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- 22 - PALEOECOLOGY OF THE ARTHROPODA By Percy E. Raymond (Harvard University) Any account of the ecology of the extinct arthropods must of necessity be a tentative one. Relatively little has been written on the subject. Knowledge of fossil arthropods is so incomplete that most students of the phylum are still preoccupied by taxonomic work. Until the species are de- scribed, paleontologists will have little time to devote to the study of the relationship of the various animals to each other and to their environment. Such paleoecological information as is available at the present time relates chiefly to the habits, and secondarily to the habitats, of various arthro- pods. Inferences are drawn from the morphology of the fossils, and from the lithological characteristics of the strata in which they are found. In most instances our knowledge of the morphological characteristics is incomplete, and there is considerable difference of opinion about the environment of deposition of certain sediments. In this first report, the writer may perhaps be pardoned if he re- fers more fully to problems which require solution, than to actual accom- plishments. Trilobites Most students are agreed that the Trilobita are the most primitive of the Crustacea. They have teen extinct since the end of the Paleozoic, and have no near relatives among existing animals. More than 99 per cent of than are incompletely known, for it is but rarely that specimens retain- ing the appendages are found. Until more actual facts have been accumulated, present ideas as to habits and habitats must be regarded as speculations rather than deductions. Such speculations, as had been printed up to 1918 have been reviewed by the writer. A series of papers by Rudolph Richter ap- peared too late to be incorporated in my paper. These are critically re- viewed in the manuscript article by William E. Schevill, appended to this report. The broad, depressed, flattened shell of the trilobite suggests that almost all members of this group were primarily dwellers in the zone at or near the sea floor. The shape of the test suggests that most were equally efficient as crawlers or swimmers. This suggestion is borne out by the shape of the appendages of such forms as retain them. All appendages, other than those whose function was tactile, are biramous, the lower, (inner) ones suitable for use in crawling or burrowing, the outer for respiration, or possibly for swimming. Only by means of detailed study of the appendages is it possible to infer habits. For example, the flattened segments of the posterior endopodites of Triarthus suggest that it was a more successful swimmer than crawler, whereas the stout limbs of Neolenus indicate that
- S3 - it was able to walk about on its "toes," i.e., the terminal spines of each endopodite. Trails on Upper Cambrian sandstones (Proticnites) were prob- ably made by similar trilobites. The blind or nearly sightless trinucleids probably sought their food in the superficial layers of the bottom ooze, ploughing along Just be- neath the surface of the mud, as does Ltaiulus. The broad head-shield and bowed endopodites are very similar to those of the modern horse-shoe crab. Other trilobites, particularly those which, are more or less shovel-faced and high-eyed, probably profiled about in the mud seeking food. That any trilobites were strictly burrowing animals, in 1he sense that they exca- vated and lived in burrows, is doubtful. Walcott described certain Cambrian fossils as- trilobitan burrows, but none has ever been found with a trilobite still within it, or with any markings which could have been produced by no other animal's than trilobitee. Most burrows and trails are more or less permanent puzzles. However ingenious the explanations of their origin, absolute proof is always lacking. . ⢠»- ⢠It should perhaps be emphasized that the trilobites were no more specialized in their habits than they were in their structure. When one says that Cryptolithus was a burrowing animal, one does not mean that it could not crawl or swim. It means merely that Cryptolithus seems to have been somewhat better adapted for burrowing that most of the other trilobites. TO cite another instance, the writer has argued that the broad axial lobe and large pygidium of Isotelus indicate that this animal was capable of propelling itself forward and backward in lobster-like fashion, using the pygidium as a swimming telson. But I have also described trails on sand- stone which I interpreted as having been made by an Isotelus crawling on its gnathobases. No trilobite yet knotm is so specialized as to suggest that it was confined to a; single environment. Nor does the history of any genus OP family indicate any progressive adaptation to a particular sort of environment. Possibly a case ire exception to this sweeping statanent might be made for the Homalonotinoe, for the Devonian Dipleura certainly has a very much wider axial lobe than. was (probably) possessed by its as yet un- known Cambrian Calymene-like ancestors. But the Ordovician members of the sub-family had axial lobes proportionally as wide as those of Dipleura. The Calymeniae and the Homalonotinse from earliest Ordovician times appear to have had different habits. As the writer interprets their structure, the former were crawlers, the latter dominantly swimmers. But there is. nothing to suggest that with the passage of time the one group produced better crawlers, or the other better swimmers. As a matter of fact, the coarse silty and sandy nature of the strata in which Devonian Hcmalonotinee are most common, both in Eastern llorth America and Germany, suggest that the later members of the group were chiefly crawlers. It is an as yet unsolved problem as to how much weight is to be given to the nature of the sediment in which fossils are found, in judging the habits and actual habitats of the animals when alive. It is true that some genera and species are found only in sandstone, in shale, or in limestone. On the other hand, there are many that are found in all kinds of sedimentary rocks. Calymene and Isotelus, for instance, are just as common in Ordovician sandstones and shales as in limestones, but since Ordovician carbonate rocks
are more widespread than argillaceous ones, most of the specimens one sees are in limestone. As a matter of fact, the sandstone, shale, and limestone environ- ments are probably not so different as the names of the rocks suggest. It is probably true that most limestone was deposited in relatively warm water, but it is Just as true that. sandstone and shale were formed under the same . conditions. Most sandstones were deposited in shallow water, but so were many shales and most limestones. In judging the nature of the environment from the sediments, texture is much more important than chemical composition. Even if this be kept in mind, it may be difficult to infer the orig- inal conditions from the consolidated sediments. Many-a coarse-grained limestone may have been deposited as grains of almost colloidal proportions. Two argillaceous shales, equally fineâgrained, may contain very different sorts of fossils, for one may have been deposited under. such conditions as to maintain a relatively firm sea-floor, whereas another may have presented a fluffy, soft, bottom on which no benthonic life was possible. It is well known that some modern "sandy" (argillaceous or calcareous) sea-floors teem with crawling and burrowing animals, whereas others on which the sand is constantly shifting, are devoid of life. In some instances, fossil animals do show definitely the effects of "bottom-control." Perhaps the most striking example is in the Oriskany fauna. It is well known that the same genera, perhaps the same species of brachiopods occur in the Oriskany of New York and the Oriskany of Gasps'. Yet in most instances the species from the thin coarse-grained sandstone of New York have approximately four times the volume of those found in the limestone of Canada. The Oriskany of New York and Maryland is noted for the large and thick shells of its brachiopods. It has been suggested that the animals "needed" them, to withstand the vigorous wave action near shore. It is more probable that the animals grew larger because of the greater supply of food near shore. As has been intimated, most trilobites could live under a variety of conditions. Only rarely is the nature of the rock in which they are found of any significance, at present known, in regard to their habits and habitat. In support of the first statement it is interesting to compare the geographic range of Tropidoleptus and Phacops. They are.the most abun- dant fossils of the sandy and shaly Mid-Devonian strata of New York and the Allegheny plateau, but although Phacops occurs in considerable abundance in the calcareous Hamilton equivalents in the Mississippi basin, Tropidoleptus seems to be rare or absent west of New York. The one instance in which there seems to be a definite correlation between the type of trilobite and the nature of the sediment is in the fine- grained black shale and fine-grained black limestone of the Ordovician. The muds, argillaceous or calcareous, which produced these strata seem to have been too soft to support any ordinary benthonic fauna; their fossils are, for the most part, remains of holo- or epiplanktonic, or nectonic creatures, although there are a few which probably lived on the bottom. The trilobite
- 25 - fauna is not a large one, but it. is interesting in that it contains blind forms such aa agnostids, trinucleids, raphiophorids, and dionideids, animals with very large eyes, 'such as Remopleurides and Robergia, and a very few with normal, medium-sized eyes, chiefly species of Triarthrus. The natural assumption is that the blind trilobites were either inhab- itants of deep seas or dwellers in the mud. This may not be true of all the agnostids, for their remains are not confined to fine-grained shales, but are found in strata oP all sorts. It is not to be supposed that all members of so large a group would have the same habits. Each species must be judged by itself. The facial sutures indicate that the ancestors of the raphio- phorids had eyes. One judges that they lost their sight through adaptation to life Us. the aphotic zone, probably in rather deep water. The long glabella and genal spines, coupled with a short and rather feeble body, suggest that this sort of animal would not be very successful in shallow waters, where it would be constantly in danger of gettimg tangled up with sea-weeds. Prob- ably they puttered about, swimming feebly, in rather deep water* As for Remopheurides and Robergia, their forms and their tests would suggest a float- ing existence. However, their very large eyes give one the impression that, like certain modern crustaceans mentioned by Dollo, their natural habitat was deep water, but that they were nocturnal visitors near the surface. Very likely they fed on graptolites, and were rather efficient swimmers. The ex- traordinarily long hypostoma awaits explanation in terms of function. .'.Triarthrus was rather a generalized trilobite, at home everywhere, equally good at swimming and crawling. Its constant association with grapto- lites suggests that they were its favorite food. The implications of the association of these trilobites with the par- ticular sort of sediments in which their remains are found are perhaps of some interest. After the publication of the "Challenger" reports, geologists became interested in deep-sea deposits,. and all such fine-grained sediments as contained Foreman if era or Radiolaria were for a time thought to have been formed at considerable depths. All chalks and fine-grained limestones and shales were generally accepted as deep-sea depositsi Then the pendulum swung back. Geologists found ripple-marks and shrinkage cracks in these so-called oceanic strata, and rather suddenly, about twenty years ago, opinion reached the other extreme;"almost all Paleozoic strata, at any rate, were supposed to be of shallow-water origin. Did not the pendulum swing too far? The evi- dence obtained from lithology and fauna seems to indicate that certain shales and limestones were founed in rather deep water; certainly so deep as to be beyond the depth to which sunlight can penetrate. Other Arthropods . . This report has been devoted particularly to a discussion of trilobites, since they are the arthropods in which the writer has been. most interested. Other groups within the phylum present problems even more important. Probably of greatest general interest is the discussion as to whether the merostomos inhabited fresh or salt waters during the Ordovician and Silurian parts of their history. A detailed discussion of the evidence will be reserved till a
- 26 - later date. As is well known, T. C. Chamberlin, Marjorie O'Connell, and A. W. Grabau have maintained that the eurypterids were, from the beginning of their history, fresh-water animals, which implies, of course, that they were the first fresh-water arthropods. If, as Chamberlin has suggested, life originated in the soil and organisms first evolved in fresh water, the eurypterids may have been the first and most primitive of arthropods. The discussion becomes doubly important, if we accept Homer's ideas about the influence of the eurypterids upon the armor of the ostracoderms, early fish-like creatures. This group, like the merostomes, was according to Chamberlin, of fresh-water origin. The general foxia of the O'Connell-Grabau argument is well known. Specimens of Ordovician eurypterids are found in marine sediments, but all are very fragmentary, hence the animals lived in rivers, and pieces of their skeletons were occasionally carried into the sea. Specimens found in late Silurian strata are remarkably complete, which shows that -tine animals were at home in fresh water, and were killed immediately if by any chance they were carried "by currents into salt water. Many paleontologists still believe, despite the facts so ably mar- shalled in support of this theory, that the eurypterids were originally marine and did not become adapted to lifo in fresh water till late Silurian or early Devonian times. The mere fact that their remains ore found in marine strata is, however, no proof of the theory that they were buried where they lived. Since some new evidence on this question is Just now coming to light, further discussion. is best postponed to a later report. The question about the habitat of the eurypterids is largely academic, for their only possible descendants, the scorpions, had become terrestrial by Pennsylvanian times. Much more important, although almost nothing has been written about it, is the history of those animals vjhich for convenience are termed the schizopods, late Paleozoic crustaceans which were presumably ancestral to amphipods, isopods, anaspides, and malacostracans. We know practically nothing of the history of this group till its sudden appearance in the Carboniferous, at which time all its members appear to have been in- habitants of fresh water. One of the lines, that of the anaspides, appears to have remained in fresh water, being represented today by small and rare animals in Australia and Tasmania. The other groups are, and have been, so far as the geological record tells us, chiefly marine sir.ce Triassic times. Some isopods and some malacostraceans now living are fresh water or terrestrial, but such forms are rare as compared with their marine relations. How are we to interpret this record? Were the ancestors of the modern crus- taceans marine animals which shifted their habitat to the fresh waters in late Paleozoic times, only to return to the seas during the Triassic? If so, do the modern fresh water and air breathing members represent a second migration to the land? These groups are so poorly represented as fossils that we may never know the full history. If the modern Crustacea originated in fresh water, then the Arthropoda must be polyphyletic, for the oldest trilobites and t)t5jyjhipods are certainly marine.. One history, which is fairly well documented, and which may parallel that of the recent Crustacea, is that of the Xiphosura. If cui- ideas about relationships are correct, these animals were marine from Mid-Cambrian till late Silurian times, fresh water during the Carboniferous and Permian, and
- 27 - marine again from the Triassic to the present. Of course it may be that there were marine as -well as non-marine representatives of the group in existence in Carboniferous times, but if so, vfay have their remains not been found? We know thousands of marine for every fresh-water Carbonifer- ous fossil. This report touches upon but a few of the subjects ifaich are involved in the study of the paleoecology of the Arthropoda. An attempt has been made to show that something can be inferred as to the possible habits and habitats of the trilobites, and attention has been called to the need for further investigation of some other croups. A brief bibliography is appended.
- 28 - BIBLIOGRAPHY Trilobita Beecher, C. E.: On the mode of occurrence, and the structure and develop- ment of Triarthrus becki; Am. Geol., vol.13 (1894), pp.38-43. Burling, L. D.: Was the Lower Cambrian trilobite supreme? Ottawa Natural- ist, vol. 31 (1917), pp. 77-79. Dollo, L.: La paleontologie Qthologique; Bull. Soc. Beige de Geol., Pal. et d'Hydrol., vol.23 (1910), pp. 377-421. Finch, G. E.: Notes on the position of the individuals in a group of Nileus vigilans found at Elgin, Iowa; Proc. Iowa Acad. Sci. for 1903, vol.11 (1904), pp. 179-181. Miller, S. A.: Silurian ichnolites, with definitions of new genera and species; Jour. Cincinnati Soc. Nat. Hist., vol. 2 (1880), pp. 217-218. Raymoad, P. E.: On two new trilobites from the Chazy near Ottawa, Ontario; Ottawa Naturalist, vol. 24 (1910), pp. 129-134. The appendages, anatomy, and relationships of Trilobites; Mem. Conn. Acad. Arts and Sciences, vol. 7 (1920), pp. 1-169. Richter, Rudolf: For references to his papers, see bibliography appended to Mr» Schevill's article. Staff, Hans v. and Reck, Hans: Ueber die Lebenweise der Trilobiten. Eine entwicklungsriechanische Studie. Gesscl. naturforsch. Freunde, Sitzb. (1911), pp. 130-146. Walcott, C. W. : Appendages of Trilobites. Smitlison. Miscl. Coll., vol. 67 (1918), No. 4, pp. 115-226. Merostomata Clarke, J. M. and Ruedemann, R.: The Euryptcrida of New York; New York State Mus. Mem. 14 (1912), pp. 1-439. O'Connell, Marjorie: The habitat of the Eurypterida. Buffalo Soc. Nat. Sci., Bun. 11 (1916), pp. 1-277. General Ruedemann, R.: Paleozoic Plankton of North America; Geol. Soc. of Amer- ica, Mem. 2 (1934), pp. 1-140.
