Slow news flash - Annelida not 'dead'
g.read at niwa.cri.nz
Wed Apr 2 00:43:29 EST 1997
Eibye-Jacobsen,Danny; Nielsen,Claus (1997 (1996)): Point of view. The
rearticulation of annelids. Zoologica Scripta. 25(3), 275-282.
Abstract it? Sorry, even the authors passed on that one. But it aims to
"discuss the profound influence the choice of input taxa has on the
results of a cladistic analysis, provide an alternative interpretation of
Rouse and Fauchald's data, and ultimately take issue with their conclusion
regarding the [non?] monophyly of Annelida."
Some of the Europeans closer to the action than New Zealand will know
when that issue appeared. It's just arrived here and although
it's a mid '96 dated issue, was seemingly delayed till '97.
While we're thinking of matters evolutionary:
Westheide,W (1997): The direction of evolution within the polychaeta.
Journal of Natural History. 31(1), 1-15.
"The phylogenetic systematics of the polychaetes, i.e. the nonclitellate
annelids, depend on which characters are regarded as belonging to the
bauplan of the annelid stem species. The main competition is between two
diametrically opposed hypotheses: the stem species was either (1) an
errant, epibenthic organism with well-developed prostomium and prostomial
appendages (antennae and palps), many homonomous segments, biramous and
well differentiated parapodia, and numerous well-structured chaetae, or (2)
a burrowing organism with small prostomium lacking appendages, which had
many homonomous segments without parapodia, and only a few simple chaetae.
(A third hypothesis, which is based primarily on certain morphological
peculiarities and the presence of exclusively monociliary cells in Owenia,
and which postulates a sessile stem species, is mentioned only
peripherally: for the present.) From a decision in favour of Hypothesis 2
it would follow that the Clitellata should be considered the most primitive
annelids, so that the possession of parapodia and many extremely
differentiated chaetae, for instance, would be interpreted as a highly
derived character state. The consequence for the phylogenetic systematics
of the Polychaeta is that oligochaete-like taxa would have to be considered
more primitive than, for example, nereidid-like taxa. On the basis of
Hypothesis 1, the evolution of these structures would have proceeded in the
opposite direction, and polychaete systematics would have the reverse
arrangement. The most important evidence for Hypothesis 2 comes from
functional morphological considerations; namely the inference that
metamerism has arisen from a burrowing mode of life. It is shown here that
(1) this hypothesis rests partly on ignorance of the close relationship
between reproductive biology and morphology in the clitellates, (2) the
notion that metamerism, and hence the stem species of the Articulata,
originated from a burrowing life in the marine environment is unconvincing,
and (3) the origin of metamerism can be explained quite differently with
reference to modern ultrastructural findings. According to these findings,
septa, which are the fundamental structural elements for annelid
segmentation, evolved as a morphological prerequisite for the development
of transversely running blood vessels; other purposes of septa (e.g.
subdivision of the hydrostatic skeleton), therefore, have to be regarded as
secondary. A highly complex blood vascular system may have been the
consequence of the development of lateral parapodia-like appendages. Thus,
parapodia are assumed to be part of the ground pattern of the Articulata
and hence were present in the stem species of the Annelida. This is
consistent with the traditional interpretation of annelid systematics,
which places the errant polychaete taxa at the base of the system
Geoff Read <g.read at niwa.cri.nz>
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