The kinds of sediments in which trilobites are entombed have so far afforded little evidence as to their habitat. Frech (Lethæa palæozoica, 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 Trinucleidæ, the dysphotic, large-eyed Remopleuridæ and Telephus, the blind nektonic Agnostidæ 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 larvæ.
A list of the methods of life noted above is given by way of summary, with examples.
| Pelagic |  | Planktonic | Primarily Secondarily | Earliest protaspis of all trilobites Deiphon, Odontopleura, etc. | ||||
| Nektonic |  | Primarily | Later protaspis of all trilobites. | |||||
| Secondarily | | Probably many thin-shelled trilobites with large pygidia (only partially nektonic) | ||||||
| Cyclopygidæ |  | (nektonic dysphotic) | ||||||
| Remopleuridæ | ||||||||
| Benthonic | | Crawlers and slow swimmers | Most trilobites with small pygidia. Triarthrus, Paradoxides, etc. | |||||
| Crawlers and slow swimmers | Most trilobites with large pygidia. Isotelus, Dalmanites, etc. | |||||||
| Crawlers and slow swimmers | Trilnuceidæ, Harpedidæ, some Mesonacidæ, etc. | |||||||
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 canal it is possible to infer something of the kind of food used by various trilobites.
The narrow glabellæ and axial lobes of the oldest trilobites would seem to indicate a carnivorous habit, while the swollen glabellæ 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 glabellæ of such genera as Deiphon or Sphærexochus 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 Palæozoic 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, competition 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 carcass, 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 passed 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 competition 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, passing 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: