Bilaterally symmetrical animals
CLASS: Synapsids
As the tree of biological classification expands there becomes confusion is the status of groupings. For example, the class of synapsida has evolved from what is considered to be a superorder of reptiliomorphs. Although classes are ranked higher than orders in biological classification.
This is due to the fact that what were originally single genus when they evolved millions of years ago have since evolved into a vast amount of animals, thus making it unavoidable to promote its classification.
The line in which this website is following is not a line of each animal's biological classification so much as its evolutionary history.
With this in mind we can describe synapsida as an amniote which first evolved around 320 million years ago, and have since evolved to become to vast array of mammals which have ever existed.
Original synapsids are recognised by the presence of an opening in the skull further back in the skull than the eye orbit.
Synapsids developed different styles of tooth among their full set of teeth. Notably the incisor, canine and molar teeth which we can recognise in mammals today.
The complexity of amniotes jawbone structure started showing signs of alteration in synapsids which would eventually in mammals become one solid jawbone. The mouth and nasal cavity also began to separate significantly from each other.
It is supposed that skin alterations where also a feature during the evolution of synapsids, as the more typical scaly skin of reptiles slowly altered into leathery mammalian style skin.
The evolution of synapsids is also the supposed start of this transition from the cold blood of reptiles into the warm blood of mammals.
Pre-mammalian and non-mammalian orders of synapsids include pelycosauria and therapsida, and it is thought that there are mammals that have evolved from these orders.
Pelycosaurs became the dominant land vertebrates for many millions of years. Some clades grew elongated vertebral spines for the purpose of body temperature regulation and possibly as a mating attraction.
Therapsids initially were dominated by the pelycosaurs, but over time became the new dominant land vertebrate. Their legs developed in a more vertical fashion beneath their bodies and they developed powerful and effective teeth.
The evolution of the synapsids (the ancestors of mammals) and sauropsids (which later become dinosaurs) together was an extensive and complicated one as they had to overcome the Great Dying of 250 million years ago where there was a mass extinction of many of the animals alive at that time. Terrestrial tetrapods, marine animals and insects all suffer a major extinction percentage which all undoubtedly impacted of the ecosystems. In other words, as particular animal groups perished, this would have had a knock on affect on other animal group on which they were dependent for survival. Subsequently, the evolution of all the world's animals was hugely delayed.
Until the Great Dying, synapsids were the dominant land vertebrates. However, the sauropsids became more dominant in the aftermath.
To enable synopsids to survive the sauripsids' dominance of the land, they took advantage of their evolved metabolism which enabled them to become nocturnal due to having the ability to stay warm at night.
It is through this adaptation that synapsids began their evolution into mammals.
Features of true mammals include advancement in brain development, skin hair and sweat glands. Mammals also developed mammary gland, but this is only present in the female.
Humans are mammals.
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PELYCOSAURIA Synapsids with outstretched limbs and some with vertebral spines.. |
THERAPSIDA Synapsids with more vertical limbs and powerful jaws with effective teeth. |
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Vertebrates with hair, sweat glands and advanced brain form. |
