Pterotrigonia thoracica vs Trigonia eufaulensis

[Fullux]

Not exactly sure whether this is new information, has already been observed, or if this is just me misinterpreting fossils, but I recently noticed that there are two species of the Coon Creek Formation that look very similar. That being Pterotrigonia thoracica and Trigonia eufaulensis. I noticed while observing a gallery made by @Herb that each species looks very similar, and are almost indistinguishable. Upon observing other examples of each species, I came up with this conclusion: P. thoracica typically has 14-15 ribs on its shell, while T. eufualensis has 16-17. Let me know what you all think of this!

 

The P. trigonia is a specimen that I found in 2018, while the T. eufualensis is from @Herb's gallery.

Edited March 3, 2023 by Fullux

[Fullux]

Just noticed that the other two ribs had been worn down on most of my P. thoracicas...

[historianmichael]

Gabbigonia eufalensis tends to be much smaller than Tennessiella thoracica

Edited March 4, 2023 by historianmichael

[Fullux]

13 minutes ago, historianmichael said:

Pterotrigonia eufalensis tends to be much smaller than Tennessiella thoracica

Is Tennessiella the same as P. thoracica?

[historianmichael]

46 minutes ago, Fullux said:

Is Tennessiella the same as P. thoracica?

Yes.

 

 

Cooper2015Pterotrigoniidae.pdf

[ClearLake]

On 3/3/2023 at 3:46 PM, Fullux said:

I noticed while observing a gallery made by @Herb that each species looks very similar, and are almost indistinguishable. Upon observing other examples of each species, I came up with this conclusion: P. thoracica typically has 14-15 ribs on its shell, while T. eufualensis has 16-17. Let me know what you all think of this!

That is a good observation by you and the number of ribs is indeed a feature that has long been used (along with many others, as is the standard) to differentiate the species.  There is a good old (1926) publication about the Coon Creek fossils that can be found here:

https://pubs.usgs.gov/pp/0137/report.pdf

 

There may be other, more recent comprehensive publications, I have never studied this formation, so I'm not familiar with everything out there.

 

Under Trigonia thoracica on page 60 there is this description:

Gabb in 1860 described· this species as follows:· Inequilateral, truncated posteriorly, alated; obliquely ribbed, the most anterior ribs almost parallel with lunule; ribs about 17, compressed laterally, nodulose except near the origin, where they are· entire and sharp; edge deeply serrate, each process corresponding to one of the ribs; anterior end slightly truncated obliquely; lunule marked by a series of finely nodulated ribs, which point posteriorly; surface between the ribs marked by delicate lines of growth. Locality and position

 

But when Morton first described the species in 1834 he wrote of it:

Specific character: Obovate, with oblique very prominent ribs, about 12 in number.

 

In the report, there is further discussion of the ribs and other features which distinguish the various species.  One area of concern, which you can probably see in your illustration above, is what counts as a rib and what doesn't.  This is why most species are described with multiple features, not just one.  Sometimes there is a lot of variability in ornamentation within a species.

 

Under Trigonia eufalensis, this was included in the description:

Gabb describes this species as follows: Subtriangular; resembles T. alaeformis 'Sowerby in outline not quite so elongate anteriorly; beaks posterior; lunule distinct; surface marked by about 14 ribs, the more anterior of which proceed from the lunule anteriorly and then cross the shell at right angles .with the lunule, exhibiting a tendency to being nodose, especially near the lunule;

 

As you can see, and @historianmichael already pointed out, genus names have changed more than a few times over the years.  I know it can be frustrating, but delving into the literature is often the only way to be certain of an identification (and even then it is sometimes tricky) if that certainty matters to you (many folks are happy to be "close enough", and that is fine for ones personal collection).  Websites are very useful, and I use them all the time, but you always have to keep in mind that sometimes they could have items mis-identified or be unaware of recent changes, etc, etc.