Returning to a one of a kind Horseshoe Crab Trackway site in the Naco

[Arizona Chris]

Last weekend, we went back to a site we had discovered about 10 years ago, along Highway 260 east of Payson, Arizona.  The Naco is pretty much all pure carbonate limestone, grey in color and in many places loaded with crinoidal material.  But when we discovered this site on satellite photo images, we knew it might be very different.  What appeared to be very fine laminar layers in a small outcrop on a side road appeared in the photos.  It turned out when we finally went there with our 4WD vehicle, it was a very finely laminated mudstone, with a high terriginous content.  (read: dirt from the nearby land).  Nothing was at first found, not even one fossil.  But then we discovered certain layers contained very small finely detailed trace fossils.  They were tracks and undertracks of Kauphichnium - the trace fossils left by Limulids.  (Horseshoe Crabs).  

 

There is no other site we have ever found in the Naco with such fossils, and we have traveled over hundreds of miles of exploration in both Northern Arizona and Southern Arizona in the Naco formation.  This site is unique.   After a hiatus of 10 years, it was time to go back and get some better photos of this site and using our new Sigma 150mm macro lens (what a sweet lens!)  obtain better shots of the trace fossils.  Since our first trips, the road had extensive road construction, and the side road was now completely blocked off by a guard rail.  Now where to park.  We parked about a quarter mile down the road on a side road, and walked to the locality.  It is on the back of a ridge, and cannot be seen from the roadway.  When we finally got to the site, it was smaller than I had remembered!  Gee, it seemed to be so big last time.  The shales were right there, a beautiful yellow green color, nothing like any other Naco site.  But we were surrounded on all sides by crinoidal limestones typical of the formation.    When the sun is out you can split the shales, and when you find the right layers - holding the slabs at a steep angle to the sun allowed us to see the most common trace here, the Limulid underprints.  Let me explain a bit what an underprint actually is.  When an animal prances through the mud on the surface of a hardground, it leaves the foot and toe impressions, and in the layers just under the prints, you get compression of the muds.  This change in density is retained in the fossils when the mud lithifies and turns to mudstone.  When the shale is split, you can have several thin layers of shale just below the actual prints with what appears to be dents or slot shaped marks.  Those are the underprints.   We of course also find the actual surface tracks as well, Kauphichnium is a Y shaped or forked print.  These horseshoe crabs were small, dime to quarter sized.  

 

After a few hours of splitting shales and finding some trackways with telson and carapace drags as well, we left once again, hoping to return soon and spend more time at this unique site in search of its trackway treasures!

 

Below are some photos of the expedition and some of the material found.  Please visit our much more extensive write up on our web page for more images and details!  It can be found here:  http://www.schursastrophotography.com/paleo/NacoCrabTrax062020.html

 

Photos:

Here is the locality as we approached on the side road.  the Outcrop is on the left.

 

The Outcrop. The thin shales here are precious - and unlike much of the Naco in the surrounding area.  It is an island of different material from a shallower water deposit with sedimentary structures. and wave ripples.

 

Montage of Kauphichnium found at site, close ups with the new lens.

 

small slab in positive hypo relief of both undertracks and actual prints.

 

Another slab in negative epirelief, of the slot shaped undertracks.  This is part of a trackway sequence.

 

Finally, a Limulid roadway.  On the left is a long series of tracks with a teleson drag, and with other underprints on the right.  The round prints on the right edge are also Kauphichnium, representing the tips of the toes so to speak piercing the sediment.

 

Once again, I encourage you to visit our web site and read more on this exciting site:  http://www.schursastrophotography.com/paleo/NacoCrabTrax062020.html

 

Thanks for looking, 

Chris

[Fossildude19]

Excellent and informative write up! 

Thanks for posting this Chris!   

Great to see these ichnofossils. 

Nice website, as well. 

 

[Monica]

Cool ichnofossils!  And that Euproops sp. shown on your website is awesome!!! 

[FossilDAWG]

Great writeup!  Also your website is fantastic!

Do you ever work with professional paleontologists to get some of your discoveries published?

 

Don

[Arizona Chris]

Thanks Everyone!  So nice to share our latest trips with a group that has a common interest.  Although we find the more common "worm burrows" not too exciting, once in a while there are some great trace fossils to surprise us!

 

Chris