Russian Ordovician Trilobites
Overview & history
Trilobites constitute a class of fossil marine animals belonging to the Arthropoda group. It is a completely extinct group. The rests of trilobites are met only in the sea sediments of Paleozoic era. The time period of the existence of trilobites is about 350 million years (from the beginning of the Cambrian period and almost up to the end of the Permian period). Many morphological features of trilobites correspond to certain modern Arthropoda, for example to the kindred horseshoe crabs (Limulidae), however the majority of morphological features of trilobites are incident only in them and they are so original, that they deserve a detailed consideration.
The whole body of the trilobite is covered from above with a firm dorsal shield, which in the absolute majority of cases is a sole remaining thing, and usually, only by the shell, it is possible to reconstruct the structure of a trilobite.
The thickness of the shell, with rare exceptions, does not exceed 1 mm and consists of a number of parallel, extremely thin layers of calcium carbonate or calcium phosphate, formerly containing chitin. The surface of the shell often has a more or less expressed sculpture in the form of tiny pores, pits or very fine roller-like bands (the so-called terrace lines), etc. The sculptural elements of the surface are probably the parts of a sensory system of trilobites. The form of the shell is usually slightly flattened or convex, often with oblong-oval outlines. Very frequently the same genus is represented both by broader and by rather longer and narrower forms. This fact is usually understood as a manifestation of sexual dimorphism. Often the shell is decorated with various spines, barbs and nodes (Hoplolichas, Cybele, Cheirurus, Chasmops, etc.). At its margins, the shell usually has a doublure, which runs further to the bottom part, in the form of a rather broad and somewhat convex border.
By two longitudinal, sometimes almost parallel dorsal furrows running along the whole trunk and embracing a larger or smaller part of the head and a tail, the shell is divided into three parts or lobes (the name trilobite comes from here). The middle, the most convex, unpaired part made an axis of the animal. Under the axial part, there were probably the most important viscera of a trilobite (neural band, heart, intestine, etc.).
Two, usually flatter parts, located on both sides of the axis, are called pleural parts.
Across, a trilobite is also divided into three parts:
Cephalon
The cephalon includes the whole part of the shell, which is located in the anterior part of the trunk and movably joint with it. The cephalon was formed during the evolution as a result of merging several anterior segments. Usually, the cephalon has more or less rounded outlines with more or less straight posterior border. Normally the cephalon is much larger than the pygidium, however, certain forms (Asaphus, Megistaspis, lllaenus) have practically equal shields.
Along the border of the cephalon, there is a more or less broad strip or roll, which forms a sort of border. The border is divided into an anterior, lateral and posterior parts). Sometimes the border and the frontal field are expanded into broad limb (Harpes).
Glabella
The axial, the most convex part of the cephalon, limited at each side by dorsal furrows, is called glabella. By its form and relative sizes, it strongly varies with various forms. At the sides, the glabella is usually dissected by several paired lateral furrows, the so-called lateral glabellar furrows, which divide the glabella into several lateral glabellar lobes. The part of the glabella, usually slightly oblong, located in front of the anterior pair of lateral glabellar furrows, is called a glabellar lobe.
According to the degree of development, number and direction, lateral glabellar furrows vary with various forms of trilobites and are one of the main attributes used for systematization. In some forms they may be completely absent (lllaenus, Onchometopus); in others, on the contrary, they may be very deep (Pliomera, Cybele). Sometimes the lateral glabellar furrows are located in the middle and run parallel to the posterior margin of the cephalon. In some forms, the lateral glabellar furrows are skewed so strongly backwards, that, merging with subsequent furrows, they form longitudinal furrows (Metopolichas, Hoplolichas).
Behind, the glabella is separated from the occipital ring by a so-called occipital furrow, which often is very similar to the thorax segments. Sometimes on the glabella, there are traces of a bonding of internal muscles (the so-called muscle prints), which on the surface are seen as pits and thickenings usually of lighter (or darker) colour. At present we know, that under the glabella there was a stomach of trilobite, and judging by the size of the glabella one can tell, what was the size of the animal’s stomach.
Genae
A part of the cephalon, which is outside the dorsal furrows, is called genae. Behind, on each side of the cephalon, the genae form genal angles, which in some forms are extended into genal spines. The genae can be much reduced as a result of an excessive development of the glabella. The majority of trilobites have the genae combined of two parts. Their structure in various forms depends on the character of peculiar seams (facial sutures), which are dissecting the cephalon in form of sharp, thin lines. Till now it is not quite clear whether these seams added any mobility to the cephalon, however one may assert with confidence that they played an important role in the ecdysis of trilobites.
Facial sutures
The facial sutures are the major attributes for classification. The facial sutures usually cross the eye of a trilobite and divide the genae into mobile, or free (librigenae) and fixed (fixigenae) parts. The latter, usually closely connected with the glabella, form a middle part of the cephalon, the so-called cranidium. According to the character of a section of the cephalon, the facial sutures are divided into 4 groups:
- Proparian – the facial sutures which cross by their posterior (located behind the eye) branches the margin of the cephalon in front of the genal angles, so the librigenae do not participate in the formation of genal angles;
- Opisthoparian-the facial sutures which, by their posterior branches, cross the posterior margin of the cephalon, so that the genal angles are on the librigenae;
- Gonatoparian -the facial sutures which, by their posterior branches, run into the middle of a genal angle;
- Metaparian – non-functional facial sutures, characteristic for some Cambrian trilobites.
Eyes
For the majority of trilobites the presence of a pair of eyes on both sides of the glabella is confirmed, however there are many blind forms (Agnostus, Ampyx, etc.). The eye is usually dissected by facial sutures, so its convex part, which in fact is a visual surface, remains on the librigenae, and the internal part is often strongly raised and swollen; the so-called palpebral lobe remains on the fixigenae. The eyes of the majority of the forms are on special genal prominences, the height thereof differs considerably. Some forms have a more or less large swelling in the basis of an eye (Asaphus cornutus, Chasmops wrangeli, etc.). Sometimes the basis of an eye extends in form of a stalk, which for some forms is very high (periscope-like) and lifts the visual surface high above the body of trilobite (Asaphus kowalewskii, Cybele panderi, etc.). In certain forms (Harpes, Harpides, many Cambrian trilobites) the eyes are set on the ends of roller-like swellings (the eye ridges), leaving the glabella near its anterior end and crossing the fixigenae.
Visual surface
The eyes of trilobites are very varied in the structure of the visual surface. Eyes of the majority of trilobites consist of numerous, close to each other prismatic lenses (facets) covered with a common transparent envelope (holochroal eyes). The number of lenses may be 15000 and more. In some forms (mainly the representatives of the genus Phacopidae) the eye consists of rather large (up to 0.5 mm) and not very numerous (14- 600) rounded multangular lenses, each of them covered with an individual envelope, and is separated from the other ones (schizochroal eyes). Some trilobites (Harpes) have the eyes consisting only of 2-3 lenses. Some paleonthologists hold that the occipital node, existing in many Ordovician and Silurian trilobites is a middle, single ocellus.
Hypostoma
In its frontal part, the head shield is turned up; a convex shield-like plate, the so-called hypostoma, is connected to the inner margin of a doublure formed by it. The form and size of the hypostoma strongly varies with various forms. On the hypostoma they distinguish a central part (central body), anterior and posterior lobes and a number of furrows crossing the body of the hypostoma. Often in the posterior part of the hypostoma there is a pair of small oval formations, the so-called maculae. Sometimes on their surface there are elements reminding eye lenses by their form, therefore some paleontologists think that the maculae are certain ocular organs on the bottom surface of trilobite. Some trilobites in front of the hypostoma have a plate, smooth or with spines, the so-called rostral shield or rostrum. The size of the rostrum depends on the distance between the anterior ends of facial sutures and the width of the doublurea.
Thorax
The thorax, unlike the non-separated head shield, consists of connected mobile segments, their number varying in different genera between 2 and 42. The number of segments sometimes varies in one genus (Ampyx, Cheirurus). There is a definite reciprocal ratio between the number of the thorax segments and the size of the pygidium. If the pygidium is large, the number of the body segments is usually small (8-12), and vice versa. Each segment is divided by dorsal furrows into a middle part, an axial ring, and two lateral parts, or pleurae. Every axial ring is immovably connected with the pleurae, and usually is very convex. In front the ring has a lower arch-like prominence (articulating half-ring). The ring connects every successive segment with a previous one. The posterior margin of every axial ring is slightly concave.
Pleuras
The pleurae of the majority of trilobites have on their surface diagonal (secant the pleura askew) or longitudinal (running parallel to the margin of the pleura) furrows (pleural furrows). Some trilobites (Pliomera) have roller-like pleurae (with a convex roll running along them). In some trilobites the pleural furrows are very poorly expressed (Nileus) or are completely absent (lllaenus). The pleurae of various genera may be regularly broad and uniform, or may have twitches, swellings (Cheirurus), or they may be covered with spines (Hoplolichas). The ends of the pleurae may be blunt (rounded), sharp, or drawn back in form of acinaciform outgrowths (Cheirurus, Basilicus). Sometimes the ends of the pleurae are prolonged into long thin spines (Cybele). Free ends of outer parts of the pleurae always consist of two layers of the shell i.e. they have a turn (doublure).
At this point, considering the internal parts of trilobites one should mention one important fact, inherent practically in all trilobites, beginning with the Ordovician times – it is their ability to roll-up. Trilobites acquired this feature only in Ordovician; the Cambrian forms did not have this ability (or it was not necessary). Undoubtedly the rolling-up was a peculiar form of shielding the underside of a body against any sort of damages, and most likely against their potential enemies – cephalopodae. While rolling-up, the body segments supplied with an articulating half-ring, were slightly displacing, the ends of the pleurae approached each other and shielded a soft body of trilobite also from the sides. The margin of the pygidium was closely pressed to the edge of the cephalon (the size of the cephalon was approximately equal to that of the pygidium), or the pygidium intruded deeply into the cephalon (the size of the cephalon equaled the size of the pygidium). Some trilobites had the means for a more reliable shielding. Sometimes they are called “locks”. Basically, these instruments are perforations or cut-ins in the doublurea of the posterior part of the librigenae and pleurae of the thorax. The anterior margin of these cut-ins is in form of a node, which serves as a detent for the anterior part of a subsequent segment of trilobite. In some trilobites (mainly the representatives of a group Asaphidae) the “lock” is very well developed, and its shows on the doublurea of the genae and the segments are called “panderian organs” in honor of a Russian paleonthologist Christian Pander, who was the first to describe this system for the genus Asaphus.
Pygidium
The tail shield or pygidium consists of a continuous part of a shell, formed in the same way as the cephalon as a result of mergence of several segments, and flexibly joined with the thorax. On the surface of the pygidium, as well as on the trunk a more or less sharply outlined by dorsal furrows middle part (rachis) and two lateral parts (the pleural parts of the pygidium) are distinguished. Sometimes the pygidium in appearance and size reminds the cephalon (Agnostus). In other trilobites, on the contrary, the pygidium so much looks like the segments of the thorax, that it becomes difficult to find a border between the thorax and the pygidium. One encounters the forms with strongly smoothed segmentation of the pygidium (lllaenus, Nileus). By its outlines the pygidium is more often semicircular or parabolic, less often triangular or trapezoidal.
Border
In the same way as in the cephalon, along the margins of the marginal border of the pygidium there is a more or less flattened strip (less often a roll). Sometimes the margin expands into a rather broad flat limb. Like on the cephalon, along the margins of the underside of the pygidium runs a doublurea, which in some trilobites (Asaphidae) is rather wide.
Rachis
The rachis of various genera and forms greatly varies in length, form, and a degree of segmentation. Sometimes it may be absent (Nileus). On the pleural parts of the pygidium the segmentation can be strongly expressed or completely absent.
The underside and soft tissues of trilobites
Usually as one is carefully dissecting the bottom (ventral) part of a trilobite, in the overwhelming majority of cases one sees only an empty hollow of the inner shell with a hypostoma attached (“suspended”) to the doublurea of the cephalon. However in very favorable conditions of a burial place (usually in argillaceous slates), samples with the partially preserved limbs are found. Based on the unique findings in Canada (Ordovician limestones near Ottawa), USA (the Ordovician Utica Shale near Rome, N.Y.) and several other exclusive places (in Germany and Czechia), the ventral part of trilobite has been well enough reconstructed. It has been definitely established that trilobites had a very thin abdominal wall (membrane), attached to the margin the doublurea of the cephalon, thorax and pygidium. Probably it was supported by a pair, gradually thickening with age, limified outgrowths, growing away from the bottom part of the dorsal shield in the dorsal furrows region. A channel in the visceral cavity located under the rachis represents the digestive system. It started with a mouth orifice behind the posterior wing of a hypostoma, bent backwards and, while extending, formed a stomach, which was under the glabella. On both parts of the stomach there were liver outgrowths (vascular prints), especially clearly seen on the reverse part of the genae. Further, gradually narrowing, the intestines extended under the rachis almost up to the end of the pygidium. An extremely rarely observed long multi-chamber vessel, lying above the digestive channel, is sometimes thought to be a heart of trilobite. A structure of the nervous system of trilobites is absolutely unknown, however certain evidences allow to assume the presence of a rather complex sensory system in many trilobites. Its existence presumes an existence of a complex nervous system. The surface of the shell of many trilobites was covered with sensitive penicilluses and vibrissae; their traces are preserved in a form of thin perforated pores and pits. Every penicillus was connected to nerve-endings, which in their turn were connected to the central nervous chain, apparently located under the axial part. The nerve-endings probably were also along the terrace lines.
The limbs of trilobites have been studied now on the examples of many specimens. On the underside of the head there was a pair of long, multi-limb antennae, located on each side of the hypostoma, behind which there were four pairs of short splitted limbs (2-3 pairs of them are considered to serve as mouth limbs). Further, under each segment of the thorax there were two pairs of doubled motor limbs. An external branch of each limb, or exopodit was covered with a rich fimbria of placoid penicilluses. Probably the exopodits were the external parts of the gills of trilobites, though it is possible that those were natatory limbs (specific “ores”). The internal branch of a limb, or endopodit, by its structure, served only for crawling.
