Guildayichthys carnegiei, Lund, 2000

Lund, Richard, 2000, The new Actinopterygian order Guildayichthyiformes from the Lower Carboniferous of Montana (USA), Geodiversitas 22 (2), pp. 171-206 : 173-179

publication ID	https://doi.org/ 10.5281/zenodo.4664626
persistent identifier	https://treatment.plazi.org/id/03ECD041-AA28-FF9E-3EC7-FB7F10A9367B
treatment provided by	Felipe
scientific name	Guildayichthys carnegiei
status	gen. nov.

Treatment

Guildayichthys carnegiei n. gen. n. sp. ( Figs 1-4 View FIG View FIG View FIG View FIG ; 14A View FIG )

HOLOTYPE. — CM 41071 .

REFERRED SPECIMENS. — MV 6045, 6046, 6932, 7671, 7758, 7795. CM 27293, 27297, 35217, 37548, 37549-37550, 41010, 46091, 46095, 46131, 46293, 46294, 46295, 46296, 46297. ROM 36560, 41039, 41042, 41047, 43012. FMNH PF10026.

ETYMOLOGY. — Named in honor of Andrew Carnegie, founder of Carnegie Museum of Natural History, Pittsburgh, Pennsylvania.

HORIZON AND LOCALITY. — Bear Gulch Limestone lens, Bear Gulch Limestone member of the Heath Formation, Big Snowy Group, south of Becket, Fergus County, Montana.

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

REMARKS

All skull bones have ganoine sculpturing of long ridges; either the mesial or the anterior face of each ridge lies at a low angle to the plane of the bone while the opposite face meets the bone approximately perpendicularly. Ganoine ridges on the gulars are of the reverse orientation ( Fig. 2 View FIG ). Scales of the anterior trunk have pegand-socket joints and have ganoine sculpturing of faint marginal vertical lines; all trunk scales are pectinated and the deepened ventral abdominal scales are particularly strongly pectinated. Only the central flank scale rows are markedly deepened (rows 2-5 below the lateral line). The anteri- or scales of these rows average 3.85 times deeper than wide. Squamation at the bases of dorsal and anal fins is of small thin scales, extending as lobes over and under the caudal peduncle.

The statistics of Guildayichthys ( Tables 1; 2) are based upon an inadequate sample size and may be misleading. Standard length-maximum height correlation is strong, and caudal peduncle proportions also correlate. No other recorded parameters show meaningful correlations.

DESCRIPTION

Lateral aspect of skull ( Figs 2-4 View FIG View FIG View FIG )

The paired premaxillae (P, Figs 3 View FIG ; 4 View FIG ) are high, narrow and separated posteriorly in the midline where they meet the rostral. They are firmly attached only to the first infraorbital posteriorly. Short styliform to conical teeth are borne in a narrow band along the oral margin. The maxilla is elongate and narrow (height one seventh of length), extends to midorbital level and overlaps the posterodorsal rim of the mandible. Short teeth are borne only upon the anterior half of the maxilla.

There are five to six bones in the infraorbital lateral line canal series (IO, Figs 3 View FIG ; 4 View FIG ). The most anterior is narrow and forms the ventral edge of the posterior nostril. The second is large and moderately expanded anteriorly. The remaining elements are quite narrow, the most posterodorsal of which is tightly associated posteriorly with a thin and large bone. The latter two bones cover the dilator opercular fossa and articulate with the skull roof. A series of thin suborbitals extend from the skull roof to midorbital level.

There is a short dorsal and a taller ventral preoperculum, both anteroposteriorly narrow, carrying the preopercular canal from the margin of the skull roof to the posterior end of the lower jaw (POPD, POPV, Figs 3 View FIG ; 4 View FIG ). Two small postspirac- ular bones (see Polypterus, Pehrson 1958 ) lie dorsal to the tall, narrow, principal bone of the opercular series. The opercular is flanked ventrally by a series of branchiostegals, two dorsally concave, the next a posteriorly widened triangle, followed by 4-5 ventrally concave, deeply overlapping bones and a ventral plate (posterolateral gular). The plate is divided anteroposteriorly in one specimen ( Fig. 3 View FIG ); a narrow lateral gular lies anterior to the plate and mesial to the mandible.

A series of five mobile, overlapping interopercular rays ( Fig. 2 View FIG ; I, Fig. 3 View FIG ) extends from the posterior margin of the mandible to the anteroventral edge of the opercular. There is strong evidence in MV 6045 of either lateral line pores or pit-organs in the two rays closest to the mandible. The lateral surface of the mandible consists of a large dentary and smaller, posterior, angular. No surangular is visible. Short conical teeth occupy the anterior half of the oral margin of the dentary. The mandibular lateral line canal traverses the mandible near its ventral margin, and bends abruptly dorsad in the angular. The articular facet is located slightly below a projection of the dorsal margin of the jaw that presumably functioned as a coronoid process.

Skull roof ( Figs 3 View FIG ; 4 View FIG ; 14A View FIG ; 15C View FIG )

The small median rostral (R, Figs 3 View FIG ; 4 View FIG ) is followed by a paired postrostral that is notched for the mesial margin of the dorsal nostril. An ethmoid commissure has not been observed. A moderately large median second postrostral extends past the level of the anterior edge of the parietals. The supratemporal commissure crosses the midline anterior to the posterior margin of the supraoccipital ( Figs 3 View FIG ; 14A View FIG ).

The posterior border of the anterior (dorsal) nasal opening is formed by the most anterior of a series of two to three narrow bones. This series extends lateral to the postrostrals, and mesial to the nasal and supraorbital, and abuts posteriorly against the frontal, the intertemporal, or both ( Figs 3 View FIG ; 4 View FIG ). A thin nasal bone bridges the space between nostrils. The course, or even the presence of the supraorbital canal in the nasal region is uncertain; the canal can only be followed in the frontal and parietal and may lie under the suture between the pre-frontal and the more lateral series. The supraorbital canal in MV 6045 bears a short posteromesial branch in the frontal before continuing on to the parietal; the canal does not reach the transverse pit line. There are three bones in the otic canal series, the largest and most anterior, the intertemporal (IT, Figs 3 View FIG ; 4 View FIG ), receiving the infraorbital canal from the posterodorsal infraorbital. Pores associated with a profundus branch of the otic canal (Poplin 1973) extend forward within the intertemporal from the junction with the infraorbital canal in MV 6045 ( Fig. 3 View FIG ). Two to three pores in a transverse row extend mesially across the supratemporal and parietal toward the transverse pitline but there is no evidence for an underlying canal branch. The posterior margin of the skull roof in MV 6045 is bordered by one pair of bones bearing the canal intersection characteristic of the extrascapular. In CM 41010, however, two pairs of bones are evident in the same space, posttemporal and extrascapular ( Fig. 4 View FIG ). In neither condition do these bones extend to the dorsal midline.

The braincase and the floor of the olfactory cup are well ossified. Details, however, are obscured by overlying bones. A short process, possibly a palatobasal (basal) process, is visible in the orbital region of CM 41010. Neither palate nor visceral skeleton is visible.

Postcranial

Shoulder Girdle. The posttemporal is abutted anteroventrally by a presupracleithrum (Lund & Melton 1982) and posteroventrally by a large supracleithrum. The supracleithrum-cleithrum contact is masked by the operculum; a tall, thin postcleithrum is present and may have served to strengthen what appears to be a very limited area of contact. The cleithrum is deeply notched for the insertion of the pectoral fin. The scapulocoracoid is ossified in a thin sheet and supports one radial for each pectoral fin ray. Clavicles are absent.

Scales have strong peg-and-socket articulations, smooth ganoine coatings, and are slightly pectina- ted. Anterior flank and abdominal scales are moderated deepened. There is an abrupt caudal inversion, and the scaled body axis continues past the last caudal fin ray.A complete series of median dorsal scutes extends from the head to the origin of the dorsal fin; as the series approaches the dorsal fin, the scutes increase in height and the sharp posteriorly directed spine of each overlaps the spine of the following scute. 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 scutes between the end of the dorsal fin and the caudal lobe, followed by a series of dorsal caudal scutes that extend to the end of the caudal axis. There is a series of short, stout, strongly serrated abdominal scutes that extend from the shoulder girdle to the pelvic fins.

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 moderate in length, and the caudal fin is slightly forked and slightly inequilobate. There are no fin fulcrae. No postcranial endoskeletal detail is available.