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The Appendages, Anatomy, and Relationships of Trilobites
The Appendages, Anatomy, and Relationships of Trilobites
BY PERCY E. RAYMOND, Ph.D. ASSOCIATE PROFESSOR OF PALAEONTOLOGY, AND CURATOR OF INVERTEBRATE PALAEONTOLOGY IN THE MUSEUM OF COMPARATIVE ZOOLOGY, HARVARD UNIVERSITY NEW HAVEN, CONNECTICUT PUBLISHED BY THE CONNECTICUT ACADEMY OF ARTS AND SCIENCES AND TO BE OBTAINED ALSO FROM THE YALE UNIVERSITY PRESS THE TUTTLE, MOREHOUSE & TAYLOR COMPANY TO THE MEMORY OF CHARLES EMERSON BEECHER SKILLFUL WITH HAND, BRAIN, AND PEN; REVEALER OF THE MYSTERIES OF TRILOBITES; THIS MEMOIR IS DEDICATED...
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FOREWORD.
FOREWORD.
By CHARLES SCHUCHERT. Trilobites are among the most interesting of invertebrate fossils and have long attracted the attention of amateur collectors and men of science. These "three-lobed minerals" have been mentioned or described in books at least since 1698 and now several thousand species are known to palæontologists. To this group of students they are the most characteristic animals of the seas of Palæozoic time, and even though they are usually preserved as dismembered parts, thousands upon
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PREFACE.
PREFACE.
The primary object of this memoir is, as has been stated by Professor Schuchert, to. rescue from oblivion the results of the last few years of Professor Beecher's investigations on the ventral anatomy of trilobites. Since he left his data in the form of drawings and photographs, without even rough notes, it became necessary, in order to write a text to accompany the plates, to restudy the entire subject. Under these circumstances, it seemed best to include all that is known about the appendages
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HISTORICAL REVIEW.
HISTORICAL REVIEW.
The beginning of the search for the limbs of trilobites was coeval with the beginning of scientific study of the group, knowledge of the appendages being essential to the proper systematic allocation of the animals. The early search was so barren of results that negative evidence came to be accepted as of positive value, and it was for many years generally believed that such organs as may have been present beneath the dorsal test were so soft as to be incapable of preservation. This view is best
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PART I.
PART I.
THE APPENDAGES OF TRILOBITES. Terminology. The terminology employed in the succeeding pages is essentially the same as that used by Beecher, with two new terms added. Beecher assigned to the various segments of the limbs the names suggested by Huxley, but sometimes used the name protopodite instead of coxopodite for the proximal one. It is obvious that he did not use protopodite in the correct sense, as indicating a segment formed by the fusion of the coxopodite and basipodite. The usage employe
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The Appendages of Isotelus.
The Appendages of Isotelus.
HISTORICAL. The first specimen of Isotelus with appendages was described orally by Billings before the Natural History Society of Montreal in 1864, and in print six years later (1870, p. 479, pls. 31, 32). The specimen is described in detail on a later page. Billings recognized the remains of eight pairs of legs on the thorax, a pair for each segment, and he inferred from the fact that the appendages projected forward that they were ambulatory rather than natatory organs. He was unable to make o
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The Appendages of Triarthrus.
The Appendages of Triarthrus.
Triarthrus becki Green. (Pls. 1 - 5 ; pl. 6, figs. 1-3 ; text figs. 1 , 10 , 11 , 33 , 42 .) (Also see Part IV .) Illustrated: Matthew, Amer. Jour. Sci., vol. 46, 1893, pl. 1, figs. 1-7;—Trans. N. Y. Acad. Sci., vol. 12, pl. 8, figs. 1-7.—Beecher, Amer. Jour. Sci., vol. 46, 1893, text figs. 1-3;—Amer. Geol., vol. 13, 1894, pl. 3;—Amer. Jour. Sci., vol. 47, pl. 7, text fig. 1;—Amer. Geol., vol. 15, 1895, pls. 4, 5;—Ibid., vol. 16, 1895, pl. 8, figs. 12-14; pl. 10. fig. 1;—Amer. Jour. Sci., vol. 1
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Historical.
Historical.
Specimens of Triarthrus retaining appendages were first obtained by Mr. W. S. Valiant from the dark carbonaceous Utica shale near Rome, New York, in 1884, but no considerable amount of material was found until 1892. The first specimens were sent to Columbia University, and were described by Doctor W. D. Matthew (1893). This article was accompanied by a plate of sketches, showing for the first time the presence of antennules in trilobites and indicating something of the endopodites and exopodites
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The Appendages of Ptychoparia.
The Appendages of Ptychoparia.
Ptychoparia striata (Emmrich). Illustrated: Jaekel, Zeits. d. d. geol. Gesell., 1901, vol. 53, part 1, pls. 4, 5. Jaekel has described a specimen of this species obtained from the Middle Cambrian near Tejrovic, Bohemia, which on development showed beneath the test of the axial lobe, certain structures which he believed represented the casts of proximal segments of appendages. On the basis of this specimen he produced a new restoration of the ventral surface of the trilobite, in which he showed t
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The Appendages of Kootenia.
The Appendages of Kootenia.
Kootenia dawsoni Walcott. Illustrated: Walcott, Smithson. Misc. Coll., vol. 67, 1918, pl. 14, figs. 2, 3. One specimen figured by Doctor Walcott shows the distal ends of some of the exopodites and endopodites of the right side. He compares the exopodites with those of Neolenus, stating that the shaft consists of two segments, the proximal section being long and flat, fringed with long setæ, while the distal segment has short fine setæ. The endopodite best shown is very slender, and the segments
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The Appendages of Calymene and Ceraurus.
The Appendages of Calymene and Ceraurus.
HISTORICAL. All of the work on these species has been done by Doctor Walcott, who summarized his results in 1881. In the first of his papers (1875, p. 159), Walcott did not describe any appendages but paved the way for further work by a detailed and accurate description of the ventral surface of the dorsal shell of Ceraurus. He demonstrated the presence in this species of strongly buttressed processes which extend directly downward from the test just within the line of the dorsal furrows. One pa
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Comparison of the Appendages of Calymene and Ceraurus with those of Triarthrus.
Comparison of the Appendages of Calymene and Ceraurus with those of Triarthrus.
As one may see by reading the above quotations from Doctor Walcott's descriptions, he found certain branchial organs in Ceraurus and Calymene which have not been found in other trilobites but otherwise the essential features of the appendages of all are in agreement. Spiral Branchiæ. It is now necessary to inquire if the thin sections can not be interpreted on the basis of trilobites with the same organs as Triarthrus . The interpretation of the structures seen in these translucent slices is exc
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The Appendages of Cryptolithus.
The Appendages of Cryptolithus.
Cryptolithus tessellatus Green. ( pl. 6, fig. 7 ; pls. 7 - 9 ; text figs. 20 , 25 , 45 , 46 .) (See also Part IV .) Illustrated: Beecher, Amer. Jour. Sci., vol. 49, 1895, pl. 3. When Professor Beecher wrote his short article on the "Structure and Appendages of Trinucleus " (1895), he had only three specimens showing appendages. In his later work he cleaned several more, so that there are now thirteen specimens of Trinucleus = Cryptolithus available for study, though some of these do not show muc
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Summary on the Ventral Anatomy of Trilobites.
Summary on the Ventral Anatomy of Trilobites.
COMPARISON OF APPENDAGES OF DIFFERENT GENERA. Since the appendages of Triarthrus , Cryptolithus , Neolenus , Calymene , and Ceraurus are now known with some degree of completeness, those of Isotelus somewhat less fully, and something at least of those of Ptychoparia , Kootenia , and Acidaspis , these forms being representatives of all three orders and of seven different families of trilobites, it is of some interest to compare the homologous organs of each. All in which the various appendages ar
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PART II. Structure And Habits Of Trilobites.
PART II. Structure And Habits Of Trilobites.
INTERNAL ORGANS AND MUSCLES. Granting that the trilobite is a simple, generalized, ancient crustacean, it appears justifiable to attribute to it such internal organs as seem, from a study of comparative anatomy, to be primitive. The alimentary canal would be expected to be straight and simple, curving downward to the mouth, and should be composed of three portions, stomodæum, mesenteron, and proctodæum, the first and last with chitinous lining. In modern Crustacea, muscle-bands run from the gut
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PART III.
PART III.
RELATIONSHIP OF THE TRILOBITES TO OTHER ARTHROPODA. It can not be said that the new discoveries of appendagiferous trilobites have added greatly to previous knowledge of the systematic position of the group. Probably none will now deny that trilobites are Crustacea, and more primitive and generalized than any other group in that class. The chief interest at present lies in their relation to the most nearly allied groups, and to the crustacean ancestor. Trilobites have been most often compared wi
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Crustacea.
Crustacea.
BRANCHIOPODA. The early idea that the trilobites were closely related to the Branchiopoda was rejuvenated by the work of Bernard on the Apodidæ (1892) and has since received the support of most writers on the subject. Fundamentally, a great deal of the argument seems to be that Apus lies the nearest of any modern representative of the class to the theoretical crustacean ancestor, and as the trilobites are the oldest Crustacea, they must be closely related. Most writers state that the trilobites
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Arachnida.
Arachnida.
No attempt will be made to pass in review all of the subclasses of the arachnids. Some of the Merostomata are so obviously trilobite-like that it would seem that their relationship could easily be proved. The task has not yet been satisfactorily accomplished, however, and new information seems only to add to the difficulties. So far as I know, the Araneæ have not previously been compared directly with trilobites, although such treatment consists merely in calling attention to their crustacean af
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Insecta.
Insecta.
Handlirsch (in several papers, most of which are collected in "Die Fossilen Insekten," 1908) has attempted to show that all the Arthropoda can be derived from the Trilobita, and has advocated the view that the Insecta sprang directly from that group, without the intervention of other tracheate stock. At first sight, this transformation seems almost an impossibility, and the view does not seem to have gained any great headway among entomologists in the fourteen years since it was first promulgate
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Chilopoda.
Chilopoda.
The adult chilopod lacks the antennules, and all of the other appendages, with the exception of the maxillulæ, are uniramous. The walking legs are similar to the endopodites of trilobites, and usually have six or seven segments. The appendages are therefore such as could be derived by modification of those of trilobites by the almost complete loss of the exopodites and shortening of the endopodites of the head. The position of the post-oral appendages, the posterior ones outside those closest th
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Diplopoda.
Diplopoda.
The diplopods, especially the polydesmids with their lateral outgrowths, often have a general appearance somewhat like that of a trilobite, but on closer examination few likenesses are seen. The most striking single feature of the group, the possession by each segment of two pairs of appendages, is not in any way foreshadowed in the trilobites, none of which shows any tendency toward a fusion of pairs of adjacent segments. The antennules are short, antennæ absent, mandibles and maxillulæ much mo
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Primitive Characteristics of Trilobites.
Primitive Characteristics of Trilobites.
TRILOBITES THE MOST PRIMITIVE ARTHROPODS. The Arthropoda, to make the simplest possible definition, are invertebrate animals with segmented body and appendages. The most primitive arthropod would appear to be one composed of exactly similar segments bearing exactly similar appendages, the segments of the appendages themselves all similar to one another. It is highly improbable that this most primitive arthropod imaginable will ever be found, but after a survey of the whole phylum, it appears tha
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The Simplest Trilobite.
The Simplest Trilobite.
Fig. 35. —A specimen of Weymouthia nobilis (Ford), collected by Mr. Thomas H. Clark at North Weymouth, Mass. Note the broad smooth shields of this Lower Cambrian eodiscid. × 6. In the discussion above I have placed great emphasis on the large size of the primitive pygidium, because, although there is nothing new in the idea, its significance seems to have been overlooked. If the large pygidium is primitive, then multisegmentation in trilobites can not be primitive but is the result of adaptation
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The Ancestor of the Trilobites, and the Descent of the Arthropoda.
The Ancestor of the Trilobites, and the Descent of the Arthropoda.
The "annelid" theory of the origin of the Crustacea and therefore of the trilobites, originating with Hatschek (1877) and so ably championed by Bernard (1892), has now been a fundamental working hypothesis for some years, and has had a profound influence in shaping thought about trilobites. This hypothesis has, however, its weak points, the principal one being its total inhibition of the workings of that great talisman of the palæontologist, the law of recapitulation. Its acceptance has forced t
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Final Summary.
Final Summary.
It is generally believed that the Arthropoda constitute a natural, monophyletic group. The data assembled in the preceding pages indicate that the other Arthropoda were derived directly or indirectly from the Trilobita because: (1) the trilobites are the oldest known arthropods; (2) the trilobites of all formations show great variation in the number of trunk segments, but with a tendency for the number to become fixed in each genus; (3) the trilobites have a constant number of segments in the he
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PART IV.
PART IV.
DESCRIPTION OF THE APPENDAGES OF INDIVIDUAL SPECIMENS. Triarthrus becki Green. In order to make easily available the evidence on which the present knowledge of the appendages of Triarthrus and Cryptolithus rests, it has seemed wise to publish brief descriptions and photographic figures of some of the better specimens preserved in the Yale University Museum. These specimens are pyritic replacements, and while they do not as yet show any signs of decomposition, it should be realized that it is onl
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Cryptolithus tessellatus Green.
Cryptolithus tessellatus Green.
Specimen No. 233 ( pl. 7, fig. 1 ; text fig. 45 ). This is the best preserved entire specimen. It is developed from the lower side, and shows the hypostoma, antennules, and a few fragmentary appendages of the cephalon, the outer portions of the exopodites of thorax and pygidium on both sides, and the endopodites on the left side. The hypostoma is imperfectly preserved and is turned completely around, so that the anterior margin is directed backward, and the posterior one is so much in the shadow
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BIBLIOGRAPHY.
BIBLIOGRAPHY.
Agassiz, L. 1873.—Discovery of the basal joint of legs of trilobites. Amer. Nat., vol. 7, pp. 741-742. Angelina N. P. 1854.—Palæontologia Scandinavica, pars 1, Crustacea formationis transitionis. Audouin, J. V. 1821.—Recherches sur les rapports naturels qui existent entre les trilobites et les animaux articulés. Ann. Gen. Sci. Phys. Nat. Bruxelles, vol. 8, p. 233, pl. 26. 1822. Isis (Encycl. Zeitung), Oken., vol. 10, p. 87, pl. 1, No. 4, figs. 1-5. Barrande, J. 1852.—Systême Silurien du centre d
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