Marjorie O'Connell

“ To be sure the "seraphims" of the Old Red Sandstone discovered by the stone-cutter of Cromarty and proclaimed by Agassiz to be "the remains of a huge lobster," are deservedly famous. Their size, abundance, association with the monster cephalaspid fishes, and above all the mystery attending their place in nature have shed upon the eurypterids of the Old Red Sandstone a picturesque and historical glow which makes the later discoveries of faunas merely so many cold triumphs of science. ”

“ When a given fossil fauna has shown certain peculiar characteristics, such, for instance, as a complete or almost complete absence of molluscan representatives or when the fauna has been confined to one or two classes of organisms, the custom has been and still is to say that the organisms lived in brackish water. It is, thus, necessary to determine the nature of recent faunas which are characteristic of the various bionomic realms, in order that we may, not without a fair degree of certainty, establish the criteria for determining the faunal nature of the habitats of the past. ”

“ It has been the custom to consider that all fossils are the remains of marine organisms unless obvious and indisputable evidence of their fluviatile, lacustrine, or terrestrial habitat is available; a fossil found without any other associates has been held to be marine until proved to be otherwise, but never has the suggestion been made that such a fossil was fluviatile until proved to be marine. ”

“ We can now understand that a fauna may be made up of individuals which show a fairly close relationship with faunas in neighboring areas, but may contain one species which is identical or nearly so with a species in a fauna three thousand miles distant. If these were marine fossils we could not understand such a thing, because marine faunas show whole groups of species in one region related to groups in another, and contemporaneous marine deposits in the path of migration show similar related groups. ”

“ Yet, when the early Palaeozoic periods were past these frightful scenes of wholesale destructon gave way to gentler, more pacific modes of life, so that in the Upper Siluric in central and western New York and on the Island of Oesel we find indications from the fossils that the eurypterids lived amicably to a ripe old age, dying a natural and peaceful death and enjoying a decent and fitting burial in the fine muds of those times. Thus we see again the steady progress in evolution from the early days of barbarism to the later ones of communal altruism. ”

“ If we apply the criteria for the recognition of the various types of fossil faunas, it is at once evident that neither throughout the Temeside group nor at any particular horizon in it is there a marine fauna, for we have seen that a marine fauna, whether existing under the rather uniform conditions of the open sea or under the more variable environment of the littoral zone, and whether in a sandy, muddy, or pure water facies, was yet made up of diverse classes of organisms with a scattering representation through the phyla of the Invertebrata. ”

“ While the Baltic serves to show on a large scale what happens to a marine fauna which is gradually subjected to fresher and fresher water until it passes through brackish conditions to entirely fresh ones, there is another type of brackish water, the estuary, which is often said to have a fauna of its own. An estuary may be defined as the drowned lower portion of a river in which twice daily there is a change in the water from fresh to marine and back again as the tide comes in and goes out. ”

“ When we apply such considerations to fossil faunas, to a class of organisms wholly extinct, where we have no facts of modern distribution to help us, no facts of present habitat to point past modes of life, we can see that the criteria which we apply to such fossil faunas in the determination of relationships and migrations must be quite different from the ones applied to marine fossil faunas. ”

“ Many new genera appear suddenly in this Lower Siluric horizon, and we are unable to do more than say that such and such genera came from a common ancestor. It is unfortunate, indeed, that the Ordovicic of Great Britain has not yielded such faunas as it has in America. Yet, keeping these points in mind, we are still struck by the provincial character of the Wenlock fauna. There is not a species in it which is closely related to any of the North American species except Eusarcus scoticus which foreshadows in certain respects E. scorpionis from the Bertie. ”

“ The brackish waters of estuaries are not stable enough to have a fauna which may be considered endemic. The marine fauna lives along shore and in the waters not too much disturbed by the tidal current, but the organic remains found in the estuaries are marine. Moreover, along the coasts affected by the deposition of the tidal muds, no organisms live and it is only many miles away from the estuary proper that the marine forms are found whose hard parts, carried by the tidal currents and in time comminuted, finally come to settle on the floors of the estuaries and on the river banks. ”

“ That advantage is not reserved for us, for the Silurian basin of Bohemia, so favored in all other respects, is relatively poor in fossils of the genus Pterygotus, not only because of their great rarity, but also because of the reduction of the specimens to little fragments. Since almost all of the remains are found in the large horizon of the Cephalopods, that is, in our limestone band e 2, it seems to us that one may attribute the almost total disappearance of these gigantic Crustacea to the voracity of these molluscs, against whom they were forced to maintain the struggle for existence. ”

“ Along the entire Atlantic coast of North America the muddy facies of the littoral zone swarms with life, and while many of the species are confined to that facies it certainly cannot be claimed that where muds are accumulating there is a paucity of plant and animal life. Detailed studies of restricted areas of the ocean floor have proved that a large and varied fauna flourishes even where muds pour in in great quantities from the land. ”

“ In Scotland the Upper Siluric is marked by the approaching continental conditions as evidenced in the deposits of greywackes and flagstones, some barren, some containing a sparse marine fauna and others only fish and eurypterid remains. The conditions as yet were unstable, showing the alternate dominance now of river-borne sediments and now of shore deposits. To the south, however, the sea still covered most of England, though the muds pouring in from the land had made conditions unfavorable for many of the forms of life which thrived in that region during Wenlock time. ”

“ Summarizing these data we conclude that the eurypterids lived in the sea from Cambric to Siluric time. They had then become less sensitive to changes, positive and negative, in the salinity of the water. In fact they seem to have thrived best under conditions of life that excluded most other marine groups of animals, that is, in the marginal, more or less inclosed marine lagoons, accompanied by estuaries receiving delta-forming terrestrial drainage, with prevailing arid or sub-arid climate, the waters being in some places more than normally briny, in others having less than normal salinity. ”

“ In the region of interlacing headwaters, streams of different systems are temporarily connected at times of flood and perhaps only two or three individuals of a certain species will change from one system to another, and then, when the connection is broken, the distribution of that species depends entirely upon the ability of the individual to contend with all of the new factors in the environment, and it is pure survival of the fittest which brings about the distribution of that species. ”

“ I do not mean to imply that migration of river forms all around the world can take place always in a single geological period. It is well known that certain related or identical species of fish which today are found in rivers thousands of miles apart have such a distribution because their ancestors in a former geological epoch when relations between land and sea were different had an opportunity to accomplish the migrations, all evidences of which have since been destroyed. ”

“ The alternation of limestone conglomerates and shales in the lower Potosi series indicates near-shore conditions of sedimentation, and the occurrence of the single specimen of a eurypterid, far from pointing to a marine habitat for this one individual, militates very strongly against such a mode of life. In all cases the occurrence of a single individual is one of the strong arguments against the assumption that the individual belongs to the fauna of the bed in which it is found. ”

“ This zone, extending from high water approximately to the two hundred fathom line, is the one of greatest geologic interest because nearly all of the marine formations of the past were littoral; unequivocal abyssal deposits being very rare. Since practically all of the invertebrate organisms of this prolific littoral marine fauna are protected either by shells or by exoskeletons, each individual that dies leaves its record behind in some hard part which falls to the bottom when the animal dies, or else soon comes to rest there, where it is buried by sand or mud. ”

“ Chamberlin, recalling that fishes and eurypterids are found both earlier and later than the Devonic in marine deposits, puts the following question: "Were the fishes and eurypterids primarily marine and later became adapted to fresh water, or were they primarily fresh water forms which were occasionally carried out to sea and which later became adapted to salt water?" He reminds us that we are always in the habit of considering all life at first marine, then terrestrial, but, though this is true in general, the idea should not be held to with too great tenacity in every case. ”

“ Complete individuals are never found and although it is possible from the fragments to determine that different genera are represented, more precise identifications are difficult, and in most cases species have been erected simply in order to have some way of designating the fragments belonging to the various genera. In order to understand why all of the specimens from the Siluric of England are so much more poorly preserved than are those from Scotland, it will be necessary briefly to trace the geological changes which were taking place throughout Great Britain during the later Siluric. ”

“ A short distance out to sea eurypterid remains would quickly be buried, the hollow case being soon filled by infiltrating sand. Anyone familiar with the occurrence of Limulus exoskeletons on sandy shores knows that they are easily filled by and buried in the sand and that they are preserved in toto, not broken to pieces. Thus we cannot account for the occurrences of the eurypterids on the assumption that they are marine organisms. In the Normanskill beds not a single entire specimen has been found, the whole fauna being made up mostly of carapaces with some separated abdominal segments. ”

“ A second change which takes place in the Baltic fauna and which may be correlated with the variation in salinity, is that the stenohaline forms of the marine fauna disappear altogether, while the euryhaline ones become dwarfed. Thus, the common cockle shell, Cardium edule, in the North Sea, of normal marine salinity, is the size of a small apple, at Stockholm, where the salinity is below 10 permille, the shell in the deeper, more saline water is only as large as a walnut and is even smaller along shore where the water is fresher. ”