,.r 



If^ 



parasphenoid is relatively as narrow as in Kentrocapros 

 and Strophiurichthys in the anterior half of its length 

 anterior to the prootic shelves, but it is much expanded 

 laterally just in front of the shelves, forming a partial 

 hard palate over the rear of the oral cavity. In 

 Caprichthys and Capropygia the ventral edge of the 

 parasphenoid is relatively as narrow as in Kentrocapros 

 and Strophiurichthys throughout most of its length 

 {interior to the prootic shelves, but it becomes laterally 

 expanded anteriorly behind the vomer, forming a partial 

 hard palate over the front of the oral cavity. The possible 

 phylogenetic implications of these differences and of the 

 others mentioned above stfe discussed in the section on 

 generic relationships that follows. 



Generic relationships. — Probably having been deriv- 

 ed from the same line of fishes that connects the tria- 

 canthids and balistids, the primitive aracanids also 

 probably had a laterally compressed body whose cross- 

 sectional outline would be a dorsoventrally elongate oval. 

 The body was probably fully covered with enlarged, per- 

 haps slightly overlapping, scale plates. While the 

 balistids retained a more flexible body and flexible scale 

 covering, the aracanids specialized in a thicker and less 



Figure H5.—Proaracana dubia: lateral view 



of entire specimen, composite based on three 



specimens, including the holotype, 31.4-54.5 mm SL, 



all specimens from the Eocene of Monte 



Bolca, Italy (Tyler 1973a:fig. 7). 



flexible scale covering with a concomitant reduction in 

 swimming ability but greatly increased protection from 

 the exoskeleton. The relatively complete covering of 

 somewhat flexible scales would have become con- 

 solidated into a relatively inflexible carapace of hex- 

 agonal plates with marginal interdigitations, except that 

 many of the scale plates on the check remained small 

 and free from one another to allow for respiratory move- 

 ments and that the scales around and behind the dorsal 

 and anal fins remained similarly small and free from one 

 another to allow for lateral flexion of the caudal peduncle 

 when the caudal fin was used in rapid swimming. The 

 tendency would be for the scales that originally entirely 

 covered the caudal peduncle to become reduced in size 

 and in coverage of the peduncle, within the limits of also 

 providing some protection from the bites of predators. 

 The Eocene Proaracana dubia provides a firm example 

 of an early aracanid, the carapace over most of the body 

 being fully consolidated and the caudal peduncle bearing 



204 



