Gastropods

How would you recognise a gastropod?


This is easy as the animal is still in existence and its recognised by its soft parts. This gives us a problem when considering fossil examples, as the soft tissue is usually not preserved.

How do modern gastropods live?


The gastropod has a recognisable head with tentacles for sensing its surroundings. This marks the anterior of the animal. It crawls along on a muscular foot with the rest of its soft parts stuck in the shell. To manage this the guts twist 180*, which is a biological distinguishing characteristic of a gastropod . When threatened, the whole body can be withdrawn into its shell.

As the gastropod grows it does so by laying down calcium carbonate on the lips of the aperture. The outer lip grows faster than the inner lip to achieve coiling as well as increase in size. Each complete coil is called a whorl. The soft body occupies the last whorl , which can therefore termed the body chamber. The rest coils up to the apex to make up the spire.

Types of coiling


Most of the coiling is right-handed or dextral, which ends up with a view of the open end, or aperture on the right. coiling to the left is known as sinistral. A good way to distinguish a gastropod from a cephalopod, is that there are no chambers within the shell. Not all gastropods show helical coiling, as some, notably freshwater species, are coiled in a plane (planispiral) and some strange cephalopod's show helical coiling.

Mode of life


The shape and construction of the shell is a good indication of the mode of life. Forms with a large siphonal canal have this to separate inhalent and echalent currents. Thin delicate shells, as seen in freshwater species, are an indication of low energy, conversely a thick, ribbed or ornamented shell indicates high energy.

evolution of amphibians

Amphibians were first land-dwelling creatures with four legs, known as tetrapods. They evolved from lobe-finned fish in the late Devonian to early carboniferous. They then ventured forth into terrestrial environments, where they would later evolve into reptiles, birds and mammals.

What are lobe-finned fish?


Lobe-finned fish are lungfish., they possessed both lungs and gills and had four fleshy fins supported by bones in a similar structure hand. These fish had the ability to breath both in and out of water.

Similarities between the lobe-finned fish and the early amphibians

• The four fins of the lobe- finned fish and the four limbs of the early amphibians skeletal structures were very similar.
• The limbs were in the same position on their bodies
• They both lacked claws or nails 
• The skull morphology, the jaw bone and teeth of the lobe-finned fish and amphibians were very much alike.
• The teeth of both lobe-finned fish and amphibians were complex.

Adaption to life on land

• The development of a girdle connecting the limb bones to the skeleton for better movement on land 
• A more robust skeleton strengthening the vertebral column and rib bones, for support on land.
• eyelids formed to help keep eyes moist, as it was no longer submerged in water.
• A tongue formed within its mouth
• Ears addapted so it could detect sound waves through the thin medium of air.
• tail used for balance

evolution of dinosaurs

The amniotic egg


Dinosaurs are believed to have laid amniotic eggs. These eggs are on of the most significant features in reptile evolution., as it allowed for life on land without the need for a water source in which to reproduce. Instead the egg provided the aquatic environment needed for the development of the embryo within the egg itself.

The first advantage for life on land was the development of a hard outer shell, which provided protection whilst remaining porous. This allowed the diffusion of oxygen into the egg and carbon dioxide out, allowing respiration to take place. The yolk sack provided the embryo with food and the albumin supplied water and nutrients, eliminating the need for a larval stage. Instead the embryo would develop directly into a miniature version of the adult.


Evolution of dinosaurs


Dinosaurs began to evolve after the Permian-Triassic extinction wiped out much of life on Earth, perhaps filling newly vacant ecological niches. They evolved into two separate classes, Saurischia and Ornithischia. The Saurischia were divided into two: Therapoda, including birds and the well- known dinosaur, Tyrannosaurus; and Sauropoda, including Diplodocus. Ornithischia includes the Iguanodon.


Characteristics of Saurischian dinosaurs


This group contains the well known carnivore Velocirapter and the herbivore Diplodocus.
Features include:

• The primitive arrangement of the hip bones is similar to reptiles, in which the pubis points forward.
• They have long, S-shaped flexible necks, allowing rapid and precise movement.
• Hands consisted of only three digits. The digits where asymmetrical with the first digit similar to a thumb , allowing the hand to grasp and with the second digit being the longest of the three. 

Characteristics of Ornithischian dinosaurs

• The arrangement of the hip bones are similar to birds, in which the pubis points backwards.
• The front teeth are small or absent, replaced at the front with a horny beak which later became broader , giving the name "duck-billed dinosaurs".
• Many where armored with bony plates, such as the stegosaurus. These were thought to be a defense mechanism but could have also acted as heat exchanges. The fossilised plates have tiny grooves, which may have housed blood vessels allowing heat to be given off or taken in.

Changing sea levels

Isostatic and eustatic sea-level changes


Changes in sea level due to local subsidence or uplift are referred to as Isostatic. For example, Scotland is rising as it is rebounding now that it is free of several kilometres of ice which has weighed it down since it was deposited during the last ice age. In contrast , the south east of England is dropping by almost a millimetre each year.

Changes in sea level due to changes in the volume of water in the oceans are called eustatic. This can be caused        
by melting of the polar icecaps releasing more water into the oceans. When the ice advances and more water is held in the ice caps then sea level falls. Eustatic change is also caused by rapid rate of sea floor spreading, causing the MOR to swell with magma. This causes sea level to rise.


Measuring past sea levels

• Using seismic evidence to find unconformities where transgressing seas have resulted in the sea laying down younger beds. When the sea retreats regressions can also be identified as unconformities. 
• Using exposed geology to estimate the area of flooded continents through time. When sea levels falls there will be raised beaches and cliff lines. When sea levels rises there will be submerged and marine organisms can be found in younger sediment. 
• Using oxygen isotopes ratios to assess the past temperatures and therefore the amount of ice. 

Milankovich cycles

The cycles are caused by changes in the amount of radiation reaching the earth from the sun over time. This is not because the Sun changes its output of energy, as it has remained relatively constant, but because the earths orbit around the sun varies in three predictable cycles.

• Eccentricity: the earths orbit changes shape to become more elliptical over a period of 100,000 years. At present the eccentricity is almost at a minimum with a difference of around 6% in received radiation between January and July. At maximum eccentricity this difference increases to between 20% and 30%, which has a massive effect on climate. 
• Obliquity: the tilt of the earths axis, which is responsible for our changing seasons, changes up to 3 degrees with a cycle of 41,000 years. A smaller tilt promotes the growth of ice sheets as warmer winters result in more moisture ans snow.  
• Precession: eccentricity and obliquity together cause this further cycle where the inclination of the Earths axis changes in relation to where it is on the orbit. The cycle operates in periods of 19,000 and 23,000 years. At the moment we are closest to the sun so northern winters are slightly warmer than 10,000 years ago when the planet was furthest from the sun. Slow changes in the direction of the axis of the earth as it orbits results in greater seasonal contrasts.

Evidence for Milankovitch cycles


The blue Lias and Kimmeridge clay


For many years geologists studying the Blue Lias rocks to the lower Jurassic in Lyme Regis noticed the way the layers of rock changed from clay to limestone and back again. The pattern of beds created seemed so regular that it must have an explanation.

Analysis of these rocks has shown that the change in environment from a clay-rich sea to a limestone-producing sea happened on a roughly 41,000 year cycle. This correlates with the obliquity of orbit predicted by Milankovitch cycles.

Attempts to identify a 100,000- year cycle have been met with scepticism from some experts, but work carried out on clay found at Kimmeridge bay in Dorset on Upper Jurassic rocks has identified regular Milankovitch cycles.

Climate change over geological time

We currently live in an icehouse where large continental ice sheets exist at both poles. The onset of this icehouse started in Antarctica 34 ma and in the Arctic 2ma. At least 3 times during earths history, the planet has been in a 'deep freeze', when ice sheets extended from the poles to the tropics.

Icehouse


Icehouse events are characterised by lower temperatures, ice caps and glaciers. The ice sheets from the last period of glaciation are still present. The huge increase in ice coverage then increases the drop in in global temperatures by reflecting more of the suns radiation back into space.

Greenhouse


Greenhouse events are characterised by a lack of ice coverage and an overall increase in global temperatures. They can be caused by an increase in the amount of solar radiation reaching the Earth or a change in the concentration of gases in the atmosphere.

Extinction events


The extinction of species can be influenced by climate change. Most organisms thrive in a relatively limited range of conditions, and if the conditions in an area change then the species living there will alter. If the changes happen on a global scale then whole species or groups could be wiped out. An example of this is the mass extinction at the Permian-Triassic  boundary where 96% of marine life extinct and 70% of terrestrial life became extinct. This followed a major glaciation that affected Antarctica, Africa and South America when they all joined as Gondwanaland. It was similar to the current icehouse, with continental ice sheets in the southern hemisphere and low atmospheric carbon dioxide concentrations.

Irregular echinoids

Morphology


Irregular echinoids are characterized by having the anus outside of the apical system. The anus has moved to the edge of the test, or towards the posterior. This means that irregular echinoids are heart shaped and have bilateral symmetry. these adaptations are to allow the irregular echinoid to live in a burrow. Examples include the Cretaceous form, Micraster and the modern day sand dollar.

Mouth and adjacent area


The mouth is still on the underside of the test, but often it has move away from the centre. The mouth lacks jaws and the periphrastic girdle found in regular echinoids. Instead the animal takes in particles from sea water and filters these. There is a large lip called the labrum, projecting on the lower side of the mouth. The labrum is used to direct currents and prevent unwanted sediments getting into the mouth. Behind the labrum there is a modified set of interambulacral plates, forming the plastron. The plastron has small tubercles for attachment of spines. These small spines are used to help dig a burrow or for movement within it.

Petaloid ambulacra 


The ambulacra do not extend all the way down from the top to the mouth, but form a flower-shaped structure called the petaloid ambulacra. These have many small pore pairs for tube feet on top of the echinoid. The petaloid ambulacra at the anterior of the animal are larger than the others, and form the anterior groove. This is lined by cilia, which beat to generate currents to pass food particles to the mouth, and is called the fasciole. Very long tube feet extend from the anterior ambulacra, which are used to help dig the burrow and keep it stable

Mode of life


Irregular echinoids live in soft sediment and in a low-energy enviroment(infaunal). They do not have jaws and have a reduced sized mouth called the peristome because they filter feed. Instead they dig burrows, using the spines on the plastron.