Pliotoxasters have me puzzled. Species ID help please!

[JamieLynn]

I am TRYING to figure out my pliotoxasters. I think i have three different species and am hoping for some confirmation or information! I grouped them into similar seeming "batches" and think these are the ID's but would appreciate any help. All Texas Cretaceous, various formations, mostly Walnut I think.

 

These I believe are Pliotoxaster whitei

 

Side view: 

 

These I believer are Pliotoxaster comanchei

 

Side view: 

 

These I think are Pliotoxaster inflatus: 

 

side view: 

 

[erose]

Sorting out species usually starts out with stratigraphy. Pliotoxaster comanchei will be found in the Glen Rose Formation. P. whitei and P. inflatus are younger and more common in the Fredericksburg Group and up(?).  Beyond that you have to get into morphologic details.

[grandpa]

As Eric has said above:

1 hour ago, erose said:

Sorting out species usually starts out with stratigraphy. Pliotoxaster comanchei will be found in the Glen Rose Formation [of the Trinity group]

P. whitei occurs in th Fredricksburg and lower Washita groups.  P. inflatus has been found in Goodland & Kiamichi fms. of the upper Fredericksburg Group; in the Duck Creek fm. of the lower Washita Group; and in the Grayson fm. of the upper Washita.

 

Once I know what formation the fossil came from and have ID'ed it as Pliotoxaster, I use some visual, morphological clues to differentiate the Pliotoxaster species from each other.  These features are not clearly shown in the photos above for me to make a good differentiation. 

 

Let me give you my clues for distinction so that you may make your own differentiation rather than having to take a lot more photos.  Then if you still have some specimens that you are uncertain about, you can post photos of just those.  FYI, I get my visual clues from "Collector's Guide to Texas Cretaceous Echinoids" by Wm. W. Morgan, Schiffer Pub. Ltd., 2016 and (to a  lesser degree) "Fossil Echinoids of Texas" by Wm. R. Thompson, Jr. , Pub in Canada, 2016 & "Texas Cretaceous Echinoids" by Roesmarry E. and Thomas J. Akers, HGMS, 1987 (using previous genus names of Palhemiaster, Hemiaster & Mecaster comanchei; Hemiaster and Epiaster whitei; and Washitaster, Heteraster, & Enallaster inflatus.) {Darn those changing genus names.}

 

So, the main visual/morphological clues that I key off of to separate the various species (there are certainly others beyond these) are:

P. comanchei:  (1) from a side view - top surface of "compact test" is rather flat. Posterior end has "a moderately steep slope".  [I hate qualitative statements like "moderately steep"].

                           (2) ventral view - "the interambulacra, particularly the plastron are covered with prominent tubercles."  Plastron is slightly raised.  (This is my go-to distinguishing feature for this species,  i.e., the raised plastron has a lot of little bumps.)

 

P. inflatus:  (1) from a posterior view the profile of the specimen looks (what else can I say) inflated, i.e. more spherical in profile than the other two.  (If comparing the 3 species side by side, this is quite obvious and easy to ID - it looks swollen/inflated compared to the other two.  This is my go-to distinguishing feature for this species.)

 

P. whitei: (1)  from a side view - the posterior end of the side profile is higher than the anterior of the specimen and slopes from posterior to anterior.  (This is my go-to distinguishing feature for this species.)  The slope of the posterior wall is quite steep, steeper than comanchei. 

                 (2) anterior view, end-on - "a protofasciole" ("densely packed tiny tubercles with interspersed primary tubercles') can be distinguished by a "linear" separation of little bumps on the lower part of the anterior end and a relatively smooth surface above the line.

 

Hope this helps,

 

Tom

[JohnJ]

Good information above.  Morgan, Akers, Finsley, and individual species descriptions are the most reliable, error free resources on Texas echinoids.

[erose]

Great diagnostics from Grandpa. Recently we have been noticing another Pliotoxaster in the Keys Valley Member of the Walnut Formation, Fredericksburg Group. It seems different than P. comanchei although that species is listed as occuring above the Glen Rose. 
 

Sometimes I have to remind myself that identification to just genus is still significant. 

[Uncle Siphuncle]

I’ll also add that in dorsal view, to my eye, P. whitei looks a little more elongated in form than the others mentioned, perhaps narrowing posteriorly.  There can be some intraspecies variation, but most of the Goodland specimens I’ve encountered strike me this way.

[JamieLynn]

Thanks all! I have been using the Collectors Guide to do my best at ID'ing, but the Pliotoxasters are really difficult to distinguish morphologically. I will continue to look at them carefully and also try to figure out what formations I found them in. I know the geographical areas I found them so now I need to figure out geologic. I am better now at writing down what formation I am collecting in but some of these previous specimens I did not know at the time so am having to retro fit them....Again, thank y'all so much for the help! 

[erose]

15 hours ago, JamieLynn said:

Thanks all! I have been using the Collectors Guide to do my best at ID'ing, but the Pliotoxasters are really difficult to distinguish morphologically. I will continue to look at them carefully and also try to figure out what formations I found them in. I know the geographical areas I found them so now I need to figure out geologic. I am better now at writing down what formation I am collecting in but some of these previous specimens I did not know at the time so am having to retro fit them....Again, thank y'all so much for the help! 

Best thing is to get the location info as tight as possible. Especially if you are in an area where there are obviously multiple formations stacked up. My notes will often include the color and quality of the strata. Good example here in Central Texas would be describing the bluish gray marly layers of the Walnut versus the light tan buff nodular beds of the overlying Comanche Peak.

 

If you know the formation that's fine, but no matter what the location And the notes can always be used to truly nail it down.  Since I started keeping notes 30-odd years ago I can now just take a good photo that will have the location data imbedded or use an app to do the same.