Carpoids (aka stylophorans or
The carpoids, the 4th group of putative fossil chordates, are
organisms with a calcitic (calcium carbonate) outer covering of a
particular type, the mineral being pierced by many small holes, the
creature consisting of 2 parts, a body portion that was compact, and a
long appendage that was segmented. The carpoids have been interpreted as
aberrant echinoderms by most authors, but have been proposed by one
author (Jefferies, 1986, 1997) that they are a mixed group of basal
echinoderms, cephalochordates, sea squirts, and vertebrates. According
to Benton, there are 4 criticisms of Jefferies proposal.
- There is agreement between morphological and molecular
phylogenetic analyses that doesn't correspond to the 'calcichordate'
- Anatomical structures, the function of which is still being
debated, comprise much of the basis for the 'calcichordate'
hypothesis. An example is the interpretation by Jefferies (1986,
1997) of the appendage on carpoids as a tail, his critics calling it
a locomotory stem or feeding arm. Jefferies calls a major body
opening a mouth, while other researchers call it an anus, and a
series of openings is interpreted by Jefferies as pharyngeal gill
slits, others calling them inhalant respiratory pores.
- There are also a number of major character losses that are
included in the theory, and according to the theory, the carpoid and
echinoderm calcite skeleton was apparently lost in 3 lines, along
the lines leading to the cephalochordates, tunicates and
vertebrates. Benton suggests the calcite skeleton of carpoids and
echinoderms evolved once and was subsequently retained, this being
the most economical assumption.
- The carpoids have been determined to be a monophyletic group (Ruta,
1999), that share a number of characters, such as the flattened
'bag-like' shape, as well as the appendage, many details of the
plates covering the body, and various openings. If the group is
monophyletic, as is debated, Benton suggests it cannot be
distributed in different places throughout deuterostomes phylogeny.
He suggests that it is potentially interesting that the presence of
gill slits that have been postulated in carpoids (Jefferies, 1986,
1997), as they could suggest a deuterostome character that was
subsequently lost in echinoderms, saying that if we are not
descended from carpoids, what are we descended from?
Sources & Further
- Benton, Michael J., 2005, Vertebrate Palaeontology, 3
rd ed., Blackwell Publishing.