Lancelets and Nautilus compared

The comparison seems obvious now
The origin of the nautilus links back to the early chordate lancelet (Fig. 1). Details follow.

Figure 1. The lancelet (above) and nautilus (below) still share several traits in common despite their many differences after hundreds of years of evolution. See figure 2 for the correct number of cirri (18 per side) in a lancelet. The nautilus funnel is an extension of the rectum and anus (see figure 3) now exiting anteriorly.

And that’s not all.
Lesser known and less mobile lophophorates (like Phoronis, Fig. 2) are also derived from lancelets. According to Wikipedia, “Molecular phylogenetic analyses suggest that lophophorates are protostomes, but on morphological grounds they have been assessed as deuterostomes.” (More on this issue below).

Figure 2. Lancelets compared to Phoronis the lophophore. Note the migrations of the elongate rectum back to the oral area, as in Nautilus. Don’t overlook the difference. In Lophophorates, which include bryozoans and brachiopods, the rectum is dorsal to the mouth, as in snails. In Nautilus the anus and funnel are ventral to the mouth coincident with 180º torsion of the coiled shell and loss of the tail.

According to Wikipedia,
“Lophophorate, any of three phyla of aquatic invertebrate animals that possess a lophophore, a fan of ciliated tentacles around the mouth. … The lophophorates include the moss animals (phylum Bryozoa), lamp shells (phylum Brachiopoda), and phoronid worms (phylum Phoronida, Fig. 2).”

“The lophophore is a characteristic feeding organ possessed by three major groups of animals: the Brachiopoda, the Bryozoa, and. the Phoronida. The lophophore can most easily be described as a ring of tentacles, but it is often horseshoe-shaped or coiled.”

Here the lophophore is homologous
with the buccal cirri on lancelets, the tentacles of cephalopods, and the feet of echinoderms. In humans the same circum-oral structure, the orbicularis oris, forms the lips.

According to Wikipedia,
“The “tentacles” of the nautili are actually cirri (singular: cirrus), composed of long, soft, flexible appendages which are retractable into corresponding hardened sheaths. Unlike the 8–10 head appendages of coleoid cephalopods, nautiluses have many cirri. In the early embryonic stages of nautilus development a single molluscan foot differentiates into a total of 60–90 cirri, varying even within a species. Nautilus cirri also differ from the tentacles of some coleoids in that they are non-elastic and lack pads or suckers. Instead, nautilus cirri adhere to prey by means of their ridged surface.”

Figure 3. Nautilus external and internal anatomy. Note the migration of the rectum = funnel back to the oral area, as in lophophorates.

According to Wikipedia,
“The mouth consists of a parrot-like beak made up of two interlocking jaws capable of ripping the animal’s food— mostly crustaceans— from the rocks to which they are attached.”

“Unlike many other [all more derived] cephalopods, nautiluses do not have what many consider to be good vision; their eye structure is highly developed but lacks a solid lens. Whereas a sealed lens allows for the formation of highly focused and clear, detailed surrounding imagery, nautiluses have a simple pinhole eye open to the environment which only allows for the creation of correspondingly simple imagery.”

The rectum is dorsal to the mouth,
beneath the mantle (of all places!) in slugs and likewise beneath the coiled shell in snails, as in lophophorates (phoronids Fig. 2, bryozoans and brachiopods, ). By contrast, in Nautilus the rectum and funnel are ventral to the mouth — along with 180º torsion of the coiled shell and loss of the slug and lancelet tail. That shell torsion in free-swimming Nautilus keeps the air-filled empty chambers dorsal to the body for traditional orientation of the preoral lobe (= hood) dorsal to the buccal cirri (= tentacles) and mouth.

The protostomate question.
Molluscs are protostomates (the mouth appears from the first embryonic invaginatiion, then then anus appears later). Chordates and echinoderms are deuterostomates (= anus first, mouth second). Traditionally this has been considered a major division. Both clades arise from nematode (= roundworm) ancestors in which the mouth and anus appear at the same time. Since these all develop in yolk-filled eggs, embryos don’t have to feed, digest and defecate, so the mouth, intestine, rectum and anus are useless and subject to inconsequential genetic timing changes, especially at the planula-grade.

Genes determine the timing of trait appearances.
Genes both evolve and reverse. Though still helpful, this makes the timing of the first appearance of the mouth or anus in embryos not the big deal they teach in universities. Long time readers will be expecting this: “Don’t pull a Larry Martin!” Look at the entire organism, not just one, two or a dozen traits — especially embryo traits — no matter how traditional or established in text books and lectures.

Why did someone not see this before?
Perhaps because most workers are trained in universities where they have rules students must follow or risk displeasing their professors and failing tests. We’ve already seen how a support system among academics keeps myths alive while keeping out disruptive hypotheses.

The antiquity of hagfish and lancelets
extends into the Ediacaran (= the Pre-Cambrian). On hindsight it seems rather exceptional that such ancient and primitive creatures should traditionally lead, like a simple ladder, only toward vertebrates. Instead, now consider the idea that a wider range of taxa evolved from hagfish and lancelets, creating more of a topological bush, which would be more typical of evolutionary events in other lineages.

The connection between lancelets and nautiloids
appears to be a novel hypothesis of evolutionary interrelationships. If not, please provide a citation so I can promote it here.

[I found one with citations for others. The more recent citations discuss convergence, not homology and do not mention lancelets. See below].

Shigeno et al. 2010 wrote:
“In 1830, two young naturalists, Meyranx and Laurencet, attempted a comparison of the anatomy of vertebrates and cephalopods, speculating that they have the same basic structural principle. While Geoffroy St. Hilaire adopted the idea as proof of his theory, on the unity of body plan that is composed of shared components of all animals, Georges Cuvier rejected it using questionable results of his anatomical study of an octopus (Figure 1; Appel, 1987; Le Guyader, 2004 for reviews). Ever since this pre-Darwinian academic debate, many zoologists have indulged in a long lasting discussion of how the cephalopod body plan and their organ systems can be linked to those of vertebrates (e.g. Packard, 1972; O’Dor and Webber, 1986).”

PS
Shigeno et al. 2008 document the origin of the hood + eye separate from the mantle without an operculum with this image of a 3-month-old Nautilus embryo. Note the posterior funnel = anus, prior to the U-turn it takes in adults with the funnel beneath the mouth. Here the yolk sac is the mouth, not a gastropod-like foot. So the tentacles and funnel both rotate to the front during embryological ontogeny as the yolk sac is absorbed.

References
O’Dor RK and Webber DM 1986. The constraints on cephalopods: Why squid aren’t fish. Canadian Journal of Zoology 64: 1591–1605.
Packard A 1972. Cephalopods and fish: The limits of convergence. Biological Review vol. 47, p. 241-307.
Shigeno S, Sasaki T, Moritaki T, Kasugai T, Vecchione M, Agata K 2008. Evolution of the cephalopod head complex by assembly of multiple molluscan body parts: Evidence from Nautilus embryonic development. J Morphol 269(1):1-17. doi: 10.1002/jmor.10564. PMID: 17654542.
Shigeno S, Sasaki T and von Boletzky S 2010. The origins of cephalopod body plans: A geometrical and developmental basis for the evolution of vertebrate-like organ systems. Pp. 23-34 in Tanabe K., Shigeta Y., Sasaki T and Hirano H (eds) 2010. Cephalopods – Present and Past Tokai University Press, Tokyo.

wiki/Nautilus
wiki/Lancelet
wiki/Phoronis
wiki/Lophophore = buccal cilia
wiki/Lophophorata

For an opposing view:
A giant cephalopod (Endoceras) with a straight shell has ‘tripartite operculum’ homologous with the ‘hood’ of Nautilus in figure 1, thanks to Tyler Greenfield, whose cladogram does not include lancelets, but just goes back to the suprageneric taxon, “Cephalopoda’. Details here:

https://incertaesedisblog.wordpress.com/2020/02/16/reconstructing-fossil-cephalopods-endoceras/

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9 thoughts on “Lancelets and Nautilus compared

  1. The operculum of nautiluses definitely did not originate from the head of lancelets. The operculum closes and seals the shell and is thus almost the exact same shape and size as the shell opening. Lancelets obviously do not have a shell so they would have no reason to evolve an operculum. Nautiluses (and all other cephalopods) are also not chordates and do not have a notochord, unlike lancelets.

    Orthoceratoids show the ancestral condition of a flat, oval operculum, unlike the hood-shaped operculum of extant nautiluses which is the derived condition. I have discussed the orthoceratoid operculum in greater detail in this blog post.
    https://incertaesedisblog.wordpress.com/2020/02/16/reconstructing-fossil-cephalopods-endoceras/

  2. Besides what has already been mentioned, a few other characters support the classification of the nautilus among the Cephalopoda. Firstly, their blood contains hemocyanin, which is found in other cephalopods – as well as the other well-known protostome group, the arthropods. Secondly, they possess two gill pairs, a shared trait derived from the ancestral metamerism of cephalopods.

    For a citation:
    Wingstrand, KG (1985). “On the anatomy and relationships of Recent Monoplacophora”. (It’s archived, but the link’s here: https://web.archive.org/web/20160303182836/http://www.zmuc.dk/inverweb/Galathea/Galathea_p5.html)

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