#gnathifera

Cambrian Explosion Month #18: Stem-Gnathifera

Protostomes are the other major evolutionary branch of bilaterian animals, and by far the most numerous with over a million known modern species (and probably several times more than that still undiscovered). This lineage is distinguished from the deuterostomes based on both embryo development and genetic studies, with the two groups estimated to have shared a common worm-like ancestor sometime back in the Ediacaran Period.

For the rest of this month we'll be featuring the spiralians, a branch of the protostomes that includes modern annelid worms, molluscs, and brachiopods. Meanwhile their cousins the ecdysozoans will be the focus of the entire second month of this series, later this summer, due to their incredibly rich Cambrian fossil record.

The earliest spiralians must have diverged from other protostomes more than 558 million years ago, if Kimberella and Namacalathus really were early members of the group, but more definite fossils only appear at the start of the Cambrian (~541 million years ago) with protoconodont "teeth" – once thought to be from early vertebrates, but now recognized as probably being jaw elements from a group of spiralians known as gnathiferans.

Gnathiferans include jaw worms, micrognathozoans, arrow worms, and rotifers, and are characterized by having complex mouthparts made of chitin. The fossil record for most of them is very poor, but along with protoconodont elements there are a few exceptionally well-preserved Cambrian examples of early members of this lineage.

Inquicus fellatus is known from the Chinese Chengjiang fossil deposits (~518 million years ago). About 3mm long (0.12") and shaped like a bowling pin, it seems to have been a stem-gnathiferan possibly related to both rotifers and arrow worms.

Its fossils have been found attached onto palaeoscolecid armored worms like Cricocosmia jinningensis, in what appears to have been a commensal relationship. Inquicus attached itself by a disc at the tip of its tail, didn't seem to penetrate the worm's skin, and its mouth faced away to feed from the water around them – so it probably wasn't harming its host, instead at most being a bit inconvenient in cases where worms were carrying at least 15 of these "passengers".

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Amiskwia sagittiformis is known from a small number of specimens in the Canadian Burgess Shale fossil deposits (~508 million years ago), with an even rarer related species Amiskwia sinica also known from Chengjiang (~518 million years ago).

For a long time its classification was a paleontological mystery, but some new specimens with better preservation seem to have finally revealed its identity – it was a gnathiferan, and possibly part of a stem lineage of early jaw worms.

Reaching about 2.5cm long (1"), it had a flattened body with two side fins and a paddle-like tail fin, a rounded head tipped with two tentacles, and complex mouth apparatus with jaw-worm-like teeth. It was probably an active swimmer that spent most of its time high in the water column well above the sea bed, potentially explaining why it so rarely fossilized even in places of exceptional preservation.

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Cambrian Explosion Month #19: Phylum Chaetognatha

Chaetognaths, commonly known as arrow worms, are a major component of marine planktonic ecosystems all around the world. They're a fairly small phylum in terms of diversity, with only about 120 known modern species, but in sheer numbers of individuals they're incredibly abundant – making up as much as 15% of total zooplankton biomass worldwide. They play an important role as predators, feeding on things like copepods, fish larvae, and each other, and can be so voracious that they're sometimes nicknamed "tigers of the zooplankton".

And they've been doing it for a very long time.

The appearance of protoconodont "teeth" at the start of the Cambrian (~541 million years ago) suggests that arrow-worm-like gnathiferans were some of the first active swimming planktonic predators – taking advantage of ecosystems that were becoming increasingly complex around that time, and laying the early foundations for more modern-style marine food chains.

Unfortunately we don't know much about their evolutionary origins, with their small fragile soft bodies leaving only a very patchy fossil record. Their relationship to other animals was also rather enigmatic for a long time, and they were only very recently identified as being part of the gnathiferans.

But their ancestors may have been something like Dakorhachis thambus.

Known from the Weeks Formation in Utah, USA (~499 million years ago), this little worm-like animal was up to about 3cm long (1.2"). It had a segmented body and a prominent feeding apparatus made up of a ring of at least six large triangular teeth, with various smaller elements located further inside its mouth.

It had no swimming appendages and would have lived either on the seafloor or hidden just below the surface, and it may have been an antlion-like ambush predator lunging at smaller prey and trapping it in its "basket" of teeth. It also seems to have been a fairly common element of the Weeks Formation ecosystem, with multiple individuals sometimes preserved on the same slab.

It's so unlike any other known fossil species – a Cambrian "weird wonder" – that it's not clear exactly what it was. But it's been tentatively linked to the gnathiferans and arrow worms due to its complex hard mouthparts, so it might represent a stem lineage retaining a more ancestral jaw arrangement and lifestyle.

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One of the earliest definite arrow worm fossils is Protosagitta spinosa from the Chinese Chengjiang fossil deposits (~518 million years ago). About 3.5cm long (1.4"), it looked similar to modern arrow worms, and had at least three sets of teeth in its jaws and a pair of small tentacles on its head.

Another arrow worm species from the same deposits, Ankalodous sericus, had a "multi-jawed" arrangement of multiple bundles of long spines, which may represent a transitional stage between a Dakorhachis-like ancestor and more modern arrow worms.

There was also the relatively "giant" Capinatator praetermissus from the younger Canadian Burgess Shale fossil deposits (~508 million years ago), which had another unique jaw arrangement with up to 50 spines.

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Limnognathia maerski,  Kristensen & Funch, 2000

The recently discovered Limnognathia maerski is an animal that is easily forgotten since it doesn’t share any characteristics with any other creature. Discovered in the springs of Disko Island, West Greenland (2000), the creature is the only species of his class.

The one-tenth of a millimeter long animal has a very complex set of jaw parts, which it uses to scrape the substrate. Apparently there have only been observations of female specimens, which carried eggs in their bodies, and it lead to believe this species is born as a male, and becomes a female when it’s in an adult stage.

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