|Paleozoic Era - 3
Life changed so much during the Paleozoic - from seaweed to forests, from proto-chordates to mammal-like synapsids - that it is difficult to summarize. Although Paleozoic means "ancient life" many of the organisms that lived during the later Paleozoic were much closer to those of today than many of the life-forms of the early Paleozoic. Basically, at the risk of generalization, we might say that the earlier Paleozoic was dominated by invertebrates, while the land remained barren. The middle Paleozoic saw the rise of strange armoured fish and the first land plants and insects. While the later Paleozoic was distinguished by great forests of mostly spore-bearing trees, inhabited by a rich assortment of arthropods, tetrapods and reptiles on land; and by diverse invertebrates in the sea.
Cambrian eco-systems were much simpler and less diversified than anything of today, and hence unstable and prone to easy mass-extinction. Moreover, it is possible to distinguish an earlier Tommotian type fauna (Terreneuvian) from a Middle Cambrian to Early Ordovician fauna.
The initial flowering of metazoans during the Early Cambrian (the "Cambrian Explosion") spread animal life throughout the seas. The typical Furongian marine community was dominated by trilobites, "inarticulate" brachiopods, and eocrinoids. However, the basic pattern for Ordovician and Middle Paleozoic marine communities was established in the great Early Ordovician radiation of marine metazoans. "The Palaeozoic evolutionary fauna originated and diversified during an early Ordovician radiation event. Many of the adaptations high-lighted in the Palaeozoic marine benthos are associated with soft substrates. Articulate brachiopods, stenolaemate bryozoans, stalked echinoderms (crinoids and blastoids), corals, ostracodes diversified together with graptolites within the water column. Most plankton groups may have been recruited from the benthos while events within the plankton ecosystem were shadowed by changes in benthic systems (Rigby and Milsom. 1996). The vigorous early Ordovician radiation set the agenda for much of the Palaeozoic; the majority of adaptations in the invertebrate groups had already been tried and tested by the end of the Ordovician." Benchley & Harper (1998).
Following the large end-Cambrian and end-early Ordovician extinctions, a new evolutionary fauna originated and diversified during the Ordovician radiation event. This constituted a Palaeozoic marine benthos associated with soft substrates. Articulate brachiopods, stenolaemate bryozoans, stalked echinoderms (crinoids and blastoids), corals, ostracodes all diversified. Higher up in the water column, the plankton and nekton included graptolites and conodonts, cephalopods, and later fish and medusa scyphozoa). This vigorous early Ordovician radiation set the agenda for much of the Palaeozoic; and most adaptations by the various invertebrate groups had already been tried and tested by the end of the Ordovician. By the Middle to Late Paleozoic marine ecosystems may not have been too unlike those of today. Ecosystems and energy and nutrient flow on land was much more inefficient, until the rise of reptilian herbivores at the very end of the era (late Permian).
Perhaps the most intelligent creatures to inhabit the earth over this long span of time were the cephalopods, the most intelligent and sentient of all the invertebrates. Cephalopods were extraordinarily diverse in Paleozoic seas, and were the dominant life-form until the rise of carnivorous fish during the Devonian (mid-Paleozoic). At the very end of the Paleozoic the Therapsida evolved larger brains than their contemporaries, and paved the way for mammalian intelligence during the Mesozoic and Cenozoic.
The Paleozoic witnessed a number of crises in the history of life, including an early Cambrian, a terminal Cambrian, an Ordovician one, a late Devonian one. The era was brought to an end by the terminal Permian extinction, the greatest catastrophe in the history of higher life on Earth (although far milder than the Early Proterozoic Oxygen crisis). There is still disagreement over whether it was caused simply by terrestrial phenomena like loss of geographic isolation and falling sea-levels, or whether (as I feel likely) these factors were aided by an extraterrestrial impact of some kind (similar to the one that saw off the dinosaurs at the end of the Mesozoic). MAK, ATW050819.
note: the following passage is from Patrick J. Benchley and by D. A. T. Harper, Palaeoecology (I have included a note in brackets].
"During the first 10 million years of the Cambrian Period the Tommotian fauna was replaced by a more diverse biota of larger metazoans participating in more complex communities. The Cambrian fauna was dominated by low-level suspension feeders such as the nonarticulate brachiopods and eocrinoids together with monoplacophoran and hyolith mollusks. Two parazoan groups developed colonial strategies: the archaeocyathans and possibly the sponges. Colonies tend to develop iteratively with new iterative units or modules derived by continuous growth from existing units. The colonial or iterative body plan thus contrasts with the unitary or solitary life mode of most organisms. A number of groups show clear trends towards a greater integration of individuals within the colony and in some cases a differentiation of functions across the colony.
Trilobites were the most abundant members of the Cambrian benthos; in many assemblages over 90% of the animals were trilobites [note - although trilobites represent the most common fossil that doesn't mean they were the most common animal. Trilobites had a hard carapace, molted frequently, hence they left a disproportionately numerous record of themselves compared to other creatures living at the same time]. Cambrian communities were loosely organized and considerable experimentation and morphological flexibility were features of many troops (Hughes, 1994). Cambrian Lagerstätten such as the Burgess Shale contain a wide range of apparently morphologically disparate organisms."
note: the following passage is from pp. 128-137 of the same text
The low-level Paleozoic benthos was dominated by fixed suspension feeders, mainly the brachiopods supported by the bryozoans and microcarnivores such as the rugose and tabulate corals. Mollusks were generally rare, although some bivalve-dominated associations occur throughout the Paleozoic and may have inhabited specialized habitats. The sponge-like stromatoporoids were responsible for carbonate buildups, particularly during the mid-Paleozoic, when they developed a range of growth modes including columnar, dendroid, encrusting and hemispherical forms (Kershaw, 1990).
By the mid-Ordovician, two major groups of anthozoans, the rugose and tabulate corals, were firmly established as microphages, although neither ever built substantial reef structures, lacking many of the adaptations, such as a basal plate, that helped the scleractinians to develop as the dominant reef-builders of the Modern fauna.
Paleozoic corals also advanced the development of colonial or compound life strategies. Tabulate corals developed only colonial forms, many fine-tuned for life on a soft substrate; the location, orientation. spacing and development of offsets in the corallites have been developed in a tool-kit of adaptations for different substrates.
Brachiopods dominated a range of nearshore to shelf-edge environments, suited to a variety of substrates at a range of water depths in marine conditions. The group was the principal epifauna of the Paleozoic. Certain basic aspects of the brachiopod morphology, such as the presence or absence of a pedicle, had a major control on the brachiopod life strategy. More subtle features of the animal, for example the size, shell shape and ornament, also had clear adaptive significance.
Crinoids, together with brachiopods, dominated the Paleozoic sessile benthos. The crinoids developed a range of articulatory structures allowing the stem considerable flexibility. Although most crinoids are and were fixed, rheophile organisms, orientating their crowns into the current when feeding, some such as the Recent comatulid Antedon are mobile or attached by small roots or cirri, whereas the Jurassic Saccocoma may have been either epiplanktonic or part of the free-living benthos
Evolution, initially, in the crinoids of discoidal holdfasts permitted attachment to hard substrates; however, the development of more versatile root-like holdfasts allowed the group to target soft, muddy substrates commonly located in more offshore environments (Sprinkle and Guensburg, 1995). This transition between these two types of attachment structures may have driven a large-scale diversification of the group and its expansion into deeper-water environments during the early Paleozoic.
Bryozoan diversified during the Ordovician radiation, building larger and more complex colony types. Initially colonies were low with few zooids; multi-story complexes were developed to occupy available space and evolve efficient feeding strategies.
Only a shallow infauna was well developed in the Paleozoic fauna, although there were exceptions; it included bivalves, scaphopods, trilobites and crustaceans occupying depths of up to about 100 mm. Nevertheless, by the early Carboniferous, trace fossil data suggests that depths of up to one meter of sediment were penetrated by bivalves. Bivalves first appeared during the early Cambrian as part of a shallow infauna. Throughout the Phanerozoic the group has demonstrated a remarkable spectrum of adaptive morphologies for life above, at and within the sediment-water interface. Aspects of the Paleozoic bivalve fauna heralded the intense infaunal radiation of the group during the Mesozoic Marine Revolution, when the group became much more dominant.
Tiered profiles evolved during the Paleozoic (Ausich and Bottjer, 1982). The intermediate-level benthos (50-200 mm) was dominated by sponges, corals, giant bivalves, giant brachiopods, stalked echinoderms and fixed dendroid graptolites. High-level sessile benthos (200-500 mm) contained mainly crinoids and blastoids. The stalked echinoderms concentrated on improving the efficiency of their feeding techniques with the development of more elaborate calices.
Trilobites continued to dominate the mobile benthos, although bivalves, gastropods and echinoids locally moved across and through the sediment. Cephalopod mollusks such as the orthoconic nautiloids patrolled the benthos, in a role as the main macrophagous predator.
The following sequence of bottom-living biotas, defined according to major extinctions, adaptive radiations, and community reorganizations, is suggested for this period by Arthur Boucot (Phanerozoic Extinctions: How similar are they to each other?", Lecture Notes in Earth Sciences 30, Springer-Verlag Berlin Heidelberg 1990). Note that these do not always follow or equate with the Terrestrial or Freshwater succession:
Even using an elastic and generous definition of "reef," the only reef systems prior to the Paleozoic were stromatolites: large, and sometimes very large, mounds formed by successive layers of bacterial biofilms. The first "real" reefs were formed in the last half of the Terreneuvian by Archaeocyatha, rather poorly known lacy, sponge-like creatures which became extinct in the Middle Cambrian. From that point until the Middle Ordovician, the only reefs were "mud mounds," somewhat variable mixtures of stromatolite, sponges, and a sort of filamentous red algae that bound everything together. In the Middle Ordovician, encrusting Bryozoa became part of the mix.
However, large biogenic reefs remained rare until the Late Ordovician, when an entirely new type of reef began to be constructed based on a framework of tabulate and colonial rugose corals together with stromatoporoid sponges. The hallmarks of Paleozoic marine ecosystems are probably these tabulate or rugose coral reef. These corals initially (Early and Middle Ordovician) were part of a reef-building guild which included bryozoans and stromatoporoids, among others. These mixed communities supported a community largely made up of suspension feeders. The corals probably came into their own in connection with the Late Ordovician glacial period. They were then associated with more complex ecosystems which supported a mobile fauna including eurypterids, ammonoids, and fishes with jaws.
These "tabulate-strome" reefs remained the dominant form of reef for almost 100 My, until the Late Devonian. A really good source of information on tabulate strome communities is the Milwaukee Public Museum's Virtual Silurian Reef site, from which we have borrowed the adjacent image.
In the Late Devonian, many groups turned over rapidly. We hesitate to label it a mass extinction, since species-level diversity seems to have suffered little, if at all. Nevertheless, the Carboniferous fauna was markedly different from the Late Devonian in many respects. Reef builders were no exception. The pattern may then have been reset, paradoxically, by a series of events associated with the spread of terrestrial plants and culminating in the Mississippian Ice Age. Stanley (2002) (abstract). What happened then is unclear. The nature of marine ecosystems during the Pennsylvanian and Permian seems to be unsettled, and may have differed considerably from place to place. Brachiopods were certainly an important component, as were stalked crinoids. In general, it seems likely that sessile filter feeders were again dominant as in Ordovician times
In addition to whatever factors caused the Late Devonian turnover, the world's ocean chemistry crossed the critical barrier between calcite and aragonite seas. Calcium carbonate can take either of two crystal forms: calcite or aragonite. When magnesium levels are high, aragonite is favored. That is, seawater concentrations of magnesium must have increased to the point that calcium carbonate precipitating from sea water took the form of aragonite rather than calcite. This was bad news for corals which had evolved to use calcite as a basic part of their body plan. Both tabulate and rugose corals were hard hit. Stromatoporoid sponges almost disappeared. Although both coral groups recovered somewhat in the Pennsylvanian, they gradually gave way to more modest reefs constructed by new sponge groups, bryozoans and calcareous algae. Both rugose and tabulate corals became completely extinct at the end of the Permian. See, generally, Stanley 1998).
ATW050107. Text public domain. No rights reserved.
note: the following passage is from Patrick J. Benchley and by D. A. T. Harper, Palaeoecology, p.137 (Chapman and Hall)
Acritarchs, chitinozoans and radiolaria had a dominant place in the microplankton. Graptolites formed a large part of the macroscopic plankton and nekton during the early Paleozoic together with pelagic and planktonic trilobites; whereas orthoconic nautiloids together with conodont animals were probably major predators above the sediment-water interface to be joined by both jawless and gnathostome fishes. A few species of small, thin-shelled bivalves and brachiopods may have pursued epiplanktonic strategies attached to floating organic material or even volcanic pumice.