Clarification of the Occipito-Vertebral Region of Arapaima gigas (Osteoglossomorpha: Osteoglossidae) through Developmental Osteology

The structure and composition of the ventral portion of the occipital region of the skull in Arapaima gigas (Osteoglossidae) is unique among teleostean fishes, and past comparative anatomical studies have variously interpreted it as containing only the basioccipital or the basioccipital fused to one or two vertebral centra. By studying an ontogenetic series, we show that the dominant element of the ventral occipital region of the skull in Arapaima is the first vertebral centrum and its greatly enlarged parapophyses. The parapophyses, which become fused to the centrum, extend anteriorly to suture to the lateral portions of the parasphenoid. In the adult, the anterior portion of the basioccipital is flattened, with a narrow ventrally directed keel of bone that is exposed ventrally only along the midline of the skull. Although a general enlargement of the anteriormost parapophyses appears to be a synapomorphy of the family Osteoglossidae, their arrangement in other osteoglossids does not closely resemble that described herein for Arapaima.

Figure 1

The occipital region of Arapaima gigas at early stages of development, bone stained red, cartilage stained blue. (A) Lateral view; parapophysis unossified (INPA 22779; 19.5 mm SL). (B) Lateral view (INPA 22785; 27.5 mm SL). (C) Ventral view (ZFMK 1006–1028; 29.7 mm SL). (D) Ventral view (ZFMK 948–965; 63 mm SL). (E) Lateral view; parapophysis reaches far anterior but remains autogenous from the centrum (FMNH 97450; 56.5 mm SL). (F) Lateral view; parapophysis fused to centrum (UMA F11261; 145 mm SL). Arrows point to the first parapophysis; small arrowheads point to more posterior parapophyses. Note that Baudelot's ligaments and the myosepta are not clearly visible in this figure. Anterior to left (images in A, B, E, F reversed). Abbreviations: boc, basioccipital; c1, centrum 1; exo, exoccipital.

Figure 2

Partially disarticulated occipital region of Arapaima gigas showing the typical arrangement of occipital bones in adult specimens (FMNH 99287; est. 1525 mm SL). (A, B) Dorsal view. (C, D) Lateral view. (E, F) Ventral view. This specimen was illustrated also by Lundberg and Chernoff (1992:fig. 3). Anterior facing left. Abbreviations: boc, basioccipital; c1, centrum 1; pas, parasphenoid; paspr, posterior process of parasphenoid ( = basioccipital process of Hilton, 2003); pp, parapophysis.

Figure 3

The occipital region of Arapaima gigas showing fusion between the basioccipital and first centrum (FMNH 99286; est. 1500 mm SL); compare with Fig. 2. This is the only specimen examined in which these bones were found to be fused. (A) Ventral and (B) lateral views. Anterior facing left. Scale bars = 10 mm.

Contributor Notes

(EJH) Geology Department, Field Museum, 1400 South Lake Shore Drive, Chicago, Illinois 60605; (RB) Department of Zoology, The Natural History Museum, Cromwell Road, London, England, SW7 5BD; (GDJ) Division of Fishes, National Museum of Natural History, Smithsonian Institution, Washington D.C. 20560, and (PLF) Department of Palaeontology, The Natural History Museum, Cromwell Road, London, England, SW7 5BD. E-mail: (EJH) ehilton@fieldmuseum.org; (RB) R.Britz@nhm.ac.uk; (GDJ) johnsond@si.edu; (PLF) plf@nhm.ac.uk

Received: 03 Feb 2006

Accepted: 29 Aug 2006