#penis worm

Cambrian Explosion #34: Phylum Priapulida

Named for their resemblance to human penises, priapulids (or "penis worms") are marine scalidophoran worms that live on or in muddy seafloor sediment, with some species having a surprisingly high tolerance for oxygen-poor environments and toxic levels of hydrogen sulfide. Despite being a rather low-diversity phylum with only around 20 living species, they're widespread and sometimes very numerous, with over 80 adult individuals per square meter (~10ft²) recorded in some locations.

The earliest definite modern-style priapulid in the fossil record comes from the late Carboniferous (~308 million years ago), but their ancestry was probably somewhere in the early Cambrian among the taxonomic mess of palaeoscoloecids and archaeopriapulids.

Xiaoheiqingella peculiaris was a very priapulid-like scalidophoran known from the Chinese Chengjiang fossil deposits (~518 million years ago). Around 1cm long (0.4") it had a ringed spiny body with a swollen proboscis region at the front and a bulbous rear end with a pair of long tail-like appendages.

It's generally considered to be a close relative or early representative of the modern priapulid family Priapulidae, but some analyses have instead suggested it was part of a "stem" priapulid lineage of much earlier forms.

Living alongside it in the Chengijiang region was another potential early priapulid, Paratubiluchus bicaudatus. Also about 1cm long (0.4"), this species was chunkier with a spiny proboscis, a bumpy body lacking rings, and two short rear appendages, and its overall proportions were very similar to the larvae of some modern priapulids.

It may have been closely related to the modern tubiluchid priapulids, but some studies disagree – one analysis even places it as much closer to kinorhynchs and loriciferans than to priapulids!

Both of these species were fairly rare elements of their ecosystem, and seem to have had very similar ecologies to each other. They would have been burrowing carnivores using their retractable proboscises to grab small invertebrate prey, sometimes also feeding detritivorously on decaying organic matter in the mud around them.

Their anatomical and ecological similarities to modern priapulids suggest that the group as a whole rapidly developed in the early Cambrian and then just… didn't ever really need to change their body plan or lifestyle much for the next half a billion years.

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Ferocious 'penis worms' were the hermit crabs of the ancient seas

These phallus-shaped worms were some of the top predators of the ancient seas, but even they needed protection.

The Cambrian period (543 million to 490 million years ago) brought the first great explosion of biodiversity to Earth, with the ancestors of practically all modern animals first appearing. One of the most feared among them was the penis worm.

Technically known as priapulids — named for Priapus, the well-endowed Greek god of male genitals — penis worms, as they’re commonly known, are a division of marine worms that have survived in the world's oceans for 500 million years. 

Their modern descendants live largely unseen in muddy burrows deep underwater, occasionally freaking out fishermen with their floppy, phallus-shaped bodies. But fossils dating back to the early Cambrian show that penis worms were once a scourge of the ancient seas, widely distributed around the world and in possession of extendible, fang-lined mouths that could make a snack out of the poor marine creature that crossed them.

But, fearsome as they were, penis worms themselves were not without fear. In a new study published Nov. 7 in the journal Current Biology, researchers discovered four priapulid fossils that were nestled into the cone-shaped shells of hyoliths, a long-extinct group of marine animals...

#360 Wynaut

Penis worms and the Cambrian explosion.

About 542 million years ago, something big happened in the biosphere, whose exact nature, cause and meaning are still under debate in both Earth and life sciences. An explosion in biodiversity occurred and the variety of life on our planet broadened considerably, combined with the appearance of the ability to biomineralise chemicals dissolved in seawater into shells. Shells of course mean easier fossilisation, and the history of life was largely written in the 19th century using their remains. Rare finds of soft bodied organisms such as those in the Burgess shale tell us that this radiation also occurred in unshelled life forms.

Eras are defined by the International Commission on Stratigraphy based on the first appearance of what are called 'type fossils', often from a 'type location'. While either of these are subject to revision in the face of new or better evidence (such as a geologically clearer locality or type fossils found out of place), most of the main boundaries are firmly defined on multiple lines of evidence, from plants or shells right down to the microscale of pollen grains and plankton. Obviously different fossils define the borders in marine and terrestrial sediments, but other than that the system is well nailed down.

The earliest Cambrian however has been trickier to pin down. It was once defined by the appearance of shells, but the beginnings of the Cambrian explosion seemingly pre-date biomineralisation (though new fossils and dating can overturn this statement at any time). Since complex life was entirely marine, with land life limited to bacterial biofilms, researchers have sought a marker in marine sediments to help them draw the border on the stratigraphic column.

To be of any use, such a marker must be demonstrated as common and widespread around the globe, so that geologists can easily identify the transitions between eras in the field. The wider the range of paleo environments it occurs in, the more useful it is. Agreement on a marker on the lower Cambrian was only reached in the mid 1990's.

This is where the penis worm enters the picture. Also known as priapulids (after the Roman fertility god Priapus), their borrows are thought to be the main feature of the chosen marker, a complex set of trace fossils known as the Treptichnus pedum assemblage. These complex burrows looking a bit like rope are the first trace fossil more sophisticated than a simple line on the old sea floor, and the first to pierce into the sediment below. They seem to sit astride the invisible line between the Ediacaran and Cambrian fauna. While their identification with penis worms is not certain, some modern priapulids produce identical burrows while probing through the muddy sediment for nutrients.

A study recently published in Geology has produced the first assessment of the usefulness of these burrows as a global marker. The team studied strata in South Africa, and compared them with locales in Namibia, Australia and Canada in order to understand the distribution of these fossils and the breadth of different sedimentary environments that they were made in. They found that Treptichnus pedum are found in the widest known range within the shallow marine environment of any organism of that era studied so far, making it more likely to be found in rocks elsewhere and increasing its range of application as a type marker fossil.

It also appears in different locales around the world within a narrower range of absolute dates and stratigraphic positions than any other early Cambrian organism studied so far. There are no perfect marker fossils that all appear at exactly the same absolute date or stratigraphic position for any era. Their boundaries are all defined by sets of fossils rather than single organisms in order to overcome this problem. This study however indicates that penis worms may have made the sharpest dividing line that allows us to recognise in the stony record what many have called the dawn of life's most crucial epoch. Not bad going for a penis worm....

Loz

Image credit: Jean Vannier.

http://burgess-shale.rom.on.ca/en/science/origin/04-cambrian-explosion.php

http://www.ucmp.berkeley.edu/cambrian/cambrian.php

Original paper, paywall access: http://geology.gsapubs.org/content/41/4/519.abstract

#202 Wobbuffet Wobbuffet: The penis worm. look it up. Actually, don’t. Instead, look up  Priapulida

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If I had a nickel for every time I learned about a very penis looking sea creature, I'd have two nickels - which isn't a lot, but it's weird that it happened twice.

But also how come most of the cool worms happen to be sea creatures?