Sorubim trigonocephalus Miranda-Ribeiro 1920

Arrowhead Shovelnose

Fig. 3

Sorubim trigonocephalus Miranda-Ribeiro, 1920: 12 (original description), pls. 10-11. Holotype: MNRJ 882 (317 mm SL). Brazil, Mato Grosso, Porto Velho, rio Madeira; Geraldo Kuhlmann, 1914.

Diagnosis: Distinguished from all other species of Sorubim by the following combination of characters: snout very elongated, triangular or arrowhead shaped in dorsal and ventral view; exposed portion of premaxillary tooth patch as long as wide (Fig. 3C).

Description: Mensural and meristic data are given in Tables 3-7. Largest individual 507 mm SL (USNM 194403). Dorsal-fin rays II,6 (n=3); anal-fin rays 19-22 (n=3); pectoral-fin rays I,9 (n=3); pelvic-fin rays 6, 1 unbranched and 5 branched (n=3); principal caudal rays variable, as with congenerics; gill rakers on first pharyngeal arch 13 (n=3), 3 on epibranchial. Body shape and form shown in Fig. 3. Head very elongate; distance between anterior and posterior nostrils longer than other congeners. Head length variable, 3.55-4.25 times gape width (n=3, range 3.55-4.25, mean 3.90); interorbital distance 3 times or less than eye diameter (n=3, range 2.37-3.07, mean 2.79); anal fin length 2 times or greater than adipose fin length (n=3, range 2.12-2.64, mean 2.36); premaxillary tooth patch width equal to its length (n=3, range 1.03-1.09, mean 1.06). Eyes lateral, barely visible from ventral view, if at all. Inner mental barbels just anterior to or even with gular apex (Fig. 3C). Maxillary barbels extend to pelvic fins.

Color in alcohol: Pigmentation and color as in other species of Sorubim.

Etymology: The name trigonocephalus, comes from Latin, in reference to the triangular ("trigono-") shape of the head ("-cephalus") in dorsal or ventral view.

Distribution: Specimens of S. trigonocephalus are extremely rare in natural history collections. The three specimens examined are from two Amazon tributaries, rios Madeira and Tapajos, Brazil (Fig. 9).

Remarks: In order to better assess the taxonomic status of S. trigonocephalus, it will be necessary to examine more material when and if it becomes available.

Similar species: Sorubim trigonocephalus most closely resembles S. lima but only in body shape, having a similarly deep caudal peduncle and nine branched pectoral-fin rays. The length of the exposed premaxillary tooth patch in this species is extraordinary, facilitating quick identification in the field or laboratory.