The fourth group of putative fossil chordates is much more diverse. The carpoids, sometimes called sty-lophorans or calcichordates, are a group of about 60 species of asymmetrical organisms that had a calcitic (calcium carbonate) outer skeleton of a particular kind in which the mineral is pierced by numerous small holes. They date from the Middle Cambrian to Middle Devonian (520-370 Myr). They consist of two parts (Figure 1.8), a compact body portion and a long segmented appendage. Most authors have interpreted the carpoids as aberrant echinoderms, but Jefferies (1986, 1997) argued strongly that they are a mix ofbasal echin-oderms, cephalochordates, sea squirts, and vertebrates. There are four criticisms of Jefferies' (1986,1997) 'calci-chordate' hypothesis.

1 Morphological and molecular phylogenetic analyses agree on a tree of living forms (Box 1.1) that does not

Fig. 1.8 The carpoid Mitrocystites mitrafrom the Mid-Ordovician of Bohemia (Czech Republic), dorsal view, showing the calcite plates that compose the body and the flexible 'arm' folded over at the top. Scale, specimen is about 30 mm long. (Courtesy of Bernard Lefebvre.)

correspond to the requirements of the 'calcichordate' hypothesis.

2 Much of the 'calcichordate' hypothesis depends on interpretations of anatomical structures that are disputed. For example, Jefferies (1986,1997) interprets the carpoid appendage as a tail, whereas his critics call it a locomotory stem or feeding arm. A major opening in the body is called a mouth by Jefferies, and an anus by others. He interprets a series of openings as pharyngeal gill slits, whereas others call them inhalant respiratory pores.

3 The theory also involves a number of major character losses. The calcite skeleton of the carpoids and echino-derms was apparently lost three times, on the lines to the cephalochordates, tunicates, and vertebrates. It is more parsimonious (economical) to assume that the calcite skeleton of echinoderms (and carpoids) evolved once and was not lost.

4 The carpoids have been determined as a mono-phyletic group (Ruta, 1999), sharing the flattened 'baglike' shape, the appendage, and numerous details ofthe plates that cover the body, and the various openings. If the group is monophyletic, and that is debated, it cannot be distributed in different places all through the phylogeny ofdeuterostomes.

The postulated presence of gill slits in carpoids (Jefferies, 1986, 1997) is potentially interesting, as these could then be seen as a deuterostome character that was subsequently lost in the echinoderms. If then we are not descended from carpoids, where did the chordates come from?

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