Mikrogeophagus Posted February 23, 2023 Share Posted February 23, 2023 (edited) The weather's finally warming up here in DFW, and with that, it's time to move on from the comforts of dry land and return to the ways of creek stomping. These past few months, I've been mostly hanging around cuts and construction sites within the Washita Group, and to be honest, I've gotten a little sick of it. During that period, I steadily accrued a sizable list of potential Eagle Ford and Atco locations within the metroplex, and I was itching for the opportunity to finally go and check them out. Yesterday, I circled a few spots on the map and hit the road. Well, the first couple of sites were more or less complete duds. Stop 1 was situated in the Turner Park Member of the Britton. Lots of cool stuff can be found here and it's one of the more prettier formations, but I'm still trying to figure it out. I only spent about 30 minutes there, and nothing really impressed me. Nevertheless, I was at least rewarded with my first ptychodus from DFW (yeah it took awhile). Stop 2 was a famous site within the Camp Wisdom Member of the Britton. There were some interesting cephalopod specimens to be had, but nothing worth taking home. I did not lose hope, though. I knew that my third site was the most promising of the bunch and I accordingly saved most of the day for it. Stop 1 in the Turner Park, and my first ptychodus from DFW (P. anonymous) The Atco lies just above the Arcadia Park Formation of the Eagle Ford Group in DFW. It's famous for being densely packed with a diverse array of vertebrate material dating to the Coniacian, mostly concentrated within a thin layer dubbed the "Fish Bed Conglomerate" just at the contact of the Eagle Ford and Austin groups. The key to finding this layer is to look for matrix that is highly concentrated in phosphatic pebbles and fossils. Down in Austin, I think it gets a bit overshadowed by the Eagle Ford. In DFW, however, I would say the Atco reigns supreme. After a decent drive and trek, I stepped into the creek and began my descent through the Austin Chalk in search of this coveted conglomerate. It was difficult to tell where exactly I was stratigraphically since the Arcadia Park seems to be a bit calcareous whereas the Austin Chalk can be a bit marly. Where the calcareous marl ends and marly chalk begins is tougher to differentiate in practice, especially for someone less experienced like me. I eventually reached a large gravel bar and decided to pause the walking and have a look for any clues. I didn't find anything out of the ordinary in my initial scan, and almost picked up my backpack to move on. However, I took a moment to take a closer look at what I had first subconsciously written off as concrete. In this chunk of rock, I saw hundreds of tiny rounded phosphatic pebbles. Once my eyes reached the edge of the block, I spotted the unmistakable serrated blade of a Squalicorax falcatus tooth. There was no doubt, this was Atco matrix! The material was noticeably greyer than I had expected. Down in Central TX, the Atco is about as white as the rest of the overlying chalk. With all of the odd bits and bobs intermingling with the grey rock, it really does look like something manmade. I pulled out the rock hammer and began searching the bar for more chunks of fish bed conglomerate. It's amazing the things your eyes can miss when you unknowingly tune things out. There were very obvious shark teeth encased in many of the rocks I had walked by earlier. I collected several larger pieces and sat down to begin extracting teeth. Found my nicest shark vert yet. Unfortunately, it was super fragile and lots of it broke apart in cleaning. My first oddball tooth from the Atco. It has the shape of Cretalamna appendiculata (in the strict sense), but the presence of labial striations pushes me towards a posterior tooth of Cretodus crassidens. It also notably has doubled cusplets on one side. That same tooth post extraction. It broke in half by the time it got home, so I'll be doing a bit of consolidating. Funky little guy. There were some very large slabs of Atco conglomerate washed out atop the gravel bar. I did some hammering to break them into more manageable chunks so I could rinse them in the flowing creek. As I split one of the large slabs in half, I let out an annoyed sigh as I saw a decent Cretalamna appendiculata tooth crumble from the point my hammer had hit the stone. I tried to push that misfortune out of my mind as I perused across the newly exposed surface for any other teeth. I quickly spotted some fine serrations poking out of a chip of matrix that had just come loose. It was a Squalicorax falcatus doing its best chameleon impression. Please don't break please don't break Voila! By far the largest Squalicorax in my ever-growing collection! Florida megs better watch out... My constant hunt for micros at home adequately prepared me to keep an eye out in the field. I was able to spot a couple of Ptychotrygon triangularis oral teeth throughout the day. I also managed to find a mediocre Scapanorhynchus raphiodon and my first decent Ptychodus whipplei! Managed not to lose it in the gravel. Ptychotrygon triangularis Lots of beat up Scapanorhynchus raphiodon that day Ptychodus whipplei. The bits and pieces I have from Austin look like they were crapped off a cliff by a coyote as some Texas fossilers like to say. After getting a nice sunburn, I decided to get up and explore more of the area to perhaps find a source for this matrix. I spotted a little tributary that seemed promising and had a gander. Some outcrops have a beauty that only fossil hunters can truly appreciate... You who are reading this, I know your mouth is watering A little ways farther, the source is found. As I walked up the small feeder creek, I periodically inspected the pieces of fish bed conglomerate that were scattered in the way of the flowing water. One ball of matrix had an unsuspecting appearance, but something about it wouldn't let me leave it alone. There was what initially seemed to be some part of a bivalve erupting from it, but the color and texture gave off enamel vibes. After hammering off some chalk, I still couldn't figure it out. I eventually extracted the entire thing. It was extremely smooth and came to an edge at the top. The surface was mostly bluish gray and semi translucent. It took a second for the cogs in my brain to start working and realize I'd seen something very similar before. @Jared C and I have been recently discussing Hadrodus, and, of course, one of his best discoveries is a large fragment of Hadrodus hewletti of which shares many visual characteristics with the object in my hand. I was excited to realize I was holding the tooth to a gargantuan pycnodont fish. Just after extraction Occlusal view and side pics For scale. Stay tuned for the identity of this monster After this discovery, my stomach growled, signaling it was about time to head back. This was only my first excursion in the Atco of North Texas, and it had already proven to be one to remember. For sure, this will be a site to return to. As I walked along the banks towards my exit, my eyes were still honed in on the ground as is the curse set upon every fossil hunter. I usually don't find anything on my return walks, but I was delighted to see the sheen of black enamel poking out of some Atco chalk just several feet above the fish bed conglomerate. I popped it out and quickly identified it as Ptychodus, though the species I was unsure of. At the moment, I think it is P. atcoensis as it seems to be a good match with a specimen in Hamm's paper on the Atco. It's got radiating ridges and a marginal area similar to some P. anonymous. That paper mentions Meyer (1974) suggested that the taxon could be transitional between P. mortoni and P. anonymous and I certainly understand that reasoning. It's a little beat up, but that's a rare species and a special one. In situ Radiating ridges and concentric ridges in the marginal area as expected for P. atcoensis Once I got home, I began researching more on my fish tooth to try and pin an ID. My initial idea was that it could be a Hadrodus incisor like what Jared had in his collection based on size and coloration alone. Jared was quick to point out that the morphology differed greatly between my tooth and most incisors. This was a valid hole in the argument and I went back to the drawing board. The tooth was most certainly pycnodont in origin, but the world of pycnodonts is much deeper than most people realize. When examining the edge of the tooth I noticed a characteristic I couldn't come up with a name for until I saw it in one of the papers on Hadrodus Jared had shared. "Papilla-like tubercles" were the things lining the edge, and thankfully the author of the paper offhandedly brought up another genus of pycnodont that shared this trait: Acrotemnus. After some searching, I found a recently published paper by Shimada on a newly discovered specimen of Acrotemnus streckeri from Big Bend in the Ernst Member of the Boquillas Formation. And wouldn't ya know it, Hamm stated that the Ernst Member can be correlated with the Atco Formation (though I should mention a discrepancy in that Shimada places the Ernst Member in the Turonian and not the Coniacian which Hamm asserts the Atco + Ernst to be). The Acrotemnus paper had some high quality images of the dentition of the specimen, and I found the most similarity in size and morphology with the prearticular teeth of their fish. The Shimada paper mentioned that most pycnodonts are small, only reaching about 25 cm in size. To people who are unfamiliar with pycnodonts, these teeth may not look all that crazy, being measured only as 1-2 centimeters . However, scaling the body with the size of the dentition shows that Acrotemnus streckeri reached lengths of at least 1 - 1.3 meters, making it some of the largest pycnodonts to exist at the time! This definitely shattered my preconceived notions about pycnodont fish. Already, the Atco has proven to be one of the most interesting formations in North Texas. This site is a bit of a laborious drive for me, but I'm certain many future adventures will be had and many more fascinating discoveries with it. Thanks for reading! Edited February 23, 2023 by EPIKLULSXDDDDD 1 20 Link to comment Share on other sites More sharing options...
Sjfriend Posted February 23, 2023 Share Posted February 23, 2023 Very good report and great finds! Love the different teeth you found. 1 Link to comment Share on other sites More sharing options...
RJB Posted February 23, 2023 Share Posted February 23, 2023 I'm really not a tooth guy but this was a really good read. Very good report. Love the pycnodont tooth! If only you could have found the entire fish! RB 1 Link to comment Share on other sites More sharing options...
rocket Posted February 23, 2023 Share Posted February 23, 2023 thanks a lot, great to see nice localities so far away from me 1 Link to comment Share on other sites More sharing options...
Tidgy's Dad Posted February 23, 2023 Share Posted February 23, 2023 Nice report. I enjoyed that. 1 Life's Good! Tortoise Friend. Link to comment Share on other sites More sharing options...
Jared C Posted February 23, 2023 Share Posted February 23, 2023 incredible find and enjoyable report. That fish tooth is awesome 1 “Not only is the universe stranger than we think, it is stranger than we can think” -Werner Heisenberg Link to comment Share on other sites More sharing options...
ThePhysicist Posted February 24, 2023 Share Posted February 24, 2023 20 hours ago, EPIKLULSXDDDDD said: My first oddball tooth from the Atco. It has the shape of Cretalamna appendiculata (in the strict sense), but the presence of labial striations pushes me towards a posterior tooth of Cretodus crassidens. It also notably has doubled cusplets on one side. Cool finds, this one is looking like Cretalamna to me. I'm sure you'll get a better look at it once it's cleaned up, but the "striations" look more like creases. ^ Siversson et al. (2015) 3 "Argumentation cannot suffice for the discovery of new work, since the subtlety of Nature is greater many times than the subtlety of argument." - Carl Sagan "I was born not knowing and have had only a little time to change that here and there." - Richard Feynman Collections: Hell Creek Microsite | Hell Creek/Lance | Dinosaurs | Sharks | Squamates | Post Oak Creek | North Sulphur River | Lee Creek | Aguja | Permian | Devonian | Triassic | Harding Sandstone Instagram: @thephysicist_tff Link to comment Share on other sites More sharing options...
Mikrogeophagus Posted February 24, 2023 Author Share Posted February 24, 2023 33 minutes ago, ThePhysicist said: but the "striations" look more like creases. I think you've got it with this one. There's always something I miss with these teeth, so thank you for pointing this out! Looks like the photo you've shared is for an upper lateroposterior of C. gertericorum which is one species I had overlooked. There's a lot of species covered in that paper . Right off the bat, the tooth looks very similar to mine, even having that strange doubled cusplet on the medial side along with the labial creases you've mentioned. Siversson says it coexisted with the other species I had thought this could belong to if it wasn't Cretodus, Cretalamna appendiculata. In the C. appendiculata portion of the paper, Siversson says the upper lateroposterior teeth of C. gertericorum may be distinguished from C. appendiculata by having a more distally curved cusp and tightly curved/small basal edge of the root. I believe that to be true for my specimen, so I have some confidence in saying that this could be an upper lateroposterior of C. gertericorum. Seems the paper doesn't mention the teeth existing outside of the early Turonian, but I guess that may just be due to a very limited number of supposed C. appendiculata specimens from this age being revised in their identification. I'm guessing you're way ahead of me though, given you already shared the photo from this species. Thank you for the well-informed replies. I always learn something new after reading them . Sorting out all these new species is tedious, but it feels so rewarding to end up with an ID you can feel confident in. Hopefully similarly ambitious papers will be written on other Cretaceous waste basket taxon soon! Squalicorax falcatus in particular... Link to comment Share on other sites More sharing options...
DPS Ammonite Posted February 24, 2023 Share Posted February 24, 2023 (edited) Locals please help me. I thought that the Fish Bed Conglomerate was in the upper few feet of the Eagle Ford Group, Arcadia Park Fm and not in the Atco Fm. The Atco also has a fish rich layer. In north Plano and Frisco fish teeth are common in several layers of sandstone/siltone in and amongst the upper few feet of the Arcadia Park Fm shales. In Sherman, the Arcadia Park Fm contains thicker layers of yellowish sandstone with phosphatic nodules and teeth all below the oyster shell layer. I have also seen fish parts in the marly Atco Fm. In west Plano. https://pubs.geoscienceworld.org/aapgbull/article-abstract/38/2/335/33995/Fish-Bed-Conglomerate-and-Sub-Clarksville-Sand?redirectedFrom=fulltext Edited February 24, 2023 by DPS Ammonite 1 My goal is to leave no stone or fossil unturned. See my Arizona Paleontology Guide link The best single resource for Arizona paleontology anywhere. Link to comment Share on other sites More sharing options...
Mikrogeophagus Posted February 24, 2023 Author Share Posted February 24, 2023 13 minutes ago, DPS Ammonite said: Locals please help me. I thought that the Fish Bed Conglomerate was in the upper few feet of the Eagle Ford Group, Britton Fm and not in the Atco Fm. The Atco also has a fish rich layer. In north Plano and Frisco fish teeth are common in several layers of sandstone/siltone in and amongst the upper few feet of the Britton Fm shales. In Sherman, the Britton Fm contains thicker layers of yellowish sandstone with phosphatic nodules and teeth all below the oyster shell layer. I have also seen fish parts in the marly Atco Fm. In west Plano. I got my info for the "Fish Bed Conglomerate" from Hamm's paper on the fauna of the basal Atco (2011). Perhaps there are multiple layers referred to as the "Fish Bed Conglomerate" as I can see that not being a very specific name. The one Hamm refers to and the one I believe I've found lies "within the contact zone of the Eagle Ford and Austin groups", and is considered by most to be within the Austin Group. That being said, the source you've shared is very interesting because it does correlate with some of the things I saw/mentioned in my trip. I had difficulty initially locating the conglomerate because I was simply looking for the contact of soft Arcadia Park shales and hard Austin Chalk. However, I found that the area was mostly filled with what I thought to be chalky shales and shaley chalks. When I did finally locate the conglomerate, it wasn't overlain by rock hard chalk, but instead by shale as your article agrees. I rationalized this as maybe being a softer form of the chalk perhaps like the marly sections of Atco you mention in your reply. Your reference is a bit older, so maybe that "10-35 feet" of shale separating the conglomerate from the Atco has since been reinterpreted as being a part of the Atco itself. The rest of paper is not accessible for me unfortunately, but I would have read more into it if I could. Also, you talk about the Britton Fm, but I wonder if you mean to say Arcadia Park Fm which is situated between the Britton and Atco. I could be confused though... Hopefully someone well-versed with DFW geology can set things straight Link to comment Share on other sites More sharing options...
DPS Ammonite Posted February 24, 2023 Share Posted February 24, 2023 2 minutes ago, EPIKLULSXDDDDD said: Also, you talk about the Britton Fm, but I wonder if you mean to say Arcadia Park Fm which is situated between the Britton and Atco. I could be confused though... Thanks. I did mean Arcadia Park Fm. I editing my post. The reason I question where the fish bed conglomerates are stratigraphically, is that I consider the teeth, clams, oysters, bryozoans and corals from Post Oak Creek in Sherman to be from the Arcadia Park Fm. My goal is to leave no stone or fossil unturned. See my Arizona Paleontology Guide link The best single resource for Arizona paleontology anywhere. Link to comment Share on other sites More sharing options...
Jared C Posted February 24, 2023 Share Posted February 24, 2023 (edited) 11 hours ago, DPS Ammonite said: The reason I question where the fish bed conglomerates are stratigraphically, is that I consider the teeth, clams, oysters, bryozoans and corals from Post Oak Creek in Sherman to be from the Arcadia Park Fm. It seems to be that way. In DFW, that shark tooth lens is the contact between the eagle ford group (of which, in DFW I believe the uppermoost member is the arcadia park) and the austin group, but the creek hits both formations in the process of washing out that contact too. My DFW experience is not extensive, but I notice from what folks post on the forum that the classic look of teeth from the DFW Atco is pitch black- think the teeth from the famous Atco Midlothian Quarry. Now and then, teeth with that look wash up in Post Oak Creek - my favorite Cretodus tooth from there, for example, has that midlothian look: Furthermore, folks like @PaleoPastels have found derived looking Ptychodus atcoensis in the creek - which appear firmly in the atco. That mixing in of the austin chalk rather than just hitting the upper arcadia park/overlying sharktooth lens also could explain the different color preservations of teeth there. 13 hours ago, EPIKLULSXDDDDD said: Seems the paper doesn't mention the teeth existing outside of the early Turonian, but I guess that may just be due to a very limited number of supposed C. appendiculata specimens from this age being revised in their identification. If I had to have an opinion on the Siversson paper, I'd say it's great at observing and then classifying morphological differences, but it's not very thorough at recording when and especially where those differences occur. That's no fault - there are too many teeth in the world for it to keep track of. But, the utility of the paper for us is probably best kept to just the morphological comparisons with the timelines as good guidelines but certainly not the law. But, I'm not as qualified as the man himself, so take my suggestions with a grain of salt Edited February 24, 2023 by Jared C 3 “Not only is the universe stranger than we think, it is stranger than we can think” -Werner Heisenberg Link to comment Share on other sites More sharing options...
LSCHNELLE Posted February 25, 2023 Share Posted February 25, 2023 Nice finds @EPIKLULSXDDDDD! It's always fun to go to new places and learn new things. As a geologist, I know that things are always different both locally and regionally. We may use similarities in radiometric dating, lithology, geochemical characteristics, and fossil assemblages to match formations across town and across the world. As a fossil hunter, the thing I have learned most is to be open to a new understanding of the same old formations. My favorite example is in the Cenomanian, Turonian, and Coniacian exposures in my area. The ones the civil engineers try and cover over with grass and riprap. The Eagle Ford (Kef/Basal Atco) varies in thickness across the San Marcos Platform from just 5 - 10' nearer to the San Antonio to over 45' in Central Texas. As you go west, east, and north, it thickens to 100' to 300'+. I spent the first year thinking that there were just two or three basic fossil zones in the Kef over that 45' thickness. Now, I know of six different fossil zones in the region. I thought there was one Kef/Atco conglomerate (or lag deposit). Now, I know there are multiple lag deposits within it - all with potential for vertebrate fossils. Some are certainly thicker and more widespread than others. The Austin Chalk here in Austin is usually white where weathered, but it is gray to dark gray in unweathered forms all over the state and it can weather yellow in southwest Texas. You might even confuse it for a shale (it can be marly) until you try and hammer into it. JaredC found a mosasaur in a condensed layer below the Basal Atco, but it was still within the South Bosque Turonian. This condensed layer doesn't necessarily occur in other parts of town or other parts of the State. I used to think Atco fossils were red-brown colored, but I have found some gray to black colored ones recently. It can be a new learning experience for everyone. And, hopefully we have fun while we play in the fossil layers! - while still learning anew about creatures long ago extinct or still living today in some similar form. For your passion/obsession - thanks @Jared C and @EPIKLULSXDDDDD 1 2 Link to comment Share on other sites More sharing options...
Jared C Posted February 26, 2023 Share Posted February 26, 2023 On 2/24/2023 at 10:04 PM, LSCHNELLE said: Nice finds @EPIKLULSXDDDDD! It's always fun to go to new places and learn new things. As a geologist, I know that things are always different both locally and regionally. We may use similarities in radiometric dating, lithology, geochemical characteristics, and fossil assemblages to match formations across town and across the world. As a fossil hunter, the thing I have learned most is to be open to a new understanding of the same old formations. My favorite example is in the Cenomanian, Turonian, and Coniacian exposures in my area. The ones the civil engineers try and cover over with grass and riprap. The Eagle Ford (Kef/Basal Atco) varies in thickness across the San Marcos Platform from just 5 - 10' nearer to the San Antonio to over 45' in Central Texas. As you go west, east, and north, it thickens to 100' to 300'+. I spent the first year thinking that there were just two or three basic fossil zones in the Kef over that 45' thickness. Now, I know of six different fossil zones in the region. I thought there was one Kef/Atco conglomerate (or lag deposit). Now, I know there are multiple lag deposits within it - all with potential for vertebrate fossils. Some are certainly thicker and more widespread than others. The Austin Chalk here in Austin is usually white where weathered, but it is gray to dark gray in unweathered forms all over the state and it can weather yellow in southwest Texas. You might even confuse it for a shale (it can be marly) until you try and hammer into it. JaredC found a mosasaur in a condensed layer below the Basal Atco, but it was still within the South Bosque Turonian. This condensed layer doesn't necessarily occur in other parts of town or other parts of the State. I used to think Atco fossils were red-brown colored, but I have found some gray to black colored ones recently. It can be a new learning experience for everyone. And, hopefully we have fun while we play in the fossil layers! - while still learning anew about creatures long ago extinct or still living today in some similar form. For your passion/obsession - thanks @Jared C and @EPIKLULSXDDDDD and thank you more for sharing your knowledge - I owe a huge part of what I know to you 1 “Not only is the universe stranger than we think, it is stranger than we can think” -Werner Heisenberg Link to comment Share on other sites More sharing options...
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