Vendian Extinctions

the extinction events of the Vendian tend to be overshadowed by the fact that the Vendian-Cambrian transition experienced the great est radiation of multicellular life known. A mass extinction occurs when large numbers of organisms in different habitats die out at the same time, as occurred near the end of the Cretaceous, with the extinction of dinosaurs and all other large terrestrial animals, as well as many types of marine organisms. The earliest known, reasonably well documented, mass extinction is of Vendian age. There are indications of a cluster of mass extinctions in the middle to late Vendian, although the abruptness and simultaneity of these extinctions are somewhat obscured by the rarity of fossils and by the difficulty of obtaining precise dates for Vendian sediments. Also, some Vendian extinctions are the continuation of declines that began before the beginning of the Vendian, such as the loss of many different types of stromatolites.

Stromatolites (figures 1.3 and 1.4) reached a peak in diversity (nearly 100 recognized taxa; Awramik 1982) about 850 million years ago in the late Riphean. The Riphean is an interval of Proterozoic time used by Soviet geologists to designate an interval of geologic time before the Vendian; see figure 1.1. Following this acme, stromatolites underwent a precipitous decline, starting in the late Ri-phean and continuing through the Vendian. Stromatolite diversity bottomed out at fewer than 30 taxa by the beginning of the Cambrian. Although this decline does not necessarily represent the extinction of any of the individual moneran species that participated in the construction of the stromatolites, it does indicate that the conditions for many successful types of Proterozoic benthic micro-bial communities became much less favorable. For example, well-formed specimens of the distinctive stromatolite Conophyton (figure 1.3) are unknown after the Vendian. The advent of burrowing and grazing metazoans, and disturbance of microbial mats as a result of their activities, has been hypothesized as the factor responsible for the decline of stromatolites (Awramik 1982).

This decline as a result of overgrazing is an event of immense and global significance. Stromatolites were the communities that characterized the shallow water marine basins of the earth for three billion years, and were reduced in diversity to small numbers in a relatively short period of time (Sokolov and Fedonkin 1986).

Microbial species seem to be largely unaffected by the defeat of the stromatolites. It may be difficult, however, to recognize turnover of species in floras consisting primarily of simple round and tubular monerans. Similar looking microfossils may have had radically different genetic programming or biochemical machinery —it is impos sible to tell this from the fossils alone, however. This problem is further compounded by the fact that fossilized sea floor microbiota are rare after the beginning of the Cambrian. We do know that certain distinctive microbial forms survived the Vendian without major change. The distinctive helically coiled (like a corkscrew) Obruchevella (Cloud et al. 1979), plus the earliest calcareous algae (Riding and Voronova 1982) crossed the Vendian-Cambrian boundary with impunity. Obruchevella has been recently discovered in silts of the Middle Cambrian Burgess Shale (Mankiewicz 1988).

Another Vendian sea-floor (or possibly planktonic) group that lives on into the Cambrian are the vendotaenid algae. Vendotaenids are ribbon-shaped, possibly moneran fossils, often microscopic but up to 1 mm in width and 10 cm in length. The first fossil evidence for primitive fungi (actinomycetes) are Vendian actinomycetes discovered in the process of decaying a fragment of a vendotaenid (Sokolov and Fedonkin 1986).

Acritarchs are another important group of Vendian fossils. Acri-tarchs are thought to primarily represent fossils of protists. They are organic-walled vesicles, frequently round and usually less than a tenth of a millimeter in diameter. They are studied by removing them from sediment by acid dissolution of the rock matrix. Acri-tarchs can be recovered from calcium carbonate rock (limestone) by dissolution with acetic acid, formic acid, or hydrochloric acid. More usually, however, the sediments containing acritarchs are silica-rich rocks such as a shale, and the more dangerous hydrofluoric acid must be used to dissolve the siliceous minerals (this technique is called maceration). Once removed, acritarchs can be studied by using transmitted light and scanning electron microscopes (Vidal 1984).

W. R. Evitt coined the term "acritarch" as a catchall category for small, organic, walled fossils of uncertain biological relationships. As such, the acritarch group can be though of as a "garbage can" term for microfossils that cannot be directly compared to known organisms. Often brown in color and collapsed or torn, acritarchs are not beautiful fossils; acritarchs recovered from maceration residues look just as though they came from a garbage can. Nevertheless, these fossils are crucial for biostratigraphy. Most acritarchs resemble the resting cysts made by unicellular, planktonic protists called dinoflagellates. Dinoflagellate blooms are responsible for the so called "red tides" that can have disastrous effects on fish living nearby. When a living dinoflagellate experiences unfavorable conditions for growth, it encloses itself in a resistant vesicle of a tough organic compound, called sporopollenin, and sinks to the bottom to await better conditions (Evitt 1986). By comparison to modern dinoflagel-lates, most acritarchs are thought to represent the resting stages of photosynthetic, free-floating protists. Acritarchs may be smoothly spherical, spiny (some look like miniature World War I-vintage floating mines), or octahedral (Vidal 1984).

The diversity of these planktonic microfossils underwent a severe decline during the middle to late Vendian, which a number of paleontologists accept as indicative of major extinctions in the eukar-yotic phytoplankton (Vidal and Knoll 1983). Diagnostic acritarch species, such as the tongue-twisting Trachyhystrichosphaera, were gone by the end of the early Vendian. These late Riphean-early Vendian acritarchs (Yankauskas 1978) were replaced by a very low diversity planktonic flora typified by vendotaenids and the distinctive acritarch Bavlinella (figure 8.1). Bavlinella resembles living colonies of spherical cyanobacteria, and is probably some type of planktonic moneran. Planktonic monerans are outnumbered in today's seas by planktonic eukaryotes such as dinoflagellates.

The sediments containing this depauperate middle to upper Vendian acritarch flora have curiously large amounts of an organic material called sapropel. The sapropel is derived from burial of huge amounts of organic material. After this low diversity interlude, acri-

FIGURE 8.1. Bavlinella faveolata, an acritarch composed of multiple spheres, may have been a colonial prokaryote. This species is common between the middle Vendian and the latest Vendian. Width of specimen 25 microns. (After Vidal 1976)

tarch diversity did not recover to its early Vendian levels until well into the Lower Cambrian, when spiny forms such as Skiagia (figure 8.2) became abundant.

Pre-Vendian animals may have existed, but we know virtually nothing about them. The Riphean-Vendian boundary is about 700 million years old, a moment at —or just before —the start of the Varangian glaciation. Animal fossils from before this glaciation are very rare. A twisting string of what have been interpreted as animal fecal pellets is known from the upper Riphean of the Ural Mountains (Sabrodin 1972). As noted in chapter 3, the billion year old "Brooksella" canyonensis from the Grand Canyon has recently been reinterpreted as a complex trace fossil (Kauffman and Fursich 1983), although this interpretation is hotly contested by those who believe that it is an inorganic sedimentary structure (Cloud 1968). The oldest burrow convincing to us is a tiny, approximately 700 million year old, backfilled burrow from South China (Awramik et al. 1985). Also from China are 740 to 840 million year old annulated tubular structures that may be the oldest record of metazoa (Sun 1986), although the dating of these fossils cannot be accepted without reservations (Cloud 1986). Although the pre-Vendian animal fossil record is very scrappy, it seems plausible that grazing animals were present as far back as a billion years ago, to accord with the decline

FIGURE 8.2. Skiagia ciliosa, a spiny acritarch known from the Lower Cambrian when acritarch diversity began to recover after the Vendian mass extinction of the marine phytoplankton. Width of specimen 24 microns. (After Knoll and Swett 1986)

FIGURE 8.2. Skiagia ciliosa, a spiny acritarch known from the Lower Cambrian when acritarch diversity began to recover after the Vendian mass extinction of the marine phytoplankton. Width of specimen 24 microns. (After Knoll and Swett 1986)

in stromatolite diversity that began about 850 to 900 million years ago (Awramik 1982). It is tempting to speculate that the evolutionary development of these pioneer animals was delayed or halted by the severe Varangian glaciation, although there is not enough evidence at this time to make any claims for a late Riphean mass extinction event. Nonetheless, Sokolov and Fedonkin (1986) infer that the Varangian glaciation interval saw mass extinction of some groups of invertebrates of which we know nothing.

Survivors of this "great cold" diversified into the distinctive soft-bodied Ediacaran fauna. On the Russian Platform (where the Vendian was first recognized), the Vendian is divided into three horizons, the lower Redkino, the upper Kotlin, and the uppermost Rovno. Sokolov and Fedonkin (1986) see a rapid expansion of the fauna in the Redkino after the glaciation, followed in the Kotlin by an episode of extinction. Only rare problematic forms of metazoa and small trace fossils are known from the Kotlin Horizon of the Russian Platform. In the Rovno (uppermost Vendian), there is an abrupt increase in the dimensions of animals, as is indicated by the sizes of the trace fossils they left behind. Trace fossils became larger, more complicated, and deeper, indicating a greatly increased level of colonization of the sea floor by animals (Fedonkin 1978). Animals with resistant tubular skeletons appear in the Rovno. Sabelliditid tubes are common in the Rovno. These thin organic-walled (not mineralized) tubes presumably housed a worm-like, filter-feeding organism; the Vendian genus name Sabellidites recalls modern filter-feeding, tube-dwelling annelid worms called sabellids. Since they are non-mineralized, sabelliditid tubes cannot properly be called shelly fossils.

The first mineralized fossils on the Russian Platform are the tubular shelly fossils of the Lontova Horizon (the strata immediately overlying the Rovno Horizon; Sokolov and Fedonkin 1984). Many of these resemble a sabelliditid that has become mineralized. Platyso-lenites is a simple mineralized shell that may have been originally siliceous. Onuphionella (figures 8.3 and 8.4) is a simple tube formed of imbricate mica flakes. Also occurring in the Lontova is Aldanella, a probable snail. These biomineralized shelly fossils are characteristic of the beginning of the Cambrian explosion.

Throughout the world, both biomineralization and increased burrowing were part of the animal explosion. The increase in the diversity of burrowers occurred slightly before that of shelly fossils. Together, these radiations signify a profound change in the ecology of

Imbricating Layer
FIGURE 8.3. Onuphionella durhami, a tubular fossil formed by imbricate layers of mica flakes. Length of tube 7.5 cm. (From Signor and McMena-min 1988)
Onuphionella
FIGURE 8.4. Reconstruction of Onuphionella in probable life position in the Early Cambrian sea of eastern California.

the marine biosphere. This change, however, is not solely expressed by metazoan burrowers and shells. Calcareous algae appeared at the end of the Vendian (Riding and Voronova 1982), and later radiated to become, in conjunction with the archaeocyathans, the co-creators of the first skeletal, wave-resistant reefs (Rowland 1984). The skeletons of the first calcareous algae discouraged grazers that were conditioned to consuming softer tissue. Somewhat later, the relationship between grazing and calcification acquired a curious twist. Soleno-pores, a group of calcareous algae that appeared in the Cambrian, are thought by Steneck (1983) to have required grazing to remain free of epiphytes. Solenopores underwent a sharp decline during the Jurassic (about 180 million years ago), just before their presumed descendants (coralline algae) underwent an explosive radiation. Modern coralline algae are so dependent on grazing (chiefly by parrotfish) that reproduction of some forms cannot occur without it (Steneck 1985). The descendants of Cambrian solenopores and their grazers have become mutually dependent.

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