Hagfish and nematodes side-by-side in detail for the first time

Summary for those in a hurry
After this comparison, nematodes and hagfish need to be added to the base of the vertebrate/ echinoderm/ deuterostome family tree as outgroup taxa. In other words, hagfish are big nematodes with a notochord. And in turn, so are we.

Figure 1. The hagfish Myxine in vivo patrolling the sea floor.

Figure 1. The hagfish Myxine in vivo patrolling the sea floor. Note the nematode-like tentacles surrounding the mouth end at lower left.

Hagfish (clade: Myxini)
are very low on the vertebrate family tree. According to Wikipedia, They are the only known living animals that have a skull but no vertebral column, although hagfish do have rudimentary vertebrae.”

With origins in the Cambrian or Ediacaran,
we know of only one fossil hagfish, Gilpichthy greenei (Bardack and Richardson 1977, FMNH PE18703, 5cm; Fig. 2) from the famous Mazon Creek Formation, Late Carboniferous, 307 mya.

Figure x. Gilpichthys, a Pennsylvanian hagfish, enlarged and full scale.

Figure 2. Gilpichthys, a Pennsylvanian hagfish, enlarged and full scale.

Without vertebrae,
the Atlantic hagfish (genus Myxine, Linneaus 1758, 50cm, other genera up to 127cm) nest between Vertebrata and more basal taxa. (Not yet added to the LRT).

Outgroup taxa include
lancelets and nematodes (= round worms).

Yesterday
one of those insightful bells rung when I realized nematodes have eversible teeth made of keratin, as in hagfish. Something obvious had, once again, been overlooked. Peters 1991 listed nematodes as vertebrate ancestors based on overall morphology. Hagfish were not included then.

Now
let’s see what other details link nematodes to hagfish, a relationship overlooked by all prior authors, probably due to the great size difference (most nematodes are <2.5mm long), or perhaps due to taxon exclusion. According to Wikipedia, “Taxonomically, they [nematodes] are classified along with insects and other moulting animals in the clade Ecdysozoa,”

Figure x. Nematodes and hagfish side-by-side, focusing on the eversible mouth parts and keratin teeth.

Figure 3. Nematodes and hagfish side-by-side, focusing on the eversible mouth parts and keratin teeth.

Classification
According to Wikipedia, “The classification of hagfish had been controversial. The issue was whether the hagfish was a degenerate type of vertebrate-fish that through evolution had lost its vertebrae (the original scheme) and was most closely related to lampreys, or whether hagfish represent a stage that precedes the evolution of the vertebral column (the alternative scheme) as is the case with lancelets. Recent DNA evidence has supported the original scheme.”

We have learned time and again, you can never trust DNA evidence, especially when taxon exclusion is in play. Instead, look at the traits of the taxa under study. And look at lots of taxa to make sure none of them share more traits.

Smithsonian Magazine listed 14 (edited to 7) fun facts about hagfish.

  1. Hagfishes live in cold waters around the world, from shallow to 1700 m.
  2. Hagfish can go months without food.
  3. Hagfish can absorb nutrients straight through their skin.
  4. Hagfish have two rows of tooth-like structures made of keratin they use to burrow deep into carcasses. They can also bite off chunks of food. While eating carrion or live prey, they tie their tails into knots to generate torque and increase the force of their bites.
  5. No one is sure whether hagfish belong to their own group of animals, filling the gap between invertebrates and vertebrates, or if they are more closely related to vertebrates.
  6. The only known fossil hagfish, [Gilphichthys, above] looks modern.
  7. Hagfish produce slime. When harassed, glands lining their bodies secrete stringy proteins that, upon contact with seawater, expand into the transparent, sticky slime.
Figure x. Illustration of a nematode with labels.

Figure 4. Illustration of a nematode with labels from corodon.com. This model has been based on the fresh-water nematode Ethmolaimus. Compare to the hagfish in figure 1.

How does the hagfish compare to an aquatic nematode?

  1. Tail — The post-anal region forms a tail in both
  2. Mucus — Moens et al. 2005 report, “Many aquatic nematodes secrete mucus while moving.” The authors did not mention hagfish, which are famous for mucus. Some nematodes also exude adhesive from post-anal, tail tip glands.
  3. Sensory tentacles — The mouth is in the centre of the anterior tip and may be surrounded by 6 lip-like lobes in primitive marine forms, three on each side. Primitively the lips bear 16 sensory papillae or setae.
  4. Burrowing into their prey — Both hagfish and nematodes attach their lips to larger prey, make incisions and pump out the prey’s contents with a muscular pharynx.
  5. Swimming — In water nematodes swim by a graceful eel-like motion as they throw their stiff but elastic bodies into sinusoidal curves by contracting longitudinal muscles (the elasticity of the cuticle and hydrostatic skeleton more or less returns the body to its original straight shape). The notochord in the hagfish gives the same sort of elasticity to the famously wriggly body capable, as in nematodes, to form corkscrews and knots.
  6. Niche — Nematodes represent 90% of all animals on the ocean floor, not counting hagfish. Both play important roles in dead vertebrate decomposition.
  7. Embryo development — An alternative way to develop two openings from the blastopore during gastrulation, called amphistomy, appears to exist in some animals, such as nematodes.
  8. Size –– some species of hagfish and nematode reach 1m in length, though most nematodes are <2.5mm
  9. Eyes — A few aquatic nematodes possess what appear to be pigmented eye-spots, but most are blind. So are hagfish.
  10. Reproduction — Usually male and female, sometimes hermaphroditic
  11. Tough skin and subcutaneous sinus — largely separated from underlying tissue

Evolution from nematode to hagfish

  1. Head — radially symmetrical evolves to bilaterally symmetrical
  2. Mouth — three or six lips with teeth on inner edges reduced to two
  3. Skin and skeleton — Hydroskeleton and cuticle evolve to notochord and ‘eelskin’
  4. Nerve chord —Dorsal, ventral and lateral in nematodes, reduced to just dorsal in hagfish
  5. Brain – circular nerve ring in nematodes, dorsal concentration in hagfish

Pikaia gracilens
(Walcott 1911, Middle Cambrian, Fig. Z) has been compared to lancelets and hagfish. Like hagfish, Pikaia retained twin tentacles, but also had cirri instead of rasping eversible teeth.

Figure z. Pikaia gracilens from Mallatt and Holland 2013 showing hagfish and lancelet affinities.

Figure z. Pikaia gracilens from Mallatt and Holland 2013 showing hagfish and lancelet affinities.

Added 24 hours later
as the question of mouth and anus origin from the original blastopore (Fig. zz) arises again in the comments section.

Figure z. Blastopre evolution to produce an anus and mouth at the same time in a marine nematode. This is the transitional taxa from protostome nematodes to deuterostomes.

Figure zz. Blastopre evolution to produce an anus and mouth at the same time in a marine nematode. This is the transitional taxon from protostome nematodes to deuterostomes. This is how it happened. This is how it was ignored in many Western textbooks.

Malakov 1997 writes,
“The blastopore initially has a spherical Caenorhabtitis sp. (Ehrenstein & Schierenberg, shape, but then stretches to become an elongated 1980). oval-shape (Fig. 2). Subsequent development results Embryogenesis in enoplids appears to have several in the lateral edges ofthe blastopore approaching and u.nusual features. Firstly, variability occurs in the eventually connecting with the centre. Two openblastomere arrangement in the stages of early cleavings, one at the anterior end the other at tl1e posterior age. At the four-cells stage various configurations end of the embryo, are persistent remnants of the have been observed, viZ., tetrahedral, rhombic, Tblastopore. The anterior opening provides the beginshaped. These configurations have been variously ning of the definitive mouth, and the posterior one, encountered in the development of nematodes bethe definitive anus.”

See figure z (above). Hagflish and vertebrates arose form marine nematodes exhibiting this form of early cell division. This is how deuterostomes arose.

Malakov 1997 reports, “From these results it may be concluded that enoplids represent an early evolutionary branch, which seperated (sic) from the ancestral nematode stem prior to all other groups of nematodes.”

Figure x. Medial section of Acipenser (sturgeon) larva with temporary teeth from Sewertzoff 1928.

Figure 5. Medial section of Acipenser (sturgeon) larva with temporary teeth from Sewertzoff 1928. Note this specimen has marginal teeth and deeper teeth.

Getting back to baby sturgeon teeth…
Several months ago I cited Sewertzoff 1928 (Fig. 5) who found tiny teeth in the tiny lava of the large sturgeon, Acipenser. Those tiny teeth disappear during maturity, as you might recall. The question is: are those teeth homologs of keratinous hagfish + nematode teeth? Or homologs of enamel + dentine shark and bony fish teeth? McCollum and Sharpe  2001 in their review of the evolution of teeth reported, “The aim of this review is to see what this developmental information can reveal about evolution of the dentition.”

Unfortunately McCollum and Sharpe 2001 delivered the usual history of citations that indicate teeth started with sharks, overlooking sturgeon, nematode, lamprey and hagfish teeth. Phylogenetic bracketing indicates that baby sturgeon teeth are keratinous, not homologous with dentine + enamel shark teeth, which phylogenetically evolve later, first in sharks and later retained by bony fish. Let me know if this is incorrect.

Figure 3. Ventral view of the GLAHM V830 specimen of Thelodus. This appears to have fang-like teeth, but these may be sharp cilia. The mandible appears to be a dead end experiment convergent with the mandible of all other vertebrates.

Figure 6. Ventral view of the GLAHM V830 specimen of Thelodus. This appears to have fang-like teeth, but teeth are too soon. These are barbels = cirri.

Sturgeon barbels:
Are they homologs of hagfish + nematode barbels? Soft tissues, like barbels, are unlikely to fossilize, but one intervening bottom-dwelling taxon, Thelodus (Fig. 6), preserves barbels anterior to the ventral oral opening. Open water thelodonts do not preserve barbels. Catfish barbels appear to be a reversal because a long line of more primitive taxa do not have barbels. The same can be said of the catfish-mimic eel ancestor, the cave fish Kryptoglanis.

The relationship between hagfish and nematodes
should have been known for decades, but apparently this hypothesis of interrelationships has been overlooked, ignored or set to the side until now. If someone else recovered this hypothesis of interrelations previously, let me know so I can promote that citation.


References
Bardack D and Richardson ES Jr 1977. New aganathous fishes from the Pennsylvanian of Illinois. Fieldiana Geology 33(26):489–510.
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.
Malakov VV 1998. Embryological and histological peculiarities of the order Enoplida, a primitive group of nematodes. Russian Journal of Nematology 6(1):41–46.
Mallatt J and Holland ND 2013. Pikaia gracilens Walcott: stem chordate, or already specialized in the Cambrian? Journal of Experimental Zoology, Part B, Molecular and Developmental Evolution 320B: 247-271.
McCollum M and Sharpe PT 2001. Evolution and development of teeth. Journal of Anatomy 199:153–159.
Moens T et al. (6 co-authors) 2005. Do nematode mucus secretions affect bacterial growth? Aquatic Microbial Ecology 40:77–83.
Morris CS, Caron JB 2012. Pikaia gracilens Walcott, a stem-group chordate from the Middle Cambrian of British Columbia. Biological Reviews 87: 480-512.
Nielsen C, Brunet T and Arendt D 2018. Evolution of the bilaterian mouth and anus. Nature Ecology & Evolution 2:1358–1376.
Nielsen C 2019. Blastopore fate: Amphistomy, Protostomy or Dueterostome. In eLS (eds) John Wiley & Sons Ltd.  DOI: 10.1002/9780470015902.a0027481
Peters D 1991. From the Beginning – The story of human evolution. Wm Morrow.
Sewertzoff AN 1928. The head skeleton and muscles of Acipenser ruthensus. Acta Zoologica 13:193–320.

wiki/Hagfish
wiki/Nematode
wiki/Pikaia
cronodon.com/BioTech/Nematode.html
pterosaurheresies.wordpress.com/2020/08/07/chordate-origins-progress-since-romer-1971/
Hagfish Day, occurs every year on the third Wednesday of October:
smithsonianmag.com/science-nature/14-fun-facts-about-hagfish-77165589/

Hagfish YouTube video 

Didazoon and Vetulicola: Early Cambrian larvacean and echinoderm ancestors

Urochordates and larvaceans are the relatives
we almost never talk about. Here those long, lost relationships are reestablished.

Didazoon haoae (Figs. 1, 2; Shu et al. 2001; ELI 0000197–0000217) was described as a member of the Ventulicolia, an “enigmatic phylum” of Cambrian deuterostomes of otherwise uncertain affinities. It was a simple, barrel-shaped animal with a line of gill openings dotting a large atrium followed by a down-angled tail. There was no head.

From the Diagnosis:
“Bipartite, cuticularized body, anterior segmented region and voluminous mouth, ventral margin ¯attened, widens posteriorly. On either side the anterior bears fve circular structures, in the form of a cowl with posteriorly directed opening and basin-like interior, apparently connected to interior. A prominent constriction separates dorsal region of anterior from posterior section. The latter, composed of seven segments, tapers in both directions, with rounded posterior termination. Internal anatomy includes alimentary canal, possibly voluminous in anterior, and in posterior narrow intestine, straight or occasionally coiled. Dark strand located along ventral side of anterior section, possibly representing endostyle.”

Figure 1. Didazoon in situ and reconstructed. The 'intestine' may be a notochord remnant because it extends to the tail tip, distinct from Metaspringgina (Fig. 2). The tail was down in all larvaceans.

Figure 1. Didazoon in situ and reconstructed. The ‘intestine’ may be a notochord remnant because it extends to the tail tip, distinct from Metaspringgina (Fig. 2). The tail was down in all larvaceans.

Comparisons to MetasprigginaArandaspis and larvacea
(Fig. 2) were not made by Shu et al. 2001. Here the ‘cuticularized body’ is transitional between Branchiostoma (Fig. 2) and extant larvacea (Fig. 2 which share several traits with these vetulicolians.

In all these taxa, the tail hangs down
and the mouth opens slightly ventrally, beneath what was the rostrum in lancelets. Prior reconstructions at Wikipedia have the tail up and the animal upside-down.

Figure 2. Branchiostoma, Metaspriggina, Didazoon, Vetulicola, Arandaspis and Larvacea to scale. Note the presence of an atrial pore/posterior atrial opening in Branchiostoma and the vetulicolians while Metaspriggina and Arandaspis had fish-like gill openings. The tail of larvaceans hangs down.

Figure 2. Branchiostoma, Metaspriggina, Didazoon, Vetulicola, Arandaspis and Larvacea to scale. Note the presence of an atrial pore/posterior atrial opening in Branchiostoma and the vetulicolians while Metaspriggina and Arandaspis had fish-like gill openings. The tail of larvaceans hangs down.

According to Wikipedia,
Vetulicola (Figs. 2, 3) “is the eponymous member of the enigmatic phylum Vetulicolia, which is of uncertain affinities, but may belong to the deuterostomes. Vetulicola cuneata could be up to 9 cm long (shown at only 6cm in Fig. 2).

Earlier we looked at the parallel development of the enlarged gill chamber in craniates and gnathostomes like the manta ray (Manta) and whale shark (Rhinchodon) that also fed on plankton, but on a much larger scale.

Figure 3. Vetulicola in situ and flipped in vivo configuration.

Figure 3. Vetulicola in situ and flipped in vivo configuration. The shape is transitional to both the larvacea (Fig. 2) and to the crinoid (Fig. 5). Note the four-part separation of the mouth parts, like a budding flower.

According to Wikipedia,
“Vetulicola’s taxonomic position is controversial. Vetulicola cuneata was originally assigned to the crustaceans on the assumption that it was a bivalved arthropod like Canadaspis and Waptia, but the lack of legs, the presence of gill slits, and the four plates in the “carapace” were unlike any known arthropod.”

“Shu et al. placed Vetulicola in the new family Vetulicolidae, order Vetulicolida and phylum Vetulicolia, among the deuterostomes. Shu (2003) later argued that the vetulicolians were an early, specialized side-branch of deuterostomes.”

“Dominguez and Jefferies classify Vetulicola as an urochordate, and probably a stem-group appendicularian (= Larvacea, Tunicata). Like a common tunicate larva, the adult Appendicularia have a discrete trunk and tail.”

“In contrast, Butterfield places Vetulicola among the arthropods.”

“The discovery of the related Australian vetulicolian Nesonektris, from the Lower Cambrian Emu Bay Shale of Kangaroo Island, and the reidentification of the “coiled gut” of vetulicolians as being a notochord affirms the identification as an urochordate.”

Figure 5. Lavacea diagram showing the tadpole organism and the house it builds from cellulose and protein. This is a highly derived organism, based on two parts that originated in the Cambrian with Vetulicola.

Figure 4. Lavacea diagram showing the tadpole organism and the house it builds from cellulose and protein. This is a highly derived organism, with origins in the Cambrian with Vetulicola and close to the salp taxa in figure 3,. Here the larvacea retains the swimming muscles and notochord that extends to the tail tip.

According to Wikipedia:
“Larvaceans have greatly improved the efficiency of food intake by producing a test, which contains a complicated arrangement of filters that allow food in the surrounding water to be brought in and concentrated prior to feeding. By regularly beating the tail, the larvacean can generate water currents within its house that allow the concentration of food. The high efficiency of this method allows larvaceans to feed on much smaller nanoplankton than most other filter feeders.”

“The immature animals resemble the tadpole larvae of ascidians, albeit with the addition of developing viscera. Once the trunk is fully developed, the larva undergoes “tail shift”, in which the tail moves from a rearward position to a ventral orientation and twists 90° relative to the trunk. Following tail shift, the larvacean begins secretion of the first house.”

There are no other chordates that shift the tail 90º relative to the trunk and build an oceanic house of cellulose and protein. Nevertheless Vetulicolia is not a new phylum, but a transitional set of taxa linking lancelets to larvacea and to crinoids. Vetulicolians (Figs. 1–3) demonstrate the down-angled tail appeared in the Early Cambrian pointing to an Ediacaran origin for lancelet-like chordates. That means previously unknown, more active, less benthic (= sea floor) niche full of free-swimming chordates was also present in the Ediacaran.

Figure 7. Vetulicola compared to a fossil crinoid. Note the splitting of the mouth parts into separate 'arms' has only just begun here. The crinoid stalk is the segmented 'tail' of Vetulicola.

Figure 5. Vetulicola compared to a fossil crinoid. Note the splitting of the mouth parts into separate ‘arms’ has only just begun here, starting with four. The crinoid stalk is the segmented ‘tail’ of Vetulicola.

When lips become arms… and tails become stems…
A clade of vetulicolians (former lancelets) also developed a down-hanging ‘tail’ (= stem) that ultimately evolved to become a holdfast: the crinoids within the echinoderms. Remember, adult lancelets also plant their tails in the substrate to become sessile plankton feeders. Crinoids are the ancestors of starfish (Fig. 6), a clade that flips, mouth-side down, and concurrently loses the tail (= stem). Not all echinoderms have a pentagonal morphology. The mouth of Vetucolia has four slightly separating mouth parts. The atrium and tail have just a trace of homologous armor, ringed in the case of the tail and stem.

Figure 3. Chordate evolution, changes to Romer 1971 from Peters 1991. Here echinoderms have lost the tail and gills of the free-swimming tunicate larva.

Figure 6. Chordate evolution, changes to Romer 1971 from Peters 1991. Here echinoderms have a stem-like tail with a holdfast, later lost in starfish, similar to the tail and gills of the free-swimming tunicate larva and adult larvaceans.

Garcia-Bellido et al. 2014 concluded
“Phylogenetic analyses resolve a monophyletic Vetulicolia as sister-group to tunicates (Urochordata) within crown Chordata. The hypothesis suggests that a perpetual free-living life cycle was primitive for tunicates. Characters of the common ancestor of Vetulicolia + Tunicata include distinct anterior and posterior body regions – the former being non-fusiform and used for filter feeding and the latter originally segmented – plus a terminal mouth, absence of pharyngeal bars, the notochord restricted to the posterior body region, and the gut extending to the end of the tail.” 

Garcia-Bellido et al. nested tunicates and vetulicolians as sisters to craniates, both derived from lancelets (cephalochordates). That seems to be correct based on the above figures in their tail-down orientation.

Cameron, Garey and Swalla (2000) reported,
“The nesting of the pterobranchs within the enteropneusts dramatically alters our view of the evolution of the chordate body plan and suggests that the ancestral deuterostome more closely resembled a mobile worm-like enteropneust than a sessile colonial pterobranch.”

Actually Peters 1991 (Fig. 6) published that hypothesis earlier… without the use of DNA.

Now we have long-sought evidence in the form of homologous traits that link vetulicolians to chordates (Fig. 2) and to echinoderms (Fig. 5). All have been understood as deuterostomes. The connection between larva was indicated earlier, according to Cameron, Garey and Swalla 2000. Now we have previously overlooked adult homologs to study and compare. This may be a novel hypothesis of interrelationships. If not, please provide a citation so I can promote it here.

Vetulicolians will not enter the LRT.
They diverge from the vertebrate lineage while keeping some basal lancelet traits and feeding patterns. The degree of their divergence indicates an ancient split from lancelets + fish, just as the divergence of starfish indicate a similar ancient split from lancelet deep time ancestors. They no longer resemble lancelets, except for their cirri-lined mouth parts and attendant mucous strands.


References
Aldridge RJ et al. (4 co-authors 2007. The systematics and phylogenetic relationships of vetulicolians. Palaeontology. 50 (1): 131–168.
Cameron CB, Garey JR and Swalla BJ 2000. Evolution of the chordate body plan: New insights from phylogenetic analyses of deuterostome phyla. PNAS 97(9):4469–4474.
Garcia-Bellido DC et al. 2014. A new vetulicolian form Australia and its bearing on the chordate affinities of an enigmatic Cambrian group. BMC Evolutionary Biology 2014, 14:214 http://www.biomedcentral.com/1471-2148/14/214
Peters D 1991. From the Beginning – The story of human evolution. Wm Morrow.
Shu D-G et al. (8 co-authors) 2001. Primitive deuterostomes from the Chengjiang Lagerstätte (Lower Cambrian, China) Nature 414:419–424.

wiki/Didazoon
wiki/Vetulicolia
wiki/Vetulicola
wiki/Larvacea

.https://www.youtube.com/watch?v=ZXCOZ2_blb8

Chordate origins: Progress since Romer 1971

Added a few days after posting:
There was a spirited discussion in the comments section (below) regarding the origin of the mouth and anus in the proposed ancestor of chordates: a round worm (Fig. 2). The most recent progress on this subject can be found in the following citations:

https://en.wikipedia.org/wiki/Embryological_origins_of_the_mouth_and_anus
It includes a third possibility, ‘amphistomy” which is a new word for me, and the pertinent comment: “An alternative way to develop two openings from the blastopore during gastrulation, called amphistomy, appears to exist in some animals, such as nematodes.”

And here is a citation for a 2019 paper that seems to sum things up to date: https://onlinelibrary.wiley.com/doi/abs/10.1002/9780470015902.a0027481


When the great paleontologist of the ’30s to early ’70s,
Alfred S. Romer, wrote his widely disseminated abridged textbook, The Vertebrate Body – Shorter Version (1971) he proposed the following scenario (Fig. 1) for the origin of chordates (animals with a notochord).

Figure 1. From Romer 1971 the origin of chordates.

Figure 1. From Romer 1971 the origin of chordates. Note the impossibly complex basal taxon, the sessile arm-feeder at bottom. Here wriggling is highly derived. Compare to figure 2.

Unfortunately
Romer 1971 started with a notoriously complex stalked ‘sessile arm feeder’ (Fig. 1), way more derived than the simple and tiny flat worms, ribbon worms and round worms that were the most evolved animals at the time. Since these worms were soft-bodied they left few to no fossils in pre-Cambrian and pre-Ediacaran strata.

Figure 3. Chordate evolution, changes to Romer 1971 from Peters 1991. Here echinoderms have lost the tail and gills of the free-swimming tunicate larva.

Figure 2. Chordate evolution, changes to Romer 1971 from Peters 1991. Here echinoderms have lost the tail and gills of the free-swimming tunicate larva.

Things changed with Peters 1991
who traced the human lineage from molecules to cells to worms and vertebrates. The round worm, a deuterostomate (mouth arising anew opposite the existing anus, Fig. 4), was put forth in that lineage as a placeholder taxon arising from more primitive flatworms and microscopic organisms. Then this wriggling “intestine wrapped in a layer of skin” (Fig. 2), developed a mesoderm and a notochord to restrict telescoping and so became a primitive chordate. This served as the first step toward swimming in the laterally undulating manner of lancelets and fish.

Added the day of publication when reader CB and Wikipedia both note nematodes are protostomates, not deuterostomates: That’s important to document and perhaps the reason why extant nematodes (roundworms) were never considered before. Given that only chordates, hemichordates and echinoderms are currently considered deuterostomes, we have to ask, which roundworm-like and ribbonworm-like taxa preceded these three derived clades? None, according to current thinking. None. That can’t be possible based on the need to get from a flatworm (mouth and anus are the same) to a lancelet. At this stage the roundworm (mouth on one end, anus on the other) serves as a model ancestor for all higher taxa. The solution to this problem: Either deuterostomate roundworm-types all became extinct, or one kind of protostomate roundworm became a deuterostomate, or deuterostomate roundworms are still around, but remain untested regarding their embryology. Thanks for bringing this fact to the surface. Here’s a problem that needs a better solution than we have now.

By contrast,
in Romer’s chart (Fig. 1), wriggling comes last.

In Peters 1991
sessile and free-floating forms, including those without an undulating tail, evolved far and away from simple notochord worm and lancelet bauplans. Some emphasized developing the cilia and mouth parts (echinoderms), while others emphasized developing the atrium (tunicates). These retained a primitive mobile wriggling bauplan as juveniles (betraying their ancestry, not their future), then metamorphosed into sessile adults.

Figure 2. Chordated evolution from Rychel et al. 2005.

Figure 3. Chordated evolution from Rychel et al. 2005.

Back to Academia… Rychel et al. 2005
likewise started with a benthic worm “with gill slits and acellular gill cartilages” (see Fig. 2 from Peters 1991), but were less clear in showing how complex and distinct starfish, tunicates, etc. evolved directly from that simple form. More to their focus, Rychel et al. 2005 write: “Chordates evolved a unique body plan within deuterostomes and are considered to share five morphological characters,

  1. a muscular postanal tail,
  2. a notochord,
  3. a dorsal neural tube,
  4. an endostyle,
  5. and pharyngeal gill slits.

No extant echinoderms share any of the chordate features, so presumably they have lost these structures evolutionarily. Hemichordates and cephalochordates, or lancelets, show strong similarities in their gill bars, suggesting that an acellular cartilage may have preceded cellular cartilage in deuterostomes. Our evidence suggests that the deuterostome ancestor was a benthic worm with gill slits and acellular gill cartilages.”

Figure 5. From Peters 1991 a diagram splitting deuterostomates from protostomates.

Figure 4. From Peters 1991 a diagram splitting deuterostomates from protostomates.

More recently, Satoh et al, 2014 write:
“Although the origin and evolution of chordates has been studied for more than a century, few authors have intimately discussed taxonomic ranking of the three chordate groups themselves.

“Accumulating evidence shows that echinoderms and hemichordates form a clade (the Ambulacraria), and that within the Chordata, cephalochordates diverged first, with tunicates and vertebrates forming a sister group. Chordates share tadpole-type larvae containing a notochord and hollow nerve cord, whereas ambulacrarians have dipleurula-type larvae containing a hydrocoel.

“We propose that an evolutionary occurrence of tadpole-type larvae is fundamental to understanding mechanisms of chordate origin.”

Satoh et al, 2014 discuss the four hypotheses then circulating. 

(1) “The paedomorphosis scenario: was the ancestor sessile or free-living?”

(2) “The auricular hypothesis – According to this view, the pterobranch-like, sessile animals with dipleurula (auricularia-like) larvae led to the primitive ascidians (as the latest common ancestor of chordates) through morphological changes both in larvae and adults.”

(3) “The inversion hypothesis – Recent debates on the origin of chordate body plans have focused most attention on inversion of the dorsal–ventral (D-V) axis of the chordate body, compared with protostomes”

(4) The aboral-dorsalization hypothesis – “Embryological comparison of cephalochordates with nonchordate deuterostomes suggests that, because of limited space on the oral side of the ancestral embryo, morphogenesis to form the neural tube and notochord occurred on the aboral side of the embryo (the side furthest from the mouth).“Namely, the dorsalization of the aboral side of the ancestral embryo may have been a key developmental event that led to the formation of the basic chordate body plan.”

Some of these recent hypotheses are still stuck in Romer’s world
of 50 years ago, replicating ‘primitive sessile arm feeders’ at the expense of ignoring wriggling round worms.

So, round worm exclusion has been going on for at least that long.
Is it because textbooks rule? New ideas are too expensive to bring to the classroom? Nobody questions the professor? No one else is thinking about this issue?

Funny that no competing hypotheses
consider the extremely simple, primitive, wriggling, telescoping, round worm as a suitable starting point in chordate origins. Evidently in 1991 this was heretical and remains so today. I thought, at the time, it was just being logical.

Next time, in Academia,
let’s start with a simple round worm, then discuss how a mesoderm and notochord developed incrementally over deep time as some roundworms evolved to become chordates and their kin back in the Cambrian or earlier.


References
Peters D 1991. From the Beginning. The story of human evolution. Wm Morrow, Morrow Jr Books, New York. FromTheBeginning book.pdf
Romer AS 1971. The Vertebrate Body – Shorter Version 4th ed. WB Saunders.
Rychel AL, Smith SE, Shimamoto HT and Swalla BJ 2005. Evolution and development of the chordates: collagen and pharyngeal cartilage. Molecular Biology and Evolution 23(3): 541–549. https://academic.oup.com/mbe/article/23/3/541/1110188
Satoh N 2008. An aboral-dorsalization hypothesis for chordate origin. Genesis 46(11):614-22
Satoh N, Rokhsar D and Nishikawa T 2014. Chordate evolution and the three-phylum system. Proceedings of the Royal Society B Biological Sciences. https://doi.org/10.1098/rspb.2014.1729

Origin of chordates webpage

Somites = bilaterally paired blocks of paraxial mesoderm along the head-to-tail axis in segmented animals.

 

Fish cladogram: Cambrian period to the present day

When one layers established time periods
over the fish portion of the large reptile tree (LRT, 1673+ taxa; Fig. 1) the surprising length of certain ghost lineages and the ability of several clades to survive several hundred million years becomes apparent.

Figure 1. Subset of the LRT focusing on basal vertebrates (= fish). Colors indicate time periods. This chart documents the lack of fossils for several clades and genera in the Silurian and Devonian.

Figure 1. Subset of the LRT focusing on basal vertebrates (= fish). Colors indicate time periods. This chart documents the lack of fossils for several clades and genera in the Silurian and Devonian.

The antiquity of Silurian members in the highly derived lungfish clade
(Guiyu and Psarolepis) helps one understand the coeval Silurian appearances of so-called primitive fish, like acanthdians and placoderms (Entelognathus). Traditional cladograms assumed early taxa must be more primitive, not realizing that phylogenetic analysis indicates a vast undiscovered radiation of taxa in the Silurian (Fig. 1). Most of these are still waiting to be discovered.

What do Silurian and Early Devonian fossil fish in the LRT have in common?
Many were flat bottom dwellers with small eyes.

By contrast, coeval spiny sharks had large eyes and were free-swimmers. Even so they lost their flexible fin rays, they lost large teeth, they kept a large mouth, and they had vestigial skeletons. Such traits are associated today with slow-moving deep sea fish.

So known Silurian fish were not open sea visual predators with great swimming skills. Their ecological absence must have a reason. I wonder if such taxa were gobbled up before they could drift to muddy or silty anoxic regions of the sea floor where they could wait undisturbed to be buried for fossilization? Even a few exceptions are lacking. Very puzzling…

According to Google:
“In North America geologic activity over the last 417 million years has removed or covered up most Silurian rocks. Well-preserved fossils from Silurian reefs can be found in the Great Lake States of Minnesota, Wisconsin, Michigan, and Illinois.” So Silurian exposures are comparatively rare.

How do left column fish differ from right column (Fig. 1) fish?
As a general rule (allowing for many exceptions) left column fish do not appear to be the fast, open water swimmers seen in the majority of primitive right column fish in the Silurian and Devonian. It is noteworthy that not one taxon in the right column has a Silurian through Permian representative. I will add them as they come to my attention. It is also noteworthy that the left column has very few living representatives. I count nine.

Traditional cladograms
put more emphasis on time and exclude extant taxa. That’s why traditional cladograms often nest spiny sharks and placoderms near the base of the basal vertebrates, prior to sharks and bony fish. And they attempt to add tube-feeding sturgeons somewhere in the middle of bony fish. In the LRT taxon exclusion is minimized and more natural evolutionary patterns are recovered based on phenomics (traits).

Some previously unrecognized relationships recovered by the LRT include:

  1. The wide radiation of clades in the Silurian.
  2. Devonian taxa take us rapidly to tetrapods, documented by Middle Devonian tracks
  3. Note the proximity of Silurian lobefins to Viséan (Early Carboniferous) tetrapods, including reptiles.
  4. Note the unbalanced fossil record with regard to the major dichotomy splitting bony fish
  5. Proamia is known from the Devonian while a sister taxon, Amia, is known from extant taxa, separated by 360 million years. This is the closest we get to a right column fish fossil in the Silurian or Devonian.
  6. The time span between tiny Silurian Loganiella and giant extant sisters Rhincodon + Manta is about 430 million years.
  7. A similar time span splits Hemicyclaspis from living sturgeons.
  8. A longer time span (~500 my) splits Branchiostoma from its Cambrian precursors.
  9. When comparing the LRT to traditional cladograms, check to make sure they have similar outgroup taxa. Too often taxon exclusion is an unaddressed issue in those papers, which make them fitting subjects for the next few blogposts.

Cautionary note:
The choosing of fish taxa for the LRT has not been random, but was made on the basis of availability and possible importance. At present the fossil record is skewed toward left column fish prior to the Permian. As more taxa are discovered and added, the subjective second reason will hopefully pale to become less of a factor.

 

What Sparked The Cambrian Explosion?

Off topic, I know…
but a Nature article (Fox 2016) highlighting Ediacaran and Cambrian fossil fauna sparked this quick blog post.

Missing from the published hypothesis 
are all the various unfossilized flat, ribbon and round worm morphologies that were precursors to the various deuterostomates and protostomates that are found in Cambrian sediments. You can read more about those in my 1991 book, From the Beginning, pdf here).

References
Fox D 2016 (journalist). What sparked the Cambrian explosion? Nature 7590 pdf

Metaspriggina walcotti, a 500 million year old finless, jawless fish

Not a reptile, but we’ll make room for this one:
The biggest news in paleontology in the last six months (IMHO) is not the discovery, but the redescription of Metaspriggina as a VERY basal Burgess Shale (Cambrian) fish (chordate) without fins, without jaws, but with two anterodorsal eyes (Fig.1). Metaspriggina is considered to represent a primitive chordate, transitional between cephalochordates and the earliest vertebrates.

Figure 1. Diagram of Metaspriggina, the basalmost chordate. Lectotype –USNM198612 and former holotype 198611 in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.

Figure 1. Diagram of Metaspriggina, the basalmost chordate. Lectotype –USNM198612 and former holotype 198611 in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA. This free-swimming “worm” is close to the ancestry of vertebrates, including humans, ultimately.

From the abstract:
“This primitive fish displays unambiguous vertebrate features: a notochord, a pair of prominent camera-type eyes, paired nasal sacs, possible cranium and arcualia, W-shaped myomeres, and a post-anal tail. A striking feature is the branchial area with an array of bipartite bars. Apart from the anterior-most bar, which appears to be slightly thicker, each is associated with externally located gills, possibly housed in pouches.”

Figure 2. A hypothetical basal fish based on a lamprey larva and the cephalochordate, Amphioxus. Image lifted from "From the Beginning, the Story of Human Evolution".

Figure 2. A hypothetical basal fish based on a lamprey larva and the cephalochordate, Amphioxus. Image lifted from “From the Beginning, the Story of Human Evolution”. Such a guess is pretty easy based on phylogenetic bracketing. The tail is off. It should have come much later.

 

Set aside by Walcott for further study, the two known specimens of this species were briefly examined by Conway Morris (1979). Simonetta and Insom (1993) described one of the two specimens (the original holotype specimen) as a potential relative of the Ediacaran organism Spriggina (Eidacaran segmented worm), whereas the second specimen (now the lectotype) was interpreted as a potential chordate. A chordate interpretation for both specimens was proposed (Janvier, 1998; Smithet al., 2001) and a detailed redescription was eventually instigated by Conway Morris (2008) with both specimens being included in the same genus and species.

Figure 2. A variety of basal fish from the Ordovician, Silurian, Devonian and the present day (for the lamprey larva).

Figure 2. A variety of very basal fish from the Ordovician, Silurian, Devonian and the present day (for the lamprey larva). The fish head is hypothetical.

I was not aware of similar specimens coming out of early Cambrian China. Apparently Metaspriggina was close to Haikouichthys and Myllokunmingia (Fig. 3).

Figure 3. A clade of finless chordates that became armored.

Figure 3. A clade of finless chordates that became armored.

And of course, Pikaia, the Cambrian cephalochordate, is also from the Burgess Shale.

Let’s remind ourselves at this point that urochordates and other sessile organisms have no place in the lineage of vertebrates other than as renegade cousins of lancelets that stopped chasing after food and let food come to them.

References
Conway Morris S and Caron, J-B 2014. A primitive fish from the Cambrian of North America”. Nature (London: Nature Publishing Group). doi:10.1038/nature13414. ISSN 0028-0836.
Conway Morris, S 2008. A Redescription of a Rare Chordate, Metaspriggina walcotti Simonetta and Insom, from the Burgess Shale (Middle Cambrian), British Columbia, Canada”. Journal of Paleontology (Boulder, CO: The Paleontological Society) 82 (2): 424–430. doi:10.1666/06-130.1. ISSN 0022-3360. Retrieved 2014-06-13.

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