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The Structure of the Mouth of the Oldest Known Vertebrates

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The Structure of the Mouth of the Oldest Known Vertebrates T he Structure of th e m o u t h of th e o l d e st know n VERTEBRATES, PTERASPIDS AND CEPHALASPIDS By Johan K iaer (Oslo). (Eingelangt am 30. Juli 1927.) The structure of the mouth of the oldest known vertebrates — the Si- lurian-Devonian Pteraspids and Cephalaspids — has been hitherto unknown, notwithstanding the fact that many of these forms are not very uncommon in some regions; partly also in a fairly good state of preservation. This fact has led the greater number of authors to presume that the afore-mentioned forms had a soft mouth, capable of protruding and devoid of ossified parts, whereon some have based further conclusions. I shall describe in the following some new finds, which show that these conclusions have been too hasty, as these forms have in reality pos­ sessed an ossified mouth-skeleton of an extremely peculiar development. One of these finds, that of a new Pteraspis-form, was made on Th. V ogt’s Spitsbergen-Expedition in 1925. This finding will be described in detail in the monographical description which I am now preparing out of the considerable Norwegian material of Pteraspid fishes from the Down- tonian and Devonian series of Spitsbergen. This work I hope will be finished next year. I consider this find to be one of the most interesting that has been made in recent years, and I shall therefore give a preliminary report of it. The other find is of a Cephalaspid, from the rich Downtonian fauna which I discovered some years ago in Ringerike in the Oslo region. Of this fauna I have hitherto described the Anaspida. In connection with these finds not before described I shall shortly give the details of the mouth structure of some earlier known forms. Earlier opinions regarding the structure of the mouth of Pteraspids and Cephalaspids. A complete summary of the views which have been earlier advanced regarding the structure of the mouth of the Pteraspids and Cephalaspids will not be given here. I shall only cite the opinions of those authorities who have made a special study of these forms. In his well-known monograph Lankaster does not express himself defi­ nitely regarding this question. In connection with his reconstruction of Pteraspis he thinks it very improbable that the mouth of this form was 118 J. Kiaer: provided with hard teeth. He thinks, consequently, that it had a suctorial mouth evidently assuming the same to be the case in the Cephalaspids. This conclusion was obviously founded on the fact that no trace of solid skeleton parts indicative of jaws or other parts of the mouth could be found. Smith-Wood ward speaks with more caution in the second volume of his Catalogue. He finds the non-existence of a bony structure around the mouth adequately proved; but nevertheless his opinion is not that jaws were missing. A similar presumption is also maintained by Traquair in his summary of the Ostracoderms in 1898. Meanwhile there have already been found in Estland some remains of the peculiar Cephalaspid Tremataspis in such a good state of preservation that a surer assumption regarding the structure of the mouth was made pos­ sible. Rohon was in 1893 able to describe a specimen with an intact ventral side and could prove that an area of small plates was developed underneath the snout. Posteriorly these plates bordered the gill openings, in front they constituted the lower side of the mouth opening. The latter was thus, accor­ ding to Rohon , bordered by the lower edge of the dorsal shield and under­ neath by a number of mouth-plates the construction of which, however, could not be ascertained. Rohon speaks with great caution and uncertainty regarding this mouth structure. He writes that we here evidently have to do with „einem geschlossenen Mundli Herewith he obviously implies that the mouth consisted of ossified skeleton parts. Not many years after follows Traquair ’s well-known description of „The lower Devonian Fishes of Gemiinden“ Therein he describes some greatly compressed, but otherwise completely preserved specimens of the genus Drepanaspis, which is a later pteraspidian offshoot. Traquair disco­ vered that the mouth in this genus formed a transversal opening, which was directly bordered by skeleton plates; he could not, however, prove the exi­ stence of a tooth construction of any sort whatever. According to this it was possible to define the function of the mouth in different ways and the greater number of authors deemed it still probable that these earliest fish-like forms had a soft suctorial mouth. The last opinion has especially been emphasized by Jaekel in a number of his interesting works on the earliest fishes and their structure. In „Die Wirbeltiere“ (1911) he has placed the forms here dealt with together with the Cyclostomata in a special class which he calls Malacostomata; these are fish types of the lowest order; their main characteristic is a soft, evidently toothless and suctorial development of the mouth. In the Palaeostraci to which belong Pteraspids, Cephalaspids and An- aspids, he assumes that the parts of the mouth were unossified, and that the mouth in the earliest forms was a protractile, suctorial mouth as in the amphibian-larvae of the present day. He maintains the same opinions in his later treatise called „Die Mundbildung der Placodermen “ (1919). Quite a different explanation is maintained by Patten , who, as is known, believes the Vertebrates to be derived directly from the Arachnids and Merostomes, and assumes the Ostracoderms to be a transitional group, the Peltocephalata, possessing distinct transitional characters. The Structure of the Mouth of Pteraspids and Cephalaspids. 119 Of importance to this study is his work „On the structure and classi­ fication of the Treraataspidae“ (1903), in which he gives an excellent recon­ struction of this peculiar form. Patten was able to point out a great number of interesting details which I shall mention later. He reached the result that the mouth was not bordered above by the lower edge of the dorsal shield, as Rohon maintained, but was situated centrally in the oral area of small plates; further that the whole construction of the mouth was in accordance with that of the Arachnids and hot with that of the Vertebrates. He also mentions that he has discovered in the Cephalaspis „indications of the pre­ sence of a pair of heavy crushing mandibles, ©o situated that they must have acted against each other, at right angles to the sagittal plane, instead of parallel with it, as in true Vertebrates.“ I have, however, been unable to find a closer description of the material to which Patten here alludes. The same assumption regarding the mouth structure of Tremataspis we may find again in his large work „The Evolution of the Vertebrates and their Kin“ (1912). By this short summary we may see the uncertainty apparent in the interpretation of the structure of the mouth in these earliest known verte­ brates. T he structure of the mouth in Pteraspis Vogti nov. sp. (m a n u s cr.). Norwegian expeditions to Spitsbergen have collected a huge material of Palaeaspids and Pteraspids in a large number of new forms which will greatly enlarge our knowledge of these primitive vertebrates. The greater number of these forms have been hitherto only found in the usual state of preservation with isolated plates and scales. On the expedition made by Th. V ogt in 1925, several forms were at last found almost completely pre­ served. One of the most important of these finds is that of a new Ptera­ spis, which was found by the Paleontologist of the expedition, A. Heintz , in the Red Bay Series (Downtonian). In honour of the leader of the expe­ dition I have called this form Pteraspis Vogti. This find is highly peculiar, as, in the small piece of reddish-brown limestone, not bigger than a hand, the remains of 6 completely preserved specimens were found. Most of these specimens are highly eroded, but together afford, after being prepared by Conservator Heintz , a nearly com­ plete reconstruction of the animal. In two specimens the mouths apparatus ■ was prepared out, in the one from the outside, and in the other from the inside. The preparation was carried out with the help of fine needles under a strong binocular instrument and took a very long time. One of these spe­ cimens after preparation (on the outside) may be seen in PI. XII, fig. 1, about 4 times enlarged. A third specimen was used to a section series affording an excellent material for the study and reconstruction of the different plates and scales. I shall not here give any general description of this new species. I only mention that it represents a comparatively small form, the length of the dorsal shield being not longer than -about 50 mm. This shield is rather broad and flat with quite short cornual plates and an uncommonly short, flat, and broad rostral-area (Fig. 1). 120 J. Kiaer: Underneath the rostral-area there appeared a hitherto quite unknown system of small plates, which serve to fill out the space between the front edge of the ventral shield and the lower edge of the rostral plate". It was at once made clear that this system of plates constituted a highly peculiar mouth apparatus in these ancient forms. In order to understand how these plates join with the remaining plates I RPi. •i i Fig. 1. Dorsal shield ofPteraspis primaevus nov. sp. (Manuscr.), seen from above, X 3, Diagrammatical sketch after specimens in Pal. Mus. Univers. Oslo from Red Bay series (Downtonian), Spitsbergen. Lateral line system (stippled lines) after grinding preparations of the author.
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