PIETSCH: OSTEOLOGY AND RELATIONSHIPS OF TETRABRACHIUM 



Rostrum 



Figure 



Sulcus 



-Medial view of right sagitta of Tetrahrachium 

 occllatum.AMS IB.7178, 61 mm SL. 



grooved. The rostrum is poorly developed, and an 

 antirostrum is absent. 



Mandibular arch (Figures 8, 9). — The premax- 

 illae (Figure 8) are each characterized by having a 

 narrow ascending process, nearly as long as the 

 tapering toothed portion of the bone; a rounded 

 articular process; and an elongate, spatulate post- 

 maxillary process (pmpmx of Rosen and Patterson 

 1969, fig. 56A). The ascending and articular pro- 

 cesses together form an oblique angle with the 

 postmaxillary and toothed processes. The toothed 

 portion of each premaxilla bears a single row 

 of 22 to 25 depressible teeth, the largest at 

 the symphysis, becoming progressively smaller 

 posteriorly. 



Each maxilla consists of a broad posterior por- 

 tion (completely hidden from behind by a thick 

 fold of skin when the mouth is closed), and an 

 expanded anterior head that, in turn, consists of 

 an anterior process that overlaps the respective 

 premaxilla and a medially directed process that is 

 attached by a short ligament to the articular 

 process of the respective premaxilla. The den- 

 taries, articulars, and angulars (Figure 9) are 



Maxilla 



Premaxilla 



Figure 8. — Upper jaw bones of Tetrabrachium ocellatum , AMS 

 IB.7178, 61 mm SL. AP = anterior process of maxilla; ARP = 

 articular process of premaxilla; ASP = ascending process 

 of premaxilla; MP = medial process of maxilla; PMP = post- 

 maxillary process of premaxilla; PP = posterior process of 

 maxilla. 



similar to those described for other lophiiforms 

 (Gregory 1933, fig. 265, 266, 269-271; Pietsch 1972, 

 1974). Each dentary bears approximately 35 de- 

 pressible teeth arranged in two rows. 



Palatine arch (Figure 9). — Each metapterygoid is 

 in contact with four other bones: dorsally and 

 posterodorsally with the hyomandibular, postero- 

 ventrally with the upper half of the symplectic, 

 and ventrally with the quadrate and ectoptery- 

 goid. The ectopterygoid is large and T-shaped, 

 overlapping the medial surface of the metaptery- 

 goid dorsally, the quadrate ventrally, and the 

 palatine anteriorly. The mesopterygoid (cartilag- 

 inous or ossified) is absent. The palatine is un- 

 usually large, approximately twice the length of 

 the ectopterygoid. Palatine teeth are absent. 



Hyoid arch (Figures 9, 10). — Dorsally, each hyo- 

 mandibular is forked forming two heads, both of 

 which articulate with the cranium: an anterior 

 head fits into a concavity formed by the sphenotic 

 and prootic, and a posterior head articulates on 

 the ventrolateral face of the pterotic (Figures 4, 5, 

 9). The symplectic is separated from the hyoman- 

 dibular by cartilage dorsally, and lies within a 

 shallow groove on the medial surface of the 

 quadrate ventrally. The dorsal head of the quad- 

 rate is narrow, considerably less than the width of 

 the metapterygoid. The interhyal bears a prom- 

 inent medial, posterolaterally directed process 

 that wraps around the posterior margin of the 

 respective preopercle when the interhyal rotates 

 upward (e.g., during a feeding event). This contact 

 between the interhyal and the preopercle limits 

 the dorsal rotation of the interhyal and, in turn, 

 limits the extent of abduction of the lower jaw 

 via ligamentous connections to the respective 

 interopercle. 



The epihyal and ceratohyal do not differ sub- 

 stantially from those described for other lophi- 

 iforms (Pietsch 1974, 1979). There are two hypo- 

 hyals on each side (Figure 10), both of which 

 are connected to the ceratohyal by a posteriorly 

 directed strut. The dorsal hypohyal is further 

 connected to an anterodorsal extension of the 

 ceratohyal by a cylindrical piece of cartilage. 



There are six branchiostegal rays all borne on 

 the ceratohyal (Figure 10); the two anteriormost 

 rays articulate on the medial surface, the four 

 posterior rays on the lateral surface. Branchios- 

 tegal rays 3 and 4 are curved in an anteroventral 

 direction, in contrast to the posterodorsal direc- 



393 



