Collette and Gillis: Osteological differences between two species of Grammatorcynus 



17 



205 



200 290 380 470 560 o\ : : 



FL , mm 



Figure 4 



Length of first dorsal fin base (BID) compared with 

 forl< length (FL) in Grammatorcynus. Open circles = 

 G. bilineatus, squares = G. bicarinatus. 



widening posteriorly. It is intermediate in shape be- 

 tween the elongate neurocranium of Acanthocyhium, 

 Scomber, and Rastrelliger , and the shorter, wider 

 neurocranium of Thunnus. The posterodorsal surface 

 is marked by a median ridge (supraoccipital crest), with 

 two parallel ridges on either side. These five thin ridges 

 of bone form six grooves, three on each side: dilator 

 (very shallow), temporal (quite deep), and supratem- 

 poral (most easily seen in lateral view) (Allis 1903:49). 



The median ridge originates just posterior to the thin, 

 oval pineal foramen located between the posterior, 

 median edges of the frontal bones. This ridge becomes 

 larger posteriorly, and forms the supraoccipital crest. 

 Internal or temporal ridges originate at the posterior 

 portion of the frontals (midlevel of the orbit), continu- 

 ing posteriorly to the epiotic. External or pterotic 

 ridges also originate near the posterior margin of the 

 frontals, continuing posteriorly to the pterotic. 



Neurocrania of the two species of Grammatorcynus 

 differ in size, relative to fork length. Length of the 

 neurocranium, measured from the anterior tip of the 

 vomer to the posterior margin of the basioccipital, is 

 slightly longer in G. bilineatus (14-16% FL) than in 

 G. bicarinatus (13% FL). 



Ethmoid region This region is composed of the 

 ethmoid, lateral ethmoid, and vomer. The nasal bone 

 lies lateral to the ethmoid and lateral ethmoid, and, 

 therefore, is included here. 



Ethmoid The ethmoid (dermethmoid) has a 

 smooth flat dorsal surface that is partially overlapped 

 by the frontals. It connects ventrally to the vomer, 

 posteriorly to the lateral ethmoids, and anterolateral- 

 ly to the nasals. Its anterior border is nearly straight, 

 with an anteromedian projection, unlike the relatively 

 smooth, concave border in Scomberomorus and Acan- 

 thocybium. The ethmoid is clearly visible in dorsal view 

 (Fig. 6), and is wider, relative to the length of the 

 neurocranium, in G. bicarinatus (width 25-28% of 

 length) than in G. bilineatus (19-21%). 



Lateral ethmoid The lateral ethmoids (pareth- 

 moids) are massive, paired bones that extend down- 

 ward from the middle region of the frontals and form 

 the anterior margin of the orbit and the posterior and 

 mesial walls of the nasal cavity. The ventral surface 

 of the lateral ethmoid bears an articulating surface for 

 the palatine, and the posterolateral process serves as 

 an articulation surface for the lachrymal. The lateral 

 expansion of the bone is greater in G. bicarinatus 

 (45-50% of neurocranium length) than in G. bilineatus 

 (39-42%) (Fig. 8). 



Vomer The anterior process of the vomer bears 

 a circular or oval patch of fine teeth on its ventral sur- 

 face. Its pointed posterior end is firmly ankylosed dor- 

 sally with the parasphenoid. The anterior process is 

 wider in G. bicarinatus (16-18% of neurocranium 

 length) than in G. bilineatus (13-15%) (Fig. 8). 



IMasal The nasal bones are flat, elongate bones 

 that articulate with the lateral edge of the frontals. 

 They project out beyond the ethmoid and, from a dor- 

 sal view, reach about as far anteriorly as the vomer. 

 There is no such projection of the nasal bones in 

 Scomberomorus or Acanthocybium. Length divided by 

 width is 2.8-3.4 in Grammatorcynus, which is inter- 

 mediate between the ranges oi Scomberomorus (2.0- 

 3.1) and Acayithocybiuyn (3.1-4.2). The anterior end 

 of the bone forms a short, slightly angled arm. No 

 differences were found between the nasals of the two 

 species of Grammatorcynus. 



Orbital region The orbit (Fig. 7) is surrounded 

 by the posterior wall of the lateral ethmoid, the ven- 

 tral side of the frontal, the pterosphenoid, sphenotic, 

 prootic, suborbital, and lachrymal bones. The left and 

 right orbits are partially separated by the basisphenoid. 

 The sclerotic bones enclose the eyeballs. 



The orbit of G. bilineatus is larger than that of 

 G. bicarinatus (Fig. 7), reflecting the difference in orbit 

 length (Fig. 3). The maximum height of the orbit 

 measured from the parasphenoid to the pterosphenoid 

 is 24-25% of neurocranium length in G. bilineatus vs. 

 16-17% in G. bicarinatus. Orbit length in G. bilineatus 

 is 51-54% of neurocranium length vs. 47-49% in 

 G. bicarinatus. 



