a Phyllacanthus with colorful verbiage and dramatic tone? .....leaving that out was clearly a case of writer's license... :blush:

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Thank you, Lance.

Bob, at this rate, you will single-handedly take Phyllacanthus texanus off the "rare" list! :D

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