Part 4

Numerous isolated basal plates of the pelvic girdle have revealed details of structure but no information on the orientation. Presumably the basal plates of Synaptotylus had essentially the same orientation as those of other coelacanths (Moy-Thomas, 1937:395). The most complete basal plate is K. U. no. 788 (see fig. 6B). The three apophyses diverge anteriorly; the horizontal one is best developed and the dorsal one is least well developed. A median process (Schaeffer, 1952:49), denticulate on several specimens, articulates with the corresponding process of the opposite plate. The expanded part that articulates with the skeleton of the fin extends caudally. The posterior expanded part is nearly square in outline, resembling the dorsal, rectangular projection. One side bears ridges leading to the extremities of the apophyses, and faint crenulations on the median process. This may be the medial view. The other view displays a smooth surface, usually without indication of the ridges seen in the reverse view. These specimens differ somewhat from the basal plates of Rhabdoderma and appear to be intermediate between Rhabdoderma and Coelacanthus (Moy-Thomas, 1937:fig. 10A, B). The apophyses are not free as in Rhabdoderma but webbed with bone almost to their extremities, as in Coelacanthus.

The pelvic fin is seen in only two specimens (K. U. nos. 786F, 788). That on no. 788 is lobate and has 25 lepidotrichia, jointed for approximately the distal half, and 2.5 to 13.0 mm. in length. Total length of the fin is 25.0 mm. There is no trace of the internal skeletal structure or of the articulation to the basal plate in either specimen. For a description of the fin on no. 786F, see Hibbard (1933:281).

Unpaired fins

A few isolated bones on K. U. no. 788 (fig. 7) are interpreted as basal plates of the unpaired fins. For additional description of the unpaired fins on the type, K. U. no. 786F, see Hibbard (1933).

Two of these bones are flat, smooth and oblong, bearing a diagonal ridge that extends in the form of a projection. Orientation is completely unknown. These may be basal plates of the anterior dorsal fin. The fin on no. 786F that Hibbard (1933:281) interpreted as the posterior dorsal fin is now thought to be the anterior dorsal fin.

One distinctive bone may represent the basal plate of the posterior dorsal fin. This incomplete specimen shows two projecting curved processes, bearing low but distinct ridges, which diverge, probably anteriorly. The central portion is narrow. The two ridges continue onto the posterior portion. This has been broken off, but shows that the ridges diverge again. The surface is smooth, except for the ridges. As before, orientation is uncertain. On no. 786F this fin was interpreted by Hibbard (1933:281) as the anal fin.

Only part of one basal plate of the anal fin was preserved on K. U. no. 11450. That plate is oblong and has an expanded anterior end. The narrow, constricted part bears two oblique ridges and a few tubercles. The posterior part has nearly straight margins (represented by impressions) and the posterior margin is oblique, sloping anteroventrally. The flared anterior part has a smooth surface. This basal plate is more nearly like those of Coelacanthus, according to the descriptions given by Moy-Thomas (1937:399). The basal plate is associated with seven apparently unjointed, incomplete lepidotrichia. The anal fin on no. 786F is interpreted as the anterior dorsal fin (Hibbard, 1933:281).

The caudal fins are preserved on K. U. nos. 786F, 787F, and have a total of 24 lepidotrichia, 12 above and 12 below. These are jointed for the distal half or two-thirds, and are up to 16.0 mm. in length. In specimen no. 787F the supplementary caudal fin has at least seven lepidotrichia, the longest of which is 11.0 mm. but incomplete. Anterior lepidotrichia appear unjointed but the posterior ones are jointed for the distal two-thirds (?) (these are broken off). The supplementary caudal fin is approximately 1.5 mm. long and 8.0 mm. or more wide. The supplementary caudal fin on K. U. no. 786F described by Hibbard (1933:281) could not be observed; this part of the caudal fin is missing.

Squamation

In the suite of specimens isolated scales are numerous, but patches of scales are rare. Only two specimens (K. U. nos. 786F, 787F) are complete enough for scale counts, but preservation permits only partial counts. In general the scales resemble those of Rhabdoderma elegans (Newberry).

The scales are oval. The exposed posterior part of each bears posteriorly converging ridges; the anterior part is widest and shows a fine fibrillar structure. There are at least six scale-rows on either side of the lateral line. Lateral line scales show no pores, and except for slight irregularities in the orientation and length of the posterior ridges, closely resemble the others. Central ridges on the lateral line scales are shorter and tend to diverge from the center of the impression of the canal. The lateral line canal shows only as the impression of a continuous canal 0.7 mm. in diameter. Preservation is poorest in scales along the line of the neural and haemal arches; therefore lateral line scales are rarely preserved. Isolated scales are of two types: those on which the posterior ridges converge sharply and form the gothic arch configuration mentioned by Hibbard (1933:282), and those which do not. Both types of scales can be present on one fish, as shown by specimen no. 788. This is not apparent on nos. 786F and 787F; all of the scales on these specimens appear to be much alike. Both Moy-Thomas (1937:385) and Schaeffer (1952:51, 52) have remarked on the variation of the scales on different parts of the same fish. Because the number of ridges and amount of convergence of the ridges is not related to size of the scale, it is concluded that these characters are not of taxonomic significance.

The strong resemblance of the scales of the Garnett specimens to those of Rhabdoderma elegans (Newberry) caused Moy-Thomas (1937:399) to add Hibbard's two species to the synonymy of R. elegans. But at that time only the scales could be adequately described. If the shape of the scale and the number and pattern of ridges can vary with age, size and shape of the scale, it follows that assignment of isolated scales to a species should not be attempted. Assignment to genus should be made only with caution.

Discussion.--The relationship of Synaptotylus to other coelacanths is obscure at present. The knoblike antotic processes on the basisphenoid are unlike those of any other known coelacanth. The palatoquadrate complex is shaped like that of Rhabdoderma elegans but consists of fewer bones, probably because of fusion. The scales resemble those of Rhabdoderma. With regard to general shape of fin girdles, the pectoral girdle resembles that of Eusthenopteron more than that of Rhabdoderma, but the cleithrum is more nearly like the cleithrum of Rhabdoderma. The pelvic girdle appears to be midway between those of Rhabdoderma and Coelacanthus in general appearance. Regarding the basal plates of the remaining fins, those of Synaptotylus appear to resemble basal plates of both Rhabdoderma and Coelacanthus. Considering the structure of the sphenethmoid region of the braincase, Synaptotylus is probably more closely related to Rhabdoderma than to other known coelacanth genera.

COMMENTS ON CLASSIFICATIONS

Classification of Carboniferous coelacanths has been difficult, partly because the remains are commonly fragmentary, and significant changes in anatomy did not become apparent in early studies. In general, coelacanths have been remarkably stable in most characters, and it has been difficult to divide the group into families. As Schaeffer (1952:56) pointed out, definition of coelacanth genera and species has previously been made on non-meristic characters, and the range of variation within a species has received little attention. For example, Reis (1888:71) established the genus Rhabdoderma, using the strong striation of the scales, gular plates and posterior mandible as the main characters of this Carboniferous genus. Moy-Thomas (1937:399-411) referred all Carboniferous species to Rhabdoderma, redescribed the genus and compared it to Coelacanthus, the Permian genus. He cited as specific characters the ornamentation of the angulars, operculars and gular plates (Moy-Thomas, 1935:39; 1937:385). Individual variation in some species has rendered ornamentation a poor criterion. This variation is apparent in Synaptotylus newelli (Hibbard), some specimens having little or no ornamentation; others having much more. The number of ridges and pattern of ridges on the scales also varies. Schaeffer (1952:56) has found this to be true of Diplurus also. Moy-Thomas (1935:40; 1937:385) realized that the type of scale is a poor criterion for specific differentiation. In the search for features useful in distinguishing genera of coelacanths, Schaeffer and Gregory (1961:3, 7) found the structure of the basisphenoid to be distinctive in known genera, and thought it had taxonomic significance at this level. Higher categories should have as their basis characters that display evolutionary sequences. A recent classification (Berg, 1940), followed in this paper, reflects two evolutionary trends in endocranial structure of coelacanths: reduction of endocranial ossification and loss of the basipterygoid processes. Because there has been little change in other structures in coelacanths, Berg's classification is the most useful. Berg (1940:390) includes Rhabdoderma in the suborder Diplocercidoidei because of the presence of the basipterygoid processes, and in the single family, Diplocercidae, but remarks that because of the reduced amount of endocranial ossification the Carboniferous Diplocercidae "probably constitute a distinct family." In considering this concept of classification, the subfamilies Diplocercinae and Rhabdodermatinae of the family Diplocercidae are proposed above. The subfamily Rhabdodermatinae includes at present Rhabdoderma and Synaptotylus. The principal characters of the subfamily Rhabdodermatinae, named for the first known genus, are the retention of the basipterygoid processes and the reduction of endocranial ossification. Application of this classification based upon endocranial structure would probably change existing groupings of species of Carboniferous coelacanths; the entire complex of Carboniferous genera should be redescribed and redefined. It will be necessary to consider endocranial structure in any future classification.

The greater part of the evolution previously mentioned appears to have been accomplished during the Carboniferous; thereafter coelacanth structure became stabilized. The trend progressed from Devonian coelacanths which had two large unpaired bones in the endocranium, and both antotic and basipterygoid processes on the basisphenoid, to Carboniferous fishes in which ossification was reduced to a number of paired and unpaired bones embedded in cartilage, and retaining both processes, and then post-Carboniferous kinds with reduced ossification and no basipterygoid processes. The Pennsylvanian was evidently the time of greatest change for the coelacanths, and they have not changed significantly since, in spite of the fact that since the Jurassic they have shifted their environment from shallow, fresh water to moderate depth in the sea (Schaeffer, 1953:fig. 1). The changes in endocranial structure appear to be significant, and are perhaps related to higher efficiency of the mouth parts in catching and swallowing prey (see p. 482).

ENVIRONMENT

The coelacanth fishes from the Rock Lake shale are part of the varied fauna collected from Garnett. Peabody (1952:38) listed many elements of the fauna and flora, and concluded that the deposits are of lagoonal origin. In addition to numerous invertebrates (including microfossils) and arthropods, a number of vertebrates other than coelacanths have been found. These include at least one kind of shark, Hesperoherpeton garnettense Peabody, one or more kinds of undescribed labyrinthodonts and the reptiles Petrolacosaurus kansensis Lane, Edaphosaurus ecordi Peabody, and Clepsydrops (undescribed species). This is indeed a rich vertebrate fauna, and the earliest known reptilian fauna. Much of the rock contains plant remains. The flora that has been identified is adapted to growing in a well-drained soil; although it contains some elements considered characteristic of the Permian, it is of Pennsylvanian age (Moore et al., 1936). Peabody (1952:38-39) discusses the features of these lagoonal sediments. Much of the fauna and flora suggests continental origin, but the many marine invertebrates at some horizons indicate that at least some of the sediments were of marine origin.

Little can be said about the actual environment of the living fishes of the genus Synaptotylus. Remains of these fishes occur in layers containing marine invertebrates, as well as in those containing plant remains and vertebrate skeletal parts, and in those nearly completely composed of dark carbonaceous material. Most of the remains are fragmentary and consist of isolated bones, isolated scales, and dissociated skulls; only one specimen and half of another are nearly complete. Many published statements on Rhabdoderma, a related genus, indicate both marine and fresh-water environments. Wehrli (1931:115) regarded Rhabdoderma elegans (Newberry) as a euryhaline species, and cited its occurrence with both marine and fresh-water fossils. Aldinger (1931:199) also found this to be the case with other species, and Fiege (1951:17) quotes others as giving the same information. Keller (1934:913) thought that few Carboniferous fishes were exclusively marine, and stated that the majority of them became adapted to fresh water during the late Carboniferous. Later, Schaeffer (1953:175) stated that all Carboniferous and Permian coelacanths were fresh-water fishes, and that many were from swamp deposits. If Keller is correct, then members of the genus Synaptotylus may have inhabited the lagoon, the adjacent sea, or the streams draining into the lagoon. Perhaps these fishes swam upstream, as modern salmon and tarpon do, although there is no direct evidence for this. Possibly they lived in the lagoon at times of scant rainfall and little runoff, when the salinity of lagoon water approached normal marine values or the fishes may have lived in the streams, and after death were washed into the lagoon. As numerous remains of land plants and animals were washed in, perhaps this best accounts for the presence of the fish in nearly all layers of the deposits, not only the marine strata.

SUMMARY

A new genus of Pennsylvanian coelacanths, Synaptotylus, is described and a previously named species, Coelacanthus newelli Hibbard, 1933 (C. arcuatus Hibbard, 1933, is a junior synonym), is referred to this genus. All specimens of Synaptotylus newelli (Hibbard) were collected from the Rock Lake shale member of the Stanton formation, Lansing group, Missouri series, six miles northwest of Garnett, Anderson County, Kansas. Synaptotylus is distinguished from all other coelacanths by a basisphenoid having large, knoblike antotic processes each connected by a low ridge to a small basipterygoid process. Synaptotylus is most closely related to Rhabdoderma, but is intermediate between Rhabdoderma and Coelacanthus in shape of the fin girdles and basal plates. Two new subfamilies, Diplocercinae and Rhabdodermatinae, of the family Diplocercidae, are proposed. Synaptotylus and Rhabdoderma are included in the subfamily Rhabdodermatinae, because both exhibit reduced ossification in the endocranium and retain basipterygoid processes.

Loss of the basipterygoid processes in post-Carboniferous coelacanths may reflect the development of a more efficient feeding mechanism, by allowing the palatoquadrate complex and mandible to swing farther laterally and expand the oral cavity.

Synaptotylus newelli (Hibbard) may have occupied either the sea or fresh water; these fishes occur in lagoonal deposits with reptiles and amphibians, arthropods, marine invertebrates and remains of land plants.

Because scale patterns on Synaptotylus and Rhabdoderma are so nearly similar and vary with size of the scale and its location on the fish, it is recommended that isolated scales not be assigned to a species, and to a genus only with great caution.

LITERATURE CITED

ALDINGER, H.

1931. Ueber karbonische Fische aus Westfaelen. Paleont. Zeit., 13:186-201.

BERG, L. S.