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 

Last common ancestor of hemichordates YouTube video

Here’s a YouTube video featuring
Dr. Karma Nanglu (Smithsonian National Museum of Natural History) showing and discussing Cambrian taxa from the Burgess Shale ancestral to living hemichordates, pterobranchs (= graptolites and kin) and enteropneusts = acorn worms).

Nanglu reports, 
“This talk will guide you through a series of recent studies using Burgess Shale fossils that shine a light on hemichordate origins, one of the most mysterious parts of the animal tree of life. These exceptional fossils reveal unanticipated combinations of morphological and ecological characteristics early in the history of this animal group, including surprising combinations of those found in their modern relatives.”

Unfortunately, when it came time to present last common ancestors at an hour into the presentation, Nanglu left much of the work undone. His graphic showed question marks at the ancestral nodes (Fig. 1).

Figure 2. Frame from Nanglu talk on YouTube (see above) showing question marks on his cladogram of chordate/hemichordate origins.

Figure 1. Frame from Nanglu talk on YouTube (see above) showing stars and question marks on his cladogram of chordate/hemichordate origins.

By contrast and thirty years ago
Peters 1991 found hemicordates arose from basal chordates (Fig. 3) like the lancelet, Branchiostoma (Fig. 2), itself derived from nearly featureless roundworms (Fig. 3). You might recall that adult lancelets are sessile feeders, anchoring themselves tail first into sandy and muddy substrates, distinct from their free-swimming tiny hatchlings that more greatly resemble tiny fish in their activity. All this occurred during the Cambrian.

Distinct from chordates,
sessile (= essentially immobile) pterobranchs emphasize and enlarge the suspension feeding cirri made sticky with mucous strands (Fig. 3).

Distinct from chordates,
worm-like enteropneusts emphasize the rostrum (= proboscis, Fig. 3).

Both hemichordates
gave up the chevron-shaped swimming muscles and internal gill basket found in lancelets and fish. However, enteropneust hatchlings present a vestigial post-anal tail that is resorbed or transformed in adults.

Figure 2. Extant lancelet (genus: Amphioxus) in cross section and lateral view. The gill basket nearly fills an atrium, which intakes water + food, sends the food into the intestine and expels the rest of the water.

Nanglu 2021 confirms this 30-year-old hypothesis of interrelationships
(Fig. 3) as he nests chordates basal to hemichordates and echinoderms.

Nanglu also presents
a tube-building, vermiform last common ancestor between pterobranchs and enteropneusts, with post-anal attachment and possible tube building. In Peters 1991 pterobranchs are basal to crinoids, blastoids and other echinoderms, taxa that further emphasize and enlarge the gracile cirri that encircles the mouth of lancelets until the cirri comprise the entire anatomy of the starfish. So starfish are walking on their greatly enlarged and elaborate mouth parts, having given up or absorbed the rest of the ancestral lancelet anatomy.

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

We looked at chordate origins
in more detail earlier here (summarized in Fig. 3).


References
Peters D 1991. From the Beginning – The story of human evolution. Wm Morrow.
Romer AS 1971. The Vertebrate Body – Shorter Version 4th ed. WB Saunders.

wiki/Acorn_worm
wiki/Pterobranchia
wiki/Hemichordate

 

 

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.

 

Drepanolepis, an Early Devonian ‘fork-tail’ fish, enters the LRT

I dreaded this one, fearing its weirdness…
but after a little DGS coloring (Fig. 1) came to realize this tiny furcacaudiforme (Early Devonian fork-tail fish?) was just like another fish already in the large reptile tree (LRT, 1677+ taxa), only shorter and narrower. After analysis the two nested together.

Figure 1. Drepanolepis, traced from Wilson and Caldwell 1998, has a ventral oral cavity and nests with Birkenia in the LRT.

Figure 1. Drepanolepis, traced from Wilson and Caldwell 1998, has a ventral oral cavity and nests with Birkenia in the LRT. I was surprised to see that ventral oral cavity.

Drepanolepis maerssae (Wilson and Caldwell 1993, 1998; Early Devonian; 2cm in length) is a traditional ‘thelodont’ and a member of the Furcacaudiformes (forked tails). In the LRT Drepanolepis is derived from Birkenia (Fig. 2), but with a taller, shorter, more angelfish-like body. They both have a ventral mouth and a hypocercal tail, somewhat elaborated in Drepanolepis with several posterior processes. The gill atrium remains quite large and the nasal extends from the orbit down to the oral cavity, which remains like that of a lancelet, without jaws. Without jaws there is no premaxilla, maxilla quadrate, articular, angular and dentary.

Birkenia is not traditionally considered a thelodont.
All other traditional thelodonts, like Thelodus and Loganiella, have a low, wide morphology. Thelodus has a ventral oral cavity and nests with osteostracans and sturgeons in the LRT. Loganiella has a wide terminal mouth and nests with whale sharks and mantas in the LRT. Drepanolepis and other furcacaudiformes do not nest with these traditional thelodonts in the LRT and should no longer be considered thelodonts.

Figure 2. Birkenia in situ with precursor facial bones labeled. This Middle Silurian taxon is basal to Furcacaudiformes and all other vertebrates.

Figure 2. Birkenia in situ with precursor facial bones labeled. This Middle Silurian taxon is basal to Furcacaudiformes and all other vertebrates.

Birkenia and furcacaudiformes 
bridge the gap between lancelets and gnathostomes. The have the body and bones of a basal fish, but retain a lancelet-like oral cavity that cannot be called a proper mouth. That comes later.

FIgure 1. Birkenia in situ and diagrams.

FIgure 3. Birkenia in situ and diagrams. Note the hypocercal tail as in Drepanolepis (Fig. 1).

By the Early Devonian
fish had evolved to such an extent that some had lobefins (the sarcopterygians) and others had exoskeletons (the placoderms).

Let’s talk about the traditional ‘terminal mouth’ of Furcacauda.
Wilson and Caldwell 1998 restored a terminal mouth on Furcacauda (Fig. 4) but did so by guessing. The skull is missing from the specimen (Fig. 4). No other furcacaudiformes have a terminal mouth (Figs. 5,6). All other furcacaudiformes (Figs. 1,5,6) described and figured by Wilson and Caldwell have an overlooked ventral oral cavity, like that of Birkenia. In addition I rotated one image, that of Sphenonectris (Fig. 6), to bring the dorsal side to the top for proper orientation.

It has been 22 years since Wilson and Caldwell 1998 was published.
Perhaps in the meantime someone else has noticed these issues. If so, let me know and I will promote that citation.

Figure 4. Furcacauda fredholmae specimen in situ along with Wilson and Caldwell 1998 diagram imagining a face and terminal mouth for this taxon. No other sisters have a terminal mouth.

Figure 4. Furcacauda fredholmae specimen in situ along with Wilson and Caldwell 1998 diagram imagining a face and terminal mouth for this taxon. No other sisters have a terminal mouth.

As in lancelets
the oral cavity of Birkenia and furcacaudiformes can never close and is surrounded by oral cirri that work as sand filters.

Figure 5. Pezzopallichthes has a ventral oral cavity (green circle).

Figure 5. Pezzopallichthes has a ventral oral cavity (green circle).

As in Birkenia
precursors to tetrapod skull bones can be found in furcacaudiformes for the first time phylogenetically. Even so, the short, narrow Furcacaudiformes displayed here today are members of a terminal clade with no descendants later than the Devonian. Closer sisters to Birkenia evolved to become basal vertebrates (fish and tetrapods).

Figure 6. Sphenonectris with facial bones colored. Here the oral elements are displaced. The orbit is close to the anterior margin as in Drepanolepis (Fig. 1).

Figure 6. Sphenonectris with facial bones colored. Here the oral elements are displaced. The orbit is close to the anterior margin as in Drepanolepis (Fig. 1).

We looked at Birkenia
and the origin of dermal facial bones from splintery scales earlier here. using similar DGS methods.


References
Traquair RH 1898. Report on fossils fishes. Summary of Progress of the Geological Survey of the United Kingdom for 1897: 72-76.
Wilson MVH and Caldwell MW 1993. New Silurian and Devonian fork-tailed ‘thelodonts’ are jawless vertebrates with stomachs and deep bodies. Nature. 361 (6411): 442–444.
Wilson MVH and Caldwell MW 1998. 
The Furcacaudiformes, a new order of jawless vertebrates with thelodont scales, based on articulated Silurian and Devonian fossils from northern Canada. Journal of Vertebrate Paleontology 18 (1): 10-29.

wiki/Thelodus
wiki/Birkenia
wiki/Drepanolepis
wiki/Furcacaudiformes

Gill chambers in basal chordates and vertebrates, pt. 1

Adding taxa without skulls
to the the large reptile tree (LRT, 1611+ taxa) would seem to bring with it a slew of problems. As a solution, the scores “skull absent”, “orbit absent” and “jaws absent” were added last weekend to several of the traits created eight years ago.

A primitive – hypothetical – chordate,
just developing a coelum (middle tissue between skin and intestine) is shown here (Fig. 1). It is not a taxon in the LRT, but serves as a zero point from which derived traits can be added in the present report. Think of it as a worm stiffened longitudinally with a notochord.

No one knows the size of its gill chamber. Early members of this clade likely lacked gills. Oxygen would have been absorbed both externally and internally on this tiny wriggling worm made up of not much more than skin over intestine with a notochord ventral to the dorsal central nerve chord. Tiny food particles would have been processed sometime during the trip from mouth to anus/cloaca. Few to no sensory organs appeared near the oral opening (not quite a mouth yet).

Figure 1. Hypothetical chordate ancestor to known chordates. This is the starting point for looking at gills and throats in basal chordates and vertebrates.

Figure 1. Hypothetical chordate ancestor to known chordates. This is the starting point for looking at gills and throats in basal chordates and vertebrates. The ‘actual size’ is also hypothetical.

A primitive extant chordate,
the lancelet (genus: Branchiostoma; Fig. 2) documents the next stage in chordate evolution and serves as the new outgroup taxon for the LRT. Still lacking a head or anterior sensory organs, Branchiostoma has ring sets of cilia both outside and inside the oral cavity. These are followed by a large atrium / gill chamber lined with slender gill bars for gas exchange. Ventrally a stiff rod, the endostyle, creates a mucous strand that captures food and, using microscopic cilia, carries the particles posteriorly to the simple, linear intestine which terminates in an anus / cloaca, no longer at the tip of the tail (see Fig. 1). Water from the atrium is expelled from a single ventral opening, the atriopore, anterior to the anus/cloaca. The swimming muscles that envelope Branchiostoma from tip to tip evolve to a chevron shape.

Figure 2. Extant lancelet (genus: Amphioxus) in cross section and lateral view. The gill basket nearly fills an atrium, which intakes oxygen, water + food, sends the food into the intestine and expels the rest of the water. It also lacks a head and anterior sensory organs.

A Middle Cambrian ‘lancelet with eyes,’ aka primitive fish,
Metaspriggina (Fig. 3), apparently loses the cilia and atriopore of the lancelet (Fig. 2) and develops seven gill openings lateral to the relatively smaller gill chamber. Those tiny eyes both direct this tiny predator to tinier prey and serve to locate predators, setting off alarms that spur the tail to wiggle seeking a hiding place or to put distance between it and any approaching marauder.

Figure 1. An early jawless, finless, lancelet-like fish from the Cambrian, Metaspriggina. Compare the placement of the eyes here with Birkenia in figures 2 and 3.

Figure 3. An early jawless, finless, lancelet-like fish from the Cambrian, Metaspriggina. Compare the placement of the eyes here with Birkenia in figures 2 and 3.

Evolving in a different direction
the tunicate (Fig. 4) is mobile only as a juvenile. It become sessile (attached to the seafloor) as an adult. The tail is resorbed and the gill chamber takes over the majority of the body. Traditional workers consider this stage primitive to the lancelet, but they don’t employ a simple worm-like chordate as an outgroup taxon. Tunicates are quite derived relative to lancelets.

FIgure 4. A tunicate diagram turned on its side to replicate the morphology of the lancelet (Fig. 2). Here the atriopore enlarges to become a exit siphon. The cilia are reduced. The gill chamber is greatly enlarged.

FIgure 4. A tunicate diagram turned on its side to replicate the morphology of the lancelet (Fig. 2). Here the atriopore enlarges to become a exit siphon. The cilia are reduced. The gill chamber is greatly enlarged.

Sessile tunicates gave rise to
barrel-shaped, planktonic  tunicates, otherwise known as salps (Fig. 5). They seem simple, but that’s because they have gotten rid of nearly every body part but the atrium / gill chamber. Salps alternate between sexual and asexual generations, each distinct in morphology. The atrium comprises the entire animal. Gonads, an endostyle, simple brain and digestive organs all migrate nside the atrium. The atriopore (exit siphon) rotates opposite to the entrance siphon, creating a little jet engine. Several morphologies have evolved (Fig. 5).

FIgure 5. Salp variety. Here the atrium is the animal with other organs inside the atrium.

FIgure 5. Salp variety. Here the atrium is the animal with other organs inside the atrium. Color helps link homologous elements.

Traditional cladograms
of chordate relationships (Fig. 6) nest tunicates basal to fish. Here both fish and tunicates are derived from lancelets, each evolving in different directions (mobile vs. sessile + planktonic). Tunicates have simplified and degenerated with fewer parts, distinct from the general trend in vertebrate evolution.

Figure 3. Traditional cladogram from Lingham-Soliar 2014.

Figure 3. Traditional cladogram from Lingham-Soliar 2014. Missing from this cladogram are the lancelets. The LRT does not agree with this tree topology.

The above boneless, headless and finless taxa
are all filter feeders with large gill chambers, as are several primitive fish (= armored and bony lancelets) in the LRT. We’ll look at those in future blogposts.