A Cambrian holdout

Although you don't tend to hear much about it, the Ordovician Period was a very peculiar time in Earth's history.

From beginning (485 million years ago) to the end (444 million years ago) it experienced one of the biggest global climatic swings the Earth has ever seen.  In the early Ordovician the climate was a sauna -- an intense greenhouse effect caused the highest temperatures the Paleozoic Era would see, and glacial ice all but vanished.  By the end, the center of the supercontinent of Gondwana was near the South Pole, and glaciers covered much of what is now Africa and South America, resulting in a massive extinction that wiped out an estimated sixty percent of life on Earth.

At this point, life was confined to the oceans.  The first terrestrial plants and fungi wouldn't evolve until something like twenty million years after the beginning of the next period, the Silurian, and land animals only followed after that.  So during the Ordovician, the shift in sea level had an enormous impact -- as the period progressed and more and more ocean water became locked up in the form of glacial ice, much of what had been shallow, temperate seas dried up to form cold, barren deserts.  And that was all there was on land -- thousands of square kilometers of rock, sand, and ice, without a single living thing larger than bacteria to be found anywhere.

Somehow, despite the extreme climatic swings that happened during the Ordovician, life in the oceans diversified, and rebounded after the dramatic dieoff at the end.  And along the way, there were some really peculiar life forms.

One of these was discovered not long ago in the Castle Bank Formation in the middle of Wales.  (Ordovician outcrops in Wales are what gave the period its name; the Ordovices were a tribe that lived there around the time of the Roman conquest of Britain.)  The animal was small -- the fossil measures only thirteen millimeters from tip to tail -- but it was one odd-looking critter:

A reconstruction of Mieridduryn bonniae [Image credit: Franz Anthony]

Aficionados of paleontology will no doubt immediately recognize the similarity to Cambrian animals called Opabinia and Anomalocaris; Mieridduryn looks almost like a hybrid of the two.  (If you're a linguistics geek like myself, you might be interested to know that the genus name Mieridduryn comes from Welsh words meaning "bramble snout.")  And it does seem to be a holdover from the Cambrian Explosion fauna, which also produced such weird forms as Hallucigenia (the name means "comes from a hallucination"), which is so bizarre that at first, paleontologists reconstructed it upside down, until some better-preserved fossils made them realize their error.

[Image licensed under the Creative Commons Qohelet12, Hallucigenia, CC BY-SA 4.0]

By the Ordovician, however, a lot of the stranger (to our eyes, at least) life forms had gone extinct, and the wipeout at the end of the Ordovician finished off the last of them.  At that point, what was left -- arthropods, primitive vertebrates, mollusks, echinoderms, annelids, and so on -- would have begun to look a lot more familiar to us.

But during the mid-Ordovician, when Mieridduryn was snorking about in the mud of shallow, warm oceans, there were still some mighty peculiar animals.  If you hopped a time machine and went back there, you might well think you were on a different planet.  It reminds me of the poem by Irish geologist John Joly, which he was inspired to write while looking at a the fossil of a long-extinct animal, and seems a fitting place to end:

Is nothing left?  Have all things passed thee by?

The stars are not thy stars.  The aged hills

Are changed and bowed beneath the ills

Of ice and rain, of river and sky;

The sea that riseth now in agony

Is not thy sea.  The stormy voice that fills

This gloom with man's remotest sorrow shrills

The memory of thy lost futurity.

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