Old Earth Ministries Online Earth History Curriculum
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Chapter 8 - The Permian Period
Lesson 41: The Permian Extinction, Part 1
The Permian–Triassic (P–Tr) extinction event, iformally known as the Great Dying, was an extinction event that occurred 251.4 million years ago, forming the boundary between the Permian and Triassic geologic periods. It was the Earth's most severe extinction event, with up to 96 percent of all marine species and 70 percent of terrestrial vertebrate species becoming extinct; it is the only known mass extinction of insects. Fifty-seven percent of all families and 83% of all genera were killed.
Chapter 8 - The Permian Period
|Graph showing extinction events from the Cambrian to the present. The extinction at the end of the Permian is the largest extinction in the history of our planet. (Graph source)|
Because so much biodiversity was lost, the recovery of life on earth took significantly longer than after other extinction events. This event has been described as the "mother of all mass extinctions". The pattern of extinction is still disputed, as different studies suggest one to three different pulses. There are several proposed mechanisms for the extinctions; the earlier peak was likely due to gradualistic environmental change, while the latter was probably due to a catastrophic event. Possible mechanisms for the latter include large or multiple bolide impact events, increased volcanism, or sudden release of methane hydrates from the sea floor; gradual changes include sea-level change, anoxia, increasing aridity, and a shift in ocean circulation driven by climate change.
Dating the extinction
Until about the year 2000 it was thought that rock sequences spanning the Permian-Triassic boundary were too few and contained too many gaps for scientists to estimate reliably when the extinction occurred, how long it took or whether it happened at the same time all over the world. However, a study of uranium/lead ratios of zircons from rock sequences near Meishan, Changxing, Zhejiang Province, China date the extinction to ±0.03 Ma, with an ongoing elevated extinction rate occurring for some time thereafter. A large (-9‰), abrupt global change in the ratio of 13C to 12C, denoted δ13C, coincides with this extinction, and is sometimes used to identify the Permian-Triassic boundary in rocks that are unsuitable for radiometric dating.
It has been suggested that the Permian-Triassic boundary is associated with a sharp increase in the abundance of marine and terrestrial fungi, and that this was caused by the sharp increase in the amount of dead plants and animals fed upon by the fungi. For a while this "fungal spike" was used by some paleontologists to identify the boundary to define the Permian-Triassic boundary in rocks that are unsuitable for radiometric dating or lack suitable index fossils, but even the proposers of the fungal spike hypothesis pointed out that "fungal spikes" may have been a repeating phenomenon created by the post-extinction ecosystem in the earliest Triassic. More recently the very idea of a fungal spike has been criticized on several grounds, including that: Reduviasporonites, the most common supposed "fungal spore", was actually a fossilized alga; the spike did not appear worldwide; and in many places it did not fall on the Permian-Triassic boundary. The algae which were mis-identified as fungal spores may even represent a transition to a lake-dominated Triassic world rather than an earliest Triassic zone of death and decay in some terrestrial fossil beds. However, newer chemical evidence agrees better with a fungal origin for Reduviasporonites, diluting these critiques.
Uncertainty exists regarding the duration of the overall extinction and about the timing and duration of various groups' extinctions within the greater process. Some evidence suggests that the extinction was spread out over a few million years, with a very sharp peak in the last 1 million years of the Permian. Statistical analyses of some highly fossiliferous strata in Meishan, South China suggest that the main extinction was clustered around one peak. Recent research shows that different groups went extinct at different times; for example, while difficult to date absolutely, ostracode and brachiopod extinctions were separated by between 0.72 and 1.22 million years. In a well preserved sequence in east Greenland, the decline of animals is concentrated in a period 10 to 60 thousand years long, with plants taking several hundred thousand further years to show the full impact of the event. An older theory, still supported in some recent papers, is that there were two major extinction pulses 5 million years apart, separated by a period of extinctions well above the background level; and that the final extinction killed off "only" about 80% of marine species alive at that time while the other losses occurred during the first pulse or the interval between pulses. According to this theory the first of these extinction pulses occurred at the end of the Guadalupian epoch of the Permian. For example, all but one of the surviving dinocephalian genera died out at the end of the Guadalupian, as did the Verbeekinidae, a family of large-size fusuline foraminifera. The impact of the end-Guadalupian extinction on marine organisms appears to have varied between locations and between taxonomic groups - brachiopods and corals had severe losses.
The event had a profound effect on the terrestrial ecosystem, which is still being felt today, a quarter of a billion years later. In the late Permian, there were many sorts of
Marine invertebrates suffered the greatest losses during the P–Tr extinction. In the intensively-sampled south China sections at the P-Tr boundary, for instance, 280 out of 329 marine invertebrate genera disappear within the final 2 sedimentary zones containing conodonts from the Permian.
Statistical analysis of marine losses at the end of the Permian suggests that the decrease in diversity was caused by a sharp increase in extinctions instead of a decrease in speciation. The extinction primarily affected organisms with calcium carbonate skeletons, especially those reliant on ambient CO2 levels to produce their skeletons.
Among benthic organisms, the extinction event multiplied background extinction rates, and therefore caused most damage to taxa that had a high background extinction rate (by implication, taxa with a high turnover). The extinction rate of marine organisms was catastrophic.
Marine invertebrate groups which survived include: articulate brachiopods (those with a hinge), which have suffered a slow decline in numbers since the P–Tr extinction; the Ceratitida order of ammonites; and crinoids ("sea lilies"), which very nearly became extinct but later became abundant and diverse.
The groups with the highest survival rates generally had active control of circulation, elaborate gas exchange mechanisms, and light calcification; more heavily calcified organisms with simpler breathing apparatus were the worst hit. In the case of the brachiopods at least, surviving taxa were generally small, rare members of a diverse community.
The ammonoids, which had been in a long-term decline for the 30 million years since the Roadian (middle Permian), suffered a selective end-Guadalupian extinction pulse. This extinction greatly reduced disparity, and suggests that environmental factors were responsible for this extinction. Diversity and disparity fell further until the P-Tr boundary; the extinction here was non-selective, consistent with a catastrophic initiator. During the Triassic, diversity rose rapidly, but disparity remained low.
The range of morphospace occupied by the ammonoids became more restricted as the Permian progressed. Just a few million years into the Triassic, the original morphospace range was once again occupied, but shared differently between clades.
The Permian had great diversity in insect and other invertebrate species, including the largest insects ever to have existed. The end-Permian is the only known mass extinction of insects, with eight or nine insect orders becoming extinct and ten more greatly reduced in diversity. Palaeodictyopteroids (insects with piercing and sucking mouthparts) began to decline during the mid-Permian; these extinctions have been linked to a change in flora. The greatest decline, however, occurred in the Late Permian and were probably not directly caused by weather-related floral transitions.
Most fossil insect groups which are found after the Permian–Triassic boundary differ significantly from those which lived prior to the P–Tr extinction. With the exception of the Glosselytrodea, Miomoptera, and Protorthoptera, Paleozoic insect groups have not been discovered in deposits dating to after the P–Tr boundary. The caloneurodeans, monurans, paleodictyopteroids, protelytropterans, and protodonates became extinct by the end of the Permian. In well-documented Late Triassic deposits, fossils overwhelmingly consist of modern fossil insect groups.
This is a long lesson. This point is a good division point to turn it into two lessons if you desire.
Plant ecosystem response
The geological record of terrestrial plants is sparse, and based mostly on pollen and spore studies. Interestingly, plants are relatively immune to mass extinction, with the impact of all the major mass extinctions "negligible" at a family level. Even the reduction observed in species diversity (of 50%) may be mostly due to taphonomic processes. However, a massive rearrangement of ecosystems does occur, with plant abundances and distributions changing profoundly.
At the P–Tr boundary, the dominant floral groups changed, with many groups of land plants entering abrupt decline, such as Cordaites (gymnosperms) and Glossopteris (seed ferns). Dominant gymnosperm genera were replaced post-boundary by lycophytes - extant lycophytes are recolonizers of disturbed areas.
Palynological or pollen studies from East Greenland of sedimentary rock strata laid down during the extinction period indicate dense gymnosperm woodlands before the event. At the same time that marine invertebrate macrofauna are in decline these large woodlands die out and are followed by a rise in diversity of smaller herbaceous plants including Lycopodiophyta, both Selaginellales and Isoetales. Later on other groups of gymnosperms again become dominant but again suffer major die offs; these cyclical flora shifts occur a few times over the course of the extinction period and afterwards. These fluctuations of the dominant flora between woody and herbaceous taxa indicate chronic environmental stress resulting in a loss of most large woodland plant species. The successions and extinctions of plant communities do not coincide with the shift in δ13C values, but occurs many years after. The recovery of gymnosperm forests would take 4-5 million years.
The Coal Gap
No coal deposits are known from the Early Triassic, and those in the Middle Triassic are thin and low-grade. This "coal gap" has been explained in many ways. It has been suggested that new, more aggressive fungi, insects and vertebrates evolved, and killed vast amounts of trees. However these decomposers themselves suffered heavy losses of species during the extinction, and are not considered a likely cause of the coal gap. It could simply be that all coal forming plants were rendered extinct by the P/Tr extinction, and that it took 10 million years for a new suite of plants to adapt to the moist, acid conditions of peat bogs. On the other hand abiotic factors (not caused by organisms), such as decreased rainfall or increased input of clastic sediments, may also be to blame. Finally, it is also true that there are very few sediments of any type known from the Early Triassic, and the lack of coal may simply reflect this scarcity. This opens the possibility that coal-producing ecosystems may have responded to the changed conditions by relocating, perhaps to areas where we have no sedimentary record for the Early Triassic. For example in eastern Australia a cold climate had been the norm for a long period of time, with a peat mire ecosystem specialising to these conditions. Approximately 95% of these peat-producing plants went locally extinct at the P-Tr boundary; Interestingly, coal deposits in Australia and Antarctica disappear significantly before the P-Tr boundary.
Even the groups that survived suffered extremely heavy losses of species, and some terrestrial vertebrate groups very nearly became extinct at the end-Permian. Some of the surviving groups did not persist for long past this period, while others that barely survived went on to produce diverse and long-lasting lineages. There is enough evidence to indicate that over two-thirds of terrestrial amphibian, sauropsid ("reptile") and therapsid ("mammal-like reptile") families became extinct. Large herbivores suffered the heaviest losses. All Permian anapsid reptiles died out except the procolophonids (testudines have anapsid skulls but are most often thought to have evolved later, from diapsid ancestors). Pelycosaurs died out before the end of the Permian. Too few Permian diapsid fossils have been found to support any conclusion about the effect of the Permian extinction on diapsids (the "reptile" group from which lizards, snakes, crocodilians, dinosaurs, and birds evolved).
Possible explanations of these patterns
The most vulnerable marine organisms were those which produced calcareous hard parts (i.e. from calcium carbonate) and had low metabolic rates and weak respiratory systems - notably calcareous sponges, rugose and tabulate corals, calciate brachiopods, bryozoans, and echinoderms; about 81% of such genera became extinct. Close relatives which did not produce calcareous hard parts suffered only minor losses, for example sea anemones, from which modern corals later evolved. Animals which had high metabolic rates, well-developed respiratory systems and non-calcareous hard parts had negligible losses - except for conodonts, in which 33% of genera died out.
This pattern is consistent with what is known about the effects of hypoxia, a shortage but not a total absence of oxygen. However, hypoxia cannot have been the only killing mechanism for marine organisms. Nearly all of the continental shelf waters would have had to become severely hypoxic to account for the magnitude of the extinction, but such a catastrophe would make it difficult to explain the very selective pattern of the extinction. Models of the Late Permian and Early Triassic atmospheres show a significant but protracted decline in atmospheric oxygen levels, with no acceleration near the P-Tr boundary. Minimum atmospheric oxygen levels in the Early Triassic are never less than present day levels - the decline in oxygen levels does not match the temporal pattern of the extinction.
The observed pattern of marine extinctions is also consistent with hypercapnia (excessive levels of carbon dioxide). Carbon dioxide (CO2) is actively toxic at above-normal concentrations, as it reduces the ability of respiratory pigments to oxygenate tissues, and makes body fluids more acidic, thereby hampering the production of carbonate hard parts like shells. At high concentrations, carbon dioxide causes narcosis (intoxication). In addition to these direct effects, CO2 reduces the concentration of carbonates in water by "crowding them out", which further increases the difficulty of producing carbonate hard parts.
Marine organisms are more sensitive to changes in CO2 levels than are terrestrial organisms for a variety of reasons. CO2 is 28 times more soluble in water than is oxygen. Marine animals normally function with lower concentrations of CO2 in their bodies than land animals, as the removal of CO2 in air-breathing animals is impeded by the need for the gas to pass through the respiratory systems membranes (lungs, tracheae, and the like). In marine organisms, relatively modest but sustained increases in CO2 concentrations hamper the synthesis of proteins, reduce fertilization rates, and produce deformities in calcareous hard parts.
It is difficult to analyze extinction and survival rates of land organisms in detail, because there are few terrestrial fossil beds that span across the Permian-Triassic boundary. Triassic insects are very different from those of the Permian, but there is a gap in the insect fossil record spanning approximately 15M years from the late Permian to early Triassic. The best known record of vertebrate changes across the Permian-Triassic boundary occurs in the Karoo Supergroup of South Africa, but statistical analyses have so far not produced clear conclusions.
Earlier analyses indicated that life on Earth recovered quickly after the Permian extinctions, but this was mostly in the form of disaster taxa, such as the hardy Lystrosaurus. The most recent research indicates that the specialized animals that formed complex ecosystems, with high biodiversity, complex food webs and a variety of niches, took much longer to recover. It is thought that this long recovery was due to the successive waves of extinction which inhibited recovery, as well as to prolonged environmental stress to organisms which continued into the Early Triassic. Recent research indicates that recovery did not begin until the start of the mid-Triassic, 4 million to 6 million years after the extinction; and some writers estimate that the recovery was not complete until 30M years after the P-Tr extinction, i.e. in the late Triassic.
During the early Triassic (4-6M years after the P-Tr extinction), the plant biomass was insufficient to form coal deposits, which implies a limited food mass for herbivores. River patterns in the Karoo changed from meandering to braided, indicating that vegetation there was very sparse for a long time.
Each major segment of the early Triassic ecosystem — plant and animal, marine and terrestrial — was dominated by a small number of genera, which appeared virtually worldwide, for example: the herbivorous therapsid Lystrosaurus (which accounted for about 90% of early Triassic land vertebrates) and the bivalves Claraia, Eumorphotis, Unionites and Promylina. A healthy ecosystem has a much larger number of genera, each living in a few preferred types of habitat.
Disaster taxa (opportunist organisms) took advantage of the devastated ecosystem and enjoyed a temporary population boom and increase in their territory. For example: Lingula (a brachiopod); stromatolites, which had been confined to marginal environments since the Ordovician; Pleuromeia (a small, weedy plant); Dicroidium (a seed fern).
Changes in marine ecosystems
Prior to the extinction, approximately 67% of marine animals were sessile and attached to the sea floor, but during the Mesozoic only about half of the marine animals were sessile while the rest were free living. Analysis of marine fossils from the period indicated a decrease in the abundance of sessile epifaunal suspension feeders, such as brachiopods and sea lilies, and an increase in more complex mobile species such as snails, urchins and crabs.
Before the Permian mass extinction event, both complex and simple marine ecosystems were equally common; after the recovery from the mass extinction, the complex communities outnumbered the simple communities by nearly three to one, and the increase in predation pressure led to the Mesozoic Marine Revolution.
Bivalves were fairly rare before the P–Tr extinction but became numerous and diverse in the Triassic and one group, the rudist clams, became the Mesozoic's main reef-builders. Some researchers think much of this change happened in the 5 million years between the two major extinction pulses.
Crinoids ("sea lilies") suffered a selective extinction, resulting in a decrease in the variety of forms in which they grew. Their ensuing adaptive radiation was brisk, and resulted in forms possessing flexible arms becoming widespread; motility, predominantly a response to predation pressure, also became far more prevalent.
Lystrosaurus, a pig-sized herbivorous dicynodont therapsid, constituted as much as 90% of some earliest Triassic land vertebrate faunas. Smaller carnivorous cynodont therapsids also survived, including the ancestors of mammals. In the Karoo region of southern Africa the therocephalians Tetracynodon, Moschorhinus and Ictidosuchoides survived but do not appear to have been abundant in the Triassic.
Archosaurs (which included the ancestors of dinosaurs and crocodilians) were initially rarer than therapsids, but they began to displace therapsids in the mid-Triassic. In the mid to late Triassic the dinosaurs evolved from one group of archosaurs, and went on to dominate terrestrial ecosystems for the rest of the Mesozoic. This "Triassic Takeover" may have contributed to the evolution of mammals by forcing the surviving therapsids and their mammaliform successors to live as small, mainly nocturnal insectivores; nocturnal life probably forced at least the mammaliforms to develop fur and higher metabolic rates.
Some temnospondyl amphibians made a relatively quick recovery, in spite of nearly becoming extinct. Mastodonsaurus and trematosaurians were the main aquatic and semi-aquatic predators during most of the Triassic, some preying on tetrapods and others on fish.
Land vertebrates took an unusually long time to recover from the P-Tr extinction; one writer estimates that the recovery was not complete until 30 million years after the extinction, i.e. not until the Late Triassic, in which dinosaurs, pterosaurs, crocodiles, archosaurs, amphibians and mammaliforms were abundant and diverse.
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