GttfWihB M. utib iHanj Baiix Walrott Stwrarrlf 3Ftmb 



THE BRACHIOPOD SUPERFAMILY 

 STENOSCISMATACEA 1 



By Richard E. Grant 

 Geologist, United States Geological Survey 



INTRODUCTION 



The Stenoscismatacea constitute a long-ranging and remarkably 

 conservative group of Paleozoic rhynchonelloid brachiopods. Internal 

 structures of the earliest known species in the Middle Devonian differ 

 only slightly from those of species that lived near the end of the phyletic 

 line in the Late Permian. External features underwent moderate, but 

 significant, changes during the history of the group ; they provide the 

 basis for its differentiation into 2 families, 4 subfamilies, 11 genera, 

 and more than 200 species. 



The superfamily is unified by the presence of a spondylium duplex 

 formed by fusion of the dental plates in the pedicle valve and by a high, 

 spoon-shaped camarophorium in the brachial valve. These two struc- 

 tures differ in detail within the superfamily, but their fundamental form 

 and construction remain nearly unchanged throughout the group. 



Families within the Stenoscismatacea are distinguished by presence 

 or absence of a peculiar external marginal fringe that extends from 

 parts of one or both valves. This fringe, here called the stolidium, is 

 absent from genera of the family Atriboniidae n. fam. The stolidium is 

 heralded in earliest genera of that family by slight protrusion of the 

 valve edges in some species, but is not unequivocally present in species 

 earlier than Mississippian members of the family Stenoscismatidae 

 Oehlert (1887). Species in the Stenoscismatidae that occur near the 

 phyletic bifurcation of the two families have the stolidium sporadically 

 present and erratically developed. It becomes an increasingly consistent 

 feature of adult shells through the development of the Stenoscismatinae, 

 and no species of Stenoscisma, the culminating Permian genus, lacks a 

 stolidium. 



Most genera of the Atriboniidae have no trace of a stolidium, not 

 even the slight protuberance of the valve edges seen in some species of 



1 Publication authorized by the Director, United States Geological Survey. 

 Smithsonian Miscellaneous Collections, Vol. 148, No. 2 



