Pseudancistrini, Armbruster & Lujan, 2024

Tribe Pseudancistrini , new tribe urn:lsid:zoobank.org:act:CE2CF701-3692-40FF-A9FE-D926399A8807

Included genera.

Pseudancistrus Bleeker, 1862:2 . Type-species: Hypostomus barbatus Valenciennes, 1840 (in Cuvier, Valenciennes, 1840).

Phylogenetic diagnosis. With Pseudancistrus megacephalus, Pseudancistrini is diagnosed by hyomandibula contacting only the compound pterotic (no prootic contact; 35:0>1), walls of metapterygoid channel tall (56: 0>1), straight, spoon-shaped anterior process on metapterygoid (58: 0>1, shared with Stellantini ), sphenotic lacking external contact with posteriormost infraorbital (117: 0>1).

All taxa except P. megacephalus are diagnosed by: anterohyal greatest width greater than half length (1: 0>1), hyomandubula deflected beyond posterior margin such that posterior margin is visible when mesial surface of hyomandibula is viewed (46: 0>1), almost vertically-oriented preopercle (61: 1>0), reduction in gap between anterior process of compound pterotic and main body (111: 2>1), forward extension of anterior process of compound pterotic halfway or greater through orbit (112: 0>1), distal margin of rib of sixth vertebral centrum flared distally such that tip is wider than shaft (128: 0>1), reversal to moderately evertible cheek plates (184: 2>1), hypertrophied odontodes along snout margin (188: 0>1), sheaths of snout odontodes long and well separated from odontode, forming tentacules (208: 0>1).

Comparative diagnosis. Pseudancistrini (except P. megacephalus ) differs from most other Hypostominae except Corymbophanes , Cryptancistrus Fisch-Muller, Mol & Covain, 2018 , some Guyanancistrus Isbrücker, 2001 (in Isbrücker et al., 2001), Hopliancistrus , Lithoxancistrus Isbrücker, Nijssen & Cala, 1988 , Neblinichthys Ferraris, Isbrücker & Nijssen, 1986 , and Pseudolithoxus by having hypertrophied odontodes along snout margin of nuptial males; from Corymbophanes by having hypertrophied, evertible cheek odontodes (vs. no evertible cheek odontodes); from Cryptancistrus , Guyanancistrus , and Hopliancistrus by having snout odontodes evenly arranged along entire snout margin and thin (vs. only at anterolateral corners and thick); from Araichthys , Corymbophanes , and Neblinichthys (and maybe Paulasquama and Yaluwak Lujan, Armbruster in Lujan et al., 2019) by lacking hypertrophied odontodes on top of snout; from Lithoxancistrus and Colossimystax by lacking enlarged papillae behind dentary teeth); and from Stellantini by having slight keel on midventral plate row on caudal peduncle, with dorsal laminae of plates largely flat (vs. plates strongly keeled with dorsal laminae strongly convex).

Geographical distribution. Found in north-flowing drainages of the eastern Guiana Shield from the Caroni and Essequibo basins east to the Oyapock, the south-flowing Branco, Negro, and Trombetas drainages, more eastward northern tributaries of the lower Amazon, and the Tapajós and Xingu rivers draining the northern Brazilian Shield.

Remarks. Placement of P. megacephalus in Pseudancistrus is uncertain, thus the phylogenetic diagnosis above is provided with and without P. megacephalus . Pseudancistrus megacephalus has a deeper body than congeners, lacks hypertrophied odontodes along the snout, and the skull ( Fig. 4) contains significant differences. These include the posterior shelf of the pterotic being largely parallel with the main body of the hyomandibula in P. megacephalus (vs. bent medially), the anterior process of the pterotic being longer and more separated from the main body of the pterotic, and the lateral wall of the pterygoid channel being differently shaped. The orientation of the suspensorium (turquoise in Fig. 4, also indicated) forms almost a right angle at the posteroventral corner when viewed laterally (vs. forming almost a straight line), the preopercle lacks a process for articulation with the canal plate (vs. process present; the canal plate is the fulcrum for rotation of the evertible cheek odontodes), and the metapterygoid condyle to the suspensorium is tall (vs. short). Pseudancistrus megacephalus has not been collected within its range in Guyana and Suriname since 1909 despite several fairly intensive collecting efforts, suggesting it may be either regionally extirpated or extinct (Armbruster, 2023a). The MBUCV collection has five lots and 19 specimens identified as Pseudancistrus megacephalus from the Cuyuni River in Venezuela, all collected in 1987 and 1991. Thus the Cuyuni may remain a refuge, although it has suffered from even worse mining impacts than other parts of the species’ range.