Reptiles – The First Amniotes

REPTILES The First Amniotes

EVOLUTIONARY PERSPECTIVE (reptos, to creep)

Reptiles were the first vertebrates that possess amniotic eggs. Amniotic eggs have extraembroynic membranes. These membranes performs the following functions:

1. They Protect the embryo from dessication.

2. These support the embryo like cushion.

3. These membranes promote gas transfer.

4.  These membranes remove the waste materials.

The amnioitic eggs of reptiles and birds also have following parts:

1. Shells: Birds and reptiles have hard or leathery shell. This shell protects the developing embryo.

2. Albumin: The albumen cushions the embryo. It also provides moisture and nutrients to the embno.

3. Yolk: The yolk supplies food to the embryo.

Reptiles egg

All these features are adaptations for development on land. The amniotic egg is the major synapomorphy (distinguishing characteristic). It distinguishes the reptiles, birds, and mammals from nonaminote vertebrates. The amniotic egg has played an important role in success of vertebrate in terrestrial habitats. Thus the members of this class flourished on land. Living reptiles are turtles. lizards, snakes, worm lizards. crocodilians and the tuatara.

1a

Adaptive radiations in reptiles

Fossil records of mans reptiles are abundant. But much is not known about the origin of reptilian. The ancestral amniote has not et been discovered.     The adaptive radiation of the early amniotes began in the late Carboniferous and early Permian periods. The adaptive radiation of insects also occurred at the same time. Insect were the major prey of early amniotes. There are many lineages of reptiles. These lineages can he distinguished with the help of skull structure and modifications in jaw, muscle attachment.

1. Sub Class Anapsida (an, without + hapsis, arch): These reptiles lack openings or fenestrae in the temporal region of the skull. The turtles represent this lineage today.

Recent evidence suggests that the ancipsid lineage does not have close evolutionary ties to other reptiles. Changes have occurred in their long evolutionary history. The fundamental skull and shell is found in 200-million- year-old fossils. Evidence of the anapsid lineage  was found in 245-million-year-old rocks from south Africa.


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1b

 2. Diapsida (Or. di, two): A second group of reptiles are diapsid. They have upper and lower openings in the temporal region of the skull. Some taxonomists believe that this condition has a single lineage. Some divide this group into  two subclasses.

(a) Subclass Lepidosauria: It includes modern snakes, lizards and tuataras.

(b) Subclass Archosauria: Extensive evolutionary radiation occurred in it in the Mesozoic era. It includes the dinosaurs.  Most archosaurs are now extinct. Living archosours are crocodilians and birds. Birds are closely related to dinosaurs.

3. Synapsids (syn, with): They possess a single dorsal opening in the temporal region the skull. All of the synapsids have become extinct. But the are most important to evolutionary point of view. A group of synapsids is therapsids. It gave rise to the mammals.

CLADISTIC INTERPRETATION OF THE AMNIOTIC LINEAGE

Cladistic method

Cladistic taxonomic methods have reexamined and reinterpreted the amniotic lineage. Cladistics believes that the amniotic lineage is monophyletic. The birds and the mammals have a common ancestor with the reptiles. It is the rule of the cladistic analysis that animals with most recent common ancestor must be placed in a particular taxon. Birds and mammals have common ancestor with reptiles. But traditional classification does not include birds and mammals in class Reptilia. According to cladistic  interpretations, birds should be classified as reptiles” with dinosaurs. Similarly, cladistic interpretations also develop close relationship between mammals and ancient synapsid reptiles.

Evolutionary systematics

1.Evolutionary systematists disagree that these cladistic interpretation. These explain that the birds and mammals have important morphological, behavioral and ecological characteristics. For example, feathers and endothermy in the birds, hair,  mammary glands and endothermy in mammals. These characteristics suggest that birds and mammals should be placed in separate classes. Evolutionary systematists take them as important characters. They conclude that these characteristics have great importance in the taxonomy of these groups.

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