How the shark lost its bones video on YouTube

From Martin Brazeau and the Imperial College London,
here’s a new YouTube video (53 minutes) on how and maybe why sharks lost their bony exoskeleton.

The phylogenetic context is wrong. Without testing, Brazeau et al. considered placoderms basal to sharks and bony fish.That’s a traditional mistake. In the large reptile tree (LRT, 1795+ taxa) placoderms are bony fish close to catfish. In the LRT sharks evolved from sturgeons (Fig. 2). Bony fish evolved from hybodontid sharks. The Silurian is when all this happened.

We looked at this subject earlier here (Borrell 2014) and here Brazeau et al. 2020.

Unfortunately, as you’ll see
Brazeau et al. include only fossil taxa to determine which taxa were present in the Silurian.

Figure x. Shark skull evolution.

The jawless,
(by reversal) anapsid-mimic placoderm, Minjinia (Fig. 3) was featured in Brazeau’s paper and video.

Figure 1. Subset of the LRT focusing on the branch of the Osteichthys that includes placoderms and their relatives.
Figure 2. Subset of the LRT focusing on the branch of the Osteichthys that includes placoderms and their relatives.
Figure 1. Minjina in 4 views, mirror-image and colors added.
Figure 1. Minjina in 4 views, mirror-image and colors added.

Ironically
Brazeau illustrates his talk with an image of the exoskeleton and endoskeleton of the sturgeon Acipenser. which entered the LRT here. He reports the endochondral bone was lost in sturgeons. That is a traditional mistake as revealed by the LRT.

Brazeau correctly reports
the origin of bone precedes sharks and is lost in sharks. He just did not realize that placoderms are descendants of sharks, not their ancestors.


References
Brazeau et al. (7 co-authors) 2020. Endochondral bone in an Early Devonian ‘placoderm’ from Mongolia. Nature Ecology & Evolution. https://doi.org/10.1038/s41559-020-01290-2
Hu Y, Lu J and Young GC 2017. New findings in a 400 million-year-old Devonian placoderm shed light on jaw structure and function in basal gnathostomes. Nature Scientific Reports 7: 7813 DOI:10.1038/s41598-017-07674-y

https://cosmosmagazine.com/nature/evolution/new-thoughts-on-how-sharks-evolved/http://www.sci-news.com/paleontology/minjinia-turgenensis-08823.html

Disc-head placoderms with tiny lateral eyes are anapsid mimics

Phyllolepid placoderms
like Cowralepis (Figs. 1, 2) and Minjinia (Fig. 3) have simple disc-like skulls and lack jaws. In this way they mimic older ‘jawless fish’ (= anapsida), like Drepanaspis (Fig. 4).

Figure 2. Cowralepis was first described as a growth series.

Figure 1. Cowralepis was first described as a growth series.

Anapsids are actually derived armored lancelets,
filtering food with oral cavities prior to the genesis of jaws.

Figure 1. Cowralepis plate and counter plate showing the medial view of the ventral and dorsal halves of this disc-like placoderm.

Figure 2. Cowralepis plate and counter plate showing the medial view of the ventral and dorsal halves of this disc-like placoderm. Red dots are resorted eyeballs.

In counterpoint,
Cowralepis and its Early to Middle Devonian allies had ancestors with jaws according to the large reptile tree (LRT, 1759+ taxa; subset Fig. x). Minjinia (Fig. 3) was originally considered a placoderm close to the shark/bony fish split. In the LRT Minjinia nests with the nearly blind phyllolepid placoderm bony fish, far from sharks.

Figure 1. Minjina in 4 views, mirror-image and colors added.

Figure 3. Minjina in 4 views, mirror-image added.

Phyllolepid placoderms also reduce
their diphycercal tail, pelvic fins and lose  their dorsal fins as they adapt to bottom feeding again, going through a process of reversal and convergence that could be misinterpreted without a wide gamut cladogram like the LRT.

Figure 4. The large gill chamber (cyan) of Early Devonian Drepanaspis.

Figure 4. The large gill chamber (cyan) of Early Devonian Drepanaspis.

Phyllolepid placoderms had their origins
with a Silurian placoderm, Entelognathus (Fig. 5), which has tiny eyes, but not yet a disc-like morphology. Entelognathus was originally misinterpreted (Zhu et al. 2013, 2016) as a placoderm at the genesis of jaws. By contrast, in the LRT Entelognathus was losing its jaws, a process that terminates with disc-like phyllolepid placoderms.

Figure 5. Entelognathus in dorsal and lateral views. This taxon also has tiny lateral eyes and is basal to the phyllolepid placoderms.

Figure 5. Entelognathus in dorsal and lateral views. This taxon also has tiny lateral eyes and is basal to the phyllolepid placoderms.

Entelognathus primordialis (Zhu et al. 2013, 2016; Late Silurian, 419 mya)

Drepanapis gemuendenensis (Schlüter 1887; Gross 1963; Early Devonian 405mya)

Minjinia turgenensis (Brazeau et al. 2020; Early Devonian)

Cowralepis mclachlani (Ritchie 2005; Carr et al. 2009; Middle Devonian)

Figure 1. Subset of the LRT focusing on the branch of the Osteichthys that includes placoderms and their relatives.

Figure x. Subset of the LRT focusing on the branch of the Osteichthys that includes placoderms and their relatives.

Figure 1. Wuttagoonaspis from Fletcher 1973. Colors added here.

Figure 1. Wuttagoonaspis from Fletcher 1973. Colors added here.

Catfish also produced a similar morphology.
Wuttagoonaspis (Fig. 6) “is a genus of primitive arthrodire placoderms from Middle Devonian Australia” according to the fish workers posting in Wikipedia. In the LRT (subset Fig. x) it nests with the walking catfish, Clarias. Expand the taxon list, let catfish in, and see for yourself where “What-a-goon-aspis” nests.


References
Brazeau et al. (7 co-authors) 2020. Endochondral bone in an Early Devonian ‘placoderm’ from Mongolia. Nature Ecology & Evolution. https://doi.org/10.1038/s41559-020-01290-2
Broili F 1929. S. B. Bayer. Akad. Wiss., 1
Carr RK, Joahnson Z and Ritchie A 2009. The phyllolepid placoderm Cowralepis mclachani: Insights into the evolution of feeding mechanisms in jawed vertebrates. Journal of Morphology 270(7):775–804.
Ritche A2005. Cowralepis, a new genus of phyllolepid fish (Pisces, Placodermi) from the Middle Devonian of New South Wales. Proceedings of the Linnean Society of New South Wales 126:215–259.
Schlüter EF 1887. Panserfische, etc. Niederrhein. Ges., Bonn, 1887, 120.
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/Cowralepis
wiki/Entelognathus
wiki/Drepanaspis

 

 

 

Adding taxa updates the origin of placoderms

A year ago
when fish (= basal vertebrates) were first added to the large reptile tree (LRT, now with 1757+ taxa; subset Fig. 1), the extant walking catfish, Clarias, nested with the Silurian placoderm, Entelognathus, rather than any other extant bony fish when there were very few other bony fish to nest with. Since then, adding taxa has separated these two, but they still nest as charter members of the unnamed catfish-placoderm clade. That was a heretical hypothesis then, and it remains so today.

Traditional fish paleontologists
consider placoderms basal to sharks and ratfish + bony fish. Stensioella was considered the most basal placoderm by Carr et al. 2009, who did not list outgroup taxa. These hypotheses are not supported by the LRT (subset Fig. 1) where placoderms arise from coelacanths among the bony fish, far from sharks and ratfish.

The LRT divides placoderms into four clades;

  1. Arthrodira (open ocean predators like Dunkleosteus, Coccosteus, Fig. 3)
  2. Antiarchi (armored jawless bottom dwellers like DicksonosteusBothriolepis, Fig. 2)
  3. Ptyctodontida (chimaera-like taxa like Australoptyctodus, Fig. 2)
  4. Phyllolepida (tiny-eye taxa like Entelognathus, Cowralepis)

Several traditional placoderms nest elsewhere in the LRT.

  1. Rhenanida – nests with catfish in the LRT
  2. Wuttagoonaspis – nests with catfish in the LRT
  3. Stensioellida – nests with Guiyu-like lobefins in the LRT
  4. Brindabellspida – nests with the tetrapodomorph Elpistostege

Several traditional placoderms have not yet been tested in the LRT.

  1. Petalichthyida (includes Diandongpetalichthys)
  2. Acanththoraci (closely related to rhenanids, nesting with catfish)
  3. Pseudopetalichthyida (similar to rhenanids, nesting with catfish)

After testing
in the LRT (subset Fig. 1) placoderms are still bony fish close to catfish and this clade still arises from coelocanths.

Figure 1. Subset of the LRT focusing on the branch of the Osteichthys that includes placoderms and their relatives.

Figure 1. Subset of the LRT focusing on the branch of the Osteichthys that includes placoderms and their relatives.

The pertinent taxa in the first list
(Fig. 2) start with the small, Early Devonian spiny shark Diplacanthus and end with the rather flat nearly jawless placoderm, Dicksonesteus also from the Early Devonian. That tells us that every taxon between them was part of the Early Devonian fauna. That also tells us the radiation of taxa in figure 2 must have occurred much earlier, sometime in the middle of the mysterious Silurian, which preserves very few gnathostome fish fossils.

Figure 2. Taxa from the LRT nesting prior to the clade Placodermi.

Figure 2. Taxa from the LRT nesting prior to the clade Placodermi. See figure 3 for the arthrodire clade within Placodermi. Robustichthys is basal to catfish and lacks a squamosal.

Phylogenetic miniaturization
occurs at the origin of placoderms with the smallest specimen in figure 2, Romundina, half the size of its predecessor, Eurynotus. In like fashion, the smallest placoderm in figure 3is the unnamed ANU V244 specimen, is also half the size of its predecessor, the aforementioned Eurynotus.

Figure 3. Arthrodires and their ancestor, Euryodus. See figure 2 for Euryodus ancestors. Note the phylogenetic miniaturization at the origin of the arthrodires.

Figure 3. Arthrodires and their ancestor, Euryodus. See figure 2 for Euryodus ancestors. Note the phylogenetic miniaturization at the origin of the arthrodires.

Phylogenetically, the lack of marginal teeth
in placoderms goes back to a late-surviving taxon from the Jurassic, the angelfish-mimic,  Cheirodus (Fig. 1). Note the hidden palatine teeth in Cheirodus that in the arthrodires, Coccosteus and Dunkleosteus become visible and act as marginal teeth/plates. The Silurian ancestors of Cheirodus may not have been so uniquely angelfish-like. That shape is apomorphic due to the separation in time.

Ptyctodonts, like Austroptyctodus,
(Fig. 2) do not nest with other traditional placoderms in the LRT, but nest closer to Cheirodus.  These are the sort of results the LRT recovers only because it tests more taxa.


References
Carr RK, Johanson Z and Ritchie A 2009. The phyllolepid placoderm Cowralepis mclachlani: Insights into the evolution of feeding mechanisms in jawed vertebrates. Journal of Morphology. 270 (7): 775–804.
Hu Y, Lu J and Young GC 2017. New findings in a 400 million-year-old Devonian placoderm shed light on jaw structure and function in basal gnathostomes. Nature Scientific Reports 7: 7813 DOI:10.1038/s41598-017-07674-y
Miles RS and Young GC 1977. 
Placoderm interrelationships reconsidered in the light of new ptyctodontids from Gogo Western Australia. Linn. Soc. Symp. Series 4: 123-198.
Young GC 1980. A new Early Devonian placoderm from New South Wales, Australia, with a discussion of placoderm phylogeny: Palaeontographica 167A pp. 10–76. 2 pl., 27 fig.
Zhu et al. 2012. An antiarch placoderm shows that pelvic girdles arose at the root of jawed vertebrates. Biology Letters Palaeontology 8:453–456.
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/Entelognathus
wiki/Bothriolepis
wiki/Dicksonosteus
wiki/Romundina
wiki/Qilinyu
wiki/Parayunnanolepis
wiki/Lunaspis
wiki/Coccosteus
wiki/Mcnamaraspis
wiki/Dunkleosteus

Revisions to the catfish + placoderms subset of the LRT

Things were not quite right in the catfish-placoderm clade,
so a critical examination of the traits and scores was due.

As longtime readers know,
every new taxon added to the LRT is a new experience, scored to the best of my nascent ability each time. When the first few taxa were scored, I had little to no experience with any fish. Now, with a substantial taxon list, comparisons can be reexamined that were overlooked or not present before.

Because evolution works gradually,
bones and proportions that appear on one taxon should also appear on closely related taxa. Here tetrapod labels were put on all fish skull bones, so the traditional published fish skull bone labels were not as helpful as they will be once other workers adopt this several times earlier proposed nomenclature standard.

Figure 1. Subset of the LRT focusing on the branch of the Osteichthys that includes placoderms and their relatives.

Figure 1. Subset of the LRT focusing on the branch of the Osteichthys that includes placoderms and their relatives.

The basic tree topology in this clade has not changed. 
A few of the taxa have been rescored (Fig. 1).

Figure 2. Menaspis armatas in situ. Colors added to bones and skin.

Figure 3. Menaspis armatas in situ. Colors added to bones and skin. White area above restores the displaced mandibles relative to one another.

Change #1:
A former odd Permian ‘placoderm’ with barbels, Menaspis (Fig. 2), moved over to the Siluriformes (catfish clade). That only makes sense.

Menaspis armata (Ewald 1848; Late Permian; > 15cm long) was described as the ‘last known arthrodire placoderm’. Here it nests with the catfish, Clarias and Wuttagoonaspis. The former skull spine is the displaced mandible. The former ‘horns’ are barbels. The orbit is somewhere under the barbels. The entire ventral half of the skull is missing here on the counterplate. This is a ventral view of the dorsal skull plates.

Figure 3. Tiny unnamed arthrodire, ANU V244-3 in various views.

Figure 3. Tiny unnamed arthrodire, ANU V244-3 in various views. The upper left image lacks jaws. The jaws are upper right. Palatal view at middle right.

Change #2:
A tiny arthrodire ANU 244  now nests basal to the open water predatory clade of large to giant placoderms.

ANU V244 (Hu, Lu and Young 2017; Early Devonian) is a tiny basal arthrodire. The authors provided several views of the skull, even dividing it in half to show upper and lower elements separately (Fig. 3). The authors followed tradition in the proposal that placoderms were basal to gnathostomes not realizing placoderms have lost the maxilla, like their sisters, the catfish.

Change #3:
Several cheek bones on other placoderms were re-identified following this holistic look at several taxa all at once. Each specimen contributed to the understanding of the clade. Placoderms are highly derived leaving no descendants in the Mesozoic or thereafter. Traditional cladograms nesting placoderms basal to sharks and bony fish are in error, according to the LRT, which tests a wide gamut without prejudice.

On that note: the traditional ptyctodontid ‘placoderms’, Astroptyctodus and Campbellodus (Fig. 4), still nest outside the clade that includes the other placoderms.

Figure 2. Cheirodus and Campbellodus to scale. These two nest together in there LRT.

Figure 4. Cheirodus and Campbellodus to scale. These two nest together in there LRT.

The radiation of catfish and placoderms
must have happened deep in the Silurian with late survivors among the tested taxa.

Placoderms developed internal fertilization
with claspers and live birth of a few large young, convergent with sharks and manta rays.

On the other hand, catfish retained external fertilization
with thousands of eggs produced by a single female through several spawning periods. Typically 10% develop and survive. The first few spawnings produce none or fewer than five eggs.


References
Ewald J 1848. Über Menaspis, eine neue fossile Fischgattung. Berichte Über die zur Bekanntmachung Geeigneten Verhandlungen der Königlich-Preussischen Akademie der Wissenschaften zur Berlin 1848:33-35.
Hu Y, Lu J and Young GC 2017. New findings in a 400 million-year-old Devonian placoderm shed light on jaw structure and function in basal gnathostomes. Nature Scientific Reports 7: 7813 DOI:10.1038/s41598-017-07674-y

https://animaldiversity.org/accounts/Clarias_batrachus/

Marjanović 2018 suggested homologizing fish and tetrapod skull bones

As longtime readers know,
tetrapod bone colors were used here on fish skull photos and diagrams as they were added to ReptileEvolution.com (Figs. 1a, b) over the past two years. That was necessary in order to score several dozen new fish taxa for a growing online phylogenetic analysis that, until then, included only tetrapods.

Figure 1. Amia juvenile with DGS colors added. Image from Digimorph.org and used with permission.

Figure 1a. Amia juvenile with DGS colors added. Image from Digimorph.org and used with permission.

Somehow it all worked out.
I was pleasantly surprised from the start at how readily tetrapod bone colors could be applied to fish skulls (Figs. 1a, b).

Figure 4. Skull of the extant bowfin (Amia). Compare to figure 3.

Figure 1b. Skull of the extant bowfin (Amia).

A few days ago, I learned that back in 2018,
Dr. David Marjanović (researcher, Museum für Naturkunde, Berlin) suggested fish workers do the same in an SVP abstract: “It is difficult to tease apart the homologies of bones across Osteichthyes, often even within Actinopterygii. For a long time, it seems, anatomists gave up the attempt; numerous separate—sometimes contradictory—nomenclatures were used in different decades for different taxa or by different authors. However, a flood of recent discoveries provides grounds for optimism.”

The time to do this is now. It is a great idea, a necessary idea.

“The tetrapod stem is much more densely sampled than 25 years ago, confirming
unambiguously that the large bones of the actinopterygian skull table—which lie in roughly
the same places as the frontal and parietal of crown-group tetrapods—are homologous to
the parietal (the “preorbital” of “placoderms”) and the postparietal. This affects the next
more lateral series as well: as recently proposed, the “dermosphenotic”/“infraorbital 5” is
the intertemporal (which participates in the orbit margin in a few early tetrapods), the
“dermopterotic”/“intertemporal” is the supratemporal and the “supratemporal” is the
tabular.”

Fish nomenclature can get confusing if you’re a tetrapod fan. Here (Figs. 1a, 1b, 2) I don’t identify fish nomenclature at all. I let colors tell the tale: pink for nasals, cyan for jugals, orange for postfrontals, yellow green for intertemporals (= prootics), etc. (Fig. 1). That way if bones split or appear as a result of a split, they can be identified in several views. Unfortunately many earlier tetrapods were colored in a more slipshod manner, not in accord with these standards. Over time these will be repaired.

“Further, the base of the tetrapod stem clarifies the original spatial relationships of other
bones: the bone dorsal of the (anterior) naris is plesiomorphically not the nasal, but the so called anterior tectal, and the one ventral to it is the so-called lateral rostral (apparently
homologous to the septomaxilla of crown-group tetrapods), making it likely that these are
the homologs of the actinopterygian “nasal” and “antorbital” respectively. Unlike in
tetrapods, the squamosal of many other sarcopterygians has a long contact with the maxilla and could be homologous to the (second) “supramaxilla”.

Beyond actinopterygians, tetrapod homologies must be extended back to all craniates and gnathostomes.

Figure 2. Eurynotus is another platysomid, basal to the placoderms Coccosteus and Entelognathus.

Figure 2. Eurynotus is another platysomid, basal to the placoderms Coccosteus and Entelognathus, the most derived of these taxa, not the least derived.

“Outside the tetrapod stem, the placoderm-grade animal Entelognathus has shown that some homologies can be traced beyond Osteichthyes.”

By contrast, the LRT nests Entelognathus (Fig. 2) deep within the Osteichthys, close to extant catfish, a traditionally excluded set of taxa.

Nothing else can proceed unless a valid phylogenetic cladogram, like the LRT, has been established.

“I further propose that the unpaired “vomer” of various actinopterygians is the “prerostral plate” seen in “placoderms” and the Silurian osteichthyan Guiyu, the actual paired vomers being represented by the “vomerine toothplates”. 

The present cladogram (subset Fig. 3) and the colors traced on taxa in ReptileEvolution.com documents the splitting and fusion of the tetrapod homologs of bones in various fish. Sometimes two or three bones represent the lacrimal. The squamosal and the quadratojual result from such bone splits. The maxilla and premaxilla appear on the lower rim of the lacrimal as new bones holding the marginal teeth. In the LRT (subset Fig. 3) Guiyu is a derived taxon close to coelacanths.

Figure x. Subset of the LRT focusing on fish.

Figure 3. Subset of the LRT focusing on fish.

“The braincase remains underresearched even within crown-group tetrapods, and
neomorphic bones seem more common there than in the dermal skeleton; still, it seems
clear that the best candidates for homologs of the opisthotic are the “autopterotic” and/or
perhaps the “epiotic”/“epioccipital” of actinopterygians, not the “intercalary” sesamoid.”

“I propose further homologies throughout the skeleton based on ontogenetic data and the
rich fossil record, and hope to start a discussion on this promising field. Confidently
identified homologs would give a boost to phylogenetics and evolutionary biology.”

I agree!


References
Marjanović D 2018.  Yes, we can homologize skull (and other) bones of actinopterygians and tetrapods. Abstracts Society of Vertebrate Paleontology 2018.

Placoderm double penis: not the primitive trait they think it is

Summary, for those in a hurry:
Contra the title of this Long et al. 2014 paper, the presence of bony claspers (shark-like dual lateral penises) does not signal the origin of gnathostome internal fertilization on this tiny, jawless placoderm, Microbrachius (Fig. 1). Contra Long et al., internal fertilization never gave rise to external fertilization in gnathostomes. Long et al. added Microbrachius to a cladogram invalidated by the exclusion of extant taxa (Fig. 2).

Massive taxon exclusion
in Long et al. 2014 recovered an incomplete and invalid cladogram (Fig. 2) that mistakenly (yet traditionally) nested placoderms with osteostracans and spiny sharks with sharks by excluding all extant taxa and several pertinent extinct taxa.

By contrast
the large reptile tree
(LRT, 1744+ taxa, subset Fig. 3), minimizes taxon exclusion and separates all four clades. In both cladograms (Figs. 2–4) claspers and internal fertilization are phylogenetically separated and thus arose at least twice by convergence in sharks and placoderms.

Figure 1. Microbrachius dicki female and male (with dual lateral penis) specimens several times larger than life size.

Figure 1. Microbrachius dicki female and male (with dual lateral penis) specimens several times larger than life size.

From the Long et al. 2014 abstract
“Reproduction in jawed vertebrates (gnathostomes) involves either external or internal fertilization1. It is commonly argued that internal fertilization can evolve from external, but not the reverse. Male copulatory claspers are present in certain placoderms, fossil jawed vertebrates retrieved as a paraphyletic segment of the gnathostome stem group in recent studies.”

This is incorrect, according to the LRT (subset Fig. 3). A valid phylogenetic context is missing in Long et al. 2014 (Fig. 2) largely due to excluding extant taxa. In Long et al. 2014 Microbrachius is a basal placoderm, basal to sharks and bony fish. By contrast, in the LRT Microbrachius is a derived taxon, a sister to Bothriolepis, leaving no descendants.

Figure 2. Cladogram from Long et al. 2014 lacking any extant taxa and suffering from massive taxon exclusion relative to the LRT in figure 4.

Figure 2. Cladogram from Long et al. 2014 lacking any extant taxa and suffering from massive taxon exclusion relative to the LRT in figure 4.

The Long et al. 2014 abstract continues:
“This suggests that internal fertilization could be primitive for gnathostomes, but such a conclusion depends on demonstrating that copulation was not just a specialized feature of certain placoderm subgroups.”

This is incorrect. According to the LRT sturgeons (Pseudoscaphirhynchus) are pre-gnathostomes and they produce millions of eggs through external fertilization. Paddlefish (Polyodon) are basal gnathostomes and they produce hundreds of thousands of eggs through external fertilization. Between them the Chondrosteus clade includes whale sharks (Rhincodon) and manta rays (Manta) and those two produce only a few young with live birth after internal fertilization. In the same clade, Loganellia (Early Silurian) is the earliest known fossil gnathostome and its reproductive strategy remains unknown at present.

Figure x. Subset of the LRT focusing on fish.

Figure 3. Subset of the LRT focusing on fish. Here, by including extant taxa, sharks are not related to spiny sharks and placoderms are not related to jawless fish.

The Long et al. 2014 abstract continues:
“The reproductive biology of antiarchs, consistently identified as the least crownward placoderms and thus of great interest in this context, has until now remained unknown.”

The addition of fish to the LRT postdates the publication of Long et al. 2015, so the authors did not realize their cladogram became erroneous by excluding extant taxa. After adding taxa (in the LRT) antiarchs are bottom-dwelling placoderms that reduce or lack mandibles as a derived, not a primitive condition.

The Long et al. 2014 abstract continues:
“Here we show that certain antiarchs [placoderms] possessed dermal claspers in the males, while females bore paired dermal plates inferred to have facilitated copulation. These structures are not associated with pelvic fins. The clasper morphology resembles that of ptyctodonts, a more crownward placoderm group, suggesting that all placoderm claspers are homologous and that internal fertilization characterized all placoderms.”

In the LRT ptyctodonts (like Campbelllodus) are closer to catfish (like Clarias) rather than the rest of the traditional placoderms.

In their efforts to simply illustrate their hypothesis,
the cladogram diagram in Long et al. 2014 (Fig. 4) cherry-picked taxa and “Pulled a Larry Martin” by focusing on copulatory organs to the exclusion of a suite of other traits. Excluding all extant taxa is the main problem with this paper. The LRT includes extant taxa and falsifies the cladograms of Long et al. 2014 (Figs. 2, 4) restricted to a select list of extinct taxa, that was the traditional list until the LRT expanded to include fish.

Figure 4. Cladogram diagram from Long et al. 2014 cherry-picking taxa and concentrating on one trait while ignoring extant taxa.

Figure 4. Cladogram diagram from Long et al. 2014 cherry-picking taxa and concentrating on one trait while ignoring extant taxa.

The Long et al. 2014 abstract continues:
“This implies that external fertilization and spawning, which characterize most extant aquatic gnathostomes, must be derived from internal fertilization, even though this transformation has been thought implausible.”

No. That conclusion arises from a phylogeny invalidated by taxon exclusion. In the LRT, by minimizing taxon exclusion the transformation from internal to external fertilization is not documented. External fertilization is always more primitive. Internal fertilization occurred by convergence several times in vertebrate phylogeny.

Just so you know…

  • Sea lampreys produce 30 thousand to 100 thousand eggs
  • Sturgeons produce 100 thousand to 3 million eggs
  • Manta rays give birth to 1.5m babies
  • Whale sharks give birth to .5m babies
  • Paddlefish produce 70 thousand to 300 thousand eggs
  • Moray eels produce 10 thousand eggs
  • Catfish produce 4 thousand to 100 thousand eggs
  • Coelacanths give birth to 5 babies
  • Basal tetrapods, like frogs and salamanders, produce dozens of eggs

References
Long J Mark-Kurik E, Johanson Z et al. 2014. Copulation in antiarch placoderms and the origin of gnathostome internal fertilization. Nature 517, 196–199 (2015). https://doi.org/10.1038/nature13825

wiki/Incisoscutum
wiki/Microbrachius

 

Did shark skeletons evolve from bony ancestors?

Did bone precede cartilage in sharks? 
Or did shark-like cartilage precede bone in bony fish?

Good question.
A good answer will come from a cladogram that accurately mirrors evolutionary events.

Brazeau et al. 2020 bring us
a new, partial placoderm skull, Minjinia turgenensis (Fig. 1), that preserves a great deal of internal bone, and not a lot of dermal bone. Brazeau et al. think their specimen answers the above questions because they think placoderms phylogenetically precede sharks + bony fish.

Figure 1. Minjina in 4 views, mirror-image and colors added.

Figure 1. Minjina in 4 views, mirror-image, tail, pectoral fins and colors added for clarity.

From the Brazeau et al. 2020 text:
“Chondrichthyans (sharks and their kin) are the living sister group of osteichthyans and have primarily cartilaginous endoskeletons, long considered the ancestral condition for all jawed vertebrates (gnathostomes). Phylogenetic analyses place this new taxon [Minjinia turgenensis] as a proximate sister group of the gnathostome crown. These results provide direct support for theories of generalized bone loss in chondrichthyans. Furthermore, they revive theories of a phylogenetically deeper origin of endochondral bone and its absence in chondrichthyans as a secondary condition.”

What came first? The large reptile tree (LRT, 1733+ taxa; subset Fig. x) supports the hypothesis that the absence of endochondral bone in sharks and ratfish is a primitive trait retained from more primitive sturgeons (Pseudoscaphorhynchus) and paddlefish (Polyodon).

According to Wikipedia, sturgeons “are unique among bony fishes because their skeletons are almost entirely cartilaginous.”

According to the Caddo Lake Institute, “The only bone in the [paddle] fish’s body is the jawbone.”

What about placoderms? The LRT nests placoderms deep within one branch of osteichtheys close to catfish. The internal and external placoderm skeleton is made of strong bone. Not sure why this major item of evidence has been traditionally overlooked.

Brazeau et al. continue:
“The absence of bone in modern jawless fishes and the absence of endochondral ossification in early fossil gnathostomes appear to lend support to this conclusion.”

Not really. Sturgeons are pre-gnathostomes in the LRT (subset Fig. x). They are at the genesis of jaws, rather than derived taxa losing their jaws, as commonly thought.

Unfortunately,
extensive taxon exclusion ruins the basics of Brazeau et al. 2020.

Instead
the LRT nests Minjinia with the small, unnamed and better preserved bottom-feeding placoderm ANU  V244 specimen (Fig. 2), a more complete taxon not mentioned by Brazeau et al. 2020. Both nest between the more famous bottom-dwelling placoderms Entelognathus and Bothriolepis.

Figure 1. The tiny ANU V244 specimen in various views. Note the scale bars.

Figure 2. The tiny ANU V244 specimen in various views. Note the scale bars.

Considering the fact that sturgeons and paddlefish have so little bone,
sharks and ratfish don’t have that much bone to lose. We just have to remember to take sturgeons and paddlefish out of the clade of bony fish and put them where the LRT (Fig. x) indicates they nest.

Like other fish workers,
Brazeau et al. 2020 used an out-dated traditional cladogram missing so many pertinent taxa that placoderms nested basal to jawed fish. In the LRT (Fig. x) placoderms nest alongside catfish deep within one branch of the Osteichthyes.

Figure x. Subset of the LRT focusing on fish.

Figure x. Subset of the LRT focusing on fish.

The publicity for Minjinia has been extraordinary.
Sci-News.com reported,
“This discovery suggests the lighter skeletons of sharks may have evolved from bony ancestors, rather than the other way around.”

While true, as shown by the LRT (Fig. x), the phylogenetic context of this placoderm fossil was greatly in need of additional taxa.

From cosmosmagazine.com:
“This 410-million-year-old fossil with a bony skull uncovered in Mongolia may force a rethink of how sharks evolved. Minjinia turgenensis, a new species, is an ancient cousin of both sharks and animals with bony skeletons, the researchers say – and that suggests the lighter skeletons of sharks may have evolved from bony ancestors, rather than the other way around.”

Too few taxa mar this study. In the LRT Minjinia does nest with placoderms, but placoderms nest far from sharks, closer to catfish.

Co-author Martin Brazeau was reported as saying,
“Conventional wisdom says that a bony inner skeleton was a unique innovation of the lineage that split from the ancestor of sharks more than 400 million years ago, but here is clear evidence of bony inner skeleton in a cousin of both sharks and, ultimately, us.”

Not related to sharks. Add taxa and placoderms move close to catfish.

“M. turgenensis belongs to a broad group of fish called placoderms, out of which sharks and all other jawed vertebrates – animals with backbones and mobile jaws – evolved.”

False. The loss of the mandible in one branch of the placoderms should not be confused with the genesis of the mandible in the clade Gnathostomata following sturgeons, a clade at the genesis of jaws in the LRT.

Again, from cosmosmagazine.com:
“The new find suggests the ancestors of sharks first evolved bone and then lost it again, rather than keeping their initial cartilaginous state for more than 400 million years, the researchers say.”

Not exactly true.  Sturgeons and paddlefish are more primitive and have very little bone. Placoderms, like Minjinia (Fig. 1) have lots of bone and nest deep within bony fish.

Sometimes scientists rush off to get publicity
BEFORE waiting a suitable amount of time for feedback (confirmation or refutation). In this case the peer-review process apparently failed because everyone was working from an old playbook. So did the publicity process.


References
Brazeau et al. (7 co-authors) 2020. Endochondral bone in an Early Devonian ‘placoderm’ from Mongolia. Nature Ecology & Evolution. https://doi.org/10.1038/s41559-020-01290-2
Hu Y, Lu J and Young GC 2017. New findings in a 400 million-year-old Devonian placoderm shed light on jaw structure and function in basal gnathostomes. Nature Scientific Reports 7: 7813 DOI:10.1038/s41598-017-07674-y

https://cosmosmagazine.com/nature/evolution/new-thoughts-on-how-sharks-evolved/
http://www.sci-news.com/paleontology/minjinia-turgenensis-08823.html

 

Vaškaninová et al. 2020 test placoderms to describe the origin of marginal teeth

Vaškaninová et al. 2020 
employ several partial placoderms from Czechoslovakia to demonstrate the antiquity of lingual tooth growth (= from the inside out as in modern fishes; Fig. 1).

Unfortunately taxon exclusion mars this study.
Following tradition, the team thought derived placoderms (in the process of losing their teeth) were primitive taxa just gaining teeth (Fig. 1). Like other workers before them, they omitted too many taxa.

By contrast and using a wider gamut of taxa,
we looked at the origin of marginal teeth earlier here. Marginal teeth first appeared in the late-surviving basal paddlefish, Tanyrhinichthys (Fig. 2). The outgroup taxon, late-shriving Chondrosteus, (Fig. 3) lacked teeth and tooth-bearing bones (the premaxilla, maxilla and dentary).

From the Vaškaninová et al. 2020 abstract:
“The dentitions of extant fishes and land vertebrates vary in both pattern and type of tooth replacement. It has been argued that the common ancestral condition likely resembles the nonmarginal, radially arranged tooth files of arthrodires, an early group of armoured fishes. We used synchrotron microtomography to describe the fossil dentitions of so-called acanthothoracids, the most phylogenetically basal jawed vertebrates with teeth, belonging to the genera Radotina, Kosoraspis, and Tlamaspis (from the Early Devonian of the Czech Republic).

Note: In the LRT these taxa are placoderms in the process of losing their teeth. Teeth developed much earlier in the family tree (Fig. 4).

“Their dentitions differ fundamentally from those of arthrodires; they are marginal, carried by a cheekbone or a series of short dermal bones along the jaw edges, and teeth are added lingually as is the case in many chondrichthyans (cartilaginous fishes) and osteichthyans (bony fishes and tetrapods). We propose these characteristics as ancestral for all jawed vertebrates.”

Figure 3. Omitting many pertinent taxa, Vaskaninova et al. constructed this cladogram of tooth evolution. The LRT uses a wider gamut of taxa and recovers a different tree topology.

Figure 1. Omitting many pertinent taxa, Vaskaninova et al. constructed this cladogram of tooth evolution. The LRT uses a wider gamut of taxa and recovers a different tree topology. See figure 4.

In the Vaškaninová et al. 2020 study
basal fish, both jawless and not, are all armored.

Here
in the large reptile tree (LRT, 1707+ taxa) the origin of jaws lacking teeth is close to Chondrosteus (Fig. 3), a derived sturgeon (Fig. 10). In Chondrosteus the upper jaw is the lacrimal. The premaxilla and maxilla have not appeared yet. The lower jaw likewise lacks a dentary and is composed of the surangular and angular.

Figure 2. Skull of Tanyrhinichthys (above) with two bones relabeled. The other fish, Saurichthys, is clearly unrelated.

Figure 2. Skull of Tanyrhinichthys (above) with two bones relabeled. The other fish, Saurichthys, is clearly unrelated. The origin of tiny marginal teeth is close to Tanyrhinnichthys, a basal paddlefish (Fig. 2), the next moreb derived clade in the LRT. The tooth bearing bones (premaxillla, maxilla and dentary) originate as slender dermal layers on the lacrimal and surangular carrying tiny teeth, not much larger than skin denticles.

Adding taxa in the LRT
separates armored Devonian placoderms from armored Silurian jawless fish.

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.

Chronology is not as helpful as phylogeny
in figuring out these transitions, so much so that extant taxa need to be added to fill out the tree topology (Fig. 4).

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

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

Members of the Placodermi
like their relatives the catfish, are relatively derived taxa in the LRT (Fig. 4). Marginal teeth are missing in catfish and placoderms because they both have lost the maxilla along with their last common ancestor, taxa near late-surviving Diplacanthus.

Figure 5. Radotina is a basal taxon in the Vaskaninova et al. cladogram (Fig. 1).

Figure 5. Radotina is a basal taxon in the Vaskaninova et al. cladogram (Fig. 1). Compare to Romundina (Fig. 6) another basal taxon in Vaskaninova et al.

Basal taxa in the Vaskaninova et al. cladogram,
Romundina (Fig. 6) and Radotina (Fig. 5) are rather specialized terminal taxa in the LRT, leaving no descendants. Chondrosteus and Tanyrhinichthys are more generalized and primitive. All living fish, other than sturgeons (Fig. 10), whale sharks and mantas, are derived from Silurian sisters to these two taxa in the LRT.

Figure 10. What little we know of Radotina and where the same bone appears on the more complete Romundina, a terminal taxon in the Placodermi.

Figure 6. What little we know of Radotina and where the same bone appears on the more complete Romundina, a terminal taxon in the Placodermi.

Vaškaninová et al. provide the parts for Kosoraspis
(Fig. 7), a basal taxon without resolution in figure 1. Here (Fig. 8) I provide a possible restoration in which the large curved green bone identified as the ‘preopercular’ is re-identified as a postfrontal (orange in Fig. 8) based on similarities to Clarias, the walking catfish (Fig. 9).

Figure 8. From Vaškaninová et al. 2020, the parts for Kosoraspis. See figure 9 for a reconstruction where the largest bone here (green preopercular) is relabeled a postfrontal.

Figure 7. From Vaškaninová et al. 2020, the parts for Kosoraspis. See figure 9 for a reconstruction where the largest bone here (green preopercular) is relabeled a postfrontal.

Figure 9. Kosoraspis restored as a Devonian catfish like Clarias (Fig. 10).

Figure 8. Kosoraspis restored as a Devonian catfish like Clarias (Fig. 10). Those tooth plates are similar to those in catfish.

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 9. 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.

Determining when teeth and jaws first appeared
in basal vertebrates has been a contentious issue largely because pertinent taxa have been left out of the solution. Apparently Vaškaninová et al. left out several taxa key to understanding this transition from toothless jaws to toothy jaws. They considered taxa in the process of losing teeth, but placed them at the genesis of developing teeth.

Once again,
more taxa resolve problems like this better than more characters do.

Figure 1. Top to bottom: Thelodus a soft jawless fish with a ventral oral opening and gill slits, perhaps a hint of diamond-shaped armor laterally. Hemicyclaspis, adds extensive armor. Acipenser, a sturgeon with a protrusible tube for a mouth and reduced armor.

Figure 10. Top to bottom: Thelodus a soft jawless fish with a ventral oral opening and gill slits, perhaps a hint of diamond-shaped armor laterally. Hemicyclaspis, adds extensive armor. Acipenser, a sturgeon with a protrusible tube for a mouth and reduced armor.

If this helps,
here again (Fig. 10) are three taxa preceding the origin of jaws with marginal teeth. These interrelationships have gone unnoticed by fish workers who continue to nest sturgeons with jawed fishes. The next taxon following these three had large jaws: Chondrosteus (Fig. 3).

Figure 11.  Manta compared to Thelodus (Loganellia) and Rhincodon. All three have a terminal mouth essentially straight across, between the lateral eyes, distinct from most fish. Note the lack of teeth. 

Figure 11.  Manta compared to Thelodus (Loganellia) and Rhincodon. All three have a terminal mouth essentially straight across, between the lateral eyes, distinct from most fish. Note the lack of teeth.

Here again are whale sharks and mantas
(Fig. 11) on their own branch derived from Silurian sisters to Thelodus and LoganelliaThese taxa have jaws, but lack marginal teeth, similar to Chondrosteus (Fig. 3).

As mentioned above,
it is so important to include a wide gamut of taxa, including extant taxa.


References
Vaškaninová V, Chen D, Tafforeau P, Johanson Z, Ekrt B, Blom H and Ahlberg PE 2020. Marginal dentition and multiple dermal jawbones as the ancestral condition of jawed vertebrates. Science 369(6500): 211-216 DOI: 10.1126/science.aaz9431
https://science.sciencemag.org/content/369/6500/211

placoderm jaws

News:
https://phys.org/news/2020-07-advanced-technology-evolution-teeth.html

Placoderm jaw de-volution

About an hour ago, 
the question of pelvic girdles before jaws in vertebrate (more specifically, placoderm) evolution was reviewed in light of the LRT.

Now let’s re-examine
another tiny placoderm whose interrelationships were originally misinterpreted due to taxon exclusion.

Hu, Lu and Young 2017
studied jaw structure in a really tiny unnamed Devonian placoderm, ANU V244 (Fig. 1, shown 3x larger here) preserved in 3D. Unfortunately, Hu, Lu and Young followed tradition when they thought placoderms represented the genesis of jaw evolution, preceding the appearance of jaws in sharks and bony fish.

By contrast,
the large reptile tree (LRT, 1697+ taxa, subset Fig. 2) recovers sturgeons and Chondrosteus at the genesis of jaws, immediately preceding sharks + bony fish. In the LRT placoderms nest deep within bony fish, after the great dichotomy. Placoderms like the ANU specimen represent a reduction of jaw elements, not the acquisition. Placoderm precursors, like Cheirodus and Eurynotus, lose or fuse the maxilla. The ANU specimen also loses the premaxilla and reduces the mandible and dentary, which retains teeth.

Figure 1. The tiny ANU V244 specimen in various views. Note the scale bars.

Figure 1. The tiny ANU V244 specimen in various views. Note the scale bars.

Figure 2. Subset of the LRT focusing on catfish + placoderm clade.

Figure 2. Subset of the LRT focusing on the catfish + placoderm clade, starting with a spiny shark, Diplacanthus.

The ANU specimen nests with the much larger Romundina (Fig. 2), a bottom feeder with reduced jaw elements and large cheeks.

The ANU specimen
is only one of several placoderms with reduced jaws (Fig. 3).

The LRT has been adding fish taxa over the past year, when the first catfish was nested with the first few placoderms. As it stands now, catfish are still closely related to placoderms in the LRT (subset Fig. 2).

The origin of placoderms would make a great PhD thesis, seen from all angles.

Figure 2. A sample of taxa related to Autroptyctodus with homologous skull bones color identified

Figure 3. A sample of taxa related to Autroptyctodus with homologous skull bones color identified.

Once again,
a valid phylogenetic analysis that includes a sufficient number of pertinent taxa is key to understanding interrelationships. Don’t get turned around by using the traditional list of too few taxa. Don’t assume your predecessors and professors are correct. Test their hypotheses. Add taxa to provide, determine and validate the proper phylogenetic context in all cases. In can tell you from experience, it will be rewarding.


References
Hu Y, Lu J and Young GC 2017. New findings in a 400 million-year-old Devonian placoderm shed light on jaw structure and function in basal gnathostomes. Nature Scientific Reports 7: 7813 DOI:10.1038/s41598-017-07674-y

Antiarch placoderms: pelvic girdles before jaws? No.

Zhu et al. 2012
report that “An antiarch placoderm (Fig. 1) shows that pelvic girdles arose at the root of jawed vertebrates.” They are wrong according to the the large reptile tree (LRT, 1697+ taxa).

Contra Zhu et al. 2012,
jaws were just disappearing, not just appearing, in this taxon. Pelvic fins and their pelvic anchors are known in many more primitive taxa in the LRT.

Taxon exclusion, once again,
rises to the top of paleontological sins (of omission).

Figure 1. Parayunanolepis, an antiarch placoderm and the subject of the Hu et al. paper.

Figure 1. Tiny Parayunanolepis, an antiarch placoderm and the subject of the Zhu et al.2012 paper, shown more than 2x life size.

From the Zhu et all. 2014 abstract:
“To date, it has generally been believed that antiarch placoderms (extinct armoured jawed fishes from the Silurian–Devonian periods) lacked pelvic fins. Parayunnanolepis xitunensis represents the only example of a primitive antiarch with extensive post-thoracic preservation, and its original description has been cited as confirming the primitive lack of pelvic fins in early antiarchs. Here, we present a revised description of Parayunnanolepis and offer the first unambiguous evidence for the presence of pelvic girdles in antiarchs.”

Figure 2. Subset of the LRT focusing on catfish + placoderm clade.

Figure 2. Subset of the LRT focusing on catfish + placoderm clade.

By contrast,
in the LRT tiny  Parayunnanolepis nests with the much larger Bothriolepis, a highly derived placoderm. Several taxa preceding these two have pelvic fins and jaws.

A valid cladogram
is the most important tool in recovering the order of gradually accumulating traits.

Earlier you may remember,
placoderms arose from ordinary fish, not the other way around. The LRT has reordered many tree branches, all due to taxon inclusion. In this fashion the LRT helps recover overlooked hypothetical interrelationships.

References
Zhu M, Yu X-B, Choo B, Wang J-Q and Jia L-T 2012. An antiarch placoderm shows that pelvic girdles arose at the root of jawed vertebrates. Biology Letters Palaeontology 8:453–456.