The Extraordinary Occipito-Vertebral Skeleton and Swim Bladder of South American Hypophthalmus Catfishes (Siluriformes: Pimelodidae): Improved Illustration, Description, and Interpretation
The pimelodid catfish genus Hypophthalmus has an unusually modified Weberian apparatus, reduced swim bladder, and associated bones of the occiput and pectoral girdle. Using high resolution X-ray tomography and traditional skeletal preparations of adult and juvenile specimens, we reexamined and illustrated this occipito-vertebral complex. Comparisons were made to the plesiomorphic architecture of the complex exhibited by phylogenetically deep pimelodid genera. Hypophthalmus has the swim bladder reduced to a pair of separate bilobed sacs, each doubly encapsulated by outer and inner layers of bone. The anterior vertebrae and occipital bones are shortened and contorted with apparent loss or fusion of the first centrum and development of novel joints between the basioccipital and complex vertebra. Two pairs of the Weberian ossicles are lost: claustra and intercalaria. The posttemporal-supracleithrum is reshaped and solidly united to a recurved arm of the 4th parapophysis with both extended far posteriorly to brace the outer swim bladder capsule and articulate with the cleithrum. The extrascapula is found in juveniles and apparently fuses with the adjacent pterotic. The third supraneural is present between the supraoccipital and complex vertebra. We point out conflicting results and interpretations among four previous studies and our treatment of the occipito-vertebral complex and swim bladder of Hypophthalmus. Based on unambiguous morphological and molecular evidence, Hypophthalmus belongs to the sorubimine clade of the family Pimelodidae. In this evolutionary context the morphology of the occipito-vertebral complex and swim bladder of Hypophthalmus is distinctly divergent from that of other sorubimines. The complex of Hypophthalmus is also distinct from the partially encapsulated swim bladders of the Calophysus and Megalonema clades. The morphology of the occipito-vertebral complex and swim bladder of Hypophthalmus is uniquely autapomorphic.
Hypophthalmus marginatus, left side lateral view of ANSP 187103, Suriname, Paramaribo, Surinam River. Photo by Mark Sabaj Pérez.
Osteology of the skull, anterior vertebrae, and posttemporal-supracleithrum of Hypophthalmus. (A) Hypophthalmus edentatus, ANSP 192939, dry skeleton, dorsal view, anterior to left. (B–D) Hypophthalmus n. sp. 1, ANSP 180621, 3-D HRXCT volume rendered models of the posterior part of the skull, anterior vertebrae, and posttemporal-supracleithrum, anterior to left. (B) Dorsal, (C) ventral, (D) lateral. Abbreviations: ac, aortic canal; ag, aortic groove flanked by parasagittal ridges; boc, basioccipital; boc-eoc-otl, basioccipital-exoccipital-ossified transcapular ligament joint; boc ms, basioccipital median sulcus; C5, 5th centrum; C6, 6th centrum; epf, epaxial muscle fossa; epo, epioccipital; icsb, inner capsule of swim bladder; na3, 3rd neural arch; na4, 4th neural arch; na6, 6th neural arch; otl, ossified transcapular ligament; pcf, posterior cranial fontanelle; pp6, 6th parapophysis; pp4-ptt-scl-cl, 4th parapophysis-posttemporal-supracleithrum-cleithrum joint; pto, pterotic; pto pp, pterotic posterior process; ptt-scl, posttemporal-supracleithrum; ptt-scl clp, posttemporal-supracleithrum cleithral process; ptt-scl ptop, posttemporal-supracleithrum pterotic process; ptt-scl vp, posttemporal-supracleithrum vertebral process; r, rib; sn3, 3rd supraneural; soc k, supraoccipital vertical keel; soc pp, supraoccipital posterior process; vr ocsb, vertical ridge of outer swim bladder capsule.
Osteology of the skull, anterior vertebrae, and posttemporal-supracleithrum of Steindachneridion scripta. (A) MZUSP 78463, dry skeleton, dorsal view, anterior to left. (B–D) ANSP 168862, 3-D HRXCT volume rendered models of the posterior part of the skull, anterior vertebrae, and posttemporal-supracleithrum, anterior to left. (B) Dorsal, (C) ventral, (D) lateral. Abbreviations: ag, aortic groove; boc, basioccipital; boc-C1, basioccipital-1st centrum joint; boc-eoc-otl, basioccipital-exoccipital-ossified transcapular ligament joint; dfr1, dorsal-fin 1st radial; eoc, exoccipital; ext, extrascapula; na4, 4th neural arch; os, os suspensorium; otl, ossified transcapular ligament; pp4, 4th parapophysis; pp4 a, 4th parapophysis anterior limb; pp4 p, 4th parapophysis posterior limb; pp4-scl, joint between 4th parapophysis and posttemporal-supracleithrum; pp5, 5th parapophysis; pp6, 6th parapophysis; pr, parasagittal ridges flanking aortic groove below complex and 5th centra; ptow, pterotic wing; ptt-scl, posttemporal-supracleithrum; ptt-scl clp, posttemporal-supracleithrum cleithral process; ptt-scl ptop, posttemporal-supracleithrum pterotic process; ptt-scl vp, posttemporal-supracleithrum vertebral process; sb, outline of swim bladder; sn4, 4th supraneural; soc, supraoccipital; tr, tripus; v for, vagus nerve foramen.
Hypophthalmus and Steindachneridion, 3-D HRXCT volume rendered models of the posterior part of the skull, anterior vertebrae, and posttemporal-supracleithrum, anterior to left. Hypophthalmus n. sp. 1, ANSP 180621. (A) Dorsal view of horizontal section through occiput and Weberian apparatus. (B) Lateral view of sagittal section at midline with insert of a single midsagittal CT slice through basioccipital-vertebral joint, Steindachneridion scripta, ANSP 168862. (C) Dorsal view of horizontal section through occiput and Weberian apparatus. (D) Lateral view of sagittal section at midline. Abbreviations: ac, aortic canal; ag, aortic groove; boc, basioccipital; boc con-cc con, dashed line along basioccipital-vertebral joint; c, claustrum; CC, complex centrum, pointer on centrum’s midpoint; C1, 1st centrum; C5, 5th centrum, pointer on centrum’s midpoint; C6, 6th centrum, pointer on centrum’s midpoint; eoc br, exoccipital bridge; f mag, foramen magnum; icsb, inner capsule of swim bladder; na3, 3rd neural arch; na4, 4th neural arch and spine; nc, neural canal; nl, neural lamina; ocaw, outer capsule anterior wall; ocpw, outer capsule posterior wall; sb, swim bladder outline; sc, scaphium; si, cavum sinus impar; sn3, 3rd supraneural; soc k, supraoccipital vertical keel; tr, tripus.
Species of Phractocephalus and Hypophthalmus, detail of basioccipital and anterior vertebrae. (A) Phractocephalus hemioliopterus, ANSP 197129, dry basioccipital, dorsal view, anterior to top. (B) Hypophthalmus n. sp. 1, ANSP 197124, dry basioccipital, dorsal view, anterior to top. (C) Same, matching Weberian complex vertebra, anterior view. The basioccipital condyle and basioccipital paired parasagittal ridges of B articulate with the complex centrum condyle and complex centrum paired parasagittal ridges of C. Abbreviations: ac, aortic canal; boc con, basioccipital condyle; boc pr, basioccipital right-side parasagittal ridge; cc con, complex centrum condyle; cc pr, complex centrum left-side parasagittal ridge; con p, occipital condyle middorsal process; na3, 3rd neural arch; ocsb d, roof of outer capsule of swim bladder; ped, basioccipital pedestal; pp4 af, 4th transverse process anterior fenestra; pp4 facet, 4th transverse process facet; sac, sagitta chamber.
Hypophthalmus edentatus, juvenile size series showing osteology of the skull, anterior vertebrae, and posttemporal-supracleithrum, anterior to left. (A–D) ANSP 197126. (A1, A2) 15 mm SL in lateral and dorsal views; (B1, B2, B3) 30 mm SL in lateral, dorsal, and ventral views; (C1, C2, C3) 43 mm SL in lateral, dorsal, and ventral views; (D1, D2, D3) 62 mm SL in lateral, dorsal, and ventral views; (E1, E2) ANSP 197127, 110 mm SL in dorsal and ventral views. Abbreviations: ag, aortic groove; boc plp, basioccipital posterior lateral process; C5, 5th centrum; ext, extrascapula; icsb, inner capsule of swim bladder; lfdr, lateral fenestra dorsal rim; na6, 6th neural arch; ns5, 5th neural spine; ocsb d, roof of outer capsule of swim bladder; pcf, posterior cranial fontanelle; pp5, 5th parapophysis; sb, swim bladder; sn3, 3rd supraneural; soc k, supraoccipital vertical keel; tr, tripus.
Contributor Notes
Associate Editor: G. Arratia.