TRILOBITA

TRILOBITA

General Considerations

The trilobites constitute an extinct group of exclusively Paleozoic arthropods in which the body was divided into a variable number of somites and partly encased in a supposedly chitinous integument. This integument consisted of a dorsal covering of mineralized chitin, a turned – under ventral doublure , one or more anteroventral plates (hypostome;² epistome), and a ventral membrane that is never preserved and probably consisted of non – chitinous and no mineralized fleshy substance.

The dorsal part of the integument, which is the part of the trilobite exoskeleton, most commonly preserved, typically has a prominent transverse trilobation, a characteristic that gives the name to the class. Ventrally, the living trilobite had a pare of biramous appendages on each somite and a pair of long delicate antennules extending anteriorly from beneath the head.³ only under expectation ally favorable conditions, however, were these delicate appendages and antennules preserved (Fig. 13-26).

The average trilobite was a small creature, usually 50 to 70 mm. (2 to 3 in.) in length, but some were quite tiny (less than 10mm.) and a few giants (e.g., Teratapis ; frontispiece) attained a length of more than half a meter (67.5cm., or 27 in.).⁴

Trilobites seem to have been exclusively marine, since their remains are always associated with those of typical salt - water animals (e.g., corals, crinoids, brachiopods, and cephalopods), and they are supposed to have been largely vagrant bottom dwellers, although some delicate spiny forms [e.g.,] odontopleurisds largely are thought to have been planktonic. Some were probanly scavengers; others may have been filter feeders.

Appearimg as widely differentiated and highly developed forms in earliest Cambrian seas, they were dominant among the invertebrates until the cephalopods displaced them in the Late Ordovician. Thereafter they declined gradually to the close of the Paleozoic when they became extinct. They attained world-wide distribution and are of great value as index fossils for local, continental, and intercontinental correlation. More than1,500¹ genera have been described, and these have been variously grouped into orders, super families, etc.

Little is known about the soft body that was covered by the strong dorsal integument. Most of what has been written on the soft parts has been inferred from structures apparent in unusually well preserved specimens or deduced by analogy or homology with living arthropods. Some aspects of the soft parats of ceraurus pleurexanthemus, a Middle Ordovician species, are shown in Fig. 13-27. because of the paucity of information on the soft parts of the trilobite, chief emphasis is laid on the exoskeleton in the following discussion.

In general, the parts of the exoskeleton where no movement took place were thickenend and strengthened with calcium carbonate; elsewhere the integument was thin and flexible. It has been assumed that these thinner places acted as joints for movement and some as places where adjacent exoskeletal parts could separate during molting.

In life the trilobite exoskeleton consisted of a trilobate dorsal shiled a small, ventral liplike plate, the hypostome, similar to the labrum of the crustacean; a thin ventral membrane (never found preserved) and numerous biramous, jointed appendages (Figs. 13-26, 13-30).

In most typical trilobites the dorsal shield is divided by two axial furrows into a prominent convex axial region, or axial lobe, flanked on each side by a pleural region, or pleural lobe. When viewed in transverse section (Fig. 13-27A), the trilobate profile of the dorsal shield is quite apparent. It should be noted, however, that the three regions are not freely separated as the integument is continuous across the entire exoskeleton.

Longitudinally the dorsal shield is divided into three distinct parts-the head, or cephalon the flexible middle part, or thorax and the abdominal part, or pygidium. Each of these exhibits transverse trilobation with a strongly conves axial part and more flattened pleural areas (Fig.13-28).

The axial part of the cephalon is termed the glabella and is generally arched prominently above the two lateral cheeks. The cheek areas of most trilobites are divided along a facial structure into outer free cheeks, which commonly become detached, and inner fixed cheeks, which are marginally continuous with the glabella. If eyes are present, they lie along the facial structure on the free cheeks. The thorax is composed of a series of imbricating segments that are supposed to have been united during life by thin flexible integument. Each segment, through divisible transversely into axial and pleural regions, is a single rigid plate. The pygidium is a segmented or smooth plate that consists of several posteriour segments more or less completely fused together. In some species the posteriour marginal area of the pygidium is prolonged into a caudal spine.

Few fossil trilobites are completely enough preserved to show the ventral side hence knowledge respecting appendages and other ventral structures is lacking in all but a few genera. In a few, however, the ventral side is preserved, and the number and nature of appendages have been determined from these specimens. In these there are five cephalic appendages-an anterior pair modified into antennules, followed by four pairs of essentially similar, biramous appendages. So far as know each thoracic and pygidial segment was also provided with a pair of biramous limbs, regardless of whether the dorsal shield segment is separate or fused with adjacent segments. The evolutionary importance of these biramous appendages is discussed later.

Students of trilobites have developed a complex nomenclature for designating the many different parts of the exoskeleton and for describing the wide variations shown by these parts. As yet there seems to be no universal agreement as to the use of many terms, but recent efforts at standardizing the nomenclature give hope that ultimately general agreement may be reached. An example is the recent attempt to standardize the nomenclature for Cambrian trilobites. In the following discussion only a few of the more general terms are used, but even these may seem to the beginning student to be numerous.

The Cephalon. The cephalon of most trilobites is a more or less rigid plate formed from the fusion of the five to seven anterior cephalic segments. It is typically somewhat arched in longitudinal profile, and in transverse profile shows the trilobation characteristic of the entire dorsal shield. Along the side and front margins it is ventrally reflexed to a greater or less degree, and the turned-under portion is designated the doublure. In outline the cephalon ranges from semicircular or semi elliptical to triangular, and the posterior margin is usually straight or but slightly curved. The angle included between the posterior and lateral margins is designated the genalangle, and if this part of the cephalon is prolonged posteriorly, the prolongation is the genal spine. The angle may be acute or obtuse, and the spine short and broad or long and pointed.

The axial portion of the cephalon constitutes the glabella. This may be segmented or smooth. It is divided by transverse furrows into an anterior lobe, one to three pairs of lateral lobs, and, if the lateral furrows do not join across the median line, a median lobe, and an occipital furrow separating the occipital ring. It has been suggested that the five glabellar somites represent integumental plates covering the five anterior somites of the trilobite ancestor, and that they became fused to form the glabella.

Furrow development and lobe shape exhibit considerable variation. In some species the glabella is quite small, whereas in others it constitutes most of the cephalon. It may be only a fraction of the length of the cephalon. It may expand forward, be parallel-sided, or narrow forward; it may extend to or even slightly beyond the anterior border. The furrows may be quite distinct, indistinct, or absent altogether, and they may extend entirely across the glabella or appear as notches on the sides. In some forms they join longitudinally, in which case they separate small lobes from the side of the glabella.

The occipital furrow is almost always well defined, even if the others are distinct.

The glabella is flanked laterally by the longitudinal axial furrows. It is also separate from the frontal margin of the cephalon by a part of the cephalon by a part of the marginal furrow.

The two fixed cheeks together with the glribclia constitute the cranidiiua. The fixed cheeks may be large or small depending upon the position of the facial suture. Is Conocoryphe (Fig. 13-36) they constitute more than half the cranidium, whereas in Aib&lelia (Fig. 13-36) they arc quite small. The asiai or dorsa) furrows arc vvell denned in some genera e.g., Calymene, Fig. 1 3-36), faint in others e.g.. Isotelus, Fig. 13-36), and almost invisible in still others (e.g.., Bamasius, Fig. 13-36). The fixed checks do not include the genai angles or spines unless the facial suture cuts the lateral margins of the cephaiors. About midway along the lateral margin of each fixed cheek is a bean-shaped elevation, the palpebral lobe. This may be connected with the frontal lobe of the giabelia, and it lies upward and inward from the eye itself which, if present is situated on the adjacent inner margin of the free cheek.

Sutures of several different kinds mark on the cephalon lines along which component parts are joined. These cephalic sutures are of considerable taxonomic and cntogenctic interest, and are illustrated in Fig. 13-29. The most conspicuous of these, and the ones generally most easily seen, are the facial swtures. These consist of two lateral branches that start symmetrically on the posterior margin or lateral doublures, cross the border and then take symmetrical courses forward over the dorsal surface of the cephalon to the anterior herder where they unite on the dorsal surface or along the anterior margin, or continue ventrally across the doublure as the connective sutures. In some cases the two branches meet the anterolateral angles of a special median rosti'ai shield, or rostrum (also designated the epistome), which interrupts the doublure (Fig. 13-29) and is typically ventral in position.

The anterior suture hounding the rostrum is the rostral suture. It is typical for the hypostome to be separated anteriorly from the cephalic doublure or the rost6rum by a transverse cleft, the hypostomal suture. In many Cambrian forms, however, this suture Is closed and marked only by an impressed line, so that the hypostome is continuous with the doublure. In many irilobites {e.g.. Cydopygidse; Phacopidae) the faciai suiurcs supposedly join amero-vei-(rally and are .not united with the hypostofnai suture, so that no rostrum is delineated and the doublure is continuous acroa me anteroventral part of the cephalon. In other forms, however, the fadal sutures are joined to the hyposroinaj suture by the connective sutures. There may, however, be only a single axial suture, as in the Asaphidac, in which case the designation median suture is useci. The reader interested in suture evolution is referred to the excellent discussions by Raw (1927) and StubbJefieM (1956).

The facial suture separates the cheek into an inner fixed cheek and an outer free cheek, and passes on the axial side of the eye which, if present, lies on the free cheek. The nature and position of facial sutures were once considered of great taxonomic iropo: tance, and trilobites were divided into three orders based upon whether the suture is ventral or marginal (Hypo-paria), whether it cats the posterior margin (Opisthoparia), in which case the free cheeks bear the genal angles or spines, or whether it cuts the lateral margin (Proparia), in which case the fixed cheeks carry the genal angles or spines. This classification has had to be modified because the assumptions on which it was based have been shoivn t" be invalid in important particu­lars. The t;vo terms cpisthoparian and proparian, however, are still useful for describing the facial suture.

The free cheeks separate easily from the cranidium in many genera (e.g., Ogygopsis, Fig, 13-366'), but in some {e.g., Ohmllus, Fig. 13-35) they are completely fused with the cranidium, and In a few others their presence is doubtful. It is thought that "the facial sutures served as places along which parts of the cephalon could separate in molting.

The eyes of trilobites are complex organs that vary greatly in size, position, and structure. Some trilobites were eyeless; some (e.g., Cryptoh'tkus; Harpes) had a tubercle with one or two facets; but by far the majority (e.g., Pkacops) had a multiple-face ted compound eye like that common among-many modern arthropods. The eye surface was convex outward, hence the trilobite had a wide range of vision. A few trilobites (e.g., Emriwrus, Fig, 13-375, and ERmnsToides, Tig. 13-3L4) had the eyes on short stalks.

Compound trilobite eyes are of two kinds. In one group the entire eye is covered with a transparent chitinous covering! the cornea. This covering may be quite smooth and give no indication of the compound nature of the eye beneath it, or it may be granular, thereby reflecting the underlying eye • facets. Eyes of this type, which are described as holochroal, are the ones found on most trilobites. The second type of eye, the schizochroal, has a separate cornea for each individual eye facet. The corneas may be circular or polygonal. Facets range in size from less than 0.10 mm. to as much as 0.50 mm. and in number from H to 600 among schizochroal eyes to more than 15,000 in some holochroal types. In most eyes the faces are regularly arranged with greatest economy of space (Fig.13-30).

Eye development of unusual nature is shown in the Ordovician Telephus (Fig. 13-30). The eyes are quite large and are situated on small free cheeks along the lateral margins of the cephalon. The eye surface is hemillipsodal; hence it is assumed that the animal could see what was going oh in every direction along or above the botton. The eyes of many trilobites (e.g., Nevadia; Wanneria) are situated on the ends of eye ridges that extend posteriorly over the free cheeks from the frontal lobe of the glabella.

The border of the cephalon is commonly set apart from the remainder of the shield, with which it is continous, by the marginal furrow. Along the outer margin the integument is turned under to form the doublure (Figs. 13-27B, 13-30), which may be interrupted along the anterior part by the median rostrum (Fig. 13-29). The underlip, or hypostome, is a subfrontal plate lying directly in front of the mouth. In front of the hypostome, in some forms is the rostrum or epistome, isolated by satures. A third ventral plate, the metastome, is situated directly behind the mouth (Fig.13-26A). The mouthitself faces backward, that is, toward the pygidium, an arrangement which is in the harmony with the assumption that the food was brought from that direction.

The Thorax. The thoracic part of the trilobite exoskeleton consists of a variable number of trilobed segments (from two in Agnoslus, Fig. 13-34, to 44 in Paedeumias robonensis). Which articulate with one another (fig. 13-27C) and thereby allow enrollment. Transversely the segments are a single piece but divisible to an axial and two pleural lobes, the Latter of which may be elongated into posterior spines., or pleura (singular, Pleuron) (Fig. 13-26C). Each segment has a frontal extension of axial portion that is inserted beneath the posterior margin of the segment directly in front (fig. 13-27C). These flanges permitted articulation of the segments and also protected the thoracic part of the body when the exoskeleton was enrolled. The axial part of each segment has a furrow at the enterior margin, and pleural furrows may also be present.

There seems to be a significant relation between number of thoracic segments and size of pygidium. Forms with few segments have relatively large pygidia (e.g. Agnostus, Eodiscus, and Isotelus; Figs. 13-35. 13-36), whereas those with many tend to have relatively small pygidia (e.g., Paradoxides, with 16 to 20, and Harpes, with 29. Fig. 13-36). As the evolutionary trend semms to have been toward reduction in number of thracic segments and increase in the number fused together to make the pygidium, it has been suggested that such form as Agnoslus and Eodiscus are actually advanced and specialized rather than primitive.

As with other a arthoropods, each new thoracic segement was added between the pygidium and the last segment of the thorax.

The pygidium. The pygidium constitutes the third and posterior part of the exosekeleton and in life covered the abdominal part of the body. It is a single transversely trilobate shield composed of a variable number (2 to 29) of segments firmly fused together. The individual segments may or may not be distinct. In some species it closely resembles the cephalon in shape an d size, Whereas in others it may differ greatly in these respects. Olenellus (fig 13-35D) seems to have lacked a pygidium altogether, whereas the pygidia and cephalons of some forms (e.g., agnostides) are so much alike that it is difficult to tell on from the other.

The axial lobe may extende the full length of the pygidium, as in Calymene; may range to only a fraction of it as in Scutellum 9fig. 13-36J); or may merge so completely with the remainder of pygidium that its identity as a distinct feature is lost, as in Bunastus (Fig.13-36I). Segmentation may be conspicuous, indistinct, or lacking altogether on the axial lobe and may be present or absent on the pleural lobes. The pygidial border may be continuously smooth or variously frilled with spines. There is usually a marginal furrow and the marginal border is reflexed to form a doublure, which is commonly of considerable width. Caudal and marginal spines are characteristic of certain groups.

The Ventral Side. The ventral side of a trilobite, except for the parts covered by the doublures and the plates around the mouth, seems to have been covered in life by nothing more that a soft epidermis or memberane which is never preserved. An axial groove extends from the posterior margin to the mouth and is bordered on each side by the appendages. Inward extensions of the basal segments of the appendages may have aided in conveying food to the mouth and in mastication (Fig.13-27A). Appendages are known is only a few trilobites, either because they were not preserved or because they are not visible in the fossil specimens. In some specimens (e.g., Triarthus) they were revealed only after careful removal of the hard rosk in whch the underside of the trilobite was embedded. Figure 13-26 shown such a form. Ventral Structures are now weel known in Calymene ( Flexicalymene). Ceraurus, Isotelus, Neolenus, and triarthrus, and imperfectly in a few other genera, but it seems likely that similar structures in other genera will come to light with further collecting. Reference is made in an earlier paragrph to the bypostome, epistome, and metastome, all situated around the mouth.

Appendages. Specimens preserving the Ventral side of a trilobite show five pairs of appendges on the cephalon (Fig.13-33) and a pair of biramous appendages on each thoracic and pygidial segment (fig. 13-26)

The typical trilobitan appendage (Fig. 13-32) is biramous and consists of two strikinglay different branches, the endopodite and exopodite, which spring from the outer extremity of a large basal segment, the coxopodite. The coxopodite is prolonged

inwardly as an endobase (gnathobase on the four cephalic appendages) and is articulated with a special process, the appendifer, fromed by invagination of the dorsal shield at the extremities of the transverse furrows of the axial lobe (Fig. 13-13).

The endopodite is a long slender jointed appendage composed of six segments, the outermost of which bears three bristles. The innermost or basal segment, which is termed the basipodite, articulares with the coxopodite along its inner extremity and partly supports the exopodite with which it is articulated along part of one side. the exdopodite seems to have been used for walking.

The exopodite, though somewhat variable in structure, typically cinsists of a flattened shaft with closely spaced bristle, or setae, along one margin. It articulates with the coxopodite, as well as with part of the basal segment (basipodite) of the endopodite, and in life it is probable that there was little individual freedom of movement between the two branches of the appendage. The exopodite is believed to have been used for both respiration and swimming.

The appendages of the thorax and pygidium are all essentially alike except for size, but the five pairs of cephalic appendages are somewhat modified and deserve some discussion. A point of possible evolutionary significance is the fact that the trilobite seems to have had only five cephalic somites and five pairs appendages, whereas arachnoids have six of each.

The anterior pair of cephallc appendages are uniramous, jointed antenules (Fig. 13-33). These are attached ventrally to the axial furrows on each side of the anterior glabellar lobe and are directed forward beyond the front margin of the cephalon. the other four pairs, which are postoral in position, are biramous and of the same fundamental structural plan as the thoracic and abdominal appendages, except that there were more or less modified for mastication.

The whole appendage articulated with the appendifer and must have been controlled in life by powerful muscles. The inner extensions of the four pairs of cephalic appendages are termed gnathobases because they are thought to have functioned in mastication; similar extensions on the thoracic and pygidial segments (endobases0 possibly aided in moving food forward along the axial groove toward the mouth.

Classification

In the different classifications of the Subclass Trilobita, the following characteristics, all exoskeletal, have been recognized as more or less important is differentiating major divisions,

1. Ontogeny as revealed in series of growth stages.
2. Nature and positions of facial sutures.
3. Numbers of thoracic segments.
4. Nature of cephalon or pygidium of both.
5. Absence or presence of eyes, and their structure.

Of these the first is the most important far because the serious of growth stages indicate the characteristics of the very young trilobites, which in may cases are quite different from the same features in adults. For example, the young of Calymene have indisputable proparian facial sutures, whereas in the adults the facial suture may bisect the genal angle, thus leaving the investigator who is dealing with only adult exoskeletons in doubt as to whether he should call it propoarian or opisthoparian.

For the first third of the present century the classification proposed by Beecher (1897) was widely used. On the basis of his classic studies, which in part consisted of the laborious excavation and preparation of the ventral sides of may specimens of Triarthrus becks (Fig. 13-26), he concluded that the trend and orientation of the facial suture and its positions with respect to the genal angle, together with the position of the free cheeks and the nature of the eves, affordeda fundamental basis for classification. Accordingly he subdivided the Trilbita into the following three Orders:

Order Hypoparia. The free cheeks form a continuous marginal ventral plate on the cephalon, extending in some form onto the dorsal side at the genal angeles, and the facial suture ranges from marginal submarginal.

Order opisthoparia. The ficaial suture cuts the posterior margin of the cephalon, and the free checks are limited to the dorsal side and include the genal angles or spines.

Order Proparia. the facial suture ends on the lateral margins, the free checks are dorsal in position, and the fixed checks bear the geneal angles or spines.

Although Beecher’s classification was the best that could have been made at the time and with the material available, it has had to be considerably modified and expanded in the light of important new discouveries1. Of particular evolutionary importance among these discoveries are the exquisitely. preserved specimens from the Middle Cambrian Burgess shall and the crtically important early growth stages that are being released from

1 Beecher (1901) thought that the free checks were at first ventral and the facial suture marginal, and that there were only eve lines and eye Cells(Ocelli) rather that true eves. This primitive conditions represents his hypoparian state. The facial suture is then supposed to have migrated from the margin onto the upper surface of the cephalon, with the genal angles on the free cheeks, and eyes are supposed to have apeared first along the margin of the cephalon and then to the moved inward toward the facial suture on the dorsal surface of the free checks. This stage represents Beecher’s opoisthoparian condition. Finally the facial suture migrated laterlly and then anteriorly until it cut the posterolateral border of the cephalon in front of the genal angles, which then became a part of the fixed check. This condition he supposed to be the most advance, and he designated it proparian.

Subsequent investigators, having the advantage of many excellently preserved specimens discovered isnc Beecher’s time, have found that some of the Hypoparia (Eodiscidae) had descendants with proparian cheeks, whreas other (Cryptolithidae) are through to have had an ancestor with the opisthoparian suture. These and other discoveries have necessiated abandoning the order Hypoparia, with the result that genera formerly included in this group have been assigned to several new order. Likewise, the ontogeny of leptoplastus salteri shows a proparian suture in youthful stages but an opisthoprian condition in adults, and this led Raw (1925;1927) to suggest that trilobites with proparian sutures in the adults stage are examples of arrested development.

Ordovician limestone by acid treatment 9Lalicker, 1935; Whittington,1941; 1941a; 1947; Ross, 1951).

The following classification, which is based on the discoverios and suggestions of many students of trilobite evolution and taxonomy,l assumes that the Class Trilobita is a natural group of rather closely related ancient arthropods, some of which may have been ancestral to other classes of Arthropoda;

order1. Agnostida.

Order2. Eodiscida.

Order 3. Olenelida.

Order 4. Opisthoparia.

Order 5. Proparia.

It is to be noted particularly that Beecher’s orders Opisthoparia and Proparia are retained, his order hypoparia is abandoned, and three additional orders are included – Agnostida, Eodiscida, and Oleneellida. The interest reader is referred to Rasetti (1948) for a recent discussion of these order and for pertinent references to the literature.

Order a. Agnostida. The agnostids are quite smal eyeless1 trilobits characterized by strong similarity of cephalon and pygidium, constant number (Two) of thoracic segments, complete lack of facial sutures and free cheeks, and highly specialized pygidium consisting supposedly of a constant number of grealy differentiated segments fused into a single rigid plate. Agnostids differ most strikingly from all other trilobites in the small and consistant number of thoracic segments and highly specialized pygidium. Thes last named feature represents the same tendency towards fusion of segments that is characteristic of the cephalons of all trilobites, and it is takes as proof that the agnostides are a highly specialized group that were differentiated from the ancestral trilobitan stock far back in the Pre-Cambrian, possibly before they acquired a hard test (White house, 1939) Lower Cambrian through Ordovician.

Agnotus (Fig. 13-34) is Typical of the order.

Some students of trilobites now believe that the agnostide are highly specialized trilobites that became blind secondarily.

Order2. Edisida. These are quite small trilobites characterized by equal sized but conspicuouly dissimilar cephalic and pygidial shields, small number of thoracic segments (two or three, as contrasted with 5 to 42 among other trilobites, except for the agnostides which invariably have two), and either total lack of facial sutures or small free cheeks of proparian type (Pagetia; Fig. 13-35). Some were blind, whereas other had eyes.

The cephalon has a tapered glabella, which never has the typical agnostid bilobation, and the pygidium is typically trilobitan with numerous segments apparent in the axial alobe and in few froms on the pleural lobes. The eodiscide do not seem to have had any post-Cambrian descendants. The North American species of the Family Eodiscidae were recently revised by Rasetti (1952). Lower and Middle Cambrian.

Eodiscus and Pagetia are examples of this small order (Fig. 13-34).

Order 3. Olenellida (also Called Mesonacida). The olenellids are typical trilobites in most respects, but differ in lacking distinguishable facial sutures (Rasetti aaould not include any genera with well-developed facial sutures) The cephalon is large, the pygidium small and simple, and the throax of many segments (13 to 27) . The eyes are large and the prominient curved palpebral lobes extend to the glabella. The olenellidas are among the oldest of trilobites ancestral stock long before the beginning of the Cambrian, and possibly to have given rise to both the Crustacea and the Arachnoidea.

Lower Cambrian.

Mesonacis (Fig. 13-35), with the third thoracic segement enlarged and the pleura extended as promoinent spines, and with the fifteenth segment having a large spine on the axial lobe, is typical of the order, wich includes among other the following familiar genera: Elliptocephala, Paedeumias, Olenellus, Molmia, Wanneria, and Callavia (Fig 13-35)

Order 4. Opisthoparia. This is the largest by far ofa all trilobite orders, and it includes among its members some of the olders (e.g., Bonnia; L. Cambrian) and all the latest (e.g., Neogriffithides; Permian) genera. All members of the order, which as presently constituted is almost certainly not a natural taxonomic group, share in common an adult opisthoprian facial suture. The eyes are holochroal and are situated on the free cheeks, which commonly bear promonent genal spines Lower Cabrian through Permian.

More than two dozen families of opisthoparian trilobites have been proposed, and these have been grouped into half a dozen superfamilies by different authors 9Swinnerton, 1915; Richter, 1933; Rasetti, 1948). Genera illustrating the variations characteristic of the order are shown in Fig. 13-36.

Order 5. Proparia. In proparian trilobites the facial sutures extend from the lateral margins of the cephalon in front of the genal angles, inward and forward cutting the front margin separetely, or uniting in front of the glabella; in the first case the two free cheeks come free from the cephalon as separate pieces, whereas in the second they from a single yokelike piece. In all proparian genera the geneal angle or genal spine is a part of the fixed cheek. Eyes are carried on the free cheeks of most forms, but a few of the more primitive genera seem to have been blind. The Proparia appeared in earliest Ordovician seas, differentiated rapidly during the Silurian and 'Devonian, and became extinct before the beginning of the Mississippian. A few Cambrian genera with proparian facial sutures [e.g., Pagetia, Fig. 13-34; Pagelidis) have been referred to this order, but they are so different from typical Proparia that they have been retained in the Eodiscida, with which they seem to have closer affinities. Lower Ordaaidan through Devonian.

A few representatives of the Proparia are^illustrated in Figs. 13-36 and 13-37

Ontogeny and Phylogeny

Trilobites are assumed to have laid eggs, but this has not been definitely-established (Wakott, 1879). Certain ovoidal bodies associated with trilobite. remains have been interpreted as fossil egg3, but their true origin is unknown. It would seem that the trilobite egg hatched out at an early stage in the ontogeny, because the youngest known larval stages differ greatly from the adult. Having attained fhe'first larval stage, the young crilobite, designated a protaspis, then passed through a succession of growth stages ending¹ finally in the adult. Forronately, specimens representing the different stages have been preserved, and it has been possible to reconstruct the ontogeny of a number of species. In such cmrogenetic studies, it has become customary to recognize three general periods of. Growth-protaspid, meraspid, and holaspid⁵-which mark, respectively, the beginning and the completion of the thorax (Fig. 13-38).

The protaspid period include all the earliest growth stages from the hatching to the appearance in the dorsal shield, or protaspis, of the first transverse suture, which divided the protaspis into two separate parts, the cephalon and transitory pygidiucs. The cephalon was complete with its five fundamental segments in the youngest protaspid stage; postcephalic segments constituting what has been designated a transitory or rudimentary pygidium appeared in later protaspid stages.

The meraspid period began with the separation of the cephalon and transitory pygidium by the transverse suture and ended with the completion of the last segment of the thorax. It consists, therefore, of a definite number ¹ of stages, or decrees, each of which was preceded by a molt and sharked by the appearance of a new segment. The degrees are designated as 0 to (;>. - 1), in which sequence 0 marks the beginning stage, when the cephalon and transitory pygidi^m were separated but before the first thoracic segment t«sd appeared, and n represents the number of segments in the completed thorax. Each new thoracic segment was added from the anterior border of the.pygid-sum, and if the trilobite molted"before forming the segment, it follows thai the number erf niclts during the period would be the same as the number of segments in the complete, thorax.

With completion of the thorax the larval stage of development ended and the trilobite, now an adult in all essential particulars; entered the holaspid period. Daring this period its most obvious change was great increase in size, although it also underwent further development It, form, culminating in the completely developed adult individual. It is assumed that the trilobite con­tinued to molt throughout the holaspid period, but it is not possible to determine exactly how many molts followed completion of the thorax. B.aw ''1927} estimate? that an individual of Leptopiastus salitri molted 3 times during the protaspid period, 12 times during the metaspid, and 14 times¹ during the for a total of 29 molts.

Although the three periods just described are defined by the development of the thorax, other important changes also took place during growth. Of special interest in this regard are the changes shown by the cephaJon. Peltura scurabaeoides, an opisthoparian, may be taken as an example to illustrate fhfjse changes. Figure 13-38 -shows two protaspid stages (A,B), two meraspid cephala (CD), one transitional meraspid-holaspid cephalon (E), and an adult cephalon of the holaspid stage. It is important to note that this species of Peltura has a proparian suture in the larval stages {i.e., during the meraspid period), but an opisthoparian suture in the adult stage. On the basis of this kind of development, Stubblefield (1936) has suggested that the proparian condition may be regarded as arrested development.

Two fundamentally different views of the significance of trilobite ontogeny have developed during the past half century. One view holds that the larval stages of development fairly closely represent the ancestry (£*., that the ontogeny recapitulates the phylogeny); the other considers them as second­ary adaptations to the larval conditions of life. Raw (1927) probably is not far wrong when he sug'gests that 'he truth lies between the two and that the position differs greatly in different cases. It might be added that many more ontogenies need to be worked out before the fundamental divisions of the Trilobita can be discriminated satisfactorily.

There arises from the preceding discussion the moot question, what is the ancestry and what are the descendants, if any, of the Trilobita? There now seems to be a wide consensus that the trilobites descended from a simple archetypal arthropod, which in turn evolved from an annelidan ancestor, and Raw (1927) concludes that the protaspid is predominantly embryonic and larval and that the paucity of its segments is ascribable partly to its small size and partly to inheritance from the larval stage of the ancestral annelid, Although the protaspis is comparable with the annelid larva,- it its quite different from the younger larvae of all classes of arthropods except the Arachnoidea; it is strikingly different !n some respects from the naupHus larva (Fig. 13-1) typical of the Crustacea, for example,, but it is somewhat similar to the -embryo of Limulus, the sole surviving genus of the Merc-stomata. This latter resemblance has led some investigators to suggest that the earliest arachnoids descended from a trilobitan ancestor (the Olenellida) (Raw, 1927).

Stormer has lai\g been a proponent of the ar.ichnoiciean affinities of the Trilobita, baling¹ his conclusion mijhe nature of the appendages in the two classes, and his views-a.nd"classifkation have been presented in several recent publications" (Stonner, 1933; 1939; 1942; 1944; 1949: 1951). In the restudy of specimens investigated earlier by Beecher (5895), Raymond (1920), and Walcott (1921, etc) Stormer (1939) claimed to have found at the base of the appendage a "pre-co*ai segment" to which the branchial ram us (i.e.\ the exopodite) is said to be attached. The branchial ramus is regarded as a pro-epipodke, and hence not homologous with the exopodite of the crustacean appendage. Extending this concept to the Trilobita in general, and also " considering the nature of the arachnoidean appendages, he reached the conclusion that "the trilobite limb probably is to be regarded as a prototype of the appendages in the Arachnomorpha [ = Merostomata + Arachnida + Trilobita, etc.,]"¹ and oniy remotely resembles the postulated biramous

Paleoecology¹

Inasmuch as trilobites are extinct, nothing can be directly determined about their habits and habitats by observation. Much about their paleo-ecology, however, uan be inferred from their faunal associates, their methods of burial and preservation) and their different exoskeletal structures. In a group of invertebrates as large and varied as the Trilobita, it is to be ex­pected that there must have been many adaptations to every marine en-? vironment. If trilobites ever lived in fresh waters or upon the land, either they left no record of that existence, or the forms that lived in those habitats have not yet been recognized as trilobites. In the sea there were certainly some . forrns adapted to both soft and hard bottoms (i.e., mud and sand on the one, hand and solid rock on the other). Others lived along the strand line, where ~ they wandered over the surface and burrowed in the soft sediments as do . some modern crabs. Still others were almost certainly swimmers or floaters. . Some of these probably lived near the bottom and spent more or less time on the bottom, whereas others, particularly spiny forms, may well have beenij; planktonic. Modified and specialized structures of different trilobites indicate-^ that species were adapted to many environments.

Certain types of trilobltes lived w great numbers on Paleeseic biohmns_: and in reef enviroEmeats that have been studied extensively the same genera are rnedy found both in f.'ia biofterm and in ilie interbiohenrial beds. Like­wise, tritobites seem to'have shown preference for different kinds of deposi­tions! environments- but the critical evidence for determining the nature arid extent uf this preference k only now beginning to be assembled.

The food of trilobites is unknown and their mode of feeding can only be conjectured. It is known, however, that trilobites Jived in close association with simple plants (algae), protozoans, sponges, worms of many kinds, bryo-zcaas, branchiopods, and mollusks, and it is assumed that some forms selected their food from among their-living associates. Soros were quite likely herbivorous, feeding on the simple plants of the earlier Paleozoic seas. Others may have been scavengers, cleaning up organic debris included in the soft sediments through xvhich they ploughed and burrowed.

Triiobites almost certainly served as food for other animals, and it may well bs that this was one of the important factors leading to their extinction. Among possible enemies were cephalopods, early chordates, sharks, and possibly even cannibalistic members of their own class. It seems more than a mere coincidence that trilobites underwent a marked decline in the Late Silurian and in the. Devonian with the advent of sharks and other early fishes, and it may well be that these new invaders soon swept the seas almost bare of triiobites, particularly of those forms thought to have been denizens of clear and open waters.

The original development of the exoskeleton was possibly a response to ar. unfavorable environment, though there may be other equally plausible reasons. The development of spines has also been ascribed to environmental conditions. Some investigators have suggested that the earliest trilobites had no exoskeleton and that cannibalistic forms may have attacked one another, since no other enemies are Shown. Such attacks would have eliminated those without protective "structures and would have put a premium on those that developed a protective armor. The armor would have served well until the sharks and other fish appeared in the Silurian and Devonian. The appearance of these early chordates may well have caused, or at least hastened, the ulti­mate extinction of trilobitesj since thereafter trilobite remains are not com­mon. Nek tonic forms probably were the first to go, as these would fall an . easy prey to the new enemy that moved swiftly through the water with mouth wide open to capture the swimming trilobites. There were also probably bottom-browsing fish which rooted and ploughed through the soft »«iiments in search of food just as modern carp still do. To such fishes the fflud-dweUing triiobites probably provided a good source of food. Many triiobite3 could enroll, thereby protecting their undersides, but this practice, ironically enough, instead of savingtihe animal may actually have aided the fi*n in capturing and swallowing it.

The ability to enrollseems to have been developed early, as a few Cam-wian forms are known to have had this ability. It became common in the Ordovician, Silurian, and Devonian, but was not acquired by all contemporary- triiobites. In enrollment the pygidiurn was opposed to the eepha--Ion in such a way as to cover the sntire ventral surface in the manner of an armadillo. The doiibJures adjusted themselves to fit one another and v/ere so perfectly opposed That no animal could reach the ventral part of the trilobite. It has bee:i suggested, hut not proved, that enrollment was de­veloped chiefty for protection against animal enemies,

Trilobkes, like many other- arthropods, molted periodically, but the number of times of molting is unknown, As a consequence of molting, ex-oskeietons greatly outnumbered actual individual organisms on the sea " bottom_s and it h likely, therefore, that the parts or the whole of several exoskeletons in a faunsl assemblage may have been made and sbed by the spime individual. The many fre.ft cheeks, cranidia, thoracic segments, and pygidia scattered sporadically through the sedimentary rocks should proba­bly be regarded In large part as fragments of molts rather than as parts of exoskeletons to which the trilobites were attached at the time of death and burial. The molts were no doubt of low specific gravity, with large surface area relative to volume, and hence could be transported long distances by slowly moving waters of low competency. In this way the remains might well have been buried far from the places where the animals lived or died and in localities where trilobites did not live and could not have lived. Such trans- -portation of molts must be given serious consideration in efforts to interpret the environments of trilobite life. Entire specimens- (/:«., wiu£ the ventral as well as dorsal structures preserved) indicate that the animal-^as inside at the time of burial and, further, that it probably lived and died quite near the place of burial.

Fossil Record and Stratigraphic Range

A complete trilobite exoskeleton is one of the rarest of fossils and when ■ found is likely to be more or less altered. Entire dorsal shields are relatively common, but antennules and appendages are rarely preserved. The usual j finds are separate cephala, cranidia, free cheeks, hypostomes, thoracic seg- j ments, and pygidia (Fig. 13-39). Such remains probably represent fragments of molted exoskeletons that came apart after being abandoned.¹ Certain ^ ovoidal bodies associated with trilpbite remains have been interpreted as-':* eggs; peculiar markings have been questionably identified as "nests," and . some tracks and trails have been ascribed to trilobites; but all these enigmatic ^: fossils are of uncertain origin.

The fossil record of trilobites is commonly in the form of chkinoid frag­ments or, if the actual exoskeletai substance ;is missing) impressions of the upper or under sides of the exoskeletai parts. In many cases the general integument is so well preserved that surface markings are still apparent, and in a few cases most of the original structure and substance of the eyes are* preserved.

The oldest known trilobite remains have been collected from Lower to the end of the Paleozoic, when the last few stragglers of the Permian seas disappeared and the race became extinct. The question is often asked of paleontologists, why did a vigorous and varied group of animals like the trilobites, which had a protective covering, wide range of adaptation, and large representation in types and numbers of individuals, decline so rapidly after such extensive development in Cambrian and Ordovician seas? No satisfactory answer has yet been given this question, but it should be noted that they were adapted to a biologic environment in which agile forms 'ike the fish were not present. Cannibalistic relatives may have assisted in the Qttline. Perhaps the giant cephalopoda of the Ordovician were also a factor. However, the sharp decline in genera and species with the advent of fish Wrongly suggests a causal relationship. Certainly after the Devonian, trilo bites were rare, and only a few (less than a dozen genera) rather generalized forms persisted through the Carboniferous into the Permian.

Trilobites had world – wide distribution and many are excellent index fossils. Cosmopolitan penera like agenostus, Eodiscus, Olenellus, Paradoxides, Cryptolithus, Bumastus, Phacops, Flexicalymene, and Dalmanitus are useful for intercomtinental correlation, and more restricted genera are equally valuable for continental and provincial correlation. Some formations locally contain large numbers of well -preserved remains (e.g., Burgess shale, Lodi siltstone, Utica shale, Niagaran limestone).

NAME : MAHESWARAN. J

SUBJECT : APPLIED PALEoNTOLOGY

TOPIC : TRILOBITA