LITHOXINI, Lujan & Armbruster & Lovejoy, 2018

Lujan, Nathan K., Armbruster, Jonathan W. & Lovejoy, Nathan R., 2018, Multilocus phylogeny, diagnosis and generic revision of the Guiana Shield endemic suckermouth armoured catfish tribe Lithoxini (Loricariidae: Hypostominae), Zoological Journal of the Linnean Society 184, pp. 1169-1186 : 1176-1178

LITHOXINI TRIB. NOV.

Composition: This taxon is established for the clade containing the described genera Exastilithoxus and Lithoxus and for the newly resurrected or described genera Avalithoxus and Paralithoxus ( Fig. 3).

Diagnosis: Externally, Lithoxini can be diagnosed from all other Loricariidae except Leporacanthicus by having a distinctly round and flat oral disc, in which the anterior lip does not fold ventrally and the mandibular barbels are anterolaterally positioned and directed ( Fig. 1; vs. typically ovoid oral disc, with an anterior lip that folds ventrally and mandibular barbels that are

Asterisks (*) indicate corrected values that differ from those in the original description and have been confirmed via personal correspondence with the authors. Plate counts are given as modes, with less common variations from the mode in parentheses. Abbreviation: n.a., not applicable.

laterally positioned and posterolaterally directed); from all non-Hypostominae loricariids plus Corymbophanes , Hypostomus and most Pterygoplichthys by having a well-developed cluster of evertible cheek odontodes on each side of the head; from all other members of the Hypostominae except Hypancistrus , Leporacanthicus , Panaqolus , Panaque , Pseudacanthicus , Pseudoqolus , Scobinancistrus and Spectracanthicus by having <13 teeth in either the dentary or the premaxilla; from Hypancistrus , Leporacanthicus , Panaqolus , Panaque Pseudacanthicus , Pseudoqolus , Scobinancistrus and Spectracanthicus by having an extremely depressed body with maximum head depth> 7.2 times in standard length (SL; vs. <7 times; head depth measured from immediately posterior to the supraoccipital to a point directly ventral on the ventral surface); from Leporacanthicus , Panaque , Pseudacanthicus , Pseudoqolus , Scobinancistrus and Spectracanthicus by growing no larger than 60 mm SL (vs.> 90 mm SL); from Panaqolus , Panaque , Pseudoqolus and Scobinancistrus by having flattened, acute tooth cusps (vs. spoon shaped or broadly truncate); and from Leporacanthicus by having all teeth approximately coequal in length (vs. two enlarged and elongate premaxillary teeth much longer than dentary teeth).

Cladistically, Armbruster (2004) found 23 mostly internal osteological changes along the branch leading to Lithoxini , including many reversals. These changes included the following: a decrease in the length and width of the accessory process of ceratobranchial 1 (characters 7-1, 8-1), loss of the accessory process of epibranchial 1 (14-0), loss of the posterior shelf of epibranchial 4 (17-0), elongation of hypobranchial 1 (23-1), loss of the interhyal (26-0), reversal to a round upper pharyngeal tooth plate with evenly distributed teeth (30-0), reversal to a pointed lateral edge of the posterohyal (32-0), incorporation of the posterior process of the hyomandibula into the main body of the hyomandibula (41-1; unique to the clade), loss of the lateral wall of the metapterygoid channel (52- 1), longitudinal ridge on the quadrate (68-1), dentaries forming an acute angle (69-1), a bowling pin-shaped maxilla (71-1), reduction in the size of the metapterygoid disc (100-1), presence of bifid hemal spines (122-1), first neural spine anterior to first dorsal-fin pterygiophore (125-1), reduced exposure of the cleithral process (157-1), reversal to curved anterolateral processes of the pelvic basipterygium meeting or nearly meeting at the midline (167-0), loss of the posteroventral ridge of the pelvic basipterygium (173-0) and the presence of enlarged teeth (205-2). Lithoxini was among the best-supported clades described by Armbruster (2004) with a Bremer decay index ( Bremer, 1988) of 17.

Phylogenetic relationships within Lithoxini ( Fig. 3): Molecular data strongly support monophyly of Lithoxini (node 15: BI, 1.0; ML, 100) and its inclusion of the respectively strongly monophyletic genera Exastilithoxus (node 13: BI, 1.0; ML, 100) and Paralithoxus (node 8: BI, 1.0; ML, 100) and the monotypic genera and species L. lithoides and Avalithoxus jantjae gen. nov. Relationships among these lineages were weakly supported, albeit topologically consistent. Both Bayesian and maximum likelihood analyses found weak support for A. jantjae to be sister to a clade containing all other species (node 14: BI, 0.85; ML, 27), and within this latter clade Exastilithoxus was consistently found to be sister to a clade in which L. lithoides was sister to Paralithoxus (node 9: BI, 0.88; ML, 31).

Atthebasalnodewithin Exastilithoxus , anundescribed species from the Cuao River in southern Venezuela was found to be sister to a strongly supported clade (node 12: BI, 1.0; ML, 100) containing E. fimbriatus , from headwaters of the Caroni River on the eastern Venezuelan Gran Sabana plateau, and three undescribed lineages from tributaries of the upper Orinoco in southern Venezuela. Intriguingly, E. fimbriatus was strongly supported as sister (node 10: BI, 1.0; ML, 100) to an undescribed lineage from the far upper Ventuari River. Both these species originate from streams atop the Guiana Shield escarpment that are> 450 km apart from east to west ( Fig. 2). Despite this disjunct distribution and great distance, these lineages exhibited only ~2.4% Cytb sequence divergence. The two remaining, undescribed Exastilithoxus lineages, respectively from the Iguapo and Soromoni rivers, were also strongly monophyletic (node 11: BI, 1.0; ML, 100). The Iguapo and Soromoni rivers are lower-elevation tributaries that enter the right bank of the upper Orinoco River upstream of the Casiquiare Canal and have mouths that are <25 river km apart ( Fig. 2). Despite the geographical proximity of these localities and the hydrological connectivity of these habitats via the Orinoco River main channel, the Iguapo and Soromoni rivers drain opposite (east–west) sides of Cerro Duida [elevation, 2358 m above sea level (a.s.l.)], and their Exastilithoxus populations also exhibit ~2.4% Cytb sequence divergence. All Exastilithoxus lineages were restricted to drainages west of the Essequibo River watershed ( Fig. 2).

At the basal node within Paralithoxus , P. planquettei (from the Oyapock River along the border between French Guiana and Brazil) was sister to a strongly supported clade (node 7: BI, 1.0; ML, 100) containing species distributed from the Maroni (French; Dutch: Marowijne) River in the northeast to the Ireng River, which is> 650 linear km to the west of the Maroni, and the Maicurú River, which is> 650 linear km to the south of the Maroni ( Fig. 2). Within this latter clade, a clade containing Paralithoxus stocki morphotypes from the geographically distant and disjunct Maroni and Maicurú rivers was found to be strongly monophyletic (node 6: BI, 1.0; ML, 100), albeit with ~4.9% Cytb sequence divergence between lineages. The P. stocki clade was found to be sister to a strongly supported clade (node 5: BI, 1.0; ML, 94) containing all remaining species. Within this last clade, a clade of P. pallidimaculatus morphotypes from the Saramacca and Marowijne drainages in Suriname was sister to a strongly supported clade (node 3: BI, 1.0; ML, 97) containing P. bovallii morphotypes. Within the P. bovallii clade, a lineage from the Coppename River in Suriname was sister to a strongly supported clade (node 2: BI, 1.0; ML, 100) containing an undescribed species from the Konawaruk River in Guyana, and the strongly supported clade (node 1: BI, 1.0; ML, 100) containing P. bovallii topotypes from the Ireng River and P. sp. nov. aff. bovallii from the Courantijne River. The undescribed Courantijne River lineage exhibited ~3% Cytb and 1.1% ND2 sequence divergence from topotypic P. bovallii . The undescribed Konawaruk River species exhibited ~2.8% ND2 sequence divergence from both topotypic P. bovallii and the undescribed species from the Courantijne (Cytb was not obtained from the Konawaruk lineage).