Palaeos | ![]() |
Vetulicolia |
Deuterostomia | Vetulicolia |
Page Back | Unit Back | Unit Home | Unit References | Unit Dendrogram | Metazoa Glossary | Taxon Index |
Page Next | Unit Next | Metazoa | Metazoa References | Metazoa Dendrogram | Pieces | Time |
DEUTEROSTOMIA |--Vetulicolia | |--+--Xidazoon | | `--Didazoon | `--+--Pomatrum | `--Vetulicola `--+--CHORDATA `--Ambulacraria |--Hemichordata `--+--Vetulocystidae `--HOMALOZOA (grade) `--ECHINODERMATA |
In
2001, Shu et
al. suggested that a small
group of Cambrian animals which they named Vetulicolia represented the stem
group of modern deuterostomes. Vetulicolia possess a distinctly bipartite body
plan with a voluminous anterior section bearing five pairs of lateral openings
(interpreted as gills) and a seven-segmented “tail”. The gut runs to the
posterior end of the tail. The junction between the two sections is constricted
in the genera Xidazoon and Didazoon,
which have an oval-shaped anterior bearing a large mouth with a strengthened
rim, and a leaf-shaped tail. The genus Vetulicola
has a more rectangular anterior with a “carapace” of four rigid
cuticular plates. The posterior section is slender, strongly cuticularised and
placed dorsally. The whole animal bears a superficial resemblance to a
phyllocarid crustacean which lead to Vetulicola
being originally regarded as an arthropod. However, the complete lack of
appendages, the gill openings and the four-plated nature of the carapace are
distinct from all known arthropods. Finally, the genus Pomatrum
has an anterior like Didazoon and Xidazoon,
but a slender tail like Vetulicola.
The
position of vetulicolians relative to other deuterostomes is still contentious.
Shu et al. did not identify anything resembling a notochord, which may
indicate a position outside the deuterostome crown. However, they identified a
dark ventral structure in the anterior part of Xidazoon and Didazoon as a
possible endostyle, a structure currently known only from chordates.
Vetulicolians bear a superficial resemblance to a tunicate “tadpole” larva,
and also to the divided “somato-visceral” animal hypothesized by Romer as
the ancestor of vertebrates. Shu et al. (2001). Lacalli (2002) suggested that the vetulicolian cuticle was
homologous to the urochordate tunic.
Other fossil animals may be assignable to the Vetulicolia or their near
relatives. The worm-like Banffia resembles
Vetulicolia in its bipartite body, as does the recently described Skeemella (Briggs et
al., 2005), both from the Cambrian. Both these genera, however, have
considerably more segments in the posterior section (43 in Skeemella) and no sign of gill openings. Briggs et
al. argued on this basis that the assignment of vetulicolians to
deuterostomes was probably unjustified. It is possible, of course, that these
two genera are not related to Vetulicolia, or that they are further down on the
deuterostome stem and predate the origin of gill openings.
When Xidazoon was first described as an unplaced problematicum (Shu et al.,
1999) it was
compared on the basis of the rim of plates around the mouth to the
“agnathan” Pipiscius from the
Carboniferous Mazon Creek of Illinois. Other features such as possession of gill
rays would retain Pipiscius in the
chordates.
Finally, the Burgess Shale problematicum
Nectocaris resembles Vetulicola in its
possession of an arthropod-like carapace
with a
chordate-like tail and no
appendages.
The Yunnanozoa are another group of Cambrian animals known from the
Chengjiang Lagerstatte of China. Two genera are known, Yunnanozoon
and Haikouella. Like
vetulicolians, they have a bipartite body plan with a gill-bearing (six pairs)
anterior and a segmented posterior. Unlike vetulicolians, the segmented
section overlays the slender canoe-shaped anterior to a significant degree. The
two sections are connected by a median zone which apparently had a thinner
cuticle, probably allowing the anterior region to expand and contract in height.
The gills were external and an outwardly- and downwardly-directed “skirt”
surrounded the anterior section. Yunnanozoans have generally been regarded as
chordates, and Dzik (1995) identified structures on the head of Yunnanozoon with a notochord running ventrally to the “muscle
blocks” (the posterior segments). These structures were not identified in Haikouella
by Shu et al. (2003),
and Dzik's “eyes” probably correspond to the front of the skirt around the
anterior. Other authors identify both a dorsal and a ventral nerve cord, which
might support a position closer to hemichordates.
Images:
Vetulicola from 岐阜大学教育学部
理科教育講座地学教室
(Gifu University Department of Education Science Education. Geography
Classroom). Three
vetulicolians from the Internet Museum of
Natural History. Vetulicola gill structures from Shu
et al. (2001).
Page Back | Page Top | Unit Home | Page Next |