Friday, 16 October 2009

The Jurassic Coast – Sidmouth, Devon, England

This photograph was taken looking south from Sidmouth which is a small town on the English Channel coast in Devon, South West England. The town lies at the mouth of the River Sid in the East Devon district, 15 miles (24 km) south east of Exeter. It has a population of about 15,000, of whom 40% are over 65, and a large part of the town has been designated as a Conservation area. Sidmouth's rocks contain fossils and so this stretch of coast is part of the Jurassic coast world heritage site.

The town is a tourist resort and a gateway town on the Jurassic Coast World Heritage Site. The Jurassic Coast consists of Triassic, Jurassic and Cretaceous cliffs, spanning the Mesozoic Era, documenting 180 million years of geological history, and the characteristic red-coloured rocks around Sidmouth indicate the arid conditions of the Triassic period.
The Triassic is a geologic period and system that extends from about 251 to 199 Ma (million years ago). As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events. The extinction event that closed the Triassic period has recently been more accurately dated, but as with most older geologic periods, the rock beds that define the start and end are well identified, but the exact dates of the start and end of the period are uncertain by a few million years.
During the Triassic, both marine and continental life show an adaptive radiation beginning from the starkly impoverished biosphere that followed the Permian-Triassic extinction. Corals of the hexacorallia group made their first appearance. The first flying vertebrates, thepterosaurs, evolved during the Triassic.
The Triassic climate was generally hot and dry, forming typical red bed sandstones and evaporites. There is no evidence of glaciation at or near either pole; in fact, the polar regions were apparently moist and temperate, a climate suitable for reptile-like creatures. Pangaea's large size limited the moderating effect of the global ocean; its continental climate was highly seasonal, with very hot summers and cold winters. It probably had strong, cross-equatorial monsoons
Three categories of organisms can be distinguished in the Triassic record: holdovers from the Permian-Triassic extinction, new groups which flourished briefly, and other new groups which went on to dominate the Mesozoic world.
On land, the holdover plants included the lycophytes, the dominant cycads, ginkgophyta (represented in modern times by Ginkgo biloba) and glossopterids. The spermatophytes, or seed plants came to dominate the terrestrial flora: in the northern hemisphere, conifers flourished. Glossopteris (a seed fern) was the dominant southern hemisphere tree during the Early Triassic period.
Marine fauna
In marine environments, new modern types of corals appeared in the Early Triassic, forming small patches of reefs of modest extent compared to the great reef systems of Devonian times or modern reefs. The shelled cephalopods called ammonites recovered, diversifying from a single line that survived the Permian extinction. The fish fauna was remarkably uniform, reflecting the fact that very few families survived the Permian extinction. There were also many types of marine reptiles. These included the Sauropterygia, which featured pachypleurosaurs and nothosaurs (both common during the Middle Triassic, especially in the Tethys region), placodonts, and the first plesiosaurs; the first of the lizardlike Thalattosauria (askeptosaurs); and the highly successful ichthyosaurs, which appeared in Early Triassic seas and soon diversified, some eventually developing to huge size during the late Triassic.
Terrestrial fauna
Temnospondyl amphibians were among those groups that survived the Permian-Triassic extinction, some lineages (e.g. Trematosaurs) flourishing briefly in the Early Triassic, while others (e.g. capitosaurs) remained successful throughout the whole period, or only came to prominence in the Late Triassic (e.g. plagiosaurs,metoposaurs). As for other amphibians, the first Lissamphibia, characterized by the first frogs, are known from the Early Triassic, but the group as a whole did not become common until the Jurassic, when the temnospondyls had become very rare.
Archosauromorph reptiles — especially archosaurs — progressively replaced the synapsids that had dominated the Permian. Although Cynognathus was a characteristic top predator in earlier Triassic (Olenekian and Anisian) Gondwana, and both kannemeyeriid dicynodonts and gomphodont cynodonts remained important herbivores during much of the period. By the end of the Triassic, synapsids played only bit parts. During the Carnian (early part of the Late Triassic), some advanced cynodont gave rise to the first mammals. At the same time the Ornithodira, which until then had been small and insignificant, evolved into pterosaurs and a variety of dinosaurs. The Crurotarsi were the other important archosaur clade, and during the Late Triassic these also reached the height of their diversity, with various groups including the phytosaurs,aetosaurs, several distinct lineages of Rauisuchia, and the first crocodylians (the Sphenosuchia). Meanwhile the stocky herbivorous rhynchosaurs and the small to medium-sized insectivorous or piscivorous Prolacertiformes were important basal archosauromorph groups throughout most of the Triassic.
Among other reptiles, the earliest turtles, like Proganochelys and Proterochersis, appeared during the Norian (middle of the Late Triassic). TheLepidosauromorpha—specifically the Sphenodontia—are first known in the fossil record a little earlier (during the Carnian). The Procolophonidae were an important group of small lizard-like herbivores.
Archosaurs were initially rarer than the therapsids which had dominated Permian terrestrial ecosystems, but they began to displace therapsids in the mid-Triassic. 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
Triassic-Jurassic extinction event
This marks the boundary between the Triassic and Jurassic periods, 199.6 million years ago, and is one of the major extinction events of the Phanerozoic eon, profoundly affecting life on land and in the oceans. A whole class (conodonts), twenty percent of all marine families and all large crurotarsans (non-dinosaurian archosaurs), some remaining therapsids, and many of the large amphibians were wiped out. At least half of the species now known to have been living on Earth at that time went extinct. This event vacated ecological niches, allowing the dinosaurs to assume the dominant roles in the Jurassic period. This event happened in less than 10,000 years and occurred just before Pangaea started to break apart. This marked the divide between the Triassic dinosaurs and the Jurassic dinosaurs.
Statistical analysis of marine losses at this time suggests that the decrease in diversity was caused more by a decrease inspeciation than by an increase in extinction.
Several explanations for this event have been suggested, but all have unanswered challenges:
§ Gradual climate change or sea-level fluctuations during the late Triassic. However, this does not explain the suddenness of the extinctions in the marine realm.
§ Asteroid impact, but no impact crater has been dated to coincide with the Triassic–Jurassic boundary (the impact responsible for the annular Manicouagan Reservoir occurred about 12 million years before the extinction event).
§ Massive volcanic eruptions, specifically the flood basalts of the Central Atlantic Magmatic Province, would release carbon dioxide or sulphur dioxide which would cause either intense global warming (from the former) or cooling (from the latter).
The isotopic composition of fossil soils of Late Triassic and Early Jurassic show no evidence of any change in the CO2composition of the atmosphere. More recently however, some evidence has been retrieved from near the Triassic–Jurassic boundary suggesting that there was a rise in atmospheric CO2 and some researchers have suggested that the cause of this rise, and of the mass extinction itself, could have been a combination of volcanic CO2 outgassing and catastrophic dissociation of gas hydrate. Gas hydrates have also been suggested as one possible cause of the largest mass extinction of all time; the so-called "Great Dying" at the end of the Permian Period.
Photograph Details: Nikon D40, Focal Length 18mm, ISO-200, exp: 1/640 sec, F-stop f/13

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  1. Man - you've been busy and I've been neglagent!! My apologies :)

    Beautiful picture!

  2. haha, that's alright. I've not been as active as I'd like, but that's always the way. Just not got the time! Good to see a familiar face again though. Trust everything's alright?


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