When phylogenetic analysis brings together taxa that had not been tested together before, non-traditional relationships can result (Fig. 1). This can bring new insights into the family tree of life and resolve old issues.
Recent efforts to understand a possible connection between cephalopods (like Nautilus, Fig. 2) and basal chordates has raised the possibility that traits once thought convergent (like the similar eyeballs in cephalopods and chordates) may instead be homologous. Here (Fig. 1) Nautilus nests as a sister to the hagfish (Myxine, Fig. 2)) rather than nesting with chitons, clams and snails (Figs. 1–4). This possibility started to emerge earlier here, here and here. So far, adding taxa has not changed these new nestings.
According to Wikipedia [Mollusca], “They are highly diverse, not just in size and anatomical structure, but also in behavior and habitat. The three most universal features defining modern molluscs are a mantle with a significant cavity used for breathing and excretion, the presence of a radula (except for bivalves), and the structure of the nervous system. Other than these common elements, molluscs express great morphological diversity, so many textbooks base their descriptions on a “hypothetical ancestral mollusc”.
That “hypothetical ancestral mollusc” turns out to be a straw dog. So it is no longer a useful concept and must be discarded. Analysis indicates three separate origins for traditional molluscs. That makes this clade name no longer useful.
In the small mollusc tree relationships are based on a last common ancestor, not three traits that turn out to be convergent. Basing a clade on a few to a dozen traits is called “Pulling a Larry Martin”. Don’t do that. Run the analysis. Report the results. Figure out the sometimes surprising convergence.
Employing flatworms and roundworms as outgroup taxa has clarified interrelationships here. The following three figures (Figs. 2–4) illustrate the taxa used in clades recovered in Fig. 1.
Earlierthe large morphological difference between basal chordates and basal cephalopods was illustrated with fossil taxa and hypothetical soft tissue not preserved in the fossil.
Interesting note: Berthelinia (Fig. 3) is a type of snail with a bivalve, clam-like shell, rather than a spiral shell, as in Helix (Fig 3). Analysis indicates this is convergent with clams, as traditionally considered, not a transitional taxon, despite appearances.
A few basal echinoderms were added to this study because they were the traditional sister clade to Chordata. However, when tested in this small analysis (Fig. 1) the two echinoderm sea cucumbers nested closer to traditional molluscs, the Aplacophora and Polyplacophora (chitons) than to tested basal chordates.
Hagfish slime and octopus ink Both are ejected when the animal feels threatened. Nautilus does not express slime or ink. Extant cephalopod ink sacs are located beneath the intestine and open internally close to the anus, prior to the muscular funnel. Ink is almost pure melanin bound by mucous particles so it lumps after ejection. By contrast, hagfish slime is released form dermal pores that run the length of its body. As you can see, there is no connection between the two defense mechanisms. These two taxa split from each other some 500 million years ago, plenty of time for both to evolve independently.
In summary, traditional molluscs had not one, but three origins. Since the traditional clade Mollusca is no longer monophyletic it should be dropped from usage. This cladogram (Fig. 1) clears up relational issues within the traditional ‘Mollusca’, unresolved until today.
These are tentative results employing relatively few taxa and few characters (none borrowed from the LRT, LPT or TST). Even, so, this appears to be a novel hypotheses of interrelationships. If not, please provide a prior citation so I can promote it here.
Konietzko-Meier, Tanczuk and Teschner 2021 report “Ozimek volans is one of the most mysterious representatives of the Late Triassic fauna from Krasiejow.”
Focusing on its non-existent gliding abilities, is what has made Ozimek mysterious for these three workers. In the large reptile tree the most fragile reptile of all time, Ozimek (Fig. 1, Dzik and Sulej 2016; Late Triassic) nests with Early Triassic Czatkowella (also from Poland, Fig. 2) and Early Triassic Prolacerta (Fig. 1), also with gracile limbs and a fused pectoral girdle. Ask first what Prolacerta was able to do. Then discuss Oziimek.
Konietzko-Meier, Tanczuk and Teschner 2021 continue: “Phylogenetically it belongs to “protorosaurians”, however, the elongated limbs and proposed gliding abilities are not known among that group and make Ozimek more similar to Pterosauria, than to any other protorosaurians.”
Ozimek was originally described as a Sharovipteryx-like glider (Fig. 1), but it is not related and Ozimek is not a glider. Gliders need strong limbs, especially for those rough landings. Ozimek does have extremely skinny limbs, but so does Prolacerta (Fig. 1). Pterosaurs are another matter. They are lepidosaurs and therefore not related to protorosaurians like Ozimek.
“Thus, it is interesting, if the unusually for protorosaurians mode of life is also reflected in the bone histology.”
“The goal of that study is to investigate if bones of Ozimek, beside the morphological elongation, show any specific histological adaptations to flying. Two long bones of Ozimek (femur UOPB 1148a and humerus UOPB 1148b) were sectioned to obtain details about histological framework. In both bones, large medullary cavity and thin walls built from lamellar bone, with rare, simple vascular canals are visible. However, the most characteristic feature of cortex are numerous lamellae, which are visible as regular, dense packed rings around the entire section.”
Dont draw any conclusions before examining the bone sections of closely related taxa, like Prolacerta.
“The bones of Ozimek, on the histological level, are more similar to bones of small bats, with compact structure, low or moderate vascularization and slow remodelling, than to birds and pterosaurs with high vascularized fibro-lamellar bones.”
First let us see (or hear about) the histological structure of Prolacerta bones first. Too many wrong assumptions here.
“Probably, bats and also Ozimek grew too slowly to form laminar bone, but compact, low vascularized cortex could be well adapted to deal with high loading, with the simultaneous limitation of the weight. In that case, well ordered collagen fibers in successive lamellae seems to be a key adaptation to better distribution of the stress, originating during the gliding, along the bone.”
Or not. As mentioned earlier, Ozimek is so slender some other possibilities should be considered. It looks ill-equipped to walk, let alone climb or glide or crash. Hard to tell what Ozimek did, but whatever it did involved a minimum of locomotion. Consider a remora-like relationship with a larger sit-and-wait predator as one possibility (FIg. 4). Or, since it looked like a spider…
References Konietzko-Meier D, Tanczuk A and Teschner EM 2021. Mystery histology of the long bones of Ozimek volans — gliding member of later Triassic fauna from Krasiejow. EAVP abstrstracts 2021.
You can recover surprising results when you add taxa and you let them nest wherever they want to. After they settle in, sometimes your cladogram can then tell you where you might be seeing things a little too traditionally. Scoring ‘exceptions’ or apomorphies might instead be scoring mistakes. Take a moment. At least review the scores and see if a possibility might pan out.
Take, for example, the siphuncle of Nautilus, a primitive cephalopod (Fig. 1) with a buccal cirri (= tentacles without suckers) surrounding the oral cavity, similar to hagfish and lancelets (Fig. 3).
The siphuncle is a tubular structure that starts from the posterior of the U-shaped digestive system of the nautilus and extends to the very tip of the coiled portion, tapering unimpeded through all the intervening septa.
Snails don’t have a siphuncle. Coiled snail shells developed by convergence according to the small mollusc tree (SMT, 18 taxa; subset Fig. 4). So, phylogenetically, what is a siphuncle?
The shape and placement of the siphuncle in Nautilus corresponds to the placement of the notochord in hagfish and lancelets where it serves as a stiffening agent, preventing peristalsis of the worm-like body wall, and acting as a springy energy saver for lateral undulations, the sort fish and lancelets use for swimming.
When tiny Nautilus ancestors started stiffening their posterior torso with layers and chambers of external shell (Fig. 3), a notochord was no longer needed for its original internal stiffening purpose. Thereafter the renamed siphuncle was retained to ever so slowly (at the level of molecular exchange) transfer water out of the growing chambers and replacing the liquid with lighter gases, thereby enhancing buoyancy in the shell portion.
According to coronodon.com, the hagfish notochord “is a pressurized tube, consisting of a core cylinder of pressurized. vacuolated cells surrounded by a thin but dense fibrous collagenous sheath. A basement. membrane (a thin membrane of glycoproteins) forms a tube around the core, immediately inside the fibrous sheath.”
Primitive nautiloids, like Trilacinoceras (Fig. 2, Sweet 1958; Ordovician) had a straighter shell with a coiled posterior. Similar taxa had a completely straight tapered shell, always with siphuncle perforations at the axis. So, a straight, conical shape is the primitive shape (Fig. 3).
As usual, this blogpost is presenting research on a day-to-day basis as more taxa and traits are added. As usual, things may change when more taxa and traits are added. Heretical hypotheses, like this one, are posted to encourage criticism. Additional thinking fine tunes rough drafts and weeds out less than parsimonious ideas.
PS. Found this cephalopod eyeball data a day later in Wikpedia Cephalopods, as active marine predators, possess sensory organs specialized for use in aquatic conditions. They have a camera-type eye which consists of an iris, a circular lens, vitreous cavity (eye gel), pigment cells, and photoreceptor cells that translate light from the light-sensitive retina into nerve signals which travel along the optic nerve to the brain. For the past 140 years, the camera-type cephalopod eye has been compared with the vertebrate eye as an example of convergent evolution, where both types of organisms have independently evolved the camera-eye trait and both share similar functionality. Contention exists on whether this is truly convergent evolution or parallel evolution. Unlike the vertebrate camera eye, the cephalopods’ form as invaginations of the body surface (rather than outgrowths of the brain), and consequently the cornea lies over the top of the eye as opposed being a structural part of the eye. Unlike the vertebrate eye, a cephalopod eye is focused through movement, much like the lens of a camera or telescope, rather than changing shape as the lens in the human eye does. The eye is approximately spherical, as is the lens, which is fully internal.
Cephalopods’ eyes develop in such a way that they have retinal axons that pass over the back of the retina, so the optic nerve does not have to pass through the photoreceptor layer to exit the eye and do not have the natural, central, physiological blind spot of vertebrates.
The crystalins used in the lens appear to have developed independently from vertebrate crystalins, suggesting a homoplasious origin of the lens.
Most cephalopods possess complex extraocular muscle systems that allow for very fine control over the gross positioning of the eyes. Octopuses possess an autonomic response that maintains the orientation of their pupils such that they are always horizontal.
Homologous with chordates, parallel with vertebrates, according to the data above and earlier.
References Sweet WC 1958. The Middle Ordovician of the Oslo region, Norway 10. Nautiloid Cephalopods. Norsk Geologisk Tidsskrift38:1-178
Rabit et al. 2021 studied a new Cretaceous hardshell turtle and came up with contradictory conclusions.
From the abstract: “Whether advanced marine adaptations like that of extant sea turtles (Chelonioidea) evolved once or twice in turtles remains unresolved owing to the contested relationships of Protostegidae, a Cretaceous extinct pelagic clade.”
The Rabit et al. 2021 abstract continues: “Fossils of protostegids are globally rare and the absence of species showing a transitional stage between littoral and pelagic adaptation precludes rigorously testing whether this clade is related to extant sea turtles or represents an earlier, convergent marine radiation.”
You don’t have to wait for that transitional fossil to be discovered. Add taxa. Run an analysis. This is exactly why cladograms are useful.
“We report a new protostegid turtle from the Early Cretaceous Aptian Apón Formation of Venezuela based on a single, three dimensionally preserved, near-complete skull. This still unnamed taxon represents one of the oldest protostegids and is characterized by a narrow interorbital space, dorsolaterally oriented and relatively small-sized orbits, anteriorly sloping skull roof, relatively deep lower and upper temporal emarginations, and reduced vomer and cavum tympani. These traits are unlike those of other protostegid or chelonioid sea turtles but approximate the condition seen in freshwater turtles;”
Always wonderful to see new taxa — but wait a minute. The authors say this is “one of the oldest protostegids,” but they also say, “These traits are unlike those of other protostegid or chelonioid sea turtles.” What’s going on here?
“We hypothesize them [= these traits] as plesiomorphic. Parsimony analysis recovers this species as a basal protostegid on the stem-lineage of crown-sea turtles, indicating a single pelagization event during turtle evolution.”
Not a ‘single pelagization event’ according to the LRT. There were at least three entries of turtles into the sea. (Fig. 1). Freshwater turtles are mentioned (above), but snapping turtles are not mentioned in this abstract. Did they miss something? or deliberately holding back? Softshell turtles had their own marine entry, Ocepechelon.
“However, further (less derived) transitional forms are needed to rigorously test the global relationships of Protostegidae. The Venezuelan taxon nevertheless fills a considerable morphological gap in the early evolution of the group, perhaps corresponding to a littoral [=shore dwelling] phase. It represents only the third described protostegid from the Early Cretaceous southwestern Atlantic.”
I wish they hadn’t said, “These traits are unlike those of other protostegid or chelonioid sea turtles” and then concluded they had “the third described protostegid“. Confusing and contradictory.
BTW, the “considerable morphological gap” disappears in the LRT with taxon inclusion. Be careful with unwarranted hyperbole. Looking forward to seeing this specimen and its cladogram published.
Carlisle, Sivestro and Donoghue 2021 report, “Recent molecular clock analyses have suggested that placental mammals originated in the mid- to late Cretaceous, before the Cretaceous-Paleogene (K-Pg) mass extinction. However, there are no unequivocal fossils of placental mammals from the Cretaceous to support this.”
Incorrect. The large reptile tree (LRT, 1890+ taxa; subset Fig. 1) recovers several Cretaceous and Late Jurassic mammals as placentals (Fig. 1). Run your own tests. University textbooks must be behind the times if this is what students and workers believe coming out of Bristol.
Carlisle, Sivestro and Donoghue 2021 continue, “Definitive fossils of placental mammals only appear after the K-Pg boundary, at which point they rapidly radiate leading into the ‘Age of Mammals’.”
This should be the last time this myth is repeated or printed.
“Here we use theBayesian Brownian Bridge modelto estimate the age of origin of placental mammals based on the fossil record. The model uses fossil diversity through time to inform a random walk from the clade’s present-day diversity back to the estimated origin of the clade within a Bayesian framework. This model works well with clades that have poor fossil records, such as the early placental mammals, and does not require a phylogeny, thereby mitigating the lingering uncertainty over the branching pattern at the root of the placental tree of life.”
No reason to estimate. Just create a cladogram. It’s a powerful tool that readily answers many questions like this. Not sure why more students and professionals don’t do this. Leaving issues like this for retired amateurs is no way to run a profession. And don’t borrow someone else’s cladogram. Create your own. You’ll have a powerful tool you can use the rest of your career into retirement.
“Our results support a Cretaceous origin for placental mammals, in agreement with the molecular data, and demonstrate that the group was already present before the K-Pg mass extinction and experienced a radiation during the Paleogene. The Bayesian Brownian Bridge model can therefore help to reconcile paleontological data with molecular data when estimating the origin of clades.”
Who knows what their molecular data recovers? We do know that molecule clades, like Afrotheria and Laurasiatheria, too often deliver false positives. Please use traits, not deep time DNA. The title of this abstract is a wee bit misleading. The authors did not use fossils from Cretaceous and Jurassic strata. They used estimates.
References Carlisle E, Silvestro D and Donoghue P 2021. The origin of placental mammals according to the fossil record. EAVP abstracts 2021.
Cerny and Natale 2021 sought to clarify the ‘time tree’ of shorebirds (= Charadriiformes).
Unfortunately, severe taxon exclusion (Fig. 2) and the use of genes (Fig. 1), rather than traits (Fig. 2), mars this bird study.
From the abstract: “Shorebirds (Charadriiformes) are a globally distributed clade of modern birds and, due to their ecological and morphological disparity, a frequent subject of comparative studies.”
Understatement. The large reptile tree (LRT, 1890+ taxa) indicates the morphological disparity of Charadrius relatives was MUCH greater (Fig. 3) than Cerny and Natale imagined.
From the abstract: “While molecular phylogenies have been instrumental to resolving the suprafamilial back bone of the charadriiform tree, several higher-level relationships, including the monophyly of plovers (Charadriidae) and the phylogenetic positions of several monotypic families have remained unclear.”
If you want to clarify relationships, don’t use molecules. Too often they deliver false positives in deep time studies. Cerny and Natale also used WAY too few taxa based on comparisons to the LRT, which employs a wider gamut of birds in order to minimize taxon exclusion.
“The timescale of shorebird evolution also remains uncertain as a result of extensive disagreements among the published divergence dating studies, stemming largely from different choices of fossil calibrations.”
Use traits. Not molecules.
“Here, we present the most comprehensive non-supertree phylogeny of shorebirds to date, based on a total-evidence dataset comprising 336 ingroup taxa (89% of all extant species), 24 loci (15 mitochondrial and 920 nuclear), and 69 morphological characters.”
That’s what they all say. If the Cerny and Natale study is “the most comprehensive” study, how did these authors manage to omit so many taxa published years earlier (Fig. 2)? They were blinkered (= having their blinders on).
“Our node-dating analyses consistently support a mid-Paleocene origin for the Charadriiformes and an early diversification for most major subclades.”
Probably earlier. We have Mid-Paleocene penguins, so penguin ancestors (Fig. 2) needed time to evolve from more primitive charadriformes.
Note Charadrius (Fig. 2) is a phylogenetically miniaturized version of longer-legged ancestral taxa, like Balearica and Burhinus. Once again, this is neotony at work, creating new taxa, completely overlooked by Cerny and Natale who likely relied on textbooks to determine which birds were traditional charadriiformes and which were not.
Test your textbooks to make sure your textbooks are valid if they repeat results gained from gene studies. This is basic science, so you can find this out for yourself using your own observations. It doesn’t take a PhD or expensive equipment.
References Černý D and Natale R 2021. Comprehensive taxon sampling and vetted fossils help clarify the time tree of shorebirds (Aves, Charadriiformes) bioRxiv 2021.07.15.452585; doi: https://doi.org/10.1101/2021.07.15.452585
From the Rossi et al. 2021 abstract: “Tridentinosaurus antiquus Leonardi 1959 is a nearly complete reptile-like tetrapod (possibly a member of the Protorosauria group) found in the Early Permian volcanic succession in Trentino Alto Adige, Italy.”
From the Rossi et al. 2021 abstract: “Its phylogenetic position is currently uncertain.”
See above. Don’t be lazy. A valid phylogenetic context is essential and, by their own admission, missing from this abstract.
The abstract continues: “Soft tissues are reported in this specimen but their nature remains unclear. The specimen shows a defined black coloured body outline, alluding that most of the soft tissues are organically preserved. In the proximity of the shoulder and pelvic girdle, three-dimensionally preserved integumentary scales are evident; these are relatively small (ca. 1 x 2 mm) and rhomboidal in shape. Our study reveals that the integumentary scales are in fact osteoderms, formed by apatite with a pitted texture; no ultrastructure of the integument is preserved. The body outline and the abdomen are formed by anhedral crystals of apatite coupled with a small amount of carbon.”
And now, the kicker: “We suggest that the body outline and the abdomen have been covered with a layer of black paint (e.g., Bone Black) perhaps to consolidate/protect the specimen. Our findings indicate the absence of soft tissues preserved in T. antiquus but the discovery of small rhomboidal osteoderms uncovers a new biological character that will support future phylogenetic studies of this ancient tetrapod.”
Future? There is a current online phylogenetic study (Fig. 2) into which Tridentinosaurus was added based on bone traits (not soft tissue outlines) back in 2016.
Tridentinosaurus antiquus (Early Permian, Dal Piaz 1932, Leonardi 1959, 26.5cm long; Museum of Paleontology of the University of Padua 26567). Ronchi et al. described the specimen as “a beautiful but biochronologically useless specimen of which only the out−line of the soft tissues is well preserved.” The volcanic sediments in Sardinia occur in Cisuralian / Sakmarian deposits 291 million years old.
References Dal Piaz Gb. 1932 (1931). Scoperta degli avanzi di un rettile (lacertide) nei tufi compresi entro i porfidi quarziferi permiani del Trentino. Atti Soc. Ital. Progr. Scienze, XX Riunione, v. 2, pp. 280-281. [The discovery of the remains of a reptile (lacertide) in tuffs including within the Permian quartz porphyry of Trentino.] Leonardi P 1959.Tridentinosaurus antiquus Gb. Dal Piaz, rettile protorosauro permiano del Trentino orientale. Memorie di Scienze Geologiche 21: 3–15. Ronchi, A., Sacchi, E., Romano, M., and Nicosia, U. 2011. A huge caseid pelycosaur from north−western Sardinia and its bearing on European Permian stratigraphy and palaeobiogeography. Acta Palaeontologica Polonica 56 (4): 723–738. Rossi V et al. 2021. New analyses of the “soft tissues” of the Italian tetrapod Tridentinosaurus antiquus. Insight on taphonomy and conservation history. EAVP abstract 2021.
Recovered in exquisite detail by splitting a small, round, brown nodule, tiny Joermungandr bolti (Fig. 1) was recently described by Mann, Calthorpe and Maddin 2021.
The authors did not describe the skull (Fig. 2) precisely. Nor did they include pertinent sister taxa, like Kirktonecta in their analysis. As a result their cladogram was unable to correctly nest their new discovery. The large reptile tree (LRT, 1890+ taxa) nested tiny Joermungandr correctly and with complete resolution. Rather than assuming expertise, sometimes its better to pretend you don’t know what a taxon is in order to expand your taxon list to minimize the possibility of taxon exclusion. Omitting taxa results in phylogenetic chaos.
From the abstract: “Here, we describe a new long-bodied recumbirostran, Joermungandr bolti gen. et sp. nov., known from a single part and counterpart concretion bearing a virtually complete skeleton. Uniquely, Joermungandr preserves a full suite of dorsal, flank and ventral dermal scales, together with a series of thinned and reduced gastralia. Investigation of these scales using scanning electron microscopy reveals ultrastructural ridge and pit morphologies, revealing complexities comparable to the scale ultrastructure of extant snakes and fossorial reptiles, which have scales modified for body-based propulsion and shedding substrate. Our new taxon also represents an important early record of an elongate recumbirostran bauplan, wherein several features linked to fossoriality, including a characteristic recumbent snout, are present.
Wikipedia reports,“Not all phylogenetic analyses recognize Recumbirostra as a valid grouping.” Worse yet, Kirktonecta is not listed among the Recumbirostra. Worse yet, Joermungandr does not have a recumbi rostrum. If the authors were counting on a certain kind of snout on an elongate taxon, they were “Pulling a Larry Martin” to reduce the number of taxa competing to be sister taxa. Don’t do that.
“We used parsimony phylogenetic methods to conduct phylogenetic analysis using the most recent recumbirostran-focused matrix.
The authors borrowed a cladogram. Don’t do that. Use your own.
“The analysis recovers Joermungandr within Recumbirostra with likely affinities to the sister clades Molgophidae and Brachystelechidae.
The published cladogram (their figure 5) has Synapsida and Eureptilia for outgroup taxa. According to the LRT, those are unrelated to Microsauria. According to the LRT, the authors needed more basal tetrapod outgroup taxa, omitted from the authors’ cladoram.
From the abstract: “Finally, we review integumentary patterns in Recumbirostra, noting reductions and losses of gastralia and osteoderms associated with body elongation and, thus, probably also associated with increased fossoriality.”
That’s nice, but without a valid phylogenetic context, such studies end up a waste of time. Microsaurs are not reptiles (= amniotes; see publicity title below). Microsaurs are basal tetrapods, not far from Reptilomorpha. The only living microsaurs are caecilians, burrowing, heavily scaled and limbless.
References Mann A, Calthorpe AS and Maddin HC 2021.Joermungandr bolti, an exceptionally preserved ‘microsaur’ from the Mazon Creek Lagerstätte reveals patterns of integumentary evolution in Recumbirostra. Royal Society Open Science 8(7):
Molluscs and chordates are getting closer and closer lately. Earlier we looked at nautilus and lancelet similarities. Also earlier we looked at garden slug and hagfish similarities.
Today we look at lancelet (Fig. 1) and clam (Fig. 2) similarities.
At first clams seem odd and inscrutable, but when you simplify their structures (Fig. 2), many previously overlooked similarities to lancelets begin to appear. Lancelets (Fig. 1) have a straight intestine terminating below a terminal tail. Clams also have a terminal tail, but it is traditionally called a foot. In clams the stomach and intestine arch dorsally and terminate dorsal to the ciliated mouth (as in lophophorates), expanding to produce a funnel, as in Nautilus (which has a ventral funnel, likely due to a close, but separate ancestry from the clam).
The phylogenetic origin of the bilateral clam shell remains a mystery at present. Ontogeny (Fig. 3) provides clues. Clam shells develop during the clam’s planktonic (= free-swimming) embryo stage, shortly after feeding commences and prior to settling on or burrowing tail first into the sea floor (like a lancelet, Figs. 1, 2).
All molluscs are traditionally considered protostomates, but note a subtle difference: the traditional protostomate trochophore (= early embryo) has a mouth that appears at the so-called equator (Fig. 3). By contrast the clam mouth appears in the middle of buccal cilia, at the oral pole, as in the lancelet. In clams, as in the nautilus, octopus and starfish, the buccal cilia double as organs of locomotion. This is distinct from lancelets that depend on their tail, not their mouth, to swim and dig. By this evidence, this early stage (Fig. 3) is where the switch from one to another took place, if ontogeny recapitulates phylogeny.
Both clams and lancelets have an atrium for filtering plankton from sea water. Both have a stomach opening posterior to the atrium to collect plankton captured on mucous strands traveling along the atrial walls.
The benthic, burrowing, plankton-feeding lifestyle of a clam (Fig. 2) remains very much like that of its unarmored ancestor, the lancelet (Fig. 1). The armored body looks extremely different, from the outside, but take away the armor and the similarities become more noticeable.
Lancelets came first. They are closer in morphology to their elongate nematode ancestors (Fig. 4) and only develop buccal cilia as they near adulthood. In clams the buccal cilia appear early in embryology and take over as swimming organs. Timing is everything. And it looks more and more like the traditional phylum Mollusca is polyphyletic, like traditional diapsids, protorosaurs, pterodactyloids, turtles and whales.
This, too, appears to be a novel hypothesis of interrelationships. If not please provide an earlier citation so I can promote it here.
References Mansfield JH, Halaler E, Holland ND and Brent AE 2015. Development of somites and their derivatives in amphioxus, and implications for the evolution of vertebrate somites. EvoDevo 6(21): DOI 10.1186/s13227-015-0007-5
The comparison seems obvious now The origin of the nautilus links back to the early chordate lancelet (Fig. 1). Details follow.
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 asdeuterostomes.” (More on this issue below).
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.”
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.
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: