Cretaceous microfossil sorting Field Museum of Natural History in Chicago October 30-November 11, 2024

[Joseph Fossil]

I've been continuing my awesome Internship work with Late Cretaceous microfossils at the Chicago Field Museum of Natural History (aka the Best Museum in Chicago). The micro fossils are 68-66 Million Years old and are from the Maastrichtian Cretaceous age Hell Creek Formation in what is now the U.S. States of North Dakota, South Dakota, and Montana.

 

Also want to give a shoutout to @ThePhysicist for his awesome diagram detailing denticle differences in Tyrannosaur tooth specimens from the Hell Creek Formation! It's really helped a a lot recently with the sorting efforts.

 

Here are some more interesting micro-fossils I've found recently these past few weeks.

 

Tyrannosaurus sp. tooth specimen 1 

 

 

 

 

Unknown Theropod tooth specimen 1

 

 

 

 

 

 

Myledaphus specimen 1

[Joseph Fossil]

Unknown Fish Specimen 1 (possible fish tooth plate)

 

 

Unknown Fish Specimen 2

 

 

Unknown Fish Specimen 3

 

 

Unknown Fish Specimen 4

 

 

Unknown Possible Mammal tooth Specimen 1

 

 

Assorted Micro fossils

[Joseph Fossil]

Tyrannosaurus sp. tooth specimen 2

 

 

 

 

Possible Gar Fish Jaw specimen

 

 

 

 

Unknown Lizard sp. Jaw Specimen

 

 

Myledaphus Specimen 2

 

[ThePhysicist]

 

1 hour ago, Joseph Fossil said:

Also want to give a shoutout to @ThePhysicist for his awesome diagram detailing denticle differences in Tyrannosaur tooth specimens from the Hell Creek Formation! It's really helped a a lot recently with the sorting efforts.

Thank you, but I believe this diagram isn't a complete picture - I've seen variation among position, location along the carinae, size/age; wear is also a factor. Perhaps I should make an update that also includes other theropods.

 

1 hour ago, Joseph Fossil said:

 

I'd actually say these more closely resemble juvenile dromaeosaurid anteriors:

 

1 hour ago, Joseph Fossil said:

 

 

This one I believe is Richardoestesia isosceles:

 

1 hour ago, Joseph Fossil said:

Unknown Fish Specimen 1 (possible fish tooth plate)

This is a fish scale.

[jpc]

I agree on the fish scale ID.  

 

Not sure about the gar jaw... gars have very small teeth, and pointed but not recurved, and there are also occasional much bigger teeth in the gar tooth row.   Have oyu looked into catfish or stingray spine for that one.  

[Joseph Fossil]

23 hours ago, ThePhysicist said:

 

Thank you, but I believe this diagram isn't a complete picture - I've seen variation among position, location along the carinae, size/age; wear is also a factor. Perhaps I should make an update that also includes other theropods.

 

I'd actually say these more closely resemble juvenile dromaeosaurid anteriors:

 

 

This one I believe is Richardoestesia isosceles:

 

This is a fish scale.

 

@ThePhysicist Thank you so much, I appreciate it! I did check with my colleagues at the Field Museum and they fully agreed with your assessment that the theropod teeth I categorized as "Tyrannosaurus sp. tooth specimen 1" and "Unknown Theropod tooth specimen 1" are indeed Dromaeosaurid teeth specimens of the genus Acheroraptor sp.. 

 

However, they did say that the specimen I categorized as "Tyrannosaurus sp. tooth specimen 2" is indeed a Tyrannosaurus sp. tooth specimen.

[ThePhysicist]

On 11/12/2024 at 1:10 PM, Joseph Fossil said:

are indeed Dromaeosaurid teeth specimens of the genus Acheroraptor sp.. 

I don't see how they can be positively attributed more specifically to Acheroraptor without the longitudinal ridges. They may well be Acheroraptor, but how I perform IDs, if it lacks diagnostic features included in the holotype, I cannot assign it. 

 

On 11/12/2024 at 1:10 PM, Joseph Fossil said:

However, they did say that the specimen I categorized as "Tyrannosaurus sp. tooth specimen 2" is indeed a Tyrannosaurus sp. tooth specimen.

I respectfully disagree with your colleagues. There is a distinct morphological difference in the denticles, and while you didn't provide a scale, I'll bet I can accurately guess how big it is.

 

Below I made a quick graphic with to-scale comparisons. Note the denticle height/width ratios and the deeper interdenticular sulci in the tyrannosaurids. Should your fragment indeed be R. isosceles, it will have a large denticle density compared to Tyrannosaurus (between 6-8 denticles / mm). I would guess the length of the carina on your fragment is roughly 4-6 mm (and not 10-18 mm).

 

[Joseph Fossil]

On 11/13/2024 at 9:43 PM, ThePhysicist said:

I don't see how they can be positively attributed more specifically to Acheroraptor without the longitudinal ridges. They may well be Acheroraptor, but how I perform IDs, if it lacks diagnostic features included in the holotype, I cannot assign it. 

 

I respectfully disagree with your colleagues. There is a distinct morphological difference in the denticles, and while you didn't provide a scale, I'll bet I can accurately guess how big it is.

 

Below I made a quick graphic with to-scale comparisons. Note the denticle height/width ratios and the deeper interdenticular sulci in the tyrannosaurids. Should your fragment indeed be R. isosceles, it will have a large denticle density compared to Tyrannosaurus (between 6-8 denticles / mm). I would guess the length of the carina on your fragment is roughly 4-6 mm (and not 10-18 mm).

 

 

@ThePhysicist I appreciate the input and I'll make sure to look over the specimens again to get a proper Theropod genera ID for them. Also yesterday I did some more micro fossil sorting and found some pretty intriguing Theropod teeth you might be interested in (some I have an idea of what genera they belong to, others I have almost no idea).

 

Unknown Theropod tooth Specimen 1 (I'm thinking it's either a Troodontid sp. or a young Tyrannosaurus sp. specimen)

 

 

 

 

 

 

Unknown Theropod tooth Specimen 2 (I have no idea what theropod genus this belongs to)

 

 

 

 

 

Unknown Theropod tooth Specimen 3 (I think it could be a Acheroraptor sp. specimen)

 

 

 

 

 

Unknown Theropod tooth Specimen 4 (Likely Richardoestesia/"Richardomorph" sp. specimen)

 

 

 

 

 

 

Unknown Theropod tooth Specimen 5 (Now this based on the denial shape and size I think is a Tyrannosaurus rex specimen)

 

 

 

 

 

 

Do you have any ideas on the Proper IDs for these tooth specimens?

[Joseph Fossil]

@ThePhysicist Do you have any ideas on the Proper IDs for these tooth specimens?

[ThePhysicist]

1. Neither Tyrannosaurus or troodontid. It's indeterminate. Tyrannosaurid teeth do not have basal constriction (troodontids do) and have denticles on both carinae, but the denticle morphology isn't right for troodontids. Similar morphs are discussed in the recent publication on putative avian teeth from the Late Cretaceous (Mohr et al. 2023).

2. Likely Richardoestesia isosceles. In the future, it's best for ID'ing purposes to see the full tooth. There are some that have longitudinal ridges that appear to be transitional between the R. isosceles and Paronychodon morphotypes, which makes sense, since they are (purportedly) from the same animal.

3. Indeed probably Acheroraptor, you can check the denticle density to confirm.

4. Indeed probably R. isosceles, you can check the denticle density to confirm.

5. Not a tyrannosaurid, it would have prominent denticles on both carinae. I would say indeterminate dromaeosaurid, not sure how you can get much from the denticles, there's only like one intact. It does appear to lack the basal constriction in troodontids.

 

Mohr SR, Acorn JH, Currie PJ (2023) Putative avian teeth from the Late Cretaceous of Alberta, Canada, are more likely from crocodilians. PLoS ONE 18(3): e0283581. https://doi.org/ 10.1371/journal.pone.0283581