Ecdysozoa () is a group of protostome animals,
A large study in 2008 by Dunn et al. strongly supported the monophyly of Ecdysozoa.
The group Ecdysozoa is supported by many morphological characters, including growth by ecdysis, with moulting of the cuticle – without mitosis in the epidermis – under control of the prohormone ecdysone, and internal fertilization.[ ]
The group was initially contested by a significant minority of Biologist. Some argued for groupings based on more traditional taxonomic techniques,
Etymology
The name
Ecdysozoa is scientific Greek, derived from (
) "shedding" + () "animal".
Characteristics
The most notable characteristic shared by ecdysozoans is a three-layered
cuticle (four in
tardigrade) composed of organic material, which is periodically molted as the animal grows. This process of molting is called
ecdysis, and gives the group its name. The ecdysozoans lack locomotory
Cilium and produce mostly amoeboid sperm, and their
do not undergo
spiral cleavage as in most other protostomes. Ancestrally, the group exhibited sclerotized teeth within the foregut, and a ring of spines around the mouth opening, though these features have been secondarily lost in certain groups.
An unpaired ventral nerve cord, present in Priapulida and Nematoida, appear to be the ancestral condition, making the paired ventral nerve cord found in Panarthropoda, Kinorhyncha and Loricifera a derived trait.
[ Preservation and early evolution of scalidophoran ventral nerve cord] A respiratory and circulatory system is only present in
and arthropods (often absent in smaller arthropods like mites); in the rest of the groups, both systems are missing.
Phylogeny
The Ecdysozoa include the following phyla:
Arthropoda,
Onychophora,
Tardigrada,
Kinorhyncha,
Priapulida,
Loricifera,
Nematoda, and
Nematomorpha. A few extinct taxa have been classified as
stem group ecdysozoans, such as
Uncus dzaugisi and
Acosmia. Other groups such as the
, have been considered possible members but lack the main characters of the group, and are now placed elsewhere. The Arthropoda, Onychophora, and Tardigrada have been grouped together as the
Panarthropoda because they are distinguished by segmented body plans.
Dunn
et al. in 2008 suggested that the tardigrada could be grouped along with the nematodes, leaving Onychophora as the sister group to the arthropods.
The non-panarthropod members of Ecdysozoa have been grouped as
Cycloneuralia but they are more usually considered
paraphyly in representing the primitive condition from which the Panarthropoda evolved.
A modern consensus phylogenetic tree for the protostomes is shown below.
The phylogenetic tree is based on Nielsen et al.
Older alternative groupings
Articulata hypothesis
The grouping proposed by Aguinaldo
et al. is almost universally accepted, replacing an older hypothesis that
Panarthropoda should be classified with
Annelida in a group called the Articulata, and that Ecdysozoa are
polyphyletic. Nielsen has suggested that a possible solution is to regard Ecdysozoa as a sister-group of
Annelida,
though later considered them unrelated.
Inclusion of the
within the Ecdysozoa was initially contested
[ — in support of clade Ecdysozoa]
Coelomata hypothesis
Before Aguinaldo's Ecdysozoa proposal, one of the prevailing theories for the evolution of the Bilateria was based on the morphology of their body cavity. There were three types, or grades of organization: the Acoelomata (no coelom), the Pseudocoelomata (partial coelom), and the Eucoelomata (true coelom). Adoutte and coworkers were among the first to strongly support the Ecdysozoa.
Further reading
External links
