BRIEF COMMUNICATION The occipital region in the basal bony fish Erpetoichthys calabaricus (Actinopterygii: Cladistia) K. M. CLAESON*† AND J. W. HAGADORN‡ *Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, 1 University Station C1100, Austin, TX 78712-0254, U.S.A. and ‡Department of Geology, Amherst College, Amherst, MA 01002, U.S.A. (Received 9 September 2006, Accepted 26 June 2008) Erpetoichthys calabaricus has unusual cranio-vertebral anatomy, with an occipital centrum forming a component part of the compound basiexoccipital bone, and a ‘free-floating’ occipital neural arch that differs from accessory arches found in some teleosts. The occipital neural arch bears autapomorphic lateral projections that articulate with small rod-like bones resembling the spatial relationship of parapophyses and ribs, a feature normally restricted to vertebral centra. Based on analyses of cleared and stained specimens, computed tomography and histology, it is hypothesized that the lateral projections and associated rod-shaped bones are structures that share developmental homologies to the unique ‘dorsal ribs’ of Polypteridae. # 2008 The Authors Journal compilation # 2008 The Fisheries Society of the British Isles Key words: horizontal septum; intermusculars; myosepta; Polypterus. Polypteridae is the basalmost extant lineage of Actinopterygii and includes the ropefish Erpetoichthys calabaricus Smith, 1865 and bichirs, Polypterus Lacepe` de, 1803 (Patterson, 1982; Schultze & Cumbaa, 2001; Venkatesh et al., 2001; Nel- son, 2006). Anatomical studies of ropefish and bichirs can offer important in- sights for the systematics and morphology of living and fossil bony fishes. Anatomical details of many living polypterids remain elusive, though hypoth- eses of a generalized bauplan for the family exist based on studies of the cranial anatomy of several species of Polypterus (Allis, 1922; de Beer, 1935; Jollie, 1984; Britz & Johnson, 2003) and E. calabaricus (Claeson et al., 2007). By integrating observations in this study from clearing and double staining (C&S), X-radio- graphic microfocus computed tomography (CT), histology and dissection, an un- usual aspect of neural arch morphology in E. calabaricus is recognized. Analysis of this neural arch morphology contributes to evolutionary interpretations of †Author to whom correspondence should be addressed. Tel.: þ1 512 471 5172; fax: þ1 512 471 9425; email: kclaeson@mail.utexas.edu Journal of Fish Biology (2008) 73, 1–8 doi:10.1111/j.1095-8649.2008.02007.x, available online at http://www.blackwell-synergy.com 1 # 2008 The Authors Journal compilation # 2008 The Fisheries Society of the British Isles 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 B J F B 2 0 0 7 Journal Name Manuscript No. Dispatch: 2.8.08 Journal: JFB CE: Vidhya Author Received: No. of pages: 8 ME: Nagalakshmi