Part 2

Members of this subfamily differ from those of the subfamily Diplocercinae in having several paired and unpaired elements in the sphenethmoid region of the endocranium, instead of only one larger ossification. They differ from those of the suborder Coelacanthoidei in the retention of basipterygoid processes.

Synaptotylus is more closely related to Rhabdoderma than to the Diplocercines because the anterior portion of the endocranium contains only a basisphenoid, parasphenoid, and probably ethmoids. The sphenethmoid region was certainly not a large, unpaired unit as in the Diplocercines. Probably the posterior part, the otico-occipital region (not known in Synaptotylus), was much more nearly like that of Rhabdoderma, which consisted of unpaired basioccipital and supraoccipital, and paired prootics, exoccipitals, and anterior and posterior occipital ossifications (Moy-Thomas, 1937: figs. 3, 4). Moy-Thomas (1937:389) points out that in Rhabdoderma the occipital region is "considerably more ossified" than in any coelacanths other than the Devonian forms. Berg (1940:390) thought that the Carboniferous coelacanths should be placed in a separate family because they did not have two large, unpaired bones in the endocranium. Rhabdoderma and Synaptotylus represent another stage in evolution of the endocranium in coelacanths, and, if classification is to be based on endocranial structure, then this stage (represented by the two genera) may later be given family rank as Berg suggested. Because Rhabdoderma and Synaptotylus have only part of the sphenethmoid region ossified and because they retain basipterygoid processes, they are considered to be related and are included in the subfamily Rhabdodermatinae.

~Synaptotylus~, new genus

Type species.--Synaptotylus newelli (Hibbard).

Horizon.--Rock Lake shale member, Stanton formation, Lansing group, Missouri series, Upper Pennsylvanian.

Diagnosis.--Late Pennsylvanian fishes of small size, having the following combination of characters: on basisphenoid, knoblike antotic processes connected by a low ridge to basipterygoid processes; entire ventral surface of parasphenoid toothed; anterior margin of parasphenoid notched and no evidence of hypophyseal opening. Dermal bones of skull smooth or with low, rounded tubercles and striae; fronto-ethmoid shield incompletely known but having one pair of large rectangular frontals with posterolaterally slanting anterior margins; intertemporals large, the lateral margins curving laterally; postorbital triangular, apex downward; subopercular somewhat triangular; squamosal carrying sensory canal that curves down posteriorly and extends onto a ventral projection; opercular generally triangular; supratemporals elongate, curving to fit lateral margin of intertemporals; circumorbital plates lightly ossified. Palatoquadrate complex consisting of endopterygoid and ectopterygoid (both toothed on medial surface), quadrate, and metapterygoid, the latter smooth and having widened border for articulation on anterodorsal margin. Pectoral girdle consisting of cleithrum and clavicle (supracleithrum not seen); small projection on medial surface of posterior portion of cleithrum; horizontal medial process on clavicle. Pelvic plate bearing three anteriorly diverging apophyses, and one denticulate ventromedian process for articulation to opposite plate. Lepidotrichia jointed distally, but not tuberculated. Scales oval, having posteriorly converging ridges on posterior exposed parts.

The name refers to the most distinctive character of the genus, the connected antotic and basipterygoid processes on the basisphenoid, and is derived from Greek, synaptos--joined, tylos (masc.)--knob, projection.

Synaptotylus is excluded from the advanced suborder Coelacanthoidei by the retention of basipterygoid processes on the basisphenoid. Synaptotylus differs from Rhabdoderma in several characters of the basisphenoid, the most important being: knoblike antotic processes (those of Rhabdoderma are wider, more flattened and more dorsal in position); small, lateral basipterygoid processes (in Rhabdoderma these are larger and farther ventral in position).

~Synaptotylus newelli~ (Hibbard)

Coelacanthus newelli Hibbard, 1933, Univ. Kansas Sci. Bull., 21:280, pl. 27, figs. 2, 3.

Coelacanthus arcuatus Hibbard, 1933, Univ. Kansas Sci. Bull., 21:282, pl. 26, fig. 8; pl. 27, fig. 1.

Rhabdoderma elegans Moy-Thomas, 1937 (in part), Proc. Zool. Soc. London, 107(ser. B, pt. 3):399.

Type.--K. U. no. 786F.

Diagnosis.--Same as for the genus.

Horizon.--Rock Lake shale member, Stanton formation, Lansing group, Missouri series, Upper Pennsylvanian.

Localities.--The specimens studied by Hibbard (K. U. nos. 786F, 787F, 788) and no. 11457 were taken from the Bradford Chandler farm, from the original quarry in SW-1/4, SE-1/4, sec. 32, T.19S, R.19E. The remainder were collected from University of Kansas Museum of Natural History locality KAn-1/D, a quarry in sec. 5, T.19S, R.19E. Both of these are approximately six miles northwest of Garnett, Anderson County, Kansas.

Referred specimens.--K. U. nos. 786F, 787F, 788, 9939, 11424, 11425, 11426, 11427, 11428, 11429, 11430, 11431, 11432, 11433, 11434, 11449, 11450, 11451, 11452, 11453, 11454, 11455, 11457.

Preservation.--Preservation of many of the specimens is good, few are weathered, but most of the remains are fragmentary and dissociated. One specimen (the type, no. 786F) and half of another were nearly complete. Specimens are found scattered throughout the Rock Lake shale (see p. 498).

Morphology.--Terminology used for bones of the skull is that of Moy-Thomas (1937) and Schaeffer (1952).

Endocranium and parasphenoid

The basisphenoid (see fig. 1) has been observed in only one specimen (K. U. no. 9939) in posterodorsal and ventral views. The basisphenoid, although somewhat crushed, appears to be fused to the parasphenoid. Both antotic and basipterygoid processes are present, and are connected by a low, rounded ridge. The antotic processes are large, bulbar projections. These processes in Rhabdoderma are wider and more flattened (Moy-Thomas, 1937:figs. 3, 4). The antotic processes are at mid-point on the lateral surface, not dorsal as in Rhabdoderma, and both the processes and the ridge are directed anteroventrally. The basipterygoid processes are smaller, somewhat vertically elongated projections, situated at the end of the low connecting ridge extending anteroventrally from the antotic processes, and are not basal as are those of Rhabdoderma. The sphenoid condyles, seen in posterior view, issue from the dorsal margin of the notochordal socket. The margins of the socket are rounded, and slope down evenly to the center. A slight depression situated between and dorsal to the sphenoid condyles is supposedly for the attachment of the intercranial ligament (Schaeffer and Gregory, 1961:fig. 1). The alisphenoids extend upward, anterodorsally from the region above the sphenoid condyles, and may connect to ridges on the ventral surface of the frontals. The lateral laminae are not preserved, and their extent is unknown.

In viewing the changes in the endocranium of Carboniferous and Permian coelacanths, it would be well to consider the mechanical relationship of the loss of the basipterygoid processes to the effect on swallowing prey. Evidently many of the coelacanths, Latimeria for example, are predators (Smith, 1939:104); to such fishes a more efficient catching and swallowing mechanism would be an adaptive improvement. Stensioe (1932:fig. 14) presents a cross section of the ethmosphenoid moiety of the endocranium of Diplocercides kayseri (von Koenen) showing the metapterygoid of the palatoquadrate loosely articulated to both the antotic and basipterygoid processes. According to Tchernavin (1948:137) and Schaeffer and Rosen (1961:190) the swallowing of large prey depends on the ability of the fish to expand its oral cavity by allowing the posteroventral portion of the palatoquadrate and the posterior end of the mandible to swing outward. Where the palatoquadrate articulates with the basisphenoid at the antotic and basipterygoid processes, as in the Devonian coelacanths, it can not swing so far laterally as where it articulates with only the dorsal, antotic process. Perhaps the loss of the basipterygoid articulation reflects the development of a more efficient mechanism for swallowing prey in these fishes. Schaeffer and Rosen (1961:191, 193) show that in the evolution of the actinopterygians several changes improved the feeding mechanism: some of these changes are: (1) freeing of the maxilla from the cheek, giving a larger chamber for the action of the adductor mandibulae; (2) development of a coronoid process on the mandible; and (3) increase in torque around the jaw articulation. In coelacanths, at least some comparable changes occurred, such as: (1) loss of the maxillary, thus increasing the size of the adductor chamber; (2) development of the coronoid bone, affording a greater area for muscle attachment; (3) development of an arched dorsal margin on the angular; (4) modification of the palatoquadrate complex, with resultant loss of the basipterygoid processes. In Synaptotylus the basipterygoid processes are small, not basally located, and perhaps not functional. A more efficient feeding mechanism developed rapidly during the Carboniferous and has remained almost unaltered.

The parasphenoid (see fig. 2) is a shovel-shaped bone having a wide anterior portion and a narrower posterior portion of nearly uniform width. Most of the ventral surface is covered with minute granular teeth. The anterior margin is flared and curved posteromedially from the lateral margin to a median triangular projection. The lateral margins curve smoothly from the greatest anterior width to the narrow central portion, where the margins become somewhat thickened and turned dorsally. Posterior to this the lateral margins are probably nearly straight. The external surface of the anterior section is nearly flat and has a central depressed area the sides of which slope evenly to the center. The internal surface is smooth and centrally convex. Because of the fragmentary nature of all four observed specimens, total length was not measured but is estimated to be 15 to 20 mm. The opening of the hypophyseal canal was not present, possibly because of crushing. Ethmoidal ossifications were not preserved in any of the specimens studied. The parasphenoid differs from that of Rhabdoderma elegans (Newberry) in being more flared and widened anteriorly and more concave centrally.

Dermal bones of the skull

Various portions of the cranial roof are preserved in several specimens (see fig. 3). For comparisons with Rhabdoderma elegans, see Moy-Thomas (1937:fig. 1).

The premaxillaries and rostral elements are not preserved in any of the specimens. Only one pair of relatively large frontals have been observed; they are 5.5 to 9.0 mm. long and 2.0 to 3.5 mm. wide. These are nearly flat bones, with the greatest width posteriorly 0.1 to 1.0 mm. wider than the anterior portion. The midline suture is straight, the lateral margins are nearly straight, the anterior margin slopes evenly posterolaterally, and the posterior margin is slightly convex to straight. The anterior margin in R. elegans is essentially straight. Ornamentation consists of sparse, unevenly spaced, coarse tubercles or short striae. In one specimen both bones have small clusters of tubercles near the lateral margins and about 2.0 mm. from the posterior margin. None of these bones has alisphenoids or ridges on the ventral surface, as Stensioe (1921:65, 97) described for Wimania and Axelia.

Only six supraorbitals have been preserved (see fig. 3). These are nearly square, flat, thin bones lying nearly in place adjacent to a frontal on K. U. no. 788. The smallest is anterior; the margins of all are nearly straight. The bones are unornamented. Each bears a pore of the supraorbital line just below the midline. The supraorbitals of R. elegans have a triangular outline and do not bear pores.

Intertemporals (fig. 3) on several specimens vary from approximately 9.0 to 15.0 mm. in length, 2.0 to 2.7 mm. in anterior width, and increase to 4.5 to 8.0 mm. in maximum posterior width. The midline suture is straight, the anterior margin is concave and the lateral margin proceeds laterally in a concave curve to the widest portion. In R. elegans only the anterior half of the corresponding margin is concave. The posterior margin is slightly rounded and slopes anteriorly toward the lateral margin. Ornamentation is usually of randomly oriented tubercles and striae, although striae are more common in the posterior third and may be longitudinal, whereas tubercles occur mainly on the anterior section. No evidence of sensory pores, as seen on the intertemporal of R. elegans, has been found.