Cambrian

Cambrian time represents perhaps 80 million years, and is important as providing the first rocks containing abundant fossils of animals with hard parts (see Precambrian chapter). However, few Cambrian fossil assemblages have been described from Britain and north-west Europe; they are thus represented in this book by only four sea floor reconstructions designed to give a few glimpses of Cambrian life rather than to be a comprehensive sample.

The Cambrian system takes its name from the Cambrian mountains of Wales, where it was first recognized by Adam Sedgwick. The Harlech Dome in North Wales consists of a thick sequence of largely deep-water sediments in which fossils are hard to find, and similar rocks are also found in the tectonically deformed Caernarvonshire Slate Belt. However, sediments deposited under relatively shallow water, in which fossils are more common, occur as small outcrops in south Wales, particularly near St David's, in the Welsh Borderland and in the English Midlands, particularly near Nuneaton (Rushton, 1974). Our four reconstructions are typical of these shallower water facies.

Cambrian fossils are rare in Scotland, but molluscs and trilobites from near Durness, Sutherland, also occur in rocks of the same age in Spitzbergen, Greenland, eastern Canada and Arctic Canada, indicating, as in the Ordovician, a northern continent separated by the Iapetus Ocean from England, Wales and Scandinavia (Fig. b).

1 Cruziana-Lingulella Community

In many localities throughout the world, Cambrian sediments show cross-bedding, ripples and other signs of deposition under relatively shallow water. In many places the only fossils are tracks and trails, which were partly due to the work of sabellid worms, and partly to the various activities of trilobites (Crimes, 1970; Seilacker, 1967). These sediments seldom contain the remains of the trilobites themselves, which suggests that these arthropods, like some today, spent only part of their time in shallow-water areas, and retreated,

Fossil Buried Sediment Discovering
c Rusophycus (trilobite trace)

perhaps with the tides, to deeper waters at other times. Trilobites have an exoskeleton which is commonly preserved as a fossil and consists of a thick dorsal body cover of calcite set in a framework of protein, and a very thin ventral skin of scleroprotein (which also covers the limbs), which is only preserved under exceptional conditions. The dorsal cover includes segments which are best developed in the middle part of the trilobite; in some trilobites the relative movement of these segments was slight, but in others there was considerable flexibility between adjacent segments so that the animal could roll up (like a woodlouse) for protection. On the ventral side, where protection was less important, the cover was very thin, but the appendages had sufficient strength for burrowing, swimming and feeding. Trilobites were mostly deposit feeders, but a few were probably carnivores (Bergstrom, 1973).

In the Cruziana-Lingulella Community the trilobite burrows take various forms. Some are elongate traces probably made while the animal was ploughing through the sea floor, and these forms are known as Cruziana. (In palaeontology it is more convenient to name animal traces by a different set of generic and specific names from those for the animals themselves; this allows a stable nomenclature to develop, even though it may be quite uncertain which trace belongs to which trilobite species). Cruziana is characterized

Fig. b. Geography of the North Atlantic region in Cambrian times (oceans after McKerrow and Ziegler, 1972; land after Cowie, 1974, p. 150). The trilobite faunas of North America are distinct from those of northern Europe;

Durness Trilobites

Community 8 is North American, but the remainder of the Cambrian and Ordovician communities illustrated are from northern Europe. The details of regions away from outcrops of Cambrian rocks are speculative.

by a double set of oblique grooves which were made by a set of appendages on each side of the trilobite. Extra oblique grooves, presumably made by further appendages, can also be present. Sometimes longitudinal grooves can also be seen, which are thought to have been produced by rigid spines dragging over the sea floor. Cruziana is the result of a trilobite in motion through the sediment, but sometimes trilobites dug resting places in the sediment; they then produced an oval trace, known as Rusophycus. Leg impressions are also known where trilobites appear to have disturbed the sediment surface only gently.

Fossils other than trilobite traces are rare in this community, apart from the inarticulate brachiopod Lingulella, which can be very abundant in some areas, often crowded along bedding planes. Lingulella occurs thus in the Upper Cambrian rocks of Wales, where shallow marine environments became widespread during the deposition of the Merioneth Series. Lingulella is generally not so elongate as its later relative Lingula (which survives with little change in its hard parts from the Ordovician to the present day). Lingula has a burrowing mode of life unique among brachiopods. Early Cambrian Lingulella is not known to occur in burrows, but may nevertheless have lived partially buried in the sediment.

2 Shallow Water Shelly Community

In slightly more open sea environments, Cambrian bottom-dwelling animals become more diverse than in the Cruziana-Lingulella Community. Trilobites are the dominant group of fossils. Paradoxides was a large trilobite (up to 40cm long), which lived in a range of marine environments; Solenopleura was smaller and had fewer segments than Paradoxides and it also had a larger tail; Bailiella was very similar to Solenopleura, but had no eyes; nor did Agnostus. Many blind trilobites appear to have had long and succes-ful evolutionary histories; so eyes were clearly not essential to all these animals. Trilobite eyes often covered a wide field of vision (Clarkson, 1966, 1966a). They were probably primarily useful in the detection of predators, but we have little idea what the predators were. However, in general, the majority of the blind trilobites seem to have been forms living in deeper water, where the amount of light available is small; shallower water blind forms like Bailiella must have relied on other senses for protection.

In addition to the trilobites, hyolithids and brachiopods are usually present in this community. Hyolithids are conical calcareous shells, sometimes with a covering plate called an operculum, which were probably inhabited by some epifaunal mollusc. They do not appear to have been closely related to any living molluscan class, h

Brachiopod Articulate And Inarticulate

and it is not certain how the animals functioned. The Cambrian brachiopods are mainly inarticulate forms (with no hinge teeth) and have a dark phosphatic shell. Most inarticulate brachiopods are epi-faunal, being attached to the sea floor by their pedicle, but a few, like Micromitra, had no pedicle and probably rested on the sea floor. Some articulate brachiopods also occur, differing from in-articulates in having a tooth and socket system which keeps the two valves in place. Articulate brachiopods all have a shell made of calcite. Many later forms developed a variety of internal plates and other calcite structures, but most Cambrian articulates, like Billingsella, have no internal hard parts other than teeth and sockets, and Billingsella is probably ancestral to the great majority of later articulates. All brachiopods, both inarticulate and articulate, are filter feeders. The trilobites, hyolithids and brachiopods can all occur in sands, shales or limestones; some species were tolerant of varied shallow marine environments, whilst other species are known only from one particular kind of rock.

Calcareous algae, which can be of any size from microscopic to a meter or so in diameter, are generally restricted to clear-water areas. Girvanella is a small intertwined tube-like form, which is locally common in Britain.

Fig. 2 Shallow Water Shelly Community a Paradoxides (Arthropoda:

Trilobita) b Bailiella (Arthropoda:

Trilobita) c Solenopleura (Arthropoda:

Trilobita) d Lingulella (Brachiopoda:

Inarticulata) e Billingsella (Brachiopoda:

Articulata: Orthida) f Micromitra (Brachiopoda:

Inarticulata) g Hyolithes (Caliptoptomatida) h Hyolithellus (uncertain affinities)

agnostid (Arthropoda: Trilobita)

3 Deeper Shelf Community

In those deeper parts of the shelf sea floor which were still well oxygenated, the Cambrian bottom-dwelling faunas show an increase in diversity. Apart from the especially high diversities associated with shallow coral reefs, this increase in number of species with increase in depth is generally characteristic of benthic communities from the Cambrian to the present day. In the deeper parts of the shelf fifteen or more genera of trilobites may occur at a single locality. Since food is scarcer at great depth, a greater variability and size range of animals is developed to exploit to the full the different types of possible feeding strategy available.

Trilobites are the most abundant group of animals in the deep shelf areas of the Cambrian. The four genera illustrated all occur in the Tremadoc, a time division which is placed at the end of the Cambrian in Britain, but at the start of the Ordovician in some parts of the world. Euloma is similar to its older relative Soleno-pleura. Agnostus is a small trilobite (usually less than 1cm long) which is often found with its tail folded up under its head, though it was probably a deposit feeder like many other trilobites and may only have curled up when threatened. Although nothing is known of the limbs of Agnostus, there is good reason to believe that Agnostus swam near the sea floor, in the same way as many arthropods today. Asaphellus, like Agnostus, had a head and tail equal in size, but it is a larger trilobite than Agnostus, with a flattened rim to its carapace. It probably lay flat on the muddy sea floor, and the rim could have helped prevent it from sinking into the mud. Platypeltoides is similar to Asaphellus, but it has no flattened rim and is smoother. This smooth outline may have assisted the animal to burrow through the sediment.

Other bottom-dwelling organisms present in this well-oxygenated deeper shelf include brachiopods and molluscs. Articulate brach-iopods are nearly all restricted to shallower areas during the Cambrian, but inarticulate forms, not very different from those found in shallow water, occur sporadically. Two groups of uni-valved molluscs can be present in small numbers: the hyolithids, such as Hyolithes, whose straight conical shells are generally more abundant in shallow-water sediments, and the gastropods, which have coiled shells. Piano-spiral bellerophontids are common in the Cambrian and Ordovician; those with thick shells probably crawled on their foot along the sea floor, just like many modern marine snails.

The earliest graptolites, which were bottom-encrusting forms, occur near the top of the Middle Cambrian. However graptolites were not widespread until planktonic forms evolved. One grapto-

Clase Graptolithina

Fig. 3 Deeper Shelf Community a Agnostus (Arthropoda: Trilobita)

b Dictyonema flabelliforme (Graptolithina: Dendroidea)

c Broeggeria (Brachiopoda: Inarticulata)

d Euloma (Arthropoda: Trilobita)

e Asaphellus (Arthropoda: Trilobita)

f Platypeltoides (Arthropoda: Trilobita)

g Hyolithes (Calyptoptomatidida)

Fig. 3 Deeper Shelf Community a Agnostus (Arthropoda: Trilobita)

b Dictyonema flabelliforme (Graptolithina: Dendroidea)

c Broeggeria (Brachiopoda: Inarticulata)

d Euloma (Arthropoda: Trilobita)

e Asaphellus (Arthropoda: Trilobita)

f Platypeltoides (Arthropoda: Trilobita)

g Hyolithes (Calyptoptomatidida)

Dictyonema Mode Life

lite with a world-wide distribution in Tremadoc time was Dictyonema flabelliforme, which either floated by itself or may have been attached to some other floating object, such as seaweed, by a long thread.

4 Olenid Community

The Olenid Community is characteristic of relatively stagnant bottom conditions under a variety of water depths. When bottom currents are absent, excess plant material accumulates on the sea floor, and the action of bacteria leads to oxygen deficiency and a high sulphide content in the bottom sediments. This environment occurs particularly in seas which were wholly or partially isolated from the main oceans, like the Black Sea today, and is represented by rocks in places throughout the stratigraphical column. The sediments are usually bituminous shales; currents are seldom strong enough to transport any coarser material. In these adverse conditions the diversity of bottom dwellers is never high, regardless of water depth. In the Cambrian the fauna of stagnant environments usually consisted of just a few trilobites; only olenids and some agnostids appear to have adapted enough to be able to tolerate these conditions, such as the small agnostid Lotagnostus, and the larger Olenus, Peltura and Ctenopyge. Characteristically only one or two of the forms are common at any one locality.

Agnostids can occur in various environments, but the other tri-lobites are only common when bottom circulation is reduced. The olenid family is characterized by having a broad head and a small tail, and in most olenids, such as Peltura and Ctenopyge, the eyes are situated in the normal place, one each side of the glabella, on slightly raised areas on the upper surface. However, in some other olenids the eyes are spherical and lie far back, or down on the sides of the head, much increasing the all-round field of vision. Ctenopyge has many prominent spines, and probably swam just above the sediment surface. None of these trilobites burrowed; there was probably an abundance of organic food on the sea floor. Their problem would not have been a shortage of food, but a shortage of oxygen for respiration. Compared with other trilobite groups, olenids have a relatively large number of segments and, since each segment would have had its own gill branch, this would no doubt have assisted them to obtain enough oxygen.

Few other fossils occur in this environment during the Cambrian, but some flattened masses with lath-shaped components can be found; these may be algal filaments (Taylor and Rushton, 1971). Olenid communities have also been described from the Ordovician (Fortey, 1975).

Fig. 4 Olenid Community a Peltura (Arthropoda: Trilobita) b Ctenopyge (Arthropoda: Trilobita) c Lotagnostus (Arthropoda: Trilobita)

Fig. 4 Olenid Community a Peltura (Arthropoda: Trilobita) b Ctenopyge (Arthropoda: Trilobita) c Lotagnostus (Arthropoda: Trilobita)

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Responses

  • alvaro
    What are the only other fossils better known than trilobites?
    6 years ago

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