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 When threatened, the whole body can be withdrawn into its shell.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.