Basal Atco Double Reptile Trouble

[Mikrogeophagus]

Took a short visit today to my Basal Atco site for the third time. Seems like every visit yields something new and totally unexpected (first visit = Acrotemnus prearticular tooth, second visit = huge assoc. fish verts, and now these). I'm fairly confident both of these are reptilian, but unfortunately I don't know much about identifying tetrapods. The first specimen I found is now my only non-Campanian mosasaur tooth. It's quite small (1 cm tall) and broke into 3 pieces, but the b72 seems to have done the job in making it presentable. Not amazing quality as expected for the Basal Atco, where much of the material is reworked.

 

I'm not completely familiar with the genera present in the earliest Coniacian. I have read about a chunk of Tylosaurinae maxilla being found near if not in the Basal Atco of Grayson county and the subfamily is thought to have started in the Lower Coniacian altogether. I want to say Russellosaurus made it to the Atco although I believe my tooth is a little large for this ID. I think Clidastes was also around in the Coniacian, although not sure how early. And there's a lot of genera I am not mentioning, so hopefully someone in the know can fill in.

 

The tooth itself seems to have both a mesial and distal non-serrated carina. The enamel isn't faceted as far as I can tell. I know it's not much to go off of and a precise ID is unlikely, but my hope is to at least cross off some of the genera that don't fit the description with yall's help.

 

Mosasaur tooth (1 cm tall)

 

Going from a relatively old mosasaur, the second specimen is a surprisingly young Coniasaurus (or so I think). This might be surprising to some, but Conisaurus is found beyond the Cenomanian. It actually has been documented to extend through the Coniacian all the way into the Middle Santonian (though the documentation is few and far between I must admit)! In other words this is a rare sort of find, but not necessarily unheard of. I don't believe any species have been named extending past the Turonian, so this may be an undescribed species. My specimen has that classic bulbous shape as well as textured enamel. It sure pays to look closely at every chunk of matrix.

 

Coniasaurus sp.? (3 mm tall)

 

One thing I would like to note is that I have since found a paper that helped to clarify the geology of this site which was a point of confusion in my previous trip reports. I made the mistake of referring to this place as the "Fish Bed Conglomerate" when it turns out that title has been used for at least a couple different layers, namely the base of the Austin Chalk and a phosphatic pebble bed that exists a short ways below the base documented by Taff and Leverett way back in 1893. This mixup has caused some confusion it seems in old academia. Anyways the stratigraphic chart (Fig. 2) in the source below shows these two phosphatic layers and some info on them. I kinda had a mind blown moment as I have actually visited both of these layers while looking for the Basal Atco at a couple of locations. The site where these teeth were found along with all my other written Atco adventures did actually in fact occur in the Atco or earliest Coniacian. However, the "second location" noted in the report I've linked below was actually at this Arcadia Park phosphatic pebble bed. These two layers are quite distinct and my "Atco" site definitely better fits the base. The sea levels paper specifically notes the layer as being ripple bedded which I coincidentally took a picture of in my old report! So satisfying when everything clicks into place...

 

Hancock, Jake M., and Ireneusz Walaszczyk. “Mid-Turonian to Coniacian Changes of Sea Level around Dallas, Texas.” Cretaceous Research 25.4 (2004): 459–471. Web.

 

 

As a bonus for reading through this, here are a couple of nice sharks teeth I found from the same layer. They are my first larger complete Cretalamna from the location and a new species on top of that. They compare best with Cretalamna ewelli which Siversson notes as being present in the Late Coniacian. I have some less impressive Cretalamna from the location that I previously identified as C. gertericorum, but I may have to double check those since it would be weird for two species to show up on top of one another in my opinion. They are very similar-looking species so I wouldn't be surprised if I was mistaken before.

 

Cretalamna ewelli with beautifully splayed cusplets

 

Looking forward to hearing yall's opinions! Thanks

[Jared C]

12 hours ago, EPIKLULSXDDDDD said:

the second specimen is a surprisingly young Coniasaurus (or so I think). This might be surprising to some, but Conisaurus is found beyond the Cenomanian. It actually has been documented to extend through the Coniacian all the way into the Middle Santonian (though the documentation is few and far between I must admit)!

Your surprise and possible suspicion is well warranted. IMO the paper about the santonian "large dolichosaur" tooth is dubious at best. Shimada and Everhart are very good paleontologists, but if I have any room for opinion, I find it much more likely that the tooth in the paper comes from an undescribed fish.  

 

There are several reasons - one of them was explained to me well by a marine reptile expert who has doubts about the paper as well. 

 

1) The tooth is far larger than what can be expected from even the largest confirmed dolichosaurs. 

2)The obvious temporal gap you've observed

3) If the two above don't kill the case, what does is basal views of the tooth in their paper. Fish typically replace their teeth like stacked cups, pushing one up to reveal another directly below. Marine reptiles (or at least mosasaurs and dolichosaurs) come in at angle, pushing inwards and then upwards. That's why, in my humble opinion, the morphology of the tooth in the santonian dolichosaur paper matches fish much better:

 

Observe how a thin wall of enamel follows the entire circumferences of the base of the tooth. That's where the enamel was ankylosed to the bone, a feature common in fish. That thin wall may survive with the fish replacement mechanism from the bottom, but the reptile replacement mechanism would not allow those paper thin ankylosed walls to survive. 

 

That said - your coniasaur tooth does indeed look like coniasaur. Most researchers tend to think that coniasaurs didn't really make it out of the turonian, so that makes your tooth interesting. Find a whole skeleton so you could demonstrate it's not reworked  

 

Edited May 5, 2023 by Jared C

[Mikrogeophagus]

@Jared CWow that is some great info! You are quickly becoming quite the reptile expert. Hopefully we get in some more research soon on early mosasaurs and related Coniasaurus because having to differentiate these teeth feels almost impossible. I forgot to even mention Dallasaurus which adds another layer of complexity and haziness to the smaller tooth (though I think Coniasaurus is the better fit for now).

 

A whole skeleton in the Basal Atco is quite a challenge, but who knows what might show up after a rock is flipped over. 

[Jared C]

8 minutes ago, EPIKLULSXDDDDD said:

A whole skeleton in the Basal Atco is quite a challenge, but who knows what might show up after a rock is flipped over

You might enjoy this trip report from @Heteromorph as much as I did then 

 

Edited May 5, 2023 by Jared C