Palæos:		Paleozoic
PALEOZOIC ERA		Paleozoic Era - 3

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The Paleozoic - 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. 

Reef Systems

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.

Plants

Plant life on land follows a very different evolutionary pattern - in terms of primary ecosystems - to both land animals and marine life. Whilst the division of the main evolutionary stages of multicellular life into Palaeozoic, Mesozoic, and Cenozoic is described in many popular science books and educational websites, few people are aware that these terms apply to animal life only. Plant life followed a different route, and the paleobotanical time-eras of Palaeophytic, Mesophytic, and Cenophytic are only approximately equivalent. Palaeophytic refers to the primarily spore-bearing (and a few primitive seed-bearing) Paleozoic vascular flora, which appeared in the Middle Ordovician period and die out quite a few millions of years before the end of the Paleozoic (latest early Permian and earlier). In the middle Permian the gymnosperm-dominated Mesophytic flora emerges (although Mesophytic type plants go back to the Carboniferous, just as some Paleophytic plants survive even to this day), and this flourishes right up until the middle and later Cretaceous. MAK010115

Arthropods

Arthropods apparently got off to an early start.  By the time of the Burgess Shale in the Middle Cambrian arthropods and their immediate relations were already the most diverse and successful group of animals.  The anomalocarids (which were either arthropods or their immediate sister group) were the world's earliest known large predator, with lengths approaching 2 meters.  Trilobites are well known from Cambrian Epoch 2 onwards, since their hard carapace fossilizes easily.  In addition to trilobites and several other high-level taxa of extinct arthropods, it is almost certain that many of the important arthropod groups of today had already diverged by this time, including the chelicerates, crustaceans, and arachnids.  Xiphosurans and some derived crustacean groups are known by the end of the Cambrian.  At that time the trilobites experienced several rounds of turnover as the nature of the sea floor environment began to change.  Of the miraculous variety of arthropod in the Burgess Shale, only 4 basic designs survived the end of the Cambrian. Of the four, the trilobites were extinguished at the end of the Permian. The remaining three are the insects (Hexapoda), the crustaceans, and the chelicerates.

By the Ordovician, some arthropods were at least partially terrestrial.  Several different types of arthropod land trackways of this age are now known.  Identifiable millipede and arachnids, including scorpion, parts are known from the Silurian.  In the seas, eurypterids also got their start during the Ordovician, but only became dominant predators in the Silurian.  Xiphosurans (hoseshoe crabs) also flourished  in the Silurian.  Trilobites were somewhat less common than in the Cambrian, but continued as an important part of the sea bottom fauna.  

Terrestrial trace fossils of apparent arthropods are known from the Wenlock (Middle Silurian).  Orr et al. (2000).  Body fossils of identifiable members of the extinct arachnid group Trigonotarbida have been found as early as the Pridoli (latest Silurian).  The manner in which arthropods came to land raises some rather deep evolutionary questions.  Terrestrial life is so different from life in in water that the barrier to evolution is high.  One might reasonably expect that that the transition would occur only rarely and that the first of any large group to accomplish the feat would radiate explosively, preventing any later group which was even remotely similar from getting a foothold on land.  So, for example, all land vertebrates descend from a single common ancestor who established itself on land in the Late Devonian.  The same can probably be said of land plants, as well as terrestrial worms (Annelida) and mollusks.  Arthropods, however, seem to have independently evolved terrestrial forms at least half a dozen times over a relatively long period of time (Pridoli to Mississippian).  See, e.g., Klok et al. (2002).   

Arthropods were well-established on land in the Devonian, and seem to have reached impressive size in some cases.  The earliest known Hexapoda -- but not yet insects -- were discovered in Devonian rocks.  Terrestrial spiders were present, but trilobites began a serious long-term decline in the seas.  The increasing biomass of land plants and higher oxygen levels by the end of the Devonian probably led to the evolution of terrestrial, herbivorous forms by the end of that period, possibly including the first insects.  

Insects were certainly present during the Carboniferous, and all of the major groups had diverged by the end of Paleozoic time.  Insects with folding wings had evolved by the Pennsylvanian (Late Carboniferous).  A few of these, particularly some dragonflies and spiders, reached very large sizes. This may have been due to abnormally high oxygen levels during the Carboniferous.  Dudley (1998).  However, most Paleozoic insects were as small as insects today.  In the seas, the trilobites and eurypterids declined and became extinct at the end of the Permian.  Crustaceans became increasingly diverse and successful.  

Image: Palmichnium image from Draganits et al. (1998)

ATW040710.  Revised ATW050914.  Text and Marella image public domain.  No rights reserved.

Links: The SUNY Cortland Geology Department has a good introduction to the Paleozoic evolution of arthropods.  Trilobites were the most succesful group of arthropods for most of the early Paleozoic.  Sam Gon has the most succesful of trilobite sites -- and with good reason.  Trigonotarbids are introduced at the UCMP site.