Discussion of Phylogenetic Relationships
Tree modified from Kerr (2000).
The evolutionary relationships of the major holothuroid lineages were, until quite recently, poorly understood. This was in part due to their lack of an integrated skeleton like that providing the extensive fossil record and numerous morphological characters of other groups of echinoderms. There have been numerous speculations about the relationships within Holothuroidea extending well back into the 19th century. The methods of modern comparative biology had not been applied to these problems until quite recently. Then Littlewood et al. (1997), in an effort to resolve class-level relationships within echinoderms, sequenced two ribosomal genes from a total of four orders. Their analyses consistently supported a close relationship between Dendrochirotida and Aspidochirotida, but they could not resolve the phylogenetic position of Elasipodida and Apodida (Figure 3: A, B). Smith (1997) subsequently argued that the Elasipodida are more closely related to (Dendrochirotida + Aspidochirotida) than the Apodida (Figure 3: C). This hypothesis recalls an early speculation (Semper 1868) whereby Apodida is sister to the remaining holothuroids.
Figure 3. Recent hypotheses about holothuroid relationships.
A. Tree based on complete 18S rDNA sequences (Littlewood et al., 1997).
B. Tree based on partial 28S rDNA sequences (Littlewood et al., 1997).
C. Interpretation of the 18S and 28S rDNA data favored by Smith (1997).
More comprehensive cladistic analyses of morphological and DNA data (Kerr, 2000) agree with Smith's hypothesis. Further, it appears that Dendrochirotida is paraphyletic due to the Dactylochirotida lineage arising from within the Dendrochirotida. This arrangement of the two orders is so far supported by few characters, and an alternate arrangement may ultimately prevail. Kerr (2000) also places Molpadiida as sister to Dendrochirotida plus Dactylochirotida. Together with Aspidochirotida, the aforementioned orders form a group united, most notably, by the presence of respiratory trees. The placement of two rare families currently referred to the Molpadiida, Eupyrgidae and Gephyrothuriidae, is uncertain; they may turn out to be only distantly related to one another and other ordinal level groups of holothurians. Based on the presence of respiratory trees, however, they are unlikely to be closely related to either the Apodida or Elasipodida, which lack such structures. The remaining features of the higher level relationships shown in the figure at the top of this page, though, appear solidly supported and unlikely to change with the consideration of new characters.
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