dorsal surface of the skull, basically the same condition 

 as found in the hollardiins and in contrast to that of the 

 triacanthodins. The acanthurid epiotic is separated for a 

 short distance from the frontal by a parietal, a bone lost 

 in all plectognaths, but the epiotic in acanthurids ap- 

 proaches the frontal closely enough to be more similar to 

 the condition in hollardiins than to that in triacan- 

 thodins. The indication from the acanthurids is that the 

 domelike supraoccipital excluding the epiotics from the 

 midline of the dorsal surface of the skull in hollardiins is 

 the more generalized of the two and ancestral to the 

 flattened condition of the supraoccipital found in 

 triacanthodins. 



The phylogenetic implications of the shaftlike pelvis of 

 hollardiins versus the basinlike pelvis of triacanthodins 

 are unknown, for these differentially shaped portions of 

 the pelvis are long extensions behind the level of the 

 pelvic fins, and most fishes, including acanthurids, have 

 no such structure with which that of triacanthodids can 

 be compared. 



While the triacanthodins are more speciose and 

 anatomically diversified than the other Recent subfami- 

 ly, from which they were derived, they apparently are not 

 ancestral to any other group of plectognaths, for it is the 

 hollardiins that are ancestral to the triacanthids, and 

 that same line probably is ancestral to the balistoids and 

 ostracioids as well, while the gymnodonts are derived 

 from the eoplectins, as discussed below. The hoUardiin 

 ancestry of the triacanthids is attested to by the similari- 

 ty in the domelike shape of the supraoccipital, which ex- 

 cludes the epiotics from meeting on the dorsal surface of 

 the skull, and in the shaftlike nature of the pelvis pos- 

 terior to the pelvic spines found in this subfamily of 

 triacanthodids and in the derived triacanthids. 



Since the nature of the supraoccipital and pelvis is not 

 known in the fossil Eoplectinae and Spinacanthinae, it is 

 impossible to say whether one was more closely related 

 to the hollardiin or triacanthodin line of triacanthodid 

 radiation than the other. However, it is clear that the 

 eoplectins were ancestral to the gymnodonts and that the 

 spinacanthins were perhaps an unsuccessful experiment 

 of the early triacanthodids that gave rise to at least 

 one species with some superficial similarities to os- 

 tracioids but which became extinct without derivatives 

 alive today. 



The phylogenetic significance of the Eoplectinae is dis- 

 cussed in detail by Tyler {1973b) and need be only briefly 

 summarized here. It is assumed to be extremely unlikely 

 that the highly specialized condition of small and 

 numerous dental units intimately incorporated into the 

 matrix of the premaxillaries and dentaries, forming a 

 parrotlike crushing beak, has arisen independently in 

 two or more lines of plectognath radiation. It is more par- 

 simonius to assume that this highly specialized condition 

 arose in only one basic line of plectognaths, that leading 

 to the gymnodonts, whose most generalized family is the 

 Triodontidae, and that all plectognaths with this condi- 

 tion are phylogenetically related. Eoplectus basically is 

 triacanthodid and generalized (general body shape, well- 

 developed pelvic and spiny dorsal fins, rounded i, 10, i 



caudal fin) in most of its structure, but has a distinctly 

 triodontid and highly specialized dentition and jaw 

 structure. It represents almost precisely the kind of 

 Eocene ancestral line that has been speculated (Tyler 

 1962a:793) between the triacanthodids and triodontids, 

 and thus between the basal scleroderms and the deriva- 

 tive gymnodonts. 



The conversion of an Eoplectus-like form into a 

 Triodon-Vike one involves mostly reductive tendencies, 

 which are well-known to be of great importance in the 

 evolutionary diversification of the plectognaths, as 

 follows: 1) complete loss of pelvic fin but retention of 

 pelvis; 2) reduction of spiny dorsal fin and its basal 

 pterygiophores into a posteriorly migrated rudimentary 

 structure; 3) reduction in number of soft dorsal and anal 

 fin rays; 4) great increase in length and decrease in depth 

 of caudal peduncle concomitant with conversion of 

 rounded into forked caudal fin; 5) decrease in height of 

 neural and haemal spines of caudal peduncular vertebrae 

 and development of anterolateral flanges on neural 

 spines, associated again with elongation and slenderiza- 

 tion of peduncle for more powerful swimming with forked 

 caudal fin; 6) increased covering of scales with more 

 elaborate surface spinulation and development of ex- 

 pansible dewlap of skin between finless pelvis and anal 

 region; 7) increase in size of prefrontal and of its sutural 

 connections to surrounding snout bones for stronger but- 

 tressing of crushing beak; 8) probable similar increase in 

 size of palatine for same reason; 9) decrease in depth of 

 ventral flange of parasphenoid; 10) straightening of 

 vertebral column associated with elongation of body. 



Triodon has only two anatomical features known to be 

 more generalized than in Eoplectus: the presence of 

 procurrent caudal fin rays and of well-developed ribs and 

 epipleurals. It is unlikely that these are de novo acquisi- 

 tions of Triodon, and the ancestry of Triodon should have 

 these structures. In fact, the Eocene Zignoichthys, ap- 

 parently closely related to Eoplectus, does have 

 procurrent caudal fin rays, indicating that these were 

 simply lost by Eoplectus after it gave rise to the line 

 leading to Triodon. It is possible that small ribs and 

 epipleurals were present on the obscured anterior ab- 

 dominal vertebrae of Eoplectus, but it still must be 

 assumed that well -developed ribs and epipleurals were 

 lost by Eoplectus only after it gave rise to the Triodon- 

 like line. Thus, E. bloti must be considered a slightly 

 specialized member of the Eoplectinae line which gave 

 rise to the Triodon line, and not the immediate direct 

 ancestor of Triodon. 



The number of dorsal and anal fin rays in Eoplectus is 

 in the high part of the range for triacanthodids, but the 

 number of basal pterygiophore supports is in the low part 

 of the range, and even lower for the dorsal pterygio- 

 phores. The fact that there are many more rays than 

 pterygiophores, especially in the posterior regions of 

 these fins, may indicate a prelude to an eventual reduc- 

 tion in the number of rays from posteriorly to anteriorly 

 in the series leading to the Triodon condition. This 

 reduction in number of rays would also tend to lengthen 

 the caudal peduncle, which would probably continue to 



