My New Megatooth Site!.. And Making Sense of the Kef/Kau Boundary

[Mikrogeophagus]

Last year, I discovered my very first Atco site in a small suburban creek on the quiet side of town. The place treated me well, and within the span of a handful of visits, I came away with a nice bounty of new shark species and some reptiles to boot. Upon posting a few trip reports to this forum, one thing that became quite clear was the confusing nature of the Kef/Kau boundary.

 

Comparing my finds in the little creek with other, more famous Atco sites, revealed a large discrepancy in terms of fossils. Post Oak Creek, for example, boasts staggering quantities of rather large Cretodus sp., Scapanorhynchus sp., and Ptychodus whipplei. The same can be said for the Midlothian quarries (when granted access at particular stretches of time). This can hardly be said for my spot, where in these departments my many hours culminated in only a single Cretodus, tiny/sparse goblins, and some small and rather badly eroded P. whipplei. So what gives? I wondered if maybe the basal Atco was spotty in nature and had some regions that were simply more stingy than others. With big teeth in mind, I decided to search for some new Atco sites.

 

My second locality of the mission was also reported about a year ago. It took me to a special sandstone layer located a ways down the stratigraphic column, multiple meters beneath the basal Atco. The most notable aspect of this layer was the beautifully preserved ripple marks as shown below:

 

Ripple Marks

 

Visualizing an ancient sea floor couldn't be any easier. Anyways, this site yielded some neat reddish teeth from a section adjacent to these ripples, but failed to deliver everything I had wanted in the fossil department (the basal Atco was obscured by manmade intervention). However, I am thankful for this particular layer because it introduced me to an extremely helpful article that has since been indispensable.

 

"Mid-Turonian to Coniacian changes of sea level around Dallas, Texas" by Jake Hancock and Ireneusz Walaszczyk does a really great job of explaining the Eagle Ford - Atco geology of the local area in terms that are "somewhat easily" understandable to a novice like me. Their second figure was what initially caught my attention, as it reflected a little bit of the confusing things I had been observing firsthand. Namely, the above ripple-marked sandstone layer (yellow below) as well as its relation with the basal Atco conglomerate (blue below) of my original creek site.

 

Blue = Basal Atco Conglomerate; Red = Speculative Megatooth Layer; Yellow = Sandstone Ripple Bed

 

If you already read my caption to the stratigraphic column above, you may have noticed that the red layer is conspicuously labeled as "speculative megatooth layer". Well, let's go ahead and explain that. Fast forwarding to just a couple days ago, I decided to end my break from hunting the Atco. I had a juicy prospect circled on the map and took an early morning drive to check it out. Pretty quickly, it became apparent that this was exactly the site I was looking for. Only my second tooth of the day was a huge Ptychodus whipplei and one of the finest preserved I had so far seen in person.

 

Ptychodus whipplei

 

Crawling on all fours through freshly watered shales, I slid my way in a meandering zigzag, stumbling upon tooth after tooth. Unlike my original creek site where goblins were relatively small and not super abundant, this place seemed to be dominated by big Scapanorhynchus sp. fragments. More and more, things were beginning to look like POC and Midlothian.

 

Scapanorhynchus sp.

 

After much meticulous collecting of tiny teeth and bigger goblins, I nearly crawled right over the first major find of the day. Just peeking from a foxhole of muddy shale was the immaculate root of what I believe to be Cretodus sp (he's a little strange... *extremely* subtle lingual striation and odd root shape).

 

Cretodus sp. and tiny P. whipplei in situ

 

Cretodus

 

Not too long after I came across one of the biggest (literally) heartbreakers I've ever found. What would have been a pristine condition Cretodus and by far the largest shark in my collection...

 

Cretodus. Cusp nowhere to be found...

 

Fortunately for me, redemption came not long after. While preparing to dig a little Squalicorax out of fast-flowing stream, I spotted just the faintest outline of a cusplet from the neighboring shale. I took an in situ photo so you guys can challenge yourselves with spotting it:

 

 

Too hard? This should make it a little easier!

 

 

With a little tapping of the hammer and chisel...

 

Cretodus!

 

Wrapping up with an even bigger P. whipplei, suffice it to say I was feeling pretty good on the drive home.

 

3 Cretodus sp. and 1 P. whipplei

 

All of the large teeth I've shown came from a single distinct thin layer of phosphatic sandstone that roughly corresponds to the one I have circled red in the chart. There was an additional thin sandstone just above it that seemingly lacked the same levels phosphate which also tracks with what's seen in the chart. Both of these sandstones were located a short distance (1 meter?) beneath the basal Atco conglomerate which was shown at the same location and looked identical to the conglomerate of my original creek site. It seems this stratigraphic figure is extremely accurate at least for the sites I have visited!

 

Now, what is the significance of relating these sandstones and conglomerates to the chart? Well, hopefully, they can tell us a thing or two about the ages of the fossils of course. This is important because the precise ages for fossils in and around the Kef/Kau boundary has been contentious and confusing with lots of contradicting information in circulation. Are they Turonian, Coniacian, or some sort of mixture?

 

The megatooth layer (red circle) is located firmly within the zone of Prionocyclus macombi which is set around the Late Middle Turonian and Early Upper Turonian (boundary contentious per the aforementioned article). Commonly associated with P. macombi and quite prolific at this site (even condensed in distinct layers near to the thin sandstone) was the oyster Cameleolopha bellaplicata which has been mentioned by our own @ThePhysicist as being evidence for a Middle Turonian presence in the fossil record of POC. This is almost certainly the case, and it would appear that the presence of large P. whipplei, Scapanorhynchus, and Cretodus may be mostly thanks to these Middle/Upper Turonian sandstones as opposed to the actual basal Atco conglomerate.

 

Before getting to the age of the basal Atco conglomerate (blue circle) itself, I would first like to have a gander at the hard Atco Formation chalks that overlie it. The article sets these chalks in the zone of Cremnoceramus deformis erectus which is a bivalve species that is easy to find at my original creek and this new megatooth location (after traveling further up the geological record). This belongs solidly to the Earliest Coniacian which is a noticeable temporal leap from the P. macombi zone. Within these C. deformis erectus chalks of the Atco, by far the most common shark tooth is none other than the famed Ptychodus mortoni (at least one form of this paleobucket, but that's another can of worms) as it makes its first appearance in the Texas record. I have been lucky enough to find them in the smooth Atco boulders a few times. Importantly, P. mortoni is not known from the Middle Turonian Kamp Ranch of Texas (Collignoniceras woollgari zone) and it would be strange to expect its immediate appearance in the nearby P. macombi zone as opposed to something slightly more basal in morphology.

 

Ptychodus mortoni of the Earliest Coniacian Atco Formation

 

So we have framed the basal Atco Conglomerate neatly between the P. macombi zone and C. deformis erectus zone (Middle/Upper Turonian + Earliest Coniacian). What should we expect to find?

 

Well, let's first establish that the conglomerate is a lag deposit/condensed bed. This was a concept that confused me for a long time before it finally clicked, so I will try to explain in the terms that are easy to understand. There are many scenarios in which a paleoenvironment may favor erosion. This means, more sediment will be taken away from a location than deposited. Like a cheese grater, the passage of time will grind down the bedrock and steadily the layers of rock (and fossils) will be worn away. At some point, the rate of erosion is outpaced as the paleoenvironment transitions into one that might favor sedimentation/deposition instead. For a brief period of time, the erosion is unable to completely remove the bigger chunks of dugout bedrock (and fossils) and can only manage to do away with the finer fragments, so you end up with what could be thought of as a "bed of gravel". The gravel of course has the chance to freely roll around on the sea floor and get damaged (hence why teeth of lag deposits are often abraded). Then deposition takes over and the gravel bed is covered and protected with younger sediments and fossils. Sometimes it takes awhile for sedimentation to really get going which means there is more chances for burrowing creatures to mix around the young and old fossils. Slower sedimentation also creates condensed zones as more temporal space is compacted into a tighter volume of bedrock. In the end, the resulting condensed zone may yield remains from animals in the deepest layer touched by erosion in addition to animals from the time in which deposition resumed.

 

In the case of the basal Atco conglomerate, we know that erosion reached all the way down to the P. macombi zone (I should note that this could vary in other parts of the state as different zonations would have been reached in different places, but I am sticking to P. macombi because of the presence of the specific phosphatic sandstones at my site), so we should expect many fossils of that age to be present in the lag. The other fraction of fossils ought to hail from the period in which sedimentation resumed. This is difficult to pinpoint in the literature precisely. The Jake Hancock and Ireneusz Walaszczyk article places this within the zone of Cremnoceramus waltersdorfensis waltersdorfensis which is Latest Turonian, but Shawn Hamm and David Cicimurri have stated the presence of the ammonites Peroniceras lepeei and Allocrioceras hazzardi as evidence of an Earliest Coniacian age. Either way, the temporal proximity of the resumed sedimentation to that of P. mortoni in the overlying Atco chalks would make it unsurprising to find P. mortoni present within the just slightly older basal conglomerate.

 

In fact, this appears to be the case for the basal Atco conglomerate, as I have come across numerous fossils of older C. bellaplicata oysters in varying states of preservation in combination with younger species such as P. mortoni and Squalicorax hartwelli (a more derived member of the genus that does not occur in the Middle Turonian Kamp Ranch). To really test this model, I will need to sample more of the thin phosphatic sandstones of the P. macombi zone. The expectation is for younger species like P. mortoni and S. hartwelli to not be present. If things work out, this could be a useful method for determining the ages of certain teeth found in places like POC, if they are noted as being exclusive to the basal Atco conglomerate or found in both.

 

C. bellaplicata and mosasaur from the basal Atco conglomerate

 

I have a lot more to say, but it's getting really late! Hope you guys enjoy this dive into the Atco and if not, please at least enjoy the teeth  

 

Thanks for reading!

[Jared C]

This is very interesting! The eagle ford is seemingly so variable through out the state. Moving south, the completely different members come with completely different exposed ages. In central Texas, as you hit Waco and move on southward, the upper member of the kef (south bosque) seems to finish without leaving the C. woollgari zone. It transitions into a thin, highly bioturbated consensed zone, which itself is P. hyatti zone. On top is the massive basal atco conglomerate. Lots to learn in this state, I love it

 

Also, I’m so jealous of your Cretodus I could die. Amazing work. Looking forward to seeing what else you have to say about the geology 

[old bones]

Very fine trip report with great photos and interesting explanation of the formations! Awesome teeth!

[ThePhysicist]

Spectacular finds, I'd totally be flabbergasted if I spotted any of those Cretodus. Thanks for the discussion on the geology, it's an interesting puzzle. 

 

14 hours ago, Mikrogeophagus said:

This is almost certainly the case, and it would appear that the presence of large P. whipplei, Scapanorhynchus, and Cretodus may be mostly thanks to these Middle/Upper Turonian sandstones as opposed to the actual basal Atco conglomerate.

Indeed, I have a sandstone chunk from POC I haven't mentioned before that has associated C. bellaplicata and P. whipplei. I'm actually headed to POC soon to see what else I can find in the way of geology.

[LSCHNELLE]

@Mikrogeophagus

I love your fossil trip report on the Atco in North Central Texas. I've shared my past Central Texas experiences here:

https://www.thefossilforum.com/topic/125189-fickle-basal-atco-member-summer-2022-hunt/

 

I wish the Basal Atco was that uniform down here. It's so wishy washy. See the following examples:

(1) In two or three places the lowest or highest part of the contact zone is lenticular limey sandstone layers under the massive Atco Chalk that split easily with a chisel. Within them, sometimes you find Ischyrhiza spines, Ptychodus latissimus, and P. atcoensis.

(2) In three different outcrops, there is a nodular marl void of fossils or with tiny fossil fragment residue.

(3) Still another outcrop had massive conglomerate with phosphatic nodules and multiple weathered Cretoxyrhina, common Ptychodus mortoni, and rare P. latissimus.

(4) Another has conglomeratic erosional troughs cutting into the South Bosque with multiple limonite pockets within and sparse Cretodus and Squalicorax and rare Ptychodus mammillaris inside.

 

Of course, the alternative is searching through the Austin Chalk for rare vertebrate layers among the abundant Pycnodont oyster layers and inoceramus fragments. So, thanks fickle Atco! Thanks @Mikrogeophagus.

 

[Jaybot]

Great trip report!  I enjoy seeing the ptychodus teeth you texans seem to find on a regular basis

 

 

On 6/6/2024 at 5:21 PM, ThePhysicist said:

I have a sandstone chunk from POC I haven't mentioned before that has associated C. bellaplicata and P. whipplei.

Woah!  Now that's something I'd like to see!