A Reptilian Jaw from Kakanui, South Island, New Zealand

Transactions and Proceedings of the Royal Society of New Zealand, Volume 65, 1936, Page 232

A Reptilian Jaw from Kakanui, South Island, New Zealand By Professor W. B. Benham, F.R.S., F.R.S.N.Z., University of Otago. [Read before the Otago Institute, November 13, 1934; received by the Editor, February 9, 1935; issued separately, December, 1935.] (With 8 figures.) Dr. J. A. Thomson, in his paper on “The Gem Gravels of Kakanui,”* J. A. Thomson, Trans. N.Z. Inst., vol. 38, p. 482, 1905. after enumerating the fossil mollusca found in the Blue Clay, suggests that the bed belongs not to the Oamaru System, but to the Pareora, and on p. 491 states: “A whale was also found”—a phrase suggesting that something approaching an entire skeleton or at least a skull had been seen by him. At any rate this sentence refers without doubt to the specimens about to be described, for Dr. Thomson's collection from Kakanui was handed over to the Otago University Museum in 1903, and amongst the other fossils is a jaw-bone, or rather fragments of the jaw containing teeth, as well as some broken, isolated teeth. These specimens are labelled in Thomson's writing—“whale from Blue Clay, All Day Bay, Kakanui.” I had therefore placed them with other remains of fossil whales in the collection here, but when I came to study the material last year, while preparing a memoir on the Extinct Whales, it was at once apparent that this jaw and teeth do not belong to the Cetacea, but are reptilian. I. Description. The available material consists of two small blocks of greyish clay, each of which contains part of a jaw-bone bearing one or more teeth. The clay was readily cleaned away and the details of bone and teeth exhibited. One of these blocks (a) after being cleared of the matrix contained a single bone measuring 11.5 cm. in length and 4 cm. in height on the inner face at the broken end near the base of the tooth; and the outer face is 5 cm. in height. From above the matrix there protruded the tip of a black tooth, while its basal end was exposed near the lower edge of the bone. I exposed the tooth for its full length. The tooth (Fig. 1) is curved, compressed from within outwards; its apex is directed slightly backwards and towards the outer side of the bone; its internal face is slightly more convex than its outer. Its total length (as exposed) is 5.5 cm., of which the crown occupies 2.2 cm.; the diameter of the root is 1.5 cm. The triangular crown is sharply pointed, covered with a shining black enamel with 5–6 longitudinal ridges along it; its margins are narrow and trenchant. The posterior margin is produced into a series of denticulations; a basal group of 12 closely-set, small, triangular denticles or serrations which occupy 1.2 cm. from the base of the crown, and a distal group of 3 larger, sharply-pointed teeth or cusps; the two proximal of these are stout, but the

most distal is much smaller and lies about half-way between the former and the tip of the tooth. The anterior edge presents no such denticulations. It is quite smooth (but amongst the loose teeth a somewhat different arrangement is met with, as will be shown later). The “root” is broken across at the base. It is covered with a pale buff cementum, and is loosely embedded in the alveolar groove, the floor of which it nearly reaches. At the broken end it is nearly oval in section, but rather broader at one end of the oval. There is a small pulp cavity traversing the black dentine (Fig. 1a). I must emphasise the absence of sockets in this portion of the jaw, for in the other portion they are distinct. Nor is there any furrow on the outer surface to indicate the presence of more than one bone. In transverse section this portion of the jaw which, as I shall show, is more anterior than the other portion (b), is V-shaped (Fig. 2); Fig. 1.—A tooth from the jaw-fragment “a” after removal of the matrix. 1a.—View of the broken end of this tooth, showing the pulp-cavity. Fig. 2.—The end of the jaw-bone “a” viewed in perspective, showing the tooth in situ. Fig. 3.—Crowns of two isolated and broken teeth. one limb, which I regard as external, is taller and stouter than the other and somewhat thickened along the upper half, whereas the internal limb of the V is thin and tapers slightly upwards. Amongst the isolated teeth there is one with the crown entire; this has a series of seven larger and smaller denticulations along one edge, which are spaced, and there are none of the finer ones at the base (Fig. 3). Another crown shows denticulations on both the anterior and posterior margins; those on one edge, probably the posterior, are larger and fewer than on the other edge. Unfortunately this crown is incomplete. The other fragment of the jaw (b) consists of two mandibles lying close together, with their inner surfaces facing, and are evidently in their natural position. One, that of the right side, is

rather longer than the other, as the fracture at one end has not gone right across the two. The longer, or right mandible (Fig. 4) measures 12 cm. in length. Its height at one end (anterior) is 3.5 cm., and at the other end is 4 cm. The shorter of the two mandibles (Fig. 5) is 9 cm. in length and 3.5 cm. in height. The outer surface of the mandible is traversed by a narrow, shallow furrow which passes obliquely downwards and backwards, starting at a distance of 10 cm. from upper edge anteriorly, to a point 23 cm. below at the posterior end. In transverse section an extremely fine line can be traced inwards from this furrow, so that the jaw seems to be composed of two bones closely in contact (see later). Fortunately, one end, the posterior, of the fragment shows a clean fracture across the two mandibles and gives a picture precisely like the figure of a transverse section of the pair of mandibles of Ichthyosaurus. The thickness of the single mandible is 3.3 cm. Fig. 4.—Side view of the right mandible of fragment “b” showing the external furrow separating the two constituent bones of the jaw. A, the anterior end. It will be noticed in the figure that the right and left alveolar grooves face outwards, away from one another; yet the association of the two mandibles must be natural, for no accidental association could, I imagine, produce so symmetrical a figure. The upper surface of the jaw presents a deep alveolar groove whose outer wall is thinner and of less height than the inner, and the latter is traversed by a small circular vascular foramen (the bones show up very distinctly when the cut surface is wetted, for they appear nearly black against the grey matrix). The longer, that is, the right jaw, supports the base of a broken tooth, situated about 4 cm. from the posterior end; it projects about 1.8 cm. and has a diameter of 1.2 cm. The root is embedded in the bone. Its long axis is oblique, the broken crown being directed towards the anterior end of the jaw. This tooth is solid, for the pulp cavity does not extend upward so far as the crown, for the crown is not preserved—

it is only the root that remains. The dentine is black, covered with cementum of buff colour, rather lighter than the colour of the bone. The shorter (left) mandible also contains the base of a broken tooth (it is possible that some of the loose teeth belong to this jaw). The fracture is evidently recent and perhaps was made while Thomson was excavating the bone from the matrix. This tooth is at a distance of 5.5 cm. from the posterior end, and thus corresponds with that of the right jaw. At the anterior broken end is the broken root of another tooth, oval in section and surrounded by bone, and covered above by a bony mass which blocks up the alveolar groove Fig. 5.—Side view of the shorter, left mandible of fragment “b.” The upper edge of the bone had been broken away, hence the less height of this as compared with the right mandible. The tooth has been broken short, and the lower edge chipped showing matrix. Fig. 6.—Upper surface of the left mandible, showing the curious septal tissue crossing and filling the alveolar groove before and behind the base of the tooth. The alveolar groove at posterior end has been freed from the matrix. (Fig. 7). When the matrix was scraped away so as to explore this groove the fact was revealed that the groove is not continuous, but is blocked at intervals by irregular masses of bone which extend apparently from the inner surface outwards, and in one case ends in a claw-like ridge on the upper surface of the outer wall of the jaw-bone (Fig. 6). These “sockets” are thus very irregular in shape and extent, and in the degree to which the septa are developed on the two mandibles, for on the right one they are but feebly developed, while on the left they are very stout. The appearance presents some similarity to those figured by Owen for Megalosaurus bucklandi, with which, however, this fossil has no affinity. * J. Hector, Trans. N.Z. Inst., vol. vii, 1884.

I removed the matrix from the rest of the alveolar groove in the hope of finding other teeth, but without result; there are none. The teeth then are widely-spaced along each mandible, there being only two teeth in a distance of 9 cm. on the left jaw, and 12 cm. on the right. II. Discussion. I regard the fragments as those of the mandibles of a reptile possessed of a long rostrum or snout. The portion (a) shows no sign of being composed of more than a single bone, and therefore must be near the anterior end and near the symphysis of the two mandibles. It has become separated from its fellow, no doubt during the excavation by Thomson; for, of course, if the bones in the more posterior region are in contact right with left, the same condition will hold in the anterior region as well. Fig. 7.—View of the anterior fractured end of the left jaw, with the broken tooth covered by the septal tissue. Fig. 8.—The posterior end of the two mandibles of fragment “b.” The alveolar groove is filled with matrix. The fragment (b) I take as being nearer the skull, and consequently the two bones of which it is made up would be the Dentary and either the Splenial, which in some fossil reptiles does appear on the outer surface, or the Surangular, which in Ichthyosaurus extends forwards for some distance from the base of the jaw. The obliquity of the groove and its height above the lower margin of the jaw suggest the latter, for the Splenial is usually confined to a very narrow portion of the outer surface. Possibly, again, this lower bone is the Angular, which sometimes extends some little distance in front of the Articular bone (as in Dimetrodon). From the fact that the mandible is composed of two bones, it is quite certain that it is not that of a Cetacean, apart from the form of the teeth, for I know of no Cetacean, living or extinct, which has teeth of the serrated character described above. In looking through what literature is available—viz., Owen's Odontography and certain palaeontological works such as those of Pictet, Nicholson, Zittel (all of whom copy Owen's figures), the volumes of the

Palaeontological Society, those of Palaeontologica Indica, and other works—it seemed at first that, if one judged from the character of the tooth, it might be Megalosaurus (cf. Owen's fig. 6, pl. 62A, M. bucklandi; or fig. 7 of Palaeosaurus platyodon). But below I give reasons against this identification. The tooth does not resemble any of the fossil reptiles described and figured by Hector* J. Hector, Trans. N.Z. Inst., vol. VII, 1884. as occurring in New Zealand. I have searched the Canadian Fossil Vertebrates and Owen's African Reptiles without finding any tooth serrated as these teeth are. But it cannot be that of Megalosaurus, for, in the first place, this genus does not range higher than the Wealden in England, or rocks in India which correspond to the Upper Chalk of Europe. In the second place, Megalosaurus had no rostrum, for as in other Dinosaurs, the skull is short and compact. But the Kakanui skull was evidently produced into a long and narrow rostrum such as occurs in the Ichthyosaurs and in some of the Crocodilia. It does not seem to be Ichthyosaurus, for in this genus the teeth form a continuous series, close together, and, moreover, they were set in an open groove, without sockets; the tooth is smooth, with a tumid root usually stouter than the crown. The transverse section of the two mandibles in our fossil resembles closely the representation of a similar section across the mandibles of I. intermedia.† R. Owen, Liassic Reptiles, Part III, Pal. Soc. Mem., pl. xxvi, fig. 7. But the drawing of the external surface of the jaw shows it to be composed of several bones—angular, surangular, splenial, and dentary at the hinder end, and only the dentary anteriorly. Owen's figure of I. longispinis indicates that in that species the teeth were socketed, but closely placed. But in I. tenuirostris the teeth, which are slender and of course unserrated, appear to be in groups of three or four, separated by spaces. The absence of serrations and the fact that, with the sole exception of I. longispinis, sockets were absent seem to negative the association of the Kakanui fossil with this genus. The only other reptiles in which sockets exist are:—Plesiosauria, where the teeth are simple curved cones; Crocodilia, where the teeth are smooth, not curved, but upright, closely set along the jaw. Some of these, such as the living Gavialis and the extinct Mystriosaurus, have a long, narrow rostrum; but the latter is of Triassic age. The Dinosauria had deeply-socketed teeth, but they are placed in an open alveolar groove; and these animals had no rostrum. A consideration of the various possibilities seems to converge on the Ichthyosauria; but if so, it is a new genus so far as the literature available here allows me to come to any conclusion. I have considered the possibility of the fragment (a) being part of the upper jaw, showing the maxilla and either the nasal or the premaxilla, but the transverse sections seem to negative this. III. Decision. I suggest a name for the fragments—no doubt a hazardous proceeding—but it is necessary to have some label for the fossil, as it seems impossible to allot it to any known genus. I am informed by

my colleague, H. D. Skinner, that the Maori name for the “god of the waters” is Tangaroa; so I suggest the title Tangaroasaurus for the genus, and will name it specifically after the locality. Tangaroasaurus kakanuiensis, n. gen.: n. sp. An Ichthyosaurid reptile with a long narrow rostrum bearing teeth at wide intervals apart. These are compressed cones, with distinct serrations on the posterior or on both edges; the bases of the teeth are implanted in sockets of peculiar and irregular form in the proximal region, but in the more distal region the base of the tooth is free in the alveolar groove. Locality: All Day Bay, Kakanui. Geological horizon: Mid-Tertiary. Explanation of Figures. A.—anterior end of the jaw-bone. al.—alveolar groove. d.—dental canal. g.—external furrow separating the two constituent bones. i.—inner face of the jaw-bone. j.—jaw-bone in section. k.—crack in the bone. m.—matrix filling spaces in the bone. o.—outer face of the jaw. s.—septal tissue. su.—suture between the constituent bones. t.—tooth. t'.—broken tooth at fractured end of jaw. The figures represent the structures of the natural size; the drawings are somewhat crude, as ink is a more difficult medium than pencil. The bony septal tissue is indicated by short undulating lines; matrix by dots; bone by short parallel lines.