تریلوبیت-Trilobita
Trilobita
class of extinct Paleozoic arthropods, occurring in marine rocks of Early Cambrian through late Permian age. Their closest living relatives are the chelicerates, including spiders, mites, and horseshoe crabs (Xiphosura). About 3000 described genera make trilobites one of the most diverse and best-known fossil groups (Fig. 1). Species diversity peaked during the Late Cambrian and then declined more or less steadily until the Late Devonian mass extinction. Only four families survived to the Mississippian, and only one lasted until the group's Permian demise. Their dominance in most Cambrian marine settings is essential to biostratigraphic correlation of that system. See also: Cambrian; Chelicerata; Devonian; Permian
Trilobites are typically represented in the fossil record by the mineralized portion of their exoskeleton, either as carcass or molt remains. The mineralized exoskeleton (Fig. 2) was confined mostly to the dorsal surface, curved under as a rimlike doublure (Figs. 1h and 2b); a single mineralized ventral plate, the hypostome, was suspended beneath the median region of the head (Fig. 2b). The mineralized exoskeleton was composed of low magnesian calcite and a minor component of organic material. Most of the ventral exoskeleton, including the appendages, was unmineralized.
| Fig. 1 Trilobite diversity and preservation. (a) Olenellid, Lower Cambrian (British Columbia). External mold of exoskeleton in shale. (b) Ogygopsis, Middle Cambrian (British Columbia). Molt assemblage, with cranidium and pygidium aligned, but librigenae, hypostome, and rostral plate inverted and rotated backward. Thorax is missing. Internal mold. (c) Slab of Upper Cambrian limestone (Sweden) with abundant disarticulated trilobite sclerites, mostly cranidia of Olenus. (d) Triarthrus, Upper Ordovician (New York). Exoskeletons replaced by pyrite, preserving antennae. (e–h) Struszia, Silurian (Northwest Territories), dorsal and anterolateral views of a cephalon, and ventral views of a cranidium and partial cranidium with attached librigena. Exoskeletons replaced by quartz; silicified fossils freed by dissolving limestone in acid. (i) Phacops, Devonian (Ohio). Enrolled exoskeleton, showing large lenses of schizochroal eye. (j) Griffithides, Mississippian (Indiana). Dorsal view of exoskeleton. |
Fig. 2 Morphological features of Trilobita. (a) Calymene, Silurian (
Morphology of exoskeleton
The term "trilobite" refers to the longitudinal division of the body into an axial lobe and two lateral pleural regions (Fig. 2c); axial furrows separate the three divisions. The head shield, consisting of up to six fused segments and an anterior presegmental region, is called the cephalon. Its median (axial) lobe contains the glabella, typically convex and indented by a transverse occipital furrow and several pairs of lateral glabellar furrows (Fig. 2a). In primitive trilobites, segments of the palpebro-ocular (eye) lobes can be traced across the anterior region of the glabella (Fig. 1a).
Trilobites preserve the oldest known visual system in the history of life. Most had rigid compound eyes analogous to those of a housefly. The eyes are situated on the pleural field (genae, or cheeks). Most trilobites had a large number of small eye lenses that shared a single corneal covering (holochroal eye) [Fig. 1e and f.] The suborder Phacopina, a major Ordovician-through-Devonian group, had large separated lenses (schizochroal eye) [Fig. 1i].
In most trilobites, a facial suture, used for molting the exoskeleton, is developed on the dorsal side of the cephalon; it passes from the ventral side usually in front of the glabella, separates the visual surface of the eye from the palpebral lobe, and exits the cephalon in front of, through, or behind the genal angle. These different configurations of the suture are termed proparian (Fig. 1e–h), gonatoparian (Fig. 2a), and opisthoparian (Figs. 1j and 2c), respectively. The area between the axial furrow and facial suture is the fixigena (fixed cheek); together with the axial region of the cephalon (including the glabella), this single skeletal part (or sclerite) is the cranidium (Fig. 1g). The librigenae (free cheeks) represent the pleural areas outside the facial suture. Most trilobites had the anterior branches of the facial suture separated on the doublure by a rostral plate (Fig. 1b and f), although some had a median suture and others lost the ventral sutures and fused the doublure medially. The hypostome was rigidly sutured to the roof of the doublure in some groups (the conterminant condition; Fig. 2b), but in others it was free and supported by soft tissue (the natant condition).
Anterior wings on the lateral part of the hypostome bear processes that permitted its attachment to a stalk in the cephalic axial furrow (Fig. 2b). The thorax is composed of from 2 to more than 60 articulated segments (although typically 6 to 16), each consisting of an axial ring and pleural band. Articulation of the thorax, via processes and sockets on adjacent pleurae, allowed flexibility for enrollment (Fig. 1i). The pygidium is a posterior sclerite composed of one or more fused segments. Primitively, it is much smaller than the cephalon, but is enlarged in many groups (Fig. 1b), and may bear spines along its margin. The cephalic doublure sometimes has notches or furrows that accommodated the pygidium and thoracic tips when the trilobite enrolled (Fig. 2b).
Appendages, preserved by pyrite or phosphate replacement or as films on shale, are well known for only a few trilobite species. A single pair of long, jointed antennae (Fig. 1d) projected forward from beneath the hypostome. Known Cambrian and Ordovician species have three pairs of postantennal cephalic appendages, while a Devonian example has four. In most cases, these show little structural differentiation from each other, or from postcephalic appendages on each segment along the length of the body (Fig. 3). The appendages are biramous, consisting of a jointed walking leg, or telopodite, and a filamentous exite, which attach toward the body axis to a spine-bearing coxa. Appendage-related musculature attached to the ventral exoskeleton at knoblike apodemes (Fig. 1e and h), just inward of the axial furrow. Enrollment and outstretching were achieved by flexor and extensor muscles; longitudinal, dorsoventral, and horizontal muscles have been observed, as well as a system of intersegmental bars. The exite (gill branch) functioned as a respiratory organ. The mouth opening was positioned above the rear margin of the hypostome and was directed posteriorly. The gut looped backward beneath the glabella, with the digestive tract extending along the axis to a posterior anus.
Fig. 3 Triarthrus eatoni, Upper Ordovician (New York). Reconstruction with dorsal exoskeleton removed on right side to show appendages. Antennae are incomplete (compare Fig. 1d). Exites of first nine postantennal appendages are removed to show structure of telopodite. The mouth was positioned above the posterior margin of the hypostome. (After H. B. Whittington and J. E. Almond, Appendages and habits of the Upper Ordovician trilobite Triarthrus eatoni, Phil. Trans. Roy. Soc. Lond., B317:28, 1987)
Development and molting
Embryonic development of trilobites is unknown. Phosphatized arthropod eggs, which may be those of trilobites, have been discovered in Cambrian rocks. The term "protaspis" is applied to the earliest calcified larval stages, in which the cephalon and protopygidium are fused as an unjointed dorsal shield (Fig. 4). Several molts may occur within the protaspid period. The meraspid period is defined by articulation of the cephalon and transitory pygidium as separate sclerites; successive degrees are marked by the release of segments from the anterior part of the transitory pygidium to form the thorax. The holaspid has the complete adult complement of thoracic segments; development in this period is marked by continued increase in size and by changes in shape, but without further addition of segments to the thorax. Adult size ranges from 1.5 mm to 70 cm (0.06 to 28 in.); 2–5 cm (0.8–2 in.) is typical.
Fig. 4 Flexicalymene senaria, Middle Ordovician (Virginia). Complete exoskeletons of protaspid larvae obtained from silicified residues. (a) Dorsal view and (b) ventral view of second of four protaspid instars for this species. (c) Dorsal view and (d) ventral view of fourth protaspid instar. Holaspides closely resemble the related genus Calymene (Fig. 2a). (After B. D. E. Chatterton et al., Larvae and relationships of the Calymenina (Trilobita), J. Paleontol., 64:259, 1990)
Trilobites show the typical arthropod solution to the problem of increasing size with a stiffened exoskeleton: they molted at regular intervals throughout the life cycle. In most species, this was effected by shedding the librigenae along the facial suture and shedding the hypostome. The soft-bodied animal emerged from the resulting gap. Several different molt strategies were employed by different trilobite groups, however, including shedding the entire cephalon, and inverting and rotating various skeletal elements (Fig. 1b). Molting results in the typical preservation of trilobite remains as disarticulated sclerites (Fig. 1c).
Ecology and macroevolution
Most trilobites were benthic deposit feeders or scavengers, living on the sediment-water interface or shallow-burrowing just beneath it. Some were evidently carnivores, equipped with sharp spines and processes projecting ventrally from their appendages. A few Cambrian and Ordovician groups acquired giant eyes coupled with narrow, streamlined bodies. The morphology and broad geographic and environmental ranges of these groups suggest they were active swimmers. Through their history, trilobites became adapted to all marine environments, from shallow high-energy shorefaces to deep-water, disaerobic habitats.
Trilobites are the most common marine fossils of the Cambrian Period, and their remains typically account for more than 90% of preserved Cambrian fossil assemblages. They were important through the Early Ordovician, but their numerical contribution to onshore communities was much reduced as a result of the Ordovician Radiation of marine life. This event saw filter-feeding organisms (for example, articulate brachiopods, bryozoans, crinoids) proliferate and rapidly evolve to dominate marine communities, a pattern that would last through the Paleozoic Era. Trilobites remained major components of deeper-water communities through the Silurian. Within-habitat, species diversity was generally constant in all environments from the Cambrian through the Silurian, despite their increasingly reduced relative abundance. This indicates that trilobites were largely unaffected by the major events of the Early Paleozoic and that their decline in importance was largely a function of increases in other groups.
Global trilobite diversity increased rapidly following the acquision of hard parts during the Cambrian Explosion, and it peaked during the Late Cambrian. Overall diversity gradually declined during the Ordovician, although a major subset of trilobite groups experienced an evolutionary burst during the Ordovician Radiation. The end-Ordovician mass extinction decimated the group, cutting their global diversity by about half. Surviving families were mainly those that had radiated during the Middle Ordovician. Global diversity continued to decline during the Silurian, although the most speciose trilobite faunas ever found occurred in this period. By the Devonian, trilobites were a relatively minor group, absent from many marine faunas, although still sometimes locally abundant. The Late Devonian mass extinction all but obliterated the trilobites, as only a handful of lineages survived. During the Late Paleozoic, trilobites were typically rare and confined to a limited number of facies. The last trilobites became extinct during the great end-Permian mass extinction. See also: Cambrian; Ordovician; Paleozoic; Silurian
Classification
Trilobita is usually assigned the ranking of class within Arthropoda. Affinities with Chelicerata are expressed by their grouping as Arachnata. Older classifications recognized a phylum or subphylum Trilobitomorpha, grouping Trilobita with an unnatural assortment of trilobite-, chelicerate-, or crustacean-like taxa lumped as Trilobitoidea. The soft-bodied Early-Middle Cambrian order Nectaspida is the closest relative (sister group) of the calcified Trilobita.
The high-level classification of trilobites remains controversial. Post-Cambrian groups (for example, orders Phacopida, Odontopleurida, Lichida, Proetida, Aulacopleurida) are well understood and are grouped into orders or suborders based on distinctive adult and larval morphologies. Cambrian trilobites are generally less well known (despite their abundance as fossils) and have tended to be classified in a small number of large unnatural orders such as Ptychopariida. A particular problem is a lack of understanding of the origins of post-Cambrian orders among Cambrian taxa, a phenomenon termed cryptogenesis. The result is that relationships between named orders of trilobites are essentially unknown. Recent progress has resulted from study of silicified life histories (Fig. 4), but inferring the high-level phylogeny of trilobites remains the cardinal problem in the paleobiology of the group.
A group of blind marine arthropods, the Agnostida, has traditionally been recognized as an order of trilobites. Agnostids share a calcified dorsal exoskeleton with Trilobita, but otherwise lack most diagnostic trilobite features, including a calcified protaspid stage, facial sutures, articulating thoracic segments, and a true transitory pygidium. The appendages of agnostids are also fundamentally unlike those of trilobites. Their affinities are currently debated, with some workers defending their position as ingroup trilobites and others considering the agnostids to be stem group Crustacea. See also: Arthropoda; Taxonomy
Gregory D. Edgecombe
Jonathan Adrain
Bibliography
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R. A. Fortey, Ontogeny, hypostome attachment and trilobite classification, Palaeontology, 33:529–576, 1990
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R. A. Fortey, Trilobite! Eyewitness to Evolution, 2000
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R. L. Kaesler (ed.), Treatise on Invertebrate Paleontology, pt. O (rev.), vol. 1, 1997
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H. B. Whittington, Trilobites, 1990Alifazeli=egeology.blogfa.com
Additional Readings
What are Trilobites?
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