Help me understand the River Bend Formation and the age of H.serra in North Carolina

[DevilDog]

I have been researching why my specimens of H.serra teeth from Topsail Island and Lee Creek look different. Apparently, H.serra from the early Oligocene are smaller, less robust, and have finer serrations compared to later, Miocene H.serra teeth. Is my conclusion accurate?

 

The H.serra from Topsail Island are supposedly from the River Bend Formation. There is conflicting information on the internet about the age of this formation. Some sources say "early" or "lower" Oligocene, some say "middle-late" Oligocene. Which is correct? If "early" is correct, when was it formed? Closer to 33 mya or more recent?

 

For H.serra found in North Carolina, what is the range in age? Oligocene-Miocene /33.9-5.3 mya? Is it possible to narrow down that range more accurately?

[grandpa]

4 minutes ago, DevilDog said:

Apparently, H.serra from the early Oligocene are smaller, less robust, and have finer serrations compared to later, Miocene H.serra teeth. Is my conclusion accurate?

A note of caution here.  Drawing conclusions from two separate sites of two separate ages is, well . . . inconclusive.  Rather than age of formation, other factors could be involved such as environmental conditions of the two sites.  If, however, you can show the same differences in species by geologic age across multiple, geographically 'distant' sites, you have cause to argue for evolutionary changes over time in the given species.  Make sense?  

 

That said, you pose an interesting hypothesis.  I'd like to hear more chime in on evidence they may have found.

[caldigger]

@MarcoSr

[DPS Ammonite]

32 minutes ago, DevilDog said:

I have been researching why my specimens of H.serra teeth from Topsail Island and Lee Creek look different. Apparently, H.serra from the early Oligocene are smaller, less robust, and have finer serrations compared to later, Miocene H.serra teeth. Is my conclusion accurate?

 

The H.serra from Topsail Island are supposedly from the River Bend Formation. There is conflicting information on the internet about the age of this formation. Some sources say "early" or "lower" Oligocene, some say "middle-late" Oligocene. Which is correct? If "early" is correct, when was it formed? Closer to 33 mya or more recent?

Both might be correct. See Geolex for papers on this formation. Later papers suggest that based on microfossils the formation spans the entire Oligocene. The age probably varies based on the exact location and stratigraphic layer you are in.

 

For those of you not familiar with Geolex, Geolex gives summaries of cited USGS references about geological groups, formations and members. It includes ages, and current and former names of most stratigraphic units. Just search for the unit name along with Geolex.

 

https://ngmdb.usgs.gov/Geolex/UnitRefs/RiverBendRefs_3575.html

[Al Dente]

24 minutes ago, DevilDog said:

H.serra from Topsail Island are supposedly from the River Bend Formation. There is conflicting information on the internet about the age of this formation. Some sources say "early" or "lower" Oligocene, some say "middle-late" Oligocene. Which is correct? If "early" is correct, when was it formed? Closer to 33 mya or more recent?

The River Bend Formation probably has two transgressive cycles so it can be two different ages. The Hemipristis from the River Bend were identified as Hemipristis wyattdurhami by Gerard Case. We would call these Hemipristis curvata today. It is possible that they are transitional between curvata and serra since curvata is usually thought of as an Eocene species.

[DevilDog]

24 minutes ago, Al Dente said:

The River Bend Formation probably has two transgressive cycles so it can be two different ages. The Hemipristis from the River Bend were identified as Hemipristis wyattdurhami by Gerard Case. We would call these Hemipristis curvata today. It is possible that they are transitional between curvata and serra since curvata is usually thought of as an Eocene species.

 

How should I properly catalog/describe the Hemipristis teeth I have from Topsail island then? H.curvata, H.serra, H.curvata-transitional?

 

Does it make a difference one way or another if they were found during/after the beach re-nourishment a few years ago? Did the dredging hit a certain area of the formation that could narrow down it's date?

[Al Dente]

When I label fossils that I’m not sure of, I will write the ID I feel most comfortable with and then put a question mark after the name.

[DevilDog]

8 minutes ago, Al Dente said:

When I label fossils that I’m not sure of, I will write the ID I feel most comfortable with and then put a question mark after the name.

I have nowhere near your experience in this area. I am not comfortable with either ID at this point.

 

I am sure you have Hemipristis from Topsail Island. How are yours labeled?

[MarcoSr]

The following excerpts are from "Fossil Fishes", Volume III of IV, North Carolina Fossil Club, 2015:

 

 

 

 

 

 

 

 

Marco Sr.

 

[DevilDog]

Thank you Marco Sr.

 

Very helpful !