From jawless fish to toothless jaws: Hemicyclaspis to Chondrosteus

Figure 1. Top to bottom: Hemicyclaspis, an extensively armored ostracoderm. Thelodus a soft jawless fish with a ventral oral opening and gill slits, retaining diamond-shaped armor laterally. Acipenser brevirostrum, a short-listed sturgeon with a protrusible tube for a mouth and reduced armor. Chondrosteus, a fish with jaws, but no marginal teeth.

Adding jawless fish
to the large reptile (LRT, 1611+ taxa) sheds new light on the origin of jaws and the basic topology at the base of the LRT.

Osteostraci,
like Hemicyclaspis (Fig. 1), have a ventral opening at the front of ventral surface of the skull, similar to their ancestors, like Birkenia, which retain lancelet-like cilia surrounding the oral opening. Perhaps Hemicyclaspis did, too. The oral cavity is poorly preserved.

Thelodus
(Fig. 1) was crushed to a thin film with a ventral exposure. Here the round lacrimal and angular oral opening is highlighted. The lateral armor (green) is barely ossified.

Sturgeons,
like Acipenser (Figs. 1, 2), have a longer rostrum and a posterior tube mouth. The maxilla and dentary are not yet present. Those bones grow teeth. Teeth are not present. Neither are the bones that grow them. So the lacrimal and surangular create the protrusible rim of that tube mouth and neither connects to the quadrate. Nesting sturgeons at the base of fish with teeth is the opposite of traditional cladogram topologies, in which sturgeons are considered ‘aberrant’ or ‘regressive’ (see below).

Figure 1. Old woodcut illustration labeling the upper mouth tube bone the lacrimal. Mn = mandible. h = quadrate. g = hyobranchial. Weave of bones above the lacrimal are palatal bones (pterygoid, ectopterygoid, palatine and vomer, plus a remnant gill bar. This taxon really exaggerates the rostrum, similar to the related spoonbill.

Figure 2. Old woodcut illustration labeling the upper mouth tube bone the lacrimal. Mn = mandible. h = quadrate. g = hyobranchial. Weave of bones above the lacrimal are palatal bones (pterygoid, ectopterygoid, palatine and vomer, plus a remnant gill bar. This taxon really exaggerates the rostrum, similar to the related spoonbill.

As you can see (Fig. 2), I am not the first worker 
to determine that the traditional ‘maxilla’ on sturgeons is instead the lacrimal.

Sturgeons, continued.
Gill covers (operculum) appear. While feeding on the bottom with the mouth buried in sediment, water cannot enter the mouth. So instead water enters the top of the operculum and exits out the back for respiration.

Note the close correspondence
between the torso ossifications, fin placement, tail shape and skull shape on the sturgeon and its osteostracan ancestor, Hemicyclaspis (Fig. 1).

Figure 3. Chondrosteus animation (2 frames) in situ and reconstructed in lateral view. This is the transitional taxon linking sturgeons to bony fish + sharks.

Figure 3. Chondrosteus animation (2 frames) in situ and reconstructed in lateral view. This is the transitional taxon linking sturgeons to bony fish + sharks.

Are sturgeons jawless fish?
In the LRT sturgeons are transitional between jawless fish and traditional gnathostomes.

Jollie 1980 reported in his growth study on sturgeons,
“It is a conclusion that the endocranium has been drastically altered in form and in the reduction of its ossifications but that the dermal head skeleton is basically that of an actinopterygian fish which shows many regressive tendencies such as the variable multiplication of ossified units. The jaws in this group are unique both in terms of suspension and in lacking a premaxilla. The post-temporal of the pectoral girdle has a unique relationship with the endocranium which involves the exclusion of the lateral extrascapular. An interclavicle is present. In spite of such features, the developmental story and adult ossifications of the sturgeon support the idea of a common, and understandable, bone pattern in actinopterygians and osteichthians.”

Jollie did not place Acipenser and Hemicyclaspis
in a phylogenetic context. In the LRT (subset Fig. 4) Pseudoscaphorhynchus is a tested sturgeon.

Figure 5. Subset of the LRT focusing on basal vertebrates. Note the clade Holocephali nests apart from Chondrenchelys and kin, including moray eels.

Figure 5. Subset of the LRT focusing on basal vertebrates. Note the clade Holocephali nests apart from Chondrenchelys and kin, including moray eels.

Are sturgeons bony fish?
Not according to the LRT. Much of their skeleton is cartilaginous and they nest basal to cartilaginous taxa. So between cilia and jaws, the transitional trait is a tube. Marginal teeth seem to have appeared three times by convergence in this scenario and once gained, were quickly lost in placoderms + catfish. Add to those palatal tooth carpets found in catfish, mantas and whale sharks.

Apologies for earlier errors.
As I’ve often said, I’m teaching myself vertebrate paleontology one taxon at a time using the LRT as a terrific tool for figuring things out.


References
Jollie M 1980. Development of head and pectoral girdle skeleton in Acipenser. Copeia 1980(2):226–249.

Actinopterygii = ray fin fish
Osteichthyes =  bony fish

wiki/Gnathostomata

Dinosaurs in the Wild video, plus a backstory video

There’s a new(?) dinosaur exhibit
in England and several visitors have uploaded YouTube videos of it. Most of these are at least one year old, so I may be the last one to learn about this.

Visitors go back in time
and every so often put on 3D glasses to see dinosaurs outside the ‘windows’ of the exhibit. Looks like a thrill a minute with up-to-date dinos.

Plus
Dr. Darren Naish provides a behind-the-scenes YouTube video.

Figure 1. When they had to animate Quetzalcoatlus, they got rid of that membrane down to the ankles.

Figure 1. Evidently, when they had to animate Quetzalcoatlus, they got rid of that membrane down to the ankles, distinct from all previous illustrations of Quetzalcoatlus, but only when standing. Baby steps…

References

For more YouTube listings click here.

 

A catfish with barbels from the Silurian

Finally
a catfish from the Silurian with preserved barbels (Figs. 1, 2).

Ironically
catfish are members of the order Siluriformes (from ‘silurus’ Latin = large river fish). Previous oldest member of this clade: Late Cretaceous, 100mya.  Sir Roderick Murchison (1792–1871), a wealthy Scottish aristocrat, named the Silurian Period after an ancient Welsh Celtic tribe, the Silures. Appears to be a coincidence. The slow genesis of plants and arthropods on land occurred in the Silurian, along with a rise in oxygen levels, a rise in temperature and a rise in sea levels after the massive glaciation of the Ordovician.

Figure 1. Originally considered another Silurian Thelodus, this specimen nests with catfish in the LRT.

Figure 1. Originally considered another Silurian Thelodus, this specimen nests with catfish in the LRT. Here’s where DGS tracing helps pick out the details from a ‘fish silhouette’ fossil.

Not sure what the museum number is on this one.
In the large reptile tree (LRT, 1602 taxa; subset Fig. A) this taxon is labeled ‘unnamed Sil. catfish‘ (in the purple clade). In the LRT the new taxon is not as primitive as the armored catfish, Hoplosternum. Worthy of note, basal catfish in the LRT are air breathers employing the intestine or modified gill arches, not their air bladder, which they need to swim upright. Clarias is the famous walking catfish (Figs. 3–5) able to traverse land in search of other ponds. The spiny pectoral fins (Fig. 4) keep it upright and act as ground undulates as it wriggles from pond to pond.

Figure A. Subset of the LRT focusing on basal vertebrates (fish).

Figure A. Subset of the LRT focusing on basal vertebrates (fish). The base of the LRT will change by the next time you see it with the addition of several jawless fish. 

The identity of this specimen
might have been overlooked because it appears like a silhouette, offering little detail. Digital Graphic Segregation (DGS) enables details to be colored, identified and later scored in the LRT.

Figure 3. Silurian catfish face

Figure 2. Silurian catfish face. Note the left barbel is aligned with a crack.

Extant velvet catfish,
members of the clade Diplomystidae, are considered primitive.

Figure 6. Clarias head with barbels in vivo.

Figure 3. Clarias head with barbels in vivo.

Clarias batrachus (Linneaus 1758, up to 50 cm in length) is the extant walking catfish. The skull bones are nearly identical to those in the placoderm, Entelognathus. The spiny pectoral fins keep the walking catfish upright as it wriggles from pond to pond. No scales or bones appear on the surface. The teeth are short bristles on pads. The maxilla is absent.

FIgure 1. Clarias, the walking catfish is a living placoderm with skull bones colorized as homologs of those in Entelognathus (Fig. 2). Here the mandible shifts forward and the opercular shifts backwards relative to Entelongnathus in the Silurian.

Figure 4. Clarias, the walking catfish skull bones identified. Note the ossified spines at the leading edge of the pectoral fin. 

Figure 3. Clarias batrachus, the walking catfish, in vivo. The pelvic fin is tiny. The single dorsal fin is elongate. The anal fin is also elongate. The skull is flat and provided with sensory barbels.

Figure 5. Clarias batrachus, the walking catfish, in vivo. The pelvic fin is tiny. The single dorsal fin is elongate. The anal fin is also elongate. The skull is flat and provided with sensory barbels.

Generally recognized fossil catfish
include Qarmoutus hitanensis from the same Eocene North African beds as the early whale, Basilosaurus. Reported by NatGeo.com (citation below): Even though the fossil is relatively old in the way we ordinarily think of ages in millions of years, it is still essentially anatomically modern and directly comparable to living catfishes,” says John Lundberg of Drexel University’s Academy of Natural Sciences. “It’s one of the best preserved and oldest of its family.”

Afterthought about fish with spines in their fins
Spiny sharks (Acanthodii), like Brachyacanthus (Fig. 6), also briefly appeared in the Silurian and Devonian. Since the walking catfish uses its spiny fins to ‘walk’ on land, I wonder if spiny sharks, especially those with longer, thinner pectoral and pelvic spines, did the same, perhaps on the sea floor, not on land?

Figure 1. Surprising homologies in Pteronisculus and Brachyacanthus indicate a close relationship, despite the spiny fins.

Figure 6. Brachyacanthus has short, thick spiny fins, distinct from the long spines found in the walking catfish.

That might explain
why those extra spines appeared between the pectoral and pelvic fins, as extra hooks in the substrate?


References

https://en.wikipedia.org/wiki/Catfish

https://www.nationalgeographic.com/news/2017/03/ancient-egypt-catfish-fossil-palaeontology-science/

https://pubs.geoscienceworld.org/gsa/geology/article-abstract/5/4/196/195354/Fossil-catfish-and-the-depositional-environment-of?redirectedFrom=fulltext

Tiny Late Cretaceous Najash: basal to burrowing snakes

Garberoglio et al. 2019
bring us long awaited skull data and several new partial skeletons of a Late Cretaceous snake with legs, Najash rionegrina (Figs. 1, 2). It must be said, the only evidence of legs supplied by the current authors was a caption labeled tibia on a tiny straight bone near the edge of the matrix. Nevertheless, legs and hips were described earlier in other headless specimens of Najash (Apesteguía and Zaher 2006; Fig. 1).

Figure 2. The pelvis of the protosnake with legs, Najash, compared to Heloderma (burrower) and Adriosaurus (swimmer). Heloderma appears to share more traits with Najash.

Figure 2. The pelvis of the protosnake with legs, Najash, compared to Heloderma (burrower) and Adriosaurus (swimmer). Heloderma appears to share more traits with Najash.

Most of the new specimens
were found in layered sandstone related to migrating aeolian dunes, along with abundant rhizoliths (root systems encased in desiccated mineral matter) and burrows.

Figure 1. Najash compared to Tetrapodophis (the last snake with legs) and Loxocemus, an extant burrowing snake without legs.

Figure 2. Najash compared to Tetrapodophis (the last snake with legs) and Loxocemus, an extant burrowing snake without legs.

From the authors’ abstract:
“the evolutionary versatility of the vertebrate body plan, including body elongation, limb loss, and skull kinesis. However, understanding the earliest steps toward the acquisition of these remarkable adaptations is hampered by the very limited fossil record of early snakes.”

That’s not true.
In the large reptile tree (LRT, 1602+ taxa, subset Fig. 3) snakes have a well documented ancestry back to Cambrian lancelets. The cladogram presented by the nine co-authors was steeped in tradition and lacking in appropriate outgroup taxa. Contra Garberoglio et al. 2019, Varanus and its monitor lizard kin are not part of snake ancestry in the LRT.

Figure 3. Subset of the LRT focusing on geckos and their sister snake ancestors.

Figure 3. Subset of the LRT focusing on geckos and their sister snake ancestors.

Garberoglio et al. continue:
“These new Najash specimens reveal a mosaic of primitive lizard-like features such as a large triradiate jugal and absence of the crista circumfenestralis, derived snake features such as the absence of the postorbital, as well as intermediate conditions such as a vertically oriented quadrate. The new cranial data also robustly resolve the phylogenetic position of this crucial snake taxon, along with other limbed snakes.”

  1. The authors’ cladogram did not nest Najash with burrowing snakes, as in the LRT, but at a much more primitive node.
  2. Perhaps this is so because Tetrapodophis and Barlochersaurus were not mentioned in the text.
  3. The quadrate was sharply bent posteriorly at a right angle, a trait only found in burrowing snakes.
  4. I found no primitive lizard-like features here, other than legs and hips, traits found in Tetraphodophis and Barlochersaurus, the last common ancestors of all living snakes.
  5. Najash is a crown-group snake in the LRT until additional untested taxa move it out.

Najash rionegrina (Apesteguía and Zaher 2006; Garberoglio FF et al. 2019; Late Cretaceous) is a tiny burrowing snake that retained a pelvis and hind limbs, transitional between Tetrapodophis and Loxocemus. The premaxilla was tiny, as in terrestrial snakes. The mandible rose anteriorly, as in burrowing snakes. The jugal and vomers were retained.


References
Apesteguía S and Zaher H 2006.A Cretaceous terrestrial snake with robust hindlimbs and a sacrum. Nature. 440 (7087): 1037–1040.
Garberoglio FF et al. (eight co-authors) 2019.
New skulls and skeletons of the Cretaceous legged snake Najash, and the evolution of the modern snake plan. Science Advances 2019(5):eaax5833, 8pp.

wiki/Najash

Is ‘Vjushkovia triplocosta’ a jr synonym for Garjainia prima?

In other words,
are the two erythrosuchid holotypes (Fig. 1) sufficiently alike to be congeneric or conspecific? Garjainia was published first.

Butler et al. 2019 reported
“Two species of Garjainia have been reported from Russia: the type species, Garjainia prima Ochev, 1958, and ‘Vjushkovia triplicostata’ von Huene, 1960, which has been referred to Garjainia as either congeneric (Garjainia triplicostata) or conspecific (G. prima).”

“…little work has been conducted on type or referred material attributed to ‘V. triplicostata’. However, this material includes well-preserved fossils representing all parts of the skeleton and comprises seven individuals. Here, we provide a comprehensive description and review of the cranial anatomy of material attributed to ‘V. triplicostata’, and draw comparisons with G. prima. We conclude that the two Russian taxa are indeed conspecific, and that minor differences between them result from a combination of preservation or intraspecific variation.”

Figure 1. Vjushkova holotype compared to Gargainia. These two nest together in the LRT, but not by much. Both the antorbital and lateral temporal regions differ greatly.

Figure 1. Vjushkova holotype compared to Garjainia. These two nest together in the LRT, but not by much. Several areas, including the antorbital and lateral temporal regions differ greatly. The dorsal view of both are quite distinct, overlooked by Butler et al. 

Combining elements from seven specimens
bears some risk of creating a chimaera. Since Butler et al. felt confident in doing so, and there is no alternative, then I do, too. Given the data presented by Butler et al. I reconstructed the skull from separate elements (Fig. 1), something Butler et al. did not do.

Although the two skulls are extremely similar
and the two taxa nest together in the large reptile tree (LRT, 1602 taxa) a few traits seem to distinguish these two taxa apart from one another, at least at the species level and perhaps at the generic level. Note the larger antorbital fenestra in Vjushkovia. Note the pinched upper portion of the lateral temporal fenestra. Note the concave posterior maxilla. Note the taller, narrower orbit. Note the much more robust quadratojugal and quadrate. Note the greater arch of the posterior postorbital. Note the posterior process of the squamosal. These differences appear to support the separation of these taxa at the generic level, IMHO. The lack of a reconstruction in Butler et al. 2019 may have hampered their decision in this case. The lack of graphic comparison in the paper (no images of the Garjainia holotype are shown side-by-side with those of Vjushkoiva) is also regrettable.


References
Butler RJ, Sennikov AG, Dunne EM, Ezcurra MD, Hedrick BP, Maidment SCR, Meade LE, Raven TJ and Gower DJ 2019.
Cranial anatomy and taxonomy of the erythrosuchid archosauriform ‘Vjushkovia triplicostata’ Huene, 1960, from the Early Triassic of European Russia. Royal Society Open Science 6: 191289. http://dx.doi.org/10.1098/rsos.191289

Criticisms of other papers by Butler as co-author:

https://pterosaurheresies.wordpress.com/2018/06/25/the-rise-of-the-ruling-reptiles-ezcurra-and-butler-2018-fiasco/

https://pterosaurheresies.wordpress.com/2019/10/21/teyujagua-paradoxa-still-no-paradox-in-the-lrt/

https://pterosaurheresies.wordpress.com/2019/04/05/mythbusting-prorotodactylus/

https://pterosaurheresies.wordpress.com/2019/02/06/what-is-gracilisuchus-add-more-taxa-to-find-out/

https://pterosaurheresies.wordpress.com/2018/12/12/ezcurra-et-al-2018-review-garjainia/

Birkenia and the origin of facial bones

The most important taxa
in the large reptile tree (LRT, 1601 taxa; Fig. A) are the basal forms at each clade and the basalmost forms at the base of the LRT. Trying to understand where we all came from, I’ve been adding fish to the LRT using generalized reptile traits with some success. Novel topologies have appeared due to testing taxa that have not been tested together before.

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 1. Not much of a face here. An early jawless, finless, lancelet-like fish from the Cambrian, Metaspriggina. Compare the protruding placement of the eyes here with Birkenia in figures 2 and 3.

Now the LRT is adding
a mid-Cambrian lancelet/fish without bones, Metaspriggina, (Simonetta and Insom 1993; Fig. 1) and Silurian jawless fish, like Thelodus and Birkenia elegans (Figs. 2, 3; Traquair 1898; Middle Silurian, 1.5-10cm long) in an effort to make sure the base of the LRT is not biased due to taxon exclusion. Turns out it was the right thing to do (Fig. A).

FIgure 1. Birkenia in situ and diagrams.

FIgure 2. Birkenia in situ and diagrams. Diagram from Blom et al. 2001. Closeup in Fig. 3.

With that short intro, on to today’s topic
About half of the traits in the LRT come from the skull. It is the one part of the vertebrate body that changes the most in evolution.  Metaspriggina is a Cambrian lancelet with eyes, muscles, guts and gill slits, but no bones. The LRT tests bones, principally. So let’s figure out where and how facial bones first appeared in the vertebrate fossil record. Up to this point, no one, it seems, has put in the effort to do so.

Certain clades of jawless fish enclose themselves
in turtle-like shells (e.g. Arandaspis) with only a scaly or bone armored tail sticking out the back. Those genera are not useful to the present purposes and may never be added to the LRT.

On the other hand, one of the earliest taxa with facial bones
is jawless Birkenia (Figs. 2, 3), a relative to extant lampreys. Here (Fig. 3) the bones are not sutured plates, the sort we expect to see. Rather, in Birkenia areas of parallel, interwoven and concentric splints, whether cartilaginous or bone, form the primordia on which bones are phylogenetically later produced.

Distinctly different,
the squamosal, quadrate and dentary are tiny splints, too (Fig. 3), but they are internal and little different from the other concentric gill bars behind them that remain gill bars before they eventually turn into jaw and throat elements in tetrapods.

Figure 2. Birkenia in situ with facial bones labeled.

Figure 3. Birkenia in situ with facial bones labeled. Frontals and quadratojugals evolve later.

Frontal and quadratojugal
bones are not present in finless Birkenia. Those appear in more derived taxa like Gogonasus. In Birkenia lamprey-like gill openings are retained along with the developing, transitional gill bars.

Birkenia is reported to have
a terminal, rather than ventral, sucking mouth. This specimen (Fig. 3) does not have a terminal mouth. Rather it has a ventral permanent opening, like a lancelet does. “Sucking” was not possible. Perhaps other specimens attributed to Birkenia have different mouth morphologies or some oddly crushed specimens (Fig. 2) have been misinterpreted.

Figure A. Subset of the LRT focusing on basal vertebrates (fish).

Figure A. Subset of the LRT focusing on basal vertebrates (fish).

Today’s post may represent a novel observation
with regard to the origin of facial bones. If there is a similar earlier paper, let me know and I will cite it for proper credit.

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 4. Ventral view of the GLAHM V830 specimen of Thelodus. This appears to have fang-like teeth and a typical mouth. On the other hand, the mandible in this taxon appears to be a dead end experiment convergent with the mandible of all other vertebrates.

Final note:
The thelodont Thelodus (Fig. 4) appears to have a mandible and teeth similar to, but phylogenetically distinct from all other vertebrates. Those ‘teeth’ are probably derived from cilia, since basal vertebrate teeth are not like these. Crushing may have given this fossil the illusion of a mandible. Or this may represent a convergent appearance of a mandible that is not phylogenetically related to the jaws of other tested vertebrates. Or it may represent an early appearance of the mandible, since pectoral fins are also present here, distinct from Birkenia.


References
Blom H, Märss T and Miller CG 2001. Silurian and earliest Devonian birkeniid anaspids from the Northern Hemisphere. Earth and Environmental Science Transactions of the Royal Society of Edinburgh 92.03 (2001): 263-323.
Simonetta AM and Insom E 1993. New animals from the Burgess Shale (Middle Cambrian)and the possible significance for the understanding of the Bilateria. Bolletino Di Zoologia 60:97–107.
Traquair RH 1898. Report on fossils fishes. Summary of Progress of the Geological Survey of the United Kingdom for 1897: 72-76.

wiki/Birkenia 
wiki/Metaspriggina

 

Origin and evolution of gnathostome dentitions

Updated January 2, 2021 and June 3, 2022
with a new reconstruction of Gemuendina (Fig. x), which now appears to nest basal to Manta, close to Jagorina, but will not be entered into the LRT due to the large amount of skin and scale covering bone.

Figure x. Undistorted Gemuendina face.

Figure x. Undistorted Gemuendina face.

Figure y. Gemuendina distorted face.

Figure y. Gemuendina distorted face.

Johanson and Smith 2005
looked at the questions of teeth and pharyngeal denticles in placoderms.

Unfortunately
the large reptile tree (LRT, 1597+ taxa; subset Fig. 1) does not confirm the first sentence of the authors’ abstract: “The fossil group Placodermi is the most phylogenetically basal of the clade of jawed vertebrates but lacks a marginal dentition comparable to that of the dentate Chondrichthyes, Acanthodii and Osteichthyes (crown group Gnathostomata).”

The LRT nests placoderms along with catfish
between Hybodus and spiny sharks, deep into the Gnathostomata. Catfish are not mentioned in the Johanson and Smith text. They do mention, “the rounded or pointed denticles described for the Arthrodira may only be present in a limited number of taxa (Gemuendina (Fig. 2) Traquair, 1903).” Regrettably the authors did not know that some members attributed to this generic wastebasket of Gemuendina are catfish (Fig. 1), a clade closely related to traditional placoderms. So, taxon exclusion, once again, becomes a major issue.

Johanson and Smith also err
when they state, “The Arthrodira is a derived taxon within the Placodermi, hence origin of teeth in placoderms occurs late in the phylogeny and teeth are convergently derived, relative to those of other jawed vertebrates.” The LRT notes that Coccosteus is a basal placoderm, one that is closer to the outgroup taxon, Gregorius than are other less predatory taxa. This exemplifies a problem with this, and many other papers in that without a proper and validated cladogram, it is nearly impossible to determine whether the absence of teeth, or any other trait, represents a vestigial loss or a vestigial genesis situation.

Johanson and Smith report, 
“Tooth sets and tooth whorls in crown-group gnathostomes are suggested to derive from the pharyngeal denticle whorls, at least in sharks, with the patterning mechanisms co-opted to the oral cavity. A comparable co-option is suggested for the Placodermi.”

Figure 1. Whale shark (Rhincodon) tooth pads, not that much different from catfish tooth pads (Fig. 2).

Figure 2. Whale shark (Rhincodon) tooth pads, not that much different from catfish tooth pads (Fig. 2).

The authors do not mention
the tooth carpets of Rhincodon (Fig. 2) and Manta. The LRT indicates that these taxa represent the origin of teeth within the jaws, not on the margins, which remain toothless, but on the palate, reusembling shark skin.

The authors likewise do not mention
the angel shark Squatina. The LRT indicates this taxon represents the origin of teeth along the margins of the jaws.

The LRT indicates
placoderms lose teeth and sometimes develop sharp, turtle-like gnathal plates, some of which retain vestigial tooth-like bumps. Their sister clade, the Siluriformes (catfish) lose the maxilla and retain tooth carpets only in the mandible (Fig. 1). This begins with the basalmost catfish, traditionally considered a basal placoderm, Entelognathus.


References
Johanson Z and Smith MM 2005. Origin and evolution of gnathostome dentitions: a question of teeth and pharyngeal denticles in placoderms. Biology Review 80:1–43.

Loganellia scotica: even more like a tiny whale shark

For over a century
taxon omission overlooked this homology.

Figure 1.  Manta compared to Thelodus (Loganellia) and Rhincodon. All three have a terminal mouth essentially straight across, between the lateral eyes, distinct from most fish.

Figure 1.  Manta compared to Thelodus (Loganellia) and Rhincodon. All three have a terminal mouth essentially straight across, between the lateral eyes, distinct from most fish.

Earlier I added
the Silurian thelodont jawless fish, Thelodus (Fig. 2), as the outgroup for the large reptile tree (LRT, 1597+ taxa; subset Fig. 6). The whale shark (Rhincodon) and the manta ray (Manta) (Fig. 1) nested as the first outgroup taxa. In my mind there is a little confusion over which specimens belong to Thelodus and which belong two Loganellia or otherwise.

Figure 2. Thelodont with whale shark shape including dorsal fin. Image from OldRedSandstone.com. This appears to be Loganellia, not Thelodus (Fig. 7).

Figure 2. Thelodont with whale shark shape including dorsal fin. Image from OldRedSandstone.com. This appears to be Loganellia, not Thelodus (Fig. 7).

After seeing an online photo
of another Silurian thelodont, Loganellia scotica (Traquair 1898, Early Silurian, 430 mya; Fig. 3), I was able to draw out more details than the traditional line drawing offered using DGS methodology. Many similar Silurian thelodonts are known (Fig. 2). Some, evidently, have changed names over the last 120 years, probably because details are difficult to glean. I have not been able to ascertain any museum numbers for these taxa.

Figure 1. Loganellia scotica in situ and after DGS coloring. Several details are recovered here that identify this as a little sister to Rhincodon, the whale shark. Photo from Birkknowesaffairs.com. Specimen from museum?

Figure 3. Loganellia scotica in situ and after DGS coloring. Several details are recovered here that identify this as a little sister to Rhincodon, the whale shark. Photo from Birkknowesaffairs.com. Specimen from museum?

After DGS, Loganellia appears closer to Rhincodon
than the traditional ink diagram by Traquair (Figs. 1, 3) indicates. Pelvic fins are present. One or two dorsal fins are present. An anal fin and cutwaters are present. A mandible is present. That comes as a surprise for a supposedly jawless taxon.

Rücklin et al. 2019 studied Loganellia
and reported, “The available evidence suggests that internal odontodes evolved through the expansion of odontogenic competence from external to internal epithelia.” Neither Rhincodon nor Manta are found in the text. Both of these taxa have the sort of scale-like tooth carpet the authors were able to see using synchrotron radiation X-ray tomographic microscopy (SRXTM). They report, “We reveal that the internal scales are organized into fused patches and rows, a key distinction from the discrete dermal scales. 

Figure 1. Whale shark (Rhincodon) tooth pads, not that much different from catfish tooth pads (Fig. 2).

Figure 4. Whale shark (Rhincodon) tooth pads, similar to those described for Loganiella.

Rücklin et al. continue:
“Internal scales, where present, are always located near to external orifices; the sequential development of pharyngeal scales in Loganellia is peculiar among thelodonts and other stem gnathostomes. It represents a convergence on, rather than the establishment of, the developmental pattern underpinning tooth replacement in jawed vertebrates.”

Perhaps Rücklin et al. were influenced by
the traditional hypothesis that placoderms were the first jawed vertebrates, derived from osteostracan jawless vertebrates (Fig. 3). The LRT (subset Fig. 6) does not support the traditional hypothesis.

Figure 4. From Zhu et al. 2016, overlay added based on LRT topology.

Figure 5. From Zhu et al. 2016, overlay added based on LRT topology.

Figure 5. Subset of the LRT focusing on basal vertebrates. Note the clade Holocephali nests apart from Chondrenchelys and kin, including moray eels.

Figure 5. Subset of the LRT focusing on basal vertebrates. Note the clade Holocephali nests apart from Chondrenchelys and kin, including moray eels.

A final note:
Long 1995 identified this specimen (Fig. 7) as Thelodus. Distinct from Loganellia (Fig. 3), this specimen assigned to Thelodus appears to have an underslung jaw and long, raking teeth, perhaps to sift through seafloor sand, long pectoral fins and other distinct traits. Some thelodonts do not have fins and jaws.

Figure 7. Specimen identified as Thelodus by Long 1995. It has an underslung mandible and long rostral teeth along with long pectoral fins, distinct from Loganellia.

Figure 7. Specimen identified as Thelodus by Long 1995. It has an underslung mandible and long rostral teeth along with long pectoral fins, distinct from Loganellia.

According to the National History Museum in London, “Shark-like scales from the Late Ordovician have been found, but no teeth. If these were from sharks it would suggest that the earliest forms could have been toothless. Scientists are still debating if these were true sharks or shark-like animals.”

References
Long JA 1995.The Rise of Fishes. 500 million years of evolution. The Johns Hopkins University Press, Baltimore and London.
Rücklin M, Giles S, Janvier P and Donoghue PCJ 2011.
Teeth before jaws? Comparative analysis of the structure and development of the external and internal scales in the extinct jawless vertebrate Loganellia scotica. Evolution & Development 13(6):523–532.
Traquair RH 1898. Report on fossils fishes. Summary of Progress of the Geological Survey of the United Kingdom for 1897: 72-76.
Žigaite· Ž & Goujet D 2012. New observations on the squamation patterns of articulated specimens of Loganellia scotica (Traquair, 1898) (Vertebrata: Thelodonti) from the Lower Silurian of Scotland. Geodiversitas 34 (2): 253-270.

wiki/Loganellia
oldredsandstone.com
nhm.ac.uk/discover/shark-evolution-a-450-million-year-timeline.html

Lunaspis: a flat, wide placoderm

Updated September 21, 2022
with more taxa (2106) and a better understanding of the skull bones, Lunaspis now nests with Heterosteus, a similar bottom feeder, in the LRT.

Always considered an odd sort of placoderm,
(Fig. 2) Lunaspis heroldi (Fig. 1) nests in the large reptile tree (LRT, 1597+ taxa, then 2106 on June 2, 2022; subset Fig. x) between tiny Bianchengichthys and giant Heterosteus.

Lunaspis heroldi (Broili 1929; latest Early Devonian, 405mya) is traditionally considered a petalichthyid placoderm. Here that clade nests between Entelognathus (Zhu et al. 2013) and the arthrodire clade (Figs. A, B). The tail, missing from this fossil, but known from other specimens, was long and whip-like, as in extant rays. None of the specimens preserve mouth parts. As in catfish and Entelognathus, the maxilla is absent. Note the serrated leading edge of the spike-like pecotral fins, also found in catfish.

The LRT recovers a different origin of jaws (Fig 3b).

Figure 2. Origin of jaws from the ostracoderm, Hemicyclaspis, Thelodus, Acipenser (sturgeon) and Chondrosteus.

Figure 3b. Origin of jaws from the ostracoderm, Hemicyclaspis, Thelodus, Acipenser (sturgeon) and Chondrosteus.

Osteostraci and Galespida|
(Figs. 2, 3) are widely considered basal to placoderms, which, in turn, are widely considered basal to sharks (Fig. 3). Unfortunately Osteostraci and Galespida skulls generally do not have bone sutures and if any sutures are present, they are not homologous to those in gnathostomes (jawed vertebrates). Missing from the above taxon lists are thelodonts, like Thelodus, which is shaped like a tiny whale shark (Rhincodon) and angel shark (Squatina). These taxa preserve gill bars alongside jaw bones and document the appearance of jaws prior to the appearance of marginal teeth. The LRT (subset Fig. 4) shows that figures 2 and 3 are upside-down, nesting basal taxa as derived taxa and vice versa, and they are missing the taxa that would upend them.

References
Broili F 1929. S. B. Bayer. Akad. Wiss., 1.
Long JA 1995. The Rise of Fishes. 500 million years of evolution. The Johns Hopkins University Press, Baltimore and London.
Zhu M, Yu X-B, Ahlberg PE, Choo B and 8 others 2013. A Silurian placoderm with osteichthyan-like marginal jaw bones. Nature. 502:188–193.
Zhu M et al. 2016. A Silurian maxillate placoderm illuminates jaw evolution. Science 354.6310 (2016): 334-336.

wiki/Lunaspis
wiki/Entelognathus

Mutation vs. natural selection

Gemma Tarlach, writing in Discover Magazine online 2014 wrote:
“Mutation, Not Natural Selection, Drives Evolution. Molecular evolutionary biologist Masatoshi Nei (2013) says Darwin never proved natural selection is the driving force of evolution — because it isn’t.”

“In 1972, he devised a now widely used formula, Nei’s standard genetic distance, which compares key genes of different populations to estimate how long ago the groups diverged. In the early ’90s, Nei was a co-developer of free software that creates evolutionary trees based on genetic data. Molecular Evolutionary Genetics Analysis, or MEGA.”

So that’s the scientist who brought us genomics. Perhaps there were others, too.

Nei continues:
“Natural selection occurs sometimes, of course, because some types of variations are better than others, but mutation created the different types. Natural selection is secondary.”

Someone is forgetting sexual selection, which encourages certain phenomic changes over others. Someone is forgetting extinction by disease, comet impact, etc., which clears the Earth for survivors.

Nei continues:
“My position is mutation creates variation, then natural selection may or may not operate, it may or may not choose the good variation and eliminate the bad one, but natural selection is not the driving force.”

Nei continues:
“I had developed the genetic distance theory [in the ’70s] because I wanted to make a phylogenetic tree, and distance can be used for making trees. But I was also interested in statistics. So I combined the two methods. To test it, first we did computer simulations: We generated a DNA sequence for an evolutionary tree where we already knew where the tree branched.

Or did they? The large reptile tree (LRT, 1596+ taxa) upsets many traditional relationships among the taxa its tests. The LRT cladogram was not available to Nei in 2011 and was probably overlooked in 2013.

Nei continues:
“Then we used statistics, the neighbor-joining method, to reconstruct the tree and test whether it resembled the actual phylogenetic tree. It did, and that’s how we knew this method gave a pretty good idea of how species evolved and diverged.”

So, false positives matched false positives. Think Afrotheria. Laurasiatheria. And remember no fossil taxa are used in genomic studies. Think what a difference fossil taxa make.

Nei continues:
“But any time a scientific theory is treated like dogma, you have to question it. The dogma of natural selection has existed a long time. Most people have not questioned it. Most textbooks still state this is so. Most students are educated with these books.”

“You have to question dogma. Use common sense. You have to think for yourself, without preconceptions. That is what’s important in science.”

You also have to add fossil taxa. And where was the common sense that nested elephants and kin as basal placentals? And who would nest flamingoes with grebes, except genomic workers?

In the comments section of this online article,
medical laboratory scientist James Kohl wrote: “In “Roles of Mutation and Selection in Speciation: From Hugo de Vries to the Modern Genomic Era” (2011), Nei ignored ecological factors as he did in his book “Mutation-Driven Evolution”. He also ignored the biophysical constraints on protein folding that prevent mutation-driven evolution. Instead, in his book he evoked ‘constraint-breaking mutation” as the source of all biological innovations and species diversity in the world. Now, with the lesser role for natural selection, mutation-driven evolution “just happens” due to constraint-breaking mutations. Is there a model for that? Is there a model organism that exemplifies it?”

“Species diversity is is obviously nutrient-dependent and it is controlled by the metabolism of nutrients to species-specific pheromones, which control the physiology of reproduction in species from microbes to man. Chemical ecology and olfactory/pheromonal input link the epigenetic landscape to the physical landscape of DNA in the organized genome of species from microbes to man. There is no reason to add the biologically implausible role of constraint-breaking mutation until experimental evidence ecologically validates the earlier versions of untested theories about evolution that are still based on population genetics instead of on experimental evidence of conserved molecular mechanisms sans mutations.”


References
Nei M 2013. Mutation-Driven Evolution.
Nei M and Nozawa M 2011. In “Roles of Mutation and Selection in Speciation: From Hugo de Vries to the Modern Genomic Era”,

http://discovermagazine.com/2014/march/12-mutation-not-natural-selection-drives-evolution

https://www.nature.com/scitable/knowledge/library/speciation-the-origin-of-new-species-26230527/