Good tidings and well-wishes!
In yesterday’s Burgess Shale post, I gave mention to the fact that, in the words of Cambrian paleontologist Simon Conway Morris, “Current research is showing that a number of species from the Burgess Shale cannot reasonably be accommodated in any extant phylum”. While this fact greatly assists the incomparable intrigue of this bizarre and delightfully unique collection of organisms, it often serves as a simultaneous source of irritation for scientists and enthusiasts alike. Perhaps the most infuriating example of this taxonomic ambiguity is embodied by the case of Canadaspis sp., a creature long believed to have not only been a crustacean, but a primitive malacostracan: a retrospective image shattered by subsequent research and debate.
Although this blog has historically covered phylogeny before anatomy and functional morphology, the confusion surrounding the former discipline with regard to this particular animal forces me to abandon this trend here. Instead, I’ve opted to commence with the general description.
In very general terms, the creature’s appearance has been described as “shrimp-like”. Its head, along with most of its “trunk”, is covered by a 2cm-long hinged, bivalved carapace (the entire animal was a mere 3 cm in length). This shell, along with the rest of Canadaspis‘ exoskeleton, is composed primarily of chitin, as is the case with all arthropods. The abdomen (rear section) sports several segments with a spine-flanked telson.
The thorax (mid-section) is a matter of considerable debate. Some paleontologists, such as Derek Briggs, have argued that this portion of Canadaspis‘ anatomy bears eight segments whilst several of his colleagues maintain that it actually houses ten segments. This academic difference of opinion is significant because the former interpretation would strongly assist any effort to place Canadaspis within the phyllocardia subclass of the Crustacea. A related point of disagreement concerns the following matter: living phyllocardians maintain a 2:8 ratio of limbs anchored on the posterior head and thorax respectively, yet the arrangement is somewhat vague in most fossil specimens.
The beast’s head displays a pair of stalked eyes along with a pair of anteriorly-projecting antennae. Unlike those of modern crustaceans, these antennae are uniramous, meaning that they exist as two separate sets of segments (alliteration unintended) rather than two branched sets united near the base as in extant lobsters, crabs, shrimp, and kin (this setup is scientifically referred to as “biramous”).
It should be noted that Canadaspis enjoys a far greater fossiliferous range than most Burgess shale genera, having also been unearthed in Utah and, most interestingly, in the older Chengjiang lagerstatte of China’s Yunning Province where it is represented by C. laevigata (which may be synonymous with C. eucalla, another species described from the formation). The best-known and most well-preserved species, however, is the Burgess’ C. perfecta .
Having discussed the controversial and relatively cryptic anatomy of this frustrating little species, the time has come to examine its functional morphology. The presence of a slit in what is believed to be the animal’s gut suggests that Canadaspis may have fulfilled its dietary needs by devouring organic material in ingested sediment. This geological bedding may have easily been shuffled into the creature’s maw by virtue of Canadaspis‘ gill flaps attached to each limb: these may also have been used to enable swimming. Furthermore, C. perfecta is frequently found in association with the miniscule trilobite Ptychagnostus which is often found within the rounded head shield of Canadaspis and a few other arthropods, suggesting that they may have been a relatively common prey item. Much more theoretically, Andrew Parker has suggested that Ptychagnostus may have even been a cranial parasite of sorts, though there appears to be no way of validating (or even expanding upon) this interpretation.
On a final note, although Canadaspis can no longer be comfortably placed within any extant crustacean group, the fact that it was almost certainly allied with the subphylum renders its classification considerably less mysterious than those of a great many Burgess Shale residents. Unfortunately, many people seem to invariably assume that “mysterious” is synonymous with “interesting” and, consequently, that any degree of familiarity associated with a given object or phenomenon robs it of intrigue. We would all do well to consider Stephen Jay Gould’s view on the subject:
“Canadaspis is both a key and an anchor to the Burgess story, a creature every bit as important as any of [its] weird wonders. Suppose that every Burgess animal were a bizarre denizen of a lost world. What then would we make of the assemblage? A failed experiment, a washout, a first attempt totally bypassed by a reconstituted modern fauna, and therefore offering no clues and no connection to the origin of later life. But the presence of Canadaspis, and other creatures of [relatively] modern design, suggests a different and more enlightening view. The Burgess fauna does include modern prototypes, and, in this key respect is an ordinary Cambrian fauna; but the vastly broader range of designs that disappeared may reveal the most important of all patterns in life’s early history.”
May the fossil record continue to enchant us all!