Cambroraster: documents how some flatworms became trilobites

Moysiuk J and Caron JB 2019
brought us a new semi-segmented ‘radiodont’ that bridges the gap between unsegmented flatworrms and segmented trilobites: Cambroraster (Fig. 1), a middle Cambrian late survivor of an Early Cambrian or earlier radiation.

We first looked at the origin of Anomalocaris
and trilobites via flatworms earlier here.

The traditional origin of trilobites according to trilobites.info:
The Early Cambrian included several orders of trilobites, so their genesis among artrhopods must have been earlier, in the Ediacaran. “Probably the key distinguishing character, one that also allowed trilobites to be preserved so well (and which accounts for their sudden prominence in the Cambrian), is calcification of the exoskeleton.”

Two opposing questions arise:
1. Did trilobites arise from slender velvet worm-types, essentially outer tubes over internal tubes with one leg pair per segment and a terminal mouth, and thereafter widen and flatten and rotate the oral cone ventrally to become a trilobite? Or…

2. Did trilobites start off as wide flatworms with a ventral oral cone and thereafter segment their bodies and grow legs and gills arising from each segment?

We already know
that all segmented worms arose from unsegmented round worms (nematodes with a mouth on one end and an anus on the other), and all nematodes arose from flatworms (with a more primitive mouth = anus). The transition from flatworm to trilobite via anomalocarid skips the roundworm and segmented (annelid) worm steps.

So, is this true?
Was there a third flatworm radiation that went directly to segmentation without rotating the mouth anteriorly while developing a separate anal opening posteriorly. The present evidence indicates this is a strong possibility.

Unfortunately, all trilobite clade members are now extinct,
but that fact may be part of today’s solution. If the present hypothesis (that trilobites, anomalocarids and Cambroraster were all gentle algal mat grazers) is correct, then the extinction of this clade can be explained by the near extinction of the algal mat during a planet-wide extinction event, like the end Permian. Thereafter algae could have slowly returned from distant and remote refugia not inhabited by now extinct trilobites and their relatives.

Trilobites.info concludes:
“The likely scenario is that trilobites arose from Precambrian bilaterians, arguably arthropods, that gave rise to Cambrian arachnomorphs, among them trilobites. The evidence is neither clear nor unambiguous. The fossil record is spotty, but suggestive. Perhaps it is the simple, dorsally unsegmented Precambrian fossil, Parvancorina, that offers the most reasonable link to arachnomorphs.”

Parvancorina bears a distinct resemblance to Cambroraster.
Moreover, Early Cambrian trilobites seem to be known better from head shields lacking ‘post-cranial’ thorax material (Figs. 2, 3). Note: This is what we see in Cambroraster (Fig. 1).

If the evidence points in a certain direction
maybe that’s where we should be looking for more data. Perhaps someone else can dive a little more deeply into this issue given this new direction.

The takeaway:
As usual, add a few taxa and see where it take you.

References
Hagadorn JW 2009. Taking a Bite out of Anomalocaris. In Smith MR, O’Brien LJ, Caron J (eds.). Abstract Volume. International Conference on the Cambrian Explosion (Walcott 2009). Toronto, Ontario, Canada: The Burgess Shale Consortium (published 31 July 2009).
Moysiuk J and Caron JB 2019. A new hurdiid radiodont from the Burgess Shale evinces the exploitation of Cambrian infaunal food sources. Proc. R. Soc. B 286:20191079.
http://dx.doi.org/10.1098/rspb.2019.1079

wiki/Cambroraster
wiki/Bigotinella
trilobites.info/feeding.htm
www.trilobites.info/origins.htm
phys.org/news/2010-11-ancient-shrimp-monster-fierce.html
phys.org/news/2010-11-earth-great-predator-wasnt.html
sciencemag.org/news/2019/07/millennium-falcon-predator-soared-across-ocean-floor-dawn-animal-life

Publicity for Cambroraster:
‘Millennium Falcon’ predator soared across ocean floor at dawn of animal life
By Joshua Sokol Jul. 30, 2019 , 7:01 PM

“The ‘ship’ was one of the largest known animals of its day to churn up the sea floor. It sailed in fleets over muddy ocean sediment, plying its unusual claws in the hunt for small prey.

“Cambroraster had a round mouth lined with toothlike plates, fronted with comblike claws it could hold out like a basket. Its eyes sat in deep notches that give the carapace its signature “spaceship” look.

“Hagadorn said the most likely diet of Anomalocaris was similar to that of modern arthropods such as crabs, lobsters and shrimps, which mostly eat soft items such as worms in the mud or microorganisms or plankton in the water. It could have eaten very small trilobites and recently molted trilobites whose new shells had not yet hardened, but the vast majority of trilobites would have broken Anomalocaris’ mouth parts.”