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.
Maysiuk and Caron report,
“Cambroraster’s morphology is consistent with a nektobenthic lifestyle. The broad, vaulted H-element is convergent with the head armature of limulids , carcinized crustaceans and ‘filter chamber’ resuspension feeding trilobites, but also the head shields of some ostracoderm fish.”
the authors omit segmented trilobites and unsegmented flatworms from their cladogram.
the authors include several segmented, multi-legged, velvet worm-like taxa, including Hallucigenia and Opabinia. Their cladogram indicated these two Cambrian legged worms were ancestral to semi-segmented, legless anomalocarids. Thus, with regard to anomalocarids, the authors presented an upside-down cladogram, with primitive legless taxa at derived nodes. This comes from taxon exclusion: excluding flatworms, in this case.
The authors conclude,
“The inferred ecology of Cambroraster indicates that the evolution of large nektobenthic consumers, alongside smaller carnivores like trilobites, occurred in tandem with the radiation of these prey, in line with hypotheses emphasizing the catalysing effects of escalation during this radiation. As large and abundant nektobenthic carnivores, hurdiids like Cambroraster likely had a considerable impact on the local benthic community through both predation and bioturbation.”
The authors mistakenly assume
that Cambroraster (and other anomolacarids) used its oral cone as a predatory organ for biting large prey. Others have considered this unlikely given the largely immobile circular construction of the oral cone ventral to the cephalon, as in flatworms and trilobites (Fig. 1). Rather than a predator, Cambroraster likely fed like a trilobite on immobile, ubiquitous and defenseless algal mats.
What and how did trilobites eat?
According to trilobites.info. “There has been a long history of speculation about the feeding habits of trilobites, ranging from predators, scavengers, filter-feeders, free-swimming planktivores, and even parasites or hosts of chemoautrophic symbionts. Using modern-day crustaceans as an analog, it is reasonable to suggest that the majority of trilobites may have been predator-scavengers, as the majority of marine crustaceans are today. Fortey and Owens suggest indicate a shift away from predation and into particle feeding, which includes scavenging for bits of benthic detritus (as the group of olenids below might be doing), or perhaps grazing on beds of algae.” On their well-researched webpage, trilobite.info provides other forms of feeding on tiny and/or buried prey or particles.
Carapace before limbs
Cambroraster displayed a carapace and began to produce segments before it had legs. That means Cambroraster was producing chitin, the material that covered the limbs of trilobites. That’s the next transitional taxon to look for: a Cambroraster with tiny segment buds arising near the ventral midline (Fig. 1).
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.
“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.
As usual, add a few taxa and see where it take you.
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.
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.”