Discoserra pectinodon, Lund, 2000

Lund, Richard, 2000, The new Actinopterygian order Guildayichthyiformes from the Lower Carboniferous of Montana (USA), Geodiversitas 22 (2), pp. 171-206 : 180-185

publication ID	https://doi.org/ 10.5281/zenodo.4664626
persistent identifier	https://treatment.plazi.org/id/03ECD041-AA23-FF98-3C9A-FCBA10F031DB
treatment provided by	Felipe
scientific name	Discoserra pectinodon
status	gen. nov.

Treatment

Discoserra pectinodon n. gen. n. sp. ( Figs 5-13 View FIG View FIG View FIG View FIG View FIG View FIG View FIG View FIG View FIG ; 14B View FIG )

HOLOTYPE. — CM 30621 .

REFERRED SPECIMENS. — MV 2772, 2773, 2956, 2984, 3579, 3810, 3811, 7669, 7670, 7756, 7757, 7793, 7794. CM 27290-27292, 27294-27296, 27298, 27333, 35206-35216, 35409, 35547, 37545-37547, 37665, 41009, 44500, 46201-46206, 48650, 48651, 48717-48720, 48841, 62794-62802. ROM 36562, 41030, 41169, 43003, 43115, 43976.

ETYMOLOGY. — Pectinodon, in reference to the long teeth.

DIAGNOSIS. — For meristics see Table 1. Other characters as for genus, only species.

REMARKS

The statistics of Discoserra ( Table 3) are remarkable.The number of dorsal fin rays correlates with the numbers of anal and caudal rays as well as those of the precaudal ridge scales. The number of caudal rays also correlates with the number of lateral line scales and a size parameter, maximum height. Pelvic position in scale rows along the lateral line negatively correlates with number of lateral line scales. Scale rows above the lateral line correlates with size parameters, while scale rows below the lateral line do not. Maximum height correlates well with most morphometrics, and excellently with caudal peduncle width. Caudal peduncle length correlates only with caudal angle. Despite these correlations, Discoserra pectinodon is morphologically and statistically the most variable fish in the Bear Gulch fauna. It has not proven possible to consistently isolate variant morphologies that would suggest separate populations either through space or time; in fact several bone complexes such as branchiostegals and suborbitals are not bilaterally consistent within all individuals.

DESCRIPTION

Ganoine

Skull bones have ganoine sculpturing of coarse, irregular longitudinal to circumferential convex ridges. The central areas of broad bones such as the opercular may bear ganoine sculpturing that grades from tubercles centrally to ridges peripherally ( Fig. 6 View FIG ).

Scales

Lateral flank scales are up to 3.6 times deeper than wide and bear strong peg-and-socket joints as well as internal anterior thickenings. Most lateral scale rows are deepened. Ganoine sculpturing varies on the flank scales from few, relatively coarse circumferential grooves near the dorsum to finer circumferential striations at midflank. The stout, deep ventral abdominal scales are strongly serrated in addition to bearing coarse circumferential grooves. Squamation at the bases of dorsal and anal fins is of small, thin scales, extended into lobes that project over and under the caudal peduncle. A significant bulk of dorsal and anal fin radial musculature is indicated.

Lateral aspect of skull ( Figs 7 View FIG ; 8 View FIG )

The premaxillae (P, Figs 7 View FIG ; 8 View FIG ) are about five times longer than wide, and are not firmly sutured either across the midline or to any other bones. There appears to have been a tendency for them to fuse, in some individuals, to the anteriormost infraorbital posteriorly. The premaxillae bear a single row of long, fine, styliform, closely set teeth. The maxilla (M, Figs 7 View FIG ; 8 View FIG ) is most often triangular in shape, varying from 1: 4 to 1: 5 in length: height ratio. It is not firmly held to any other skull bones. A mesial view of the maxilla of one specimen is available, that shows what appears to be a slight anterior articular facet (CM 27290). The single row of long, closely fit teeth diminishes abruptly in height and ends at one third of the distance to the posterior end of the bone.

The infraorbital lateral line canal bones are very variable in number (I0, Figs 7-9 View FIG View FIG View FIG ). The first infraorbital is short and in close contact with the premaxilla. The second element is wedge shaped, of variable length, and may either be absent or fused to the large third anterior infraorbital in some specimens. When present, it forms the anteroventral margin of the posterior nostril. The third anterior infraorbital is large and trapezoidal, with pores distributed irregularly and remote from the course of the infraorbital canal. As many as five to seven additional infraorbitals, most only surrounding the infraorbital canal, rim the orbit. Pores from the infraorbital bones below the orbit are irregularly distributed upon the suborbital bones ventrally. The most posterodorsal infraorbital is firmly associated with a large, thin bone that covers the dilator opercular fossa.

Suborbitals ( Figs 7 View FIG ; 8 View FIG ) on the posterodorsal cheek are scale-like. Under the orbit, the suborbital bones vary in number and shape from one long element (CM 35215) to a series of seven (CM 27292). Normally, a trapezoidal element or elements lie dorsal to the articular region of the mandible, followed by a variable number of small posterodorsal bones; in several specimens an anterior triangular element intervenes between the third infraorbital and the trapezoidal bone or bones (CM 27294, 35206, 35211).

The dorsal of the two preopercular bones contacts the skull roof; it is shorter and thinner than the ventral element. The preopercular canal branches into an anteroventral and a ventral branch near the mandibular end of the bone ( Fig. 8 View FIG ). There is one tall, narrow principal opercular bone, ventral to two to four considerably smaller bones and a large presupracleithrum (PSP, Figs 7 View FIG ; 8 View FIG ). The branchiostegals are very variable and are not always bilaterally symmetrical. A typical pattern is one dorsally concave ray below the opercular, followed ventrally by one that widens posteriorly, four to six ventrally concave rays and a single ventral plate (posterolateral gular). Variations are detailed in Figure 10 View FIG . One to three small interopercular bones (I, Figs 7 View FIG ; 8 View FIG ) are found between the posterior end of the mandible, the posteroventral end of the ventral preopercular and the anteroventral end of the opercular. There is clear evidence for a branch of the mandibular lateral line canal extending under the anterior of these bones, and some indication that the ventral branch of the preopercular lateral line canal extended ventrally between the interoperculars. This is best displayed in CM 35206.

The external aspect of the lower jaw consists of a large dentary and a small angular, with a single row of long, slim teeth along the anterior oral margin of the dentary. A surangular seems to be absent. The internal aspect of the mandible consists of two elements, an extensive prearticular that seems to lack teeth, and an articular with a strongly elevated anterior wall and a dorsomesial pit for adductor musculature insertion (CM 27290). The mandibular lateral line canal lies near the ventral border of the dentary; the canal is L-shaped in the angular but a posterior continuation of the canal is evident in several specimens.

Skull roof ( Figs 7 View FIG ; 8 View FIG ; 14B View FIG )

The long, narrow median rostral (R, Figs 7 View FIG ; 8 View FIG ) extends back to the level of the anterior border of the nostrils. While lateral line canals enter the median rostral from either side, a complete ethmoid commissure has not been seen. The paired postrostrals extend along the dorsal midline between the nostrils, receiving the posteromedian end of the median rostral between them. The posterior median postrostral contacts the supraoccipital posteriorly and excludes the frontal and parietal bones from the dorsal midline ( Fig. 14B View FIG ). The supraoccipital is a significant bone of both the braincase, the skull roof, and the posterior cranial surface, where it bears a strong posteriorly projecting crest. The supratemporal commissure does not enter the supraoccipital.

The more anterior of two pre-frontal bones (PF, Figs 7 View FIG ; 8 View FIG ) forms the posterior border of the anterior nostril, and a small nasal bone (N, Figs 7 View FIG ; 8 View FIG ) extends between the anterior and posterior nasal openings. The posterior prefrontal is narrow and flanked laterally by a single supraorbital bone. The supraorbital lateral line canal extends from the posterior pre-frontal through the frontal and parietal; the canal cannot be traced anterior to the posterior pre-frontal. The parietal bears a short transverse pitline, which may be continued laterally onto the supratemporal, the second of the three bones of the otic series, in the form of a few pores or pits. Behind the skull roof the supratemporal canal intersects with the extrascapular canal in a bone lateral to two paired extrascapulars, but there is no evidence for a complete commissure.

Visceral skeleton

The palate and suspensorium are displayed only in CM 27290 ( Fig. 11 View FIG ). No palatal teeth can be seen. The hyomandibula (H, Fig. 11 View FIG ) has a ventral process one half of the total height of the bone and has a prominent central foramen for the hyomandibular branch of nerve VII. The metapterygoid (ME, Fig. 11 View FIG ) is attached only to the anterior edge of the ventral process of the hyomandibula, leaving ample room for a large spiracle. There is an indication of a symplectic and a palatal groove for this bone near the posterior edge of the quadrate. The quadrate condyle is poorly preserved but faced forward; a prominent ventral facet posterior to the condyle may have been for attachment of a strong quadrate-mandibular ligament. A stout ceratohyal is visible, with branchiostegal rays attached, in FMNH PF10207.

There are eight to ten sclerotic bones in the eye, the largest of which are the dorsal and anterior bones.

Braincase

The braincase is well ossified and is devoid of sutures that would delineate individual bones. No occipital fissure is evident, unless the small foramina (OC, Fig. 12 View FIG ) are remnants of such a structure. Structures are best displayed in CM 27295, although CM 27290 and CM 35547 have also contributed information ( Figs 11 View FIG ; 12). The parasphenoid (PS, Figs11 View FIG ;12) extends the entire length of the braincase. It has small lateral wings, no obvious palatal articulations, and is strongly V-sha- ped in transverse section through most of its length. The ethmoid region is sheathed by two thin ossifications, the dorsal one also forming the floor of the nasal capsule and enclosing a foramen for the olfactory nerve. The sphenoid region is strongly notched for the exit of the optic nerve (II, Fig. 12 View FIG ). The sphenotic ossification area projects laterally beyond the skull roofing bones and is excavated by a deep pit with an apparent narrow foramen in its floor. This pit clearly is of the form and position of the dilator opercular fossa (DO, Fig. 12 View FIG ) of teleosts and seems to shows no relationship to a spiracular canal as reported for some chondrosteans (Patterson 1975). Foramina for branches of cranial nerves V, VII, and X are visible ( Fig. 12 View FIG ). A strong posteriorly directed supraoccipital crest projects beyond the posterior face of the braincase.

Postcranial

Clavicles are absent in the shoulder girdle. The cleithrum is deeply notched for the insertion of the pectoral fin and has a strong peg-and-socket articulation with the supracleithrum. The postcleithrum is small.

All scales have strong peg-and-socket articulations, smooth ganoine coatings, and have finely pectinated posterior borders ( Fig. 13C, D View FIG ). The anterior flank and abdominal scales are tall and narrow.There is no caudal inversion, and the unscaled body axis extends to the last caudal fin ray and bears only long, fine dorsal scutes. A complete series of median dorsal scutes extends from the head to the origin of the dorsal fin; each scute taller than the preceding one and bearing a small forwardly facing hook. The bases of the first jointed fin rays originate behind the last of the dorsal scutes and level with the bases of the scutes. There are two to three dorsal and ventral scutes between the end of the dorsal fin and the caudal lobe. There is a series of tall, thick, strongly serrated abdominal scutes that extend from the shoulder girdle to the pelvic fins, and there are two to three pre-anal scutes.

All fins are composed of well-spaced and jointed rays. The pectoral, dorsal and anal fins are supported on finely scaled lobes. The pelvic fin is very small, and the caudal fin is rounded but not dorsoventrally symmetrical. There are no fin fulcrae.

Postcranial endoskeleton ( Fig. 13 View FIG )

The pectoral fin base is lobed.Up to11 to 12radials are visible supporting the pectoral fin rays. The pelvic fin is very small and no endoskeleton is known. The axial skeleton contains 12 to 13 precaudal segments, 13 to 14 caudal segments anterior to the caudal fin endoskeleton and 11 segments are visible in the sharply upturned caudal lobe. Precaudal neural arches are paired, and while there are blocky precaudal ventral arch elements, there are no ossified interarcuals. Caudal neural arches are fused across the midline. Neural spines are not fused to the arches but extend from occiput to the end of the caudal as a complete separate series. Dorsal to the neural spines, a complete supraneural series extends from the occiput to the posterior end of the dorsal fin, the predorsal supraneurals each bearing a dorsal facet for articulation with the dorsal ridge scales ( Fig. 13A, B View FIG ). Approximately two dorsal fin rays are supported by each supraneural (baseost); axonosts are not evident. Haemal arches are fused across the midline and articulate with separate haemal spines; these in turn support the infrahaemal series. The origin of the anal fin is supported by two haemal and infrahaemal elements. The anal fin ray: infrahaemal ratio is also approximately 2: 1.