#phosphatocopida

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Cambrian Explosion #59: Stem-Crustacea – Actual Ancient Aliens & Bivalved Buddies

The majority of known fossils of Cambrian crustaceans are in the form of minuscule microfossils with "Orsten-type preservation" – formed in oxygen-poor seafloor mud and exceptionally well-preserved in three-dimensional detail. They can only be discovered and studied after dissolving away the rock around them with acid and picking through the residue under a microscope, then they're scanned with an electron microscope to see their fine details.

And it turns out some of these tiny early crustaceans looked really weird.

Cambropachycope clarksoni is known from a small number of specimens from the Swedish Orsten Lagerstätte (~497 million years ago). And at first this stem-crustacean seems like it's easily one of the most bizarre of all Cambrian animals, looking more like a speculative alien creature design (or possibly a pokémon) than a real once-living animal.

Up to 1.5mm long (0.06"), it had a large pointed "head" bearing a single huge compound eye with up to 150 facets. Its long tapering body had one pair of simple appendages at the front, then three pairs of biramous appendages, and then four more pairs of paddle-like appendages that decreased in size further back.

…But its strange anatomy starts to make a little bit more sense once you figure out what's actually going on: that's not a head.

The "head" is actually just an eyestalk with a pointy projection on the back. Cambropachycope's actual head encompasses the first third or so of its body, with the first appendages being antennae and its mouth located on its underside just behind them. The next three pairs of biramous limbs are other head appendages – probably corresponding to the second antennae, mandibles, and maxilluae of other pancrustaceans – and then the "paddles" are its thorax limbs.

With its huge cycloptic eye and swimming paddles it was probably an active predator preying on smaller planktonic animals, possibly convergently similar to some modern predatory water fleas.

A second very similar-looking stem-crustacean was also found in the same deposits – Goticaris longispinosa – hinting that there may have been an entire weird lineage of these little alien swimmers in the late Cambrian oceans, which the Orsten fossils are only giving us the smallest glimpse at.

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Meanwhile some of the most abundant animals in Orsten deposits are the phosphatocopids, with around 30 different species known from between about 518 and 490 million years ago.

These tiny arthropods were less than 1cm long (0.4") and had bivalved carapaces completely covering their bodies. They're mostly known just from their empty shells, but some specimens do preserve the rest of them, mostly represented by various early larval stages with the exact anatomy of later larvae and adults being less well understood.

They were traditionally regarded as being true crustaceans and part of the ostracods, and sometimes were lumped together with various other similar Cambrian bivalved groups like the isoxyids and bradoriids – but the similarities between them seem to have been superficial and convergent. More recently phosphatocopids have been considered to be stem-crustaceans instead, very closely related to the true crustaceans but not quite part of that lineage.

Klausmuelleria salopiensis is one of the earliest known phosphatocopids, found in the Comley Limestone deposits in England (~511 million years ago). Just 340μm long (~0.01") it's only known only from a single very early larval stage – often referred to as a "head larva" because it's essentially just a swimming head, with all four of its pairs of appendages corresponding to the head region of older individuals.

Like other phosphatocopids it would have lived just above the seafloor in oxygen-poor waters, and probably mostly fed on suspended particles of organic detritus. Some species had particularly strong spiny gnathobases at the bases of their appendages, however, suggesting they may have also been crushing and manipulating harder food items, possibly "micro-scavenging" on dead small planktonic animals.

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