PHYLOGENY OF SUPERFAMILY STENOSCISMATACEA 



The superfamily Stenoscismatacea ranges from the earliest Middle 

 Devonian through the Permian (fig. 3). The progenitor of the group 

 is not known, but evidence in the earliest genus, Atribonium, points to 

 a rhynchonelloid ancestry. The ancestor probably was a small septate 

 rhynchonelloid with well-developed dental plates converging toward 

 the floor of the pedicle valve, such as the camarotoechiid genus Hir- 

 cinisca illustrated by Havlicek ( 1961, p. 66, pis. 4, 7) . 



No descendants of the Stenoscismatacea are known ; apparently the 

 group became extinct at the end of the Permian. By Cretaceous time 

 other rhynchonelloids were mimicking some of the external characters 

 of Stenoscisma and Torynechus (see pi. 7, figs. 1, la), but the defining 

 internal features of the superfamily appear not to have been repeated. 



FAMILY ATRIBONIIDAE 



The earliest genus is Atribonium n. gen. which is characterized by 

 its relatively flat and short camarophorium, blunt anterior shell margin, 

 little or no overlap of posterolateral valve margins, and delthyrium 

 with deltidial plates and consistently open pedicle foramen. This genus 

 is structurally capable of having given rise to the early stenoscismatid 

 genus Coledium n. gen., and to all the later atriboniids. The family 

 Atriboniidae extends into the Permian of Timor, the Salt Range of 

 Pakistan, far eastern Siberia, and the northern Caucasus. 



Subfamily Atriboniinae. — The Mississippian genus Sedenticellula 

 Cooper probably derived from Atribonium by addition of costae, com- 

 pression of the shell, thus lowering the angle of meeting of the anterior 

 edges of the valves (as in juveniles of Atribonium) , and continuation 

 of the nepionic sessile spondylium of Atribonium into a later onto- 

 genetic stage. Internal features of the brachial valve maintained their 

 Atribonium-like aspect, with an undivided hinge plate and a well- 

 developed intercamarophorial plate. 



The smooth genus Camerisma n. gen., with its narrowly arched fold, 

 resembles Atribonium in the internal structures of both valves. The 

 structures are stronger, with more reinforcement by secondary shell 

 material, but the spondylium is elevated and the intercamarophorial 

 plate robust. 



Subfamily Psilocamarinae. — Mississippian representatives of this 

 subfamily are unknown ; the earliest genus is Psilocamara Cooper, a 

 smooth Pennsylvanian shell with a high, arched fold that probably 

 derived from the similarly smooth and arched Camerisma of the Atri- 



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