-24- 



Contraction-Rotation Contractions 



Figures 10F and 10G represent two arrangements resulting from inter- 

 action of contraction and rotation. In figure 10G, the ozarkodinids have 

 been moved anteriorly to the position of the platform elements (contraction) 

 and the combination has been rotated 90 degrees with respect to the hindeo- 

 dellid-synprioniodinid units. In figure 10F, the ozarkodinids have been 

 translated anteriorly and the resulting platform-ozarkodinid combination, as 

 well as the hindeodellid-synprioniodinid unit, has been rotated 90 degrees. 



Examination of the various patterns and their intergradations has 

 led us to make some speculations concerning the organization and musculature 

 of the animal that bore the natural assemblage Scottognathus . 



(1) The platform, ozarkodinid, and hindeodellid-synprioniodinid 

 units appear to have been separate biological units, individ- 

 ually bound by connective tissue such as muscle or cartilage. 



(2) The hindeodellid-synprioniodinid units consisted of six sub- 

 units, four lateral pairs of Hindeodella and two lateral 

 Synprioniodina individuals. The synprioniodinids were the 

 least tightly held and generally occur farthest from the mid- 

 line of symmetry. 



(3) Ozarkodinids, although clearly representing a separate unit, 

 tend to be associated with the platform unit whereas the 

 hindeodellid-synprioniodinid unit rarely is. 



Possible Causes of Post-Mortem Arrangement of Assemblages 



Movement by water provides the most obvious explanation for post- 

 mortem movement and rearrangement of natural assemblages. However, in the 

 Bailey Falls collection the fine-grained lithology of the black shale, the 

 absence of sedimentary structures indicative of current action, and the 

 common association of light and heavy, big and small elements tend to dis- 

 count any significant work by currents. The common occurrence of orderly 

 patterns, on the other hand, can in most cases be explained by muscle relax- 

 ation-contraction effects or by compression-decomposition effects that 

 translated elements after death from their life position down to the sur- 

 face of the sea floor on which the animal lies. 



Assemblages from Bailey Falls 



In addition to being the type locality for Scottognathus , Bailey 

 Falls is also the outcrop from which the holotype of Duboisella Rhodes, 1952 

 was obtained. Duboisella is rare here, however. Rhodes reported it as rep- 

 resenting only two of the approximately eighty assemblages collected from 

 the locality, and this rarity is consistent with our data as well. 



In view of present progress, the most significant natural assemblages 

 found at Bailey Falls are those specimens that contain more than one pair of 

 platforms per natural grouping. Forms with three or four platforms are in 

 our collections, and although not proved to be natural groupings, they have 



