Back to enigmatic invertebrates today.
Let’s take a look at Middle Cambrian Nectocaris (Figs 1–3, Morris 1976 ), known from a single specimen for a long time. Now it is known from 90 additional specimens.
Smith and Caron 2010 wrote:
“Nautiloids, traditionally considered basal within the cephalopods, are generally depicted as evolving from a creeping Cambrian ancestor whose dorsal shell afforded protection and buoyancy. Although nautiloid-like shells occur from the Late Cambrian onwards, the fossil record provides little constraint on this model, or indeed on the early evolution of cephalopods. Here, we reinterpret the problematic Middle Cambrian animal Nectocaris pteryx as a primitive (that is, stem-group), non-mineralized cephalopod, based on new material from the Burgess Shale. This clade extends the cephalopods’ fossil record by over 30 million years, and indicates that primitive cephalopods lacked a mineralized shell, were hyperbenthic [= above the sea floor], and were presumably carnivorous. The presence of a funnel suggests that jet propulsion evolved in cephalopods before the acquisition of a shell. The explosive diversification of mineralized cephalopods in the Ordovician may have an understated Cambrian ‘fuse’.”
in 1988 Alberto Simonetta wondered, “Is Nectocaris pteryx a chordate?,” based on the single original specimen. Smith and Caron 2010 did not address that hypothesis, but did cite that paper.
I sent the following email to Dr. Martin Smith, Ontario, Canada,
author and co-author of several Nectocaris papers (cited below).
Dear Dr Smith:
Thank you for publishing on Nectocaris.
I took the liberty of apply colors to the head and funnel region of one of your closeup images of Nectocaris. See attached. It appears to follow an earlier hypothesis of a chordate origin for cephalopods. Here the notochord is the cuttlebone. The long twin ‘tentacles’ are not mouth parts, but sensory structures, as in hagfish, which also evert their mouth parts during feeding. This could be a variation on that Bauplan.
Longtime readers might remember
an earlier hypothesis presented here linking lancelets (Branchiostoma) with nautiloids (Nautilus, Fig 5) and a cladogram that linked hagfish (Myxine) with Nautilus (Fig 6). Among other traits, the presence of large eyes (below the skin in extant hagfish) was a cephalopod synapomorphy not shared with blind lancelets.
Getting back to chordates, Simonetta 1988 wrote,
“A revision of the morphology of Nectocaris pteryx Conway Morris, 1976, and a comparison with the morphology of living Chordates supports the inclusion of Nectocaris in the phylum Chordata. The supposed somewhat crustacean‐like valves, which sheath the forepart of the animal are probably better considered as being the dermo‐epidermal folds that limit the peribranchial cavity of most lower Chordates, while the tail closely resembles the tail of the larval Tunicata and of Branchiostoma. The large eyes are a unique feature.”
An affinity with hagfish is perhaps more appropriate given the new data from dozens of other specimens. That’s where Nautilus nested here back in July 2021.
Here’s a problem worth noting.
Hagfish swim like lancelets and fish: with vertical tail fins and lateral undulations. By contrast shell-less cephalopods, like squids, do not undulate the torso and swim by undulating horizontal fins, as in Nectocaris (Figs 1, 2).
Simmonetta interpreted the original (holotype) Nectocaris
with myomeres, vertical fins and a subterminal anus, as in lancelets and fish. That’s not the case with the latter 90 specimens presented by Smith and Caron 2010 and Smith 2013, the ones under study here (Figs 1, 2). So, getting back to the vertical vs horizontal fin problem…
The solution was presented here a year ago
in which a step-wise reduction in lateral undulation and a stiffening of the notochord (Fig 5 transition 1) ultimately evolving into an immobile cuttlebone (= siphuncle), whether a shell was present or not, or later lost. Thereafter, to increase mobility and stability, lateral fins developed de novo in Nectocaris – just as they did by convergence in armored placoderms and again by convergence in armored osteostracans (Hemicyclaspis) followed by less armored Thelodus and sturgeons. So fin orientation from hagfish to cephalopods is not an insurmountable problem.
Simonetta AM 1988. Is Nectocaris pteryx a chordate? Bollettino di Zoologia. 55 (1–2): 63–68.
Smith MR and Caron JB 2010. Primitive soft-bodied cephalopods from the Cambrian. Nature 465 (7297): 469–472.
Smith MR 2013. Nectocaridid ecology, diversity and affinity: Early origin of a cephalopod-like body plan”. Paleobiology. 39 (2): 291–321.
Smith MR 2019. An Ordovician nectocaridid hints at an endocochleate origin of Cephalopoda. Journal of Paleontology. 94: 64–69.
“On May 27th, 2010 paleontologists Martin Smith and Jean-Bernard Caron announced that they had found a spectacular solution to one of the fossil record’s long-running mysteries. Since its description in 1976, the 505 million year old fossil Nectocaris pteryx from British Columbia’s famous Burgess Shale had vexed scientists. Known from a single specimen – appearing as little more than a smear on a rock slab – this creature seemed to be equal parts chordate and arthropod. No one could say what it was. Thanks to the discovery of nearly 100 additional specimens, however, this Cambrian oddball could finally be reexamined and its affinities resolved. In the pages of Nature, Smith and Caron presented Nectocaris as the early, Cambrian cousin of all other cephalopods, informally promoted as the ur-squid.”