Three researchers, two from China, have come up with a strange new fossil, which they describe in the 14 November 1996 Nature. Degan Shu, X.-L. Zhang, and Simon Conway Morris name their fossil Cathaymyrus diadexus, the 'Chinese eel of good fortune'.
But Cathaymyrus is not the fossil of an eel. At just 5 cm long, but 535 m.y. old, it is the earliest known chordate. Dogs and dinosaurs, fish and frogs, cats and chickens are all chordates. So are some animals that one doesn't meet every day, such as sea-squirts [tunicates].
Cathaymyrus comes from the Chengjiang fossil fauna from Yunnan in southern China. This incredible collection of fossils offers a window on what the world was like in the 'Cambrian Explosion', when most modern kinds of animal, including chordates, made their first appearance, together with a lot of strange creatures that became extinct soon afterwards. The Chengjiang collection contains more than 10,000 fossils, but of Cathaymyrus there is but a single specimen. The researchers have given it the name diadexus (from the Greek word for good fortune) in the hope that more specimens will be found.
The Chengjiang fauna is similar to the celebrated Burgess Shale fauna of Canada, which is slightly younger (525 Ma). The Burgess Shale has yielded more than a hundred specimens of an eel-like animal called Pikaia gracilens. This is generally seen as a chordate, similar in some ways to the lancelets. These primitive chordates are shy, marine creatures, without fins, skull, jaws, backbone or a proper head. They look like animated anchovy fillets. Lancelets are popular food items, supporting a fishery in the South China Sea, but they are known worldwide. The researchers think that Cathaymyrus, like Pikaia, is a fossil relative of modern lancelets.
What makes a chordate, such that it is different from other animals? Chordates are usually long and narrow, with a stiffening rod of cartilage running along the back. In vertebrates this rod, the notochord, is replaced by bone to become the spinal column. In other chordates, such as sea-squirts, the notochord is present only in the larva. Adult sea-squirts have no notochords. Lancelets are somewhere in between, with a notochord that remains throughout life. There is good evidence that Cathaymyrus and Pikaia had notochords.
But the matter is more debatable for the strange fossil Yunnanozoon, also from Chengjiang. Some researchers think that Yunnanozoon represents a whole group of extinct chordates. Dr Shu and colleagues disagree, placing it among distant relatives of chordates called hemichordates. But this just shifts the problem one step away: the existence of notochords in modern hemichordates (obscure, marine worm-like creatures) is likewise debatable.
Chordates have other defining features. In one, the throat is modified to make a kind of filter, in which particles of food entering the mouth are strained from the water, the 'exhaust' leaving through the gills. Sea-squirts and lancelets are highly developed filter-feeders. Vertebrates have generally lost the full filter-feeding system, though intriguing traces remain, For example, in sea-squirts and lancelets, an organ called the endostyle secretes mucus that is used to trap food particles. The larvae of lampreys also have this organ, but it becomes transformed, in adults, into the thyroid gland. The thyroid makes hormones such as thyroxine that are rich in iodine (a function of the endostyle is to concentrate iodine from seawater). The existence of goitres and cretinism, conditions related to iodine deficiency, can be blamed on our filter-feeding heritage.
Cathaymyrus preserves faint traces of gill slits, but these have not yet been seen in Pikaia. Neither have gills been traced in conodonts, a large, extinct group of vaguely fishy creatures with formidable teeth. Conodonts have been known since the dawn of geology, but only from their teeth. Only in the past 20 years have fossils been found showing conodont teeth at the sharp ends of eel-like creatures not dissimilar to Pikaia and Cathaymyrus. However, conodonts had fins, and proper heads with eyes, putting them firmly among the vertebrates.
© Macmillan Magazines Ltd - NATURE NEWS SERVICE 1996
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