Family Triacanthodidae 



Comparative dia^osis (contrast with that of the 

 Triacanthidae) (modified from Tyler 1968:58-62).— A 

 not especially strongly sutured skull and a variety of 

 body forms none of which are built for particularly ac- 

 tive, strong swimming; many areas of cartilage visible on 

 the external surface of the skull between the limited areas 

 of suturing between the bones in the otic and occipital 

 regions; teeth in the jaws various, conical or truncate, 

 in a single series or with a numerous outer series fol- 

 lowed by a few teeth in an inner series, or absent al- 

 together (in adults of one of the long-snouted genera, 

 Macrorhamphosodes) in the upper jaw, or of numerous 

 rounded dental units incorporated into the matrix of the 

 premaxillary and dentary in beaklike jaws in the Eocene 

 Eoplectus, but never as heavy incisor teeth (except per- 

 haps somewhat so in the Eocene Protobalistum); a nor- 

 mal ethmoid-frontal complex for support of jaws without 

 massive dentition (except in the Eocene Eoplectus, with 

 a modified ethmoid-vomer-palatine complex to support 

 the massive beaklike jaws); the prefrontal without a long 

 anterior extension sutured to the ethmoid and vomer, 

 and the vomer without posterolateral extensions toward 

 the prefrontal; premaxillary pedicels when retracted 

 reaching most of the way along the dorsal surface of the 

 ethmoid, almost to the tip of the frontals, except in both 

 of the long-snouted genera (Halimochirurgus and Mac- 

 rorhamphosodes), in which the premaxillary is far 

 removed from both the ethmoid and frontal, and in the 

 Eocene Eoplectus, which has only very short premaxil- 

 lary pedicels and a nonprotractile jaw; supraoccipital 

 either domelike with a convex posterior surface, or most- 

 ly flattened with a small dome or laterally compressed 

 crest; epiotics meeting one another in the midline on the 

 dorsal surface of the skull and separated from the fron- 

 tals by the sphenotics in one group (those with a flat- 

 tened supraoccipital) but separated from one another on 

 the dorsal surface of the skull in the other group by the 

 supraoccipital and meeting medially only on the pos- 

 terior surface of the skull, and articulating anterolateral - 

 ly with the frontals (for illustrations see Tyler 1968:fig. 

 4a-d); pterosphenoids not meeting in the midline of the 

 posterior wall of the orbit; neural processes of the first 

 vertebra not meeting in the midline above the neural 

 canal, forming in conjunction with the exoccipitals an 

 only partially enclosed bony region (without a bony bot- 

 tom) in which the shaftlike end of the first basal pte- 

 rygiophore of the spiny dorsal fin is immovably held; 

 parasphenoid in region of orbit relatively straight or with 

 a ventral arch and with a poorly developed ventral flange 

 below the orbit, the flange not deeper than the depth of 

 the shaftlike portion of the bone, except in the Eocene 

 Eoplectus, with a much deeper flange; hyomandibular 

 without a prominent groove and crest along its lateral 

 surface; pterotic without a ventral process overlying the 

 hyomandibular; supracleithrum usually placed obli- 

 quely to the horizontal axis of the skull, only the approx- 

 imate lower half of its length overlying the cleithrum; 

 mesopterygoid always present as a separate element. 



sometimes sutured along its ventral edge with the 

 metapterygoid; olfactory cavity between the ethmoid 

 and prefrontal not well defined by bony outlines; basi- 

 hyal present; lower two branchiostegal rays more or less 

 like the others, not enlarged; operculum more or less 

 triangular; air bladder usually thin walled, somewhat 

 rounded, extending posteriorly to no more than the level 

 of about two-thirds the length of the abdominal cavity; 

 pelvis either a sturdy shaft more or less triangular or 

 broadly heart-shaped in cross section (the apex or round- 

 ed surface ventrally) just behind the level of the pelvic 

 spine or a flat basin with upturned edges; the two halves 

 of the pelvis only lightly sutured to one another; the side 

 of the pelvis at the level of the flange of the pelvic spine 

 either smooth and allowing for only a single position of 

 erection of the spine or with numerous small grooves 

 allowing for numerous, continuous positions of erection 

 of the spine, but never with a single relatively large ob- 

 lique crest allowing for only two positions of erection; the 

 ventrolateral surface of the pelvis at the base of the spine 

 with a complete foramen through which the two sides of 

 the base of the spine meet medially; the haemal arch and 

 spine of the penultimate vertebra usually autogenous 

 (occasionally extensively sutured to the centrum), the 

 other haemal arches and spines fused to their centra; the 

 epural, parhypural, and five hypurals usually separate 

 elements articulated by connective tissue to each other 

 and to the centrum, but occasionally a few of the middle 

 hypurals may be partially to fully fused to one another; 

 usually two pairs of uroneurals present; slender 

 epipleurals present from the first or second abdominal 

 vertebra to the seventh or eighth abdominal vertebra, 

 and, sometimes, on the first or second, rarely third, 

 caudal vertebra, except in the Eocene Eoplectus, in 

 which epipleurals, if present, probably did not occur on 

 at least the more posterior abdominal vertebrae; fifth 

 basal pterygiophore of spiny dorsal fin present, normally 

 developed; first basal pterygiophore of spiny dorsal fin 

 with anterior and posterior medial flanges well 

 developed, many times the width of the relatively slen- 

 der shaftlike portion of the bone, except in the Eocene 

 Eoplectus in which the flanges are only moderately 

 developed and in Halimochirurgus in which they are 

 poorly developed (in the Eocene Protobalistum and 

 Spinacanthus the more anteriorly placed spiny dorsal fin 

 probably was supported high on the rear of the skull by 

 basal pterygiophores with short shafts and poorly 

 developed anterior and posterior medial flanges); first 

 basal pterygiophore of spiny dorsal fin with a medial 

 flange dorsally completely enclosing a foramen through 

 which the two sides of the first dorsal spine meet medial- 

 ly; second to fifth basal pterygiophores of spiny dorsal fin 

 well developed, their shafts reaching ventrally to or 

 between the distal regions of the neural spines of the 

 fourth to eighth abdominal vertebrae; at least the first 

 two basal pterygiophores of spiny dorsal fin sutured 

 together distally; spiny dorsal fin base (including rudi- 

 ments) much longer than soft dorsal fin base, except in 

 the Eocene Eoplectus, with these two bases of equal 

 length; neural spines of all eight abdominal vertebrae 



