Palaeontology

Echinoderm roots

A bold claim about the origins of the echinoderms is based on newly discovered fossils from China. But many pieces are still missing from this part of the fragmented puzzle of life's evolutionary history.

Few marine animals are so immediately recognizable as echinoderms. The five-fold symmetry of a starfish or sea urchin is striking (Fig. 1), and this pentaradiate form sets them apart from their bilaterally symmetrical relatives. The echinoderm skeleton is equally distinctive, being made of calcite plates with a microstructure that resembles a very holey Swiss cheese. Finally there is their bizarre asymmetrical transformation from larva to adult, which involves loss of the right-hand set of paired larval body chambers. But echinoderms have not always possessed these features, and unravelling their early history remains highly controversial.

Figure 1
figure1

A. B. SMITH

Sea-urchin symmetry: an example of the unique pentaradiate form of echinoderms.

On page 422 of this issue Shu et al.1 describe a new group of small fossils from the Lower Cambrian of Chengjiang, China. The fossils are some 520 million years old: Shu et al. call them vetulocystids, and interpret them as the most primitive echinoderms yet known. If correct, this links the echinoderms to an enigmatic group, the vetulicolians, remains of which are found in the same deposits of early Cambrian age.

Fossils can be made sense of only by comparison with living organisms, where the biological function of structures that become preserved can be directly observed. This is relatively easy when the fossil is rather close in structure to its extant relatives. But fossils such as the vetulocystids, whose affinity with living groups is not immediately apparent, pose a major difficulty. In the past they would have simply been hived off into their own higher taxonomic group, thereby avoiding the problem. Nowadays, palaeontologists take the harder path and strive to place such fossils into their appropriate branch in the tree of life, interpreting characters they display in the most plausible (or least implausible) way.

The vetulocystids are a fascinating but frustrating group — fascinating, because of what they may tell us about the morphology of echinoderms before they had evolved such distinctive morphology, and frustrating because of the difficulty of interpreting even their basic anatomical organization. In this they are not alone. Another group of primitive and entirely extinct echinoderms, the homalozoans, have been the source of debate amongst palaeontologists for years.

Echinoderms belong to the branch of the animal kingdom known as the deuterostomes, a group that also includes ourselves. This is a very diverse assemblage of organisms, the other members being the vertebrates (fishes and tetrapods), urochordates (tunicates and sea squirts) and hemichordates (pterobranchs and acorn worms) (Fig. 2). Molecular data strongly support this grouping, placing vertebrates and urochordates together and echinoderms and hemichordates as a second pairing2. But in terms of morphology echinoderms have always stood apart because of their aberrant symmetry and lack of structures known as gill slits. Gill slits are present in hemichordates, urochordates and the more primitive vertebrates, and are openings that pierce the wall of the digestive system just behind the mouth. They appear to have evolved for venting excess water drawn into the gut during feeding and are unique to deuterostomes.

Figure 2: The main deuterostome groups.
figure2

Shu et al.1 interpret their vetulocystid fossils as an offshoot of the echinoderm lineage that branched off before the division between the now-extinct homalozoans and the radiate echinoderms. According to this view, the vetulocystids have affinities with another mysterious group, the vetulicolians, which may have diverged from the deuterostome line before the division between the chordates and ambulacraria.

Homalozoans are important fossils because they help bridge the gap between radiate echinoderms and other deuterostomes. They have the characteristic skeleton of an echinoderm and a strongly asymmetrical development. But homalozoans never display even a hint of radiate symmetry and, more importantly, they have gill slits and a post-anal muscular appendage. These last two traits are probably primitive features common to all deuterostomes. So do the vetulocystids take us even further back towards the root of echinoderms?

Faced with such fossils, the first step is to define front and back, and identify the major body openings — mouth and anus, gill slits, and the pores involved in water circulation or reproduction (gonopores) are all possibilities that must be considered. Here the problems really start because fossils do not come ready labelled. Pictures of the fossils themselves, and the interpretations of Shu et al., appear in Figs 1–3 of their paper (pages 423–425).

In the best-preserved vetulocystid material there is evidence for a straight gut running to the posterior along a possibly segmented tail-like structure, as in vetulicolians3, and this presumably led to a terminal anus. The sack-like anterior part, or theca, has two large and more or less identical openings, both circular and taking the form of a low pleated cone. The anterior of the two circular cones is taken to be the mouth and the posterior may be the anus or gonopore. Towards the base of the theca there is a rhomboidal structure, which Shu et al. interpret as respiratory in function and possibly a gill. The identification of the vetulocystids as a ‘basal’ echinoderm group hinges on this interpretation, with the presence of a single rather than paired ‘gill’ implying echinoderm-like developmental asymmetry. There is no calcite plating.

But other interpretations are possible. The two circular structures could be gill openings (they are in a similar position in vetulicolians3), the mouth anterior and forming the elongate recessed zone, and the rhomboidal structure a folded epithelial zone for gaseous exchange (analogous to those of certain primitive stemmed echinoderms) rather than a gill. That would make the echinoderm affinities of vetulocystids much more dubious. Vetulocystids and vetulicolians share many similarities and are clearly closely related, but quite where they fit within the deuterostomes remains ambiguous because of these alternative possibilities. Such is the way with fossils.

There is now direct fossil evidence that all of the major deuterostome groups were established by about 520 million years ago. Fossil vertebrates (yunnanozoans4), tunicates (Shankouclava5) and both asymmetric and radiate echinoderms (homalozoans, helicoplacoids) have all now been discovered in early Cambrian deposits. Phlogites, a tentacle-bearing early Cambrian fossil of uncertain affinity5, might even be a hemichordate or part of the common ancestral lineage of echinoderms and hemichordates. So, if deuterostome divergence occurred around 575 million years ago, as recent molecular-clock studies suggest6, there is a 50-million-year gap in the fossil record between the origin of deuterostomes and their appearance in the fossil record. In the jigsaw of deuterostome evolution, vetulocystids represent another piece to be fitted into a puzzle where many of the pieces are still missing.

References

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Smith, A. Echinoderm roots. Nature 430, 411–412 (2004). https://doi.org/10.1038/430411a

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