Attention Texas experts! What is growing on my ammonite?

[jpenn]

Howdy!

 

Not long ago I acquired this nice 11-inch ammonite from Texas. Per seller this was collected at a quarry in Crawford- a fast glance at maps show a variety of formations in the area including Edwards group formations. From the best of my knowledge this ammo is a species of Oxytropidoceras genus and a nice-sized one.

 

Anyway... the question of this thread: Within the inner whorl is a collection of shelly material. When I purchased the piece I assumed it all to be indistinct shelly debris but in hand it appears more distinct in form and perhaps identifiable. I have my own guess to what it is, but I thought I'd have the many Texas-based collectors here at TFF have a look at it first, if it's not too rude to ask. Can the inner material be identified? Thanks in  advance!

[Innocentx]

Not from Texas but they look like oysters.

[FossilDAWG]

They resemble the oyster genus Pseudoperna.   Pseudoperna congesta is common in the Turonian and younger Late Cretaceous formations in the western interior seaway, but I don't know about Texas or about the Albian (the age for Oxytropidoceras).

 

Don

[Troodon]

They just needed a place to call home and work on some pearls  

[Aurelius]

Oysters commonly colonised vacant ammonite shells. I find them frequently.

[DPS Ammonite]

40 minutes ago, FossilDAWG said:

They resemble the oyster genus Pseudoperna.   Pseudoperna congesta is common in the Turonian and younger Late Cretaceous formations in the western interior seaway, but I don't know about Texas or about the Albian (the age for Oxytropidoceras).

 

Don

Don is correct and they are common in the Cretaceous rocks in Texas. See my post showing Pseudoperna congesta oysters from N. Texas: http://www.thefossilforum.com/index.php?/topic/58968-texas-north-sulphur-river-lagerstätte/

 

[Macrophyseter]

I agree with oyster shells. Bivalves like oysters will commonly make their homes on top of rocks and similar surfaces. In many cases, to-be-fossils like that ammonite shell will serve as a substitute. 

[jpenn]

Okay, so oysters similar to the somewhat later Cretaceous Pseudoperna are a possibility to consider.

 

I present for additional consideration another encrusting bivalve from the same area and time as this Oxytropidoceras- could this be caprinid rudists? There are structures that may suggest divided body chambers. Image included of Edwards group caprinids in cross section, and image of a couple of the encrusting structures on my ammonite to compare.

 

Of course, if it is impossible for rudists to have colonized the surface of an ammonite, or if rudist anatomy is completely different at this surface attachment point, I'd like to know. Hoping to hear from those familiar with Albian Texas. Thoughts?

 

[DPS Ammonite]

I do think that the small bivalves are Pseudoperna because their shells are thin and they occur in colonies. Note the pictures of the caprinids have thick shells with a cellular/ woody texture.

 

The real questions:

 

Are these from the Edwards Fm?

 

What species is the ammonite? Most Oxytropidoceras that I have seen have thicker ribs and a more pronounce keel.

 

Does the time range of the probable ammonite species coincide with the range of Pseudoperna in Texas. Do both species exist in the Edwards Formation. I have seen Pseudoperna congesta  on ammonites in the younger  Duck Creek and Fort Worth Formations in Denton Creek west of Fort Worth.

 

We we need the help of more experience Texas collectors. Help: @Uncle Siphuncle, @JohnJ and for good measure @BobWill

[abyssunder]

I agree with the others.

Here's what I think:

 

Rudist were reef building bivalves, so I don't think they preferred unstable substrates like cephalopods, oysters, gastropods - as oysters often prefer, excluding the case when they buildup on an existing sediment (that might be from a different geological stage or stratum) which contains cephalopods (for example).

 

" Rudist bivalves have undergone multiple Early Cretaceous episodes of extinction and the group rapidly diversified throughout the Early Cretaceous making them an important tool for biostratigraphic correlation (SCOTT and FILKORN, 2007; SKELTON, 2013; STEUBER et al., 2016).
Although rudists do not occur in the same beds as ammonites, they are in facies superposed with ammonites. The lower parts of the Fredericksburg Group, the Walnut Formation yields numerous middle Albian ammonite species (Fig. 15) (YOUNG, 1966). The upper part of the Edwards Formation spans the middle-upper Albian zones, and hosts common caprinid rudists among other rudist groups. In shelf carbonates of the West Texas Pecos River region rudist genera overlie upper Albian ammonite facies
(SCOTT and KIDSON, 1977; SCOTT and KERANS, 2004). " - R. W. Scott et al, 2016

 

The specimens of the comparative picture are all cross sections of rudist valves, while your specimens, in the posted picture, are not cross sections, so they cant be compared with that.

 I would rule out rudists, in this case.

 

The question is if Pseudoperna congesta were present in the Albian (it usually encrust inoceramid valves from later geological stages) or not, or the specimen has other encrusting bivalves similar to S. congesta, or the sample is from a different gelogical stage.

[Uncle Siphuncle]

My first guess would be Goodland formation, an Edwards equivalent in North Texas.

[jpenn]

Thank you @DPS Ammonite and @abyssunder, these are the sort of informative answers I needed to eliminate rudists as a possibility. When trying to identify a fossil I like to consider all species known to be from that time (Albian) and place (Crawford, TX.) Edwards Group formations are in the immediate area of that town but unless I was in the quarry this was taken from (I was not) I could not know for sure which strata this ammonite comes from.

 

Regarding the ammonite's identification itself, Oxytropidoceras was my call based on its size, location and form. I will of course easily and quickly defer to experts here if there is a better species. There is a keel present although it appears to be heavily worn. Photo included of the most evident portion of the keel and also the other side of the ammonite to assist with identification.

 

 

[jpenn]

2 minutes ago, Uncle Siphuncle said:

My first guess would be Goodland formation, an Edwards equivalent in North Texas.

Per the original finder of the ammonite this came from Crawford which as I understand it is too far south for Goodland.

[BobWill]

3 hours ago, jpenn said:

Regarding the ammonite's identification itself, Oxytropidoceras was my call based on its size, location and form. I will of course easily and quickly defer to experts here if there is a better species. There is a keel present although it appears to be heavily worn. Photo included of the most evident portion of the keel and also the other side of the ammonite to assist with identification.

 

The keel DPS ammonite is referring to is very broad and thin and therefore often missing. With the wear showing on the ribs of yours I wouldn't expect to see any of it remaining. Oxytropidoceras is the genus and there are at least 22 species in Texas so it can be hard to narrow it down completely without a better location and better preservation. Of those 22 species O. buarquianum, O. carbonarium, O. moorei and O. powelli have been found in various parts of McLennan County. To add to the mystery there are exposures near Crawford where Denton Clay, Ft. Worth Limestone and Duck Creek Formations appear undivided and each of these can have different species of Oxys.  I feel sure you can confidently label it to genus and county but that may be the best you can hope for without a location for the quarry, maybe even with that information.

 

[FossilDAWG]

13 hours ago, abyssunder said:

Rudist were reef building bivalves, so I don't think they preferred unstable substrates like cephalopods, oysters, gastropods - as oysters often prefer, excluding the case when they buildup on an existing sediment (that might be from a different geological stage or stratum) which contains cephalopods (for example).

One thing to note is that these fossils often have a complex taphonomic history.  I have Oxytropidoceras specimens from the Killeen area (Walnut Formation) that are somewhat deformed (flattened) internal molds completely lacking shell, with oysters and other epifauna directly attached to the mold.  The sequence of taphonomic events must have been:

1. Ammonite is buried and becomes filled with fine calcarious sediment (both living chamber and phragmocone).

2. Shell dissolves leaving the internal mold.  The mold is stiff clay, able to hold its shape, but not yet completely lithified (turned to rock).

3. As the mold is buried deeper and deeper in the sediment, pressure causes it to flatten a bit.  This is quite evident if you look at the groove formed where earlier whorls fit into later whorls.

4. The overlying sediment is eroded away, leaving the mold exposed on the sea floor.

5. The mold is colonized by oysters and other organisms seeking a firm substrate.

6. The mold and associated epifauna is once again buried and the epifauna becomes fossilized and the mold completely lithified.

7. The fossil is exposed again by erosion, and this time it is collected.

 

Don

[erose]

3 hours ago, FossilDAWG said:

One thing to note is that these fossils often have a complex taphonomic history.  I have Oxytropidoceras specimens from the Killeen area (Walnut Formation) that are somewhat deformed (flattened) internal molds completely lacking shell, with oysters and other epifauna directly attached to the mold.  The sequence of taphonomic events must have been:

1. Ammonite is buried and becomes filled with fine calcarious sediment (both living chamber and phragmocone).

2. Shell dissolves leaving the internal mold.  The mold is stiff clay, able to hold its shape, but not yet completely lithified (turned to rock).

3. As the mold is buried deeper and deeper in the sediment, pressure causes it to flatten a bit.  This is quite evident if you look at the groove formed where earlier whorls fit into later whorls.

4. The overlying sediment is eroded away, leaving the mold exposed on the sea floor.

5. The mold is colonized by oysters and other organisms seeking a firm substrate.

6. The mold and associated epifauna is once again buried and the epifauna becomes fossilized and the mold completely lithified.

7. The fossil is exposed again by erosion, and this time it is collected.

 

Don

I have been fascinated by this for many years. I first realized I was seeing epifauna on steinkerns early in my Texas collecting. Big gastropods and bivalves with serpulid worm tubes and oyster spat. Fossils on "fossils."

 

As far as oyster species is concerned...many types of oysters colonized hard surfaces like that. You need to look closely at hinge type and muscle scars to begin the ID process.  I know Pseudoperna mostly from the Upper Cretaceous Austin Chalk. They are distinct in form and these don't look like them.  I've seen Ceratostreon weatherfordensis packed together like that but not sure if the age jives.

[DPS Ammonite]

4 hours ago, FossilDAWG said:

One thing to note is that these fossils often have a complex taphonomic history.  I have Oxytropidoceras specimens from the Killeen area (Walnut Formation) that are somewhat deformed (flattened) internal molds completely lacking shell, with oysters and other epifauna directly attached to the mold.  The sequence of taphonomic events must have been:

1. Ammonite is buried and becomes filled with fine calcarious sediment (both living chamber and phragmocone).

2. Shell dissolves leaving the internal mold.  The mold is stiff clay, able to hold its shape, but not yet completely lithified (turned to rock).

3. As the mold is buried deeper and deeper in the sediment, pressure causes it to flatten a bit.  This is quite evident if you look at the groove formed where earlier whorls fit into later whorls.

4. The overlying sediment is eroded away, leaving the mold exposed on the sea floor.

5. The mold is colonized by oysters and other organisms seeking a firm substrate.

6. The mold and associated epifauna is once again buried and the epifauna becomes fossilized and the mold completely lithified.

7. The fossil is exposed again by erosion, and this time it is collected.

 

Don

I have another possible scenerio that is simpler and follows Occam's Razor principle. I think that the epibonts grew on a shell that dissolved away. While buried, the space where the shell was, collapsed and was cemented together. I note that many of my north Texas ammonites where the epibonts attached to the steinkerns also have remnants of original or replaced shell.

 

1 hour ago, erose said:

I have been fascinated by this for many years. I first realized I was seeing epifauna on steinkerns early in my Texas collecting. Big gastropods and bivalves with serpulid worm tubes and oyster spat. Fossils on "fossils."

 

As far as oyster species is concerned...many types of oysters colonized hard surfaces like that. You need to look closely at hinge type and muscle scars to begin the ID process.  I know Pseudoperna mostly from the Upper Cretaceous Austin Chalk. They are distinct in form and these don't look like them.  I've seen Ceratostreon weatherfordensis packed together like that but not sure if the age jives.

I agree with erose that there are other small oysters that formed colonies on large shells. I do not see the pointy ends that Pseudoperna have. See my pictures of them from NSR: 

 

 

[abyssunder]

Just want to say, if the ammonite is an Albian Oxytropidoceras sp., the encrusting oysters probably are not Pseudoperna (P. congesta), the geological stage might be too old for them, but if it's from younger strata, then the oysters might be Pseudoperna.