ORIGIN AND RADIATION 



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are shown in fig. 13. Fossils which look very like some of the 

 Leptostraca are known from the Devonian (about 300 million years 

 ago), and some which bear a less marked resemblance date from 

 even earlier periods. These fossils had a well developed carapace 

 just like the modern forms, and had a telson bearing paired rami. 

 The possession of these rami distinguishes the Leptostraca from all 

 other Malacostraca. The thoracic limbs are also very peculiar in 

 structure (fig. 22), quite different from those of other malacostracans, 

 yet they are thought to represent a modification of a typical 

 branched limb which has been altered by the animal adopting mud 

 burrowing habits while retaining a filter feeding mechanism (p. 31). 



Fig 13. An evolutionary tree of the Malacostraca. Based on the 

 ideas of Siewing (1956). 



Another group with early fossil relatives is the order Anaspidacea. 

 These are nowadays confined to Tasmania and south-eastern 

 x^ustralia, but fossils which agree in general body form and in 

 details of limb structure are known from the Coal Measures of 

 North America and Europe. They also have some more widespread 

 minute relatives, the Bathynellacea, which are only about a milli- 

 metre long and live in underground waters in various parts of the 

 world (p. 142). They agree with the anaspids in lacking a carapace, 

 and in not having a brood pouch under the thorax, as well as in 

 other more technical details. 



