The Bois d'arc Formation of Southern Oklahoma PArt II

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Coelospira virginia

Continuing the analysis of the brachiopod fauna of the Cravatt Member of the Bois d'arc Formation (Lower Devoninan - Helderbergian) of Southern Oklahoma to establish a strong corellation between the different regions of the Epeiric Sea, Coelospira virginia represents a species even smaller than Atrypina hami (discussed previously on page 1) but equally as valuable.

With many specimens measuring only 4 to 5 mm in diameter/height this is one of the smallest members of the Cravatt fauna. It has been found in the Birdsong Shale of Tennessee, but is replaced by C. concava (Hall 1859) in the Helderbergian of New York. The two species differ in that the New York specimens are larger, more finely ornamented and thinner (Amsden and Boucot, 1958). An reduction in the number of costae on the valves while those that remain are thickened was an adaptation also seen in Atrypina hami and is believed to represent a strategy to strengthen the shell in a higher energy environment, such as that see in the Cravatt and on the Ross Shelf. 

[Many collectors never come across specimens of Coelospira because they can be easily overlooked in the field as the eye is drawn to the large Atrypids and corals at many localities. This is why it is essential to always collect a bag or two of matrix at any site so it can be searched later using bright lights and magnification.]

This is a common species in both the Cravatt and the Birdsong Shale but is usually only found when matrix is searched under magnification. How common the species is in New York is unknown.

References:

Amsden, T.W. 1958. Stratigraphy and Palaeontology of the Hunton Group in the Arbuckle Mountain Region - Part V - Bois d'Arc Brachiopods. Oklahoma Geological Survey Bulletin 82.

Hall, J. 1859. Containing Descriptions and Figures of the Organic Remains of the Lower Helderberg Group and the Oriskany Sandstone. Palaeontology of New York, Vol. 3. 582p.

This tiny inarticulate brachiopod rarely measures more than 4 mm in length, with most specimens that are recovered being only 2 to 3 mm long. All of the specimens found by this author were picked from matrix samples using a microscope. Collection of this species in the field in impossible unless it is found attached to a larger fossil such as a crinoid stem.  So small is the species that Amsden and Boucot (1958) failed to catch it in their overview of the Cravatt fauna.

The species was originally described from the Silurian of New York (Hall, 1852), but was also reported by Hall (1859) from the Lower Devonian. Berdan (1972) confirmed the presence of this species in the Silurian and presented photographs of the specimens which appear identical to those recovered in the Cravatt. The survival of this species into the Lower Devonian makes it a rare holdover from the earlier faunas of the region. [This author has found specimens assignable to this species in the Silurian age Henryhouse Formation of Oklahoma and Brownsport Formation of Tennessee, plus the Lower Devonian Haragan and Bois d'Arc of Oklahoma and the Rockhouse Formation and Birdsong Shale of Tennessee.] Having a wider temporal range than most other species in the Epeiric Sea, when considered in isolation C. ovata is less useful in biostratigraphy than other species, but when taken in context with the rest of the fauna is indicative of the Lower Devonian (rather than middle Devonian or late Silurian). 

Regardless of the localities at which they have been found, almost all specimens of this species exhibit the same characteristics. As the specific name suggests the shells are oval in shape, but very thin in profile with the highest point being a small peak a short distance from the rear of the shell. Strong growth lines/lamellae are visible on many specimens. Most specimens are disarticulated valves, but on rare occasions articulated examples have been found (right). 

Craniops ovata is a common species in both the Cravatt (and based on this authors studies the Haragan as a whole) and Birdsong Shale.

Reference:

Berdan, J. 1972.  Brachiopoda and Ostracoda of the Cobleskill Limestone (Upper Silurian) of Central New York.  USGS Professional Publication 730.

The list that follows is based on the faunas from the Cravatt  and Birdsong Shale. The New York fauna is far more diverse and many species are endemic, and to include them all would be pointless. Instead, the smaller Oklahoma and Tennessee faunas are used as a data set to establish relationships between the three realms of the Epeiric Sea as many of these species are more widely distributed and therefore useful in correlation.

Species occurring in the Cravatt  are as follows. 

(The presence of the species, or a related form, in either Tennessee (TN) or New York (NY) is indicated in parentheses that follow the Cravatt form. If the species or related does not occur then no entry is made).

Orthostrophia strophomenoides parva (NY & TN = Orthostrophia strophomenoides)

Skenidium insigne (NY & TN)

Levenea subcarinata pumilis  (TN) (NY= Levenea subcarinata) 

Dicoelosia varica (NY & TN)

Discomyorthis oblata (NY & TN)

Isorthis pygmaea (TN) (NY = Isorthis perelegans)

Anastrophia grossa  (NY & TN = A. verneuili)

Gypidula multicostata (TN)

Leptaeana acuticuspidata (TN) (NY = L. rhomboidalis)

Leptaensica concava (TN & NY)

Stophonella bransoni (NY = S. punctulifera) (TN = S. punctulifera holladayi)

Stropheodonta (Brachyprion) gibbera

Stropheodonta (Brachyprion) arata (NY) (TN = S. purdei?)

Lissostrophia lindenensis (TN)

Leptostrophia beckii tennesseensis (TN) (NY = Leptostrophia beckii)

Schuchertella haraganensis (NY & TN = S. woolworthana)

Schellwienella mardicula

Plectodonta petila

Chonetes sp.?

Chonostrophia helderbergiae* (NY)

Anopliopsis pygmaea* (Occurs in the Helderberg of Maryland, NY unconfirmed)

Ancilloteochia haraganensis formerly Camarotoechia haraganensis (TN & NY = A. bialveata)

Camaraotechia transversa? (TN & NY)

Sphaerirhynchia lindenensis (TN) (NY = nucleolata) 

Sphaerirhynchia glomerosa (NY = S. nucleolata)

Obturamentella wadei (TN)

Trigonorhynchia acutirostella (TN)

Eatonia exserta (TN = E. fissicosta) (NY = E. medialis)

Atrypa oklahomensis (TN)

Atrypina hami (TN) (NY = A. imcricata)

Coelospira virginia (TN) (NY = C. concava)

Spinoplasia gaspensis?* 

Kozlowskiellina velvata (TN & NY = K. perlamellosa)

Howellella cycloptera (TN & NY)

Meristella atoka (TN) (NY = M. arcuata)

Nucleospira ventricosa (TN & NY)

Cyrtina dalmani nana (TN) (NY = C. dalmani)

Rhynchospirina maxwelli (TN & NY = R. formosa) 

Trematospira cf. hippolyte (NY = T. costata) (TN = T. costata angusta)

Trematospira sp.

Rensselaerina haragana (TN = R. medioplicata)

Craniops ovata (TN & NY)

* Not in Amsden and Boucot 1958, but listed in Amsden and Ventress (1963).

This list shows that there are strong similarities between all the faunas of the Lower Devonian Epeiric Sea, and that correlation of the three realms suggests the three faunas existed within only a couple of million years of each other. 

Further inspection of the list also shows that although the Cravatt represented a western fauna that had some endemic species it still shared part of its community with the Epeiric Sea as a whole. The New Scotland of New York was in exactly the same situation but represented the slightly different fauna of the eastern arm of the Epeiric Sea. The Birdsong Shale was the central realm and is slightly different in the it had a fauna that was heavily influenced by both the Oklahoma and New York faunas, containing a mix of species from both realms, as well as some endemic species/subspecies. 

Putting the Cravatt fauna in context is an important step in the study of the deposits and it shows that the species found in Oklahoma did not evolve in isolation, but had a considerable geographic range and were part of a large marine community that stretched across almost one half of what is now the United States. 

Reference: 

Amsden, T.A. and Ventress, W.P.S. 1963. Early Devonian Brachiopods Of Oklahoma. Oklahoma Geological Survey Bullet 94.