TRACE FOSSILS

AND

SEDIMENTARY STRUCTURES

The Permian Coconino Sandstone

Last Updated: January 2000

Some of the most unusual and rare trace fossils in the world come from the eolian Coconino Sandstone. These ancient fossil sand dunes contain a record of a desert vertebrate and invertebrate fauna that lived near a large inland sea. This is the same sea that produced the wonderful carbonate beds of the Kaibab and Toroweap formations. Trace fossils include Merostomatid (spider), Scorpionid, Centipede trackways, possible euripterid tracks, and multitudes of a dog sized mammal like reptile foot impressions and trackways called Laoporis.We have also found tree bark impressions, worm burrows, wave ripples and raindrop marks. Gorgeous sweeping crossbedded units are common in this preserved sand sea (erg).

Click on Thumbnail for an enlarged view.

This is one of the most spectacular combinational fossil slabs we have found yet. Along the top and decreasing toward the bottom we find raindrop impressions seen as round bowl shaped pits. In the center, a surface impression of the bark from a tree is seen crossing the slab from left to right. And along the bottom a spider trackway proceeds across the slab. The story this slab tells is interesting, perhaps 280 million years ago, a large tarantula like spider (Octopodichnus) crossed a sand flat between the dunes near a fallen tree in a large desert perhaps running for cover from the large raindrops that began to fall. Found near Ashfork.

(Octopodichnus) Spider Trackway - One of the better trackway slabs found by us up near Ashfork. Such small animals can ONLY leave their trackways on completely dry sand. This represents an inter-dunal sand flat that was wetted by a morning dew after the animal passed to solidify the surface of the sand and preserve the layer before the next sand storm buried it. Ashfork area.

Close up of a portion of the above spider trackway to show the details of the unique pattern and tread detail made by this arthropod.

Octopodichnus - Another spider trackway, but also seen here are wave ripples as might be generated by a nearshore dune field. Note how the spider moved parallel to the dune ripples. This was a very common behavior, as we noted on other occurrences of the trace fossil. Ashfork area.

Another fine impression of a spider trackway (Octopodichnus) from Ashfork. This marks the passage of the animal across an interdunal flat, perhaps dry sand near the shoreline, but NOT in the wet tidal zone. From Ashfork.

Laoporis Noblei trackway crossing a Octopodichnus trackway - The interdunal sand flats were quite busy during Permian times, as evidenced by the dog sized mammal like reptile that produced the five toed prints (Leoporis) and crossing the trackway of a large spider (Octopodichnus). Look carefully, and you'll see the spider came first, since the footprints are over the top of the trackway (see below)

Laoporis Noblei track stepping over Octopodichnus, close up from above image shows clearly the mammal like tracks left by this reptile.

Laoporis Noblei - An excellent cast of a mammal like reptile track reveals several key details. Note the front foot (manus) is smaller than the rear foot (pes), and the spacing for such large prints between sides of the animal is very narrow - characteristically mammalian, however this animal whose bones have not been found in the Coconino stood upright with feet below the body, very unlike its purely reptilian ancestors whose feet splayed out on each side producing a much wider stance.

Manus-Pes pair of Laoporis Noblei. Note the "sand crescents" behind the feet, showing how it pushed through the sand as it traveled. Such details are crucial when determining environmental reconstructions.

Paleohelcura - Scorpion trackway. This gorgeous slab from Ashfork shows the typical pattern of three on a side produced by scorpions. Also note the tail drag down the middle, modern scorpions also make this pattern. The animal was going from left to right, and also we can see the Leoporis track at the bottom which may have eaten the Paleohelcura animal as a food source on occasion.

Paleohelcura 2 - Close up of another trackway also showing clear impressions of the tail drag in the sediments. Note the slight variation in the track pattern from the above photo. Brady's experiments with live scorpions found that the temperature had a great affect on the trackway pattern made by scorpions.

Permichnium? - Centipede like trackway. Rare in the Coconino were the traces left by small arthropods and insects. The larger heavier animals such as giant centipedes and millipedes are present however in the fossil dunes as seen here. Mogollon Rim.

Permichnium? - This complex trackway may be the trail left by a centipede as well, in either softer sediment or slightly damp sand. Linear body drag marks can be seen in places along its length. Mogollon Rim.

Diplopodichnus biformus - Centipede trackway. This trackway is quite uncommon in the Coconino, and according to Brady, rarely shows the leg impressions. The track can also be trifid as well. Desert Centipedes lived on rotting vegetation and debris found in damp areas.


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