Palæos:		Arthropoda
INVERTEBRATES		Uniramia

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Uniramia

One of the three main phyla proposed by S. M. Manton as part of a paradigm of Arthropod polyphyly, the Uniramia ("single branch") are identified by their single pair of preoral antennae, possibly homologous with the antennae of trilobites. The body divided into a distinct cephalon (head) and metameric (segmented) trunk. The limbs are unbranched, hence the name of the group. There are three pairs of postoral appendages on head; chewing mouthparts (mandibles) on the second postantennal segment, and compound eyes.  They seemed to have evolved a hard exoskeleton later than other groups, but were quick to colonize the land (during the Silurian period).  Breathing is by special gas exchange organs along the side of the body called tracheae and malpighian tubules.

There are three taxa conventionally included: the Onychophora (velvet worms, which do not posses an exoskeleton, and were previously considered a distinct phylum), and the superclasses Myriapoda and Hexapoda.  These would seem to represent an evolutionary sequence, from many-legged soft-bodied forms through many-legged hard-bodied types to few legged forms with fused segments (insects).  The poorly known Cambropodus may also be an early uniramian, but this is not certain.

Recently however the status of the Uniramia as a valid taxon has been increasingly questioned and evidence presented suggesting the Crustacea are the stock from which the Hexapoda arose (i.e. Hexapods and Crustacea form a monophyletic group apart from the Myriapoda) - the Pancrustacea hypothesis, although this is still controversial. Equally if not more damaging is fossil research that shows that transitional lobopod-arthropod-like forms reveal a continuity between lobopods proper and Euarthropods, thus supporting the monophyly of the Arthropoda and invalidating the Uniramia. A limited version of the Uniramia, the Atelocerata, includes only euarthropodic Uniramia, but this would also be invalidated if the Pancrustacea theory turns out to be correct.

Uniramia Phylogeny

<==o UNIRAMIA
   `-o LOBOPODA  
     |-- ONYCHOPHORA
     `--+?- †Cambropodus
        | ?-o †EUTHYCARCINOIDEA 
        |  `--†Euthycarcinida   
        |--o MYRIAPODA
        |  |-- CHILOPODA 
        |  `--o PROGONEATA
        |     |-SYMPHYLA
        |     `--+- PAUROPODA 
        |        | ?- †ARTHROPLEURIDA
        |        | ?- †ARCHIPOLYPODA
        |        `-- DIPLOPODA
        `--o HEXAPODA  
           |--o DIPLURA
           |--+--o PROTURA  
           |  `-- COLLEMBOLA ( springtails)  
           `--INSECTA (insects)

The Uniramia family tree. Cambropodus gracilis is tentatively included between lobopods and higher Uniramia. The Euthycarcinoidea would seem to be safely uniramian but their exact relationships are not clear.

 

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