Figure 6.1. Chain of cause and effect for Toarcian Age (Early Jurassic Period) environmental change and extinction crisis.
PANGEA’S FINAL BLOW
JURASSIC GOLGOTHA
The recovery from the end-Triassic catastrophe was in many ways a much swifter and more successful affair than that which followed the end-Permian debacle. A million years after the start of the Jurassic, seafloor life was both abundant and diverse. In contrast, a million years after the start of the Triassic, life was still in the doldrums—only a handful of small gastropod and bivalve species eked a living in the hot, oxygen-poor seas. However, Jurassic recovery was not all it seemed; many of the survivors were never to regain their former glory. Taking the ichthyosaurs as an example, many types went extinct and the few survivors never reevolved the same range of body types—only dolphin-like fish lizards were to grace the Jurassic seas. It would take a range of other newly evolved marine reptiles, such as the plesiosaurs, pliosaurs, and marine crocodiles, to add to the diversity of the ocean’s top predators. The ammonoids reveal a very different response, although they too were reduced to a handful of species during the extinction. These gave rise to the ammonites, one of the most abundant and successful (and beautiful) of Jurassic fossils, but initially they were restricted to simple shells lacking ribs or fancy ornament—such embellishments would take a few million years to evolve.
Reefs are in many ways the most delicate of marine communities: highly prone to being wiped out during most mass extinctions with prolonged recovery times required in their aftermath. This was the case after the end-Triassic mass extinction. However, within 20 million years, corals were once again constructing reefs of appreciable diversity, but all this evolutionary effort was about to be undone by impending catastrophe. Trouble was about to strike in a familiar place.
The Karoo Basin of South Africa is doubly famous among the mass extinction fraternity; first, because it provides the best fossil record of the fate of terrestrial animals during the Permo-Triassic mass extinction as discussed in chapter 3, and second, because it was the site of vast flood basalt eruptions in the Early Jurassic that coincide with the final crisis in Pangea’s history. The Karoo eruptions marked the onset of the continental rifting that was to ultimately break apart southern Pangea and create the continents that now occupy the southern hemisphere. As a result, as with the CAMP volcanics, formerly united lava piles are now separated between three continents (in this case Africa, Antarctica, and Australia), with the greater part being in South Africa. The Antarctic equivalents occur in Victoria Land, where the lavas are named after the Ferrar Glacier, which was in its turn named after the geologist Hartley Ferrar. The whole LIP is named the Karoo-Ferrar Province, and its original volume is estimated to be more than 3 million cubic kilometers of lava, making it a large example of its kind.
Like many LIPs, it was originally thought that Karoo-Ferrar eruptions spanned tens of millions of years, but dramatic improvements in dating precision in the 1990s and since have shown this to be a considerable overestimate. Recent work has shown that the lavas were erupted in a mere million years or so approximately 183 million years ago. This point in time is in the Toarcian, the last stage of the Early Jurassic, a period of major oceanic and climatic changes and significant extinctions.
The best evidence for the nature of the Early Jurassic crisis comes from a series of lovely coastal outcrops in North Yorkshire, northern England. These are centered on the fishing/tourist town of Whitby, which is famous for a Benedictine abbey that sits on the hill overlooking the harbor, and for its fossils, especially ammonites. The abbey and the ammonites are linked because St. Hilda, the seventh-century founder of the abbey, is said to have turned the local snakes into the coiled stone effigies (i.e., ammonites) that are found in the local cliffs and on the beaches. These are the targets of the numerous fossil hunters who visit the area, including me. I have many happy childhood memories of holidays on the Yorkshire coast, armed with a hammer and chisel and my mother’s instructions to keep an eye on the tide. The latter is important because the tidal range of the North Sea is considerable, and it is easy to get cut off if you are not wary. However, this is to the benefit of geologists, paleontologists, and small children who like to play in rock pools because at times of low tide there are vast stretches of wave-cut platforms. During high tide one has to retreat to the amusement arcades and cafes in Whitby.
The Lower Jurassic cliffs around Whitby accumulated in a shallow sea that lay to the north of the Tethyan Ocean. In addition to ammonites, bivalves are very common, and many belong to long-lived species that first appeared at the start of the Jurassic and survived up to the start of the Toarcian Stage—a period of 20 million years. Deeper-water, dark-gray marine shales succeeded the shallow marine sediments at this time, and most of the long-lasting bivalves coincidentally disappeared—their time had finally come.
This extinction takes place in the first zone of the Toarcian, which is named after the ammonite Dactylioceras tenuicostatum. Tony Hallam was the first to draw attention to this increase of water depth coincident with extinction among bivalves, and he showed that the precise level of the losses coincides with a tough bed of black shale called the Jet Rock.17 This bed marks the onset of a phase of anoxic deposition in the lower water column that was to persist for much of the Toarcian Stage. The reason for the bivalve extinctions is therefore immediately clear: in the absence of oxygen, the seabed became uninhabitable. Oxygen remained plentiful in the upper water column, though, as shown by the abundance of nektonic (free-swimming) fossils that lived there, including ammonites, fishes, marine reptiles, and a newly evolved group called the belemnites. These squid-like animals had an internal skeleton that included toward their back end a solid calcite structure shaped like a bullet and called a guard. Belemnite guards (usually just called belemnites) are prolifically common in the Toarcian shales, although the rest of the animal is rarely preserved, and as we shall see, the guards have provided a bonanza for geochemical study.
After Tony’s pioneering work, one of my colleagues at Leeds, Crispin Little, looked in further detail at the timing of the Early Jurassic extinction losses, and he showed, along with his PhD supervisor, Mike Benton, that the crisis was not a single event but rather was spread over perhaps several million years. Further studies have resolved this further and shown that most extinction losses can be grouped into two distinct phases, especially among the ammonites. The main ammonite extinction occurs at the boundary between the Toarcian Stage and the earlier Pliensbachian Stage and thus was a little bit earlier than the bivalve losses, which were mainly late in the D. tenuicostatum Zone.
Around Whitby the change in ammonite faunas is very obvious to fossil collectors because thick-ribbed ammonites belonging to the amaltheid family are found in Pliensbachian strata and they abruptly give way, at the start of the Toarcian, to fine-ribbed dactylioceratids. These were immigrants. They had migrated from the warmer waters of the Tethyan Ocean to the south, and their appearance at Whitby can be read both as a refilling of empty habitats following the loss of the incumbents and as a response to increased warmth driving tropical faunas northward. The ammonite transition occurs precisely at the Pliensbachian-Toarcian boundary, which is marked by a thin bed of black shale called the Sulphur Band. The bed gets its name from its weathering appearance. It has a high content of pyrite, and this iron sulfide mineral decomposes readily to give a veneer of red iron oxides and the yellow of sulfur. The abundance of pyrite indicates that anoxia occurred during deposition of the Sulphur Band, and like the Jet Rock, this black shale is almost entirely devoid of fossils. Therefore both the slightly younger bivalve extinction and the main ammonite extinction coincide with anoxic conditions, although the Sulphur Band anoxia was of much briefer duration than that recorded by the Jet Rock.
Thanks to several detailed studies on this interval, we can date the sequence of Pliensbachian-Toarcian events with considerable precision. Thus the boundary between these two stages occurred a little before 183.5 million years ago, and the deposition of the Sulphur Band that followed, along with the coincidental first extinction pulse, probably lasted no more than a few thousand years. There was, then, a 300,000-year improvement in oxygenation before the next extinction and the onset of Jet Rock deposition. The second phase of intense oxygen starvation was of much longer duration; black shale deposition was to last 2 million years. As best we can tell, the onset of Karoo-Ferrar volcanism occurred early on in this prolonged anoxic episode, around 200,000 years after the mass extinction, a mismatch in timing that we will return to.
The losses recorded in the cliffs around Whitby were significant; many long-lived species disappeared, but it is difficult to claim that they were truly catastrophic in the same way as the Permo-Triassic and end-Triassic mass extinctions. The Toarcian extinctions did not cause wholesale changes in the composition of marine communities. Bivalves dominated on the seafloor, and ammonites and belemnites filled the water column both before and after this crisis. It is a similar story for other marine animals such as gastropods, forams, and crinoids—all suffered significant but not severe extinctions. The exceptions to this terrible-but-not-quite-a-catastrophe pattern are seen among the brachiopods and ostracods, two groups that faired very badly at this time.
The heyday of the brachiopods was before the Permo-Triassic mass extinction, and they never recovered their previous dominance following the disastrous losses incurred during this crisis. However, by the Early Jurassic they had attained a respectable diversity and were common in the warm waters of the Tethyan Ocean but less abundant in the shallow seas that covered England. Brachiopods show distinct changes in the early Toarcian: like the ammonites, many species migrated northward from Tethys into the seas covering southern England, suggesting a warming trend (discussed further, below), but their main story was one of extinction. Two entire orders disappeared at the end of the Pliensbachian—the Spiriferinida and Athyridida—bringing to an end two dynasties that had lasted for more than 250 million years. It is fair to say that the brachiopods were truly knocked for six18 by this Jurassic devastation: more than three-quarters of their genera and perhaps 90% of their species failed to survive. No new orders of brachiopods arose to replace the losses, and only two orders are common today—a minor memory of their Paleozoic glory days.
For the ostracods (tiny crustaceans that live encased inside two bean-shaped shells), the Toarcian crisis was even worse; an entire suborder, the Metacopina, was lost—the highest-ranking taxonomic extinction of the entire crisis—but their recovery was much more successful. Today ostracods are a common and diverse component of aquatic communities.
Beyond the shallow shelf seas of northwestern Europe, the Toarcian crisis also wrought its changes. One of the strangest groups to succumb was an unusual group of bivalves called the lithiotids. Throughout most of their history, bivalves evolved at a rather slow pace and generally restricted themselves to a limited range of forms familiar to anyone who has gone shell hunting on a beach. However, every so often large, thick-shelled forms with strange morphologies have appeared. We have already encountered one family, the Triassic megalodontids, which were victims of the end-Triassic mass extinction, and in the Early Jurassic the lithiotids were another evanescent group to appear in the shallow seas fringing Tethys. They were denied a longer history by the Toarcian crisis. I have studied their extinction in limestones in southern Tibet, where they disappear from a series of beds that record deepening. In this regard, their extinction is comparable with that in northern Europe, but the Tibetan extinction/deepening is not associated with the onset of anoxic, black shale deposition. Indeed, marine anoxia seems to have had a very patchy development in the seas of the southern Tethyan shelf.
Turning our attention to the Panthalassa Ocean, we can examine the fate of the planktonic radiolarians. The best evidence, like that for all previous Pangean crises, comes from chert beds that formed on the Panthalassa Ocean floor but are now found in the mountains of Honshu, central Japan. The cherts reveal a sharp decline in the variety of radiolarians during the Pliensbachian-Toarcian and an early Toarcian population that was characterized by relatively few species of spumellarians—one of the morphologically simple group of radiolarians. So, clearly something was happening to plankton populations in the oceans, but the changes proved to be short lived. There were no catastrophic losses for radiolarians on the scale of those in the Permo-Triassic wipeout or even the end-Triassic extinction. Instead, immediately after the spumellarian bloom, most of the radiolarian groups that had gone reappeared once more.
The clearest evidence of a Toarcian oceanic crisis comes not from the radiolarians but rather from the sediments in which they occur because a thin horizon of organic-rich shale is interbedded among the Japanese cherts. Very similar to (and contemporaneous with) the Jet Rock in Whitby, this bed also records anoxic seafloor conditions, although in this case, the oxygen starvation was developed far out in the open ocean. The Japanese black shales contain tiny pyrite framboids with the same size ranges of those that formed in the Early Triassic (see p. 57), and so like the earlier times, conditions of intense anoxia with hydrogen sulfide present in the water column had returned to Panthalassa. All told, this Toarcian oceanic anoxic event, as it has been termed by geologists, has a somewhat hit-and-miss development: although clearly expressed in European shelf seas and the open ocean, it has a very muted development in the waters of Tethys.
The radiolarians are not the only plankton that left a good fossil record in the Early Jurassic. As noted in chapter 4, the coccolithophores are a group of plankton that secrete tiny calcitic plates, called coccoliths, that form deep-sea carbonate mud. They appeared in the Late Triassic, and by the end of this period they had diversified somewhat, only to loose significant numbers of species in the mass extinction. However, they bounced back successfully and began to proliferate, initially in the shallow seas fringing Tethys and later in the open ocean waters of Tethys. At the time of the early Toarcian crisis, they were still mostly restricted to shelf seas. How did they fair at this time? Surprisingly the answer is, really quite well. Compilations of diversity show no hiccup; instead, the coccoliths’ increasing diversity continued unchecked until the end of the Cretaceous, nearly 120 million years later. The Toarcian crisis clearly did not have a long-lasting impact on the world’s plankton.
Having surveyed the changes in the Jurassic oceans, let us now turn our attention to the land. By the Early Jurassic, the dinosaurs had taken over and had diversified into forms that included large, armored herbivores; even larger, long-necked sauropods; and predatory theropods, such as Dilophosaurus. The last makes an appearance in the first Jurassic Park movie, where it has a nasty spitting habit. Unfortunately though, our knowledge of dinosaur fortunes around the time of the Toarcian crisis is not very good, which may have to do with the very high sea level at this time, making terrestrial dinosaur-bearing sediments rather rare. This lack makes it difficult to tell if there were any significant losses among the dinosaurs. There is a little bit of indirect evidence that suggests something may have happened. Matthew Carrano of the Smithsonian Institution undertook a huge, exhaustive study of the evolutionary tree of the megalosaurids—giant predatory dinosaurs—which showed that they apparently underwent a major diversification in the later Toarcian. Such radiations often follow on from mass extinctions because they remove incumbents and thus provide an impetus for evolution to refill vacated niches, so perhaps a dinosaur extinction event in the early Toarcian removed the existing predators like Dilophosaurus and allowed the megalosaurids to step forward and take their place. Unfortunately, evidence like this is no more than circumstantial. Not every evolutionary success story has to be preceded by extinction. It would be fair to say that had there been a truly major terrestrial extinction in the Toarcian, then paleontologists would probably have found the evidence by now.
Despite the patchy effects of the early Toarcian crisis on the world’s ecosystems, it is clear that the oceanographic changes were spectacular, especially the development of a phase of oceanic anoxia comparable in intensity with that of the Early Triassic oceans. There is also plenty of geochemical evidence for major changes in the atmosphere. Much of this support comes from belemnites. These are very solid, chunky bits of calcite that have altered little since they were originally grown, so the constituent carbon and oxygen atoms of the calcite retain the original isotopic ratios of the Jurassic seawater in which they formed. This is doubly fortunate because hitherto, in our study of the Pangean world, conodonts have been the fossils of choice when compiling isotopic trends. As we saw earlier, these animals disappeared forever during the end-Triassic mass extinction, to the great disappointment of geochemists (and no doubt the conodonts), but fortunately the belemnites stepped in to fill a vacancy in the category of incredibly useful fossils.
Carbon isotope measurements from belemnite calcite show a shift to heavier ratios (i.e., there is more of the carbon-13 atom) in the early Toarcian at the time of deposition of the Jet Rock and Panthalassa Ocean black shales. This change makes a great deal of sense because the abundant organic matter in these black shales is rich in carbon-12. We would expect the removal of a lot of light carbon from the oceans (in the form of the shales) to leave the remaining carbon enriched in heavy carbon, which is what the belemnite calcite carbon isotope ratios show. All well and good then, but we also have another way of measuring the isotopic record of carbon in the oceans and atmosphere, and that is by analyzing organic carbon. Remarkably, this carbon does not show the same trend as in the belemnites, even though it is coming from the same source—the atmosphere and oceans of the Early Jurassic world. The shift to the heavier isotope is still recorded in the organic carbon isotopes, but it is preceded by a major swing to values enriched in carbon-12 around the time of the Toarcian extinction. This earlier negative shift is very similar to that seen around the onset of all the other mass extinctions discussed in this book, and so we should perhaps not be surprised to find it happening again during the Toarcian crisis. Needless to say, the discrepancy between the belemnite and organic carbon isotopic values has caused a huge amount of (sometimes acrimonious) discussion among geologists—which need not concern us here—but finding the reason behind it has so far proved elusive.
If the negative shift in organic carbon isotope values is genuine, then it may record a major influx of greenhouse gases into the atmosphere as seen during other extinction events (see p. 62, for example), in which case we should find evidence for global warming. This prediction is supported by such fossil distributions as the northward migration of warm water brachiopods and ammonites from the Tethyan Ocean. It is also supported by belemnite analyses (again!), but this time of their oxygen isotope ratios. A little over ten years ago, John McArthur (University College London) and his colleagues were the first to produce a temperature curve using the oxygen isotope values from Whitby belemnites. These ratios showed that the onset of anoxic conditions, seen in the Jet Rock and other black shales, coincided with a rapid rise in seawater temperatures of up to 7°C. John speculated that the two phenomena were related as follows. Rising temperatures typically go hand in hand with increased humidity, in turn causing increased runoff from the continents (because of the extra rainfall) and thus an extra supply of terrestrial nutrients into the sea. These nutrients are food for plankton, which therefore increase in abundance. The decay of all this extra planktonic organic matter in the oceans would raise the rate at which oxygen is depleted, and voilà, you have a link between warming and anoxia. This link between nutrient supply to the oceans and anoxia has also been proposed as one of the causes of Permo-Triassic ocean anoxia, in which we saw that the links between warming and anoxia are manifold.
Seawater temperatures derived from oxygen isotopes can also be compared with evidence for warming derived from analysis of leaf stomata density. Once again Jenny McElwain has done the hard work, and her results are rather intriguing. Samples of fossil leaves from the Danish island of Bornholm revealed a rapid rise in atmospheric CO2 from 600 ppm in the middle of the D. tenuicostatum Zone to around 1000 ppm at the end of this interval—a perfect fit with the oxygen isotope trend. However, after a gap in the fossil record, the next set of leaves shows that atmospheric CO2 values plunged to 350 ppm, implying a sharp cold snap. This result does not fit with the belemnite oxygen isotope trend at all because this suggests the acme of warming occurred at the same time as the leaves record cooling. I suspect the stomatal data is at fault in this case and has been overinterpreted, because the published data is “noisy” during the so-called cool phase.
Further refinements to the Jurassic seawater temperature curve have been made using samples taken from Spanish locations. These have reproduced the original temperature curve but also include an additional, brief warm “blip” at the start of the D. tenuicostatum Zone. This neatly coincides with the first pulse of extinction (particularly of ammonites) and also the black shale that developed at this time. It seems as if the beginning of the Toarcian witnessed the first attempt to move to a world with warm climates and anoxic seas, but feedback mechanisms swung conditions back to cooler, better-ventilated seas. Nonetheless, the Pliensbachian-Toarcian boundary changes were sufficiently bad to cause problems for ammonites. Only later in the D. tenuicostatum Zone was there a more permanent shift to warmth and anoxia, and this time the extinctions were concentrated among seafloor creatures.
With these temperatures trends established, it becomes clear just how remarkably similar the Toarcian crisis was to the Permo-Triassic mass extinction. They share the following attributes:
1. Two main phases of marine extinctions separated by an interlude lasting 200,000 to 300,000 years that allowed a temporary respite and recovery
2. Coincidence between rapid warming of oceans and atmosphere and sea-level rise
3. A first extinction phase that coincides with a short anoxic interval and a second extinction phase that occurs at the start of a much longer phase of ocean anoxia lasting a few million years
4. Carbon isotope trends that become lighter (more carbon-12 relative to carbon-13), suggesting a lot of light carbon is being added to the atmosphere
And of course there is also the correlation with the eruption of a LIP, although in the case of the Karoo-Ferrar eruptions these seem to have begun 200,000 years after the second extinction—they were a little bit late. In fact, this slight mismatch in timing is also seen for the Emeishan Traps, Siberian Traps, and CAMP eruptions. The cause of the discrepancy probably lies in way the LIPs are being dated. Working out the ages of volcanic rocks involves extracting crystals from lavas and finding ones that are rich in radioactive elements. Analysis of these allows an age to be determined. There is nothing wrong with this approach—it gives the age of the main interval of eruptions—but it is likely that all the environmental damage was done by gigantic gas eruptions right at the onset of the eruption history before the main lava flows appeared. The reasons for this lie in the nature of the mantle plumes that are the source of LIP magmas: it is probable that a lot of volatile, gas-rich eruptions preceded the main lava flows.
Direct evidence in support of the giant gas eruptions can be found in the Karoo landscape of South Africa. The region is peppered with thousands of small hills called volcanic breccia, cones that measure around 100 meters in diameter and 10 to 20 meters high. There are no less than 430 of them around the small town of Loeriesfontein alone. Below the surface, the cones are connected to narrow shafts of broken-up rock that formed by the explosive release of gas; these are called breccia pipes. The magma that fed the lavas in this region ascended through continental crust that has thick layers of Permian black shales. As this hot material came into contact with the shales, it probably would have baked them and released huge volumes of CO2—suggested to be as much as 2000 gigatonnes of CO2. Because the carbon dioxide was derived from isotopically light organic matter in the shales, then it too would have had light isotopic values. It thus likely that the negative carbon isotope excursions seen in contemporary organic carbon is recording this major episode of carbon dioxide release. This is, of course, Svensen’s hypothesis all over again. It has been a regular feature in all of the proposed extinction scenarios of Pangea.
Putting all these observations and ideas together it is possible to come up with an extinction scenario that is essentially the same as that proposed for all the other Pangean extinctions, especially the Permo-Triassic crisis (fig. 6.1). A sudden eruption of flood basalts in southern Pangea released great volumes of CO2 and thereby triggered a warming trend. Additional CO2 from crustal baking (Svensen’s hypothesis) may have further contributed to this heating. Once established, the warming trend may also have initiated the release of methane from gas hydrates, although this aspect is not a compulsory feature of the kill model; the gases linked with volcanism may have been sufficient.
A point was rapidly reached at the start of the eruptions where the warming trend caused marine anoxia to become widespread (for the link between warming and anoxia, see the discussion in chapter 3, pp. 76–79). This set up negative feedback that gradually returned the world to less harsh conditions. The mechanism works as follows: anoxic conditions favor the preservation of organic matter, which causes black, organic-rich shales to accumulate. The carbon in black shales is derived from phytoplankton, which have removed carbon dioxide as they photosynthesize. Thus, organic burial is a highly effective way of removing atmospheric CO2 and reversing greenhouse trends. The shift to heavier carbon isotopes in the seawater at the time of black shale formation is evidence of this process occurring because isotopically lighter carbon is being buried in the black shales.
Figure 6.1. Chain of cause and effect for Toarcian Age (Early Jurassic Period) environmental change and extinction crisis.
Intriguingly, as with the Permo-Triassic crisis, it would appear that this cycle of warming anoxia and cooling happened at least twice: once (briefly) at the Pliensbachian-Toarcian boundary, and across a longer period beginning in the late D. tenuicostatum Zone. The implication is that there were two blasts of gas from the Karoo-Ferrar LIP.
The direct cause of extinction in this Toarcian scenario is generally thought to be the spread of anoxic conditions and the consequent loss of habitable seabed environments and water columns for marine life. The link is especially clear in northern Europe, where black shales are well developed, but less so in southern Tethys, where the oxygen deficiency was less intense but perhaps still sufficiently damaging. Were high temperatures also involved in the extinctions? Several geologists are certainly keen on this idea as a factor in the brachiopod extinction, although the temperature rise does not seem to be that extreme. Temperatures, calculated from belemnite calcite data, suggest a rise in Spanish seas from 13°C to 21°C during the extinction. If these values are reliably accurate, they are not likely to have been detrimental to marine life; indeed, they are comparable to those around the Mediterranean shores of Spain today. It is difficult to imagine such temperatures playing any role in the Toarcian crises.
As with the earlier mass extinctions of Pangea, the rapid release of carbon dioxide to the atmosphere can lead to ocean acidification and thereby inflict a further potential stress on marine life, especially among shelled invertebrates. Some evidence for this comes from the study of coccoliths in western Tethys. Emanuela Mattioli of the University of Lyon has spent a career examining coccolith fossils—something that requires a great deal of patience, because they are very small (rarely more than 10 microns in size), and a keen eye because, to nonspecialists at least, they all look rather similar. By counting their abundance in sediments from Tethyan shelf sediments, Emanuela has shown that the coccolith flux rate (how fast they were raining down to the seabed) declined in the early Toarcian as the coccoliths become smaller and lighter. It is tempting to ascribe these changes to the increased difficulty of secreting calcite in more acidic surface waters; however, other environmental factors could be at play. Coccolithophores do best in regions of low nutrient supply because they get outcompeted by other types of plankton when nutrients are abundant. The poor showing of the coccoliths during the Toarcian may therefore be more a signal of increased nutrient runoff in a warmer world rather than of a lower pH. Wisely, Emanuela has been careful not to make too emphatic a link between her observations and acidification.
Whatever was happening with the plankton in the Toarcian oceans, it was not a time of devastating crisis. Neither do the terrestrial animals appear to have suffered badly at this time. One can speculate on the reasons behind this restrained response to the Karoo-Ferrar eruptions. Marine anoxia was widespread but not anywhere near as extensive as that seen during the Permo-Triassic event, so there was plenty of sea-floor not affected by these hostile conditions. Temperatures rose and no doubt were a key factor behind the anoxia, but values do not appear to have reached the same stressful levels seen during earlier crises.
All told, the final crisis of Pangea was a subdued, muted version of those that had gone before. It is almost as if life was finally getting used to the effects of LIP volcanism. Whether true or not, there is no chance to test this notion because for the remainder of the Jurassic, no more LIPs formed. Instead Pangea began to slowly drift apart as the Central Atlantic opened, and its southern arm, Gondwana, began to split up. Not until 40 million years after the Karoo-Ferrar eruptions did LIP volcanism once again return, and by this time Pangea had separated. This was the first of many giant flood basalt outpourings to form in the Cretaceous Period. And what happened to life at this time? This is a topic for the final chapter.