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 

Anteosaurus: a killing machine? No.

Benoit et al. 2021
“reconstruct Anteosaurus [Figs. 1-4] as an agile terrestrial predator based on the enlarged fossa for the floccular lobe of the cerebellum and semicircular canals of the inner ear.” 

Yes, that’s what they wrote. Not the teeth. Not the limbs. The inner ear.

Figure 6. Anteosaurus scale model.

Figure 1. Anteosaurus scale model.

From the Material and Methods section:
“The disarticulated skull (BP/1/7074) of a juvenile Anteosaurus magnificus from the middle Permian of the South African Karoo…”

The BP/1/7074 specimen (Figs. 2–4) does not nest with Anteosaurus (Figs. 1–4) in the Therapsid Skull Tree (TST; Fig. 3) as we learned earlier here after testing (Fig. 3).

Figure 2. Kruger et al. 2017 figure 21. provided "Ontogenetic changes in the skull of Anteosaurus; A. juvenile; B, intermediate sized; C, adult sized, redrawn from Kammerer 2011. Their figure 20 labeled the intermediate sized skull as Titanophoneus. So this is a phylogenetic series, not an ontogenetic one.

Figure 2. Kruger et al. 2017 figure 21. provided “Ontogenetic changes in the skull of Anteosaurus; A. juvenile; B, intermediate sized; C, adult sized, redrawn from Kammerer 2011. Their figure 20 labeled the intermediate sized skull as Titanophoneus. So this is a phylogenetic series, not an ontogenetic one.

 

Figure 3. Therapsid Skull Tree with herbivorous clades colored.

Figure 3. Therapsid Skull Tree with herbivorous clades colored.

Figure 1. Anteosaurus magnifies compared to the smaller and coeval BP/1/7074 specimen others considered a juvenile. Other more closely related specimens in the TT are also shown alongside BP/1/7074 specimen.

Figure 4. Anteosaurus magnifies compared to the smaller and coeval BP/1/7074 specimen others considered a juvenile. Other more closely related specimens in the TT are also shown alongside BP/1/7074 specimen.

Phylogenetic context is paramount and required.
There are several problems with the conclusions of Benoit et al. 2021.

  1. The specimen is not Anteosaurus.
  2. Dinocephalians were all herbivores.
  3. Those huge teeth were as sharp as bananas, like hippo teeth.
  4. The bones of the inner ear do not determine whether you are predator or prey.

What is going on
at the universities nowadays?? Is phylogenetic analysis old-fashioned? Let’s get back to basics.


References
Benoit J, Kruger A, Jirah S, Fernandez V and Rubidge BS 2021. Palaeoneurology and palaeobiology of the dinocephalian therapsid Anteosaurus magnificus. Acta Palaeontolgocia Polonica 66(X):xxx-xxx. https://doi.org/10.4202/app.00800.2020

https://phys.org/news/2021-03-prehistoric-machine-exposed.html?fbclid=IwAR0ZhQ_5_Qgn4LscUgK0rUVcUv8XmwDAdtKdDXJJ6pHNG3M3UAjvRpw9d-s

Evolution and synonyms of the hyomandibular and intertemporal

A major issue still facing paleontology and comparative anatomy
is the different names given to homologous bones in fish, reptiles and mammals. For example:

  1. the hyomandibular of fish is the stapes in tetrapods;
  2. the sphenotic in fish is the intertemporal in basal tetrapods, the prootic + opisthotic in reptiles and mammals;
  3. in fish the supraoccipital is the postparietal in stem tetrapods. That bone splits transversely to produce a postparietal and a supraoccipital in reptiles (Fig. 9);
  4. sometimes the jugal, lacrimal, nasal, maxilla and other bones also split into two or more bones. Other times they fuse together;
  5. some bones do not appear until later, de novo or by the product of a split;
  6. likewise, marginal teeth appear, disappear, fuse, unfuse, become more complex and simpler during evolution.
  7. … and that’s not counting the bones that have been traditionally mislabeled (Fig. 10).

From the genesis of the vertebrate skeleton
in Middle Silurian Birkenia (Fig. 1), a tiny hyomandibular articulates with the intertemporal dorsally and the tiny quadrate ventrally. The hyomandibular, a former dorsal gill arch segment, would ultimately evolve to become the most robust bone in the architecture of certain basal bony fish (Fig. 2) before shrinking in stem tetrapods (Fig. 6), ultimately becoming the stapes in basal reptiles (Fig. 9), and a tiny middle ear bone in mammals and humans.

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

Figure 1. Birkenia in situ with precursor facial bones labeled. This Middle Silurian taxon is basal to Furcacaudiformes and all other vertebrates.

In the first fish with jaws,
Chondrosteus (Fig. 2) the hyomandibular pivots to thrust the jaws forward during a bite, an action originated in tube-mouth osteostracans and sturgeons.

Figure 1. Chondrosteus animation (2 frames) in situ and reconstructed in lateral view. This is the transitional taxon linking sturgeons to bony fish + sharks.

Figure 1. Chondrosteus animation (2 frames) in situ and reconstructed in lateral view. This is the transitional taxon linking sturgeons to bony fish + sharks.

In the paddlefish ancestor,
Tanyrhinichthys (Fig. 3), the hyomandibular (deep green again) is no longer as mobile.

Figure 2. Tanyrhinichthys face after color tracing.

Figure 2. Tanyrhinichthys face after color tracing.

The hyomandibular becomes a massive immobile element
in the Early Devonian bony fish and spiny shark  Homalacanthus (Fig. 4). It continues to link the intertemporal with the quadrate.

Figure 4. Homalacanthus in situ and reconstructed.

Figure 4. Homalacanthus in situ and reconstructed. The massive hyomandibular is dark green.

In the fish portion
of the large reptile tree (LRT, 1710+ taxa; Fig. x) we’ve just crossed the major dichotomy separating stem lobefins (many of which are still ray fins) from stem frog fish + mudskippers, sea robins and tripod fish, which also use their pectoral fins to walk along the sea floor. (Let’s save that bit of interest for another blogpost).

Figure x. Subset of the LRT, focusing on fish for July 2020.

Figure x. Subset of the LRT, focusing on fish for July 2020.

Just across the dichotomy,
the tiny (3cm) spiny shark, Mesacanthus (Fig. 5) has a slender hyomandibular with forked tips. Thereafter the hyomandibular is largely covered up by cheek bones.

Figure 1. Early Devoniann Mesacanthus in situ. This 3 cm fish is a typical acanthodian here traced using DGS methods and reconstructed. Distinct from other spiny sharks, this one lacks large cheek plates, as in the extant Notopterus (Fig. 3).

Figure 5. Early Devoniann Mesacanthus in situ. This 3 cm fish is a typical acanthodian here traced using DGS methods and reconstructed. The hyomandibular is dark green.

In the stem tetrapod and large osteolepid,
Eusthenopteron (Fig. 6) the hyomandibular (dark green) attaches to a largely submerged intertemporal (yellow-green) with little dorsal exposure. The quadrate (red) contact with the hyomandibular is only tentative.

Figure 5. Eusthenopteron hyomandibular (dark green) still linking a largely submerged intertemporal (yellow-green) and a small quadrate (red).

Figure 6. Eusthenopteron hyomandibular (dark green) still linking a largely submerged intertemporal (yellow-green) and a small quadrate (red). Here the pterygoid (dark red) is essentially vertical, distinct from most tetrapods (e.g. Figs. 7-9).

In the flattend skull of a basal tetrapod, like
Laidleria (Fig. 7), the hyomandibular / stapes is horizontal and the intertemporal does not have a dorsal exposure. The quadrate connection is broken as the stapes contacts the small posterior tympanic membrane.

Figure 6. Early tetrapod Laidleria. The intertemporal disappears from the dorsal skull and the hyomandibular / stapes dark green)  is oriented horizontally here without a quadrate connection.

Figure 7. Early tetrapod Laidleria. The intertemporal disappears from the dorsal skull and the hyomandibular / stapes dark green)  is oriented horizontally here, perhaps without a quadrate connection, but note the extent of the stapes in palate view vs. occiput view.

In the aquatic reptilomorph,
Kotlassia (Fig. 8), the hyomandibular / stapes is tiny and oriented dorsolaterally in contact with a large tympanic membrane filling a posterior notch. The intertemporal reappears on the dorsal surface of the skull and expands internally to form the paraoccipital process (opisthotic).

Figure 7. The reptilomorph, Kotlassia, skull. Note the reappearance of the intertemporal here called the prootic. The hyomandibular / stapes is tiny and dark green.

Figure 8. The reptilomorph, Kotlassia, skull. Note the reappearance of the intertemporal here called the opisthotic in occipital view. The hyomandibular / stapes is tiny and dark green. The stapes contacts the tympanic membrane laterally.

In the basal and fully terrestrial archosauromorph,
Paleothyris (Fig. 9), the intertemporal is no longer exposed on the dorsal surface, but is exposed in occipital view, where it is called the opisthotic. The otic notch is now absent as the eardrum is reduced and relocated posterior to the jaw hinge. The former robust hyomandibular continues thereafter to shrink, becoming more sensitive to eardrum vibrations enabling a greater range of sound frequencies to be transmitted to the inner ear and brain.

Figure 8. The early archosauromorph, Paleothyris. Here the hyomandibular / stapes is oriented ventrolaterally. The intertemporal is not exposed dorsally.

Figure 9. The early archosauromorph, Paleothyris. Here the hyomandibular / stapes is oriented ventrolaterally. The intertemporal is not exposed dorsally, only occipitally where it is called the opisthotic.

On a slightly different subject:
bone misidentification by Thomson 1966

has been something of a problem ever since that publication. Here (Fig. 10) are the original bone IDs along with revised IDs on separate frames. Principally the relabeled intertemporal and parietal move behind the dorsal braincase division (Fig. 11).

Figure 2. Ectosteorhachis skull from Thomson 1966 with layers to show the brain case and palatoquadrate. Some bones are relabeled in the revised view.

Figure 10. Ectosteorhachis skull from Thomson 1966 with layers to show the brain case and palatoquadrate. Some bones are relabeled in the revised view. Note the intertemporal becomes the prootic + opisthotic at this point.

Thomson 1966 erred
when he put these elements anterior to the split, probably in order to locate the pineal opening between the parietals, which is typical of tetrapods. In osteolepids and their kin the pineal opening is between the relabeled frontals anterior to the transverse cranial split (Fig. 11).

Figure 11. Eusthenopteron and Osteolepis with skull bones relabeled.

Figure 11. Eusthenopteron and Osteolepis with skull bones relabeled.

Why is this so?
Under this new labeling system the contact between the intertemporal and hyomandibular is maintained (Figs. 6, 10). Outgroups to these taxa, like Cheirolepis (Fig. 12) likewise run a portion of the postorbital over the orbit, separating the postfrontal from the orbit margin. Now the ostelepids follow that trait despite the two-part postorbital.

Figure 11. Cheirolepis is an outgroup taxon to the ostelepids that includes a postorbital that extends over the orbit, separating the postfrontal from the orbit margin.

Figure 12. Cheirolepis is an outgroup taxon to the ostelepids that includes a postorbital that extends over the orbit, separating the postfrontal from the orbit margin.

In earlier posts
on hyomandibular evolution. and juvenile Eusthenopteron (Fig. 13; Schultze 1984) corrections have now been made. This bit of relabeling is a new hypothesis awaiting confirmation from others. At present phylogenetic bracketing (Fig. 12) supports this interpretation.

Figure 1. Eusthenopteron juvenile in situ from Schultze 1984. Large plate ventral to the mandible overlaps a convex ventral margin. The quadratojugal is not labeled here. Several bones are re-labeled here.

Figure 1. Eusthenopteron juvenile in situ from Schultze 1984. Large plate ventral to the mandible overlaps a convex ventral margin. The quadratojugal is not labeled here. Several bones are re-labeled here.

For those interested,
these changes affected only 4 character traits out of 238. These scoring changes did not affect the tree topology.


References

Schultze H-P 1984. Juvenile specimens of Eusthenopteron foordi Whiteaves, 1881 (Osteolepiform Rhipidistian, Pisces) from the Late Devonian of Miguasha, Quebec, Canada. Journal of Vertebrate Paleontology 4(1):1–16.
Thomson KS 1966. The evolution of the tetrapod middle ear in the rhipidistian-amphibian transition. American Zoologist 6:379–397.
Westoll TS 1943. The hyomandibular of Eusthenopteron and the tetrapod middle ear. Transactions of the Royal Society B 131:393–414.

In Memorium: paleontologist Robert L. Carroll

Figure 1. Robert L. Carroll in his younger days.

Figure 1. Robert L. Carroll in his younger days.

Robert L. ‘Bob’ Carroll (1938-2020):
a warm-hearted, kind, and knowledgeable professor, always eager to answer a question.

Earlier, we looked at the impact of his major work from 1988, the textbook ‘Vertebrate Paleontology.’ That ‘must-have’ volume was a prime resource for many students and professors for decades. Some considered it ‘The Bible’ of our profession.

We all enter science to make a contribution. Carroll made his in small and large ways, not only by describing and illustrating many of his own discoveries, but by working with others to bring them all together between book covers in the pre-cladistic era. His work will remain on our library shelves. ReptileEvolution.com was built on that foundation and stands on the shoulders of this giant.


References
Use key word “Carroll” to see the index of all the taxa RL Carroll helped describe and covered in this blogpost.

A few days later this link goes into detail on RL Carroll’s career.

Headline: “Vertebrate palaeontologist who recognized and described the oldest known ancestor of all reptiles birds and mammals; the origins of terrestrial vertebrates, the origin of various amphibians such as frogs and salamanders.” 

Subhead: “Any high-school kid can go out and make fossil discoveries.”

Caveat: Some of those hypotheses have been superseded by more recent discoveries (e.g. “Hylonomus lyelli, shown here, is the oldest known reptile (315 million years)”… “Another paleontological mystery: where did turtles come from? Nobody knows.”)

Sharks and sturgeons: fish with a lateral temporal fenestra

When you add the mako shark
(genus: Isurus) to the large reptile tree (LRT, 1460 taxa) it nests with the sturgeon, (genus: Pseudoscaphirhynchus, Fig. 2) close to the bottom. Both are derived from the placoderm, Entelognathus.

FIgure 1. The mako shark (Isurus oxyrinchus) skull and skeleton. Note the confluent lateral temporal fenestra separated from the orbit by a tiny postorbital.

FIgure 1. The mako shark (Isurus oxyrinchus) skull and skeleton. Note the confluent lateral temporal fenestra separated from the orbit by a tiny postorbital. Photo copyright © Sebastian Enault, originally published by Science Connected and used with permission. https://www.gotscience.org/2015/12/preserving-soft-skeleton-backs-without-bones/ Bone colors added here.

Long separated in prior cladograms
sharks have a skeleton of cartilage, several gill openings and are fast swimmers. Meanwhile sturgeons have bone in their skeleton, a single gill opening and tiny jaws for slow motion bottom feeding.

Now would be a good time to remember
that sharks and rays are traditional relatives and rays, like sturgeons, have a small mouth beneath a flat ultra sensitive rostrum ideal for bottom feeding.

Figure 2. The small sturgeon Pseudoscahirhynchus skull in several views. Note the perforated rostrum (nasal) sensitive to prey hiding in mud. The mouth is reduced to a tiny sucking tube disconnected from the quadrate. Even so, this sturgeon nests with sharks in the LRT. Yellow, green and blue insert highlights the premaxilla, maxilla and mandible here reduced to a support a bottom-feeding extendable tube disconnected from the quadrate.

Figure 2. The small sturgeon Pseudoscahirhynchus skull in several views. Note the perforated rostrum (nasal) sensitive to prey hiding in mud. The mouth is reduced to a tiny sucking tube disconnected from the quadrate. Even so, this sturgeon nests with sharks in the LRT. Yellow, green and blue insert highlights the premaxilla, maxilla and mandible here reduced to a support a bottom-feeding extendable tube disconnected from the quadrate.

Which came first?
Both are extant taxa and no more primitive sisters have been tested so far. Even so, the shark retains strong jaws, like those of succeeding (descendant) taxa, but the sturgeon is a flattened bottom dweller, like stem tetrapods.The last common ancestor, Entelognathus, had both of these traits.


Pseudoscaphirhychus kaufmanni (Nikolskii 1900) is the extant Amu darya sturgeon. Distinct from traditional cladograms, the LRT nests this sturgeon next to the placoderm, Entelognathus, among tested taxa. Both have weak jaws and no teeth, bony armor and (presumeably) a shark-like heterocercal tail, despite the 425 million year difference. Note the confluent orbit and lateral temporal fenestra.


Isurus oxyrinchus (Rafinesque 1810; 3.2m in length) is the extant shortfin mako shark here nesting with the sturgeon, Pseudoscaphirhynchus, contra tradition. Here the nasal is extended to form a rostrum. The lacrimal, premaxilla, maxilla, jugal, squamosal and quadrate are all fused together. The postorbital is fused to the postfrontal. A deep valley divides the skull. The orbit is confluent with the lateral temporal fenestra. The premaxillary teeth are posteriorly oriented. The jugal produced a lateral flange for increased muscle attachment. The skeleton is cartilaginous.


We have a more generalized/plesiomorphic
late-surviving Jurassic sturgeon ancestor, Chondrosteus acipenseroides (Agassiz 1843, 1 meter length; Fig. 3) and it has jaws midway in size between the two above taxa. Skull bones are reduced from the primitive state in the Late Silurian placoderm, Entelognathus. Chondrosteus has not been added to the LRT yet.

Figure 3. The sturgeon ancestor, Chondrosteus, overall and focused on the skull. Note the reduction of several skull bones and the general shark-like appearance.

Figure 3. This untested sturgeon  ancestor, Chondrosteus  reduces several skull bones and has a general shark-like appearance.

In a few days we’ll look at
‘the oldest articulated chndrichthyan,’ Doliodus, along with another Devonian shark, Cladoselache to see where they nest in the LRT. Chondrichthyans include living sharks, skates, rays and chimaeras.


References
Nikolskii AM 1900. Pseudoscaphirhynchus rossikowi, n. gen, et spec. Ann. Mus. Imp. Sci. St. Petersburg 4, 257–260 (text in Russian).
Rafinesque CS 1810. Caratteri di alcuni nuovi generi e nuove specie di animali e piante della sicilia, con varie osservazioni sopra i medisimi. Per le stampe di Sanfilippo: Palermo, Italy. pp. 105, 20 fold. Pl., online

wiki/Pseudoscaphirhychus
wiki/Sturgeon
wiki/Shortfin_mako_shark
wiki/Chondrosteus