Paracanthopoma, Giltay, 1935

Pinna, Mário de & Dagosta, Fernando Cesar Paiva, 2022, A taxonomic review of the vampire catfish genus Paracanthopoma Giltay, 1935 (Siluriformes, Trichomycteridae), with descriptions of nine new species and a revised diagnosis of the genus, Papéis Avulsos de Zoologia 62, pp. 1-90 : 81-82

Monophyly of Paracanthopoma + Paravandellia

A monophyletic group composed of Paracanthopoma plus Paravandellia was originally proposed by DoNascimiento (2012). The clade is strongly corroborated by morphological evidence. It has not yet been tested by molecular data because none of the studies done so far have simultaneously included representatives of the two genera. The section below lists all characters herein identified and/or scrutinized as to their potential evidence of monophyly for the clade Paracanthopoma + Paravandellia . One of the characters proposed by DoNascimiento (2012) for this clade, the loss of the fourth ceratobranchial, is here considered as a synapomorphy for Paravandellia only (see section above on that genus).

19 Distribution of scalpelloid teeth restricted to distal end of premaxilla – Scalpelloid teeth are a highly unusual feature of Vandelliinae , present in all members of the subfamily.They have been referred to in the literature as "claw-like teeth″ and consist of a combination of anatomical modifications. The tip of the teeth is scythe-like and strongly bent relative to the tooth base. Also one of the sides of the basal portion of the tooth is expanded into a large plate extending in the same direction as the distal portion. The sinusoidal jaw teeth of stegophilines and tridentines may include elements similar to those of vandelliines and in Pareiodon (Stegophilinae) the general morphological similarity is expressed also in an equivalent plate-like expansion.While homologies of the various modifications composing the vandelliine scalpelloid dentition to similar features in close relatives is debatable, their distribution on the premaxilla shows two well-defined conditions. In Vandellia and Plectrochilus , the scalpelloid teeth are distributed along a relatively long stretch of the ventrolateral surface of the premaxilla. In Paracanthopoma and Paravandellia , contrastingly, the scalpelloid teeth are positioned at the distal end of the premaxilla ( Figs. 10B, C , 30B, C , 46B , notice that in last illustration, there are numerous partly-formed replacement teeth, but a single attached tooth). The latter condition is considered as apomorphic relative to the former because in other trichomycterids and catfishes in general, the premaxillary teeth are usually distributed along a wide portion of the premaxilla, rather than restricted to a small terminal area of the bone. The tooth distributions here considered refer only to attached teeth (i.e., their sockets),not replacement tooth caps, which may be quite numerous and broadly spread out in soft tissues of the labial bursa.

20 Presence of large, pointed, post-articular process of anguloarticular, directed straight laterally and projecting beyond lateral limits of anterior portion of suspensorium and jaw skeleton: In all species of both Paracanthopoma and Paravandellia , the lower jaw has a large process directed straight laterally, projecting to or beyond the lateral limits of the suspensorium and remainder of the jaw skeleton in ventral view ( Figs. 7B, C , 17B, C , 46B, C ). This structure is a hypertrophied dorsal post-articular process of the anguloarticular and is not homologous to the coronoid process. Despite topological relations and general position somewhat similar to the laterally-deflected coronoid process in Plectrochilus , Vandellia , Acanthopoma and Ochmacanthus , the process in Paracanthopoma and Paravandellia is a different structure, formed exclusively from anguloarticular material dorsal to its articulation with the quadrate. No part of that process is derived from coronoid process elements. Corroboration of that comes from the presence of an independent vestigial anguloarticular portion of the coronoid processes in a few taxa which also have the post-articular process (e.g., Pc. alleynei , Fig. 10A, B ). Some variations of detail exist in the morphology of the process, e.g., in Paravandellia the process is distally shallowly bifid ( Fig. 46C ), and in species of the Pc.parva -clade (see below), it is distally obliquely truncate ( Figs. 17B, C , 19B, C , 30B, C , 41B, C ).

21 Palatine ring-like, with large fenestra occupying most of its central area: The palatine in trichomycterids and other loricarioids is most often a continuous bone. Uniquely in Paracanthopoma and Paravandellia , the palatine ossification is composed mostly of its margins, leaving the central portion as a large well-delimited unossified fenestra ( Figs. 15B , 26B , 30B , 43B , 46B ). A few other vandelliines, such as some species of Vandellia and Plectrochilus diabolicus , have a fenestra on the palatine lamina. In those cases, however, the fenestra is poorly-delimited, off-center and clearly a result of gradually fading calcification.While it is possible that those conditions share homologous elements with the situation in Paracanthopoma and Paravandellia , (a view expressed in DoNascimiento, 2012: 125, character 198), the extreme condition in the latter two genera, with the fenestra being a well-delimited central feature which determines the shape and structure of the palatine, is unique. Within Paracanthopoma the degree of ossification varies markedly, with some species (e.g., Pc. carrapata , Pc. parva , Pc. saci , Pc. truculenta ) having a thick-framed palatine ( Figs. 17 , 30 , 34 , 41 ) and others (e.g., Pc. irritans , Pc. vampyra ) with the palatine ossification very thin, nearly thread-like ( Figs. 23 , 43 ). In all cases, however, the central fenestra is larger and more well-defined than in any other vandelliines outside of the Paracanthopoma + Paravandellia clade.

22 Fourth basibranchial absent: Among the various reductions of the branchial skeleton in vandelliines, basibranchial elements 2 and 3 are always absent (basibranchial 1 is generally absent in catfishes). Species of Vandellia and Plectrochilus retain one basibranchial element in the form of an elongated cartilage positioned between ceratobranchials 3 and 4, probably corresponding to basibranchial 4. Species of Paravandellia and Paracanthopoma exclusively share a loss of that single remaining element ( DoNascimiento, 2012) and thus lack any differentiated basibranchial elements.

23 Ascending process of cleithrum absent: The cleithrum in trichomycterids in general has a well-defined finger-like dorsal process which forms a major component of the biomechanical link between the pectoral girdle and the skull. The process inserts dorsally into a small orifice formed between the supracleithrum, Weberian capsule and often the epioccipital. Uniquely in Paracanthopoma and Paravandellia , this process is vestigial or entirely absent ( DoNascimiento, 2012). The contact between the cleithrum and the skull is instead implemented by a direct articulation with the ventral portion of the Weberian capsule and the basi-exoccipital complex. As part of such modifications, the corresponding orifice in the skull is also absent.

24 Presence of an elongated cartilage-like structure horizontally between the coronoid region of the lower jaw and the premaxilla: DoNascimiento (2012) described a condition exclusive to Paracanthopoma and Paravandellia where the coronomeckelian cartilage of the lower jaw extends anteriorly to the posterior region of the premaxilla, adjacent to the base of the maxillary and rictal barbel cores. Our observations confirm that there is a unique alcian-blue dense structure in that shape and position in all species of those two genera. It seems, however, that the structure is contiguous but not continuous with the coronomeckelian cartilage. We are also uncertain whether it is actually constituted of cartilaginous tissue.In our specimens it seems to be composed of a bursa filled with some loose material with affinity for alcian-blue stain, superficialy similar to barbel cores. We identified a probably homologous small roundish structure close to, but independent of, the meckelian cartilage in specimens of Vandellia and Plectrochilus examined. This structure is yet poorly understood and certainly requires further anatomical and comparative study for proper description. However, its shape, size and position in Paracanthopoma and Paravandellia are indeed unique and likely represent an additional synapomorphy for the two taxa.

Figures

Figure 7. Paracanthopoma ahriman, holotype FMNH 105525,CT scan images of head skeleton,(A) Lateral;(B) Dorsal;(C) Ventral.Specimen poorly calcified,some structures not properly shown.

Figure 10. Paracanthopoma alleynei, MZUSP 103052,CT scan images of head skeleton,(A) Lateral;(B) Dorsal;(C)Ventral.

Figure 15. Paracanthopoma capeta, holotype MZUSP 29154, CT scan images of head skeleton, (A) Lateral; (B) Dorsal; (C) Ventral. Specimen poorly calcified,some structures not properly shown.

Figure 17. Paracanthopoma carrapata, holotype MZUSP 100145, CTscan images of head skeleton,(A) Lateral;(B) Dorsal;(C) Ventral.

Figure 19. Paracanthopoma daemon, holotype,MZUSP 103047, CTscan images of head skeleton,(A) Lateral;(B) Dorsal;(C) Ventral.

Figure 23. Paracanthopoma irritans, holotype,MZUSP 126815,CT scan images of head skeleton,(A) Lateral;(B) Dorsal;(C) Ventral.Specimen poorly calcified,some structures not properly shown.

Figure 26. Paracanthopoma malevola, holotype,INPA 59836, CTscan images of head skeleton,(A) Lateral;(B) Dorsal;(C) Ventral.

Figure 30. Paracanthopoma parva, CAS 118205,Brazil,Amazonas,upper Rio Catrimani.CT scan images of head skeleton,(A) Lateral;(B) Dorsal;(C) Ventral.

Figure 34. Paracanthopoma saci, holotype, MZUSP 125624, CT scan images of head skeleton, (A) Lateral; (B) Dorsal; (C) Ventral. Specimen poorly calcified, some structures not properly shown.

Figure 41. Paracanthopoma truculenta, holotype,MZUSP_30399,CT scan images of head skeleton,(A) Lateral;(B) Dorsal;(C) Ventral.

Figure 43. Paracanthopoma vampyra, holotype,MZUSP 86953,CT scan images of head skeleton,(A) Lateral;(B) Dorsal;(C)Ventral.

Figure 46. Paravandelliaphaneronema, MCZ 35874,CT scan images of head and anterior portion of body,(A) Lateral;(B) Dorsal;(C) Ventral.