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Nature:伯吉斯页岩中的囊舌虫

“囊舌虫”是生活在泥土中的生物,其生活范围从前滩延伸到深海。它们与“羽鳃类”动物相关,后者是小型的、群居的、在管中生活的动物,从表面上看完全不同。二者都与棘皮动物(海星及同类)和脊索动物(包括我们人类自己在内的动物类群)有关。

Jean-Bernard Caron及其同事在这篇文章中介绍了来自加拿大卑诗省著名“寒武纪伯吉斯页岩”的“囊舌虫”化石。它们看起来非常像一些现代“囊舌虫”,但生活在管子中,更像现代“羽鳃类”。

这一发现增加了化石动物群落的多样性,也为“囊舌虫”和“羽鳃类”的共同祖先是什么样子的提供了线索。它也许还能推动有关脊索动物的祖先是一种自由生活的蠕虫还是一种固定、群居生活的动物的辩论。

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Tubicolous enteropneusts from the Cambrian period

Nature 28 March 2013 | doi: 10.1038/nature12017

Hemichordates are a marine group that, apart from one monospecific pelagic larval form, are represented by the vermiform enteropneusts and minute colonial tube-dwelling pterobranchs. Together with echinoderms, they comprise the clade Ambulacraria. Despite their restricted diversity, hemichordates provide important insights into early deuterostome evolution, notably because of their pharyngeal gill slits. Hemichordate phylogeny has long remained problematic, not least because the nature of any transitional form that might serve to link the anatomically disparate enteropneusts and pterobranchs is conjectural. Hence, inter-relationships have also remained controversial. For example, pterobranchs have sometimes been compared to ancestral echinoderms1. Molecular data identify enteropneusts as paraphyletic, and harrimaniids as the sister group of pterobranchs. Recent molecular phylogenies suggest that enteropneusts are probably basal within hemichordates, contrary to previous views, but otherwise provide little guidance as to the nature of the primitive hemichordate. In addition, the hemichordate fossil record is almost entirely restricted to peridermal skeletons of pterobranchs, notably graptolites. Owing to their low preservational potentials, fossil enteropneusts are exceedingly rare, and throw no light on either hemichordate phylogeny or the proposed harrimaniid–pterobranch transition. Here we describe an enteropneust, Spartobranchus tenuis (Walcott, 1911), from the Middle Cambrian-period (Series 3, Stage 5) Burgess Shale. It is remarkably similar to the extant harrimaniids, but differs from all known enteropneusts in that it is associated with a fibrous tube that is sometimes branched. We suggest that this is the precursor of the pterobranch periderm, and supports the hypothesis that pterobranchs are miniaturized and derived from an enteropneust-like worm5, 6. It also shows that the periderm was acquired before size reduction and acquisition of feeding tentacles, and that coloniality emerged through aggregation of individuals, perhaps similar to the Cambrian rhabdopleurid Fasciculitubus11. The presence of both enteropneusts and pterobranchs in Middle Cambrian strata, suggests that hemichordates originated at the onset of the Cambrian explosion.

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