On the Coast of Angaride: Classes in Paleontology

Angaride is the name of a Paleozoic continent which existed in place of Siberia between 570 and 250 million years ago called after the Angara River, where sediments throughout the catchment (and elsewhere in Central Siberia) store extremely rich Paleozoic flora of temperate climate. Time transformed the layers of dead leaves, brunches, and tree stumps into thick coal measures of the Tunguska and Kuznetsk basins. The name Angaride sounds like Atlantide, a fabulous land believed to have sunk into the sea abyss in the distant past. Angaride, on the contrary, had been rising up from the ocean for millions of years. It was encircled by shallow seas and broad shelf expanses. Continental deposition there began over 4 billion years ago. Angaride moved from the southern to northern hemisphere in a slow but steady travel, and its large crustal block was at the equator in the earliest Paleozoic. The former geographic position of the continent left an imprint on its history. Warm shallow seas were favorable to building an enormous barrier reef, about 1500 km in diameter, which enclosed a giant evaporation salt basin separated by a neck from a basin of shale deposition. Thus there arose a broad range of habitats which became home for marine organisms. It might be there, who knows, that life, the first live cell, made its appearance on the Earth…

Over half a billion years ago, at the Precambrian (Cryptozoic)/Paleozoic boundary, the Earth’s water and air chemistry suffered dramatic changes, largely due to the effect of live organisms and their ability for photosynthesis. Ubiquitous single-celled algae consumed carbon dioxide and released oxygen, and extracted dissolved carbonates from water. As a result, they reduced atmospheric CO₂ and increased O₂, and, when dead, produced kilometers-thick algal limestones. Animals responded to the chemical change by developing a skeleton, an endoskeleton (as in fishes) or an exoskeleton (as in turtles).

Archaeocyaths were the first to build a complex skeleton. They were followed by trilobites, arthropodal organisms which were dominant in the Cambrian, proliferous through Ordovician, Silurian, and Devonian time, and became extinct by the close of the Permian. The first carnivores, cephalopodal molluscs, initiated as early as in the Cambrian to become true sea terrorists later in the Ordovician.

LOWER MULTICELLULAR ORGANISMS

Lower multicellular organisms were the first in the geological history to develop a skeleton. The event occurred at the Vendian/Cambrian boundary and may have been driven by the change in water chemistry or was helped by simultaneous weakening of hard UV radiation due to better screening by atmospheric gases; or, maybe, a dramatic change in water salinity caused mass mutations responsible for the unusual ability in living organisms. All these are just hypotheses.

Some Siberian scientists assign lower metazoans to a separate kingdom of Inferibionta with two subkingdoms of Archaeata, extinct organisms, and Porifera, extant sponges.

Paleozoic Archaeata built sophisticated frameworks of a just technocratic habit with interleaved perforated cones and cylinders intricately divided by septa, a tracery of bars and tubes, geometrically lined cubes or heavy tablets, and labyrinths of peculiar cellular plates.

The nature of these fantastic fossils is sometimes hard to decipher. Paleontologists classify Archaeata either with animals or with plants; others suggest they are dividual organisms. Dividualism in this context is a special dual combination when some organismal structure behaves as an individual and at the same time as a colony of single-celled animals.

ARCHAEOCYATHS

The name comes from the Greek arche “ancient” and cyathus “cup” and refers literally to a cone-shaped stone cup, though some forms rather resemble a mushroom or a dish; or sometimes a queer waist turns a regular-shaped cone into a fancy figure baffling all description.

Archaeocyaths lived in warm shallow seas and carpeted the sea floor, either as individuals or sometimes formed colonies. They flourished briefly during the Early Cambrian and then declined drastically to be extinct already by the latest Cambrian. They had an intricate morphology consisting of two nested perforate cones (outer and inner walls) connected by a series of vertical and horizontal porous septa joined in turn by numerous calcareous pillars, tubes, or laminae. The “ancient cups” were supported by root-like long fibers called holdfasts. Some species were 70 cm in diameter and 1 m high — cups for giants! Yet most were as small as under 6 cm in diameter. Judging by their skeleton, they must have been suspension feeders and ate single-celled algae and bacteria. Food got inside the cones through the pores in the walls and in the septa. This quite boring way of eating is unambiguously diagnostic of animal origin. The workers who classified archaeocyaths with plants looked for traces of photosynthetic tissue on the outer surface exposed to light. If they are right, what would be then the function of the numerous pores in the intricately arranged space between the two cones? Having compared archaeocyaths with the extant sponges scientists however agreed to group them together. The principal metabolic processes in archaeocyaths developed between the outer and inner walls where water entered continuously through the pores. The pores on the outer wall were always smaller than those on the inner wall. According to recent modeling of water biomechanics, this structure allows passive (without cilia, flagella, or feelers) flow of water into the central cavity wherefrom water takes wastes away through the top aperture.

TRILOBITES

Having appeared at the dawn of the Paleozoic, trilobites soon settled over all seas of Angaride and remained purely aquatic animals till their extinction in the Permian. They possibly evolved from primitive worm-like ancestors but the very inception of these highly organized organisms looks rather mysterious. Already the earliest forms had a perfect structure which allowed their nearly 300 million year long survival and diversification into numerous species.

The evolution offered to living organisms the choice between exoskeleton and endoskeleton. Trilobites chose exoskeleton as a hard chitin carapace with longitudinal and latitudinal trilobation (separation into three lobes) wherefrom is the name Trilobita.

The growth process in trilobites was accompanied by molting. Growing trilobites shed their carapaces in the same way as the modern crayfish. Numerous exuviae found here and there do not necessary mean that plentiful colonies of trilobites used to live here; it is very likely that this place was good for the complicated and vital procedure of molting. Once deposited, the carapace chitin became replaced by inorganic components, most often carbonate (limestone) or less often pyrite and silica, which provided the perfect preservation of the fossils.

Some trilobite species had carapaces consisting of dozens of articulated hard segments. The body was longitudinally divided into three regions of the head shield or cephalon, the thorax, and the anal shield or pygidium. The hard cephalon was separated by sutures into the central part and two lateral segments of free cheeks. The thoracic section of the exoskeleton was made up of identical articulated tergites varying in number in different species from two to ten or more. The pygidium developed from the anal tergite in the earliest forms. In more advanced forms, the tergite became extended with new fixed segments with their structure the same as in the thoracic tergites. The abdomen of trilobites was covered with a thin chitin coat easily removable in molting and thus found only in specimens which were buried suddenly under fine silty sediments.

The longitudinal (axial) sections in those specimens record the structure of the digestive tract, and the transverse sections across the cephalic region reveal “hepatic” outgrowths of intestines. The insertion sites of muscles are distinctly marked by pits and furrows whose depth indicates the activity of different muscle systems. A feather-like brunch of appendages in the cephalic region (a brush of densely packed outgrowths) must have been gill-bearing.

Many trilobite species developed sophisticated visual organs and thus, already at the dawn of their evolution, possessed a complex brain-controlled nervous system. Silurian phacopid trilobites Phacopina studied by Clarkson had compound eyes allowing a panoramic view due to numerous lenselets arranged in files. Each lenselet terminated the outer end of a tiny prism which transmitted light to photoreceptors. Yet, compared to the eyes of modern arthropods the trilobites’ eyes, though very sophisticated, were unable to produce perfect images. The angular coverage of the visual field was discontinuous and showed only horizontally moving objects, their approximate size, speed, and direction of motion, i.e., trilobites could see an object approaching as it became recorded by ever higher-positioned eye lenses. Other phacopid trilobites, however, had biconvex lenses which made their vision similar to ours or even of higher visibility.

Trilobites were bisexual organisms. Females laid eggs in pits where they soon became covered with a thin sand layer while males followed the females and fertilized the eggs before sand filled the “nest” up. The stages of growth from the larval to the adult form are documented with great detail in the fossil record. The cephalon formed at the earliest growth stage, the pygidium appeared at the end of the early stage, and thoracic segments developed at the larval stage. Reaching the appropriate number of segments in the thorax marked the onset of the adult life stage. The growth continued during the adult stage but the skeleton no longer acquired new structures.

Molting began when the carapace became too tight for the growing soft body of a trilobite who wanted to free from the old “clothes”. The sutures that connected the hard parts of the cephalon loosened and the trilobite shed them by just shaking slightly his head and then crawled out of the old exoskeleton. Not very elegant but quite an easy way to change!

Trilobites that inhabited the warm seas of Angaride were extremely diverse, from dwarfs of a few millimeters to giants half-meter long. Their chitin coat was most likely of various colors. At least, banded and dotted patterns are often clearly notable in their fossil prints.

Trilobites were exclusively marine animals and inhabited shallow water. They were not very able swimmers but could move through water in patient search for quietness, good illumination, and food supply. The smallest forms apparently were passively carried by currents like plankton, and blind species dwelled in deep or turbid water where almost no light could penetrate.

Early trilobites were most likely vegetarians and ate slimy algae covering stones on the bottom and near the shore, as can be inferred from the strongly convex and dilated glabella at the front of the digestive tract adapted to take up large volumes of organic-rich sediment. More advanced forms with deep insertion sites of jaw muscles obviously preferred animal food. Yet the slow trilobites were scavengers rather than true predators and learned to eat remnants of their sea mates. Perhaps, they ate dead jellyfish and were responsible for the fact that jellyfish mass extinction horizons, abundant in Precambrian sediments, disappear from the younger deposits.

In the Cambrian, trilobites occupied the entire global shelf zone and reached their radiation acme due to favorable environments and the lack of enemies. They had to develop efficient protective tools, such as enrollment, only with the inception of menacing cephalopods and fishes who had good teeth. Trilobites learned to roll themselves up in a chitinous ball exposing their hard carapace and hiding the most vulnerable abdominal membrane; the thorny spine “decorations” secured more protection from the predators.

It is questionable whether the evolution of trilobites took a good way, because they were eventually extinct by the latest Paleozoic, having left attractive fossils. Yet their morphological diversity, and thus high variability, allowed them to remain the most important group of marine organisms through their 350 Myr survival.

MOLLUSCS

Molluscs is a common name for several animal groups (bivalves, snails, nudibranchs (sea-slugs), cuttlefish, octopus, etc.) related by obvious phylogenetic parentage, though they do not look much alike. Some have a calcareous exoskeleton or a shell, others have an endoskeleton, or are even free from any skeleton at all. Many became extinct, others survived and live in the modern seas and oceans, in lakes and rivers, or on the land.

Molluscs were quite rare in the Early Cambrian and reached their acme in the Ordovician. Now they yield in number of species (about 90,000) to only arthropods, especially insects. They are grouped together in a single systematic unit on the basis of subtle comparative anatomy of the soft body of the extant forms and ontogeny.

Most molluscs are unsegmented bilaterally symmetrical organisms, though there are partially segmented and asymmetrical forms. Some species are herbivores and others are carnivores; some are scavengers or detritovores. All are highly organized and well adapted to various habitats.

The Earth’s sedimentary archive stores record of skeletal molluscs, such as bivalves, gastropods, and cephalopods. The latter were good swimmers and spread globally. Skeletal remnants of cephalopods appear in sedimentary strata since the Early Cambrian. Many species of cephalopods have successfully underwent all severe trials set by nature and many catastrophes that repeatedly shook the organic world during its long history. Representatives of the very old genus of Nautilus still inhabit equatorial seas, and their nice white-ivory colored shells with red-brownish bands attract everybody’s eye, and the exotic soupe cooked in Polynesia from their soft bodies is extremely popular with natives as well as with tourists.

CEPHALOPODS: EARLIEST CARNIVORES

Even an absent-minded person would pay attention to queer shells that covered the beaches of Silurian and Devonian seas: curious barrels, peculiar stone sticks, curved calcareous horns, or surprising spirals with iridescent mother-of-pearl coat. The shells hardly can provide an appropriate idea of how the extinct organisms looked, what their vital organs were, and what the adaptive role of their morphology was. Fortunately, scientists can learn this from the habits of the extant forms.

Octopus, cuttlefish, and squid are the most highly organized cephalopods, all with endoskeletons. The modern cephalopods retain the structure of the soft body of their ancestors. Endoskeleton allowed them to accelerate considerably their locomotion, and the huge size (some specimens were 18 m long) made them really dangerous predators. The sea stories about giant squids who captured ships, together with the crew, and carried them down deep into the ocean might be an echo of our “genetic” memory.

Octopus, by the way, is an animal close to humans in many respects. It has the same structure of eyes and its sight is better than that of many other organisms. Its brain is highly developed, and the blood is… blue. Sea aristocracy indeed!

Fossil cephalopods are found mostly as uncoiled cone-shaped or coiled shells. Few intermediate forms are much less typical and were restricted to some local habitats.

Uncoiled shells belong to Nautiloidea, though the shells in modern forms are coiled. Therefore, the appearance, though an important diagnostic feature, is never the crucial one. Coiled shells represent Ammonoidea, a formerly abundant but extinct group. Their fossils are often found in Paleozoic rocks in the Altai and Salair mountains, in the Kuznetsk basin, in the Tunguska catchment, and in the Taimyr peninsula.

FOSSIL BOATS

Nautiloidea take their name from the Latin for boat. Nautilus was the name of Captain Nemo’s submarine in 20000 Leagues under the Sea by Jules Verne. The extant nautiloids have their shells coiled and divided into many hollow gas-filled cameras by septa. The last open camera is the living chamber. A special skinny tube called the siphuncle penetrates through all septa and extends the soft body. When diving, the animal uses the siphuncle to pump water into the cameras and to balance the external and internal pressure to maintain buoyancy, much like a submarine. After the animal is dead, its gas-filled cameras keep its heavy body afloat.

The appearance and life habits of extinct nautiloids were inferred from studies of their extant counterparts. The coiled shells of the extant nautilus have a wide anterior opening which hosts the animal’s head part, with the eyes and the mouth surrounded by numerous short tentacles. The adnate upper tentacles make a hood above the head and the first coils are above the hood. Most of Paleozoic nautiloids had straight or slightly sinuous shells.

The ontogeny of the extinct species was likely similar to that of the living forms. Juveniles with the embryonic shell sometimes already as large as 2.5 cm hatched out of quite large eggs. The animal grew by adding new gas-filled cameras with the siphuncle extending through the conch.

The uncoiled fossil shells are easily detectable, even in outcrops of dense rocks, from two convergent lines separated by bending septa. Separate shells or their cores fall quite easily, like heavy stone sausage, out of clayey rocks under a hammer strike. Numerous shells of nautiloids are found in Paleozoic strata in the Altai, Salair, and Kuznetsk regions and make up thick layers of Silurian sediments along the Moyero river in East Siberia.

AMMONOIDEA

The name Ammonoidea refers to their resemblance to a tightly coiled ram’s horn (the ancient Egyptian god Ammon (or Amun) was commonly depicted as a man with ram’s horns). Ammonoidea, or ammonites, are morphologically similar to nautiloids but had mostly coiled shells. The first camera was typically small (rarely up to 1.5 cm), and the number of cameras was thus much larger than in nautiloids.

Ammonites differed in the shape of the suture along which septum joins shell wall, or the suture line. The suture increased in complexity through the 300 Myr history of ammonites, and its pattern was getting ever more intricate: four main groups of sutures correspond to four evolution stages.

People called the extinct cephalopods “goblin’s arrows”, “witch’s arrows”, “thunder arrows”, “swamp stones”, “devil’s fingers”, or “golden snails”. A primitive man may have often stopped struck before a nice attractive stone. The fossils were different from everything he saw around but were obviously part of the surrounding world. Those vague thoughts, along with the mysterious appeal of the queer objects, possibly inspired the earliest idea of beauty — the eternal beauty, hard to perceive, which had existed long before man.
This story offers another reason to think over the history of the living world.

The suture line pattern is a key indicator in genealogical reconstructions. To reveal nearer and more distant ancestors of a specimen, its shell, if well preserved, is uncoiled successively till the first camera and the pattern of each camera is sketched on paper and then examined with great care. Ammonoidea are thought to have evolved in the Devonian from ancient cephalopods with the straight shell.

The diversity of coiling tightness, shape, suture pattern, and sculpture in ammonites was associated with broad species diversity. The forms with flattened, disc-shaped, streamlined shells were apparently better swimmers than the barrel-shaped forms, and the species with a narrow aperture fed preferably on plankton. However, these are only hypotheses as the soft body of cephalopods is unfortunately never preserved in fossils, though the shells of ammonites are well known to many amateurs of nature. The shells of Mesozoic species are especially popular because of their perfectly preserved iridescent mother-of-pearl coat, sculpture diversity, and peculiar saddle and lobe patterns.