The Teeth of an Extinct Whale, Microcetus hectori n. sp.
[Read before the Otago Institute, November 11, 1934; received by the Editor, March 12, 1935; issued separately, December, 1935.]
(With 13 figures.)
A Block of soft friable limestone presenting a jaw bearing a row of teeth has long been on exhibition in the Dominion Museum, and is labelled by Sir James Hector as follows:—
Kekenodon Onamata. Hector.
Jaw showing teeth.
From Maerewhenua Limestone,
(Geol. form VIb)
The specimen was found by McKay * during his exploration of the geology of this district, and he thus writes of its discovery (p. 104): “Besides these [remains of Kekenodon] there was obtained from the Otakaika limestone or middle part of the Maerewhenua limestone, part of the skull and lower jaws of a young animal, in which are a number of teeth still in position in the jaw, besides several others scattered through the matrix.” This limestone is attributed by McKay to the Upper Eocene, but is usually considered nowadays as being of Oligocene age.
I wish to take this opportunity of expressing my indebtedness to the Director, Dr. F. W. B. Oliver, for his courtesy in allowing me to examine this and other remains of fossil whales in the museum, and for very generously sending this valuable unique specimen to Dunedin for more detailed study by me. He also most kindly gave me permission to excavate the bone so as to expose the roots of the teeth.
The jaw is wrongly identified; it is certainly not that of a young Kekenodon, for apart from the size, which is a great deal smaller than in that huge whale, the shape of the teeth is quite different. Indeed, it belongs to the rare genus Microcetus Kellogg as I will show below.
A. The Teeth.
The margin of the jaw bears six teeth fairly close together, and in addition a tooth of different shape lies embedded in the matrix at the fractured end of the jaw bone, which tooth does not show from the side. These six teeth are the molars; the enamel covering the crown is brown, polished, and smooth; the crown is triangular
[Footnote] * A. McKay, Rept. Geol. Expl. N.Z. Geol. Surv., 1881.
and compressed laterally, with rounded apex, and on the posterior edge or margin is provided with two low, round-topped, conical cusps (Fig. 1). The enamel is split vertically along its lower border, and fine cracks are traceable for some distance upwards. There are, however, no ridges or other markings on the surface.
Each tooth is supported by a two-fanged root and a slight furrow extends upwards from the base of the crown in continuity with the groove separating the two fangs, which are united in their upper region, but are distinct for the greater part of their length. I was able to expose most of the root, but not the whole extent of the fangs, for I wished to avoid doing damage unnecessarily to the specimen, nor did it seem of much importance to measure the length of the root.
On the four posterior molars the accessory cusps are well defined, but on the two anterior teeth they are much less pronounced, being reduced to mere convexities, which are readily overlooked (Fig. 2).
In the squalodonts the number of molars is somewhat variable in different species, but it must be borne in mind that only in a few species have the entire series of teeth been preserved. The upper jaw contains three Incisors, one Canine, five Premolars, and either five or six or even seven Molars, i.e., the Dental formula for this jaw is 3.1.5.(5 or 6 or 7). It is, of course, impossible to state how many molars actually occur in this jaw. I did not expose the root of either of the two anterior teeth, but assume that as the base of the crown presents the same vertical groove running up from the root, it has two fangs as in the posterior teeth; that in fact is usually the case.
The anterior extremity of the jaw is broken across, and in the matrix at the fracture is a solitary tooth, smaller than the others and of simpler structure; it has a smooth conical crown, which is bluntly pointed and slightly curved, one side being slightly concave. The axis of this tooth is at right angles to the other teeth. The crown is directed backwards and the root inwards, indicating, of course, considerable displacement (Fig. 3). This tooth may be an incisor which has been forced backwards; or it may be one of the premolars. Against the latter view is the fact that in the Squalodonts, though the crown of a premolar is simpler, usually without cusps, and fairly large, the root is, in the case of the upper teeth, generally double, the two fangs being closely united and the line of union indicated by a groove. On the other hand, the root of a canine or an incisor is more or less curved—sometimes much curved. I think that this tooth is not a canine, for its comparatively slender form negatives that view. The straightness of the root seems against its being an incisor, and I am inclined to regard it as one of the premolars, perhaps one of the anterior teeth of this series, since the duplicity of the fangs is not evident.
This Cetacean was then heterodont, as are all the Squalodonts, as opposed to the extant Odontocetes.
Of the Molars—The height of the crown of the 3rd molar is 9 mm., its antero-posterior length 8 mm.
Of the Premolar—Total length, 17 mm.; but it is imperfect.
Height of crown, 7 mm.
Diameter at base, 5 mm.
The six molars are spaced as follows:—
|1st–2nd||8 mm. (end of jaw seems distorted).|
The 5th tooth leans against the 6th, which hides its posterior edge.
B. The Jaw.
The fossil is the posterior region of the maxilla of the left side.
The fossil presents a flattish bone bent along its long axis so that one surface lies flush with the stone. This is the upper or dorsal surface: the other face which is bounded by the alveolar or dentigerous border forms an angle of about 45° with the former face and is the lower or palatal surface of the bone. The upper surface (Fig. 4) is broader posteriorly where its margin is rounded, and tapers anteriorly to the point where it is broken across. The length of the upper face is 23.5 cm. and its greatest width 4.3 cm.
The outer margin is straight and rounded, bending inwards, but is crushed. This is the line along which the maxilla met the premaxilla. At its anterior end, where its fractured surface permits inspection, this edge bends inwards almost horizontally towards the alveolar border, forming a rather thick curved plate (Fig. 7).
The lower or palatal surface of the bone (Figs. 5, 6) slopes downward from the outer edge forming a slightly convex area which as it approaches the alveolar border becomes somewhat depressed so as to form a broad shallow furrow; the surface then rises up so as to produce the dentigerous ridge or alveolar border. The length of this face, as preserved in the fossil, is less than the other face, being but 16.5 cm., as it has been broken across not only anteriorly, but also posteriorly. Its height at the level of the 3rd tooth is 2.5 cm. and its greatest height behind the last tooth is 4 cm.
The six molars occupy a length of 7 cm., and the dentigerous ridge is prolonged backwards for a space of 3 cm. beyond the last tooth.
By means of a series of drawings to scale representing transverse sections through the maxilla at different levels, I have attempted to show the general features of this bone. Naturally, it is only possible to express the external appearance, for as the bone is embedded in the matrix its thickness cannot be determined.
Fig. 7 is taken at the anterior fractured end, and thus shows the true form and dimensions of the maxilla, and the isolated tooth.
Fig 8 is taken at the level of the last molar.
Fig. 9 is behind the last tooth.
Fig. 10 is near the posterior end.
It is astonishing that the maxilla should have become separated from the rest of the skull, and at first I supposed that the fossil
was a mandible, but was soon assured from its form and relations that it is a maxilla; and a comparison between this bone in a Squalodont and in modern Odontocetes indicates how it is that it was possible for it to become detached from the skull.
I made transverse section across the rostrum of Globicephalus and of Cephalorhynchus, and drawings to scale of such a section of Prosqualodon, of which I have two skulls. In the latter genus, at any rate in the species before me (though apparently not in all species, for S. calvertensis does not show this peculiarity), the outer margin of the maxilla extends well beyond the alveolar border, so that the teeth are overhung by it, like the eave of a house-roof (Fig. 11), but in modern Odontocetes the teeth are set along the outer margin of the bone.
Another difference became apparent, and that is the way in which the premaxilla makes contact with the maxilla. As will be seen in the section through Globicephalus (Fig. 13) the under-surface of the premaxilla is much curved and thrown into a number of small undulations, and the villi so produced fit into pits in upper surface of the maxilla, forming a very intimate union between the two bones. In Cephalorhynchus (Fig. 12) the union is less complete, the premaxilla having a simple ridge which fits into a groove on the maxilla. Now in Prosqualodon (Fig. 11) the premaxilla merely rests upon the maxilla; there is no interdigitation, no interlocking of the bones as far as can be seen in the two fossil skulls in my collection, though it is rather difficult in the case of that from Caversham to be certain of this owing to the infiltration of the petrifying mineral, yet in the skull from Milton the arrangement is quite clear. Consequently, assuming that the same condition holds in Microcetus, the two bones would easily separate from one another, a thing that would be impossible in Globicephalus and very unlikely in Cephalorhynchus.
The only genus of extinct cetaceans to which these teeth bear any resemblance is the genus Microcetus, of which there is but one species, M. ambiguus, imperfectly described by Meyer as Phoca ambigua in 1840, and later referred to this genus by Kellogg (p. 15) in his summary of the characteristics of all previously-described Squalodonts.*
I have not access to Meyer's original description, but according to Kellogg, who gives the history of the fossil on which the genus is founded, it has “Two one-rooted premolars, and two two-rooted molars.” It is the only genus listed by Kellogg in his “key” to the characteristic dentition of the family in which accessory cusps are present only on the posterior margin of the tooth (p. 39). The type species “was found in the Tertiary gravel of the Osnabruck basin, near Bünde, Oldenburg, Germany. Upper Oligocene.” It is consequently of about the same age as this New Zealand fossil.
In the type species the enamel presents “coarse striae at the base of the crown”; the teeth of the present species show only quite
[Footnote] * R. Kellogg, Description of Two Squalodonts recently discovered in the Calvert Cliffs, Maryland; and Notes on the Shark-toothed Cetaceans, Proc. U.S. Nat. Mus., vol. 62, Art. 16, 1923.
Note—Figures 1–10 represent Microcetus hectori.
Fig. 1.—The third molar (x 2).
Fig. 2.—The second molar (x 2).
Fig. 3.—The simple tooth (? premolar) at fractured end (x 2).
Fig. 6.—A drawing of the anterior end of the actual fossil (half nat. size); shows the shape of the six molars
Figs. 7–10.—Imaginary sections across the maxilla at different levels (of the natural dimensions).
Fig. 7.—The fractured end of the actual bone.
Fig. 8.—Section at level of the last molar tooth.
Fig. 9—Behind the last tooth.
Fig. 10.—Near the hinder end of the fossil.
Fig. 11.—Transverse section of the rostrum of Prosqualodon at level of last tooth, showing the overlapping eave of the maxilla, and the simple form of contact with the premaxilla.
Fig. 12.—Section across the rostrum of Cephalorhynchus hectori with the teeth close to the margin of the jaw.
Fig. 13.—Similar section across the rostrum of Globicephalus. In this whale the teeth are limited to the lower jaw. Face p. 242.
Fig 4—Photograph of the cast of the fossil (halt nat size). Upper surface of the maxilla; the anterior end does not come out clearly owing to the difference of level
fine striae, which indeed appear to be postmortem, as there are delicate cracks passing upwards from the base.
In Meyer's specimen only two of the molars were present, the four (or more) hinder ones being lacking.
Kellogg does not say whether the premolars have accessory cusps. As Kellogg writes on p. 5: “The absence of accessory cusps on the anterior cutting edge of the molars and the shape of those on the posterior edge shows clearly that the form is quite unlike those specimens referred to the genus Neosqualodon. Since it does not belong to any previously described genus, it may hereafter be known as Microcetus ambiguus (Meyer).” And in footnote on p. 39 he remarks that “the molars of Microcetus are quite unlike those of the other forms grouped under the Microzeuglodontidae. Perhaps they should be allocated to the Squalodontidae.”
Whether Microcetus has 7 molars we cannot, of course, determine, for the anterior end of the maxilla has been broken immediately in front of the sixth. That this genus does belong to the family Squalodontidae is evidenced by the form and extent of the maxilla, for in the family the Zeuglodontidae, the maxilla is a more massive bone; it exhibits a foramen on its facial surface, which is more nearly upright rather than flattened, and forms an angle approaching a right angle with the alveolar or palatal surface; in short, the relations of the bone approximate to that of seals or even to terrestrial mammals.
Genus: Microcetus Kellogg.
This genus has hitherto been known only from its teeth, and has been found only at one spot in Germany, where it is represented by an ill-known species. This new species may well be named after the distinguished naturalist who was the first to put on record the existence of fossil whales in New Zealand in his description, in 1881, of the huge teeth of Kekenodon.
Microcetus hectori n. sp.
It is difficult to give more than an imperfect diagnosis, partly from lack of material for comparison, partly from the imperfect series of teeth in this fossil. The molars are at least six in number, the posterior teeth having two conical accessory cusps, while the anterior ones have mere convexities. There is, in addition, a simple conical tooth with a single fang whose identity as a premolar is doubtful.
Explanation of Lettering.
a.—external margin of maxilla.
b.—premaxillary or inner margin.
d.—upper surface of maxilla.
e.—overlapping eave of the maxilla.
l.—lower or palatal surface.
t.—tooth displaced at fractured end.
1–6.—the six molars.