The Appendages, Anatomy, and Relationships of Trilobites Part 1

The Appendages, Anatomy, and Relationships of Trilobites.

by Percy Edward Raymond.

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 palaeontologists. To this group of students they are the most characteristic animals of the seas of Palaeozoic time, and even though they are usually preserved as dismembered parts, thousands upon thousands of "whole ones" are stored in the museums of the world. By "whole ones" perfect individuals are not meant, for before they became fossils the wear and tear of their time and the process of decomposition had taken away all the softer parts and even most of the harder exterior covering. What is usually preserved and revealed to us when the trilobites weather out of the embrace of their entombing rocks is the test, the hard sh.e.l.l of the upper or dorsal side. From time to time fragments of the under or limb-bearing side had been discovered, first by Elkanah Billings, but before 1876 there was no known place to which one could go to dig out of the ground trilobites retaining the parts of the ventral side.

Students of trilobites have always wanted specimens to be delivered to them weathered out of the rock by nature and revealing the ventral anatomy without further work than the collecting, but the wish has never been fulfilled. In the Utica black shales, near Rome, New York, there was finally discovered in 1892 a layer less than ten millimeters thick, bearing hundreds of _Triarthrus becki_ with most of the ventral anatomy intact. The collector's first inkling that such were present in the Utica formation came to him in a chance find in 1884, and for eight years he sought off and on for the stratum whence this specimen came. His long search was finally rewarded by the discovery of the bed, and lo! here were to be had, in golden color, prostrate specimens with the breathing and crawling legs and the long and beautifully curved feeling organs all replaced by iron pyrites. Fool's gold in this case helped to make a palaeontologic paradise. The bed contained not only such specimens of _Triarthrus becki_, but also, though more rarely, of _Cryptolithus tessellatus_ and exceptionally of _Acidaspis trentonensis_. This important discovery, which has figured so largely in unraveling the evolution of the Crustacea and even has a bearing on that of most of the Arthropoda, was made by Mr. W. S. Valiant, then curator of the Museum of Rutgers College.

There were, however, great material difficulties to overcome before the specimens revealed themselves with all of their information exposed for study. No surgeon was needed, but a worker knowing the great scientific value of what was hidden, and with endless patience and marked skill in preparation of fossils. Much could be revealed with the hammer, because specimens were fairly abundant. A chance fracture at times showed considerable portions, often both antennae entire, and more rarely the limbs protruding beyond the test, but the entire detail of any one limb or the variation between the limbs of the head, thorax, and tail was the problem to be solved. No man ever loved a knotty problem more than Charles E. Beecher. Any new puzzle tempted him, and this one of _Triarthrus becki_ interested him most of all and kept him busy for years. From the summer of 1893. when he quarried out two tons of the pay stratum at Rome, until his death in 1904, his time was devoted in the main to its solution by preparing these trilobites and learning their anatomical significance.

The specimens of _Triarthrus becki_ from Rome are pseudomorphs composed of iron pyrites, as has been said, and are buried in a gray-black carbonaceous shale. A little rubbing of the specimens soon makes of them bronze images of the former trilobite and while under preparation they are therefore easily seen. However, as the average individual is under an inch in length and as all the limbs other than the antennae are double or biramous, one lying over the other, and the outer one fringed with a filamentous beard, the parts to be revealed by the preparator are so small and delicate that the final touch often obliterates them. These inherent difficulties in the material were finally overcome by endless trials on several thousand specimens, each one of which revealed something of the ventral anatomy. Finally some 500 specimens worthy of detailed preparation were left, and on about 50 of these Beecher's descriptions of _Triarthrus_ and _Cryptolithus_ were based.

The black shale in which the specimens are buried is softer than the pseudomorphous trilobites, a condition that is of the greatest value in preparation. With chisel and mallet the trilobites are sought in the slabs of shale and then with sharp chisels of the dental type they are revealed in the rough. At first Beecher sought to clean them further by chemical methods, and together with his friends, the chemist Horace L. Wells, and the petrologist Louis V. Pirsson, several solutions were tried, but in all cases the fossils were so much decomposed as to make them useless in study. Therefore Beecher had to depend wholly oh abrasives applied to the specimens with pieces of rubber. Much of this delicate work was done on a dental lathe, but in the final cleaning most of it was done with patient work by hand.

Rubber has the great advantage of being tough and yet much softer than either specimen or shale. As the shale is softer than the iron pyrites, the abrasives (carborundum, emery, or pumice) took away the matrix more quickly than the trilobite itself. When a part was fully developed, the rubbers were cut to smaller and smaller dimensions and the abrading reduced to minute areas. So the work went on and on, helped along from time to time by the dental chisels. Finally Beecher became so expert with these fossils that after one side was developed he would embed the specimen in Canada balsam and fix it on a gla.s.s slide, thus enabling him to cut down from the opposite side. This was done especially with _Cryptolithus_ because of the great scarcity of material preserving the limbs, and two of these revealed both sides of the individuals, though they were then hardly thicker than writing paper.

Then came ill.u.s.trations, which at first were camera-lucida drawings in pencil smoothed out with pen and ink. "In some quarters," however, it has been said, "his methods unknown, their results were not accepted; they were regarded as startling, as iconoclastic, and even unreliable." He therefore decided to rework his material and to ill.u.s.trate his publications with enlarged photographs. The specimens were black, there was little relief between fossil and matrix, and the ammonium chloride process of coating them white and photographing under artificial light was unsuitable. Nevertheless, after many trials, he finally succeeded in making fine enlarged photographs of the trilobites immersed in liquid Canada balsam, with a contact cover of gla.s.s through which the picture was taken, the camera standing vertically over the horizontal specimen. Beecher had completed this work in 1903 and in the winter of 1903-1904 was making the drawings, nearly all of which are here reproduced. On Sunday morning, February 14, 1904, as he was working at home on a large wash drawing of _Cryptolithus_, death came to him suddenly, leaving the trilobite problem but partially solved.

When the writer, in the autumn of 1904, succeeded Professor Beecher in the chair of Palaeontology at Yale, he expected to find considerable ma.n.u.script relating to the ventral anatomy of the trilobites, but there was only one page. It was Beecher's method first to prepare and thoroughly study the material in hand, then to make the necessary ill.u.s.trations, and between times to read what others had written.

There was no written output until everything had been investigated and read, certain pa.s.sages being marked for later reference. Then when all was a.s.similated, he would write the headings of topics as they came to him, later cutting them apart and arranging them in a logical sequence. When the writer visited him in his home in January 1904, he was primed for his final trilobite memoir, but the writing of it had not been begun.

The writer has never made the trilobites his special subjects for study as he has the brachiopods, and therefore felt that he should not try to bring to light merely the material things that Beecher had so well wrought out. It seemed at first an impossible task to find the specialist and friend to do Beecher justice, but as the years have pa.s.sed, one of Beecher's students, always especially interested in trilobites, has grown into a full appreciation of their structures and significance, and to him has fallen the continuation of his master's work. If in the following pages he departs here and there from the accepted interpretation and the results of others, it is because his scientific training, in desiring to see with his own eyes the structures as they are, has led him to accept only those interpretations that are based on tangible evidence as he understands such. Furthermore, in seeking the relationship of the trilobites to the rest of the Arthropoda, his wide study of material and literature, checked up by the ontogeny of fossil and recent forms, has led him in places from the beaten path of supposedly ascertained phylogenies. His results, however, have been won through a detailed study of the interrelations of the Arthropoda, starting from the fact that the Trilobita are chronogenetically the oldest and most primitive. The trilobites are held by him to be the most simple, generalized, ancient Crustacea known, and the progenitors, directly and indirectly, of all Arthropoda.

It is now twenty-six years since Professor Beecher began his publications on the cla.s.s Trilobita, and in commemoration of him and his work, Professor Percy E. Raymond of Harvard University presents this memoir, to bring to fruition the studies and teachings of his honored guide. It has been with Professor Raymond a labor of love, and it is for the writer of this foreword a long-desired memorial to the man to whose position in the Museum and University he had the privilege of succeeding.

Yale University, New Haven, Connecticut.

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 circ.u.mstances, it seemed best to include all that is known about the appendages of trilobites, thus bringing together a summary of present information on the subject.

The growth of the memoir to its present size has been a gradual one.

As first completed in 1917, it contained an account of the appendages only. Thoughts upon the probable use of the appendages led to the discussion of possible habits, and that in turn to a consideration of all that is known or could be inferred of the structure and anatomy of the trilobite. Then followed an inquiry into the relationships to other Arthropoda, which ultimately upset firmly established preconceptions of the isolated position of the group, and led to a modification of Bernard's view of its ancestry.

During the progress of the work, I have had the opportunity of examining most of the known specimens retaining appendages. From the Marsh collection in the Yale University Museum were selected the forty-six specimens showing best the appendages of _Triarthrus_, _Cryptolithus_, and _Acidaspis_. Dr. Charles D. Walcott very kindly returned to the Museum of Comparative Zoology the slices of _Ceraurus_, _Calymene_, and _Isotelus_ which were the basis of his paper of 1881, and which had been loaned him for further study. He loaned also eight of the more important specimens of _Neolenus serratus_, and two of _Triarthrus becki_. At the United States National Museum I saw the specimens of _Isotelus_ described by Mickleborough and the isolated limbs of _Calymene_ from near Cincinnati. The _Isotelus_ at Ottawa I had already studied with some care while an officer of the Geological Survey of Canada.

This memoir consists, as shown in the table of contents, of four parts. The appendages of _Neolenus_, _Isotelus_, _Ptychoparia_, _Kootenia_, _Ceraurus_, _Calymene_, and _Acidaspis_ are discussed, as fully as circ.u.mstances warrant, in the first part, and new restorations of the ventral surfaces of _Neolenus_, _Isotelus_, _Triarthrus_, _Ceraurus_ and _Cryptolithus_ are included It is not supposed that these restorations will be of permanent value in all of their detail, but they are put forward as the best approximations to the real structure that the writer is able to present from the materials so far discovered. I am greatly indebted to Doctor Elvira Wood for the care and skill with which she has worked up these restorations from my rather sketchy suggestions. She has put into them not only a great amount of patient work, but also the results of considerable study of the specimens.

Part II is a discussion of the internal anatomy of the trilobite and a brief statement of some of the possible habits and methods of life of these animals. Part III, which begins with a survey of the relationships of the trilobites to other Arthropoda, is largely taken up with an attempt to demonstrate the primitive characteristics of the former, and their probable ancestral position. The form of the ancestor of the trilobite is deduced from a study of the morphology, ontogeny, and phylogeny of the group, and evidence adduced to indicate that it was a depressed, flattened, free-swimming animal of few segments.

In Part IV are included somewhat detailed descriptions of a few of the best specimens of _Triarthrus_ and _Cryptolithus_. Professor Beecher, while an observer of the minutest details, believed in publishing only the broader, more general results of his investigations. This method made his papers brief, readable, and striking, but it also resulted in leaving in some minds a certain amount of doubt about the correctness of the observations. In a matter so important as this, it has seemed that palaeontologists are ent.i.tled to the fullest possible knowledge of the specimens on which the conclusions are based. The last part is, therefore, a record of the data for the restorations of _Triarthrus_ and _Cryptolithus_.

The ill.u.s.trations in the plates were nearly all made by or under the supervision of Professor Beecher, as were also text figures 45 and 46.

In conclusion, I wish to express my thanks to Mrs. Charles E. Beecher for the use of drawings which were the personal property of Professor Beecher; to Doctor Charles D. Walcott for photographs of the limbs of _Calymene_, and for his kindness in sending me the slices of trilobites from Trenton Falls and specimens of _Neolenus_ and _Triarthrus_; to Doctor R. V. Chamberlin for suggestions and criticisms in regard to the relationship of trilobites to Insecta, Arachnida, Chilopoda, and Diplopoda; to Mr. Samuel Henshaw, Director of the Museum of Comparative Zoology, for permission to use the time which has been devoted to this work; and to Miss Clara M. Le Vene, for a.s.sistance in the preparation of the ma.n.u.script. My greatest debt is to Professor Charles Schuchert, to whom the work owed its inception, who has a.s.sisted in many ways during its prosecution, and who read the ma.n.u.script, and arranged for its publication. To him I can only express my warmest thanks for the favors which I have received and for the efforts which he has put forth to make this a worthy memorial to our friend and my teacher, Professor Charles Emerson Beecher.

Harvard University, Cambridge, Ma.s.s.

November, 1919.

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 expressed by Burmeister (1846, p. 43):

There is good proof that the feet of trilobites must have been soft membranous organs, for the absence of the slightest remains of these organs in the numerous specimens observed is of itself evidence of the fact, and it can indeed scarcely be supposed that hard h.o.r.n.y extremities should be affixed to a soft membranous abdominal surface; since they would not have possessed that firm basis, which all solid organs of locomotion require, in order that they may be properly available.

Very well reasoned, and were it not for the discovery of new material in American localities, Burmeister's views would probably never have been proved incorrect. One can not escape the suspicion that some of the accepted hypotheses of today, founded on similar "proof," may yield in time to the weight of bits of positive evidence.

The history of the study of appendages of trilobites may be divided into two periods. The first, in which there was a general belief that the appendages were soft organs, but during which numerous "finds" of limbs were reported, extended from the time of Linne to the year (1876) in which Walcott demonstrated the fact that the animals possessed jointed ambulatory and breathing organs.

The second, much more fruitful period, began with Walcott's publication of 1881, descriptive of the appendages of _Ceraurus_ and _Calymene_, and for the purposes of this memoir, closes with his great contribution on the anatomy of _Neolenus_ (1918). Beecher's brilliant productions came in the middle of the second period.

In the first period, there were at least two authentic discoveries of appendages, those of Eichwald (1825) and Billings (1870), but since neither of these men convinced his confreres of the value of his finds, the work of neither can be considered as having marked an especial epoch in the history.

As all the authentic finds will be treated in detail on later pages, only a brief resume of the first period will be given here. This has already been done by Burmeister (1843, 1846) and Barrande (1852, 1872), whose works have been my primary sources of information, but I have looked up the original papers, copies of nearly all of which are to be seen in the libraries in Cambridge and Boston. Brig.-Gen. A. W.

Vogdes, U. S. A. (retired), has very kindly placed at my disposal a number of references and notes.

Linne (1759) was the first to report the discovery of appendages of trilobites. Tornquist (1896) has pressed for a recognition of the contribution of the great Swedish naturalist to this problem, but Beecher (1896 B) doubted the validity of the find. Linne figured a specimen of _Parabolina spinulosa_ (Wahlenberg), with what he interpreted as a pair of antennae attached. He states (translation quoted from Tornquist): "Most remarkable in this specimen are the antennae in the front, which I never saw in any other sample, and which clearly prove this fossil to belong to the insects." Beecher has shown as conclusively as can be shown without access to the original specimen that the supposed antennae were really only portions of the thickened anterior border, the appearance being due to imperfect preservation. Brunnich as early as 1781 called attention to the imperfection of this specimen, and it is also referred to by Wahlenberg (1821, p. 39), Brongniart (1822, p. 42), Dalman (1828, p.

73), and Angelin (1854, p. 46).

Audouin (1821) seems to have been the first naturalist with sufficient knowledge of the Arthropoda to be competent to undertake the study of the trilobites. He concluded that the absence of ventral appendages was probably a necessary consequence of the skeletal conformation, and thought if any were discovered, they would prove to be of a branchial nature.

Wahlenberg (1821) in the same year expressed his belief that the trilobites were nearly allied to _Limulus_ and in particular tried to show that the trilobites could have had masticatory appendages attached about the mouth as in that modern "insect" (p. 20).

Wahlenberg was also the first to describe an hypostoma of a trilobite (p. 37, pl. 1, fig. 6), but did not understand the nature of his specimen, which he described as a distinct species.

Brongniart (1822, p. 40) devoted five pages of his monograph to a discussion of the affinities of trilobites, concluding that it was very probable that the animals lacked antennae and feet, unless it might be that they had short soft feet which would allow them to creep about and fix themselves to other bodies.

Schlotheim (1823) thought that the spines on _Agnostus pisiformis_ were segmented and compared them with the antennae of _Acarus_.

Stokes (1823) was the first who, with understanding, published an ill.u.s.tration of the ventral side of a trilobite, having figured the hypostoma of an _Isotelus_. He was followed in the next year (1824) by Dekay, who also figured the hypostoma of an _Isotelus_, and added some observations on the structure of trilobites. The researches of Barrande, Novak, Broegger, Lindstroem, and others have dealt so fully with the hypostoma that further references to that organ need not be included here.