A ptotally pterrific pterobranch: amazing Cambrian fossil discovered

Posted by mjs76 at Mar 24, 2011 05:00 PM |
Pterobranch? It’s a type of hemichordate. Hemichordate...? Not the most familiar group of animals, for sure, but in evolutionary terms hemichordates are very important. To understand why, we first need to consider chordates – and before that, we need to consider vertebrates…

We all know what a vertebrate is: an animal with a backbone. So that’s mammals, birds, reptiles, amphibians and fish. Chordates are a group which includes all vertebrates plus a few invertebrates that share certain distinctive vertebrate characteristics without actually having a backbone. These characteristics include a flexible rod called a notochord which supports their body, a hollow nerve cord down their back and ‘pharyngeal slits’ which in higher animals evolved into gills. All vertebrates have these features too, even humans, although they may only appear in embryos.

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The fossil is in two pieces which are mirror images of each other (or would be if they were both complete).

And hemichordates, as the same suggests, are halfway to being chordates – they have pharyngeal slits but no notochord, for example - which is what makes them such a fascinating part of the evolutionary tree of life.

Now researchers in our Department of Geology, working with colleagues in Oxford and China, have found a quite staggering fossil of an ancient hemichordate which advances our knowledge and understanding of these creatures – and hence of the early evolution of animal life itself.

The extraordinary thing about this fossil is that the soft parts of the animal are perfectly preserved – and when you don’t have a backbone, most of your body is soft parts.

A bluffer’s guide to the hemichordata

Hemichordates divide into two basic groups or classes: enteropneusts, commonly called acorn worms, and pterobranchs. There is a third class, planctosphaeroids, but that is only represented by one species. And there used to be a fourth class, graptolites, which were extremely common in Paleaozoic seas about 400-500 million years ago. Fossil graptolites are so common that they are used as an ‘index fossil’ to act as an ancient time-marker – and they can also indicate palaeoenvironmental factors such as temperature.

But the new fossil isn’t a graptolite, it’s a pterobranch, And it is, to quote the research team:

by far the best-preserved, the earliest and the largest hemichordate zooid* from the fossil record.”

Pterobranches are distinguished by their ability to secrete a collagen-like substance which builds up around them to create a rigid, upright tube (a feature seen in other marine life such as tube worms). Extant pterobranch species have from one to nine pairs of arms protruding from the top of their tube, each of which has a row of tiny tentacles along one side. They are filter feeders and use these tentacles to catch plankton.

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Photograph taken using a Canon 5D DSLR camera. This whole fossil is less than 4cm long.

One-armed boxer: why this specimen matters

Fossil pterobranchs have been discovered before, but they tend to just be a tube – because that’s the hard part of the animal. This new specimen, however, has a perfectly preserved arm with 36 clearly visible tentacles (there is a second arm, but most of that is missing). For the first time, we are able to see what a pterobranch from the Cambrian era looked like – and it looks very much like some of the 20-30 species that are known to exist today. This is a creature which found its niche on the ocean floor and has not evolved significantly in 525 million years.

Although it is the largest hemichordate fossil ever found, it is (to a human perspective) far from huge. The tube is 14mm long and the more complete of the two arms is 22.5mm, although the tip is missing. So the whole fossil is less than 4cm in length. Individual zooids in living pterobranch colonies are often only 1mm long, though. By comparison, this is a prehistoric giant.

The specimen was found in Yunnan, a province in southwest China which is rich in all sorts of fossils and has already given its name to species such as the primitive chordate Yunnanozoon and the prosauropod Yunnanosaurus. Xian-guang Hou and Xiao-ya Ma from the Yunnan Key Laboratory for Palaeobiology at Yunnan University collaborated with academics from our Department of Geology – David Siveter, Richard Aldridge, Mark Williams and Jan Zalasiewicz – plus Derek Siveter from Oxford University’s Department of Earth Sciences.

Ah, but is it…?

While a discovery like this is terribly exciting, one of the basic rules of good science is that you don’t get carried away with the neatest idea but must consider and discount other possibilities. As the research team put it:

Alternative interpretations are less convincing: tubicolous polychaetes bear either unbranched oral tentacles or numerous feather-like radioles; entoprocts lack the tube, and their unbranched tentacles form an encircling crown; bryozoans possess either horseshoe-shaped lophophoral arms with densely arranged tentacles or numerous unbranched tentacles arranged circularly; phoronids have a large number of unbranched tentacles arranged in the form of a horseshoe or, in some species, a coil; some vermetid snails bear a few unbranched oral tentacles, but their tubes tend to be corkscrew-like or irregular.”

So with that lot out of the way, it probably is a pterobranch hemichordate – the largest species ever identified. One thing which can’t be determined is how this particular zooid fitted into a larger colony, if indeed it was part of a colony at all (a few species of pterobranch live solitary lives). The bottom of the tube is missing so we have no way of knowing whether it was attached to other individuals.**

As well as the tube, the one and a bit arms and the 36 tentacles, various parts of the fossil may represent other structures such as a contractile stalk, but really it’s impossible to say for certain. What can be said with certainty is that this discovery vastly expands our knowledge of these ancient creatures, both in and of itself and in its similarity to living pterobranchs.

Naming names

Which just leaves the question of what to call the beast. The new genus which it represents has been named Galeaplumosus, meaning ‘feathered helmet’, simply because of what it looks like. The species name assigned is abilus, from ‘ab’ (away from) and ‘nubilus’ (cloudy), a clever paraphrasing of Yunnan which literally means ‘south of the clouds’.

The study of G abilus was funded by the Royal Society and the National Natural Foundation of China and a paper describing the fossil has now been published in Current Biology. In a nice touch, the paper is dedicated to Professor RB ‘Barrie’ Rickards, a Cambridge-based palaeontologist and world authority on graptolites who passed away in 2009.

*Today’s word is ‘zooid’, meaning an individual animal that is connected to others as part of a larger colony, like coral.

**The pieces that connect zooids within a colony are called stolons.