The Appendages, Anatomy, and Relationships of Trilobites Part 16

The use of the pygidium as a swimming organ, suggested by Spencer (1903, p. 492) on theoretical grounds, developed by Staff and Reck (1911, p. 141) from a mechanical standpoint, and elaborated in the present paper by evidence from the ontogeny, phylogeny, and musculature, provided the animal with a swifter means of locomotion.

By a sudden flap of this large fin, a backward darting motion could be obtained, which would be invaluable as a means of escape from enemies.

Staff and Reck seem to think that in this movement the two shields were clapped together, and that the animal was projected along with the hinge-like thorax forward. This might be a very plausible explanation in the case of the bivalve-like Agnostidae, and it is one I had suggested tentatively for that family before I read Staff and Reck's paper. In the case of the large trilobites with more segments, however, it would be more natural to think of a mode of progression in which there was an undulatory movement of the body and the pygidium, up-and-down strokes being produced by alternately contracting the dorsal and ventral muscles. Bending the pygidium down would tend to pull the animal backward, while bringing it back into position would push it forward. It follows, therefore, that one of these movements must have been accomplished very quickly, the other slowly. If the muscle scars have been interpreted properly, the ventral muscles were probably the more powerful, an indication that the animal swam backward, using the cephalon and antennules as rudders.

The chief objection to the theory of swimming by clapping the valves together is that where the thorax consists of several segments it no longer acts like the hinge of a bivalve, and a sudden downward flap of the pygidium would impart a rotary motion to the animal. Take, for example, such nearly spherical animals as the Illaenidae, and it will readily be seen that there is nothing to give direction to the motion if the pygidium be brought suddenly against the lower surface of the cephalon. A lobster, it is true, progresses very well by this method, but it depends upon its great claws and long antennae to direct its motions. The whole shape of the trilobite is of course awkward for a rapidly swimming animal. It could keep afloat with the minimum of effort and paddle itself about with ease, but it was not built on the correct lines for speed.

Dollo (1910, p. 406), and quickly following his lead, Staff and Reck (1911, p. 130), have published extremely suggestive papers, showing that by the use of the principle of correlation of parts, much can be inferred about the mode of life of the trilobites merely from the structure of the test.

Dollo studied the connection between the shape of the pygidium and the position and character of the eyes. As applied by him, and later by Clarke and Ruedemann, to the eurypterids, this method seems most satisfactory. He pointed out that in Eurypterida like _Stylonurus_ and _Eurypterus_, where there is a long spine-like telson, the eyes are back from the margin, so that a _Limulus_-like habit of pushing the head into the sand by means of the limbs and telson was possible.

_Erettopterus_ and _Pterygotus_, on the other hand, have the eyes on the margin, so that the head could not be pushed into the mud without damage, and have a fin-like telson, suggesting a swimming mode of life.

In carrying this principle over to the trilobites, Dollo was quite successful, but Staff and Reck have pointed out some modifications of his results. The conclusions reached in both these papers are suggestive rather than final, for not all possible factors have been considered. The following are given as examples of interesting speculations along this line.

_Homalonotus delphinocephalus_, according to Dollo, was a crawling animal adapted to benthonic life in the euphotic region, and an occasional burrower in mud. This is shown by well developed eyes in the middle of the cephalon, a pointed pygidium, and the plow-like profile of the head. This was as far as Dollo went. From the very broad axial lobe of _Homalonotus_ it is fair to infer that, like _Isotelus_, it had very long, strong c.o.xopodites which it probably vised in locomotion, and also very well-developed longitudinal muscles, to be used in swimming. From the phylogeny of the group, it is known that the oldest homalonotids had broad unpointed pygidia of the swimming type, and that the later species of the genus (Devonian) are almost all found in sandstone and shale, and all have wider axial lobes than the Ordovician forms. It is also known that the epistoma is narrower and more firmly fused into the doublure in later than in earlier species. These lines of evidence tend to confirm Dollo's conclusion, but also indicate that the animals retained the ability to swim well.

On the same grounds, _Olenellus thompsoni_ and _Dalmanites limulurus_ were a.s.signed the same habitat and habits. Both were considered to have used the terminal spine as does _Limulus_.

_Olenellus thompsoni_ is generally considered to be unique among trilobites in having a _Limulus_-like telson in place of a pygidium.

This "telson" has exactly the position and characteristics of the spine on the fifteenth segment of _Mesonacis_, and so long ago as 1896, Marr (Brit. a.s.soc. Adv. Sci., Rept. 66th Meeting, page 764) wrote:

The posterior segments of the remarkable trilobite _Mesonacis vermontana_ are of a much more delicate character than the anterior ones, and the resemblance of the spine on the fifteenth "body segment" of this species to the terminal spine of _Olenellus_ proper, suggests that in the latter subgenus posterior segments of a purely membranous character may have existed devoid of hard parts.

This prophecy was fulfilled by the discovery of the specimens which Walcott described as _Paedeumias transitans_, a species which is said by its author to be a "form otherwise identical with _O. thompsoni_, [but] has rudimentary thoracic segments and a _Holmia_-like pygidium posterior to the fifteenth spine-bearing segment of the thorax." A good specimen of this form was found at Georgia, Vermont, a.s.sociated with the ordinary specimens of _Olenellus thompsoni_, and I believe that it is merely a complete specimen of that species. _Olenellus gilberti_, which was formerly supposed to have a limuloid telson, has now been shown by Walcott (Smithson. Misc. Coll., vol. 64, 1916, p.

406, pl. 45, fig. 3) to be a _Mesonacis_ and to have seven or eight thoracic segments and a small plate-like pygidium back of the spine-bearing fifteenth segment. All indications are that the spine was not in any sense a pygidium. Walcott states that _Olenellus_ resulted from the resorption of the rudimentary segments of forms such as _Mesonacis_ and _Paedeumias_, leaving the spine to function as a pygidium. This would mean the cutting off of the a.n.u.s and the posterior part of the alimentary ca.n.a.l, and developing a new a.n.a.l opening on the spine of one of the thoracic segments!

If the spine of the fifteenth segment is not a pygidium, could it be used, as Dollo postulates, as a pushing organ? Presumably not, for though in entire specimens of _Olenellus_ (_Paedeumias_) it extends back beyond the pygidium, it probably was borne erect, like the similar spines in _Elliptocephala_, and not in the horizontal plane in which it is found in crushed specimens.

While this removes some of the force of Dollo's argument, his conclusion that _Olenellus_ was a crawling, burrowing animal living in well lighted shallow waters was very likely correct. The long, annelid-like body indicates numerous crawling legs, there is no swimming pygidium, and the fusion of the cheeks in the head makes a stiff cephalon well adapted for burrowing.

Staff and Reck have pointed out that _Dalmanites limulurus_ was not entirely a crawler, but, as shown by the large pygidium, a swimmer as well. This kind of trilobite probably represents the normal development of the group in Ordovician and later times. The Phacopidae, Proetidae, Calymenidae, and other trilobites of their structure could probably crawl or swim equally well, and could escape enemies by darting away or by "digging themselves in."

_Cryptolithus tessellatus_ (_Trinucleus concentricus_) is cited by Dollo as an example of an adaptation to life in the aphotic benthos, permanently buried in the mud. In this case he appealed to Beecher's interpretation of the appendages, and pointed out that while the adult is blind, the young have simple eyes and probably pa.s.sed part of their life in the lighted zone. It needs only a glance at the very young to convince one that the embryos had swimming habits, so that in this form one sees the adaptation of the individual during its history to all modes of life open to a trilobite. The habits of the Harpedidae may have been similar to those of the Trinucleidae, but the members of this family are supplied with broad flat genal spines. It has been suggested that these served like pontoons, runners, or snow-shoes, to enable the animal to progress over soft mud without sinking into it.

Some such explanation might also be applied to the similar development in the wholly unrelated Bathyuridae. The absence of compound eyes and the poor development of ocelli in the Harpedidae suggest that they were burrowers, and from these two families, Trinucleidae and Harpedidae, it becomes evident that a pygidial point or spine is not a necessary part of the equipment of a burrowing trilobite. In fact, from the habits of _Limulus_ it is known that the appendages are relied upon for digging, and that the telson is a useful but not indispensable pushing organ.

_Deiphon_ is an interesting trilobite from many points of view. Its pleural lobes are reduced to a series of spines on either side of the body, and its pygidium is a mere spinose vestige. Dollo considered this animal a swimmer in the euphotic zone, because its eyes are on the anterior margin, its body depressed, its glabella globose, and its pygidium flat and spinose. That such a method of life was secondary in a cheirurid was indicated to him by the fact that the more primitive members of the family seemed adapted for crawling. Staff and Reck have gone further and shown that the pygidium is only the vestige of a swimming pygidium, and that the great development of spines suggests a floating rather than a swimming mode of life. They therefore argue for a planktonic habitat. A similar explanation is suggested for _Acidaspis_ and other highly spinose species.

The Aeglinidae, or Cyclopygidae as they are more properly called, present the most remarkable development of eyes among the trilobites.

In this, Dollo saw, as indeed earlier writers have, an adaptation to a region of scanty light. The cephalon is not at all adapted to burrowing, but the pygidium is a good swimming organ, and one is apt to agree that this animal was normally an inhabitant of the ill lighted dysphotic region, but also a nocturnal prowler, making trips to the surface at night. Similar habits and habitat are certainly indicated for _Telephus_ and the Remopleuridae, but whether _Nileus_ and the large-eyed _b.u.mastus_ are capable of the same explanation is doubtful.

Finch (1904, p. 181) makes the suggestion that "_Nileus_" (_Vogdesia_) _vigilans_, an abundant trilobite in the calcareous shale of the Maquoketa, was in the habit of burying itself, posterior end first. He found a slab containing fifteen entire specimens, all of which had the cephalon extended horizontally near the surface of the stratum, and the thorax and pygidium projecting downward. The rock showed no evidence that they were in burrows, and the fact that all were in the same position indicates that the att.i.tude was voluntarily a.s.sumed.

They appear to have entrenched themselves by the use of the pygidia, which are incurved plates readily adapted for such use, and, buried up to the eyes, awaited the coming of prey, but were, apparently, smothered by a sudden influx of mud. The form of the eye in _Vogdesia vigilans_ bears out this supposition of Finch's. Not only are the eyes unusually tall, but the palpebral lobe is much reduced, so that many of the lenses look upward and inward, as well as outward, forward and backward. The particular food required by _V. vigilans_ must have been very plentiful in the Maquoketa seas of Illinois and Iowa, for the species was very abundant, but that its habits were self-destructive is also shown by the great number of complete enrolled specimens of all ages now found there. The soft mud was apparently fatal to the species before the end of the Maquoketa, for specimens are seen but very rarely in the higher beds.

_Vogdesia vigilans_ is shaped much like _b.u.mastus_, _Illaenus_, _Asaphus_, _Onchometopus_, and _Brachyaspis_, and it may be that these trilobites with incurved pygidia had all adopted the habit of digging in backward. As noted above, their pygidia are not very well adapted for swimming, and most of them have large or tall eyes.

Dollo's comparison of the Cyclopygidae to the huge-eyed modern amphipod _Cystosoma_ is instructive. This latter crustacean, which has the greater part of the dorsal surface of the carapace transformed into eyes, is said to live in the dysphotic zone, at depths of from 40 to 100 fathoms, and to come to the surface at night. It swims ventral side down.

The kinds of sediments in which trilobites are entombed have so far afforded little evidence as to their habitat. Frech (Lethaea palaeozoica, 1897-1902, p. 67 _et seq._) who has collected such evidence as is available on this subject, places as deeper water Ordovician deposits the "Trinucleus-Schiefer" of the upper Ordovician of northern Europe and Bohemia, the "Triarthrus-Schiefer" of America, the "Asaphus-Schiefer" of Scandinavia, Bohemia, Portugal, and France, and the Dalmania quartzite of Bohemia. .

_Cryptolithus_ and _Triarthrus_, although not confined to such deposits, are apt to occur chiefly in very fine-grained shales, in company with graptolites. These latter are distributed by currents over great distances within short periods. It is somewhat curious that the nearly blind burrowing Trinucleidae, the dysphotic, large-eyed Remopleuridae and Telephus, the blind nektonic Agnostidae and Dionide, and the planktonic graptolites should go together and make up almost the entire fauna of certain formations. Yet, when the life history of each type is studied, a logical explanation is readily at hand, for all have free-swimming larvae.

A list of the methods of life noted above is given by way of summary, with examples.

{Planktonic {Primarily Earliest protaspis of all trilobites { {Secondarily _Deiphon_, _Odontopleura_, etc.

{ Pelagic { {Primarily Later protaspis of all trilobites.

{ { _Naraoia_ { { { { {Probably many thin-sh.e.l.led { { { trilobites with large pygidia { { { (only partially nektonic) {Nektonic {Secondarily {Cyclopygidae } {Remopleuridae } (nektonic dysphotic)

{Crawlers and { slow swimmers Most trilobites with small pygidia.

{ _Triarthrus_, _Paradoxides_, etc.

Benthonic {Crawlers and Most trilobites with large pygidia.

{ active swimmers _Isotelus_, _Dalmanites_, etc.

{ {Crawlers, slow { swimmers, and Trinucleidae, Harpedidae, { burrowers some Mesonacidae, etc.

FOOD AND FEEDING METHODS.

This subject has been less discussed than the methods of locomotion.

The study of the appendages has shown that while the mouth parts were not especially powerful, they were at least numerous, and sufficiently armed with spines to shred up such animal and vegetable substances as they were liable to encounter. It having been ascertained that the shape of the glabella and axial lobe furnishes an indication of the degree of development of the alimentary ca.n.a.l it is possible to infer something of the kind of food used by various trilobites.

The narrow glabellae and axial lobes of the oldest trilobites would seem to indicate a carnivorous habit, while the swollen glabellae and wider lobes of later ones probably denote an adaptation to a mixed or even a vegetable diet. This can not be relied upon too strictly, of course, for the swollen glabellae of such genera as Deiphon or Sphaerexochus may be due merely to the shortening up of the cephalon.

Walcott (1918, p. 125) suggests that the trilobites lived largely upon worms and conceives of them as working down into the mud and prowling around in it in search of such prey. While there can be no doubt that many trilobites had the power of burying themselves in loose sand or mud, a common habit with modern crustaceans, most of them were of a very awkward shape for habitual burrowers, and how an annelid could be successfully pursued through such a medium by an animal of this sort is difficult to understand. In fact, the presence of the large hypostoma and the position of the mouth were the great handicaps of the trilobite as a procurer of live animal food, and coupled with the relatively slow means of locomotion, almost compel the conclusion that errant animals of any size were fairly safe from it. This restricts the range of animal food to small inactive creatures and the remains of such larger forms as died from natural causes. The modern Crustacea are effective scavengers, and it is probable that their early Palaeozoic ancestors were equally so. It is a common saying that in the present stressful stage of the world's history, very few wild animals die a natural death. In Cambrian times, compet.i.tion for animal food was less keen, and with the exception of a few cephalopods, a few large annelids, and a few Crustacea like _Sidneyia_, there seem to have been no aggressive carnivores. In consequence, millions of animals must have daily died a natural death, and had there been no way of disposing of their remains, the sea bottom would soon have become so foul that no life could have existed. For the work of removal of this decaying matter, the carnivorous annelids and the Crustacea, mostly trilobites, were the only organisms, and it is probable that the latter did their full share. After prowling about and locating a carca.s.s, the trilobite established himself over it, the cephalon and hypostoma on one end and the pygidium on the other enclosing and protecting the prey, which was shredded off and pa.s.sed to the mouth at leisure by means of the spinose endobases.

Even in Middle Cambrian times some trilobites (e. g., _Paradoxides_) seem to have enlarged the capacity of the stomach and taken vegetable matter, but later, in the Upper Cambrian and Ordovician, when the development of cephalopods and fishes caused great compet.i.tion for all animal food, dead or alive, most trilobites seem to have become omnivorous. This is of course shown by the swollen glabella, with reduced lateral furrows, and, in the case of a few species (_Calymene_, _Ceraurus_), the known enlargement of the stomach.

_Cryptolithus_ is the only trilobite which has furnished any actual evidence as to its food. From the fact that the alimentary tract is found stuffed from end to end with fine mud, and because it is known to have been a burrower, it has been suggested by several that it was a mud feeder, pa.s.sing the mud through the digestive tract for the sake of what organic matter it contained. Spencer (1903, p. 491) has suggested a modification of this view:

The phyllopods appear to feed by turning over whilst swimming and seizing with their more posterior appendages a little mud which swarms with infusoria, etc. This mud is then pushed along the ventral groove to the mouth. Casts, of the intestine of trilobites are still found filled with the mud.

_Ceraurus_ and _Calymene_ also must have occasionally swallowed mud in quant.i.ty, otherwise the form of the alimentary ca.n.a.l could not have been preserved as it is in a few of Doctor Walcott's specimens.

TRACKS AND TRAILS OF TRILOBITES.

Tracks and trails of various sorts have been ascribed by authors to trilobites since these problematic markings first began to attract attention, but as the appendages were until recently quite unknown, all the earlier references were purely speculative. The subject is a difficult one, and proof that any particular track or trail could have been made in only one way is not easily obtained. Since the appendages have actually been described, comparatively little has been done, Walcott's work on _Protichnites_ (1912 B, p. 275) being the most important. Since the first description of _Protichnites_ by Owen (Quart. Jour. Geol. Soc., London, 1852, vol. 8, p. 247), it has been thought that these trails were made by crustaceans, and the only known contemporaneous crustaceans being trilobites, these animals were naturally suggested. Dawson (Canadian Nat. Geol., vol. 7, 1862, p.

276) ascribed them, with reserve, to _Paradoxides_, and Billings (1870, p. 484) suggested _Dikelocephalus_ or _Aglaspis_. Walcott secured well preserved specimens which showed trifid tracks, and these were readily explained when he found the legs of _Neolenus_, which terminated with three large spines. Similar trifid terminations had already been described by Beecher, and clearly pictured in his restoration of _Triarthrus_, but the spines and the tracks had somehow not previously been connected in the mind of any observer.

Walcott concluded that the tracks had been made by a species of _Dikelocephalus_, possibly by _D. hartti_, which occurs both north and south of the Adirondacks. In a recent paper, Burling (Amer. Jour.

Sci., ser. 4, vol. 44, 1917, p. 387) has argued that Protichnites was not the trail of a trilobite, but of a "short, low-lying, more or less heavy set, approximately 12-legged, crab-like animal," which had an oval shape, toed in, and was either extremely flexible or else short and more or less flexible in outline. This seems to describe a trilobite.

_Climactichnites_, the most discussed single trail of all, has also been ascribed to trilobites,--by Dana (Manual of Geology, 1863, p.

185), Billings (1870, p. 485), and Packard (Proc. Amer. Acad. Arts and Sci., vol. 36, 1900, p. 64),--though less frequently than to other animals. The latest opinion (see paper by Burling cited above) seems to be against this theory.

Miller (1880, p. 217) described under the generic name _Asaphoidichnus_ two kinds of tracks which were such as he supposed might be made by an _Asaphus_ (_Isotelus_). In referring to the second of the species, he says: "Some of the toe-tracks are more or less fringed, which I attribute to the action of water, though Mr. Dyer is impressed with the idea that it may indicate hairy or spinous feet."

The type of this species, _A. dyeri_, is in the Museum of Comparative Zoology, and while it may be the trail of a trilobite, it would be difficult to explain how it was produced.

Ringueberg (1886, p. 228) has described very briefly tracks found in the upper part of the Medina at Lockport, New York. These consisted of a regularly succeeding series of ten paired divergent indentations arranged in two diverging rows, with the trail of the pygidium showing between each series. The ten pairs of indentations he considered could have been made by ten pairs of legs like those shown by the specimen of Isotelus described by Mickleborough, and the intermittent appearance of the impression of the pygidium suggested to him that the trilobite proceeded by a series of leaps.

Walcott (1918, pp. 174-175, pl. 37-42) has recently figured a number of interesting trails as those of trilobites, and has pointed out that a large field remains open to anyone who has the patience to develop this side of the subject.