The Eurypterida of New York/Volume 1/Mode of life
MODE OF LIFE
The mode of life of the eurypterids has thus far been touched upon incidentally in the description of the "swimming feet"; the earlier writers have generally assumed that the eurypterids were active swimmers on account of the structure of these legs which Hall very properly compared with the quite similar swimming organs of the common "lady crab" (Platyonichus ocellatus) of our eastern Atlantic coast. Woodward pointed out that the position of the eyes of Pterygotus, half above and half below the margin, indicates that it can not have been a mud grubber and he has also said of P. anglicus that its large eyes, its powerful natatory appendages and the general form of its body suggest that it was a very active animal. Laurie [1893, p. 511; 1893, p. 124], on the other hand, has repeatedly advanced the view that the eurypterids were bottom crawlers and diggers in the mud. He says of the swimming leg of Slimonia [op. cit. p. 511]:
This appendage is always described as a swimming organ, but I am inclined to doubt the correctness of this interpretation of its function. The Eurypteridae appear to me, from their general build, more fitted for crawling than swimming, and I am inclined to explain this appendage as having been used by the animal to get a firm hold on the bottom, and probably also for digging out sand and covering itself, in much the same way that Portunus uses its very similar pair of appendages.
Holm concluded from his study of Eurypterus fischeri that the last pair of legs of that species was principally adapted to swimming. He says:
Die Hauptbewegung des achten Gliedes im Verhältniss zum siebenten scheint von vorn nach hinten gewesen zu sein, indem sich, wie schon von Fr. Schmidt hervorgehoben ist, das achte Glied bei der Bewegung über die dreieckige Platte wie ein Scheerenblatt über das andere schiebt. Die beiden Glieder müssen daher beim Schwimmen wie ein einziges
We have to gather our evidence by analogy from the habits of the living relatives of the eurypterids and of similarly built marine animals, comparing the structures in which their habit finds its most distinct expression with the supposedly analogous structural features of the eurypterids. With a view to this end, we may briefly consider the habits of Limulus, the common king crab, and those of crabs with swimming legs as in the eurypterids.
An early and excellent account of the habits of the king crab was given by the Rev. S. Lockwood (cited by Owen ) who says:
It is emphatically a burrowing animal, living literally in the mud, into which it scoops or gouges its way with great facility by means of the anterior edge of its enormous cephalic shield. In the burrowing operation the forward edge of the anterior shield is pressed downwards and shoved forwards, the two shields being inflected, and the sharp point of the tail providing the fulcrum as it pierces the mud, while underneath the feet are incessantly active, scratching up and pushing out the earth on both sides. There is a singular economy of force in this excavating action, for the alternating doubling up or inflecting and straightening out of the two carapaces with the pushing purchase exerted by the tail spine, accomplish both digging and a subterranean progression. Hence the king crab is worthy to be called the "marine mole."
As to the use of the last pair of legs in burrowing and the use of the tail spine in locomotion, Owen also cites the interesting observations of W. A. Lloyd on living Limuli in the aquarium at Hamburg.
At Hamburg, specimens of Limuli were kept alive from the year 1865 to 1870. From his observations of these Mr Lloyd informs me, "the ulterior}} pair of limbs" are not employed for walking, but exclusively for burrowing. These limbs are terminated by four long, stiff, oval, or leaf-shaped lobes, jointed at their base to the leg, and capable of being opened and closed in a four-radiate manner. When it wishes to burrow these two limbs are sometimes alternately and sometimes simultaneously thrust backwards below the carapace quite beyond the hinder edge of the shell, and in the act of thrusting the lobes or plates on each leg encounter the sand, the resistance or pressure of which causes them to open and fill with sand, a load of which at every thrusting operation is pushed away from under the crab and deposited outside the carapace. The four plates then close, and are withdrawn closed, previously to being opened and charged with another load of sand, and at the deposit of every load the whole animal sinks deeper in its bed, till it is hidden all except the eyes. The great overarching shield of the carapace again prevents one from seeing whether this excavating work is being aided by the fanning motion of the branchial false feet, but I think such a fanning is going on, as I have seen signs of sand being driven out as if urged by a current of water.
Mr Lloyd thus describes the use of the tail spine in locomotion:
The animal having climbed up a rock in the aquarium till it has got near the top of the tank (which in Hamburg contained a depth of 30 inches of water), and having assumed a vertical position, it leaves go its hold on the rock and allows itself to fall back into the water; but its downward fall is instantly checked, and the creature propelled upwards by the powerful flapping of its false branchial feet, and when the impetus given by these appendages ceases the animal again sinks down, but is prevented from falling prone on the floor of the tank by alighting on the tip of its tail spine. The moment this happens, and before the creature has lost its balance on the spine, the false feet make another flap and give another impulse upwards and forwards; all this time the position of the carapace is slanting, the top inclining downwards at an angle of about 45°, the hinder shell being at another angle, and the tail spine hanging down vertically, and so it progresses by a combination of flapping and hopping till it reaches the limits of its tank and sinks to the ground. The Limulus was fond of thus going about at night, generally remaining on the sand all day.
Another use made of the tail spine was as a lever by means of which it righted itself when it fell off a rock on its back. The spine is then bent, its point is planted in the sand so that it makes an acute angle with the carapace, which is then so far raised that some of the feet are enabled to grasp a projecting surface, and the crab then turns over.
Another more recent brief account is given by H. W. Fowler which reads in effect as follows:
The king crabs prefer such conditions as are afforded in Delaware Bay, where there are muddy or sandy bottoms, and it is seldom that they are found along the ocean beach. They also prefer more or less tranquil water where there is little or no surf and burrow in these places just under the sand or mud where they find an abundance of food in the many small animals; they are said to feed principally on worms. Although without any means of offense to such animals as fishes, they are amply protected from most enemies by their hard covering. The strong tail spine is used to right themselves when overturned. They progress through the sand by crawling, and when burying themselves are assisted by means of the hinged back, and also the spine. They are good swimmers, though they may frequently be seen crawling about over the bottom. When in shoal water they may easily be discovered by the two rows of small bubbles constantly appearing at the surface of the water.
From these notes on the habits of the American king crab it may be inferred that though a good swimmer, it mostly frequents the bottom and is a crawler, but feeds by digging in the mud and sand. In an aquarium such as that in Castle Garden, New York, it can be seen to swim easily by napping the powerful sternites which thus function as breathing and swimming organs. It crawls by the use of the four pairs of endognathites, and digs by the combined use of the shovelshaped head shield, the pushing spine, the activity of the legs, especially of the last pair, which are adapted to this function and probably even by the aid of the flapping sternites.
The features of Limulus which appear to be of importance in ascertaining the habits of the eurypterids, being also observed in one or the other of their genera, are: the broad carapace, its beveled edge, the triangular concave shield in the middle of the doublure of the underside, the subapical position of the lateral eyes, the large, platelike sternites, the articulation between the carapace and abdomen and that between abdomen and telson, and the spiniform telson.
The other marine animals that have been repeatedly referred to as indicating the habits of the eurypterids by their similar swimming feet are the crabs of the family Portunidae, notably Matuta and Portunus (Huxley and Salter), Platyonichus ocellatus (Hall), and Portunus (Laurie). Our most common forms with swimming legs are the edible or blue crab (Callinectes hastatus) and the lady crab (Platyonichus ocellatus). Here the question arises as to whether their last pair of limbs which so strongly resemble the swimming legs of many eurypterids, are better adapted to swimming or to digging. A full account of the habits of Callinectes hastatus has been given by Mary J. Rathbun, and a shorter account of the habits of the edible, lady and green (Carcinus moenus) crabs by Paulmier. We learn from these descriptions that the edible crab usually wanders about crawling or hides under stones and logs and in seaweeds, but when pursued and finding speed ineffective, will rapidly bury itself in the sand to escape observation. As October draws to a close the crab moves into deep water and at this season may frequently be seen paddling near the surface as he works himself down stream with the tide. Paulmier states that "as may be supposed from the finlike posterior legs, it is a good swimmer and may often be found at the surface." On the other hand, he says of the lady crab, which has quite similar swimming legs: "Its usual habitat is the sandy beaches, even the most exposed, when at low water mark it buries itself all but the eyes and antennae and is on the watch for enemies and prey. If disturbed when feeding or if dug out, it disappears with great rapidity, burrowing backward into the sand."
We thus infer, and have verified this inference by actual observation, that they use their last legs with great ease for both digging and swimming, but are more given to a crawling and a burrowing than to a swimming habit.
On comparison of the characters similar to the eurypterids, the king crab and the brachyuran crabs here mentioned, we may first remark that both. Limulus and the crabs are highly specialized in comparison with the eurypterids. This is evinced in Limulus by the extreme broadening of the carapace, the adaptation of the last pair of legs to burrowing and the fusion of the abdominal somites; in the crabs by the excessive broadening of the cephalothorax and the reduction of the abdomen. In both cases the specialization is mainly a distinct adaptation to the crawling and burrowing habit. No such far-reaching specialization is found among the eurypterids.
In surveying the genera of the eurypterids in regard to the characters bearing on their habits, we find that they readily fall into four groups which show the following differentials:
- Compound eyes marginal, body slender, fishlike, last pair of limbs swimming legs, telson mostly broad and finlike: Hughmilleria, Pterygotus, Erettopterus, Slimonia
- Compound eyes marginal and frontal, body scorpioid, last pair of limbs swimming legs, telson spiniform: Eusarcus
- Compound eyes dorsal, subapical, body slender to broad, last pair of limbs swimming legs, telson spiniform: Eurypterus, Dolichopterus
- Compound eyes dorsal, subapical to apical, body slender, last pair of limbs exceedingly long and slender jointed, telson styliform: Drepanopterus, Stylonurus
We are disposed to believe that these four groups of genera represent four different modes of life habit.
In regard to Pterygotus and Erettopterus, the typical representatives of the first group, it has been remarked by Woodward that the marginal eyes which are half on the underside, prove that the animals could not have been mud grubbers and that the spatulate and bilobed telson was obviously adapted to swimming. Laurie [1893, p. 521] has also conceded that "the only advantage which occurs to one as possibly appertaining to the bilobed form of telson is its greater efficiency as a swimming organ." The general form of the body, notably the relatively small carapace which lacks the shoveling rim of other eurypterids, the slender body with relatively narrow preabdomen that gradually tapers into the postabdomen, are clearly adapted to greater agility, and the long slender prehensile pincers in front, show that Pterygotus was highly predaceous and not a burrower after worms as Limulus or a carrion eater as most of the crabs, which use their stout claws largely as organs of defense and for tearing pieces off their food. The poorly developed walking legs of Pterygotus attest that it was a bad walker or crawler. There, then, seems to remain by exclusion the single inference that Pterygotus was essentially a swimming creature, probably slow and therefore in need of the long prehensile pincers. The large size attained by some species (5 feet and more) would also indicate that they could hardly have been of burrowing habit.
Hughmilleria, in both its species, has a distinctly fishlike appearance in dorsal aspect, which is evidence of its agility; this is effected by the relatively long, convex, anteriorly angular carapace and the very slender form of the body, which gradually tapers to the stout tail spine. It also lacks the distinct shoveling edge of the carapace. Hughmilleria has much better developed walking legs than Pterygotus and lacks the broadening of the telson. As it is clearly a more primitive and less specialized form than Pterygotus, it is equally adapted to a crawling and swimming habit. Its telson was probably used as an organ of defense.
Slimonia, specialized in the position of its compound eyes at the anterior angles of the carapace, the broadened telson and slender form of body, seems adapted to swimming and by its well developed walking legs also to crawling.
All these three genera, in comparison with Eusarcus and Eurypterus, have relatively small and narrow swimming legs, which is the more surprising as for the rest of their organization they appear to be better adapted to swimming than the others. Quite likely the more slender body required less effort and smaller organs to paddle it.
In the group represented by Eusarcus, the carapace is subtriangular, raised in front, where it bears the marginal compound eyes, the walking legs are powerful and provided with long spines, the preabdomen is very broad and flat, the tail long and scorpionlike, with a curved spine that could be raised above the body as an organ of defense. This is a quite aberrant and highly specialized genus and doubtless its habits were different from those of the other genera. On account of its general form it is highly improbable that it was a good swimmer, in spite of the marginal, frontal eyes, and its scorpioid form indicates that it was given principally to crawling, scorpionlike, on the bottom and, as we have little doubt, to burying itself in the mud, with the eyes on the raised frontal end and the scorpioid tail projecting from the mud. It is a singular character of Eusarcus that the second pair of walking legs is longer than the rest, and the series decreases in posterior direction. This arrangement not only served to raise the eye-bearing frontal end of the carapace above the mud, but the long spiniferous legs in front undoubtedly aided also in seizing and holding the prey, the chelicerae being relatively small.
The swimming legs of Eusarcus are relatively large and heavy, the segments carrying the blade being short and stout. The legs were therefore more strong than agile and quite surely adapted to digging and anchoring the creature in the mud.
The group represented by Eurypterus and Dolichopterus is characterized by the prevailing broad carapace with dorsal eyes, broad to slender body, relatively strong swimming legs and a spiniform telson. The stout walking legs in all members of this group leave no doubt that they were able crawlers on the bottom, but there is considerable evidence to show that they burrowed in the mud and were also well able to swim; some species more given to one, some to the other habit. The evidence of a burrowing habit is seen in the relatively broad carapace with a distinct shoveling edge, the dorsal or subapical position of the lateral eyes, as in Limulus, which would allow them to project above the mud, the peculiarly long, mobile and yet strong articulation of the carapace and first body segment, and the spiniform telson.
It has been particularly pointed out by Holm [op. cit. p. 9] that in Eurypterus fischeri the articulation between the carapace and the preabdomen must have been one of great mobility, as indicated by the broad slit, closed only by a membrane between the lateral fulcra. This connection was nevertheless so strong that the first preabdominal segment is most frequently found attached to the carapace in dismembered specimens. These facts suggest that Eurypterus may have been able to use the articulation between the carapace and the abdomen somewhat like Limulus in pushing itself through the mud, a process which would be aided by the tail spine.
On the other hand, there is good evidence that the species of Eurypterus were able swimmers. We have elsewhere cited Holm's description of the adaptation of the swimming legs in E. fischeri, evident in the sharp anterior keel of the organ and the arrangement of the articulation allowing the turning of the oar blade into a vertical position. Still more conclusive is the preceding pair of spineless slender legs, which would seem to have no other function than to serve as balancers and aid in swimming.
The combined evidence of the characters of Eurypterus is that it was a still little specialized, primitive genus, not yet wholly adapted to either crawling, digging or swimming, but could perform all these functions and was on the whole a sluggish animal. As it is not provided with strong organs of offense, it probably lived on worms or carrion.
There are certain species of this genus, such as E. maria, which are distinguished by a remarkably slender form. As a rule these also have the eyes far forward on the small carapace. Their form indicates that they were more inclined to a swimming habit. Others have the broad head of a catfish and seem well adapted to resting on the mud and there awaiting their prey or digging for it as Limulus does. A good instance of such a species is E. pittsfordensis.
We consider the genus Dolichopterus well adapted to a swimming habit. This is indicated by the notably forward position of the compound eyes and the remarkable lengthening and broadening of the swimming legs. The lengthening has been produced not only by the great lengthening of the segments, but also by the development of the ninth segment (only a minute claw in Eurypterus), into a palettelike plate. We have also shown [see generic description p. 264] that the spines on the swimming legs are transformed into expanded leaflike appendages, which serve to broaden the limb. If these appendages were arranged in whorls, as in Limulus, the claim could be made that they served in digging as in the latter genus, but it is difficult to conceive that these appendages, arranged in a series on the posterior side of the limb, could have well served that purpose, since they would fail to push the mud outward as does the leg of Limulus, but would move it backward and inward. Nor is the great length of the swimming legs favorable to the digging function, for digging organs are always short and stout; and as for serving only as anchors in the mud it would not seem necessary to lengthen the limbs to such extent.
The last group, represented by Drepanopterus and Stylonurus, shows again a different adaptation. Here the legs exhibit a distinct tendency to become greatly lengthened without being broadened. It is obvious that these forms were not mud dwellers, and if we consider Drepanopterus as the ancestor of Stylonurus, they were originally crawlers.
Drepanopterus [see restoration pl. 54] possesses five pairs of walking legs which increase regularly in length backward and exhibit no differentiation, except that the frontal legs have longer spines and the last pair is spineless. The very broad shovellike carapace with its elevated eyes and the long styliform telson are, however, quite suggestive of a mud-grubbing mode of feeding.
In Stylonurus the tendency of Drepanopterus to lengthen the last pairs of legs has reached its extreme. While all its species exhibit these strikingly long last two pairs, different groups have developed differently in regard to the preceding legs. In some these have remained relatively short and shaped as in Dolichopterus, in others they have grown in correspondence with the last pairs, are highly spiniferous, the spines becoming very long and increasing greatly in number [pl. 49, fig. 6] and in others again the spines of the first three pairs show a tendency to become flat and broad.
In Woodward's well known restoration of Stylonurus the animal is given three short pairs of anterior legs and two very long, subequal pairs of posterior legs, the latter being regarded as for swimming, and the former for walking. Laurie, who recognized the probable derivation of Stylonurus from Eurypterus through Drepanopterus, considered the sixth pair as being reduced "from the typical digging foot to a purely crawling one," adding: "This may indicate more purely littoral habits, or a more active predatory existence, demanding rapid locomotion rather than firm anchorage." Beecher's life size restoration of S. excelsior accepted Woodward's conception of the posterior legs, adding only the bladelike appendages of the short first three pairs of legs observed by Hall and Clarke in the first and second endognathites of S. excelsior and suggested that these legs "served partly as swimming organs."
From the somewhat diffeient restoration of Stylonurus, at which we have arrived in this paper and which is fully set forth in another place we infer that the animal was comparable to the existing gigantic Japanese spider crab, which some of its species rivaled in size. Like that grotesque creature it probably used its long hind legs to shove itself forward over the muddy bottom, while its short front legs indicate that the head lay near the bottom, the front legs being used for walking and grasping, and perhaps also, where the spines are broadened, as swimming organs. The extremely long styliform telson frequently with a blunt extremity, may have served less as a protecting than as a supporting organ of the long abdomen, and have aided in righting the awkward creature when it was overturned.
In summing up the evidence regarding the life mode of the eurypterids, and in view of the variety of forms with incipient adaptations, it would be a legitimate inference that the whole group was not so highly specialized as the recent merostomes and was still able to use different methods of locomotion though no one of them with great proficiency. Some were best adapted to swimming, others to crawling and many to finding their food by grubbing in the mud.
Some light seems also to be shed on their probable mode of life by a consideration of the separate faunules to which they pertain. Thus the Otisville fauna is composed of notably slender forms, such as Hughmilleria shawangunk and Eurypterus maria, which indicate that a large element of the fauna consisted of agile, swimming species. The presence here of heavy beds of conglomerate with very thin shale seams between corroborates the view that these forms were but little adapted to digging in the mud. The prevalence of Stylonurus in the fauna would seem to support Laurie's suggestion that Stylonurus possessed purely littoral habits.
The sedimentary facies in which the Otisville and Schenectady faunas are involved is not a usual accompaniment of the eurypterids, as fine mud rocks constitute the prevailing sediment and relatively broad-headed forms the expression of the body.
If one tries to picture the group as a whole, the typical habit would appear to be that of the mud grubber, and the broad carapace, relatively broad preabdomen, the flippers and the tail spine will be the most important elements in producing this picture.
It is very interesting to note in this connection that a subclass of an entirely different phylum, viz, the Ostracophora (Cephalaspis, Pteraspis [Old Red Sandstone of Scotland]), among the fishes, lived at the same time with the eurypterids, is frequently associated with them in the rocks, had acquired the same mud groveling habit and a similar general form. The theory that these earliest fishlike vertebrates are derived from the arthropod stem, and have features in common with the merostomes (eurypterids) and arachnids (scorpions etc.) is still seriously defended by some competent investigators. Eastman, in an excellent essay on the evolutionary history of the fishes, has emphasized the fact that the merostomes and arachnids at this early date had already diverged too widely along certain directions from the primal trilobitic type of organism, to be the possible ancestors of backboned animals, and such resemblances as are shared by merostomes and early fishlike vertebrates are explained as "due to mimicry, or to adaptation of creatures of different grades to a similar environment."
We are here not so much interested in the problem of the possible derivation of the vertebrates from the merostomes, as in the fact of the
great exterior similarity of the eurypterids and ostracophores and the explanation of this phenomenon as resulting from adaptation to like conditions. The ostracophores have been generally regarded by paleontologists as owing their peculiar form to their mud-grubbing habit, and it may be inferred that the eurypterids, being of similar form, were of like habit and perhaps of like form because of similar habit.
GEOLOGICAL DISTRIBUTION AND BIONOMIC RELATIONS
In this chapter we shall first survey the geological distribution of the eurypterids in North America as indicated by the following conspectus, compare this distribution with that in Europe, and finally attempt a conclusion as to the physical conditions under which these strange creatures lived.
A Conspectus of American species arranged according to their geological occurrence
Beltina danai Walcott. Greyson shales, Montana
Strabops thacheri Beecher. Potosi limestone, St François county, Missouri
- Fowler, H. W. The King Crab Fisheries in Delaware Bay. N. J. State Mus. An. Rep't. 1907. p. 116.
- We may remark that in the accounts of the embryology of Limulus the great swimming ability and the swimming habit of the young are emphasized.
- U. S. Nat. Mus. Proc. 1895. 18: 368.
- N. Y. State Mus. Bul. 91, 1905, p. 142.
- The authors above cited with regard to the habits of Limulus, have not emphasized the use made by that animal of the telson in defending itself. When buried in the shore sand, at low tide, in spawning time, the erected spine protrudes through the sand, and thereby causes much annoyance to waders.
- In connection with the locomotion of the merostomes, the question suggests itself whether they may not have been in the habit of swimming on their backs; this swimming attitude being assumed by the young of Limulus, and directly asserted for the merostomes by Patten .
The peculiar attitude of the young Limulus has been described by Alex. Agassiz [1878, p. 75] in support of Walcott's claim that the trilobites swam on their backs, as follows:
"An additional point to be brought forward to show that the trilobites probably pass the greater part of their life on their back, and died in that attitude, is that the young Limulus generally feed while turned on their back; moving at an angle with the bottom, the hind extremity raised, they throw out their feet beyond the anterior edge of the carapace, browsing, as it were, upon what they find in their road, and washing away what they do not want by means of a powerful current produced by their abdominal appendages."
Patten [Origin of Vertebrates from Arachnids, p. 363] in the endeavor to correlate the neural (dorsal) surface of Pterichthys or of the fishes in general with the neural (ventral) surface of the merostomes assumes that the position of the eyes depends largely on the position of the animal in swimming, adding: "In Pterygotus, for example, where locomotion was probably largely effected by swimming on the haemal (dorsal) surface, the eyes have already become lateral—a position very unusual in Arachnids. This change is readily explained, since the original position in the embryo of all arthropods is neural; moreover, the history of arthropod eyes shows conclusively that they can assume any position the method of locomotion may demand." In citing further evidence for his claim of important resemblances between the cephalothorax of arachnids and the head of vertebrates, the same author says [p. 365]: "The trilobites probably swam, if at all, on their backs; and it is still more probable that the Merostomata, from their shape and the position of their oarlike appendages, swam in the same way. The larvae of Limulus, according to my own observations, always swim on their backs. Thus the way is prepared for the manner of locomotion in fishes." In view of the great contrast in the shape of their bodies, we doubt the propriety of comparing the swimming mode of the young Limulus with that of such eurypterids as Pterygotus and Hughmilleria. We have, however, been impressed from another consideration with the possibility of their having been able to swim on their backs. In casting about for living water animals of a shape similar to the general expression of the eurypterids, we could find no better analogy than the well known "water-boatmen" or "boat-flies" (Notonecta) of our ponds with their elongate elliptic bodies, anteriorly rounded carapace with large marginal eyes, and long natatorial feet in the middle of the body. We have again and again been struck with this analogy in looking over the young specimens of Eurypterus remipes with their outstretched swimming feet, or in wondering at the long and powerful oarlike feet of Dolichopterus. The species of Notonecta swim on their backs, but those of Corisa, a closely related genus of associated waterbugs (the "oar-feet bugs") of like outline as seen from above, swim like all other water insects on their ventral surface. It appears that the peculiar attitude of Notonecta is principally adapted to the easy accumulation and storage of air in the hairy covering of the upper (ventral) side and that the body is distinctly boat-shaped, the dorsal side being keeled, to facilitate the inverted mode of locomotion, while in Corisa as in the other swimming waterbugs the body is distinctly flat.
From analogy with Notonecta and Corisa we consider it possible that Pterygotus and other types of eurypterids could have assumed an inverted position in swimming, but there are several reasons why it is unlikely. The most important of these is that the inverted attitude is not the normal one for nektonic organisms, but distinctly an adaptation to special conditions. It is not apparent that any of these conditions existed in the adult eurypterids, and the marginal eyes of the Hughmilleria and Pterygotus group would permit, by analogy with Notonecta and Corisa, either attitude in swimming.
In regard to the probable swimming attitude of the long-legged eurypterid types, Drepanopterus and Stylonurus, we know no better analogy than the water spider of the middle European ponds (Argyroneta aquatica). Any one who has watched the ease with which this interesting arachnid swims by means of its long spider legs and even overtakes its prey of water insects, can not fail to appreciate the swimming possibilities of even the awkward looking Stylonurus. This water spider also differs from other spiders by having the median eyes raised on a round mound, and the lateral ones on oblique nodes, just as in the eurypterids and especially in Stylonurus. The water spider swims with its right (dorsal) side up, and by analogy we infer that the similar Stylonurus group did the same.
- C. R. Eastman. Iowa. Geol. Sur. Rep't 1908. 18:51f.
- There occur gigantic tracks in the Potsdam rocks of New York which, have been considered by good authorities as suggesting the presence of merostomes at that age. These tracks known as Climactichnites, were first described by Logan [Can. Nat. & Geol. 1860. v. 5] and later recorded by Hall [N. Y. State Mus. 42d Rep't. 1889. p. 25] from Port Henry, Essex co., N. Y., and by Woodworth [N. Y. State Mus. Bul. 69. 1903. p. 959] from the town of Mooers, Clinton co., N. Y. In the latter locality they assume gigantic proportions, being 6 inches wide and 15 or more feet long, terminating in an oval impression 16 inches long.
Various explanations have been suggested for these tracks. Besides having been referred to trilobites, burrowing crustaceans, plants, gastropods and annelids, they have been compared with those of the horseshoe crab, first by Dawson and recently again by Hitchcock and Patten. Sir William Dawson [Can. Nat. & Geol. 1862. 7: 271], who studied the American Limulus on the seashore, pointed out that when Limulus creeps on quicksand, or on sand just covered with water it uses its ordinary walking legs and produces a track strikingly like that described as Protichnites from the Potsdam sandstone, but in shallow water just covering the body, it uses its abdominal gill plates and produces a ladderlike track the exact counterpart of the