Polygnathus dubius Hinde, 1879

1879 Polygnathus dubius n. sp. - Hinde, pag. 362-364, Plate XVI, fig. 6-18.

Original diagnosis - The only example discovered in which the teeth of this remarkable form are grouped together has been crushed to such an extent that the individual teeth and plates can be only partially distinguished; but the various kinds are met with in a very perfect condition, as so many separate specimens, scattered through the rock. As these detached teeth occur not only in the rock in which the grouped specimen is found, but are widely distributed even in Lower Carboniferous rocks, I append descriptions and figures of the individual teeth and plates.

For the convenience of reference I refer the teeth to pectinate, fimbriate, and crested forms.

1. Pectinate teeth.—Of these the kind figured in PL XVI. figs. 6-9 has a narrow, slightly arched base and a main tooth which is sometimes produced below the level of the base; the secondary teeth are slender and acute, and vary from 14 to 20 in number. This kind averages about I line in length, and is abundant in the " Conodont-bed " of the Hamilton group as well as in the Genesee Shale at North Evans.

Another variety of pectinate compound teeth, shown in Pl. XVI. figs. 10, 11, 12, has the base straight and almost linear ; there is no distinctive central tooth, but a series of similarly shaped teeth, sometimes as many as 14, of which the central ones are the longest. Occasionally smaller denticles are intercalated. The base of these forms is about 3/4 line in length, and the longest teeth from to line long. This variety is very widely distributed. It has been described by Pander under the name of Centrodus simplex (' Monograph.' p. 31, tab. 2 A. figs. 2, 3, 5, 6), from the Lower Carboniferous in Russia ; it occurs in the same formation at Bedford, Ohio, and also appears in Genesee Shale at Kettle Point, Ontario, as well as at North Evans, New York.

2. Fimbriate teeth (Pl. XVI. figs. 13, 14). The base of these compound teeth is straight, narrow, elongate, and pointed at one extremity; near the opposite end is a delicate, needle-like main tooth with three smaller teeth on one side between it and the end of the base ; on. the other side, extending to the pointed tip of the base, is a series of very numerous, extremely minute denticles, appearing like a fringe on the upper border of the base. In some examples these small denticles are nearly uniform in size, in others every fourth tooth is larger; but there exists considerable variation in this respect, even in the same specimen. In the example figured (fig. 13), of which the base is 15 line in length, there are 70 of the small denticles. This form is very abundant and quite as widely distributed as the preceding. It appears to be identical with the fragmentary tooth named by Pander Centrodus lineatus (' Monographie,' p. 31, tab. 2 A. fig. 9), from the Lower Carboniferous in Russia ; it is in the same formation at Bedford, Ohio, and is also found at Kettle Point and Bear Creek, Ontario, and at North Evans, New York, in the Genesee Shale.

3. Crested teeth (Pl. XVI. figs. 15, 16, 17).—Of these there are two varieties present in Polygnathus dubius. The first (fig. 15) has the base compressed and nearly of equal width, save at one end, which is abruptly contracted. There are about twenty small teeth or crenulations on the base. The second variety (figs. 16, 17) has one part of the base narrow and thickened, with sometimes a row of minute crenulations on its upper edge ; beyond this the base forms a small, flattened, crest-like expansion with from 5 to 8 teeth on its border. Both these varieties are closely allied to the form named by Pander Gnathodus mosquensis (' Monographie,' p. 24, tab. 2 A. figs. 10 a, b, c). The second is the more abundant of the two, and three or four individuals can be distinguished in the crushed example of Polygnathus.

The small plates associated with the teeth in Polygnathus dubius are of an elliptical form with smooth edges (Pl. XVI. fig. 18). One surface is slightly convex, with a slight longitudinal median ridge ; the surface, as well as the ridge, is covered with small tubercles frequently with a linear arrangement ; the reverse side of the plate is smooth, with faint traces of concentric lines ; the two ends are slightly elevated, and there is a median ridge with a small diamond-shaped pit in or near the centre of the plate. Each plate is about 5/8 line long and 3/8 line wide. Six of these plates, but all apparently of the same form, can be distinguished in the specimen of Polygnathus.

Besides the teeth already referred to, there are fragments of others too imperfect to be recognized, and these may possibly belong to some forms which, occurring as detached specimens, I have described under other names.

The existence of such a variety of teeth and plates in this single example appears to make the question of the affinities of the organism to which they belonged still more complex. Great numbers of teeth compose the lingual ribbon of many mollusks ; but in none, that I am aware of, is there a similar variety of form, nor are there any bodies analogous to the tuberculated plates. Nor in existing Myxinoids, to whose teeth the Conodonts are comparable, are similar plates present. Whatever the animal may have been, it appears to have had a very wide distribution both in Devonian and Carboniferous times.

Occurrence - Genesee Shale, Hamilton Group, Kettle Point (U. S. A.).

Age range - Devonian (Hinde, 1879).

References:

- Hinde, G. J. (1879) - On conodonts from the Chazy and Cincinnati Group of the Cambro-Silurian, and from the Hamilton and Genesee-Shale divisions of the Devonian, in Canada and the United States. Geological Society of London, Quarterly Journal, vol. 35: 351-369p.