smaller unbranched rays without cross-striations in 

 Recent triacanthodids, but with the Eocene Eoplectus 

 having a spine and four long branched and presumedly 

 cross-striated rays); 16 principal caudal fin rays (12 in 

 triacanthodids, the maximum for the order); the presence 

 of a folding spine or fixed plates on the caudal peduncle 

 (folding spine lacking in all plectognaths but with fixed 

 plates in some scleroderms); a complete series of subor- 

 bital bones (none in plectognaths); nasal or other bones 

 lateral to the ethmoid and anterior to the prefrontal 

 (none in plectognaths); parietals (none in plectognaths); 

 true or pleural ribs (lacking in all plectognaths except in 

 one monacanthid and in the primitive gymnodont Trio- 

 don); a basisphenoid (lacking in all plectognaths except 

 Triodon and molids); a relatively normal percoid type 

 pelvis without a long extension posterior to the level of 

 the pelvic fin origin (a long extension of the pelvis pres- 

 ent only in triacanthoids posterior to the level of the pel- 

 vic fin origin, this portion being at least half, and usually 

 more, of the total length of the pelvis); very short 

 premaxillary pedicels and a nonprotractile upper jaw 

 (premaxillary pedicels well developed and upper jaw 

 protractile only in triacanthoids); and a dorsal fin con- 

 sisting of an anterior portion of 4 to 9 (rarely 3 or 10) 

 spines not decreasing much, if at all, in length pos- 



teriorly in the series (except in the young of some species) 

 and followed without spatial interruption or indentation 

 of the membrane by about 20 to 33 rays (as many as six 

 dorsal spines present only in triacanthoids, decreasing 

 greatly in length posteriorly in the series, except in the 

 Eocene Spinacanthus, and followed either without 

 spatial interruption or with only a slight spatial inter- 

 ruption, but always with an indentation of the mem- 

 brane to the base of the fin, by 12 to 26 rays in Recent 

 species of triacanthoids and as few as about eight in 

 Spinacanthus; no more than three dorsal spines in other 

 plectognaths). 



A number of other features of acanthurids (e.g., the 

 ridged surfaces, deep indentations, and hooks found on 

 the distal articular areas of the first basal pterygio- 

 phores of the spiny dorsal and anal fins and the laterally 

 expanded distal ends of many of the others; the usually 

 highly modified dentition; less restricted gill opening; 

 longer and more anteriorly placed ethmoid; two or three 

 more vertebrae; fewer and less well-developed branchios- 

 tegal rays anteriorly in the series, etc.) distinguish them 

 from triacanthoids and other plectognaths, but these will 

 be dealt with in detail along with their similarities to 

 triacanthoids in a subsequent work comparing the fossil 

 acanthurids and triacanthoids. 



SYSTEMATIC SECTION 



Definition and Synopsis of the Osteology of the Order Plectognathi (Tetraodontiformes) 



Until such time as most other orders of fishes have 

 been extensively studied anatomically, and especially os- 

 teologically, it is impossible to give a comparative in- 

 clusive diagnosis of the Plectognathi that would clearly 

 and succinctly distinguish them at once from all other 

 orders of Acanthopterygii, and especially from all sub- 

 groups of the Order Perciformes, of which order the plec- 

 tognaths are most likely a specialized derivative. The 

 perciform suborder Acanthuroidei has often been 

 thought to be the closest extraordinal relative of the plec- 

 tognaths, the implication being that the acanthurids and 

 plectognaths share a common ancestral line in the 

 Cretaceous, with Patterson (1964:400) having suggested 

 that this was the Pharmacichthyidae. Tyler (1968:42-43) 

 briefly pointed out that the Recent acanthurids differ 

 from the most generalized plectognaths, the triacan- 

 thodids, mainly in the retention of a number of bones not 

 now found in triacanthodids and in a greater number of 

 certain meristic elements. I subsequently studied the os- 

 teology of numerous additional Recent species (the major 

 features summarized in Tyler 1970a, c) and of a large 

 majority of the specimens of fossil acanthurids available 

 (unpubl. data), mainly from the Eocene of Monte Bolca, 

 Italy. These subsequent studies of acanthurids tend to 

 support the view that acanthurids are probably relatively 

 generalized (except in dentition, caudal peduncle ar- 

 mature, and dorsal and anal spine locking mechanisms) 



representatives of the same ancestral line which gave rise 

 to the plectognaths, with at least the majority of plectog- 

 naths becoming far more specialized than the acan- 

 thurids in most respects. 



These matters will be discussed in a subsequent paper 

 on the anatomy of fossil and Recent acanthurids by 

 J. Blot and the author, and then on their differences 

 from and similarities to the plectognaths. 



For the moment, the Order Plectognathi is defined 

 below by listings of the bones that are either constantly 

 present throughout the order or are of variable oc- 

 currence. 



Certain bones apparently are always present in plectog- 

 naths, although in many cases these bones are highly 

 variable in size and shape from one family or genus to the 

 next. Those elements which are constantly present are: 

 basioccipital; exoccipital; supraoccipital; pterotic; 

 sphenotic; epiotic; prootic; frontal; parasphenoid; eth- 

 moid; vomer (with the possible exception of diodontids); 

 hyomandibular; quadrate; metapterygoid; symplectic; 

 palatine; ectopterygoid; operculum; suboperculum; pre- 

 operculum; premaxillary; maxillary; dentary; articular; 

 angular; ventral hypohyal; ceratohyal; epihyal; four 

 branchiostegal rays (at least one in the anterior group 

 and three in the posterior group); two basibranchials (of 

 the second and third arches); three hypobranchials (of 

 the first to third arches); five ceratobranchials (support- 



