From the Canyon Edge -- :-Dustin

Wednesday, May 28, 2014

Texas Armchair Geology Research -- Lunatia Pedernalis

I have the privilege of living on a couple of acres of a beautiful limestone canyon just outside of Austin, Texas (the very canyon you see in the banner across the top of my blog and G+ page).  My wife, Kim, and I built a trail down to the very bottom of the canyon, where there's a serene little creek.  I try to take a daily walk down the canyon, to appreciate its beauty and get a breath of fresh air.

A few months ago, on one such walk, I stumbled across an unmistakable fossil, just barely poking through the very thin layer of top soil, and a little bigger than a tennis ball.

 I collected it, admired it a bit, and set it in a terrarium of air plants (Tillandsia) that graces our kitchen counter.

It has made for a nice conversation piece, though I knew very little about it until very recently.

Just yesterday, my and wife and daughters took another walk down the canyon, and while I was chasing my oldest around, I noticed another fossil jutting up above some recently exposed soil!

I found this one several hundred meters away from the first.  I can't help but wonder how many more there are littered about the canyon...

Last night, I became curious about their age and origin.  I looked around at Google images of "snail fossils Austin Texas", and spotted a few familiar lookers.

From there, I was able to chase down a likely species name -- Lunatia pedernalis, a type of moon snail  (Naticidae).
Lunatia is a genus of predatory sea snailsmarine gastropod mollusks in the family Naticidae, the moon snails.

Predatory!  That sounds awesome.  Unless you're the prey.  These salt water snails dig through sand, to find clams, drill a hole through the clam's shell, and suck the meat out of it.  Wow!  Circle of life, indeed.

Evidence of northern moon snail predation is usually much easier to find than the snails themselves:The powerful foot enables this gastropod to plow under the sand in search of other mollusks. Upon finding one, it "drills" a hole into the shell with its radula, releases digestive enzymes, and sucks out the somewhat predigested contents.[5]When empty shells of clams and snails, including other moon snails, are seen to have a neat "countersunk" hole drilled in them, this is evidence of predation by a moon snail.

I also wondered how old a fossil like this might be.  I stumbled across this 133 page gem of a PDF, Texas Fossils An Amateur Collectors Handbook, (first published in 1960, I just added it to my Kindle, and then actually ordered a print copy).  Page 62 has a couple of familiar looking images, specifically of Lunatia and perhaps Tylostoma for the second fossil.

What I found most interesting there was the classification of Createous Gastropod.  Placing these in the Createous period puts these fossils between 145 million - 66 million years old!  Holy smokes!

I found a bit more information in the 1947 publication, Studies of Some Comanche Pelecypods and Gastropods.  It specifically talks about a slightly different species, Lunatia Praegrandis, as being more prevalent in the Glen Rose Formation.

Looking a bit more into the Austin hill country's geologic history, it seems that at least some of our limestone is part of the Glen Rose Formation.
The Glen Rose Formation is a shallow marine to shoreline geological formation from the lower Cretaceous period exposed over a large area from South Central to North Central Texas. The formation is most widely known for the dinosaur footprints and trackways found in the Dinosaur Valley State Park near the town of Glen Rose, Texas, southwest of Fort Worth and at other localities in Central Texas.
If these fossils are indeed part of the Glen Rose Formation, then they're likely 115 million to 105 million years old.

And it was a about that time that I stumbled on this article from Excerpts from Jim Conrad's Naturalist Newsletter, about a Lunatia fossil snail.  And it more or less confirms what I found.  Createous, Glen Rose formation, mean gastropod that eats other mollusks.  113 million to 108 million years old.  His fossil looks like this:

Finally, while I'm rather partial to my fossils, it seems you can own your piece of 100 million year old Texas for a mere $8 on eBay.