How variable are Hell Creek/Lance Formation crocodilian teeth?

[Jurassicbro238]

I've had these crocodilian teeth from Niobrara County in the Lance Formation for a while now, but I'm not sure how if its possible to ID them beyond "Crocodilian tooth." I've generally heard that the "sharp" morphology are Borealosuchus teeth and the short bulbous teeth are Brachychampsa. Is that a safe rule to follow? I've also noticed that there are some slight differences in the teeth I have (hopefully the pics make it visibile). They're small, but the two on the lower left have noticeable raised striations (not sure what to call it). However, the one on the lower right and on the top are smooth. Is this just a difference in preservation? Identifiable traits to the species level? Positional characteristics? Individual variation?

I'd like to hear any opinions on this.

 

Thank you for your time

[Opabinia Blues]

Thoracosaurus is also present, albeit rare, and there's potential for different undescribed genera as well. Genus-level identification is possible, but take it with a grain of salt. Species-level identification based only on teeth is probably impossible, though for many Lancian crocs there's only one species present in the genus anyway. Certain Brachychampsa teeth are unmistakable, but there are other morphologies as well based on placement in the jaw. Different morphologies in teeth (especially croc teeth) can be due to placement and ontogeny, not just species differences.

 

Borealosuchus teeth are generally smooth and pointy with two unserrated carinae, but that's basically a description of a generalized croc tooth. Thoracosaurus allegedly has long needle-like teeth with wrinkled enamel (quite similar to the Ichthyovenator teeth you see on the market occasionally) though I have yet to see any actual specimens to verify this.

 

The big tooth with the broken tip and the tooth to the far right on the bottom row I would be comfortable labeling as c.f. Borealosuchus with the previous caveat in mind. The two smaller teeth in the bottom row (left and center) with the ridged enamel display a fairly common morphology seen in croc teeth from the Hell Creek/Lance Fms. and perhaps elsewhere too. These are difficult to place, and I have had one or two informal conversations with other collectors where we speculated this morphology may be ontogenic, ie they look the way they do because they come from juvenile crocs. The width of the base and the ridged enamel also makes me speculate that they might be premaxillary teeth from Brachychampsa juveniles, but that's just speculation on my part.

 

 

 

 

[Jurassicbro238]

Oh I see, makes sense. As far as I'm aware, there are 4 described croc species between Lance and Hell Creek which are Thoracosaurus, Brachychampsa, Borealosuchus, and Prodiplocynodon.

 

That's really interesting about alleged Thoracosaurus morphology. I'll definitely have to keep a look out for that.

 

About the ontogeny, do you know if that happen with modern crocodilians?

[Opabinia Blues]

27 minutes ago, Jurassicbro238 said:

Oh I see, makes sense. As far as I'm aware, there are 4 described croc species between Lance and Hell Creek which are Thoracosaurus, Brachychampsa, Borealosuchus, and Prodiplocynodon.

 

That's really interesting about alleged Thoracosaurus morphology. I'll definitely have to keep a look out for that.

 

About the ontogeny, do you know if that happen with modern crocodilians?

I hadn't heard of Prodiplocynodon, that adds another layer of complication. But nonetheless it fits with what I said above, that there are almost certainly other genera present, haha.

 

As for ontogeny and heterodonty in modern crocs, here are some papers that might shed some light onto the topic (I have only skimmed these so far):

 

https://pubmed.ncbi.nlm.nih.gov/29865481/

 

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6397764/

[Jurassicbro238]

Prodiplocynodon seems to be a lesser known croc species (I hadn't known about it until a few months ago). It makes sense since the holotype is a single skull lacking a lower jaw. Here's the description paper if you're interested.

 

Thank you! I'll definitely have to go over these papers.

1503948249_ProdiplocynodonDescription.pdf

[Troodon]

This mabe be fish, Melvius thomasi

 

It's very hard to ID croc teeth in these deposits, they do vary quite a bit and yours are indeterminate ranging from very young to juvenile teeth.

 

Posterior Brachychampsa teeth are crusher types and can be identified 

Not sure what to say about Prodiplocynodon with just one occurrence in a heavily studied deposit.   Described in 1941 and I have not seen much since then.   May be synonymized into one of the others if re-examined today who knows and may be real.  Even a 2012 paper that looked at shed teeth teeth in the HC made no mention of it.

Community structure and paleoecology of crocodyliforms  from the upper Hell Creek Formation (Maastrichtian),  eastern Montana, based on shed teeth
George E. Bennett, III

 

[Jurassicbro238]

Don't Amiid teeth usually have a translucent tip? I have seen some that are solid but they're just smooth. I'll look into it though.

 

I figured as much for my own teeth. I have yet to look at the ontogeny paper in detail but it would be interesting to see why some teeth from these deposits have differing textures on them.

 

Yeah, that's also a point I've been wondering about. It's strange that only 1 skull has been found and I'd like to see how it compares to Borealosuchus. I didnt know there was a paper on croc teeth! That'll be a fun read, thanks.

 

 

[Troodon]

Take a closer photo of that croc/ fish tooth do we can get a better look.  That croc paper basically says teeth are indistinguishable from one another at least Borealosuchus and Brachychampsa

 

[Jurassicbro238]

I think I'm wrong haha. I was able to put the tooth under the microscope and what I thought were striations look like microfractures. Maybe it is a fish tooth after all.

 

@Opabinia Blues is this what you were referring to in terms of enamel wrinkles? Or is this normal croc morphology?

 

[Troodon]

Thanks, That's croc.

[Opabinia Blues]

1 hour ago, Jurassicbro238 said:

I think I'm wrong haha. I was able to put the tooth under the microscope and what I thought were striations look like microfractures. Maybe it is a fish tooth after all.

 

@Opabinia Blues is this what you were referring to in terms of enamel wrinkles? Or is this normal croc morphology?

 

 

That first image is what I was referring to as far as enamel wrinkles, and this enamel texture shows up all over the place in different crocs. Borealosuchus adult skulls typically have smooth enamel (not like this texture). Brachychampsa and Thoracosaurus apparently have this wrinkled texture (as seen on the bulbous crusher Brachychampsa teeth). I am not sure if this actually holds up or is diagnostic, but this is the typical wisdom from fossil hunters.

[Opabinia Blues]

@Jurassicbro238 

 

Here are some personal finds from my collection. These teeth are from Hell Creek, however the faunal overlap between Hell Creek and Lance is almost 1:1. The tooth on the right is unmistakably 100% Brachychampsa. The tooth on the left I have labeled as c.f. Borealosuchus, as it is a large, conical, pointed tooth with smooth enamel and non-serrated carinae. I am pretty confident on this ID. It is essentially a larger version of the tooth that Troodon just verified as croc above.

 

However, both of my teeth are large teeth from adult crocs, and the identification of these large adult teeth is likely much more straight forward and understood than smaller, juvenile teeth (there are morphologies that show up in small croc teeth that do not show up on big croc teeth). 
 

[Jurassicbro238]

This is all super fascinating and very helpful! So a genus ID can be tentatively assigned only to large teeth then. Nice finds!

 

So for the 1st image I showed, I forgot to mention the scale bar was in cm. For that tooth, would you say it'd be safe to label that as cf. Thoracosaurus? Im also fine with Indet. Crocodilian though. It also came from the Lance.

[jpc]

4 minutes ago, Jurassicbro238 said:

This is all super fascinating and very helpful! So a genus ID can be tentatively assigned only to large teeth then. Nice finds!

 

And the rounded button teeth... these are all rear teeth of Brachychampsa.  

 

[Opabinia Blues]

1 minute ago, Jurassicbro238 said:

This is all super fascinating and very helpful! So a genus ID can be tentatively assigned only to large teeth then. Nice finds!

 

So for the 1st image I showed, I forgot to mention the scale bar was in cm. For that tooth, would you say it'd be safe to label that as cf. Thoracosaurus? Im also fine with Indet. Crocodilian though. It also came from the Lance.

It really depends on how loosey goosey you want to be with your personal labeling. Personally I would probably label it as crocodilian indeterminate but that’s only because I’ve been unable to actually take a good look at Thoracosaurus material. If what I have heard about Thoracosaurus is correct then I think that’s definitely a reasonably informed speculative guess at that tooth. I haven’t seen a tooth that large that looks like that out of these localities before, so if I had to bet on a given tooth being Thoracosaurus that particular one would probably be a good bet.

 

Just remember that assigning genus-level IDs to limited material like this will often carry some level of speculation and educated guesswork, but the standards for labeling a personal fossil collection are more lax than for an academic publication, at least for most people.

[Troodon]

41 minutes ago, Opabinia Blues said:

The tooth on the left I have labeled as c.f. Borealosuchus, 

The paper I quoted says you cannot distinguish  caniniform teeth between Borealosuchus and Brachychampsa

[Opabinia Blues]

19 minutes ago, Troodon said:

The paper I quoted says you cannot distinguish  caniniform teeth between Borealosuchus and Brachychampsa

I didn't think that particular tooth was an anterior tooth, but combing through imagery now I'm not 100% convinced you can even reliably place the difference (outside of the "crusher" posterior Brachychampsa teeth.