Crown and stem Groups – getting our basics right
Classification of Fossils is sometimes tricky as they do not follow systematics of living organisms and here come stem groups to the rescue.In order to know about stem groups we need to understand Crown Group also.
Stem groups:
Stem group organisms always lack one or more features that are present at the base of the crown group to which they are attached.
Crown Group:
A crown group is a living clade or monophyletic group which consists of the last common ancestor of all living examples and all of its descendants.But one problem comes when one try to accommodate fossils,like Archaeopteryx which, resembles clearly like a bird but it is not a descendent from last common ancestor of all living birds we find today on earth.But its origin can be traced to even more basal level and Such organisms can be referred to the stem group of a clade.The term Stem Group was introduced by the palaeontologist Dick Jefferies.So in the above example Archaeopteryx is a stem-group bird.By definition all Birds are in Crown Groups and all members of the stem-group of a clade are extinct; and thus, stem groups only have fossil members.
Archaeopteryx:
Archaeopteryx is a famous example of a transitional fossil which lived in late Jurassic Period around 155–150 million years ago.Archaeopteryx could grow to about 0.5 metres (1.6 ft) in length and Despite its small size, broad wings, and ability to fly(debated), Archaeopteryx share more common features with small theropod dinosaurs than it does with modern birds.There have been 9 specimens of Archaeopteryx found .
Similarity with Dinosaurs:
jaws with sharp teeth, three fingers with claws, a long bony tail, hyperextensible second toes (”killing claw”), feathers (which also suggest homeothermy), and various skeletal features.
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