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F. Marlétaz and Y. Le Parco
evolutionary relationships between organisms could be inferred from the similarity
of their earliest developmental stages. As a result, embryological characters were
given prominent importance (Valentine 1997). Most historical hypotheses about
animal relationships have thus been based on assumptions about the structure and
formation of the coelomic cavities.
The so-called “traditional textbook” phylogeny, which is based to a large
degree on the work of Libbie Hyman, mainly relied on the proposition that a
coelom arose independently in protostomes and deuterostomes (Hyman 1940–
1967). Accordingly, distinct mechanisms of coelom formation, schizocoely and
enterocoely, would have respectively arisen from a triproblastic acoelomate ancestor of bilaterians (see Willmer 1990). This led some to consider that the acoelomate
(platyhelminthes and nemertes) and pseudocoelomate (e.g. nematodes and rotifers)
lineages constituted early offshoots during metazoans evolution (Fig. 4.2) (Barnes
1974). But such schemes should be distinguished from the original thinking of
Libbie Hyman whose gradist approach was focused exclusively on the level of
complexity in body plans and was not necessarily aimed at precisely deciphering
evolutionary radiations (Jenner 2004).
However, a divergent interpretation of coelom evolution could lead to radically opposite views about metazoan relationships. For instance, the archecoelomate
hypothesis, commonly adopted by German workers, was built upon the assumption
that the bilaterian ancestor possessed a trimeric coelom developed by enterocoely.
These multiple coelomic cavities would have been derived from the gastric pouches
of an anthozoan-like ancestor (Remane 1963). The lineages that develop their
coeloms through outpocketing of the archenteron such as hemichordates, chordates,
lophophorates and chaetognaths were thus regarded as having diverged early among
bilaterians (Siewing 1976) (Fig. 4.2). The segmented bodyplans encountered in
annelids and arthropods would have subsequently originated through the segmentation of the posterior coelomic cavity after the loss of the anterior cavities (Tautz
2004). The body plans of acoelomates and pseudocoelomates were considered to
represent various stages of coelomic degeneration from the ancestral type (Siewing
1976).
Alternative schemes have been proposed with respect to these examples and
numerous refinements of each of these schemes have been formulated (Willmer
1990) but those cases perfectly illustrate the problem of character orientation that
underlies the historical discussion of animal phylogeny.
4.2.2 Sorting More Characters Through a Cladistic Approach
Numerous developmental and morphological characters have been considered in
the attempt to reconstruct metazoan relationships but they only contribute information about specific nodes of the tree (for an extensive review, see Willmer 1990).
It has been argued, for example, that Bilaterians acquired a third germ layer (the
mesoderm) and bilateral symmetry based on the two germ layers and radial symmetry described in basal metazoans: cnidarians, poriferans and ctenophores. But
F. Marlétaz and Y. Le Parco
evolutionary relationships between organisms could be inferred from the similarity
of their earliest developmental stages. As a result, embryological characters were
given prominent importance (Valentine 1997). Most historical hypotheses about
animal relationships have thus been based on assumptions about the structure and
formation of the coelomic cavities.
The so-called “traditional textbook” phylogeny, which is based to a large
degree on the work of Libbie Hyman, mainly relied on the proposition that a
coelom arose independently in protostomes and deuterostomes (Hyman 1940–
1967). Accordingly, distinct mechanisms of coelom formation, schizocoely and
enterocoely, would have respectively arisen from a triproblastic acoelomate ancestor of bilaterians (see Willmer 1990). This led some to consider that the acoelomate
(platyhelminthes and nemertes) and pseudocoelomate (e.g. nematodes and rotifers)
lineages constituted early offshoots during metazoans evolution (Fig. 4.2) (Barnes
1974). But such schemes should be distinguished from the original thinking of
Libbie Hyman whose gradist approach was focused exclusively on the level of
complexity in body plans and was not necessarily aimed at precisely deciphering
evolutionary radiations (Jenner 2004).
However, a divergent interpretation of coelom evolution could lead to radically opposite views about metazoan relationships. For instance, the archecoelomate
hypothesis, commonly adopted by German workers, was built upon the assumption
that the bilaterian ancestor possessed a trimeric coelom developed by enterocoely.
These multiple coelomic cavities would have been derived from the gastric pouches
of an anthozoan-like ancestor (Remane 1963). The lineages that develop their
coeloms through outpocketing of the archenteron such as hemichordates, chordates,
lophophorates and chaetognaths were thus regarded as having diverged early among
bilaterians (Siewing 1976) (Fig. 4.2). The segmented bodyplans encountered in
annelids and arthropods would have subsequently originated through the segmentation of the posterior coelomic cavity after the loss of the anterior cavities (Tautz
2004). The body plans of acoelomates and pseudocoelomates were considered to
represent various stages of coelomic degeneration from the ancestral type (Siewing
1976).
Alternative schemes have been proposed with respect to these examples and
numerous refinements of each of these schemes have been formulated (Willmer
1990) but those cases perfectly illustrate the problem of character orientation that
underlies the historical discussion of animal phylogeny.
4.2.2 Sorting More Characters Through a Cladistic Approach
Numerous developmental and morphological characters have been considered in
the attempt to reconstruct metazoan relationships but they only contribute information about specific nodes of the tree (for an extensive review, see Willmer 1990).
It has been argued, for example, that Bilaterians acquired a third germ layer (the
mesoderm) and bilateral symmetry based on the two germ layers and radial symmetry described in basal metazoans: cnidarians, poriferans and ctenophores. But